Positron Emission Tomography and Drug Discovery

PII S1536-1632(02)00017-3

Molecular Imaging and Biology Vol. 4, No. 5, 311–337. 2002 Copyright © 2002 Elsevier Science Inc. Printed in the USA. All rights reserved. 1536-1632/02 $–see front matter

REVIEW

Positron Emission Tomography and Drug Discovery: Contributions to the Understanding of Pharmacokinetics, Mechanism of Action and Disease State Characterization Michael T. Klimas, PhD Genomics and Molecular Imaging, GE Medical Systems, Waukesha, WI As an imaging modality, positron emission tomography (PET) provides unique quantitative in vivo information of value to drug discovery studies. These non-invasive studies span the pharmacokinetic/pharmacodynamic evaluation of potential drug candidates, receptor occupancy as an important determinant of efficacy, the pharmacological characterization of potential mechanisms of action, and the biological characterization of disease with well-characterized PET ligands. PET techniques are also being applied to the assessment of gene-level activities and the longitudinal evaluation of disease progression and therapeutic intervention. As the availability of PET scanners, cyclotrons, and specific PET ligands grows, the techniques highlighted in this review will become central to target validation, drug candidate selection, pharmacokinetic characterization, and clinical evaluation. (Mol Imag Biol 2002;4:311–337) © 2002 Elsevier Science, Inc. All rights reserved. Key Words: PET; Pharmacokinetics; Receptor occupancy; Disease markers; Diagnosis; Disease characterization; Longitudinal evaluation.

Introduction

D

rug discovery and development relies on a scientific understanding of pathology and biology at the molecular, cellular, system, and organism level. The sequencing of the human genome and developments in comparative genomics, proteomics, and cellular investigations has provided an extensive opportunity to understand and treat pathological states. The development of next generation therapeutics is, however, limited by information that must be obtained from animal studies and clinical evaluation. This includes pharmacokinetic parameters, exposure, and pharmacokineticpharmacodynamic (PK-PD) relationships. Animal-based disease state models, including transgenics, provide insight to the clinical state to the extent that the experimental condition can be quantified, is representative of the underlying pathology, and the PK-PD relationship scales to man. These are very significant hurdles for most disease state models, and a variety of parameters must be determined in a clinical environment. The early clinical environment is also an approximation be-

Address correspondence to: Michael Klimas, GE Medical Systems, 3000 Grandview Blvd., W-1390, Waukesha, WI 53188. E-mail: [email protected]

cause of the healthy nature of the initial test subjects and the rigorously controlled nature of the trial. Cost considerations limit the size of early trials, and this puts pressure on the statistical interpretation of any quantitative clinical measure. The challenge is magnified by diseases that are poorly understood at a biological level, are difficult to objectively measure, are slowly progressing and chronic in nature, or are incapable of approximation in animal models. Positron emission tomography (PET), the non-invasive monitoring of pharmacokinetic and functional processes in intact organisms at tracer concentrations, is a powerful technique to address a range of pharmaceutical discovery challenges. This review will sample the literature to highlight examples of PET techniques applied to disease state and pharmacological understanding. It is not meant to be comprehensive and will undoubtedly leave important papers and researchers unreferenced. The interested reader is encouraged to investigate the literature at considerably greater depth for a more thorough understanding. Most of the cited work employs a narrow set of available PET radiopharmaceuticals including 2-deoxy-2-[18F]fluoro-D-glucose (FDG; 1), [15O] water, [13N]ammonia, 6-[18F]fluoro-L-dopa (FDOPA; 2) and dopamine-related structures, [11C]dopamine receptor ligands, [11C]serotonin receptor ligands, and [11C]GABA/ 311

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namic outcome.10 PET and nuclear medicine techniques are being extended to the determination of gene function and regulation with in vivo reporter genes.11 The future of PET in drug discovery is substantial, and the functional and anatomical information it provides will be central to solving many treatment challenges.

Pharmacokinetic, Pharmacodynamic

benzodiazepine ligands. Other molecular imaging probe ligands and research investments are discussed throughout the text. The scope of this review was not confined to PET’s traditional applications in drug discovery, pharmacokinetic measurements, and receptor occupancy studies. PET evaluation of pathology and disease processes also provides an understanding of disease and the opportunity to prioritize molecular targets based on these hypotheses. PETbased receptor occupancy studies are becoming the state of art in evaluating the mechanism of action and for determining dose. PET has been applied to pharmacodynamic studies of the central nervous system (CNS)1 and is expanding into drug development as applied to cardiovascular disease and oncology.2 PET measurements can evaluate a new drug relative to standard therapies or study a series of analogs being considered for further development. In the clinical context pharmacokinetic measurements can be coupled to PET imaging to optimize the dosing schedule, evaluate clinical utility, and formulate the appropriate design for later stage trials.3 The best illustration of PET’s scientific value beyond pharmacokinetic measures is in the areas of neurology and psychiatry. The understanding of human brain pharmacology has traditionally depended on indirect assessments or models derived from animal studies. PET techniques allow a direct measure of human pharmacology and brain function.4,5 Cerebral development, neuronal plasticity, and compensatory reorganization are active areas of investigation with the capability to map progression and therapeutic response.6 PET is capable of bridging molecular biology, pathological understanding, and drug discovery.7,8 By using appropriate mathematical models and labeled compounds, PET can non-invasively measure protein synthesis and transport in addition to the well precedented measures of local metabolism, blood flow, and drug pharmacokinetics.9 PET’s application to drug discovery and development offers the opportunity to evaluate the in vivo expression of therapeutic molecular targets and the degree of molecular target occupancy as a function of the pharmacody-

PET is a sensitive technique to measure drug pharmacokinetics and pharmacodynamics non-invasively in target tissues. The technology can address several key drug discovery bottlenecks including biodistribution, absorption, plasma binding, metabolism, efflux pump activity, and drug delivery. Inhomogeneous drug distribution and drug dilution in the target tissue are likely to be limiting parameters for therapy response.12 PET measurements provide insight into the tissue distribution of drug classes (neuroleptics, antimicrobials, and antineoplastic agents) after intravenous, inhalation, or oral administration.13

Biodistribution PET has been used to determine the disposition of CNS drugs and the role of the blood-brain barrier in drug uptake.14 Hyperosmolar blood-brain barrier disruption (HBBBD), produced by infusion of mannitol into the cerebral arteries, has been used in the treatment of brain tumors to increase tumor-based drug delivery. Rubidium-82 PET was used in a study of HBBBD to optimize the timing and to determine the impact on the therapeutic index of agents such as methotrexate.15 Molecular imaging probes containing carbon-11 and fluorine-18 have been used to generate three-dimensional images of drug disposition in the lung and to investigate physiological changes in the lung as a result of therapeutic intervention.16 The distribution dynamics and kinetics of aerosolized drugs such as Azmacort® (3; using [11C]triamcinolone acetonide) have been in-

vestigated with respect to lung morphology and drug formulations.17 Pulmonary deposition and clearance of inhaled powder particles was studied by PET in chronic obstructive pulmonary disease (COPD) patients. Regional powder deposition in peripheral lung fields was

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more uneven in COPD patients, with more inhaled powder deposited centrally than in normal patients.18 PET ([11C]cyanoimipramine) has also demonstrated that lungs may function as a reservoir for antidepressants with high affinity for the serotonin transporter. Displacement of the accumulated antidepressants could complicate plasma concentration targets and interpretation of the side effect profile.19

uations of fleroxacin (6), another fluoroquinolone, cen-

Pharmacokinetics and Dosing The literature contains a larger number of pharmacokinetic studies that compare plasma pharmacokinetics with PET image-based quantitative receptor occupancy. This information is then used to optimize dosing schedules and relate pharmacological effects to receptor occupancy.20 The clearance of many drugs varies with the patient, so drug candidates with narrow therapeutic indexes (such as anticancer agents) would be better applied if the doses were individualized to achieve a target exposure.21 Pharmacokinetic studies depend on the availability of an appropriately labeled positron-emitter candidate. Recent literature examples include tumor, normal tissue, and plasma pharmacokinetics for N-[2-(dimethylamino) ethyl]acridine-4-carboxamide (DACA; 4) and its impli-

tered on lung, kidney, and prostate as the target organs.24 PET studies could distinguish tumor kinetics from normal tissue kinetics in a [11C]temozolomide (7; 3,4-di-

hydro-3-methyl-4-oxoimidazo(5,1-d)-as-tetrazine-8-carboxamide) study.25 A greater volume of distribution was demonstrated in glioma than in normal brain without appreciable differences in mean residence time. PET has been used to evaluate mu-opiate receptor blockage with [11C]carfentanil (8; [4(1-oxopropyl) phenylamino)-1cations for predicting activity and toxicity.22 Researchers evaluated the pharmacokinetics of trovafloxicin (5), a new

broad-spectrum fluoroquinolone antimicrobial agent, with emphasis on the range of concentrations that could be achieved in particular tissues.23 Their conclusion was that trovafloxicin could be useful in the treatment of a broad range of infections at diverse anatomical sites. Similar eval-

(2-phenylethyl)-4-piperidinecarboxylic acid, methyl ester]). This study was a case where the plasma half-life did not give an accurate indication of the duration of drug action at a specific receptor.26 The application of PET techniques can also suggest when something is of concern in a pharmacokinetic sense. Work with [11C]BMS-181101 demonstrated that this novel

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antidepressant label’s distribution was dominated by blood flow and that significant receptor-specific localization did not occur in any brain region.27 Specific binding to 5-HT3 receptors was not detectable in brain in vivo for [11C]MDL 72,222 (9).28 It is hypothesized that this is a

result of high lipophilicity and thus greater capacity for non-specific binding. An important step in the drug development process is the definition of the dose-response relationship and determination of the optimal dose. PET can be a cost effective tool in this activity as has been demonstrated in a number of literature papers.29 These types of investigations are oriented to clinical trial populations, but PET can also be useful on an individual basis. Quantitative PET modeling of 5-fluorouracil (5-FU; 10; an anticancer treatment) allows

tabolism for an 11C-labeled novel antitumor agent DACA.33 The variability in the disposition of risperidone (11; an

antipsychotic medication), is partly related to hepatic cytochrome P450-based hydroxylation to the active metabolite, 9-hydroxyrisperidone.34

Efflux Pumps The efficacy of brain tumor chemotherapy is poor because of the blood brain barrier and associated efflux mechanisms: P-glycoprotein (Pgp), organic anion transporters (OAT), and multidrug-resistance-associated proteins (MRP1 and 3). Efflux pumps more broadly often limit the efficacy of chemotherapy, and the evaluation of different resistance mechanisms is therefore critical.35 PET evaluation of efflux pumps is possible by comparing Pgp-mediated transport of radiolabelled substrates in the absence and presence of Pgp blockade. There is a report of the use of N-[11C]acetyl-leukotriene E4, a substrate for multidrug-resistance associated proteins (MRP), as a means of evaluating pump activity.36 PET provides functional activity that would not be available from mRNA or protein level measurements of efflux pumps. Researchers have used [11C]verapamil (12) with PET as

the adjustment of an individual’s dose and optimization of the treatment schedule.30

Absorption, Plasma Binding, and Metabolism PET is a valuable method for the understanding of intestinal absorption mechanism and for the simultaneous quantitation of absorption rate and distribution kinetics. Researchers demonstrated increases in absorption of drug candidates when amino acids were present in the intestinal lumin, suggesting that increased intestinal motility was initiated by the nutrient load.31 Plasma binding has been monitored with PET. [11C]L-703,717 has poor bloodbrain barrier permeability because of tight binding to plasma proteins. The researchers demonstrated that they could block plasma protein binding of 11C-labeled L-703,717.32 It is proposed that this may be a method to enhance the blood-brain barrier permeability of drugs with this plasma binding limitations. Labeling of potential drug candidates with positron-emitting radionuclides provides in vivo assessment of metabolism. Researchers demonstrated unexpected interspecies differences in me-

a measure of efflux pump activity.37 In vivo pharmacokinetic analysis of Pgp transport identifies the capacity of Pgp substrate modulators like cyclosporin. PET and radiolabeled Pgp substrates could be useful as a clinical tool to select patients that would benefit from the addition of a Pgp modulator to their drug regimen.38,39,40

Delivery: Liposome PET offers a unique capability to monitor drug delivery mechanisms. There have been a variety of reports evaluating FDG encapsulated within liposomes of different compositions. Liposomes composed of dipalmitoylphosphatidylcholine, cholesterol, and palmityl-D-glucuronide

PET and Drug Recovery / Klimas 315

(PGlcUA) were prepared in the presence of FDG and evaluated by PET.41 Various kinds of long-circulating liposomes (including ganglioside GM1-, polyethyleneglycol(PEG-), and glucuronide-modified liposomes) have been developed for passive targeting of liposomal drugs to tumors.42 The researchers investigated serum protein binding to positively charged liposomes as a factor in the accumulation of DMRIE-liposomes in liver. Liposomes containing DPP-CNDAC proved useful for evaluating metastatic pulmonary cancer.43 DPP-CNDAC/PGlcUA-liposomes containing FDG tended to accumulate in mouse tumor tissues to a greater extent than DPP-CNDAC/DPPG-liposomes.

being responsible for the psychotogenic effects of the noncompetitive N-methyl-D-aspar-tate antagonist ketamine (14).47 The PET study confirmed microdialysis studies in

Receptor Occupancy and Mapping

rodents and nonhuman primates that there are only small effects of acute NMDA receptor blockade on extracellular striatal dopamine concentrations. The relationship between robalzotan drug concentrations and 5-HT1A receptor occupancy was determined to be a hyperbolic function.48 This neuroimaging study suggests the value of receptor binding information as a guide to selecting the appropriate dose. The minimum effective dose of risperidone (6 mg/day) was established with measurements of dopamine D2 and serotonin 5HT2A receptor occupancy, minimizing the risk of extrapyramidal side effects as a result of unnecessarily high D2 receptor occupancy.49 The corollary has also been demonstrated in the literature, that dopamine D1 receptor occupancy was not significant at the doses used of SDZ MAR 327, a new neuroleptic agent with high in vitro affinity for D1 and D2 receptors.50 Researchers have evaluated the level of dopamine transporter occupancy required to produce the subjective high associated with regular cocaine (15) abuse. At intravenous co-

PET studies can provide quantitative estimates of receptor number and affinity, information that that can evaluate changes in receptor number, and affinity as a function of disease. The data also provide a measure of receptor occupancy by endogenous neurotransmitters and changes in receptor occupancy correlated to behavioral changes during drug treatment.44 The drug candidate can be radiolabeled directly and its anatomical distribution and binding evaluated by imaging. An indirect approach involves unlabeled drug as measured by competitive PET radioligand binding. The quantitative relationship between drug binding in vivo and pharmacodynamic effects in patients is used to validate biological targets and to optimize the clinical treatment.45

Receptor Binding and Occupancy There are a variety of publications that discuss the application of characterized PET radiolabels in the study of competitive binding pharmaceuticals of interest. The level of receptor occupancy is estimated by comparing time-activity measurements between the tracer alone and experiments involving the unlabelled compound. This surrogate marker aids the development of pharmacology models that link receptor occupancy to plasma concentrations and physiological impacts. The predictive performance of these models is sensitive to the precision of the receptor occupancy measure obtained by PET.46 The receptor occupancy studies are often driven by a hypothesis of a particular receptor affinity being responsible for an observed physiological or therapeutic observation. Researchers studied the D2 dopamine receptor activity (using [11C] raclopride; 13) as

caine doses commonly abused by humans (0.3–0.6 mg/kg), blockade of between 60% and 77% of dopamine transporter sites was observed.51

Receptor Visualization The studies described above centered on extensively characterized PET radiolabels with high affinity for serotonergic and dopaminergic receptors (and transporters). There are also a variety of research publications describing receptor selective high affinity radioligands for an expanded set of targets.52 S-Fluorocarazolol [(S)-FCZ; 16] is a PET ligand being studied to visualize central beta-adrenergic receptors in vivo.53 Efforts with RS-79948197 are working towards the goal of developing a central

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2 adrenergic receptor radioligand.54 [18F] SR144385 (17)

Pharmacological Characterization

and [18F]SR147963 (18) are being developed as mea-

PET’s strengths as a drug discovery tool extend beyond the characterization of pharmacokinetic parameters and receptor binding/occupancy studies. As a non-invasive molecular imaging technique, PET can also provide insight into the biological impact of particular drug candidates or classes of drug candidates. This type of in vivo feedback is fundamental to understanding how a particular set of compounds is acting on the biology as a whole, particularly in situations where behavioral measures are inexact or completely absent. The mechanism of action of many existing and potential drugs is often poorly understood. PET provides a means to investigate the science of drug action in vivo and complement molecular and cellular understanding of disease.

Serotonin Treatment of major depression with antidepressants is associated with a clinical response delay of several weeks. Researchers characterized the chronic treatment and clinical response of fluoxetine (20), a serotonin

sures of brain cannabinoid receptors.55 The glycine regulatory site on the NMDA receptor has been evaluated with the carbon-11 labeled analog of 4-Acetoxy L703,717, a prodrug pharmacological tool of L703,717.56 Anticholinergic drugs like biperiden (19) are used to treat extrapyramidal symptoms. Research into the muscarinic cholinergic receptor occupancy of biperiden was evaluated with [11C]N-methyl-4-piperidylbenzilate (NMPB).57 This is only a partial list of PET agents being developed to evaluate receptors and receptor number changes in vivo.

reuptake inhibitor, with a reciprocal pattern of subcorticol and limbic decreases and cortical increases in brain activity by FDG-PET.58 The work suggests a process of adaptation in specific brain regions to sustained serotonin reuptake inhibition, and pattern differences between responders and nonresponders suggest that the lack of an adaptive response is responsible for the treatment nonresponse. Pindolol (21), a 5HT1A receptor agent, might reduce the delay between initiation of selective serotonin reuptake in-

PET and Drug Recovery / Klimas 317

ferences in the methylphenidate (24) and cocaine in

hibitor treatment and antidepressant response. It is hypothesized that this effect is mediated by blockade of 5HT1A autoreceptors in the dorsal raphe nuclei. PET studies suggest that pindolol might be more potent at blocking these autoreceptors than at postsynaptic receptors, an important property underlying it biological impact.59 Other studies have also confirmed that the 5HT1A receptor may be a clinically significant target for pindolol.60 A high level of 5HT2A receptor blockade does not appear specific to clozapine (22), an atypical antipsychotic associated

with reduced extrapyramidal syndrome potential, relative to high doses of chlorpromazine (23), a typical anti–

psychotic.61 The conclusion of the study is that the distinct clinical profile of the drugs is not related to 5HT2A blockade in and of itself.

Dopamine Transporter/Reuptake The dopamine transporter has been extensively studied to better understand the nature and treatment of addiction. PET studies have been used to evaluate differences in the in vivo potency of drugs at the dopamine transporter as a component of their abuse liability. Dif-

vivo potency were not found to be responsible for their respective rates of abuse.62 It is hypothesized that other variables like methylphenidate’s side effect profile counterbalancing its reinforcing effects may be responsible for the differences. The time to peak brain uptake of oral methylphenidate corresponds to the peak behavioral effects.63 Cocaine analogs such as RTI-113 (3beta-(4chlorophenyl)tropane-2beta-carboxylic acid phenyl ester) have been studied by PET (using 8-(2-[18F]fluoroethyl)-2beta-carbomethoxy-3-beta-(4-chlorophenyl) nortropane (FECNT; 25) relative to their reinforcing effects. The

results suggest that the long-acting pharmacokinetic profile of RTI-113 may influence it abuse liability.64 Amphetamine was observed to increase the rate constant for L-[beta-11C]DOPA utilization in the brain, potentially as a result of presynaptic dopamine receptor subsensitivity.65 PET has been used to evaluate GBR12909 (26), a long-acting noncompetitive dopamine transporter

agent, as a treatment option for cocaine addiction.66 The results suggest that occupancy of the dopamine transporter is responsible for GBR12909’s ability to attenuate cocaine-induced increases in extracellular dopamine and to suppress cocaine self-administration.

Dopamine Drug candidates that alter dopamine neurotransmission are important to Parkinsonian syndromes, schizophrenia,

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and attention deficit disorders. FDOPA-PET studies have evaluated the potentiation of DOPA decarboxylation after pharmacological blockade of dopamine D2 receptors in Parkinson’s disease.67 The acute administration of L-DOPA (L-3,4-dihydroxyphenylalanine) increased the binding potential of [11C]raclopride suggesting that L-DOPA plays a more complex role than strictly a dopamine precursor in the nigrostriatal pathway.68 A report has demonstrated altered opioid transmission as part of the pathophysiology of L-DOPA induced dyskinesias and highlighted the potential to manage involuntary movements with opioid agents.69 The enzyme catechol-O-methyltransferase (COMT) plays an important role in the extraneural inactivation of catecholamine neurotransmitters. Researchers evaluated a selective and reversible inhibitor of COMT, tolcapone (27), by FDOPA-

PET in Parkinson’s disease.70 The work confirmed that talcapone has a significant blocking effect on peripheral and central COMT. The classic example of pharmacological understanding of drug therapy is in the area of dopaminergic antagonism and schizophrenia treatments. Clinical doses of all currently used antipsychotic medications cause substantial blockade of central dopamine (D2) receptors.71 The development of atypical antipsychotics, those with reduced extrapyramidal side effect liability, was initiated with investigations of clozapine. There are a wide variety of PET drug discovery reports that compare drug candidate profiles to that of clozapine. This includes an evaluation of the dose-response characteristics of olanzepine (28) and work to understand atypical antipsy-

chotic mechanisms of action.72 Que-tiapine (29) demonstrates a transiently high dopamine D2 receptor occupancy that decreases to low levels by the end of the dosing interval. The authors suggest that transient D2 occupancy is responsible for the therapeutic effect and

that the low D2 occupancy explains quetiapine’s freedom from extrapyramidal symptoms and prolactin level elevation.73 The mechanism of action of well-established antipsychotics continues to be studied by PET. It is proposed that haloperidol (30) exerts its primary antidopam-

inergic action in the basal ganglia.74 Additional changes in the thalamus and cortex are secondary to the basal ganglia-based changes. L-DOPA exhibits mood effects. Research investigated changes in rCBF produced by acute doses of dopamine D1 full agonist SKF82958 (31).75 The results suggest

that amygdala activity (and associated mood effects) is potentially mediated by dopamine D1 receptor activity. PET studies have evaluated the mechanistic framework for methylphenidate, a drug therapy for the treatment of attention deficit hyperactivity disorder in children. Using [11C]raclopride and oral methylphenidate the researchers conclude that methylphenidate amplifies weak dopaminergic signals to enhance task specific signaling, improving attention, and decreasing distractability.76 An alternative hypothesis is that methylphenidate enhances the motivational/reward features of the task thus improving performance.

Gamma-Aminobutyric Acid/Benzodiazepines The frequency and severity of seizures can be reduced with medications that increase gamma-Aminobutyric

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Acid (GABA). This includes vigabatrin (32), an irrevers-

receptor interaction theory. Intrinsic efficacy of a particular pharmaceutical could be extracted from this dataset. Benzodiazepines are associated with cognitive effects. PET studies with midazolam (35) demonstrated

ible inhibitor of GABA-transaminase. A [11C]flumazenil (FMZ; 33)-PET study demonstrated that vigabatrin in-

duces a decrease in GABA(A) receptor binding in the cortex and cerebellum of the developing epileptic brain.77 The authors highlight the importance of the GABAergic system in developmental plasticity and the reversibility and functional consequences of age-specific drug effects. PET studies with the molecular imaging probe [11C]flumazenil combined with electroencephalography techniques were applied to the evaluation of the anti/proconvulsant activity and potency of the benzodiazepine agonist diazepam (34)

and the inverse agonist methyl-beta-carboline-3-carboxylate (beta-CCM) in baboons.78 A given level of beta-CCM convulsant activity was related to different benzodiazepine receptor occupancy.79 The paper concludes that a different pharmacological potency when occupying the same number of receptors could depend on the pathophysiological state of the subject. PET was used to estimate fractional benzodiazepine occupancy by measuring the displacement of [11C]flumazenil across full agonists, partial agonists, antagonists, partial inverse agonists, and full inverse agonists.80 The work found that fractional receptor occupancy by a given drug was well correlated with its resulting graded pharmacological effects as predicted from competitive drug

changes in brain regions associated with specific cognitive operations.81 PET has been applied to the evaluation of diazepam’s impact on performance of executive compared to episodic memory tasks.82 Neuroanatomical dissociation reflects the distinct functional effects of diazepam on encoding versus ordering tasks, and the effect on ordering tasks is not simply secondary to effects on episodic memory. Triazolam (36) administration was

associated with deactiviation during encoding of brain regions (anterior cingulate cortex, cerebellum, and precuneus) that were previously associated with encoding in the absence of drug.83 It has been hypothesized that the anesthetic isoflurane (37) has its mechanism of action by inducing conforma-

tional changes in the GABA-A receptor. This hypothesis was studied with the [11C]flumazenil to quantitatively evaluate physiological effects of isoflurane.84 Propofol (38), another anesthetic, interferes with the processing of vibrotactile information at the level of the cortex before attenuating its transfer through the thalamus.85 By measuring rCBF changes by PET, it was determined that pro-

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produced measurable changes in plasma, tumor, and liver pharmacokinetics of fluorouracil as measured by probe quantities of 5-[18F]fluorouracil.92 The work highlights the biomodulation of 5-fluorouracil by other chemotherapeutic agents. This is important because the combination of interferon-alpha and 5-fluorouracil may be an alternative for patients with inoperable meningiomas.93 pofol preferentially decreased rCBF in brain regions associated with the regulation of arousal, the performance of associative functions, and autonomic control.86 The data suggests that anesthetics have a preferential and concentration dependent effect on specific neuronal networks rather than a more diffuse, nonspecific, and generalized effect on the brain.

Other Pharmacologies There are a variety of PET studies that do not cluster into large bodies of work but that are representative of the opportunity to understand pharmacological mechanism of action. Noradrenergic drugs such a clonidine (41) mediate the functional integration of attentional

Oncology Markers Although the literature is weighted to CNS applications of PET in understanding the pharmacology of drug candidates, the opportunity in oncology is significant. Researchers have evaluated the treatment efficacy of interferon-alpha on meningioma patients with postoperative residual masses and recurrent or primarily inoperable tumors.87 This particular work used [11C]-L-methionine (39) to estimate the relative methionine accumulation in the tumor as a measure of proliferative activity. PET scans for tumor hypoxia ([ 18F]misonidazole; 40) was used

to evaluate the combination of tirapazamine, cisplatin, and radiation therapy in patients with advanced head and neck cancer.88 5-[18F]Fluorouracil has been used in PET evaluations of the mechanism of action of eniluracil, particularly with respect to dihydropyrimidine dehydrogenase effects on tumor versus normal tissue pharmacokinetics.89 Repeated FDG scans revealed reductions in tumor volume and decreases in the standard uptake values of FDG on treatment with octreotide.90 It was concluded that octreotide could alter the biological activity of metastatic thyroid cancer lesions that exhibit somatostatin receptors. Many tumors are resistant to therapy with thymidylate synthase inhibitors because of the high levels of the enzyme. Thymidylate synthase activity can be non-invasively evaluated with [18F]2-F-azadeoxyuridine (FAU), and this phenotypic information can improve patient therapy choices.91 Administration of N-phosphonacetyl-L-aspartate and interferon-alpha

brain systems. PET studies have highlighted that the context sensitive nature of changes is consistent with differential effects on brain processes depending on the patients underlying arousal level.94 This study illustrates the dynamic plasticity of cognitive brain systems following neurochemical challenge. Early after the initiation of tacrine (42) treatments, an inhibitor of ace-

tylcholinesterase, there are measurable changes in nicotinic receptors (measured with [11C]nicotine; 43),

cerebral blood flow and some cognitive tests (trail making test, block design test).95 These changes precede the impacts on glucose metabolism that take several months to develop.

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Trimetazidine (44), an anti-ischemic agent for the

treatment of angina pectoris, was shown to increase total glucose utilization (oxidative and glycolytic) in myocardium without a preferential increase in ischemic tissue.96 This study showed that this effect was not mediated by changes in hemodynamic parameters but predominantly by increases in glycolysis. Cardiac uptake of tamoxifen suggests that its cardioprotective benefits may result from a combination of serum cholesterol reduction and a direct cardioprotective action.97 Studying angiotensin-converting enzyme (ACE) inhibition with [18F]fluorocaptopril (45), researchers con-

ine oxidase type B (MAO-B), demonstrates that selegiline is retained in brain regions high in MAO-B activity.101 Using [11C]clorgyline (47) and [11C]L-deprenyl (48) to measure

MAO A and B respectively, it was demonstrated the Ginkgo biloba did not produce significant changes in brain MAO suggesting an alternative mechanism for Ginkgo biloba’s physiological activity.102

PET Ligand Development

cluded that ACE inhibition might be useful as a treatment of primary pulmonary hypertension.98 Researchers have investigated the inhibitory potential of synthetic sialyl Lewis X selectin ligands and fibronectin-derived RGDS peptide analogs on lung metastases. This study was done with FDG-labeled tumor cells injected in vivo into an animal.99 Cholecystokinin tetrapeptide (CCK-4) elicits a marked anxiogenic response as measured by subjective anxiety ratings and heart rate measures. This behavioral response was evaluated experimentally by PET using a [15O]water bolus subtraction method to determine rCBF changes.100 CCK-4 induced anxiety was associated with robust and bilateral increase in extracerebral blood flow in the vicinity of the superficial temporal artery and increases in the anterior cingulated gyrus, claustrum-insular-amygdala region and the cerebellar vermis. PET studies with selegiline (46), an irreversible and selective inhibitor of monoam-

Probe development studies are numerous with specific references throughout this text and associated references. Although dopaminergic and serotonergic neurotransmission has been a dominant focus of selective tracer synthesis, storage, re-uptake, and post-synaptic binding;103 this type of tracer portfolio is developing for a range of signaling systems. Transporter proteins are particularly good tracer targets. Monoamine transporters are a major mechanism of terminating the action of released neurotransmitters and are good markers for the integrity of monaminergic innervation.104 A good example of this type of research investment is in the development of cocaine analogs as measures of dopamine transporter activity for the evaluation of neuropsychiatric disorders and biological changes associated with drug abuse.105 The study of brain penetrant radiolabels of human 5HT1A receptors includes (pyridinyl-6-halo)WAY analogs as a specific example of PET imaging agent development.106 5-Methyl-17-[18F]fluoroheptadecanoic acid (5-MFHA) has been proposed as a new myocardial imaging tracer, evaluating the position of methyl branching in the fatty acid with accumulation, metabolic, and imaging studies in heart muscle.107 Although much of the PET literature centers on low molecular weight PET ligands, there are a variety of studies evaluating immuno-PET to evaluate the targeting of monoclonal antibodies against tumor associated antigens and the development of diagnostic imaging capabilities.108 IgG has limitations as a tumor imaging agent, and studies are evaluating radiolabeled Fv fragments as alternatives.

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Biological Characterization The previous section emphasized studies that targeted an understanding of the mechanism of action or biological activity of drug candidates and pharmacological tools. The corollary is also a key contribution of PET, namely to assume a pharmacological profile for a particular PET ligand and attempt to better understand a disease state in that context.

Anxiety, Depression, Obsessive Compulsive Disorder The serotonergic 5HT1A receptor is central as a treatment target for anxiety, depression, and schizophrenia. Serotonin is predominantly located in the limbic forebrain and is involved in the modulation of emotion.109 Researchers suggest that the rostral cingulated area 24a/b in the limbic-cortical network plays a key role in abnormal mood states.110 Cingulate hypermetabolism is hypothesized to be an important adaptive response to depression, and the failure of this response may suggest a poor outcome. PET studies have implicated the orbito-frontal cortex and the head of the caudate nucleus in the mediation of obsessive-compulsive disorder (OCD) symptoms.111,112 It is hypothesized that desensitization of the terminal serotonin autoreceptor is responsible for the efficacy of serotonin reuptake inhibitors in OCD. This is a result of enhanced release of serotonin induced by prolonged and significant reuptake inhibition in the orbito-frontal cortex. PET evaluation of local cerebral metabolic rates for glucose suggested that percentage changes in OCD symptom ratings correlated significantly with right Cd/hem changes with fluoxetine and behavior therapy.113

Addiction and Withdrawal The neurochemical changes associated with drugs of abuse are poorly understood. With the development of labeled drugs and PET molecular imaging probes, it is becoming possible to evaluate the pharmacokinetic, pharmacodynamic (behavior), and toxic properties of these addictive substances and fundamentally understand the mechanisms of addiction.114 Self-reports of feeling high (a subjective metric) are correlated with rCBF activation in the hippocampus, a brain region important to the acquisition of stimulus-associated reinforcement.115 Studies with methamphetamine and cocaine suggest that dopamine D2 receptor mediated dysregulation of the orbitofrontal cortex could underlie a common mechanism for loss of control and compulsive drug intake.116 There is a quantitative relationship between levels of dopamine D2 receptor occupancy and the intensity of the high.117 Regional brain activation PET studies suggest that craving derives from the activation of the temporal insula, a brain region involved in auto-

nomic control, and the orbitofrontal cortex, a brain region involved with expectancy and reinforcing salience of stimuli.118 Difficulties in defining craving and drug use urges have resulted in neural mechanism of craving study. The study uses [15O]water-PET and imagerybased procedures.119 PET studies also have revealed abnormalities in dopaminergic and opioid system markers as a result of cocaine withdrawal.120 These studies have demonstrated a connection between the response to drug-related stimuli and the neural elements of cognition and emotion. FDOPA uptake studies suggest that during abstinence from cocaine, there is a delayed decrease in dopamine terminal activity in the striatum.121 Selective changes in glucose metabolism in the basal ganglia and orbitofrontal cortex are seen in cocaine abusers undergoing detoxification.122 This activity is consistent with proposed changes in brain dopamine activity.

Schizophrenia The understanding of schizophrenia continues to advance as a result of PET, the availability of specific molecular imaging probes and the development of a range of therapeutic options. One hypothesis has the mechanism of neuroleptic drugs leading to compensation and adaptation rather than normalization of functional activities in the brains of schizophrenics.123 The authors suggest that efficacy may result from a shift in balance of cortical to limbic cortex activity. Alterations of dopamine D2 receptor function in the extrastriatal region may underlie the positive symptoms of schizophrenia.124 The co-occurrence of high and low dopamine activity in schizophrenia potentially explains the concurrent presence of negative and positive symptoms.125 This fact has implications for the conceptualization of dopamine’s role in schizophrenia and for treatment options. PET studies demonstrate that the seven positive symptoms on the symptom scale show different correlations with regional cerebral blood flow (rCBF).126 Formal thought disorders and grandiosity correlated positively and strongly with bifrontal and bitemporal rCBF; delusions, hallucinations, and distrust correlate negatively with cingulated, left thalamic, left frontal, and left temporal rCBF. PET was used to compare metabolic patterns in schizophrenics with predominantly negative symptoms (alogia, affective flattening, avolition, and attentional impairment) and predominantly positive symptoms.127 Subjects with predominantly negative symptoms had lower glucose metabolic rates in the right hemisphere, particularly in the temporal and vental prefrontal cortices, compared to healthy controls and patients exhibiting predominantly positive symptoms. Negative symptom subscale scores were negatively correlated with glucose metabolic rates in many brain areas. Additional investigations have demonstrated a correlation between re-

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ductions in dopamine D1 receptor binding in the prefrontal cortex and the severity of negative symptoms.128 It is thought that this dysfunction of D1 receptor signaling may contribute to the negative symptoms and cognitive deficits associated with schizophrenia. The predisposition of neuroleptic treated patients to develop tardive dyskinesias was evaluated with FDG.129 The authors suggest that differences in the metabolic activity of specific brain regions are associated with vulnerability to tardive dyskinesia development. In addition to the dopamine-centric evaluation of schizophrenia, PET has been used to evaluate diminished glutaminergic neurotransmission.130 Diminished glutaminergic signaling in the hippocampal glutamatemediated efferent pathway and cerebral dysfunction in the hippocampus and its target areas, particularly the anterior cingulated cortex, may underlie some of the behavioral features of schizophrenia.

Parkinson’s Disease, Tremors, and Movement Disorders Molecular brain imaging has contributed to the understanding of the pathophysiology of movement disorders, the functional reorganization following brain lesions, and the drug induced changes of movement-related brain activity.131 Studies with [11C]CFT (49; dopamine

transporter probe) and [11C]SCH 23,390 (50; D1 re-

ceptor probe) have evaluated presynaptic and postsynaptic dopaminergic functions in both the nigrostriatal and mesocortical systems in Parkinson’s disease.132 The authors found that dopamine transporters and D1 receptors changed in parallel in the normal striatal synapses, but the association becomes asymmetrical in Parkinson’s disease because of reductions in presynaptic and relative elevations in postsynaptic markers. Alterations in synaptic regulation in the nigrostriatal

system may reflect early pathophysiological changes. Research has also investigated the functional compensation that occurs in presynaptic dopamine nerve terminals in Parkinson’s disease by [11C]dihydrotetrabenazine ([11C]DTBZ; 51), [11C]methylphenidate (labeling the

plasma membrane DA transporter), and FDOPA(reflecting synthesis and storage of dopamine) PET.133 [11C]dihydrotetrabenazine labels vesicular monoamine transporter type 2 (VMAT2). Previous studies have shown that VMAT2 density is linearly related to the integrity of substantia nigra dopamine neurons.134 The work demonstrated that aromatic L-amino acid decarboxylase is upregulated and plasma membrane dopamine transporter is downregulated in the striatum of Parkinson’s disease patients. Using the ability of endogenous dopamine to compete for [11C]raclopride, PET studies have provide evidence of a substantial release of endogenous dopamine in the striatum of Parkinson’s disease patients in response to placebo.135 It has been postulated that dyskinesias are associated with a dysregulation of basal ganglia opioid transmission resulting in an overactivity of the basal ganglia-frontal projections. This conclusion derives from the study of dyskinetic and nondyskinetic Parkinson’s disease patients with [11C]SCH23390, [11C]raclopride, and [11C]diprenorphine (52) PET.136 There are also efforts to provide insight

into the mechanisms underlying specific forms of tremor.137 Seizure and drug-related destruction of Purkinje cells have been studied by labeling benzodiazepine receptors with [11C]flumazenil.138 This work demonstrated the functional relationship between seizures and exci-

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totoxic lesions for specific changes in the GABA-benzodiazepine system.

Cognition and Neurodegenerative Disease PET studies of cognitive activation under conditions of pharmacological challenge have been used to demonstrate neuromodulation.139 This type of work includes the rCBF characterization of rapid visual informationprocessing tasks, tests of sustained attention that require working memory.140 A significant correlation is observed between impairment of nicotinic receptors in the temporal cortex and the cognitive impairment of Alzheimer’s disease patients. There is substantial interest in the imaging of nicotinic cholinergic receptors to evaluate receptor impairments even at presymptomatic Alzheimer’s disease stages. Efforts in this area include the use of [11C]nicotine and the alpha-4 beta-2 subtype selective compound A-8538 (53).141 Receptors can be

myocardial ischemia, congestive heart failure, and hypertension.147 Across many tumor types a strong relationship exists between thymidine phosphorylase expression in tumor biopsies and the clinical outcome. This expression is associated with angiogenesis although the mechanism is not well understood. Imaging and therapy determinations directed to thymidine phosphorylase have been undertaken with 5-X-Ura compounds.148

Pain

used as markers for specific neurons (or other cells) that are affected by disease progression. In Alzheimer’s disease neuronal degeneration and inflammation result in decreases in neural receptors and increases in omega-3 receptors respectively.142 Researchers have used rCBF PET measurements to characterize differences between the resting state and performance of tasks under the influence of clonidine.143 They were able to demonstrate the neuroanatomical dissociation of the noradrenergic modulation of arousal from attention-based activity. The effects of aging on drug responsiveness and the effects of drug treatment on disease associated with old age are relatively unexplored areas of functional imaging.144

Cardiology and Oncology Changes in the number and affinity of cardiac neurotransmitter receptors have been associated with myocardial ischemia, congestive heart failure, myocardial infarction, and cardiomyopathies.145 Myocardium sympathetic terminal vesicles have been labeled with 6-[18F] fluorodopamine (54) and 6-[18F]fluoronorepinephrine (55) as a means of assessing sympathetic innervation and function non-invasively.146 Studies with [11C](S)CGP 12,177 and [11C]GB67 have been initiated to evaluate changes in cardiac beta and alpha adrenoreceptors respectively as metrics of various diseases including

PET studies have resulted in the generation of physiological and neuropathic pain related brain activity maps.149 Pain-related activations have been observed in the thalamus, insula/SII, anterior cingulated and posterior parietal cortices. Activity in the right pre-frontal and posterior parietal cortices, anterior cingulated and thalami can be modulated by attention and likely subserve attentional processes rather than pain discrimination. Responses in the insula/SII cortex subserve discriminative aspects of pain perception. Drug or stimulationinduced analgesia are associated with normalization of basal thalamic abnormalities associated with chronic pain. Headache has been examined by 2-deoxy-D-[1-11C] glucose regional cerebral metabolic metrics before and after administration of reserpine.150

Disease Characterization The discussion to this point has emphasized PET’s potential to characterize the pharmacokinetic/pharmacodynamic features of potential drug candidates, receptor occupancy as an important determinant of efficacy, the pharmacological characterization of potential mechanisms of action, and the biological characterization of disease with well-characterized PET ligands. The last concept can be extended to help characterize disease in a clinical context. This includes the development of markers or combinations of markers that aid in the diagnosis of a disease state alongside behavioral and/or biological correlates. PET can also be usefully applied to the staging (severity and early disease characterization), fractionation,

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and progression of disease. The goal is to provide differential diagnosis information and to impact the choice of therapeutic options. This information aids drug development by characterizing the disease, its time course, and the effect of therapy in a rigorous and quantitative manner. Behavioral measures in particular can be difficult to evaluate and are confounded by the experimental conditions. In a clinical setting, PET measures can significantly reduce the number of patients needed for statistical significance, improve the patient segmentation, lessen the intervals required for longitudinal evaluation, and increase the quantitative metrics available to characterize a drug candidate’s efficacy. All these factors reduce the cost of clinical evaluation and improve patient care. As the portfolio of selective ligands and the biological understanding of these ligands grow, the impact of PET on clinical drug development will continue to expand.

Disease Characterization PET evaluation of pathophysiology (topography and severity of synaptic damage for example) can characterize disease at early time points when structural imaging is of less value.151 Profiles of abnormalities can be characteristic of each disorder even though individual components are not specific to each disorder. PET and an understanding of the associated pharmacology provide surrogate markers to objectively evaluate the effect of the drug candidate being tested. FDOPA-PET is a sensitive and useful technique for detecting changes in dopamine metabolism in schizophrenic patients.152 Primary negative symptoms are difficult to characterize in schizophrenia. Using [15O]water researchers have demonstrated low activation in the middle frontal cortex and inferior parietal cortex in deficit schizophrenia.153 This deficit was observed without a performance confound and may be a marker of primary negative symptomology. In bipolar disorder, PET revealed a moderate bilateral frontal hypermetabolism in the hypomanic phase and yielded normal findings for the depressive phase.154 FDG-PET and different stereo-electroencephalographic patterns were used to characterize the epileptogenic, irritative, and lesional zones associated with epileptic foci.155 A focal dystonia hypothesis suggests a dysfunction of a subcortical-cortical motor network playing a prominent role in the pathogenesis. PET studies correctly assigned patients to a clinical category by a discriminant function analysis (total accuracy 96%) based on hypermetabolism in the basal ganglia, thalamus, premotor-motor cortex, and cerebellum.156 Serotonergic system abnormalities have been implicated in depression, suicide, violence, alcoholism, and other psychopathologies. Researchers have combined the neuroendocrine (prolactin) and FDG responses to fenfluramine as measures of serotonin responsivity.157 Obsessive-compulsive disorder showed cerebral

glucose metabolic patterns that differ from controls in both the symptomatic and recovered states.158 PET provides a means to evaluate the response and prognosis of tumors prior to the administration of standard chemotherapy by measuring the distribution of radiolabeled cytostatic agents 5-[18F]fluorouracil in tumor regions.159 Patients with high uptake values were more likely to achieve disease stabilization with planned chemotherapy and demonstrate prolonged survival.

Behavior Correlate Memory tests are important behavioral measures of a variety of neurodegenerative diseases. Testing procedures can tease out specific aspects of the mnemonic process including discrimination, encoding and retention. PET activity in specific brain regions were compared with delayed matching-to-position, delayed matching-to-sample and titrating matching-to-sample procedures.160 The combination of delayed matching-to-sample and PET suggests the particular brain structures involved in mnemonic processes. PET studies have demonstrated that particular neural activity is associated with symptom provocation in specific phobics, subjects with posttraumatic stress disorder and healthy individuals experiencing panic.161 Brain glucose metabolism combined with cerebralspinal fluid (CSF) levels of 5-hydroxyindoleacetic acid (5-HIAA; 56) showed that increased aggressiveness was associ-

ated with low concentrations of CSF 5-HIAA and higher brain glucose metabolism.162 In characterizing bipolar and unipolar depression with FDG, it was observed that prefrontal hypometabolism might be part of a common neural substrate for depression independent of etiology.163 Other studies have reported increased rCBF in the amygdala in familial pure depressive disease. Using [11C]glucose-PET, researchers reported that the glutamic acid pool was reduced in cortical areas of the brain in patients with major depression.164 PET studies have shown that certain aspects of mania-like syndrome correlated positively with the metabolic changes seen in the frontal cortex, caudate nucleus, and putamen.165 In schizophrenic patients, left striatal FDOPA uptake values correlated negatively with depressive symptoms in a significant manner.166 Paranoid symptomology correlated positively with right putamen FDOPA uptake suggesting a connection between paranoia and subcorticol hyperdopaminergia.

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In addiction studies research has shown that the rate at which cocaine and methylphenidate enter the brain and block the dopamine transporter is the variable associated with the subjective high.167 The paper suggests that the level of dopamine transporter blockage predicts the intensity of the high and the rate of transporter blockade determines whether the high is perceived or not.

measures could play an important and complementary role to FDG in the evaluation of tumor oxygenation and viability. For breast cancer patients, the detection of axillary lymph node involvement is a critical issue and is indicative of distant microdiffusion. FDG-PET is a diagnostic approach to evaluating axillary lymph node involvement and leads into adjuvant chemotherapy and surgical decisions.179

Fractionation, Staging, Severity, and Progression PET allows the in vivo determination of striatal dopaminergic activity. This information provides insight to the pathophysiology of Parkinson’s disease, permits the evaluation of disease progression at a biochemical level, and provides a means to monitor potential neuroprotective interventions.168 Measurements of dopamine transporter binding can provide accurate and sensitive measures of dopamine system degeneration.169 Although there is a significant correlation between PET-based striatal radioligand binding and the degree of Parkinsonian symptoms such as rigidity and bradykinesia, the pathophysiology, and evaluation of resting tremor remains unclear.170 Parkinson’s disease patients have been evaluated by L-[11C]DOPA in a drug-free state and after a therapeutic levodopa infusion.171 The research found that there was a marked shift in the modulatory action of levodopa with the advancement of Parkinson’s disease. The findings are consistent with a less graded clinical response to levodopa in advanced Parkinson’s disease, and this data is potentially important to the understanding of motor fluctuation pathogenesis. FDOPA could be used to detect preclinical disease in clinically unaffected twins and relatives of Parkinson’s disease patients, because the FDOPA uptake is reduced by at least 35% before onset of symptoms.172 PET has demonstrated a negative correlation between regional metabolic rates and autistic or negative symptoms.173 The lower the metabolic rate, the more autistic the patient. This work did not see a correlation between metabolic rates and atrophic brain changes. In Huntington’s disease basal ganglia metabolism is highly correlated with the overall functional capacity of individual patients and with the degree of motor abnormalities.174 Evaluating myocardial viability with [13N]ammonia and FDG, physicians were able to determine evidence of jeopardized (ischemic or viable) myocardium that may benefit from revascularization.175 Evaluating regional and averaged myocardial blood flow at rest and after dipyridamole-induced vasodilation can extend this type of evaluation.176 A significant opportunity in the cardiology field is the ability to evaluate the severity of diffuse cardiovascular disease as a graded and continuous disease process.177 PET studies allow the functional evaluation of tumor tissues, distinguishing residual disease from non-viable or necrotic tumor masses.178 Nitroimidazole hypoxia

Differential Diagnosis PET contributes to the differential diagnosis of several disease states. This information more accurately defines the patient population in a clinical trial environment, as well as improving treatment choices. None of the cardinal signs of Parkinson’s disease (tremor, rigidity, bradykinesia, and asymmetric presentation) is entirely specific to Parkinson’s disease or allows a definite diagnosis.180 The most frequent challenges in diagnosing Parkinson’s disease are the conditions of essential tremor and multiple system atrophy. Initial monotherapy with a dopamine agonist is advised when functional disability appears, a patient specific threshold. Early monotherapy delays the appearance and reduces the intensity of late motor complications with subsequent levodopa treatment. PET changes in combination with presymptomatic markers of Parkinson’s disease (odor detection, handwriting, speech, personality traits, dopaminergic antibodies, and mitochondrial DNA mutation profiles) are important to investigations of neuroprotective therapies.181 [11C]- and [18F]-labeled butyrophenones and benzamides provide a differential diagnosis of Parkinson’s disease based on dopamine D2 receptors.182 Patients with early Parkinson’s disease show an increase in D2 receptor binding compared to control subjects. In contrast, patients with multiple systems atrophy show a decrease D2 receptor binding and are generally non-responsive to treatment. Extending the D2 receptor binding studies, depressive disorder patients showing a low D2 binding are likely to improve following antidepressive therapies whereas sleep deprivation is promising in patients with high D2 binding. PET evaluation can impact cancer treatments (radiation, surgical, and pharmacological). There are a variety of agents that measure tumor hypoxia including 62 Cu-labeled diacetyl-bis(N4-methylthiosemicarbazone) ([62Cu]ATSM), [18F]misonidazole and other nitroimidazoles. Uptake is based on cytosolic/microsomal bioreduction and is unrelated to uptake of FDG.183 The non-invasive differentiation of benign from malignant disease is a key diagnostic challenge. Studies with labeled antibodies are demonstrating utility for detecting colorectal cancer in patients with suspected recurrence and in those at high risk of recurrence.184 The early diagnosis of multiple drug resistance (MDR) is important to the efficacy of chemotherapeutic intervention and

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patient quality of life. PET studies with the MDR tracer, [11C]colchicine (57) have been used to evaluate the in-

of depressive disorders) and normalized ventrolateral prefrontal cortical activity have been demonstrated in patient response to serotonin reuptake inhibitors.194

Response Differentiation: Side Effect Profile and Treatment Choices Therapeutic side effects are a function of dose, ideally with sufficient therapeutic window that the side effects do not limit the application of a drug candidate. PET’s application to understanding therapeutic dose and mechanism of action can be extended to side effect mechanisms and dose-dependent behavior as well. direct effects of Pgp action on colchicines-to-tubulin binding and colchicine’s uptake into tumors.185

Therapeutic Response The evaluation of cancer states and changes in tumors as a result of pharmacological intervention are crude and often applied to heavily pretreated disease.186 PET imaging has the sensitivity to evaluate small but important changes and differential responses across classes of tumor types. The use of hypoxia agents such as misonidazole ([18F]FMISO) begins to differentiate tumors and provide a predictor of tumor response to conventional radiotherapy.187 There are studies suggesting that PET offers a unique capability for predicting and defining pituitary adenoma growth, particularly since there is no correlation between hormone release and the adenoma metabolic index.188 FDG studies of gastroesophageal preoperative chemotherapy may differentiate responding and nonresponding tumors early in the course of therapy.189 Avoiding ineffective and potentially harmful therapy will significantly improve patient care. The metabolic response to haloperidol drug challenge separates treatment responding from nonresponding schizophrenic patients and normal subjects.190 The study results suggest that subtyping of psychiatric disorders can be achieved by measuring the physiological response to an acute pharmacological challenge in vivo. Pharmacological challenge paradigms allow the study of functional interactions of different neurotransmitter systems.191 Abnormally low right anterior midprefrontal cortex metabolic rates predict better clinical response to neuroleptics while high basal ganglia rates and low mid-cingulate rates predict poor treatment response.192 The research suggests that the sustained attention pathway is playing an important role in the expression of psychotic symptoms and the modulation of symptoms by anti-psychotic therapy. Among schizophrenia responders to haloperidol, the treatment had a normalizing effect on metabolic activity in the striatum.193 Nonresponders were more likely to demonstrate a worsening of hypofrontality and no changes in the striatum after receiving medication. Positive correlations between HAM-D scores (measure

Side Effect Profile, Toxicity One of the best-studied toxicologies is associated with high levels of dopamine D2 receptor occupancy in antipsychotic medications. PET studies have demonstrated that at least 65% D2 receptor occupancy is required for clinical response and that occupancy rates exceeding 72–78% are associated with elevated prolactin levels and adverse motor effects.195 The lack of prolactin elevation associated with the atypical antipsychotic clozapine (as measured by [11C]raclopride) derives from the low dopamine D2 receptor occupancy and not effects at other receptors.196 D2 receptor upregulation is postulated to be central to tardive dyskinesias. D2 receptor binding is increased on long-term treatment with antipsychotics, and both traditional and new antipsychotics with high D2 receptor affinity are associated with this increase in D2 receptor number.197 Catalepsy and the selective suppression of conditioned avoidance response are animal model measurements correlated with extrapyramidal symptoms and antipsychotic efficacy respectively. PET determinations of the dopamine D2 receptor blockade associated with catalepsy and conditioned avoidance have evaluated the receptor occupancy levels associated with these behaviors.198 Drugs, including some tricyclic antidepressants, interact with GLUT1 and potentially other glucose transport or glucose binding proteins.199 It is postulated that these activities may contribute to tricyclic antidepressant toxicity. Sumatriptan (58), a

5-HT1D receptor agonist, is thought to treat migraine attacks by extracerebral vasoconstriction. A side effect

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of this mechanism of action is myocardial ischemia. Researchers have studied the effect of subcutaneous sumatriptan on myocardial perfusion by [13N]ammonia-PET.200

Drug Comparisons PET has been used to make direct pharmacodynamic comparisons between drug candidates. Work evaluated the MAO-A inhibition activity of esuprone (59) and moclobemide (60) on human volunteers.201 The effect of

valproic acid on global CMRGlc has been compared to carbamazepine (61), phenytoin (62), phenobarbital (63), diazepam, and combinations of valproic acid and carbamazepine.202 Valproic acid depresses cerebral metabolism to a greater degree than carbamazepine and phenytoin but less than phenobarbital. It is thought that this metabolic effect may be related to the mechanism of action.203 In the treatment of attention deficit hyperactivity disorder, a variety of stimulants produced different patterns of regional glucose increases and decreases.204 The authors discuss the interpretation of their results comparing glucose utilization changes in specific brain regions and as a property of the whole brain.

Receptor Polymorphism Polymorphisms in genes coding for neurotransmitter receptors and transporters have been associated with neuropsychiatric conditions, although it has been difficult to replicate some of these studies. PET is capable of evaluating these targets for pharmacological intervention and clarifies the functional significance of the polymorphisms.205 Brain imaging studies have evaluated the significance of polymorphisms in genes encoding the serotonin transporter, dopamine transporter, and dopamine D2 receptor in terms of expression and function. As a quantitative imaging modality, PET is capable of

evaluating biochemical and biological processes at high resolution. This approach has been used to evaluate the metabolic consequences of gene expression or gene defects, bridging molecular biology and an understanding of pathology.206 Studies of amino acid levels in children with epileptic encephalopathies (and their parents) evaluated patterns of cortical glucose metabolism by FDG-PET.207 The research developed a hypothesis around an aspartate neurotransmitter genetic abnormality and the pathogenesis of epileptic encephalopathy seizures. Taking into account the polymorphism of the D2 dopamine receptor (A1 or A1 allele) is a consideration in the evaluation of FDG based regional glucose metabolism studies of healthy nonalcoholic/non-drug abusing subjects.208 Asymptomatic carriers of DOPA-responsive dystonia have increased dopamine D2 receptors in the striatum.209 Increased D2 receptor binding in DOPA-responsive dystonia (DRD) may be a homeostatic response to the dopaminergic deficit in subjects carrying the DRD gene.

Cellular Monitoring and Gene Expression Gene and cellular therapy do not necessarily involve pharmacological or drug discovery features, but the techniques are powerful tools to validate molecular targets

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ahead of lengthy drug discovery programs. PET-coupled reporter gene constructs can be used to trace the location and temporal expression level changes of therapeutic and endogenous genes.210 Gene therapy will benefit from this methodology to locate and measure therapeutic transgene expression. Researchers have created constructs that evaluate herpes simplex virus type 1 thymidine kinase (HSV-1-tk) as a marker of gene expression by PET imaging. The efficiency of gene expression and expression is measured with radiolabeled 5-iodo-2-[18F] fluoro-2deoxy-1-beta-D-arabino-furanosyl-uracil (FIAU; 64).211 The use of multi-gene vector constructs that in-

grated in the patient’s brain and released dopamine into the striatum.216 This functional activity paralleled partial recovery of motor function in many patients including reduced rigidity and hypokinesia bilaterally but predominantly on the side contralateral to the graft. Synaptic dopamine release from embryonic nigral transplants in the striatum of Parkinson’s disease patients is monitored with [11C]raclopride.217 This type of study has also been applied to the evaluation of dopamine precursor uptake and metabolism by intracerebrally implanted polymer encapsulated PC-12 cells.218 The research determined that behavioral improvements as a result of implanted encapsulated PC-12 cells was partly the result of highly specific uptake and metabolism of dopamine precursors.

Longitudinal Evaluation and Adaptive Responses

clude a marker gene and a therapeutic gene of interest enables the PET evaluation of transgene expression.212 Parallel work with mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) uses 8-[18F]fluoropenciclovir (FPCV; 65) as the PET-reporter gene phos-

213

phorylation substrate. The aromatic amino acid decarboxylase tracer 6-[18F]fluoro-L-m-tyrosine (66) has

been applied to the monitoring of gene therapy in vivo.214 Researchers have applied PET to evaluate the viability of transplanted human fetal dopamine neurons to reinnervate the striatum of Parkinson’s disease patients.215 PET indicated that the grafts were functionally inte-

The non-invasive and quantitative nature of PET enables the evaluation of adaptive responses or pathological changes in a longitudinal fashion using the patient as their own control in a time series. Chronic treatment with the selective serotonin reuptake inhibitor fluvoxamine results in increases in the in vivo binding of fluoro-ethylspiperone ([18F]FESP; 67) in the frontal and occipital

cortex of drug naïve unipolar depressed patients.219 It is hypothesized that modifications in 5HT2 binding capacity is secondary to changes in cortical serotonin activity. Serotonin levels increase after several weeks of treatment with selective serotonin reuptake inhibitors. Studies with [18F]setoperone (68) demonstrate the

down regulation of 5HT2A receptors in young depressed subjects after treatment with paroxetine (69), but this downregulation attenuates with age.220

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duced by chronic dopaminergic therapy or occur independent of therapy as a structural adaptation of the postsynaptic dopamine system to the progressive decline of nigrostriatal neurons. Long-term treatment with the cholinesterase inhibitor tacrine increases the uptake of [11C]nicotine in a stereoselective way.225 This observation parallels increases in glucose metabolism and suggests improvements in functional activity in Alzheimer’s disease patients with long-term tacrine treatment.

The data suggests that more than six weeks of paroxetine treatment increases serotonin agonism on 5HT2A receptors in the cortex. Some of the neurotoxic effects of psychotropic drug 3,4-methylenedioxymethamphetamine (MDMA; 70) can be measured with [11C]McN5652 (71), a PET measure

Conclusion PET, in combination with a diverse array of PET ligands, provides a powerful tool to drug discovery efforts. PET can evaluate receptor occupancy and time course as a function of dose and blood levels. It has the potential to evaluate drug disposition as a function of formulation, delivery mechanisms and the functional activity of efflux pumps. PET provides a means to investigate mechanisms of pharmacological action in vivo and to develop testable hypotheses as to improved selectivity and efficacy for potential drug candidates. In a clinical context PET provides a means to quantitatively and objectively evaluate, stage, and monitor disease. Its non-invasive and quantitative nature allows for longitudinal evaluation, a key cost challenge for clinical studies, and a key scientific challenge for the treatment of chronic diseases. As the availability of PET expands, drug discovery and clinical development will benefit greatly from this imaging modality.

References of the serotonin transporter and by FDG.221 These changes can be evaluated as function of cumulative dose and time since the last dose in addition to the reversibility after ending the drug abuse. Neuroleptic treatment demonstrates changes in regional brain metabolism, blood flow, and dopamine receptor binding in schizophrenics. PET evaluation of these changes is capable of providing insight into the relationship of these actions to symptomatic changes and drug-induced side effects.222 The systemic administration of amphetamine leads to compensatory increases in dopamine synthesis capacity. FDOPA-PET has demonstrated a disparity between striatum and substantia nigra reactivity to amphetamines suggesting unique dopamine system regulatory mechanisms.223 These activities were measured at both 3–4 and 10 to 12 weeks postdrug. Longitudinal studies of Parkinson’s disease patients with [ 11C]raclopride-PET indicate long-term downregulation of striatal dopamine D2 receptor binding.224 The authors suggest that receptor changes may be in-

1. Burns, H.D.; Hamill, T.G.; Eng, W.S.; Francis B., Fioravanti, C.; Gibson, R.E. Positron emission tomography neuroreceptor imaging as a tool in drug discovery, research and development. Curr. Opin. Chem. Biol. 3:388–394; 1999. 2. Hietala, J. Ligand-receptor interactions as studied by PET: implications for drug development. Ann. Med. 31:438–443; 1999. 3. Fischman, A.J.; Alpert, N.M.; Babich, J.W.; Rubin R.H. The role of positron emission tomography in pharmacokinetic analysis. Drug Metab. Rev. 29:923–956; 1997. 4. Yanai, K.; Tagawa, M. The role of positron emission tomography in neuropharmacology in the living human brain and drug development. Nippon Rinsho. 58:2149– 2157;2000. 5. Phelps M.E., Mazziotta J.C., Baxter L., Gerner R. Positron emission tomographic study of affective disorders: problems and strategies. Ann. Neurol. 15:S149–S156; 1984. 6. Phelps, M.E. PET: a biological imaging technique. Neurochem. Res. 16:929–940; 1991. 7. Vaalburg, W.; Hendrikse, N.H.; de Vries E.F. Drug development, radiolabelled drugs and PET. Ann. Med. 31:432–437; 1999. 8. Aboagye, E.O.; Price, P.M.; Jones T. In vivo pharmacokinetics and pharmacodynamics in drug development

PET and Drug Recovery / Klimas 331

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20. 21.

22.

23.

24.

25.

using positron-emission tomography. Drug Discov. Today 6:293–302; 2001. Phelps, M.E.; Schelbert, H.R.; Mazziotta J.C. Positron computed tomography for studies of myocardial and cerebral function. Ann. Intern. Med. 98:339–359; 1983. Jones, T. Present and future capabilities of molecular imaging techniques to understand brain function. J. Psychopharmacol. 13:324–329; 1999. Haberkorn, U.; Altmann, A.; Eisenhut, M. Functional genomics and proteomics—the role of nuclear medicine. Eur. J. Nucl. Med. 29:115–132; 2002. Dimitrakopoulou-Strauss, A.; Strauss, L.G.; Gutzler, F.; et al. Pharmacokinetic imaging of 11C ethanol with PET in eight patients with hepatocellular carcinomas who were scheduled for treatment with percutaneous ethanol injection. Radiology 211:681–686; 1999. Fischman, A.J.; Bonab, A.A.; Rubin, R.H. Regional pharmacokinetics of orally administered PET tracers. Curr. Pharm. Des. 6(16):1625–1629; 2000. Foster, K.A.; Roberts, M.S. Experimental methods for studying drug uptake in the head and brain. Curr. Drug Metab. 1:333–356; 2000. Zunkeler, B.; Carson, R.E.; Olson J.; et al. Quantification and pharmacokinetics of blood-brain barrier disruption in humans. J. Neurosurg. 85:1056–1065; 1996. Dolovich, M.B. Measuring total and regional lung deposition using inhaled radiotracers. J. Aerosol Med. 14:S35–S44; 2001. Lee, Z.; Berridge, M.S.; Finlay, W.H.; Heald, D.L. Mapping PET-measured triamcinolone acetonide (TAA) aerosol distribution into deposition by airway generation. Int. J. Pharm. 199:7–16; 2000. Yanai, M.; Hatazawa, J.; Ojima, F.; Sasaki, H.; Itoh, M.; Ido, T. Deposition and clearance of inhaled 18FDG powder in patients with chronic obstructive pulmonary disease. Eur. Respir. J. 11:1342–1348; 1998. Suhara, T.; Sudo, Y.; Yoshida, K.; et al. Lung as reservoir for antidepressants in pharmacokinetic drug interactions. Lancet 351:332–335; 1998. Waarde, A.V. Measuring receptor occupancy with PET. Curr. Pharm. Des. 6:1593–1610; 2000. Rousseau, A.; Marquet, P.; Debord, J.; Sabot, C.; Lachatre, G. Adaptive control methods for the dose individualization of anticancer agents. Clin. Pharmacokinet. 38:315–353; 2000. Saleem, A.; Harte, R.J.; Matthews, J.C.; et al. Pharmacokinetic evaluation of N-[2-(dimethylamino)ethyl]acridine4-carboxamide in patients by positron emission tomography. J. Clin. Oncol. 19:1421–1429; 2001. Fischman, A.J.; Babich, J.W.; Bonab, A.A.; et al. Pharmacokinetics of [18F]trovafloxacin in healthy human subjects studied with positron emission tomography. Antimicrob. Agents Chemother. 42:2048–2054; 1998. Fischman, A.J.; Livni, E.; Babich, J.W.; et al. Pharmacokinetics of [18F]fleroxacin in patients with acute exacerbations of chronic bronchitis and complicated urinary tract infection studied by positron emission tomography. Antimicrob. Agents Chemother. 40:659–664; 1996. Meikle, S.R.; Matthews, J.C.; Brock, C.S.; et al. Pharmacokinetic assessment of novel anti-cancer drugs using spectral analysis and positron emission tomography: a feasibility study. Cancer Chemother. Pharmacol. 42:183–193; 1998.

26. Lee, M.C.; Wagner, H.N. Jr; Tanada, S.; Frost, J.J.; Bice, A.N.; Dannals, R.F. Duration of occupancy of opiate receptors by naltrexone. J. Nucl. Med. 29:1207–1211; 1988. 27. Christian, B.T.; Livni, E.; Babich, J.W.; et al. Evaluation of cerebral pharmacokinetics of the novel antidepressant drug, BMS-181101, by positron emission tomography. J. Pharmacol. Exp. Ther. 279:325–331; 1996. 28. Camsonne, R.; Barre, L.; Petit-Taboue, M.C.; et al. Positron emission tomographic studies of [11C]MDL 72222, a potential 5–HT3 receptor radioligand: distribution, kinetics and binding in the brain of the baboon. Neuropharmacology 32:65–71; 1993. 29. Offord, S.J.; Wong, D.F.; Nyberg, S. The role of positron emission tomography in the drug development of M100907, a putative antipsychotic with a novel mechanism of action. J. Clin. Pharmacol.331 39 (suppl):17S–24S; 1999. 30. Kissel, J.; Brix, G.; Bellemann, M.E., et al. Pharmacokinetic analysis of 5-[18F]fluorouracil tissue concentrations measured with positron emission tomography in patients with liver metastases from colorectal adenocarcinoma. Cancer Res. 57:3415–3423; 1997. 31. Nilsson, D.; Lennernas, H.; Fasth, K.J.; et al. Absorption of L-DOPA from the proximal small intestine studied in the rhesus monkey by positron emission tomography. Eur. J. Pharm. Sci. 7:185–189; 1999. 32. Haradahira, T.; Zhang, M.; Maeda, J.; et al. A strategy for increasing the brain uptake of a radioligand in animals: use of a drug that inhibits plasma protein binding. Nucl. Med. Biol. 27:357–360; 2000. 33. Osman, S.; Luthra, S.K.; Brady, F.; et al. Studies on the metabolism of the novel antitumor agent [N-methyl-11C] N-[2-(dimethylamino)ethyl]acridine-4-carboxamide in rats and humans prior to phase I clinical trials. Cancer Res. 57:2172–2180; 1997. 34. Nyberg, S.; Dahl, M.L.; Halldin, C. A PET study of D2 and 5–HT2 receptor occupancy induced by risperidone in poor metabolizers of debrisoquin and risperidone. Psychopharmacology (Berl) 119:345–348; 1995. 35. Strauss L.G. Positron emission tomography: current role for diagnosis and therapy monitoring in oncology. Oncologist 2:381–388; 1997. 36. Hendrikse, N.H.; Franssen, E.J.; van der Graaf, W.T.; Vaalburg, W.; de Vries, E.G. Visualization of multidrug resistance in vivo. Eur. J. Nucl. Med. 26:283–293; 1999. 37. Bart, J.; Groen, H.J.; Hendrikse, N.H.; van der Graaf, W.T.; Vaalburg, W.; de Vries, E.G. The blood-brain barrier and oncology: new insights into function and modulation. Cancer Treat Rev. 26:449–462; 2000. 38. Hendrikse, N.H.; de Vries, E.G.; Eriks-Fluks, L.; et al. A new in vivo method to study P-glycoprotein transport in tumors and the blood-brain barrier. Cancer Res. 59:2411–2416; 1999. 39. Sharma, V.; Beatty, A.; Wey, S.P.; et al. Novel gallium(III) complexes transported by MDR1 P-glycoprotein: potential PET imaging agents for probing P-glycoprotein-mediated transport activity in vivo. Chem. Biol. 7:335–343; 2000. 40. Hendrikse, N.H. Monitoring interactions at ATP-dependent drug efflux pumps. Curr. Pharm. Des. 6:1653–1668; 2000. 41. Oku, N.; Tokudome, Y.; Tsukada, H.; Okada, S. Realtime analysis of liposomal trafficking in tumor-bearing

332 Molecular Imaging and Biology, Volume 4, Number 5

42.

43.

44.

45.

46.

47.

48.

49.

50.

51.

52.

53.

54.

55.

mice by use of positron emission tomography. Biochim. Biophys. Acta. 1238:86–90; 1995. Oku, N.; Tokudome, Y.; Tsukada, H.; Kosugi, T.; Namba, Y.; Okada, S. In vivo trafficking of long-circulating liposomes in tumor-bearing mice determined by positron emission tomography. Biopharm. Drug Dispos. 17:435–441; 1996. Asai, T.; Shuto, S.; Matsuda, A.; et al. Targeting and anti-tumor efficacy of liposomal 5-O-dipalmitoylphosphatidyl 2C-cyano-2-deoxy-1-beta-D-arabino-pentofuranosylcytosine in mice lung bearing B16BL6 melanoma. Cancer Lett. 162:49–56; 2001. Frost, J.J. Measurement of neurotransmitter receptors by positron emission tomography: focus on the opiate receptor. NIDA Res. Monogr. 74:15–24; 1986. Farde, L. The advantage of using positron emission tomography in drug research. Trends Neurosci. 19:211– 214; 1996. Zamuner, S.; Gomeni, R.; Bye, A. Estimate the time varying brain receptor occupancy in PET imaging experiments using non-linear fixed and mixed effect modeling approach. Nucl. Med. Biol. 29:115–123; 2002. Kegeles, L.S.; Martinez, D.; Kochan, L.D.; et al. NMDA antagonist effects on striatal dopamine release: positron emission tomography studies in humans. Synapse 43:19– 29; 2002. Farde, L.; Andree, B.; Ginovart, N.; Halldin, C.; Thorberg, S. PET-Determination of robalzotan (NAD-299) induced 5-HT(1A) receptor occupancy in the monkey brain. Neuropsychopharmacology 22:422–429; 2000. Nyberg, S.; Eriksson, B.; Oxenstierna, G.; Halldin, C.; Farde, L. Suggested minimal effective dose of risperidone based on PET-measured D2 and 5–HT2A receptor occupancy in schizophrenic patients. Am. J. Psychiatry 156:869–875; 1999. Christian, B.T.; Babich, J.W.; Livni, E.; et al. Positron emission tomographic analysis of central dopamine D1 receptor binding in normal subjects treated with the atypical neuroleptic, SDZ MAR 327. Int. J. Mol. Med. 1:243–247; 1998. Volkow, N.D.; Wang, G.J.; Fischman, M.W.; et al. Relationship between subjective effects of cocaine and dopamine transporter occupancy. Nature 386:827–830; 1997. Pauli, S.; Sedvall, G. Three-dimensional visualization and quantification of the benzodiazepine receptor population within a living human brain using PET and MRI. Eur. Arch. Psychiatry Clin. Neurosci. 247:61–70; 1997. Roth, B.L.; Ernsberger, P.; Steinberg, S.A.; et al. The in vitro pharmacology of the beta-adrenergic receptor pet ligand (s)-fluorocarazolol reveals high affinity for cloned beta-adrenergic receptors and moderate affinity for the human 5–HT1A receptor. Psychopharmacology (Berl) 157:111–114; 2001. Hume, S.P.; Ashworth, S.; Lammertsma, A.A.; et al. Evaluation in rat of RS-79948–197 as a potential PET ligand for central alpha 2-adrenoceptors. Eur. J. Pharmacol. 317:67–73; 1996. Mathews, W.B.; Scheffel, U.; Finley, P.; et al. Biodistribution of [18F] SR144385 and [18F] SR147963: selective radioligands for positron emission tomographic studies of brain cannabinoid receptors. Nucl. Med. Biol. 27:757–762; 2000.

56. Haradahira, T.; Zhang, M.R.; Maeda, J.; et al. A prodrug of NMDA/glycine site antagonist, L-703,717, with improved BBB permeability: 4-acetoxy derivative and its positron-emitter labeled analog. Chem. Pharm. Bull. (Tokyo) 49:147–150; 2001. 57. Sudo, Y.; Suhara, T.; Suzuki, K.; et al. Muscarinic receptor occupancy by biperiden in living human brain. Life Sci. 64:PL99–PL104; 1999. 58. Mayberg, H.S.; Brannan, S.K.; Tekell, J.L.; et al. Regional metabolic effects of fluoxetine in major depression: serial changes and relationship to clinical response. Biol. Psychiatry 48:830–843; 2000. 59. Martinez, D.; Broft, A.; Laruelle, M. Pindolol augmentation of antidepressant treatment: recent contributions from brain imaging studies. Biol. Psychiatry 48:844–853; 2000. 60. Andree, B.; Thorberg, S.O.; Halldin, C.; Farde, L. Pindolol binding to 5–HT1A receptors in the human brain confirmed with positron emission tomography. Psychopharmacology (Berl) 144:303–305; 1999. 61. Trichard, C.; Paillere-Martinot, M.L.; Attar-Levy, D.; Recassens, C.; Monne,t F.; Martinot, J.L. Binding of antipsychotic drugs to cortical 5–HT2A receptors: a PET study of chlorpromazine, clozapine, and amisulpride in schizophrenic patients. Am. J. Psychiatry 155:505–508; 1998. 62. Volkow, N.D.; Wang, G.J.; Fowler, J.S.; et al. Methylphenidate and cocaine have a similar in vivo potency to block dopamine transporters in the human brain. Life Sci. 65:PL7–PL12; 1999. 63. Volkow, N.D.; Wang, G.J.; Fowler, J.S.; et al. Dopamine transporter occupancies in the human brain induced by therapeutic doses of oral methylphenidate. Am. J. Psychiatry 155:1325–1331; 1998. 64. Wilcox, K.M.; Lindsey, K.P.; Votaw, J.R.; et al. Selfadministration of cocaine and the cocaine analog RTI113: relationship to dopamine transporter occupancy determined by PET neuroimaging in rhesus monkeys. Synapse 43:78–85; 2002. 65. Hartvig, P.; Torstenson, R.; Tedroff, J.; et al. Amphetamine effects on dopamine release and synthesis rate studied in the Rhesus monkey brain by positron emission tomography. J. Neural. Transm. 104:329–339; 1997. 66. Villemagne, V.L.; Rothman, R.B.; Yokoi, F.; et al. Doses of GBR12909 that suppress cocaine self-administration in non-human primates substantially occupy dopamine transporters as measured by [11C] WIN35,428 PET scans. Synapse 32:44–50; 1999. 67. Danielsen, E.H.; Smith, D.; Hermansen, F.; Gjedde, A.; Cumming, P. Acute neuroleptic stimulates DOPA decarboxylase in porcine brain in vivo. Synapse 41:172–175; 2001. 68. Hume, S.P.; Opacka-Juffry, J.; Myers, R.; et al. Effect of L-dopa and 6-hydroxydopamine lesioning on [11C] raclopride binding in rat striatum, quantified using PET. Synapse 21:45–53; 1995. 69. Piccini, P.; Weeks, R.A.; Brooks, D.J. Alterations in opioid receptor binding in Parkinson’s disease patients with levodopainduced dyskinesias. Ann. Neurol. 42:720–726; 1997. 70. Ceravolo, R.; Piccini, P.; Bailey, D.L.; Jorga, K.M.; Bryson, H.; Brooks, D.J. 18F-dopa PET evidence that tolcapone acts as a central COMT inhibitor in Parkinson’s disease. Synapse 43:201–207; 2002.

PET and Drug Recovery / Klimas 333

71. Farde, L.; Wiesel, F.A.; Halldin, C.; Sedvall, G. Central D2-dopamine receptor occupancy in schizophrenic patients treated with antipsychotic drugs. Arch. Gen. Psychiatry 45:71–76; 1988. 72. Nyberg, S.; Farde, L.; Halldin, C. A PET study of 5–HT2 and D2 dopamine receptor occupancy induced by olanzapine in healthy subjects. Neuropsychopharmacology 16:1–7; 1997. 73. Kapur, S.; Zipursky, R.; Jones, C.; Shammi, C.S.; Remington, G.; Seeman, P. A positron emission tomography study of quetiapine in schizophrenia: a preliminary finding of an antipsychotic effect with only transiently high dopamine D2 receptor occupancy. Arch. Gen. Psychiatry 57:553–559; 2000. 74. Holcomb, H.H.; Cascella, N.G.; Thaker, G.K.; Medoff, D.R.; Dannals, R.F.; Tamminga, C.A. Functional sites of neuroleptic drug action in the human brain: PET/FDG studies with and without haloperidol. Am. J. Psychiatry 153:41–49; 1996. 75. Black, K.J.; Hershey, T.; Gado, M.H.; Perlmutter, J.S. Dopamine D(1) agonist activates temporal lobe structures in primates. J. Neurophysiol. 84:549–557; 2000. 76. Volkow, N.D.; Wang, G.; Fowler, J.S.; et al. Therapeutic doses of oral methylphenidate significantly increase extracellular dopamine in the human brain. J. Neurosci. 21:RC121; 2001. 77. Juhasz, C.; Muzik, O.; Chugani, D.C.; Shen, C.; Janisse, J.; Chugani, H.T. Prolonged vigabatrin treatment modifies developmental changes of GABA(A)-receptor binding in young children with epilepsy. Epilepsia 42:1320–1326; 2001. 78. Schmid, L.; Bottlaender, M.; Brouillet, E.; Fuseau, C.; Maziere, M. Vigabatrin modulates benzodiazepine receptor activity in vivo: a positron emission tomography study in baboon. J. Pharmacol. Exp. Ther. 276:977–983; 1996. 79. Chavoix, C.; Brouillet, E.; Kunimoto, M., et al. Relationships between benzodiazepine receptors, impairment of GABAergic transmission and convulsant activity of betaCCM: a PET study in the baboon Papio papio. Epilepsy Res. 8:1–10; 1991. 80. Brouillet, E.; Chavoix, C.; Bottlaender, M.; et al. In vivo bidirectional modulatory effect of benzodiazepine receptor ligands on GABAergic transmission evaluated by positron emission tomography in non-human primates. Brain Res. 557:167–176; 1991. 81. Reinsel, R.A.; Veselis, R.A.; Dnistrian, A.M.; Feshchenko, V.A.; Beattie, B.J.; Duff, M.R. Midazolam decreases cerebral blood flow in the left prefrontal cortex in a dosedependent fashion. Int. J. Neuropsychopharmacol. 3:117–127; 2000. 82. Coull, J.T.; Frith, C.D.; Dolan, R.J. Dissociating neuromodulatory effects of diazepam on episodic memory encoding and executive function. Psychopharmacology (Berl) 145:213–222; 1999. 83. Mintzer, M.Z.; Griffiths, R.R.; Contoreggi, C.; Kimes, A.S.; London, E.D.; Ernst, M. Effects of Triazolam on Brain Activity During Episodic Memory Encoding: A PET Study. Neuropsychopharmacology 25:744–756; 2001. 84. Gyulai, F.E.; Mintun, M.A.; Firestone, L.L. Dose-dependent enhancement of in vivo GABA(A)-benzodiazepine receptor binding by isoflurane. Anesthesiology 95(3):585– 593; 2001.

85. Bonhomme, V.; Fiset, P.; Meuret, P.; et al. Propofol anesthesia and cerebral blood flow changes elicited by vibrotactile stimulation: a positron emission tomography study. J. Neurophysiol. 85:1299–1308; 2001. 86. Fiset, P.; Paus, T.; Daloze, T.; et al. Brain mechanisms of propofol-induced loss of consciousness in humans: a positron emission tomographic study. J. Neurosci. 19:5506–5513; 1999. 87. Muhr, C.; Gudjonsson, O; Lilja, A.; et al. Meningioma treated with interferon-alpha, evaluated with [(11)C]-Lmethionine positron emission tomography. Clin. Cancer Res. 7:2269–2276; 2001. 88. Rischin, D.; Peters, L.; Hicks, R.; et al. Phase I trial of concurrent tirapazamine, cisplatin, and radiotherapy in patients with advanced head and neck cancer. J. Clin. Oncol. 19:535–542; 2001. 89. Saleem, A.; Yap, J.; Osman, S.; et al. Modulation of fluorouracil tissue pharmacokinetics by eniluracil: in-vivo imaging of drug action. Lancet 355:2125–2131; 2000. 90. Robbins, R.J.; Hill, R.H.; Wang, W.; Macapinlac, H.H.; Larson, S.M. Inhibition of metabolic activity in papillary thyroid carcinoma by a somatostatin analogue. Thyroid 10:177–183; 2000. 91. Collins, J.M.; Klecker, R.W.; Katki, A.G. Suicide prodrugs activated by thymidylate synthase: rationale for treatment and noninvasive imaging of tumors with deoxyuridine analogues. Clin. Cancer Res. 5:1976–1981; 1999. 92. Harte, R.J.; Matthews, J.C.; O’Reilly, S.M., et al. Tumor, normal tissue, and plasma pharmacokinetic studies of fluorouracil biomodulation with N-phosphonacetyl-L-aspartate, folinic acid, and interferon alpha. J. Clin. Oncol. 17:1580–1588; 1999. 93. Zhang, Z.J.; Wang, J.L.; Muhr, C.; Smits, A. Synergistic inhibitory effects of interferon-alpha and 5-fluorouracil in meningioma cells in vitro. Cancer Lett. 100:99–105; 1996. 94. Coull, J.T.; Buchel, C.; Friston, K.J.; Frith, C.D. Noradrenergically mediated plasticity in a human attentional neuronal network. Neuroimage 10:705–715; 1999. 95. Nordberg, A.; Amberla, K.; Shigeta, M.; et al. Long-term tacrine treatment in three mild Alzheimer patients: effects on nicotinic receptors, cerebral blood flow, glucose metabolism, EEG, and cognitive abilities. Alzheimer Dis. Assoc. Disord. 12:228–237; 1998. 96. Mody, F.V.; Singh, B.N.; Mohiuddin, I.H.; et al. Trimetazidine-induced enhancement of myocardial glucose utilization in normal and ischemic myocardial tissue: an evaluation by positron emission tomography. Am. J. Cardiol. 82:42K–49K; 1998. 97. Inoue, T.; Kim, E.E.; Wallace, S.; et al. Preliminary study of cardiac accumulation of F-18 fluorotamoxifen in patients with breast cancer. Clin. Imaging 21(5):332–336; 1997. 98. Qing, F.; McCarthy, T.J.; Markham, J.; Schuster, D.P. Pulmonary angiotensin-converting enzyme (ACE) binding and inhibition in humans. A positron emission tomography study. Am. J. Respir. Crit. Care Med. 161:2019–2025; 2000. 99. Saiki, I.; Koike, C.; Obata, A.; et al. Functional role of sialyl Lewis X and fibronectin-derived RGDS peptide analogue on tumor-cell arrest in lungs followed by extravasation. Int. J. Cancer 65:833–839; 1996.

334 Molecular Imaging and Biology, Volume 4, Number 5

100. Benkelfat, C.; Bradwejn, J.; Meyer, E.; et al. Functional neuroanatomy of CCK4-induced anxiety in normal healthy volunteers. Am. J. Psychiatry 152:1180–1184; 1995. 101. Heinonen, E.H.; Anttila, M.I.; Lammintausta, R.A. Pharmacokinetic aspects of l-deprenyl (selegiline) and its metabolites. Clin. Pharmacol. Ther. 56:742–749; 1994. 102. Fowler, J.S.; Wang, G.J.; Volkow, N.D., et al. Evidence that gingko biloba extract does not inhibit MAO A and B in living human brain. Life Sci. 66:PL141–146; 2000. 103. Gjedde, A. Receptor mapping in living human beings by means of positron emission tomography. Ugeskr. Laeger. 163:5199–5205; 2001. 104. Laakso, A..; Hietala, J. PET studies of brain monoamine transporters. Curr. Pharm. Des. 6:1611–1623; 2000. 105. Farde, L.; Ginovart, N.; Halldin, C.; Chou, Y.H.; Olsson, H.; Swahn, C.G. A PET study of [11C][beta]-CIT-FE binding to the dopamine transporter in the monkey and human brain. Int. J. Neuropsychopharmacol. 3:203–214; 2000. 106. Pike, V.W.; Halldin, C.; Wikstrom, H.; et al. Radioligands for the study of brain 5-HT(1A) receptors in vivo–development of some new analogues of way. Nucl. Med. Biol. 27:449–455; 2000. 107. Takahashi, T.; Nishimura, S.; Ido, T.; Ishiwata, K.; Iwata, R. Biological evaluation of 5-methyl-branched-chain omega-[18F]fluorofatty acid: a potential myocardial imaging tracer for positron emission tomography. Nucl. Med. Biol. 23:303–308; 1996. 108. Pelegrin, A.; Xavier, F.; Barbet, J.; et al. Immunotargeting of tumors: state of the art and prospects in 2000. Bull. Cancer 87:777–791; 2000. 109. Passchier, J.; van Waarde, A. Visualization of serotonin1A (5–HT1A) receptors in the central nervous system. Eur. J. Nucl. Med. 28:113–129; 2001. 110. Mayberg, H.S.; Brannan, S.K.; Mahurin, R.K.; et al. Cingulate function in depression: a potential predictor of treatment response. Neuroreport. 8:1057–1061; 1997. 111. el Mansari, M.; Bouchard, C.; Blier, P. Alteration of serotonin release in the guinea pig orbito-frontal cortex by selective serotonin reuptake inhibitors. Relevance to treatment of obsessive-compulsive disorder. Neuropsychopharmacology 13:117–127; 1995. 112. Swedo, S.E.; Pietrini, P.; Leonard, H.L.; et al. Cerebral glucose metabolism in childhood-onset obsessive-compulsive disorder. Revisualization during pharmacotherapy. Arch. Gen. Psychiatry 49:690–694; 1992. 113. Baxter, L.R. Jr; Schwartz, J.M.; Bergman, K.S.; et al. Caudate glucose metabolic rate changes with both drug and behavior therapy for obsessive-compulsive disorder. Arch. Gen. Psychiatry 49:681–689; 1992. 114. Fowler, J.S.; Volkow, N.D. PET imaging studies in drug abuse. J. Toxicol. Clin. Toxicol. 36:163–174; 1998. 115. Sell, L.A.; Morris, J.S.; Bearn, J.; Frackowiak, R.S.; Friston, K.J.; Dolan, R.J. Neural responses associated with cue evoked emotional states and heroin in opiate addicts. Drug Alcohol Depend. 60:207–216; 2000. 116. Volkow, N.D.; Chang, L.; Wang, G.J.; et al. Low level of brain dopamine D2 receptors in methamphetamine abusers: association with metabolism in the orbitofrontal cortex. Am. J. Psychiatry 158:2015–2021; 2001. 117. Volkow, N.D.; Wang, G.J.; Fowler, J.S.; et al. Reinforcing effects of psychostimulants in humans are associated with in-

118.

119.

120.

121.

122.

123.

124.

125.

126.

127.

128.

129.

130. 131.

132.

133.

creases in brain dopamine and occupancy of D(2) receptors. J. Pharmacol. Exp. Ther. 291:409–415; 1999. Wang, G.J.; Volkow, N.D.; Fowler, J.S.; et al. Regional brain metabolic activation during craving elicited by recall of previous drug experiences. Life Sci. 64:775–784; 1999. Weinstein, A.; Feldtkeller, B.; Malizia, A.; Wilson, S.; Bailey, J.; Nutt, D.J. Integrating the cognitive and physiological aspects of craving. J. Psychopharmacol. 12:31–38; 1998. London, E.D.; Bonson, K.R.; Ernst, M.; Grant, S. Brain imaging studies of cocaine abuse: implications for medication development. Crit. Rev. Neurobiol. 13:227–242; 1999. Wu, J.C.; Bell, K.; Najafi, A.; et al. Decreasing striatal 6-FDOPA uptake with increasing duration of cocaine withdrawal. Neuropsychopharmacology 17:402–409; 1997. Volkow, N.D.; Fowler, J.S.; Wolf, A.P.; et al. Changes in brain glucose metabolism in cocaine dependence and withdrawal. Am. J. Psychiatry 148:621–626; 1991. Cohen, R.M.; Nordahl, T.E.; Semple, W.E.; Andreason, P.; Litman, R.E.; Pickar, D. The brain metabolic patterns of clozapine- and fluphenazine-treated patients with schizophrenia during a continuous performance task. Arch. Gen. Psychiatry 54:481–486; 1997. Suhara, T.; Okubo, Y.; Yasuno, F.; et al. Decreased dopamine D2 receptor binding in the anterior cingulate cortex in schizophrenia. Arch. Gen. Psychiatry 59:25–30; 2002. Davis, K.L.; Kahn, R.S.; Ko, G.; Davidson, M. Dopamine in schizophrenia: a review and reconceptualization. Am.. J. Psychiatry 148:1474–1486; 1991. Sabri, O.; Erkwoh, R.; Schreckenberger, M.; Owega, A.; Sass, H.; Buell, U. Correlation of positive symptoms exclusively to hyperperfusion or hypoperfusion of cerebral cortex in never-treated schizophrenics. Lancet 349:1735– 1739; 1997. Potkin, S.G.; Alva, G.; Fleming, K.; et al. A PET study of the pathophysiology of negative symptoms in schizophrenia. Am. J. Psychiatry 159:227–237; 2002. Okubo, Y.; Suhara, T.; Suzuki, K.; et al. Decreased prefrontal dopamine D1 receptors in schizophrenia revealed by PET. Nature 385:634–636; 1997. Szymanski, S.; Gur, R.C.; Gallacher, F.; Mozley, L.H.; Gur, R.E. Vulnerability to tardive dyskinesia development in schizophrenia: an FDG-PET study of cerebral metabolism. Neuropsychopharmacology 15:567–575; 1996. Tamminga, C. Glutamatergic aspects of schizophrenia. Br. J. Psychiatry 37(suppl):12–15; 1999. Lang, W.; Hollinger, P.; Eghker, A.; Lindinger, G. Functional localization of motor processes in the primary and supplementary motor areas. J. Clin. Neurophysiol. 11:397–419; 1994. Ouchi, Y.; Kanno, T.; Okada, H.; et al. Presynaptic and postsynaptic dopaminergic binding densities in the nigrostriatal and mesocortical systems in early Parkinson’s disease: a double-tracer positron emission tomography study. Ann. Neurol. 46:723–731; 1999. Lee, C.S.; Samii, A.; Sossi, V.; et al. In vivo positron emission tomographic evidence for compensatory changes in presynaptic dopaminergic nerve terminals in Parkinson’s disease. Ann. Neurol. 47:493–503; 2000.

PET and Drug Recovery / Klimas 335

134. Frey, K.A.; Koeppe, R.A.; Kilbourn, M.R.; Van der Borght, T.M.; Albin, R.L.; Gilman, S.; Kuhl, D.E. Presynaptic monoaminergic vesicles in Parkinson’s disease and normal aging. Ann. Neurol. 40:873–884; 1996. 135. de la Fuente-Fernandez, R.; Ruth, T.J.; Sossi, V.; Schulzer, M.; Calne, D.B.; Stoessl, A.J. Expectation and dopamine release: mechanism of the placebo effect in Parkinson’s disease. Science 293:1164–1166; 2001. 136. Brooks, D.J.; Piccini, P.; Turjanski, N.; Samuel, M. Neuroimaging of dyskinesia. Ann. Neurol. 47:S154–S159; 2000. 137. Deuschl, G.; Raethjen, J.; Lindemann, M.; Krack, P. The pathophysiology of tremor. Muscle Nerve 24:716–735; 2001. 138. Savic, I.; Thorell, J.O. Localized cerebellar reductions in benzodiazepine receptor density in human partial epilepsy. Arch. Neurol. 53:656–662; 1996. 139. Friston, K.J.; Grasby, P.M.; Bench, C.J.; et al. Measuring the neuromodulatory effects of drugs in man with positron emission tomography. Neurosci. Lett. 141:106– 110; 1992. 140. Coull, J.T.; Frith, C.D.; Frackowiak, R.S.; Grasby, P.M. A fronto-parietal network for rapid visual information processing: a PET study of sustained attention and working memory. Neuropsychologia 34:1085–1095; 1996. 141. Sihver, W.; Langstrom, B.; Nordberg, A. Ligands for in vivo imaging of nicotinic receptor subtypes in Alzheimer brain. Acta. Neurol. Scand. Suppl. 176:27–33; 2000. 142. Korf, J. Imaging of receptors in clinical neurosciences. Acta. Neurol. Belg. 97:200–205; 1997. 143. Coull, J.T.; Frith, C.D.; Dolan, R.J.; Frackowiak, R.S.; Grasby, P.M. The neural correlates of the noradrenergic modulation of human attention, arousal and learning. Eur. J. Neurosci. 9:589–598; 1997. 144. Bryant, C.A.; Jackson, S.H. Functional imaging of the brain in the evaluation of drug response and its application to the study of aging. Drugs Aging 13:211–222; 1998. 145. Syrota, A. Cardiac receptors studied by positron emission tomography. Acta. Radiol. Suppl. 374:85–91; 1990. 146. Goldstein, D.S.; Eisenhofer, G.; Dunn, B.B.; et al. Positron emission tomographic imaging of cardiac sympathetic innervation using 6-[18F]fluorodopamine: initial findings in humans. J. Am. Coll. Cardiol. 22:1961– 1971; 1993. 147. Pike, V.W.; Law, M.P.; Osman, S.; et al. Selection, design and evaluation of new radioligands for PET studies of cardiac adrenoceptors. Pharm. Acta. Helv. 74:191–200; 2000. 148. Klecker, R.W. Jr; Collins, J.M. Thymidine phosphorylase as a target for imaging and therapy with thymine analogs. Cancer Chemother. Pharmacol. 48:407–412; 2001. 149. Laurent, B.; Peyron, R.; Garcia Larrea, L.; Mauguiere, F. Positron emission tomography to study central pain integration. Rev. Neurol. (Paris) 156:341–351; 2000. 150. Sachs, H.; Wolf, A.; Russell, J.A.; Christman, D.R. Effect of reserpine on regional cerebral glucose metabolism in control and migraine subjects. Arch. Neurol. 43:1117– 1123; 1986. 151. Baron, J.C. Dementias and memory impairment of degenerative origin. Rev. Neurol. (Paris)154 (suppl):S122– S130; 1998.

152. Elkashef, A.M.; Doudet, D.; Bryant, T.; Cohen, R.M.; Li, S.H.; Wyatt, R.J. 6-(18)F-DOPA PET study in patients with schizophrenia. Positron emission tomography. Psychiatry Res. 100:1–11; 2000. 153. Lahti, A.C.; Holcomb, H.H.; Medoff, D.R.; Weiler, M.A.; Tamminga, C.A.; Carpenter, W.T. Jr. Abnormal patterns of regional cerebral blood flow in schizophrenia with primary negative symptoms during an effortful auditory recognition task. Am. J. Psychiatry 158:1797–1808; 2001. 154. Schreiner, R.; Mirisch, S.; Vesely, Z.; Wiegand, M.H. Sleep and sleep-wake cycle in an 81-year-old patient with de novo ultra-rapid cycling bipolar disorder. Eur. Arch. Psychiatry Clin. Neurosci. 251:29–31; 2001. 155. Lucignani, G.; Tassi, L.; Fazio, F.; et al. Double-blind stereo-EEG and FDG PET study in severe partial epilepsies: are the electric and metabolic findings related? Eur. J. Nucl. Med. 23:1498–1507; 1996. 156. Galardi, G.; Perani, D.; Grassi, F.; et al. Basal ganglia and thalamo-cortical hypermetabolism in patients with spasmodic torticollis. Acta. Neurol. Scand. 94:172–176; 1996. 157. Mann, J.J.; Malone, K.M.; Diehl, D.J.; Perel, J.; Nichols, T.E.; Mintun, M.A. Positron emission tomographic imaging of serotonin activation effects on prefrontal cortex in healthy volunteers. J. Cereb. Blood Flow Metab. 16:418–426; 1996. 158. Baxter, L.R. Jr; Phelps, M.E.; Mazziotta, J.C.; Guze, B.H.; Schwartz, J.M.; Selin, C.E. Local cerebral glucose metabolic rates in obsessive-compulsive disorder. A comparison with rates in unipolar depression and in normal controls. Arch. Gen. Psychiatry 44:211–218; 1987. 159. Moehler, M.; Dimitrakopoulou-Strauss, A.; Gutzler, F.; Raeth, U.; Strauss, L.G.; Stremmel, W. 18F-labeled fluorouracil positron emission tomography and the prognoses of colorectal carcinoma patients with metastases to the liver treated with 5-fluorouracil. Cancer 83:245–253; 1998. 160. Paule, M.G.; Bushnell, P.J.; Maurissen, J.P.; et al. Symposium overview: the use of delayed matching-to-sample procedures in studies of short-term memory in animals and humans. Neurotoxicol. Teratol. 20:493–502; 1998. 161. Fischer, H.; Andersson, J.L.; Furmark, T.; Fredrikson, M. Brain correlates of an unexpected panic attack: a human positron emission tomographic study. Neurosci. Lett. 251:137–140; 1998. 162. Doudet, D.; Hommer, D.; Higley, J.D.; et al. Cerebral glucose metabolism, CSF 5-HIAA levels, and aggressive behavior in rhesus monkeys. Am. J. Psychiatry 152:1782– 1787; 1995. 163. Ketter, T.A.; Kimbrell, T.A.; George, M.S.; et al. Effects of mood and subtype on cerebral glucose metabolism in treatment-resistant bipolar disorder. Biol. Psychiatry 49:97–109; 2001. 164. Kishimoto, H.; Yamada, K.; Iseki, E.; Kosaka, K.; Okoshi, T. Brain imaging of affective disorders and schizophrenia. Psychiatry Clin. Neurosci. 52 (suppl):S212–S214; 1998. 165. Vollenweider, F.X.; Maguire, R.P.; Leenders, K.L.; Mathys, K.; Angst, J. Effects of high amphetamine dose on mood and cerebral glucose metabolism in normal volunteers using positron emission tomography (PET). Psychiatry Res. 83:149–162; 1998.

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166. Hietala, J.; Syvalahti, E.; Vilkman, H.; et al. Depressive symptoms and presynaptic dopamine function in neuroleptic-naive schizophrenia. Schizophr. Res. 35:41–50; 1999. 167. Volkow, N.D.; Fowler, J.S.; Wang, G.J. Imaging studies on the role of dopamine in cocaine reinforcement and addiction in humans. J. Psychopharmacol. 13:337–345; 1999. 168. Leenders, K.L.; Oertel, W.H. Parkinson’s disease: clinical signs and symptoms, neural mechanisms, positron emission tomography, and therapeutic interventions. Neural. Plast. 8:99–110; 2001. 169. Antonini, A.; Moresco, R.M.; Gobbo, C.; et al. The status of dopamine nerve terminals in Parkinson’s disease and essential tremor: a PET study with the tracer [11-C]FECIT. Neurol. Sci. 22:47–48; 2001. 170. Winogrodzka, A.; Wagenaar, R.C.; Bergmans, P.; et al. Rigidity decreases resting tremor intensity in Parkinson’s disease: A [(123)I]beta-CIT SPECT study in early, nonmedicated patients. Mov. Disord. 16:1033–1040; 2001. 171. Torstenson, R.; Hartvig, P.; Langstrom, B.; Westerberg, G.; Tedroff, J. Differential effects of levodopa on dopaminergic function in early and advanced Parkinson’s disease. Ann. Neurol. 41:334–340; 1997. 172. Brooks, D.J. Detection of preclinical Parkinson’s disease with PET. Geriatrics 46 (suppl):25–30; 1991. 173. Wiesel, F.A.; Wik, G.; Sjogren, I.; Blomqvist, G.; Greitz, T.; Stone-Elander, S. Regional brain glucose metabolism in drug free schizophrenic patients and clinical correlates. Acta. Psychiatr. Scand. 76:628–641; 1987. 174. Young, A.B.; Penney, J.B.; Starosta-Rubinstein, S.; et al. PET scan investigations of Huntington’s disease: cerebral metabolic correlates of neurological features and functional decline. Ann. Neurol. 20:296–303; 1986. 175. Siebelink, H.M.; Blanksma, P.K.; Crijns, H.J.; et al. No difference in cardiac event-free survival between positron emission tomography-guided and single-photon emission computed tomography-guided patient management: a prospective, randomized comparison of patients with suspicion of jeopardized myocardium. J. Am. Coll. Cardiol. 37:81–88; 2001. 176. Baller, D.; Gleichmann, U.; Notohamiprodjo, G.; et al. Improved coronary vasodilator capacity by drug lipid lowering therapy in patients in the early stage of coronary atherosclerosis with reduced coronary reserves and moderate LDL hypercholesteremia. Z. Kardiol. 87 (suppl):136–144; 1998. 177. Gould, K.L. New concepts and paradigms in cardiovascular medicine: the noninvasive management of coronary artery disease. Am. J. Med. 104:2S–17S; 1998. 178. Glasspool, R.M.; Evans, T.R. Clinical imaging of cancer metastasis. Eur. J. Cancer 36(13 Spec No):1661–1670; 2000. 179. Bombardieri, E.; Crippa, F.; Maffioli, L.; et al. Nuclear medicine approaches for detection of axillary lymph node metastases. Q. J. Nucl. Med. 42:54–65; 1998. 180. Gimenez Roldan, S.; Mateo, D.; Martinez Gines, M.; Iniesta, I. The problem of accurate diagnosis of Parkinson disease. Neurologia. 14 (suppl):3–16; 1999. 181. Hristova, A.H.; Koller, W.C. Early Parkinson’s disease: what is the best approach to treatment. Drugs Aging 17:165–181; 2000. 182. Larisch, R.; Klimke, A. Clinical impact of cerebral dopamine-D2 receptor scintigraphy. Nuklearmedizin. 37:245–250; 1998.

183. Takahashi, N.; Fujibayashi, Y.; Yonekura, Y.; et al. Evaluation of 62Cu labeled diacetyl-bis(N4-methylthiosemicarbazone) as a hypoxic tissue tracer in patients with lung cancer. Ann. Nucl. Med. 14:323–328; 2000. 184. Blend, M.J.; Abdel-Nabi, H. New methods for the staging of colorectal cancer using noninvasive techniques. Semin. Surg. Oncol. 12:253–263; 1996. 185. Levchenko, A.; Mehta, B.M.; Lee, J.B.; et al. Evaluation of 11C-colchicine for PET imaging of multiple drug resistance. J. Nucl. Med. 41:493–501; 2000. 186. Connors, T. Anticancer drug development: the way forward. Oncologist 1:180–181; 1996. 187. Koh, W.J.; Rasey, J.S.; Evans, M.L.; et al. Imaging of hypoxia in human tumors with [F-18]fluoromisonidazole. Int. J. Radiat. Oncol. Biol. Phys. 22:199–212; 1992. 188. Francavilla, T.L.; Miletich, R.S.; DeMichele, D.; et al. Positron emission tomography of pituitary macroadenomas: hormone production and effects of therapies. Neurosurgery 28:826–833; 1991. 189. Weber, W.A.; Ott, K.; Becker, K.; et al. Prediction of response to preoperative chemotherapy in adenocarcinomas of the esophagogastric junction by metabolic imaging. J. Clin. Oncol. 19:3058–3065; 2001. 190. Bartlett, E.J.; Brodie, J.D.; Simkowitz, P.; et al. Effect of a haloperidol challenge on regional brain metabolism in neuroleptic-responsive and nonresponsive schizophrenic patients. Am. J. Psychiatry 155:337–343; 1998. 191. Schlosser, R. Detection of neurotransmitter interactions with PET and SPECT by pharmacological challenge paradigms. Nervenarzt. 71:9–18; 2000. 192. Cohen, R.M.; Nordahl, T.E.; Semple, W.E.; Andreason, P.; Pickar, D. Abnormalities in the distributed network of sustained attention predict neuroleptic treatment response in schizophrenia Neuropsychopharmacology 19:36–47; 1998. 193. Buchsbaum, M.S.; Potkin, S.G.; Siegel, B.V. Jr; et al. Striatal metabolic rate and clinical response to neuroleptics in schizophrenia. Arch. Gen. Psychiatry 49:966–974; 1992. 194. Brody, A.L.; Saxena, S.; Silverman, D.H.; et al. Brain metabolic changes in major depressive disorder from pre- to post-treatment with paroxetine. Psychiatry Res. 91:127–139; 1999. 195. Tauscher, J.; Kapur, S. Choosing the right dose of antipsychotics in schizophrenia: lessons from neuroimaging studies. CNS Drugs 15:671–678; 2001. 196. Kapur, S.; Roy, P.; Daskalakis, J.; Remington, G.; Zipursky, R. Increased dopamine d(2) receptor occupancy and elevated prolactin level associated with addition of haloperidol to clozapine. Am. J. Psychiatry 158:311–314; 2001. 197. Silvestri, S.; Seeman, M.V.; Negrete, J.C.; et al. Increased dopamine D2 receptor binding after long-term treatment with antipsychotics in humans: a clinical PET study. Psychopharmacology (Berl) 152:174–180; 2000. 198. Wadenberg, M.L.; Kapur, S.; Soliman, A.; Jones, C.; Vaccarino, F. Dopamine D2 receptor occupancy predicts catalepsy and the suppression of conditioned avoidance response behavior in rats. Psychopharmacology (Berl) 150:422–429; 2000. 199. Pinkofsky, H.B.; Dwyer, D.S.; Bradley, R.J. The inhibition of GLUT1 glucose transport and cytochalasin B binding activity by tricyclic antidepressants. Life Sci. 66:271–278; 2000.

PET and Drug Recovery / Klimas 337

200. Lewis, P.J.; Barrington, S.F.; Marsden, P.K.; Maisey, M.N.; Lewis, L.D. A study of the effects of sumatriptan on myocardial perfusion in healthy female migraineurs using 13NH3 positron emission tomography. Neurology 48:1542–1550; 1997. 201. Bergstrom, M.; Westerberg, G.; Nemeth, G.; et al. MAO-A inhibition in brain after dosing with esuprone, moclobemide and placebo in healthy volunteers: in vivo studies with positron emission tomography. Eur. J. Clin. Pharmacol. 52:121–128; 1997. 202. Gaillard, W.D.; Zeffiro, T.; Fazilat, S.; DeCarli, C.; Theodore, W.H. Effect of valproate on cerebral metabolism and blood flow: an 18F–2-deoxyglucose and 15O water positron emission tomography study. Epilepsia 37:515–521; 1996. 203. Leiderman, D.B.; Balish, M.; Bromfield, E.B.; Theodore, W.H. Effect of valproate on human cerebral glucose metabolism. Epilepsia 32:417–422; 1991. 204. Matochik, J.A.; Nordahl, T.E.; Gross, M.; et al. Effects of acute stimulant medication on cerebral metabolism in adults with hyperactivity. Neuropsychopharmacology 8:377– 386; 1993. 205. Martinez, D.; Broft, A.; Laruelle, M. Imaging neurochemical endophenotypes: promises and pitfalls. Pharmacogenomics 2:223–237; 2001. 206. Paans, A.M.; Vaalburg, W. Positron emission tomography in drug development and drug evaluation. Curr. Pharm. Des. 6:1583–1591; 2000. 207. Ferrie, C.D.; Bird, S.; Tilling, K.; Maisey, M.N.; Chapman, A.G.; Robinson, R.O. Plasma amino acids in childhood epileptic encephalopathies. Epilepsy Res. 34:221–229; 1999. 208. Noble, E.P.; Gottschalk, L.A.; Fallon, J.H.; Ritchie, T.L.; Wu, J.C. D2 dopamine receptor polymorphism and brain regional glucose metabolism. Am. J. Med. Genet. 74:162– 166; 1997. 209. Kishore, A.; Nygaard, T.G.; de la Fuente-Fernandez, R.; et al. Striatal D2 receptors in symptomatic and asymptomatic carriers of dopa-responsive dystonia measured with [11C]-raclopride and positron-emission tomography. Neurology 50:1028–1032; 1998. 210. Phelps, M.E. Inaugural article: positron emission tomography provides molecular imaging of biological processes. Proc. Natl. Acad. Sci. U S A 97:9226–9233; 2000. 211. Jacobs, A.; Dubrovin, M.; Hewett, J.; et al. Functional coexpression of HSV-1 thymidine kinase and green fluorescent protein: implications for noninvasive imaging of transgene expression. Neoplasia 1:154–161; 1999. 212. Blasberg, R.G.; Tjuvajev, J.G. Herpes simplex virus thymidine kinase as a marker/reporter gene for PET imaging of gene therapy. Q. J. Nucl. Med. 43:163–169; 1999.

213. Gambhir, S.S.; Bauer, E.; Black, M.E.; et al. A mutant herpes simplex virus type 1 thymidine kinase reporter gene shows improved sensitivity for imaging reporter gene expression with positron emission tomography. Proc. Natl. Acad. Sci. U S A 97:2785–2790; 2000. 214. Bankiewicz, K.S.; Eberling, J.L.; Kohutnicka, M.; et al. Convection-enhanced delivery of AAV vector in parkinsonian monkeys; in vivo detection of gene expression and restoration of dopaminergic function using prodrug approach. Exp. Neurol. 164:2–14; 2000. 215. Lindvall, O.; Hagell, P. Cell therapy and transplantation in Parkinson’s disease. Clin. Chem. Lab. Med. 39:356–361; 2001. 216. Lindvall, O. Neural transplantation in Parkinson’s disease. Novartis Found. Symp. 231:110–128, 145–147; 2000. 217. Piccini, P.; Brooks, D.J.; Bjorklund, A.; et al. Dopamine release from nigral transplants visualized in vivo in a Parkinson’s patient. Nat. Neurosci. 2:1137–1140; 1999. 218. Subramanian, T.; Emerich, D.F.; Bakay, R.A.; et al. Polymer-encapsulated PC-12 cells demonstrate high-affinity uptake of dopamine in vitro and 18F-Dopa uptake and metabolism after intracerebral implantation in nonhuman primates. Cell Transplant. 6:469–477; 1997. 219. Moresco, R.M.; Colombo, C.; Fazio, F.; et al. Effects of fluvoxamine treatment on the in vivo binding of [F-18] FESP in drug naive depressed patients: a PET study. Neuroimage 12:452–465; 2000. 220. Meyer, J.H.; Kapur, S.; Eisfeld, B.; et al. The effect of paroxetine on 5-HT(2A) receptors in depression: an [(18)F]setoperone PET imaging study. Am. J. Psychiatry 158:78–85; 2001. 221. Buchert, R.; Obrocki, J.; Thomasius, R.; et al. Long-term effects of ‘ecstasy’ abuse on the human brain studied by FDG PET. Nucl. Med. Commun. 22:889–897; 2001. 222. List, S.J.; Cleghorn, J.M. Implications of positron emission tomography research for the investigation of the actions of antipsychotic drugs. Br. J. Psychiatry 22(suppl):25–30; 1993. 223. Melega, W.P.; Raleigh, M.J.; Stout, D.B.; Lacan, G.; Huang, S.C.; Phelps, M.E. Recovery of striatal dopamine function after acute amphetamine- and methamphetamine-induced neurotoxicity in the vervet monkey. Brain Res. 766:113–120; 1997. 224. Antonini, A.; Schwarz, J.; Oertel, W.H.; Pogarell, O.; Leenders, K.L. Long-term changes of striatal dopamine D2 receptors in patients with Parkinson’s disease: a study with positron emission tomography and [11C]raclopride. Mov. Disord. 12:33–38; 1997. 225. Nordberg, A. Clinical studies in Alzheimer patients with positron emission tomography. Behav. Brain Res. 57:215– 224; 1993.