Clinical Applications of Small-molecule PET Radiotracers: Current Progress and Future Outlook

ARTICLE IN PRESS

Clinical Applications of Small-molecule PET Radiotracers: Current Progress and Future Outlook Amy L. Va¯vere, PhD,* and Peter J.H. Scott, PhD† Radiotracers, or radiopharmaceuticals, are bioactive molecules tagged with a radionuclide used for diagnostic imaging or radiotherapy and, when a positron-emitting radionuclide is chosen, the radiotracers are used for PET imaging. The development of novel PET radiotracers in many ways parallels the development of new pharmaceuticals, and small molecules dominate research and development pipelines in both disciplines. The 4 decades since the introduction of [18F]FDG have seen the development of many small molecule PET radiotracers. Ten have been approved by the US Food and Drug Administration as of 2016, whereas hundreds more are being evaluated clinically. These radiotracers are being used in personalized medicine and to support drug discovery programs where they are greatly improving our understanding of and ability to treat diseases across many areas of medicine including neuroscience, cardiovascular medicine, and oncology. Semin Nucl Med ■■:■■–■■ © 2017 Elsevier Inc. All rights reserved.

Introduction

I

n 2016, the nuclear medicine community marked the 40th anniversary of the first human PET studies with [18F]FDG.1 Following its introduction in the 1970s,2 and marketing and reimbursement approval by the US Food and Drug Administration (FDA) and the Centers for Medicare & Medicaid Services in the 1990s, [18F]FDG continues to be the workhorse of PET imaging. For example, ~5000 [18F]FDG PET scans and ~800 pediatric scans are performed at the University of Michigan Health System and St. Jude Children’s Research Hospital, respectively, on an annual basis. From the mid-1970s to the mid-2000s, PET imaging was dominated by [18F]FDG oncology imaging, with only a few other PET radiotracers gaining FDA approval in this early period ([18F]sodium fluoride, [13N]ammonia, and [82Rb]rubidium chloride). From the mid-2000s to the present day, the PET landscape has changed considerably. On the manufacturing end, PET drug producers need to comply with new regulations, which govern PET drug manufacture according to the principles of current Good Manufacturing Practice.3 In the United *Department of Diagnostic Imaging, St. Jude Children’s Research Hospital, Memphis, TN. †Department of Radiology, University of Michigan, Ann Arbor, MI. Address reprint requests to Peter J.H. Scott, PhD, Department of Radiology, University of Michigan, Ann Arbor, MI 48105. E-mail: pjhscott@ med.umich.edu

http://dx.doi.org/10.1053/j.semnuclmed.2017.05.001 0001-2998/© 2017 Elsevier Inc. All rights reserved.

States, such regulations are found in the U.S. Pharmacopeia or the code of federal regulations (21CFR212: Current Good Manufacturing Practice for Positron Emission Tomography Drugs). In nuclear medicine clinics, the last decade has seen approval of numerous new PET radiotracers (Table 1). Although [18F]FDG is still the most used, the availability of all of these new radiotracers has led to an evolution in the imaging environment. Anecdotally, several PET centers now conduct more PET scans with gallium-68 tracers than with [18F]FDG. One thing that remains abundantly clear when reviewing both the therapeutic and the new PET drug markets is that small molecules continue to be extremely important in both drug discovery and radiotracer development. Although increasing numbers of biopharmaceutical companies are developing biological drugs (including immunotherapy4 and immuno-PET5), small molecules still very much dominate the pharmaceutical6 and radiopharmaceutical7 marketplaces and discovery pipelines. By way of example, small molecule drugs made up 6 of the top 10 selling drugs (Abilify [$7.9 billion], Sovaldi [$6.9 billion], Crestor [$5.9 billion], Harvoni [$5.3 billion], Nexium [$5.3 billion], and Advair Diskus [$4.7 billion]) and 9 of the top 10 most prescribed drugs in the United States in 2015.7 Most PET radiotracers approved to date by the US FDA are also small molecules (Table 1), including [18F]FDG, [11C]choline, [13N]ammonia, AMYVid (florbetapir F18), Neuraceq (florbetaben F18), Vizamyl (flutemetamol F18), and Axumin (fluciclovine F18). 1

ARTICLE IN PRESS A.L. Va¯vere and P.J.H. Scott

2 Table 1 PET Radiotracers With FDA Approval Radiotracer

Trade name

Manufacturer

Indication*

[11C]Choline [18F]FDG

N/A N/A

Mayo Clinic Multiple

[18F]Florbetapir [18F]Florbetaben [18F]Flutematemol [18F]Sodium fluoride [18F]Fluciclovine [68Ga]DOTATATE

AMYVid Neuraceq Vizamyl N/A Axumin Netspot

[13N]Ammonia [82Rb]Rubidium chloride

N/A Cardiogen-82/ RUBY-FILL

Avid/Eli Lily Piramal Imaging GE Healthcare Multiple Blue Earth Diagnostics Advanced Accelerator Applications Multiple Bracco Diagnostics/ Jubilant Draximage

Prostate cancer recurrence PET imaging agent for assessing abnormal glucose metabolism in oncolog or myocardial hibernation, and to identify regions of abnormal glucose metabolism associated with epileptic seizures Β-Amyloid plaque density Β-Amyloid plaque density Β-Amyloid plaque density PET bone imaging Prostate cancer recurrence Localization of somatostatin receptor positive neuroendocrine tumors Myocardial rest or stress testing PET myocardial perfusion imaging

*Table provides abbreviated indications only. Please refer to drug package inserts for full information of approved indications in adults and pediatric patients.

Rubidium-82, [18F]NaF, and NETSPOT (Ga68 DOTATATE) are the only exceptions that fall outside of what is usually considered a small molecule. There remains some debate over what constitutes a small molecule. Medicinal chemists typically mean drug molecules with molecular weight (MW) ≤500-550 Da that fall into Lipinski space.8 Molecular biologists on the other hand often use the term to cover species that help regulate biological processes, or which can diffuse across cell membranes, and generally talk about small molecules as those with MW ≤900 Da.9 Others use the term more liberally to distinguish such drugs from macromolecules, also known as biologics, such as therapeutic proteins. However, it is recognized that there are increasing numbers of drugs and diagnostics that break the Lipinski rules, but are not large enough to be considered biologics. For example, DOTATATE is (Tyr3)-octreotate derivatized with a DOTA group suitable for chelating radioactive metal ions such as 68Ga for diagnostic purposes or 177Lu for radiotherapy and has an MW of 1435.63 Da. The development of numerous such drugs in recent years has necessitated the introduction of new terminology, including extended Lipinski space (MW = 500-700 Da) or beyond Lipinski space (MW = 700-3000 Da).10 Whichever definition of small molecule is chosen, there is no question that Chemical Space is vast.11 Of the estimated 10200 possible chemical molecules, virtual Lipinski space alone has been estimated to contain as many as 1060 molecules, and probably about 108 of these have been synthesized and exist in public databases, molecule repositories, or corporate collections (Fig. 1).7 Unsurprisingly, small molecule space is extremely versatile and, beyond those FDA-approved radiotracers summarized in Table 1, thousands of small molecule radiotracers have been developed for applications in PET imaging, and millions of patients receive PET scans every year. Reflecting this move toward development and commercialization of PET tracers, development of new methods for their

synthesis has become an active area of research in its own right.12 Considerable intellectual property covering new tracers and methods for their production has also been issued in recent years, and the Scott group has recently reviewed intellectual property in the context of fluorine-18.13,14 With thousands of new radiotracers being developed worldwide, a comprehensive review of all of them is beyond the scope of this article, but herein we discuss current trends in clinical PET imaging. In keeping with the theme of this issue of Seminars in Nuclear Medicine, we have concentrated on the use of small molecule PET radiotracers and discuss hot topics in neurology, oncology, and cardiology.

Figure 1 The figure depicts a cartoon representation of the relationship between the continuum of chemical space (light blue) and the discrete areas of chemical space that are occupied by compounds with specific affinity for biological molecules. Reprinted from Lipinski and Hopkins.11 Copyright (2004) with permission from Macmillan Publishers Ltd.

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers

3

Neurology Many clinically overlapping neurologic disorders are often difficult to distinguish while patients are alive and, historically, have been definitively diagnosed postmortem by identification of disease-specific pathology. Developing molecular imaging approaches aimed at identifying such pathology in living patients is a very active area of research, with the goals of distinguishing such disorders as early as possible, and supporting development of disease-modifying therapies. Over the last 30 or 40 years, PET has provided unique and powerful insights into normal brain function as well as malfunction associated with various neurologic disorders and diseases.15 The developments in this time have been remarkable and are apparent when comparing articles about early central nervous system (CNS) imaging in the very first issue of Seminars in 1971,16 with the articles reviewing the latest advances in brain imaging in the first issue of Seminars in 2017,17 as well as in the present issue. Our intent with this overview is not to replicate the extensive information provided in these issues of Seminars, or recent review articles published elsewhere,18 but rather highlight important topics and current focuses of brain PET research as well as future potential. With this in mind, we discuss small molecule radiotracers targeting misaggregated proteins and neuroinflammation as hot topics that demonstrate the potential of brain PET in 2017.

Misfolded Proteins Numerous different neurodegenerative disorders are associated with 1 or more misaggregated proteins. This includes the complex dementia landscape, consisting of multiple disorders that frequently have clinically overlapping symptoms such as Alzheimer disease (AD) and dementia with Lewy bodies (DLB), but also movement disorders such as Parkinson disease (PD), multiple sclerosis (MS), Huntington disease, and traumatic brain injury (TBI). Identifying the protein pathology responsible on a case-by-case basis is therefore critical for effective therapeutic intervention and patient management across all of these disorders. Moreover, in certain cases, misaggregated protein burden is correlated with irreversible neuronal loss and associated cognitive decline. Accordingly, moving diagnostic strategy from detection of signs and symptoms of midstage disease to early detection of pharmacologic biomarkers or pre-symptomatic disease can be expected to have positive impact on patient prognosis. Diagnostic tests that allow identification of abnormal protein pathology in presymptomatic disease, and enable initiation of appropriate treatment before loss of neuronal function, are therefore of the utmost importance. In the PET field, considerable research has focused on development of radiotracers for dementia through design of radiotracers that enable quantification of misaggregated proteins such as amyloid-beta, tau, and α-synuclein.19,20 AD is the most common form of dementia, accounting for up to 60%-80% of all cases.21 It is a progressive and fatal disorder associated with memory loss and cognitive decline that has severe impact on the daily living of not only patients with AD, but also their families and caregivers. The disease is characterized by misaggregated extracellular amyloid-beta plagues

Figure 2 PET radiotracers for amyloid plaques.

and intracellular tau neurofibrillary tangles (NFTs). Drugs that target the clearance of both proteins are being considered as treatment strategies in AD.22,23 Because the amyloid hypothesis postulates that an imbalance between production and clearance of amyloid is a possible initiating factor in AD,24 early efforts developing radiotracers for AD focused on amyloid plaques (Fig. 2). Klunk et al. were pioneers in this area, developing the prototypical [11C]Pittsburgh compound B ([11C]PiB).25 Successful delineation of amyloidpositive subjects from amyloid-negative subjects has led to the development and subsequent commercialization of a range of amyloid tracers including AMYVid (florbetapir F18, Avid Radiopharmaceuticals/Eli Lilly), Neuraceq (florbetaben F18, Piramal Imaging), Vizamyl (flutemetamol F18, GE Healthcare), and [18F]NAV4694 (Navidea Biopharmaceuticals). These tracers are all labeled with fluorine-18 because the longer halflife (t1/2 C11 = 20 minutes; t1/2 F18 = 110 minutes) allows manufacture at centralized nuclear pharmacies and distribution to satellite imaging centers. Thousands of subjects with normal cognition, mild cognitive impairment (MCI), and AD have received amyloid PET scans to date, leading to large data sets such as those of the Alzheimer’s Disease Neuroimaging Initiative26 and the Dominantly Inherited Alzheimer Network.27 Moreover, strategies for standardizing the quantitative estimation of amyloid plaque burden between different PET centers have been developed, allowing comparison of data obtained using the different amyloid tracers.28 Amyloid PET has allowed, for the first time, accurate detection of amyloid plaques in living people (Fig. 3), and having this information available on a patient-bypatient basis is improving diagnostic confidence. In a healthcare setting, this is enabling clinicians to proceed with greater knowledge and assurance, giving patients (and their families) greater confidence in their diagnoses and treatment recommendations.29 PET imaging allows drug discovery teams to easily explore questions central to the evaluation of new drug candidates such as confirmation of target engagement, identification of off-target binding, and establishment of the absorption-distributionmetabolism-excretion profile (for recent reviews on the use of PET in drug discovery, see Refs. 30-32). With the wealth of information now available from amyloid PET, it is not surprising that the technology is increasingly being used to support AD drug

ARTICLE IN PRESS 4

Figure 3 Amyloid burden as assessed by amyloid-beta (Aβ) ligands. Reprinted from Villemagne et al.20 Copyright (2017), with permission from Elsevier.

discovery programs. At its simplest, this can be using the established amyloid radiotracers to confirm patient eligibility for amyloid therapy and enrollment in an associated clinical trial. Scanning patients throughout the trial also allows monitoring of patient response to therapy. For example, Rinne and colleagues used [11C]PiB in a phase 2 clinical trial to determine whether bapineuzumab, a humanized anti-amyloid-β monoclonal antibody, reduced cortical

A.L. Va¯vere and P.J.H. Scott amyloid burden in patients with AD. The trial showed that treatment with bapineuzumab reduced cortical [11C]PiB retention compared with both baseline and placebo (Fig. 4).33 Amyloid (and tau) imaging also played a pivotal role in Eli Lilly’s recent Expedition 3 Phase 3 study of solanezumab.34 Unfortunately, however, neither bapineuzumab nor solanezumab has progressed to market. Pfizer and Johnson & Johnson reported that bapineuzumab failed to outperform a placebo at moderating symptoms of mild-to-moderate AD during phase 3 trials,35 whereas Eli Lilly recently confirmed that patients treated with solanezumab did not experience slowing in cognitive decline compared with those given placebo.36 Efforts continue to develop a drug for AD, and therapeutic candidates such as Biogen’s aducanumab are currently in clinical trials that are also being supported by amyloid PET.37 PET isotopologs of drug molecules can also be prepared, enabling use of imaging to establish the pharmacodynamic and pharmacokinetic profile of the candidate and confirming suitability for further development. In a recent example, Langer and coworkers carbon 11-labeled a potential anti-amyloid drug, [11C]1,1'-methylene-di-(2-naphthol) ([11C]ST1859),38 and used PET imaging to assess CNS penetration, distribution and metabolism, and local tissue pharmacokinetics of the study drug in the brains of normal controls and patients with AD (Fig. 5).39 Beyond AD, amyloid imaging agents are also being used for investigation of other neurodegenerative disorders.40 For example, amyloid plaques are frequently found in postmortem examination of patients with TBI, and epidemiological data suggest that TBI may substantially increase the risk of developing AD later in life.41 Because TBI cases are increasing in a number of specific patient groups, including professional athletes and veterans, amyloid PET is potentially useful in understanding the pathophysiology of the condition, including patterns of amyloid deposition and possible links to AD. With studies to date demonstrating agreement between amyloid PET imaging data and postmortem distribution of amyloid in TBI,42 just

Figure 4 [11C]PiB PET images from patients treated with bapineuzumab and those given placebo and estimated change from baseline over time in mean [11C]PiB PET. Reprinted from Rinne et al.33 Copyright (2010), with permission from Elsevier.

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers

Figure 5 Transaxial magnetic resonance-co-registered PET summation images recorded from 20 to 90 minutes after intravenous injection of [11C]ST1859 into 1 control subject (left) and 1 patient with AD (right). Reprinted from Bauer et al.39 Copyright (2006), with permission from Wiley.

like AD, using PET to understand amyloid deposition and clearance in TBI can be expected to play an important role in effective management of patients with the disorder. PET imaging is also widely used in movement disorders. Frequently, this involves imaging dysfunction of the various neurotransmitters (and associated receptors and transporters) implicated in such conditions (for a recent review, see Ref. 43). However, the development of imaging agents targeting the misfolded protein deposits that characterize these neurodegenerative disorders will improve patient diagnosis and allow monitoring of therapeutic responses to new experimental treatments. In the case of amyloid PET, for example, [11C]PiB uptake has shown correlation between amyloid plaque burden and cognition in patients with PD at risk of dementia.44 Amyloid imaging is an extremely valuable tool for neuroimaging and supporting ongoing global efforts to better understand brain function. However, there are limitations associated with the technique. First and foremost, the FDA-approved radiotracers (AMYVid, Neuraceq, and Vizamyl) are not approved for the diagnosis of AD. These restrictions are in place because the presence of amyloid plaques by themselves is insufficient for a positive diagnosis of AD, as other neurodegenerative disorders such as DLB are also associated with amyloid pathology.45 Second, anti-amyloid therapy is not the only strategy being considered for AD. In fact, neuropathologists have demonstrated that cognitive decline does not correlate with amyloid burden in AD,46 and this finding has been corroborated by amyloid PET.47 Evaluation of alternative therapeutic strategies for AD is therefore extremely important, and biomarkers for such approaches are also required. Another misaggregated protein target is tau, which is found in the AD brain. Tau protein is associated with microtubules, specifically promoting their assembly and stabilizing them. However, in AD there is dysfunction of enzymes responsible for phosphorylation of tau, which gives rise to a hyperphosphorylated version that aggregates and forms insoluble NFTs.48 Although the formation of amyloid plaques is thought to occur first in the AD cascade, pathology studies have shown that tau NFTs, unlike amyloid plaques, track closely with cognitive decline in patients with AD.49 Given this correlation, as well as the number of tau-based therapies being developed,23 tau radiotracers would allow the use of PET to select

5 tau-positive patients with AD for clinical trials and to monitor their response to therapy in an analogous fashion to the amyloid tracers previously described. In addition to AD, misaggregated tau is implicated in a range of tauopathies (eg, progressive supranuclear palsy [PSP], corticobasal degeneration [CBD], and the tau variant of frontotemporal dementia [FTD]).50 A tau PET radiotracer will improve diagnostic confidence across the entire dementia spectrum. The synergistic combination of amyloid and tau PET can also be expected to be powerful in differentiating AD from these other amyloidnegative tauopathies, or non-AD dementias such as amyloidpositive or tau-negative DLB. A larger number of potential tau radiotracers have been reported in recent years (Fig. 6) and have been extensively reviewed.51-55 Tau imaging has proven more challenging than amyloid imaging for a number of reasons.56 First, tau exists in 6 isoforms. The isoforms of tau can be subcategorized into 3R or 4R variants depending on whether they contain 3 or 4 repeated microtubule-binding domains, respectively. Because 3R and 4R tau are accumulated in different tauopathies (Table 2), any new tau radiotracers need to be evaluated for their ability to bind both 3R and 4R tau to determine their clinical utility. Second, aggregated tau is found intracellularly at lower levels than extracellular amyloid. Most tau imaging has been conducted in AD to date. Therefore, it is noteworthy that for a tau radiotracer to be useful in AD, it should have good selectivity for tau over amyloid plaques as these proteins can co-localize. Early agents, such as [18F]2-(1-{6-[(2-[18F]fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene)malono nitrile (FDDNP), bind to both and so tend to be limited to imaging in pure tauopathies.57 Several newer tau radiotracers that do have good selectivity for tau over amyloid have been reported recently. Of those, [18F]AV1451 ([18F]T807, flortaucipir F18),58-60 the THK series (including [18F]THK523, [18F]THK5105, [18F]THK5117, and, most recently, [18F]THK5351),61-64 and [11C]PBB365,66 have been most used in clinical imaging studies. The binding of all 3 scaffolds to the tau NFTs found in AD has been confirmed using postmortem brain tissue samples. For example, [18F]AV1451 has been shown to correlate with NFT burden and Braak stagingin postmortem AD brain tissue.67 In clinical imaging studies with this tracer, uptake also agrees with the reported distribution of tau in Braak staging of MCI and AD (Fig. 7). Longitudinal studies have also shown that uptake of [18F]AV1451 and THK tracers correlate with NFT burden and the degree of cognitive decline in patients with MCI and AD.68,69 This is consistent with the idea that the accumulation of misaggregated tau leads to neuronal cell dysfunction and ultimately death, and also that tau burden correlates with decreased cognition and AD progression. There is also growing use of these tau tracers in non-AD tauopathies, such as PSP, CBD, and FTD. The ability to use such radiotracers to distinguish these disorders from one another is as important to neurologists as understanding tau pathology in AD. If tau radiotracers are to be useful across the tauopathy spectrum, it is necessary for them to have affinity to both 3R and 4R tau (Table 2). Preliminary findings using these radiotracers in non-AD tauopathies have proven complicated, however, and there is disagreement about which

ARTICLE IN PRESS A.L. Va¯vere and P.J.H. Scott

6

Figure 6 PET radiotracers for tau.

forms of tau they bind to, as well as inconsistencies between apparent in vivo imaging results and postmortem studies. For example, a number of groups have investigated binding of [18F]AV1451, [18F]THK5351, and [11C]PBB3 to the 4R tau found in PSP. Rabinovici and coworkers report that [18F]AV1451 uptake matches the known distribution of tau in patients with AD and PSP, but describe overlap in signal between PSP and controls.70 Similarly, Cho et al report [18F]AV1451 binding that reflects subcortical tau in PSP, but no evidence of cortical tau or correlation with severity of motor dysfunction.71 Their analysis, like that of others, was also complicated by varying uptake in controls. Meanwhile, there are numerous studies that report [18F]AV1451 binds to AD tau but shows only low (or no) binding to PSP tau in postmortem tissue samples.72-74 More promising results were obtained Table 2 Different Tau Isoforms in Common Neurodegenerative Disorders Disorder

Tau isoform Aggregated tau found in disorder

AD CBD

3R and 4R 4R

PSP

4R

Frontotemporal 3R dementia

NFTs Astrocytic abnormalities (eg, plaques) Tufted astrocytes and globose tangles Pick bodies

using [18F]THK5351 and [11C]PBB3. [18F]THK5351 has been used to detect tau in patients with PSP, where it appeared to correlate with tau pathology at postmortem and clinical severity of disease.75,76 However, conflicting reports, which state THK5351 does not bind to PSP tau postmortem, should also be considered in its evaluation.74 [11C]PBB3 on the other hand has been shown to accumulate in regions of neurologic symptoms in PSP.77 A 4R form of tau is also found in CBD, and recent reports include imaging with [ 18 F]AV1451, [ 11 C]PBB3, and [18F]THK5351.65,78,79 In the case of [18F]AV1451, McMillan and colleagues describe highest uptake in deep gray matter areas commonly associated with CBD pathology, and increased uptake was observed in a follow-up scan later in the course of disease. Kikuchi and colleagues reported that [18F]THK5351 binds to CBD tau in vivo, and also showed that [3H]THK5351 binds to tau deposits in postmortem CBD brain tissue samples. However, both of these reports conflict with recent postmortem studies suggesting that binding does not correlate with tau burden in CBD for either [18F]AV1451 or [18F]THK5351.73,74 Imaging of tau deposits in the tau variant of FTD is also being investigated. For example, [18F]AV145180,81 and [11C]PBB382 PET have both shown utility in imaging patients with suspected FTD with MAPT mutations, although once more there is disagreement between these findings and postmortem studies with FTD brain tissue.74 Imaging of tau in AD is quite well established at this point. Contrastingly, for non-AD tauopathies, the results to date are

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers

7

Figure 7 Tau images reprinted from Ariza et al.54 Copyright (2015) with permission from the American Chemical Society.

oftentimes contradictory, with notable discrepancies between in vivo imaging data and postmortem autoradiography findings. The differences could be related to study sizes. Many patients with AD have been scanned with the tau tracers. In contrast, many of the non-AD tauopathies are quite rare, and some of the studies described herein concern single-patient data. Therefore, larger scale imaging and postmortem studies will be needed to assess the utility of the different radiotracers for imaging tau in the various disorders. Moreover, despite the positive results with these tau radiotracers, and the valuable insights they have provided into tau pathology to date, there are issues with all of them that complicate their use, issues that still need to be addressed. For example, [11C]PBB3 is associated with a radioactive metabolite that complicates image analysis,83 and so the team is working to resolve that issue and also develop fluorine-18 labeled radiotracers ([18F]AM-PBB3 and [18F]PM-PBB3) more suited for commercial development.84 [18F]AV1451 and [18F]THK5351 on the other hand appear to exhibit specific off-target binding to monoamine oxidase,85-87 which can also complicate data analysis. As a result of these issues, new tau radiotracers continue to be developed. For example, the Scott lab has developed [11C]and [18F]-N-methyl lansoprazole and is currently conducting first in man studies in collaboration with the University of

Oslo and PositronPharma.88,89 New tau tracers have also recently been disclosed by a number of pharmaceutical companies including Merck ([18F]MK6240),90,91 Roche ([18F]RO6958948),92 Genentech ([18F]GTP1),93 and Piramal Imaging ([18F]PI2014).94 Although only preliminary data have been reported to date, all of these radiotracers appear to be promising candidates for detecting tau in a clinical setting, and we will follow their future development and evaluation with interest. Imaging amyloid and tau have been invaluable to our understanding of AD and related tauopathies. Building on this work, the ability to image α-synuclein deposits using PET is of critical importance to improve our understanding of synucleinopathies.95 The synucleinopathies are a group of disorders, characterized by α-synuclein accumulating in Lewy bodies and dystrophic neurites, which include multisystem atrophy, pure autonomic failure, Parkinson disease with dementia, and DLB. The Michael J. Fox Foundation Consortium, along with other research groups, has been working to develop PET radiotracers with high affinity for α-synuclein and selectivity over other protein aggregates such as amyloid,96 and a $2 million prize for the first group to validate such a radiotracer was also recently offered by the Michael J. Fox Foundation.97 Several promising leads are being investigated (Fig. 8), for both PET and SPECT imaging, and progress

ARTICLE IN PRESS A.L. Va¯vere and P.J.H. Scott

8

to support AD therapeutic development by populating clinical trials and monitoring response to experimental disease modifying therapies. A number of radiotracers are also in advanced stages of development for imaging tau across the tauopathy spectrum. Those reported to date have proven useful for imaging tau in AD, although there are off-target binding issues or metabolic stability concerns that should be optimized further. Use of the tau radiotracers for imaging nonAD tauopathies is still being investigated. Currently, there are still conflicting results about their affinity for the other types of tau associated with these disorders, possibly owing to small sample sizes of patients with these rare diseases that have been imaged to date. Finally, development of radiotracers for other misfolded proteins implicated in neurodegenerative disorders is a very active area of research, with promising radiotracers for α-synuclein in development as described herein, and development of PET radiotracers for huntingtin102 and TDP43103 also starting to attract attention. Having access to tracers for these different protein deposits will greatly increase our understanding of the pathology and progression of neurodegenerative disorders, including the numerous clinically overlapping subtypes of dementia.

Neuroinflammation

Figure 8 Potential PET radiotracers for α-synuclein.

to date has been recently reviewed.98,99 In the SPECT area, efforts have focused on [125I]SIL23,100 whereas Zhang and colleagues have prepared derivatives labeled with fluorine-18 and carbon-11.101 Although the radiotracers have reasonable affinity for α-synuclein (Ki 32-58 nM), they only have ~2- to 4-fold selectivity over tau and amyloid. A number of PET radiotracers based on different scaffolds are also being explored. For example, researchers at Tohoku and Melbourne have synthesized [11C]- and [18F]BF227, and demonstrated that they have affinity for α-synuclein in addition to amyloid plaques. In both cases, further derivatives with higher selectivity are necessary if either is to be useful as an α-synuclein imaging agent. Better selectivity has been obtained by the Mach group at the University of Pennsylvania, who reported that [18F]WC-58a has high affinity for α-synuclein, ~5- to 6-fold selectivity for α-synuclein over tau, and ~30-fold selectivity over amyloid. However, the high log P of [18F]WC-58a leads to high levels of nonspecific binding, meaning that secondgeneration derivatives of this scaffold are also required to develop an optimal α-synuclein radiotracer. In summary, developing PET radiotracers for misfolded proteins has been an extremely active area of research over the last 2 decades. Much progress has been made in amyloid imaging, with numerous radiotracers gaining FDA approval. These have been widely used to image large cohorts of patients, which has improved our understanding of AD and

Inflammation is increasingly identified as an important contributor to many different diseases and disorders. Neurodegenerative and psychiatric disorders are no different, and neuroinflammation has been implicated in stroke, AD, PD, MS, depression, and a host of other conditions.104 Molecular imaging methods for visualizing neuroinflammation are important if its role in all of these brain disorders is to be understood, including PET imaging, which has played an important part to date.105 Neuroinflammation is a complicated process that causes neuronal damage and death, and is commonly localized in specific cerebral regions for a given condition. As such, molecular imaging can provide valuable insights into inflammatory pathways being activated in different neurodegenerative disorders. Each step in the neuroinflammation pathway represents a possible target for PET radiotracers, and many different tracers for a number of these targets are being investigated (see Figure 9 for an overview). A comprehensive discussion of all of these radiotracers is not the intent of this Seminar, but the topic has been reviewed in recent years.106 Herein, we focus our attention on radioligands targeting the translocator protein 18 kDa (TSPO), formally known as the peripheral benzodiazepine receptor. Radiotracers for the TSPO are the most developed PET radiotracers for neuroinflammation to date, and have also been the subject of a recent review article.107 TSPO is a mitochondrial protein found on the outer mitochondrial membrane, and its expression in the CNS is almost entirely associated with microglia. Microglia are macrophages that function as immune cells in the CNS where they are thought to mediate an inflammatory process that has been implicated in multiple neurodegenerative disorders including AD, PD, MS, and others.108 Levels of TSPO in the healthy brain are, therefore, low, but increase when inflammatory pathways are activated, correlating with both microglial activation and neuronal damage.109,110 TSPO is,

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers

9

Figure 9 Neuroimaging targets for the major players in neuroinflammation. This figure depicts an overview of the major nuclear imaging and MRI tools to study human neuroinflammation. Nuclear imaging methods are displayed in red font, and MRI methods are in blue font. Categories are broken into (1) activation of CNS immunocompetent cells (microglia and astrocytes), (2) disruption of BBB, (3) infiltration of peripheral immune cells, and (4) consequences of neuroinflammation (eg, demyelination and cell death). ASL, arterial spin labeling; BBB, blood-brain barrier; COX-1, cyclooxygenase-1; DCE, dynamic contrast enhanced; DSC, dynamic susceptibility contrast; DWI, diffusionweighted imaging; MAO-B, monoamine oxidase B; mcMRI, multiple contrast MRI; MRS, magnetic resonance spectroscopy; MTR, magnetization transfer ratio; NAA, N-acetyl aspartate; PS, phosphatidylserine; SPIO, superparamagnetic iron oxide particle; TSPO, translocator protein 18 kDa; USPIO, ultrasmall SPIO; VBM, voxel-based morphometry. Reprinted from Albrecht et al.105 Copyright (2016) with permission from the American Chemical Society.

therefore, an attractive imaging target for monitoring disease progression in neurodegenerative disorders such as AD,111 as well as potentially monitoring patient response to antiinflammatory therapy.112 Dozens of PET radiotracers have been developed for TSPO (Fig. 10). Early efforts focused on both benzodiazepines and isoquinoline carboxamides. Although radiotracers based on the former, such as [11C]Ro5-4864, proved problematic because affinity for TSPO was not maintained across species,113 the latter showed more promise, and the prototypical radiotracer [11C]PK11195 remains in widespread use to this day.114 Despite its extensive utilization, there are issues with [11C]PK11195 including limited CNS penetration, high plasma protein binding, and poor kinetic profile. Therefore, other potential radiotracers have been developed, and the phenoxyphenyl acetamides, typified by [11C]PBR28, are the most investigated to date.107 [11C]PBR28 is noteworthy because of its very high affinity for TSPO and the excellent signal-to-noise ratio obtained in [11C]PBR28 images. The main drawback of [11C]PBR28 and related radiotracers is the variable binding to TSPO that needs to be accounted for in patient recruitment. Humans fall into

Figure 10 PET radiotracers for imaging TSPO.

ARTICLE IN PRESS A.L. Va¯vere and P.J.H. Scott

10 1 of 3 TSPO binding profile phenotypes: high binders, low binders, and mixed-affinity binders, determined by a singlenucleotide polymorphism in the TSPO gene (rs6971).115 Because [11C]PBR28 (and most other TSPO radiotracers) binds to the high-affinity state of TSPO, it is critical to determine a patient’s genotype before recruiting or interpreting PET data. Nevertheless, imaging with [11C]PBR28 has provided valuable insights into the role of TSPO in a range of disorders. For example, [11C]PBR28 binding to TSPO has been shown to correlate with severity of AD116 and is also being investigated as a possible diagnostic technique for PD,117 MS,118 amyotrophic lateral sclerosis,119 and stroke.120 In summary, as we increasingly understand the part neuroinflammation plays in a host of neurodegenerative disorders, having access to PET radiotracers that allow accurate monitoring of inflammatory processes is critical if they are to be targeted for therapeutic intervention. PET radiotracers that allow localization and quantification of areas of neuroinflammation, including phenoxyphenyl acetamide ligands for TSPO such as [11C]PBR28, have been developed and shown to be sensitive to inflammation. These agents are being used for neuroimaging in patients with a number of disorders as well as other imaging applications.

Oncology More than 4 decades have passed since the introduction of the first human PET scanner, and whereas early work focused on brain and cardiac targets, oncologic investigations began to take root in the 1990s. Malignant transformation has a profound effect on a multitude of chemical pathways within the human body. Being that PET imaging is a process-driven modality rather than anatomical, these transformed pathways become targets for scientists and clinicians to exploit for the benefit of the patient. The applications of PET in the realm of oncologic imaging are vast, and this review will not attempt

to cover them all. Instead, it will focus on the primary targets in current clinical PET imaging of cancer and highlight noteworthy reports in each of these areas. As the most common and well-known PET tracer, [18F]FDG has been investigated in every type of malignancy, but for the sake of brevity, it has been intentionally omitted as several in-depth reviews have covered this well-known radiotracer over the years.121,122

Hypoxia Reduced levels of oxygen (hypoxia) are found in all types of malignancies and have not been shown to be dependent on tumor size or stage of disease. Tumor hypoxia is attributed to a complex array of biological traits exclusive to malignant cells, and is rarely seen in normal cells. Because of their increased metabolic status, tumors often grow faster than the vasculature supporting them, resulting in a deficiency in blood vessels and poor blood supply. At the same time, they tax the surrounding environment by their demand for oxygen, depleting the immediate oxygen resources. These regions show profound resistance to chemotherapies and correlate with disease aggressiveness.123 In addition, treatment by external beam radiation is less effective because, as a result of the reduced oxygen environment, the damaging free radicals recombine before significant cell-killing effects.124 For these many reasons, delineation of hypoxia has been a focus of PET imaging for several years as a tool to tailor treatments based on the oxygenation of the tumor environment.125 [18F]Fluoromisonidazole ([18F]FMISO, Figure 11) remains the most commonly studied PET imaging agent for hypoxia.126 Its mechanism of sequential reduction of the nitro group on the imidazole ring occurs in hypoxic cells, whereas in an oxygenated cell, it goes through a reduction cycle that returns it to its initial state. The clinical utility of this radiotracer for delineating hypoxia has been demonstrated extensively in many types of cancer including head and neck, brain, breast, lung, pancreatic, and others, and has been reviewed in a recent article published

Figure 11 Mechanism of action of misonidazole-based molecules in the presence and absence of oxygen adapted from figure originally published in JNM (Padhani et al126). Copyright by the Society of Nuclear Medicine and Molecular Imaging, Inc. Inset shows 3 misonidazole-based PET tracers discussed in this review.

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers

11

Figure 12 Images show a patient affected by stage IIIA NSCLC of the left main pulmonary bronchus investigated with [18F]FDG and [64Cu]ATSM PET/CT demonstrated an increased uptake of bother tracers (arrows). The uptake in the left lateral cervical region was a very FDG-avid but with low hypoxia. Adapted from figure originally published in Clinical Nuclear Medicine. Lopci.136 Copyright by Lippincott Williams & Wilkins. The structure of [64Cu]ATSM is shown in the right.

in Seminars.127 Two recent reports examined the clinical benefit of [18F]FMISO imaging in head and neck cancer for radiotherapy monitoring. A prospective study examined untreated patients imaged before and after intensity-modulated radiation therapy and compared these with [18F]FDG images.128 They found that [ 18 F]FMISO uptake declined earlier than [18F]FDG over the course of treatment, suggesting reoxygenation of the treated tissues, which could aid in decisions on dose escalation in the future. A year earlier, a study found a correlation between tumor hypoxia by [18F]FMISO and patient outcome after primary radiochemotherapy at only 2 weeks, with statistical significance at the baseline scan and at week 2 (P = 0.031 and 0.016, respectively) for less recurrence in tumor sites with more oxygenation.129 It was also reported that using the semiquantitative value of tumor-tonormal cerebellum ratio in the analysis of [18F]FMISO uptake in brain tumors, tumor necrosis could be sufficiently predicted.130 As with any radiotracer, attempts to improve uptake and clearance of [18F]FMISO led to other notable hypoxia tracers. Not long after, another nitroimidazole—[18F]fluoroazomycin ([18F]FAZA)—was introduced. The addition of the arabinoside moiety results in a more hydrophilic molecule allowing faster clearance from nontarget tissues. One limitation of imaging with hypoxia tracers is the uncertainty that lack of uptake can be due to normal oxygenation or could just as easily be the result of poor perfusion. A recent study embarked on a multiparametric analysis using both [18F]FAZA and [15O]H2O as a marker of blood flow.131 Their findings imply that to confirm normoxia in a tumor or region with diminished [18F]FAZA uptake, sufficient blood flow to the tissue in question must also be demonstrated. Aggressive cancers such as pancreatic cancer are thought to be unusually hypoxic, and a recent study assessed the hypoxic fraction of primary and metastatic tumors while also using the initial kinetics of tracer uptake to dynamically assess perfusion.132 They determined that hypoxia levels identified by [18F]FAZA

imaging in pancreatic cancer varied greatly in the 15 patients they studied, and that clinical imaging of hypoxia could select patients who may benefit from therapies targeting hypoxia. Through structure-activity experiments, a novel 2-nitroimidazole nucleoside analog, [18F]HX4, was developed to have better water solubility and faster clearance, while maintaining the hypoxic-targeting properties. This relatively new agent was recently tested in 20 patients with head and neck squamous cell carcinoma and compared with blood biomarkers during radiotherapy.133 All patients showed a significant decrease in hypoxic fraction during treatment, which was not observable in the tested blood parameters of osteopontin, carbonic anhydrase 9, and vascular endothelial growth factor. Copper(II)-diacetyl-bis(N(4)-methylthiosemicarbazone, CuATSM (Fig. 12), is another compound exploiting an alternate, indirect pathway to assess tumor hypoxia. Capable of being labeled with one of many positron-emitting copper isotopes, 60 Cu, 61 Cu, 62 Cu, or 64 Cu, this tracer has been investigated to delineate hypoxia since it was first reported in the late 1990s.134 Under hypoxic or anoxic conditions, the redox potential of the cell is altered, and Cu(II) in the complex is reduced to Cu(I), causing the complex to fall apart. This retention mechanism is reliant on the reduction of Cu(II) to Cu(I); however, uncertainty lies in the possibility that other physiological changes caused by malignant progression and hypoxia within the cell could also affect this redox balance regardless of oxygen concentration. In a recent study of tumor redox status by [62Cu]ATSM imaging in patients with head and neck cancer, high tumor uptake significantly predicted poor prognoses for progression-free survival and could help clinicians modulate treatment based on redox status of individual tumors.135 Similar results were seen in another study using [64Cu]ATSM in both non–small cell lung cancer (NSCLC) and head and neck cancers.136 Of note, an interesting preclinical study analyzed all 4 of the hypoxia tracers presented here in a comparison of

ARTICLE IN PRESS A.L. Va¯vere and P.J.H. Scott

12

Figure 13 Structures of protein and cell membrane synthesis PET radiotracers.

microregional distribution by autoradiography in several tumor types and immunohistological assessment.137 The results show that [18F]FAZA had the lowest tumor uptake and also the lowest background, and [64Cu]ATSM had the highest tumor uptake and image contrast. In comparison with known fluorescence markers of hypoxia, the 3 nitroimidazoles ([18F]FMISO, [18F]FAZA, and [18F]HX4) all target hypoxic tumor regions, whereas the hypoxia selectivity of [64Cu]ATSM was questionable.

Protein and Cell Membrane Synthesis As cancer cells are proliferating at increased rates, the upregulation of critical pathways in the creation of cells also leads to increased uptake of choline in malignant tissues. Choline is a precursor of phosphatidylcholine, an indispensable element in the composition of the cell membrane. Developed as [11C]choline, this PET radiotracer has been used almost exclusively for prostate cancer because it is not excreted through the bladder, therefore allowing an excellent view of the prostate and pelvic regions. However, reports have also outlined its use in breast, brain, and renal cancers138 (Fig. 13). A recent Seminar in this journal focusing on PET imaging in prostate cancer beyond [18F]FDG provides a comprehensive review of radiotracers of choline, acetate, and other agents for the primary detection of disease, staging, and detection of recurrence.139 Most reports have shown that choline is an excellent tool at the point of biochemical recurrence when blood levels of prostate-specific antigen (PSA) have increased. In the past year, 2 groups presented studies investigating [11C]choline as a therapy monitoring tool of castration-resistant prostate cancer in patients treated with docetaxel. The first looked retrospectively at 61 patients imaged before and after 4-12 cycles of chemotherapy, and concluded that 44% of patients showed radiological progression of disease concurrent with a PSA response of ≥50%.140 However, a prospective study of 32 patients in the same treatment plan found no specific correlation with clinical assessment based on Response Evaluation Criteria In Solid Tumors 1.1, concluding no clinical benefit for this patient population.141 As a side note, tumor resistance to docetaxel can be problematic. In such cases, it is often associated with low drug concentrations in tumor tissue. To address this issue, van der Veldt and coworkers prepared [11C]docetaxel and demonstrated that variable [11C]docetaxel kinetics in tumors may reflect differential sensitivity to docetaxel therapy.142 This approach was demonstrated in lung cancer, but could be applicable in prostate cancer and potentially be used to predict expected response to docetaxel on a patient-by-patient basis in advance of starting chemotherapy.

A fluorine-18 labeled analog of choline ([18F]fluorocholine, Fig. 13) has also been developed, affording additional time for radiotracer clearance from systemic circulation, although this tracer does have urinary excretion. One study looked at patients with castrate-resistant prostate cancer treated with various types of chemotherapy, and reported that change in tumor burden by imaging with [18F]fluorocholine predicted PSA progression in >80% of cases (Fig. 14).143 In addition, PET/MRI with [18F]fluorocholine was investigated as a possible replacement for MRI-guided prostate biopsies and found that, although additional studies are necessary, these fusion images improved identification of clinical significant lesions over MRI alone.144 Fatty acid synthesis is upregulated in cancer cells as fatty acids are an integral part of cell membranes. With the overexpression of fatty acid synthase, acetate is primarily converted to acetyl-CoA and enters into the pathway producing fatty acids. As such, [11C]acetate (Fig. 13) has been exploited as an indirect marker of fatty acid synthesis and has been especially attractive for monitoring prostate cancer as it is not metabolized through the bladder but primarily through respiration. A retrospective analysis looked at patients with prostate cancer who experienced biochemical relapse by PSA following radical prostatectomy and who were imaged with [11C]acetate.145 The novel conclusions of this study were that maximum standardized uptake value (SUVmax) showed a linear, direct correlation with survival time, and intense uptake indicated poor survival even in asymptomatic patients with very low PSA levels. [11C]Acetate was also investigated as a marker of bone metastasis in prostate cancer in comparison with the bone scintigraphy tracer [99mTc]hydroxymethylene diphosphonate ([99mTc]HDP).146 Because [99mTc]hydroxymethylene diphosphonate will show uptake related to any increase in osteoblastic activity in the bone, the investigators hypothesized that [11C]acetate would be more specific for malignant sites. This was, in fact, demonstrated by the superior detection of bone lesions (P < 0.01) including 5 of 14 patients who showed negative uptake in the bone scintigraphy.

Transporter-targeted Agents Protein synthesis is another process that is accelerated in the malignant transformation of cells. As a part of the process, essential amino acids—known as the building blocks of proteins—are shuttled into the cell via various transporters at an increased rate over normal tissues. One of these, the L-type amino acid transporter (LAT1), is the major transporter of large neutral amino acids and has been exploited through nuclear imaging. LAT1 is overexpressed in most cancer types147 and has been correlated with tumor aggressiveness and poor prognosis.148 The primary disease site for clinical

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers

Figure 14 [18F]Fluorocholine PET, PET/CT, and maximum intensity projection (MIP) images from 65-year-old patient receiving sipuleucel-T displays cancer progression reflected by increasing net metabolically active tumor volume (MATV). Color indicates MATV contours on MIP images. Pretreatment PET/CT shows hyperactive vertebral metastases (arrows; net MATV, 42.9 cm3; PSA, 38.0 ng/ mL). PET/CT image obtained 36 days after initiation of treatment demonstrates increasing activity and new lesions in sternum and lumbar spine (arrows; net MATV, 338.4 cm3; PSA, 46.7 ng/mL). PSA level after 4 months increased to 241.4 ng/mL. Adapted from figure originally published in JNM. Lee et al.143 Copyright by the Society of Nuclear Medicine and Molecular Imaging, Inc.

13 investigation has been in brain cancers owing to the relatively low level of protein synthesis in normal brain tissue, but many other solid tumors have also been investigated including breast, bladder, head and neck, and others.138 Recently, clinicians reported the use of radiolabeled methionine, L-[methyl-11C]Methionine ([11C]MET, Figure 15), for the monitoring of proton irradiation of intracranial meningiomas.149 Although most intracranial meningiomas are benign, their locations within the brain can sometimes rule out surgery. This group found that [11C]MET was useful to confirm that proton beam irradiation was safe and effective for treatment with these lesions, and PET imaging would be a good adjunct to MRI in these cases. Over the last few years, [11C]MET has proven useful and safe for use in the pediatric population,150,151 and most recently compared with [18F]FDG for the staging and monitoring of pediatric lymphoma.152 They reported that tumor sites in the neck and chest were particularly well visualized, whereas abdominal lesions were obscured by the clearance through the liver more so than with [18F]FDG. Although [11C]MET shows excellent utility for PET imaging of amino acid transport, it has not gained widespread use owing to the need for an on-site cyclotron for production. O-(2-[18F]Fluoroethyl)-L-tyrosine ([18F]FET), a metabolically stable analog of tyrosine, was developed in the late 1990s and has been employed to image the same process while also overcoming the limitation of half-life. Because of the location of the fluorine, this tracer will not be incorporated into proteins; however, the uptake is still mediated by the same transporter. Most recently, a set of 113 patients with confirmed high-grade gliomas were analyzed retrospectively with [18F]FET to determine if pre-therapeutic uptake correlated with tumor grade and patient survival.153 The uptake heterogeneity was useful in the subgrading of tumors, and the coarseness, contrast, and busyness (all parameters in the analysis of tumor heterogeneity) were predictors of progression-free survival and overall survival (P < 0.05). Like most tracers in recent years, [18F]FET has been explored in the context of simultaneous PET/MRI. In a recent study, Henriksen and coworkers outline their assessment of patients with brain tumor using MRI, [18F]FET, and blood volume (BV) mapping.154 Results showed poor spatial agreement in tumors of patients with treated gliomas between [18F]FET and BV imaging. However, the study demonstrated the utility of using PET/MRI to evaluate structure, metabolism, and BV in a single imaging session, which could add clinical value.

Figure 15 Structures of transporter-targeted PET radiotracers.

ARTICLE IN PRESS 14 Currently, the most exciting amino acid analog is anti-1amino-3-18F-fluorocyclobutane-1-carboxylic acid ([18F]FACBC; [18F]fluciclovine; Axumin) as it has recently been approved by FDA for prostate cancer.155 Whereas LAT1 may be involved, investigations have also shown that transport of this tracer may be more like glutamine using a member of the alanine-serine-cysteine transporter family, ASCT2.156 In patients with recurrent prostate cancer, imaging with [18F]FACBC PET/CT is superior in sensitivity and accuracy to imaging with CT alone (89% vs 11% and 78% vs 35%, respectively)157 and also superior to [11C]choline for clinical and technical reasons with biochemical relapse after prostatectomy.158 Others have recently explored the potential for its use in breast cancer.159 In these small, preliminary studies, [18F]FACBC showed higher uptake in breast tumors over non-malignant tissues (including benign lesions) and the highest intensity of uptake correlated with poor prognosis.160 There was also confirmation that [18F]FACBC is effective in imaging both invasive ductal and invasive lobular breast cancers, with uptake of [18F]FACBC in invasive lobular breast cancers being higher than that seen with [18F]FDG.161 The human norepinephrine transporter (hNET) is found on the neuronal cell membrane and has been exploited for decades with [123I]meta-iodobenzyl guanidine ([123I]MIBG) SPECT-CT as the clinical standard for diagnosis and staging of neuroendocrine tumors such as neuroblastoma. Unfortunately, [123I]MIBG is limited in sensitivity by the emissions of iodine-123 and image resolution as an inherent limitation of SPECT imaging. Another drawback is the required 24-hour uptake time necessary for clearance of background radioactivity. With recent advances in the production of a fluorine analog of MIBG, meta-[18F]fluorobenzyl guanidine has become a feasible reality for routine use in the therapy monitoring of neuroendocrine tumors. Preliminary data from an ongoing clinical trial were presented at the 2016 Annual Meeting of the Society of Nuclear Medicine and Molecular Imaging, and demonstrated that targeting of lesions was possible in 4 hours or less with favorable biodistribution and kinetics.162 A more detailed analysis of the results of this clinical trial is currently underway.

Receptor-targeted Agents Somatostatin is a common neuropeptide, which acts on 5 subtypes of somatostatin receptor (SSTR1-SSTR5). The receptors are expressed in normal tissues including the brain, pituitary, pancreas, adrenal, kidney, and gut. Activation of the somatostatin receptors (SSTR) gives rise to many physiological effects including reduction in secretion of many hormones. However, increased levels of expression of SSTR have also been delineated in many neuroendocrine tumors (NETs), a group of unique neoplasms that includes pheochromocytomas, neuroblastomas, and gastroenteropancreatic (GEP) tumors. Synthetic somatostatin analogs such as [68Ga]Ga-DOTATOC and, to a greater extent, [68Ga]Ga-DOTATATE, bind to SSTR2 with high affinity while displaying much lower affinity for the other receptor subtypes (Fig. 16) and have been developed as imaging agents for neuroendocrine tumors.

A.L. Va¯vere and P.J.H. Scott [68Ga]Ga-DOTATATE was given Orphan Drug Designation by both the FDA and the European Medicines Agency in 2014 for use in GEP neuroendocrine tumors (GEP-NETs). A recent review of studies comparing [68Ga]Ga-DOTATATE with a similar agent used for SPECT imaging, [111In]In-DTPA-octreotide, in patients with pulmonary and GEP-NETs, concluded that the superior image quality of PET over SPECT images, lower radiation dose, and convenience of a 2-hour imaging time vs 2-3 days warranted the use of [68Ga]Ga-DOTATATE whenever possible.163 The same group performed a prospective study on 97 patients with the same cancer types imaged with [68Ga]Ga-DOTATATE and compared with previously obtained [111In]In-DTPA-octreotide scans.164 It was determined that the [111In]In-DTPA-octreotide did not provide additional information for any patient, and that the [68Ga]Ga-DOTATATE PET/CT affected clinical management in 37% of patients. They concluded, again, that [68Ga]Ga-DOTATATE should be used in place of [111In]In-DTPA-octreotide where available. Another study recently looked at the utility of [68Ga]Ga-DOTATATE in the imaging of pheochromocytomas and paragliomas, a much less common set of NETs, that, until this point, were imaged with the highest sensitivity using [18F]FDOPA, a tracer targeting the L-amino acid transporter.165 Although a small prospective study, the results showed very sensitive detection of paragangliomas using [68Ga]Ga-DOTATATE, whereas [18F]FDOPA failed to show uptake in some small lesions. In contrast, in pheochromocytomas, [68Ga]Ga-DOTATATE detected less lesions and was not justified to use over [18F]FDOPA PET/CT. The same institution compared imaging of patients with paraganglioma with 4 tracers—[68Ga]Ga-DOTATATE, [18F]FDOPA, [18F]fluorodopamine ([18F]FDA), and [18F]FDG—and clearly demonstrated superiority of [68Ga]DOTATATE over all other tracers tested (see Figure 17 for 1 example).166 The excellent specificity of peptide agents targeting the SSTRs has been clearly demonstrated, and this has been exploited not only for imaging but for treatment. DOTATATE and DOTATOC can also bind therapeutic radiometals 177Lu or 90Y for peptide receptor radionuclide therapy at the tumor site. Nilica et al used [68Ga]DOTATOC imaging before and after peptide receptor radionuclide therapy on neuroendocrine tumors to examine if [68Ga]DOTATOC could affect treatment decisions.167 In this retrospective study of 66 patients, they determined that [68Ga]DOTATOC on its own may not reflect progression in certain NET lesions and should therefore not be used for prognosis without [18F]FDG. The integrin αvβ3 is one of a class of cell adhesion receptors that has been shown to be integral in the mediation of tumor growth and invasiveness and highly expressed on endothelial cells during angiogenesis.168 A sequence containing the Arg-Gly-Asp (RGD) tripeptide sequence was shown to be recognized by αvβ3, and over the last few decades several radiotracers have been developed for angiogenesis imaging exploiting this target. Improvements and upgrades in radionuclide-of-choice and targeting ability have been made over the years; however, the RGD sequence remains conserved. Alfatide II NOTA-E[PEG4-c(RGDfk)]2 is one of these upgraded tracers that has demonstrated lower liver uptake and higher tumor accumulation in preclinical models.169 This tracer, labeled with gallium-68, was better than [18F]FDG at

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers

15

Figure 16 Structures of receptor-targeted PET imaging agents.

distinguishing NSCLC from tuberculosis and more likely to detect brain metastases; however, FDG was more capable of detecting liver and early-stage bone metastases.170 Much recent work has also focused on a fluorine-18 agent targeting αvβ3

that uses Al-18F chemistry to radiolabel with fluorine instead of gallium, called [18F]FPRGD2 or [18F]alfatide. Several groups have recently assessed this tracer in the clinic, and the Yuan group from Shandong demonstrated that in the assessment

Figure 17 Images of a 39-year-old man with sporadic, metastatic paraganglioma. The [68Ga]DOTATATE, [18F]FDG, and [18F]FDA PET images show very similar extensive metastatic disease involving bone, liver, and mediastinal and retroperitoneal lymph nodes. The [18F]FDOPA PET image is almost negative. Adapted from a figure originally published in EJNMMI. Janssen et al.166 Copyright by Springer.

ARTICLE IN PRESS 16 of glioblastoma, [18F]FPRGD2 allows good visualization of lesions and predicted sensitivity to concurrent chemoradiotherapy by as early as the third week of treatment. They also observed that decreased SSTR uptake resulted in increased likelihood of response.171 The same group discovered a similar outcome for patients with advanced NSCLC.172 An integral piece in the ability of cancer to metastasize is the CXCR4 receptor173 and its ligand CXCL12. Tumors often metastasize to the lung, bones, and lymph nodes, organs with high levels of CXCL12. CXCL12 functions as a chemoattractant that causes CXCR4-positive primary tumor cells to accumulate in secondary metastatic sites and ultimately the onset of metastatic lesions. The CXCR4 receptor is one of a family of G-coupled protein receptors that is overexpressed on several cancers. It has also been shown that the level of CXCR4 expression is higher in metastases than in primary tumors, making it an even more interesting target for targeted therapies and imaging. In the last few years, [68Ga]Ga-CPCR4.2, now known as [68Ga]pentixafor, a cyclic pentapeptide with a DOTA moiety for metal chelation, was developed as a PET tracer for CXCR4 expression based on an IC50 of 5 ± 1 nM. Preclinical studies were promising with CXCR4-specific uptake and high tumor-to-muscle ratios, and 2 studies recently performed preliminary imaging on patients with a variety of solid tumors known to overexpress CXCR4174 and also specifically small cell lung cancer.175 It is noteworthy that previously determined levels of in vitro expression of this receptor did not correlate with uptake in patients, which shows heterogeneity of uptake. However, in patients with small cell lung cancer, [68Ga]pentixafor performance was superior to that of SSTR-targeted imaging. Overall, with a wide distribution of uptake in various tumors, this agent could act as a biomarker for identification of patients who could benefit from CXCR4 therapies.

Protein-targeted Agents Prostate-specific membrane antigen (PSMA) is a transmembrane protein that has selective overexpression in prostate cancer that is correlated with tumor grade and stage. In the last couple of decades, PSMA has been heavily researched because of the prevalence of prostate cancer worldwide. Highaffinity antibodies, peptides, and small molecules have all been designed to image this specific target, and several recent reviews go in-depth into the nature and clinical potential of these many tracers.176,177 Introduced only 6 years ago, [68Ga]PSMA-11 (Fig. 18), a PSMA antagonist HBED-CC conjugate of GluNH-CO-NH-Lys, has gained exceptional interest above the rest as it showed excellent tumor targeting and is labeled with gallium-68, giving it broader accessibility. Importantly, [68Ga]PSMA-11 demonstrates very low false-positive rates; however, there is some concern with the false negatives that are also observed.178 Several investigations over the last few years have demonstrated this tracer’s impact on the management of newly diagnosed and recurrent prostate cancer.179 Most issues of major nuclear medicine journals over the last year have contained clinical reports of new applications or retrospective analyses of this tracer, and a few of their findings are highlighted here. In a retrospective analysis of 57 patients

A.L. Va¯vere and P.J.H. Scott with primary or recurrent prostate cancer, [68Ga]PSMA-11 was examined as a tool for radiotherapy management and found that it had a significant impact in treatment planning in more than half of cases180(Fig. 18). Another retrospective comparison of [68Ga]PSMA-11 PET/ CT with PET/MRI of lymph node and bone metastases was of interest because MRI is the modality of choice for assessment of the prostate gland. Freitag et al found good correlation between SUVmean with similar accuracy and reliability in detection of these lesions, warranting further examination.181 [68Ga]PSMA-11 PET/CT was also compared with traditional bone scintigraphy for skeletal staging and demonstrated higher sensitivity and specificity at primary diagnosis and at biochemical relapse (Fig. 19).182 This retrospective analysis of 126 patients concluded that bone scintigraphy rarely offered additional information over [68Ga]PSMA-11 PET/CT. In addition, studies compared [68Ga]PSMA-11 with both [11C]choline and [18F]fluoroethylcholine PET/CT for detection of recurrent prostate cancer. [68Ga]PSMA-11 was found to be superior to [11C]choline in detecting lesions at low PSA levels; however, at higher PSA levels, there was no statistical difference.183 In a similar study, a group of patients were initially imaged with [18F]fluoroethylcholine. During the following years, another group of patients were routinely imaged with [68Ga]PSMA-11, and these images were compared. Each of the tracers was employed before lymphadenectomy, and [ 68 Ga]PSMA-11 demonstrated superiority for sensitivity, specificity, and accuracy of detecting metastatic lesions.184 A proof-of-concept study in 6 patients outlined that [68Ga]PSMA-11 was not recommended for primary renal cell carcinoma owing to normal renal uptake; however, it was useful in detecting metastatic lesions.185 In a retrospective analysis of 45 patients with lung lesions, [68Ga]PSMA-11 PET/CT was not able to discriminate lesions of prostate cancer origin from others, with primary lung tumors showing unexpectedly high uptake.186 Another group reported that retrospective analysis of 6 patients showed accurate detection of primary lesions within the prostate structure at biochemical relapse and may be helpful for targeting lesions before biopsy or for supporting therapy planning.187 An 18F-labeled PSMA inhibitor is the urea-based N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-4-18F-fluorobenzylL-cysteine. Although the high blood pool activity of the tracer draws some concern for the detection of smaller lesions, this tracer shows clinical relevance for detection of larger, higher grade lesions. In a recent study, a mixed group of patients with hormone-naive or castration-resistant prostate cancer were imaged to evaluate sites of metastatic disease and were compared with conventional imaging.188 Of note, both groups of patients displayed no significant difference in SUVmax of lesions. Another protein-targeting radiotracer, 3'-deoxy-3'-[18F] fluorothymidine ([18F]FLT), enters the cell by active transport and is trapped after undergoing phosphorylation by thymidine kinase 1. At this point, natural thymidine would be incorporated into DNA; however, the fluorine-18 in the 3' position on [18F]FLT (as opposed to the hydroxyl in thymidine) prevents this from occurring and the molecule is trapped. As a result, [18F]FLT uptake is a marker of tyrosine kinase 1 expression and a surrogate of cellular proliferation.

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers

17

Figure 18 Structures of protein-targeted radiotracers.

Several studies have established a correlation between analysis of histopathologic tissue samples with the proliferation marker Ki-67 and the distribution of [18F]FLT.189 An extensive review of the literature pertaining to [18F]FLT as a measure of treatment response in patients with cancer was recently published.190 Of interest in recent reports, a prospective study in advanced-state B-cell lymphoma in 55 patients showed that [18F]FLT was a significant predictor of response to treatment.191 A negative [18F]FLT scan clearly identified patients with good prognosis, and a decrease in [18F]FLT uptake over the course of treatment of greater than the mean indicated a significant improvement in progression-free survival (P = 0.001). A multicenter trial studying a small number of patients with high-grade glioma was embarked upon to assess the translatability of an optimum protocol for quantitative [18F]FLT imaging across multiple institutions.192 They showed that the test-retest repeatability of [18F]FLT imaging seems to be better than that for [18F]FDG and correlates with other single-center findings with a repeatability coefficient of 23.2% with SUVmax and a slightly better repeatability coefficient of 18.5% for SUVpeak. (1-(2'-Deoxy-2'-[18F]-fluoro-beta-D-arabinofuranosyl)thymine) 18 ([ F]FMAU) is another analog of thymidine labeled with fluorine-18 in the 4 position and conserving the 3' hydroxyl group. This minor change allows the tracer to be incorporated into DNA, but increases its binding with thymidine kinase 2, which is found in other stages of the cell cycle. High tumor uptake has been seen relative to background tissues, and the majority of the tracer is cleared from the blood in 10 minutes, which permits improved imaging in a shorter time. In a recent study, [18F]FLT, [18F]FMAU, and

an additional thymidine analog, [18F]FAU (a prodrug of [18F]FMAU), were all assessed for their feasibility as imaging agents to monitor response to capecitabine, a prodrug of 5-fluorouracil that is incorporated into DNA and RNA.193 In this pilot study, [18F]FLT uptake was found to increase with treatment, demonstrating a possible flare response; however, [18F]FMAU and [18F]FAU show little difference after treatment.

Cardiology Because of the reliability of established SPECT and PET tracers for cardiac imaging, considerably less clinical reports have been published on new agents in recent years when compared with oncology and neurology. A few years ago, this journal published an extensive review on cardiac molecular imaging,194 and the recent excitement over combined PET/MRI imaging is leading to new possibilities in the realm of cardiac imaging.195,196 Cardiac PET saw its inception in the 1980s with the translation of [18F]FDG to heart imaging and the introduction of 82 RbCl, followed soon after by [15O]water and [13N]ammonia. 18 [ F]FDG PET has the highest sensitivity for assessing myocardial viability at >90% and remains the gold standard for this procedure. This analog of glucose is transported into cells similar to glucose; however, after phosphorylation to [18F]FDG-6-phosphate, it is not able to enter the tricarboxylic acid cycle for further metabolism and remains trapped

ARTICLE IN PRESS A.L. Va¯vere and P.J.H. Scott

18

Figure 19 Example of improved sensitivity of 68Ga-PSMA-HBED-CC PET vs BS with respect to affected bone regions. A 67-year-old patient with metastatic castrate-resistant prostate cancer under antiandrogenic therapy; PSA level was 500 ng/mL. Bone scintigraphy shows only limited bone involvement of the lumbar spine, ribs, pelvis, and right femur, whereas PSMA PET shows extensive osseous metastases in spine, pelvis, shoulder girdle, ribs, and all extremities, as well as lymph node involvement. Adapted from a figure originally published in EJNMMI. Pyka et al.182 Copyright by Springer.

in the cytosol.197 Over the years, extensive research has been performed to validate the kinetics and trapping of this tracer, and the details of this quantification have been reviewed elsewhere.198 Physiological changes in metabolism of the heart, which typically gets 30%-50% of its energy from aerobic glucose metabolism, can lead clinicians to specific pathologies of disease. In addition, [18F]FDG also targets myocardial inflammation because of overproduction of glucose transporters and glycolytic enzymes in inflammatory cells, and, as such, post-infarct inflammation and sarcoidosis have also benefited from regular use of this tracer in the clinical setting. Most specific diagnoses involve analysis of more than 1 parameter—with multiple tracers or a single tracer at early and late time points—because of the myriad processes involved in cardiac metabolism, blood flow and perfusion, and other disease-specific alterations in biochemical pathways.

Perfusion Myocardial perfusion is the passage of blood through the heart, and a comparison of the differences of this flow at rest vs under stress can delineate sites of occlusion or damage. The clinical standard in myocardial perfusion imaging (MPI) has been the use of the SPECT tracers [ 201 Tl]thallous chloride, [99mTc]sestamibi, or [99mTc]tetrofosmin owing to the ease of access and general reliability of the diagnoses stemming from

these tests. However, the design of PET flow tracers has sprung from the desire for quantification and the inherently higher resolution of PET, although availability to patients can be limited by cost and accessibility.199 Perfusion imaging with PET is increasing in use as its clinical utility to guide treatment decisions is increasingly recognized. Myocardial uptake of [15O]water is by passive diffusion, with typical measurements performed as dynamic scans from start of injection and 5 minutes after. As it has a half-life of 2.06 minutes, this radiotracer must be produced with an onsite cyclotron and stress test imaging must be caused pharmacologically rather than by treadmill; therefore, it has not seen widespread use beyond academic pursuits. The most common PET radiotracer for MPI is [82Rb]rubidium chloride primarily owing to its FDA approval and availability of clinical 82Sr/82Rb generators that provide radionuclide for 1-2 months, allowing MPI to be performed without the need for a cyclotron.200 Rubidium-82 is similar to potassium and enters the myocardium by passive diffusion and active transport via the adenosine triphosphate (ATP)-dependent sodiumpotassium cotransporter. Its half-life of 76 seconds allows very fast imaging and a low effective dose for patients; however, its high positron range (7 times that of 18F) causes some degradation of the resolution. Whereas these examinations have become routine, researchers have investigated specifics of the

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers test, especially test-retest variability and stress induction. Recently, Giorgi et al conducted a head-to-head comparison of 82 RbCl and the SPECT agent 99mTc-sestamibi to assess left ventricular function and perfusion after biochemically induced stress by dipyridamole.201 Dipyridamole causes blood vessel dilation and is often used as a replacement for treadmillinduced cardiac stress for MPI. Each of the 221 patients was imaged with both tracers in a single stress test session to minimize variables. Although there were differences in measured volumes between the 2 tracers in normal and abnormal perfusion images, they were limited. Although some statistically significant differences in measured parameters were observed, including left ventricular ejection fraction (LVEF), the authors provided no preference of 1 modality over the other, provided the clinician is aware of possible bias in each case. Remote ischemic conditioning (RIC) uses cycles of intentionally applied nonlethal ischemia and reperfusion to a remote site that results in protection from a sustained episode of ischemia reperfusion injury to the heart.202 In a set of patients with suspected ischemic coronary artery disease, the effects of RIC on myocardial perfusion as measured by 82RbCl were investigated.203 Although preclinical data showed an increase in coronary blood flow as a result of RIC, in the 49 patients analyzed following 4 cycles of RIC, there was no substantial effect on global myocardial perfusion. By imaging for myocardial perfusion ( 82 RuCl) and glucose metabolism ([18F]FDG), an assessment of viable, nonviable, or hibernating myocardium can be made.204 A small subset of patients with heart failure have a reverse mismatch (~15%) consisting of reduced glucose uptake but preserved perfusion that falls outside of these standard categories. A recent retrospective analysis of 91 patients found that this group had no increase in morbidity or mortality, and the mismatch is likely a benign adaptation to continued rebuilding of the myocardium. It is also important to note one might expect a less pronounced increase in LVEF in subjects with preserved perfusion; however, they found that the patients still showed the same benefits from revascularization procedures. Another FDA-approved tracer for myocardial perfusion is [13N]ammonia, which passes freely across cell membranes. Once inside the cell, it is trapped by conversion to glutamine via glutamine synthase. Because of its 10-minute halflife, a cyclotron nearby is required; however, small cyclotrons designed specifically for the production of [13N]ammonia have made production for individual patient doses possible with reduced cost.205 Typically, this short half-life would, in most cases, preclude exercise stress testing and require stress to be administered pharmacologically. However, it is possible with exercise performed in or near the PET scanner, and a recent study provided the first report of myocardial perfusion in obese patients with [13N]ammonia.206 A 10-minute rest MPI session was performed, followed by 50 minutes for decay, then treadmill exercise, injection, and imaging at stress. Although image quality is generally degraded in obese patients, this study found >95% had good image quality that is substantially higher than those obtained with SPECT (55%61%), resulting in a higher overall diagnostic accuracy of this test in the obese population.

19

Sympathetic Innervation One of the functions of the body’s sympathetic nervous system is to maintain homeostasis by controlling basic functions. In the cardiovascular system, this is mediated in part by catecholamines, and especially norepinephrine (NE), transmitted through the sympathetic nervous system. NE synthesis begins with tyrosine, which is hydroxylated to dihydroxyphenylalanine then to dopamine and NE through a series of enzymes. Sympathetic neurons release NE, causing stimulation of adrenergic receptors of the cardiac muscle and other effector organs. At these adrenergic nerve terminals is the NE transporter, or hNET, a transmembrane protein that facilitates the reuptake of NE. Whereas the overexpression of hNET has been exploited in various cancers, significant normal expression of hNET is seen in all major organs of the sympathetic nervous system, including the heart. Changes to the balance of this system have been correlated to heart disease progression and increased mortality. Radiotracers have been designed for every step of the NE biosynthesis pathway to image this process, and have shown utility in all areas of PET imaging including in the heart. [11C]Hydroxyephedrine ([11C]HED, Fig. 20) is a synthetic catecholamine analog that acts as substrate for the hNET while also being resistant to metabolism, allowing rapid reuptake of the radiotracer. This quick cycling images as a trapping mechanism with >99% of retention in cardiac muscle as intact [11C]HED, although metabolism does occur via catecholamine breakdown in the blood even 10-20 minutes after injection.207 This radiotracer has been employed as a marker to quantify sympathetic nerve terminals in the heart. Reduced regional [11C]HED uptake has been correlated with myocardial ischemia and can be monitored for observance of recovery of the sympathetic neurons after injury, which generally occurs much later than revascularization. Although use of this tracer and imaging method has become routine, new research delving deeper into [11C]HED uptake kinetics has been reported. [11C]HED kinetics can be quantified; however, to date, these methods require arterial blood sampling to correct the input function for metabolites. Recently, a group from the Netherlands proposed the use of an image-derived input function from a large blood pool structure also in the field of view with correction via venous blood samples, as opposed to the typically required arterial cannulation.208 The researchers found that by applying a mathematical transformation to the data obtained from venous samples, absolute quantification of [11C]HED kinetics was possible with only a small bias because of the slight but significant overestimation of blood pool activity when using the image-derived input function. This method will allow a more in-depth analysis of sympathetic

Figure 20 Structure of [11C]-meta-hydroxyephedrine ([11C]HED).

ARTICLE IN PRESS 20

A.L. Va¯vere and P.J.H. Scott

Figure 21 Representative [11C]HED PET images and analysis. Patient 1 with hypertensive heart disease and grade 1 diastolic dysfunction. In Patient 2, with cardiac amyloidosis and grade 2 diastolic dysfunction, the polar map of [11C]HED retention index (RI) shows more extensive impairment of myocardial sympathetic innervation than that of patient 1. Adapted from figure originally published in JNM. Aikawa et al.210 Copyright by the Society of Nuclear Medicine and Molecular Imaging, Inc.

innervation of patients with cardiomyopathy without invasive arterial cannulation. Several studies have shown that regional sympathetic denervation visualized by [11C]HED is useful in assessing arrhythmias and predictive of sudden cardiac arrest in patients with ischemic cardiomyopathy.209 A recent study assessed sympathetic innervation status in patients with heart failure but preserved LVEF, which in previous studies was reduced.210 Roughly half of patients with heart failure have preserved LVEF, and they postulated that impaired sympathetic innervation of the heart in these cases may be related to diastolic dysfunction. The results confirmed, with significance, correlation of the [11C]HED uptake retention index (uptake at the end of scan corrected for arterial blood activity) with severity of diastolic dysfunction comparing control patients, mild, and severe diastolic dysfunction (Fig. 21). Finally, a recent study analyzed diabetic patients (type 1) with no evidence of heart disease to look at the relationship between changes in sympathetic innervation of the left ventricle ([11C]HED) and myocardial oxidative metabolism ([11C]acetate).211 In this set of patients, they found that greater sympathetic function by increased [11C]HED retention index was associated with high rates of oxidative metabolism. Interestingly, they also found a distinct difference in these measurements between diabetic men and women, with women having a higher possibility for cardiomyopathy in the future.

Summary and Future Outlook [18F]FDG has been the workhorse of PET imaging for decades, and millions of PET scans are conducted with this radiotracer every year. However, [18F]FDG is not the whole story, and it has been increasingly recognized that developing specific radiotracers that allow quantification of equally specific biomarkers will greatly expand the capabilities of PET imaging. Reflecting this, today there is a wealth of small molecule PET

radiotracers available for many different targets (proteins, receptors, transporters, enzymes, etc.) that are being developed across many fields of medicine, and herein we have surveyed many of the state-of-the-art radiotracers being used for dementia imaging, cardiac PET, and cancer diagnostics. The ability to noninvasively obtain insights into the underlying pathophysiology and progression of many different diseases is invaluable to physicians as it allows early diagnosis of disease and differentiation of clinically overlapping but pathologically distinct disorders. This makes PET imaging integral to the age of personalized medicine, where therapy is increasingly tailored to the individual. Moreover, this ability to predict response to therapeutic intervention allows stratification of patients either to an appropriate approved medication, or in the event there is none for a given disease, the most pertinent experimental treatments in clinical trials. Not surprisingly, access to such treasure chests of information has also been recognized as extremely valuable by those working in drug discovery. PET is increasingly being used by pharmaceutical companies to garner information on their assets that can be fed back into research and development pipelines, but also to populate clinical trials and differentiate products from those of competitors. As a closing comment, PET has come a long way since the introduction of [18F]FDG 40 years ago, and the sophisticated imaging possible today is changing health care and drug discovery. However, there is still much work to be done. A number of today’s radiotracers are not specific enough for their intended target or purpose, and there are still many highprofile targets of interest with no available radiotracer. Therefore, developing radiotracers that are highly selective for targets, such as α-synuclein, should be a priority for PET radiochemists and imaging scientists. The other big challenge facing the community is the spiraling costs of health care and the dwindling reimbursement for advanced technologies such as PET that are perhaps seen by some

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers as expensive luxuries.212 This serves to keep access to many of the state-of-the-art small molecule radiotracers discussed in this issue of Seminar out of the hands of patients and their families, often leaving them frustrated and searching for answers about their disease that often PET could easily provide. Finding a cost-effective solution to this issue, which expands access to PET for both physicians and patients in a fashion that addresses concerns of insurers and health-care systems alike, should be high on the agenda of all of us working in PET and molecular imaging. Only then will we be able to use the life-changing technology at our fingertips to truly take us into the age of personalized medicine and support global efforts to eradicate many of the debilitating diseases that plague our society today and confine them to medical history books.

Acknowledgments We thank Dr Scott Snyder of St. Jude Children’s Research Hospital for his careful proofreading of the manuscript. PJHS also acknowledges the Alzheimer’s Association (NIRP-14-305669) for financial support.

References 1. Anonymous: SNMMI honors at 2016 annual meeting. J Nucl Med 57:12N-19N, 2016 2. Reivich M, Kuhl D, Wolf A, et al: The [18F]fluorodeoxyglucose method for the measurement of local cerebral glucose utilization in man. Circ Res 44:127-137, 1979 3. Schwarz SW, Dick D, VanBrocklin HF, et al: Regulatory requirements for PET drug production. J Nucl Med 55:1132-1137, 2014 4. Reichert JM: Monoclonal antibodies as innovative therapeutics. Curr Pharm Biotechnol 9:423-430, 2008 5. Van Dongen GA, Vosjan, MJ: Immuno-positron emission tomography: Shedding light on clinical antibody therapy. Cancer Biother Radiopharm 25:375-385, 2010 6. Brown T: 100 best-selling, most prescribed branded drugs through 12. Donnelly DJ: Small-molecule PET Tracers in Drug Discovery March. Medscape Medical News, 2015. Available at: http://www.medscape .com/viewarticle/844317. Accessed September 29, 2016 7. Chaturvedi S, Mishra AK: Small molecule radiopharmaceuticals—A review of current approaches. Front Med 3:2016, Article 5 8. Lipinski CA, Lombardo F, Dominy BW, et al: Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings. Adv Drug Del Rev 46:3-26, 2001 9. Jung H, Park M, Kang M, et al: Silver nanoislands on cellulose fibers for chromatographic separation and ultrasensitive detection of small molecules. Light Sci Appl 5:e16009, 2016 10. Doak BC, Zheng J, Dobritzsch D, et al: How beyond rule of 5 drugs and clinical candidates bind to their targets. J Med Chem 59:2312-2327, 2016 11. Lipinski C, Hopkins A: Navigating chemical space for biology and medicine. Nature 432:855-861, 2004 12. Brooks AF, Topczewski JJ, Ichiishi N, et al: Late-stage [18F]fluorination: New solutions to old problems. Chem Sci 5:4545-4553, 2014 13. Brooks AF, Drake LR, Stewart MN, et al: Fluorine-18 patents (20092015). Part 1: Novel radiotracers. Pharm Pat Anal 5:17-47, 2016 14. Mossine AV, Thompson S, Brooks AF, et al: Fluorine-18 patents (2009-2015). Part 2: New radiochemistry. Pharm Pat Anal 5:319-349, 2016 15. Jones T, Rabiner EA: The development, past achievements, and future directions of brain PET. J Cereb Blood Flow Metab 32:1426-1454, 2012 16. Seminars in Nuclear Medicine vol 1, No. 1. 1971 17. Seminars in Nuclear Medicine vol 47, No. 1. 2017

21 18. Marcus C, Mena E, Subramaniam RM: Brain PET in the diagnosis of Alzheimer’s disease. Clin Nucl Med 39:e413-e426, 2014 19. Jovalekic A, Koglin N, Mueller A, et al: New protein deposition tracers in the pipeline. EJNMMI Radiopharm Chem 1:2016, Article 11 20. Villemagne VL, Doré V, Bourgeat P, et al: Aβ-amyloid and tau imaging in dementia. Semin Nucl Med 47:75-88, 2017 21. Alzheimer’s Association: 2016 Alzheimer’s disease facts and figures. Alzheimers Dement 12:459-509, 2016 22. Kurz A, Perneczky R: Amyloid clearance as a treatment target against Alzheimer’s disease. J Alzheimers Dis 24:61-73, 2011 (suppl 2) 23. Boutajangout A, Sigurdsson EM, Krishnamurthy PK: Tau as a therapeutic target for Alzheimer’s disease. Curr Alzheimer Res 8:666-677, 2011 24. Selkoe DJ, Hardy J: The amyloid hypothesis of Alzheimer’s disease at 25 years. EMBO Mol Med 8:595-608, 2016 25. Klunk WE, Engler H, Nordberg A, et al: Imaging brain amyloid in Alzheimer’s disease with Pittsburgh compound-B. Ann Neurol 55:306319, 2004 26. Weiner MW, Veitch DP, Aisen PS, et al: The Alzheimer’s disease neuroimaging initiative: A review of papers published since its inception. Alzheimers Dement 8:S1-S68, 2012 (suppl 1) 27. Su Y, Blazey TM, Owen CJ, et al: Dominantly Inherited Alzheimer Network: Quantitative amyloid imaging in autosomal dominant Alzheimer’s disease: Results from the DIAN study group. PLoS ONE 11:2016, e0152082 28. Klunk WE, Koeppe RA, Price JC, et al: The Centiloid Project: Standardizing quantitative amyloid plaque estimation by PET. Alzheimers Dement 11:1-15, 2015 29. Doraiswamy PM, Sperling RA, Johnson K, et al: Florbetapir F18 amyloid PET and 36-month cognitive decline: A prospective multicenter study. Mol Psychiatry 19:1044-1051, 2014 30. Matthews PM, Rabiner EA, Passchier J, et al: Positron emission tomography molecular imaging for drug development. Br J Clin Pharmacol 73:175-186, 2012 31. Gunn RN, Rabiner EA: Imaging in central nervous system drug discovery. Semin Nucl Med 47:89-98, 2017 32. Marik J, Sanabria Bohorquez SM, Williams S-P, et al: New imaging paradigms in drug development: The PET imaging approach. Drug Discov Today Technol 8:e63-e69, 2011 33. Rinne JO, Brooks DJ, Rossor MN, et al: 11C-PiB PET assessment of change in fibrillar amyloid-β load in patients with Alzheimer’s disease treated with bapineuzumab: A phase 2, double-blind, placebocontrolled, ascending-dose study. Lancet Neurol 9:363-372, 2010 34. Mintun MA, Devous MD Sr, Lu M, et al: Abstract 60: PET Amyloid and Tau Imaging in a Phase 3 Study of Solanezumab. Human Amyloid Imaging Conference Book of Abstracts 2017; 181-182 35. Salloway S, Sperling R, Fox NC, et al: Bapineuzumab 301 and 302 clinical trial investigators: Two phase 3 trials of bapineuzumab in mild-to-moderate Alzheimer’s disease. N Engl J Med 370:322-333, 2014 36. The Lancet Neurology: Solanezumab: Too late in mild Alzheimer’s disease? Lancet Neurol 16:97, 2017, Editorial 37. Sevigny J, Chiao P, Bussière T, et al: The antibody aducanumab reduces Aβ plaques in Alzheimer’s disease. Nature 537:50-56, 2016 38. Langer O, Krcal A, Schmid A, et al: Synthesis of 1,1' [11C]-methylene-di-(2-naphthol) ([11C]ST1859) for PET studies in humans. J Labelled Comp Radiopharm 48:577-587, 2005 39. Bauer M, Langer O, Dal-Bianco P, et al: A positron emission tomography microdosing study with a potential antiamyloid drug in healthy volunteers and patients with Alzheimer’s disease. Clin Pharmacol Ther 80:216-227, 2006 40. Catafau AM, Bullich S: Amyloid PET imaging: Applications beyond Alzheimer’s disease. Clin Transl Imaging 3:39-55, 2015 41. Johnson VE, Stewart W, Smith DH: Traumatic brain injury and amyloid-β pathology: A link to Alzheimer’s disease. Nat Rec Neurosci 11:361-370, 2010 42. Hong YT, Veenith T, Dewar D, et al: Amyloid imaging with carbon 11–labeled Pittsburgh compound B for traumatic brain injury. JAMA Neurol 71:23-31, 2014 43. Frey KA: Molecular imaging of extrapyramidal movement disorders. Semin Nucl Med 47:18-30, 2017

ARTICLE IN PRESS 22 44. Petrou M, Bohnen NI, Müller ML, et al: Aβ-Amyloid deposition in patients with Parkinson disease at risk for development of dementia. Neurology 79:1161-1167, 2012 45. Hepp DH, Vergoossen DL, Huisman E, et al: Distribution and load of amyloid-β pathology in Parkinson disease and dementia with Lewy bodies. J Neuropathol Exp Neurol 75:936-945, 2016 46. Serrano-Pozo A, Frosch MP, Masliah E, et al: Neuropathological alterations in Alzheimer disease. Cold Spring Harb Perspect Med 1:a006189, 2011 47. Jung Y, Whitwell JL, Duffy JR, et al: Regional β-amyloid burden does not correlate with cognitive or language deficits in Alzheimer’s disease presenting as aphasia. Eur J Neurol 23:313-319, 2016 48. Iqbal K, Liu F, Gong C-X: Tau and neurodegenerative disease: The story so far. Nat Rev 12:15-27, 2016 49. Haroutian V, Davies P, Vianna C, et al: Tau protein abnormalities associated with the progression of Alzheimer’s disease type dementia. Neurobiol Aging 28:1-7, 2007 50. Iqbal K, Alonso Adel C, Chen S, et al: Tau pathology in Alzheimer disease and other tauopathies. Biochim Biophys Acta 1739:198-210, 2005 51. Watanabe H, Ono M, Saji H: Novel PET/SPECT probes for imaging of tau in Alzheimer’s disease. ScientificWorldJournal 2015:2015, 124192 52. Villemagne VL, Fodero-Tavoletti MT, Masters CL, et al: Tau imaging: Early progress and future directions. Lancet Neurol 14:114-124, 2015 53. Kolb HC, Andrés JI: Tau positron emission tomography imaging. Cold Spring Harb Perspect Biol 2016, doi:10.1101/cshperspect.a023721 54. Ariza M, Kolb HC, Moechars D, et al: Tau positron emission tomography (PET) imaging: Past, present, and future. J Med Chem 58:4365-4382, 2015 55. James OG, Doraiswamy PM, Borges-Neto S: PET imaging of tau pathology in Alzheimer’s disease and tauopathies. Front Neurol 6:2015, Article 38 56. Villemagne VL, Furumoto S, Fodero-Tavoletti MT, et al: The challenges of tau imaging. Fut Neurol 7:409-421, 2012 57. Kepe V, Bordelon Y, Boxer A, et al: PET imaging of neuropathology in tauopathies: Progressive supranuclear palsy. J Alzheimers Dis 36:145-153, 2013 58. Xia C-F, Arteaga J, Chen G, et al: [18F]T807, a novel tau positron emission tomography imaging agent for Alzheimer’s disease. Alzheimers Dement 9:666-676, 2013 59. Chien DT, Bahri S, Szardenings AK, et al: Early clinical PET imaging results with the novel PHF-tau radioligand [18F]T807. J Alzheimers Dis 34:457-468, 2013 60. Shcherbinin S, Schwarz AJ, Josh A, et al: Kinetics of the tau PET tracer 18 F-AV-1451 (T807) in subjects with normal cognitive function, mild cognitive impairment and Alzheimer’s disease. J Nucl Med 57:15351542, 2016, doi:10.2967/jnumed.115.170027 61. Fodero-Tavoletti MT, Okamura N, Fukumoto S, et al: 18F-THK523: A novel in vivo tau imaging ligand for Alzheimer’s disease. Brain 134:1089-1100, 2011 62. Okamura N, Furumoto S, Harada R, et al: Novel 18 F-labeled arylquinoline derivatives for noninvasive imaging of tau pathology in Alzheimer disease. J Nucl Med 54:1420-1427, 2013 63. Harada R, Okamura N, Fukumoto S, et al: 18F-THK5351: A novel PET radiotracer for imaging neurofibrillary pathology in Alzheimer disease. J Nucl Med 57:208-214, 2016 64. Furumoto S, Tago T, Harada R, et al: 18F-Labeled 2-Arylquinoline derivatives for tau imaging: Chemical, radiochemical, biological and clinical features. Curr Alzheimer Res 14:178-185, 2017 65. Maruyama M, Shimada H, Suhara T, et al: Imaging of tau pathology in a tauopathy mouse model and in Alzheimer patients compared to normal controls. Neuron 79:1094-1108, 2013 66. Kimura Y, Ichise M, Ito H: PET quantification of tau pathology in human brain with 11C-PBB3. J Nucl Med 56:1359-1365, 2015 67. Marquié-Sayagués M, Antón-Fernández A, Chong MST, et al: Abstract 34: [F-18]-AV-1451 binding correlates with neurofibrillary tangle Braak staging in postmortem brain tissue samples. Abstract 31: Human Amyloid Imaging Conference Book of Abstracts 2017; 106

A.L. Va¯vere and P.J.H. Scott 68. Devous MD Sr, Michael N, Siderowf A: Baseline 18F Flortaucipir SUVR, but Not Amyloid or Cognition, Predicts Cognitive Decline over 18 Months in Phase 2 Trial Subjects. Human Amyloid Imaging Conference Book of Abstracts 2017; 179 69. Ishiki A, Okamura N, Furukawa K, et al: Longitudinal assessment of tau pathology in patients with Alzheimer’s disease using [18F]THK-5117 positron emission tomography. PLoS ONE 10:2015, e0140311 70. Rabinovici G, Schonhaut D, Ossenkoppele R, et al: Initial experience with [18F]AV1451 PET in AD and non-AD tauopathies. Neurology 84:p5-p5, 2015 (suppl) 71. Cho H, Choi JY, Hwang MS, et al: Subcortical 18F-AV-1451 binding patterns in progressive supranuclear palsy. Movement Disord 32:134140, 2017 72. Gomez F, Lin Y-G, Liang Q, et al: Quantitative assessment of [18F]AV-1451 distribution in AD, PSP and PiD post-mortem brain tissue sections relative to that of the anti-tau antibody AT8. J Nucl Med 57:348, 2016 (suppl 2) 73. Sander K, Lashley T, Gami P, et al: Characterization of tau positron emission tomography tracer [18F]AV-1451 binding to postmortem tissue in Alzheimer’s disease, primary tauopathies, and other dementias. Alzheimers Dement 12:1116-1124, 2016 74. Honer M, Gobbi L, Muri D, et al: Abstract 99: In Vitro Binding of [3H]RO6958948, [3H]AV-1451, [3H]THK5351 and [3H]T808 to Tau Aggregates in Non-AD Tauopathies. Human Amyloid Imaging Conference Book of Abstracts 2017; 62 75. Ishiki A, Harada R, Okamura N, et al: Tau imaging with [18F]THK5351 in progressive supranuclear palsy. Eur J Neurol 24:130-136, 2017 76. Vettermann F, Brendel M, Schoenecker S, et al: [18F]THK-5351 PET in patients with clinically diagnosed progressive supranuclear palsy. J Nucl Med 57:457, 2016 (suppl 2) 77. Suhara T, Shimada H, Shinotoh H, et al: In vivo tau PET imaging using [ 11 C]PBB3 in Alzheimer’s disease and non-Alzheimer’s disease tauopathies. J Nucl Med 55:1824, 2014 (suppl 1) 78. McMillan CT, Irwin DJ, Nasrallah I, et al: Multimodal evaluation demonstrates in vivo 18F-AV-1451 uptake in autopsy-confirmed corticobasal degeneration. Acta Neuropathol 132:935-937, 2016 79. Kikuchi A, Okamura N, Hasegawa T, et al: In vivo visualization of tau deposits in corticobasal syndrome by 18F-THK5351 PET. Neurology 87:2309-2316, 2016 80. Bevan Jones WR, Cope TE, Passamonti L, et al: [18F]AV-1451 PET in behavioral variant frontotemporal dementia due to MAPT mutation. Ann Clin Transl Neurol 3:940-947, 2016 81. Spina S, Schonhaut DR, Boeve BF, et al: Frontotemporal dementia with the V337M MAPT mutation: Tau-PET and pathology correlations. Neurology 88:758-766, 2017, doi:10.1212/WNL.0000000000003636 82. Shinotoh H, Sahara N, Shimada H, et al: [11C]PBB3 PET visualizes tau aggregates in patients with FTDP-17 MAPT gene mutation. Alzheimers Dement 12:P135-P136, 2016 83. Hashimoto H, Kawamura K, Takei M, et al: Identification of a major radiometabolite of [11C]PBB3. Nucl Med Biol 42:905-910, 2015 84. Ono M, Kitamura S, Shimada H, et al: Development of Novel Tau PET Tracers, [18F]AM-PBB3 and [18F]PM-PBB3. Human Amyloid Imaging Conference Book of Abstracts 2017; 34 85. Vermeiren C, Mercier J, Viot D, et al: T807, a Reported Selective Tau Tracer, Binds with Nanomolar Affinity to Monoamine Oxidase A. Alzheimer’s Association International Conference 2015. 2015; Abstract ID 5422 86. Guo Q, Otterstätter B, Benninghoff S, et al: Abstract 127: Evaluation of the Selectivity of Tau PET Radioligand THK5351 in AD Brain In Vitro and Nonhuman Primate Brain In Vivo. Human Amyloid Imaging Conference Book of Abstracts 2017; 94-95 87. Harada R, Ishiki A, Furumoto S, et al: Abstract 82: Characterizing the “Off-target” Binding of 18F-THK5351 in Alzheimer’s Disease: Correlation Between Ante-mortem and Post-mortem Findings. Human Amyloid Imaging Conference Book of Abstracts 2017; 96 88. Shao X, Carpenter GM, Desmond TJ, et al: Evaluation of [11C]N-Methyl lansoprazole as a radiopharmaceutical for PET imaging of tau neurofibrillary tangles. ACS Med Chem Lett 3:936-941, 2012

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers 89. Fawaz MV, Brooks AF, Rodnick ME, et al: High affinity radiopharmaceuticals based upon lansoprazole for PET imaging of aggregated tau in Alzheimer’s disease and progressive supranuclear palsy: Synthesis, preclinical evaluation, and lead selection. ACS Chem Neurosci 5:718-730, 2014 90. Walji AM, Hostetler ED, Selnick H, et al: Discovery of 6-(Fluoro-18F)-3-(1H-pyrrolo[2,3-c]pyridin-1-yl)isoquinolin-5-amine ([18F]-MK-6240): A positron emission tomography (PET) imaging agent for quantification of neurofibrillary tangles (NFTs). J Med Chem 59:4778-4789, 2016 91. Hostetler ED, Walji AM, Zeng Z, et al: Preclinical characterization of 18 F-MK-6240, a promising PET tracer for in vivo quantification of human neurofibrillary tangles. J Nucl Med 57:1599-1606, 2016 92. Wong DF, Borroni E, Kuwabara H, et al: First in-human PET study of 3 novel tau radiopharmaceuticals: [ 11 C]RO6924963, [11C]RO6931643, and [18F]RO6958948. Alzheimers Dement 11:P850P851, 2015 (suppl) 93. Sanabria Bohorquez S, Barret O, Tamagnan G, et al: Evaluation of tau burden in a cross-sectional cohort of Alzheimer’s disease subjects using [18F]GTP1 (Genentech Tau Probe 1). Alzheimers Dement 12:1172, 2016 (suppl) 94. Muhs A, Berndt M, Kroth H, et al: Characterization and development of novel tau PET tracers for the assessment of tau spreading in Alzheimer’s disease. J Prev Alzheimers Dis 4:273, 2015 95. Eberling JL, Dave KD, Frasier MA: α-Synuclein imaging: A critical need for Parkinson’s disease research. J Parkinsons Dis 3:565-567, 2013 96. Eberling JL, Medhurst AD, Nash K, et al: Consortium to develop an alpha-synuclein imaging agent. J Cereb Blood Flow Metab 32:P096, 2012 (suppl 1) 97. Michael J Fox Foundation Press release: Alpha-Synuclein Imaging Prize. Available at: https://www.michaeljfox.org/research/imaging-prize.html. Accessed January 24, 2017. 98. Kotzbauer PT, Tu Z, Mach RH: Current status of the development of PET radiotracers for imaging alpha synuclein aggregates in Lewy bodies and Lewy neurites. Clin Transl Imaging 5:3-14, 2016, doi:10.1007/ s40336-016-0217-4 99. Shah M, Seibyl J, Cartier A, et al: Molecular imaging insights into neurodegeneration: Focus on α-synuclein radiotracers. J Nucl Med 55:1-4, 2014 100. Bagchi DP, Yu L, Perlmutter JS, et al: Binding of the radioligand SIL23 to α-synuclein fibrils in Parkinson disease brain tissue establishes feasibility and screening approaches for developing a Parkinson disease imaging agent. PLoS ONE 8:e55031, 2013 101. Zhang X, Jin H, Padakanti PK, et al: Radiosynthesis and in vivo evaluation of two PET radioligands for imaging α-synuclein. Appl Sci (Basel) 4:66-78, 2014 102. Dominguez C, Wityak J, Bard J, et al: Probes for Imaging Huntingtin Protein. WO 2016/033445. 2016 103. ALS Association Press Release: Grand Challenge: Generation of a PET Tracer for TDP43 Aggregates. Available at: http://als-ny.org/blog/grand -challenge-generation-of-a-pet-tracer-for-tdp43-aggregates/. Accessed February 6, 2017. 104. Chen W-W, Zhang X, Huang W-J: Role of neuroinflammation in neurodegenerative diseases (Review). Mol Med Rep 13:3391-3396, 2016 105. Albrecht DS, Granziera C, Hooker JM, et al: In vivo imaging of human neuroinflammation. ACS Chem Neurosci 7:470-483, 2016 106. Dupont AC, Guilloteau D, Kassiou M, et al: Radiopharmaceuticals for PET imaging of neuroinflammation. Médecine Nucléaire 40:72-81, 2016 107. Luus C, Hanani R, Reynolds A, et al: The development of PET radioligands for imaging the translocator protein (18 kDa): What have we learned? J Label Compd Radiopharm 53:501-510, 2010 108. Dheen ST, Kaur C, Ling EA: Microglial activation and its implications in the brain diseases. Curr Med Chem 14:1189-1197, 2007 109. Benavides J, Fage D, Carter C, et al: Peripheral type benzodiazepine binding sites are a sensitive indirect index of neuronal damage. Brain Res 421:167-172, 1987 110. Wilms H, Claasen J, Rohl C, et al: Involvement of benzodiazepine receptors in neuroinflammatory and neurodegenerative diseases:

23

111.

112.

113.

114.

115.

116.

117. 118. 119.

120.

121. 122.

123.

124.

125. 126. 127.

128.

129.

130.

131.

132.

Evidence from activated microglial cells in vitro. Neurobiol Dis 14:417-424, 2003 Zimmer ER, Leuzy A, Benedet AL, et al: Tracking neuroinflammation in Alzheimer’s disease: The role of positron emission tomography imaging. J Neuroinflammation 11:2014, Article 120 Walker D, Lue LF: Anti-inflammatory and immune therapy for Alzheimer’s disease: Current status and future directions. Curr Neuropharmacol 5:232-243, 2007 Bolger GT, Weissman BA, Lueddens H, et al: Late evolutionary appearance of ‘Peripheral-type’ binding sites for benzodiazepines. Brain Res 338:366-370, 1985 Hashimoto K, Inoue O, Suzuki K, et al: Synthesis and evaluation of 11 C-PK11195 for in vivo study of peripheral-type benzodiazepine receptors using positron emission tomography. Ann Nucl Med 3:63-71, 1989 Owen DR, Yeo AJ, Gunn RN, et al: An 18-kDa translocator protein (TSPO) polymorphism explains differences in binding affinity of the PET radioligand PBR28. J Cereb Blood Flow Metab 32:1-5, 2012 Kreisl WC, Lyoo CH, McGwier M, et al: Biomarkers consortium PET radioligand project team: In vivo radioligand binding to translocator protein correlates with severity of Alzheimer’s disease. Brain 136:22282238, 2013 Gerhard A: TSPO imaging in parkinsonian disorders. Clin Transl Imaging 4:183-190, 2016 Airas L, Rissanen E, Rinne JO: Imaging neuroinflammation in multiple sclerosis using TSPO-PET. Clin Transl Imaging 3:461-473, 2015 Zürcher NR, Loggia ML, Lawson R, et al: Increased in vivo glial activation in patients with amyotrophic lateral sclerosis: Assessed with [11C]-PBR28. Neuroimage Clin 7:409-414, 2015 Kreisl WC, Mbeo G, Fujita M, et al: Stroke incidentally identified using improved positron emission tomography for microglial activation. Arch Neurol 66:1288-1289, 2009 Almuhaideb A, Papathanasiou N, Bomanji J: 18F-FDG PET/CT imaging in oncology. Ann Saudi Med 31:3-13, 2011 Pinilla I, Rodríguez-Vigil B, Gómez-León N: Integrated FDG PET/CT: Utility and applications in clinical oncology. Clin Med Oncol 2:181-198, 2008 Walsh JC, Lebedeev A, Aten E, et al: The clinical importance if assessing tumor hypoxia: Relationship of tumor hypoxia to prognosis and therapeutic opportunities. Antioxid Redox Signal 21:1516-1554, 2014 Barker HE, Paget JTE, Khan AA: The tumour microenvironment after radiotherapy: Mechanisms of resistance and recurrence. Nat Rev Cancer 15:409-425, 2015 Kelada OJ, Carlson DJ: Molecular imaging of tumor hypoxia with positron emission tomography. Radiat Res 181:335-349, 2014 Padhani AR, Krohn KA, Lewis JS, et al: Imaging oxygenation of human tumors. Eur Radiol 17:861-872, 2007 Rajendran JG, Krohn KA: F-18 fluoromisonidazole for imaging tumor hypoxia: Imaging the microenvironment for personalized cancer therapy. Semin Nucl Med 45:151-162, 2014 Okamoto S, Shiga T, Yasuda K, et al: The reoxygenation of hypoxia and the reduction of glucose metabolism in head and neck cancer by fractionated radiotherapy with intensity-modulated radiation therapy. Eur J Nucl Med Mol Imaging 43:2147-2154, 2016 Wiedenmann NE, Bucher S, Hentschel M, et al: Serial [18F]-fluoromisonidazole PET during radiochemotherapy for locally advanced head and neck cancer and its correlation with outcome. Radiother Oncol 117:113-117, 2015 Toyonaga T, Hirata K, Yamaguchi S, et al: 18F-fluoromisonidazole positron emission tomography can predict pathological necrosis of brain tumors. Eur J Nucl Med Mol Imaging 43:1469-1476, 2016 Iqbal R, Kramer GM, Verwer EE, et al: Multiparametric analysis of the relationship between tumor hypoxia and perfusion with 18 F-fluoroazomycin arabinoside and 15 O-H 2 O PET. J Nucl Med 57:530-535, 2016 Metran-Nascente C, Yeung I, Vines DC, et al: Measurement of tumor hypoxia in patients with advanced pancreatic cancer based on 18 F-Fluoroazomyin arabinoside uptake. J Nucl Med 57:361-366, 2016

ARTICLE IN PRESS 24 133. Zegers CML, Hoebers FJP, van Elmpt W, et al: Evaluation of tumour hypoxia during radiotherapy using [18F]HX4 PET imaging and blood biomarkers with head and neck cancer. Eur J Nucl Med Mol Imaging 43:2139-2146, 2016 134. Vavere AL, Lewis JS: Cu-ATSM: A radiopharmaceutical for the PET imaging of hypoxia. Dalton Trans 43:4893-4902, 2007 135. Tsujikawa T, Asahi S, Oh M, et al: Assessment of the tumor redox status in head and neck cancer by 62Cu-ATSM PET. PLoS ONE 11:2016, e0155635 136. Lopci E, Grassi I, Rubello D, et al: Prognostic evaluation of disease outcome in solid tumors investigated with 64C-ATSM PET/CT. Clin Nucl Med 41:e87-e92, 2016 137. Carlin S, Zhang H, Reese M, et al: A comparison of the imaging characteristics and microregional distribution of 4 hypoxia PET tracers. J Nucl Med 55:515-521, 2014 138. Challapalli A, Aboagye EO: Positron emission tomography imaging of tumor cell metabolism and application to therapy response monitoring. Front Oncol 6:1-20, 2016 139. Schuster DM, Nanni C, Fanti S: PET tracers beyond FDG in prostate cancer. Sem Nucl Med 46:507-521, 2016 140. Ceci F, Castellucci P, Graziani T, et al: 11C-Choline PET/CT in castration-resistant prostate cancer patients treated with docetaxel. Eur J Nucl Med Mol Imaging 43:84-91, 2016 141. Schwarzenbock SM, Eiber M, Kundt G, et al: Prospective evaluation of [11C]Choline PET/CT in therapy response assessment of standardized docetacel first-line chemotherapy in patients with advanced castration refractory prostate cancer. Eur J Nucl Med Mol Imaging 43:2105-2113, 2016 142. van der Veldt AA, Lubberink M, Greuter HN, et al: Absolute quantification of [11C]docetaxel kinetics in lung cancer patients using positron emission tomography. Clin Cancer Res 17:4814-4824, 2011 143. Lee J, Sato MM, Coel MN, et al: Prediction of PSA progression in castration-resistant prostate cancer based on treatment-associated change in tumor burden quantified by 18F-fluorocholine PET/CT. J Nucl Med 57:1058-1064, 2016 144. Piert M, Montgomery J, Kunju LP, et al: 18F-Choline PET/MRI: The additional value of PET for MRI-guided transrectal prostate biopsies. J Nucl Med 57:1065-1070, 2016 145. Regula N, Häggman M, Johansson S, et al: Malignant lipogenesis defined by 11C-acetate PET/CT predicts prostate cancer-specific survival in patients with biochemical relapse after prostatectomy. Eur J Nucl Med Mol Imaging 43:2131-2138, 2016 146. Strandberg A, Karlsson CT, Ogren M, et al: 11 C-AcetatePET/CT Compared to 99m Tc-HDP bone scintigraphy in primary staging of high-risk prostate cancer. Anticancer Res 36:6475-6480, 2016 147. Zhao Y, Wang L, Pan J: The role of L-type amino acid transporter 1 in human tumors. Intractable Rare Dis Res 4:165-169, 2015 148. Kaira K, Oriuchi N, Imai H, et al: L-type amino acid transporter 1 and CD98 expression in primary and metastatic sites of human neoplasms. Cancer Sci 99:2380-2386, 2008 149. Ryttlefors M, Danfors T, Latini F, et al: Long-term evaluation of the effect of hypogractionated high-energy proton treatment of benign meningiomas by means of 11 C-L-methionine positron emission tomography. Eur J Nucl Med Mol Imaging 43:1432, 2016 150. Sörensen J, Savitcheva II, Engler H, et al: Utility of PET and 11 C-Methionine in the paediatric brain tumors. Clin Positron Imaging 3:157, 2000 151. Harris SM, Davis JC, Snyder SE, et al: Evaluation of the biodistribution of [11C]Methionine in children and young adults. J Nucl Med 54:19021908, 2013 152. Kaste SC, Snyder SE, Metzger ML, et al: Comparison of 11C-methionine and 18F-FDG PET-CT for staging and follow-up of pediatric lymphoma. J Nucl Med 58:419-424, 2017, doi:10.2967/jnumed.116.178640 153. Pyka T, Gempt J, Hiob D, et al: Textural analysis of pre-therapeutic [18F]-FET-PET and its correlation with tumor grade and patient survival in high-grade gliomas. Eur J Nucl Med Mol Imaging 43:133-141, 2016

A.L. Va¯vere and P.J.H. Scott 154. Henriksen OM, Larsen VA, Muhic A, et al: Simultaneous evaluation of brain tumour metabolism, structure and blood volume using [18F]-fluoroethyltyrosine (FET) PET/MRI: Feasibility, agreement and initial experience. Eur J Nucl Med Mol Imaging 43:103-112, 2016 155. Newsline: FDA appoves 18F-Fluciclovine and 68Ga-DOTATATE products. J Nucl Med 57:9N, 2016 156. Okudaira H, Nakanishi T, Oka S, et al: Kinetic analyses of trans-1-amino-3-[18F]fluorocyclobutanecarboxylic acid transport in Xenopus laevis oocytes expressing human ASCT2 and SNAT2. Nucl Med Biol 40:670-675, 2013 157. Odewole OA, Tade FL, Nieh PT, et al: Recurrent prostate cancer detection with anti-3-[18F]FACBC PET/CT: Comparison with CT. Eur J Nucl Med Mol Imaging 43:1773-1783, 2016 158. Nanni C, Zanoni L, Pultrone C, et al: 18 F-FACBC (anti1-amino-3- 18 F-fluorocyclobutane-1-carboxylic acid) versus 11 C-choline PET/CT in prostate cancer relapse: Results of a prospective trial. Eur J Nucl Med Mol Imaging 43:1601-1610, 2016 159. McConathy J: 18F-Fluciclovine (FACBC) and its potential use for breast cancer imaging. J Nucl Med 57:1329-1330, 2016 160. Tade FI, Cohen MA, Styblo TM, et al: Anti-3-18F (18F-Fluciclovine) PET/CT of breast cancer: An exploratory study. J Nucl Med 57:13571363, 2016 161. Ulaner GA, Goldman DA, Gönen M, et al: Initial results of a prospective clinical trial of 18F-Fluciclovine PET/CT in newly diagnosed invasive ductal and invasive lobular breast cancers. J Nucl Med 57:1350-1356, 2016 162. Pandit-Taskar N, Zanzonico P, Reidy-Lagunes D, et al: 18F-Meta Fluorobenzyl Guanidine (MFBG) PET imaging in patients with Neuroendocrine malignancies: Biodistribution, pharmacokinetics, organ dosimetry and lesion uptake. J Nucl Med 57:S25, 2016 163. Deppen SA, Blume J, Bobbey AJ, et al: 68Ga-DOTATATE compared with 111 In-DTPA-octrotide and conventional imaging for pulmonary and gastroenteropancreatic neuroendocrine tumors: A systematic review and meta-analysis. J Nucl Med 57:872-878, 2016 164. Deppen SA, Liu E, Blume J, et al: Safety and efficacy of 68Ga-DOTATATE PET/CT for diagnosis, staging, and treatment management of neuroendocrine tumors. J Nucl Med 57:708-714, 2016 165. Archier A, Varoquax A, Garrigue P, et al: Prospective comparison of 68 Ga-DOTATATE and 18 F-FDOPA PET/CT in patients with various pheochromocytomas and paragangliomas with emphasis on sporadic cases. Eur J Nucl Med Mol Imaging 43:1248-1257, 2016 166. Janssen I, Chen CC, Millo CM, et al: PET/CT comparing 68 Ga-DOTATATE and other radiopharmaceuticals and in comparison with CT/MRI for the localization of sporadic metastatic pheochromocytoma and paraganglioma. Eur J Nucl Med Mol Imaging 43:1784-1791, 2016 167. Nilica B, Waitz D, Stevanovic V, et al: Direct comparison of 68 Ga-DOTA-TOC and 18F-FDG PET/CT with the follow-up of patients with neuroendocrine tumour treated with the first full peptide receptor radionuclide therapy cycle. Eur J Nucl Med Mol Imaging 43:1585-1592, 2016 168. Haubner R, Beer AJ, Wang J, et al: Positron emission tomography tracers for imaging angiogenesis. Eur J Nucl Med Mol Imaging 37:S86-S103, 2010 169. Guo J, Lang L, Hu S, et al: Comparison of three dimeric 18 F-AlF-NOTA-RGD tracers. Mol Imaging Biol 16:274-283, 2014 170. Kang F, Wang S, Tian F, et al: Comparing the diagnostic potential of 68Ga-Alfatide II and 18F-FDG in differentiation between nonsmall cell lung cancer and tuberculosis. J Nucl Med 57:672-677, 2016 171. Zhang H, Liu N, Gao S, et al: Can an 18F-ALF-NOTA-PRGD3 PET/CT scan predict treatment sensitivity to concurrent chemoradiotherapy in patients with newly diagnosed glioblastoma? J Nucl Med 57:524-529, 2016 172. Luan X, Huang Y, Gao S, et al: 18F-alfatide PET/CT may predict short-term outcome of concurrent chemoradiotherapy in patients with advanced non-small cell lung cancer. Eur J Nucl Med Mol Imaging 43:2336-2342, 2016

ARTICLE IN PRESS Clinical applications of small-molecule PET radiotracers 173. Mukerjee D, Zhao J: The role of chemokine receptor CXCR4 in breast cancer metastasis. Am J Cancer Res 3:46-57, 2013 174. Vag T, Gerngross C, Herhaus P, et al: First experience with chemokine receptor CXCR4-targeting PET imaging of patients with solid cancers. J Nucl Med 57:741-746, 2016 175. Lapa C, Lükerath K, Rudelius M, et al: [68Ga]Petixafor-PET/CT for imaging of chemokine receptor 4 expression in small cell lung cancer—Initial experience. Oncotarget 7:9288-9295, 2016 176. Maurer T, Eiber M, Schwaiger M, et al: Current use of PSMAPET in prostate cancer management. Nat Rev Urol 13:226-235, 2016 177. Lütje S, Heskamp S, Cornelissen AS, et al: PSMA ligands for radionuclide imaging and therapy of prostate cancer: Clinical status. Theranostics 5:1388-1401, 2015 178. Lindenberg L, Choyke P, Dahut W: Prostate cancer imaging with novel PET tracers. Curr Urol Rep 17:18, 2016 179. Perera M, Papa N, Christidis D, et al: Sensitivity, specificity, and predictors of positive 68Ga-prostate-specific membrane antigen positron emission tomography in advance prostate cancer: A systematic review and meta-analysis. Eur Urol 70:926-937, 2016 180. Sterzing F, Kratochwil C, Fiedler H, et al: 68Ga-PSMA-11 PET/CT: A new technique with high potential for the radiotherapeutic management of prostate cancer patients. Eur J Nucl Med Mol Imaging 43:34-41, 2016 181. Freitag MT, Radtke JP, Hadaschik BA, et al: Comparison of hybrid 68 Ga-PSMA PET/MRI and 68Ga-PSMA PET/CT in the evaluation of lymph node and bone metastases of prostate cancer. Eur J Nucl Med Mol Imaging 43:70-83, 2016 182. Pyka T, Okamoto S, Dahlbender M, et al: Comparison of bone scintigraphy and 68Ga-PSMA PET for skeletal staging in prostate cancer. Eur J Nucl Med Mol Imaging 43:2114-2121, 2016 183. Schwenck J, Rempp H, Reischl G, et al: Comparison of 68Ga-labelled PSMA-11 and 11C-choline in the detection of prostate cancer metastases by PET/CT. Eur J Nucl Med Mol Imaging 44:92-101, 2017 184. Pfister D, Porres D, Heidenreich A, et al: Detection of recurrent prostate cancer lesions before salvage lymphadenectomy is more accurate with 68 Ga-PSMA-HBED-CC than with 18F-Fluoroethylcholine PET/CT. Eur J Nucl Med Mol Imaging 43:1410-1417, 2016 185. Sawicki LM, Buchbender C, Boos J, et al: Diagnostic potential of PET/CT using a 68Ga-labelled prostate-specific membrane antigen ligand in whole-body staging of renal cell carcinoma: Initial experience. Eur J Nucl Med Mol Imaging 44:102-107, 2017 186. Pyka T, Weirich G, Einspieler I, et al: 68Ga-PSMA-HBED-CC PET for differential diagnosis of suggestive lung lesions in patients with prostate cancer. J Nucl Med 57:367-371, 2016 187. Rahbar K, Weckesser M, Huss S, et al: Correlation of intraprostatic tumor extent with 68Ga-PSMA distribution in patients with prostate cancer. J Nucl Med 57:563-567, 2016 188. Rowe SP, Macura KJ, Ciarallo A, et al: Comparison of prostate-specific membrane antigen-based 18F-DCFBC PET/CT to conventional imaging modalities for detection of hormone-naïve and castration-resistant metastatic prostate cancer. J Nucl Med 57:46-53, 2016 189. Chalkidou A, Landau DB, Odell EW, et al: Correlation between Ki-67 immunohistochemistry and 18F-Fluorothymidine uptake in patients with cancer: A systematic review and meta-analysis. Eur J Cancer 48:34993513, 2012 190. Bollineni VR, Kramer GM, Jansma EP, et al: A systematic review on [18F]FLT-PET uptake as a measure of treatment response in cancer patients. Eur J Cancer 55:81-97, 2016 191. Schöder H, Zelenetz AD, Hamlin P, et al: Prospective study of 3’-Deoxy-3’-18F-Fluorothymidine PET for early interim response assessment in advanced-state B-cell lymphoma. J Nucl Med 57:728-734, 2016

25 192. Lodge MA, Holdhoff M, Leal JP, et al: Repeatability of 18F-FLT PET in a multi-center study of patients with high grade glioma. J Nucl Med 116:jnumed, 2016 193. McHugh CI, Lawhorn-Crews JM, Modi D, et al: Effects of capecitabine treatment on the uptake of thymidine analogs using exploratory PET imaging agents: 18F-FAU, 18F-FMAU, and 18F-FLT. Cancer Imaging 16:Article 34, 2016 194. Wollenweber T, Bengel FM: Cardiac molecular imaging. Sem Nucl Med 44:386-397, 2014 195. Robson PM, Dweck MR, Trivieri MG, et al: Coronary artery PET/MR imaging. JACC Cardiovasc Imaging 2017, doi:10.1016/j.jcmg .2016.09.029 196. Ferda J, Hromádka M, Baxa J: Imaging of the myocardium using 18 F-FDG-PET/MRI. Eur J Radiol 85:1900-1908, 2016 197. Peterson LR, Gropler RJ: Radionuclide imaging of myocardial metabolism. Circ Cardiovasc Imaging 3:211-222, 2010 198. Slomka P, Berman DS, Alexanderson E, et al: The role of PET quantification in cardiovascular imaging. Clin Transl Imaging 2:343-358, 2014 199. Maddahi J, Packard R: Cardiac PET perfusion tracers: Current status and future directions. Semin Nucl Med 44:333-343, 2014 200. Juneau D, Erthal F, Ohira H, et al: Clinical PET myocardial perfusion imaging and flow quanitification. Cardiol Clin 34:69-85, 2016 201. Giorgi MC, Meneghetti JC, Soares J Jr, et al: Left ventricular function in response to dipyridamole stress: Head-to-head comparison between 82 Rubidium PET and 99mTc-sestamibi SPECT ECG-gated myocardial perfusion imaging. Eur J Nucl Med Mol Imaging 44:876-885, 2016, doi:10.1007/s00259-016-3588-x 202. Lim SY, Hausenloy DJ: Remote ischemic conditioning: From bench to bedside. Front Physiol 3:27, 2012 203. Pryds K, Nielsen RR, Hoff CM, et al: Effect of remote ischemic conditioning on myocardial perfusion in patients with suspected ischemic coronary artery disease. J Nucl Cardiol 2016, doi:10.1007/ s12350-016-0709-7 204. Lam WC, Pennell DJ: Imaging of the heart: Historical perspective and recent advances. Postgrad Med J 92:99-104, 2016 205. Murthy VL, Leuschner M, Scott PJH, et al: Abstracts of original contributions ASNC2016 the 21st annual scientific session of the American Society of Nuclear Cardiology. Abstract 302-05: Initial validation of a highly automated superconducting mini-Cyclotron for decentralized production of 13N-Ammonia. J Nucl Cardiol 23:899-937, 2016 206. Aggarwal NR, Drozdova A, Askew JW, et al: Feasibility and diagnostic accuracy of exercise treadmill nitrogen-13 ammonia PET myocardial perfusion imaging of obese patients. J Nucl Cardiol 22:1273-1280, 2015 207. Thackeray JT, Bengel FM: PET imaging of the autonomic nervous system. Q J Nucl Med Mol Imaging 60:362-382, 2016 208. Harms HJ, Huisman MC, Rijnierse MT, et al: Noninvasive quantification of myocardial 11C-Meta-Hydroxyephedrine kinetics. J Nucl Med 57:1376-1381, 2016 209. Malhotra S, Canty JM: Life-threatening ventricular arrhythmias: Current role of imaging in diagnosis and risk assessment. J Nucl Cardiol 23:1322-1334, 2016 210. Aikawa T, Naya M, Obara M: Impaired myocardial sympathetic innervation is associated with diastolic dysfunction in heart failure with preserved ejection fraction: 11C-hydroxyephedrine PET study. J Nucl Med 58:784-790, 2017, doi:10.2967/jnumed.116.178558 211. Duvernoy CS, Raffel DM, Swanson SD: Left ventricular metabolism, function, and sympathetic innervation in men and women with type 1 diabetes. J Nucl Cardiol 23:960-969, 2016 212. Goozner M: Editorial: CMS’ PET peeve. Mod Healthc 45:24, 2015