- Email: [email protected]
Radiotracers for functional brain imaging
Radiotracers for Functional Brain Imaging Monte Blau
The rapid growth of nuclear medicine 25 years ago was in large part related to the success of brain tumor imaging using radiopharmaceuticals designed to detect changes in the blood-brain barrier (BBB). The success of computed tomography, and more recently nuclear magnetic resonance, in imaging these lesions has all but eliminated the use of radioactive agents for brain tumor detection. But, in recent years there has been a new wave of interest in isotope studies of the brain. The recent emphasis
has been on agents which enter the brain across the BBB and are designed to provide functional data ranging from regional perfusion and metabolism to the distribution of binding sites for neuroactive compounds. While none of these new radiopharmaceuticals has yet come into widespread clinical application, the research results already achieved clearly indicate that brain imaging will again be an important aspect of nuclear medicine practice.
HE MOST BASIC functional brain study is
the brain washout curves need not be corrected for recirculated xenon. Although it is usual to use multimillicurie amounts of xenon, it is still not possible to make adequate measurements with ordinary imaging devices. Xenon washout curves are determined either with a battery of probes 3 or a multicrystal imaging system with only moderate resolution. 4 Other freely diffuseable tracers have been proposed and have found limited use for regional brain blood flow imaging, but the xenon inhalation technique is by far the most widely used clinically. These agents include [~23I] iodoantipyrene, labeled nicotine, [~SF] fluoromethane, ~C butanol, and others.
T regional blood perfusion, and a number of agents have been found which provide measurements more or less directly related to regional perfusion. These fall into two general categories; freely diffuseable materials which cross the blood-brain barrier (BBB) in both directions, and agents which readily cross the BBB, but once in brain are trapped by one mechanism or another and maintain a fixed distribution. REGIONAL PERFUSION
Freely Diffuseable Tracers With the exception of a restricted number of materials which enter brain by specific facilitated transport systems (eg, glucose), entry into brain is restricted to neutral lipid-soluble molecules I of moderate molecular weight (mol wt) (< 500). 2 If these molecules do not undergo any metabolic or other changes once in brain, they leave the brain as easily as they entered. With freely diffuseable tracers, regional blood flow can be deduced either from the initial distribution or from the rate of disappearance, since both wash-in and washout are determined by the local blood flow. For practical reasons it is usually more convenient to measure the washout rates. The freely diffuseable agent in most common use for measuring regional brain perfusion by this technique is xenon gas labeled with ~33Xe or ~27Xe. Originally injected intraarterially, xenon gas is now usually administered by inhalation. In addition to providing a convenient route of administration, the lungs also serve to prevent recirculation of xenon back into brain once it has been washed out. Since xenon is essentially quantitatively removed from the blood by the lungs,
Seminars in Nuclear Medicine, Vol XV, No 4 (October), 1985
9 1985 b y G r une & S t r a t t o n , Inc.
Microsphere-Like Tracers To avoid the difficulties of measuring or imaging the rapidly changing local concentration of the freely diffuseable tracers, a series of agents have been developed that are intended to behave like microspheres. Ideally, these tracers have a 100% first-pass brain extraction with no subsequent washout. This provides fixed distribution in brain proportional to regional blood flow. Imaging this distribution is substantially less difficult than measuring washout rates of diffuseable tracers. Since none of the available agents is ideal, the quantitative determination of regional perfusion with microsphere-like agents depends on knowlFrom the Department of Radiology, Harvard Medical School Boston. Address reprint requests to Monte Blau, PhD, Department of Radiology, Harvard Medical School, 25 Shattuck St, Boston, MA 02115. 9 1985 by Grune & Stratton, Inc. 0001-2998/85/1504-0002505.00/0
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edge of (1) the first pass extraction fraction for all regions of brain at all blood flow rates from 0 to well above normal, (2) washout rates at all blood flows for all brain regions, (3) recirculation values for all flow rates and regions, and (4) counting efficiency and selfabsorption corrections to convert observed count rates to absolute tissue radioactivity. While these factors have been reasonably well studied for some of the positron-labeled agents, for the most part they are not well established for the single photon agents described below. It seems likely that for some time to come brain-blood flow determinations with these agents will be qualitative or semiquantitative, at best. The design of radiopharamaceuticals for these studies must permit ready passage across the BBB and, once inside brain, provide some mechanism which prevents washout. These two requirements are quite independent and to some extent, mutually exclusive. As with the freely diffuseable tracers, all of the microsphere-like brain perfusion agents cross the BBB because they are lipid soluble. Iodoarnphetamine. Of the single photon agents, N-isopropyl-p- [lZ3I]-iodoamphetamine (123I IMP) has had the widest application and is the best studied. First proposed by Winchell et al 5 in 1980, it was selected from a group of iodine-labeled amphetamine derivatives on the basis of its high uptake and slow washout from brain. There is evidence that the regional distribution of IMP is reasonably well correlated with blood flow.6"7 Although IMP was developed as an amphetamine analog, it is not totally clear that retention in brain is related to binding at amphetamine receptor sites. The usual criteria for receptor binding, including stereospecificity and blocking, have not been well established. Widely used in the United States and Europe for experimental studies, 123I IMP should soon be available for clinical use. A new drug application has been filed by Medi-Physics (Richmond, Calif), and this will no doubt be approved in due course. HIPDM. The second most studied (after IMP) single photon brain perfusion radiopharmaceutical is the "pH shift" agent developed by Kung, Tramposch, and BlauS: [I-123]N,N,N'trimethyl-N'- [2-hydroxy-3-methyl-5-iodoben-
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zyl]-l,3-propanediamine (HIPDM). HIPDM is one of a series of compounds designed to take advantage of the low pH in brain (pH = 7.0) compared to blood (pH = 7.4). 9 These compounds are lipid soluble and neutral at blood pH, and freely cross the BBB. Once inside brain the molecule picks up a H + and becomes charged and therefore unable to leave the brain. In spite of the totally dissimilar rationale behind the development of IMP and HIPDM, the molecular structures are remarkably similar (Fig 1). Neither compound has precisely the biological behavior predicted. Considered as amphetamine analogs, both fail to be blocked by excess carrier. 8 On the other hand, although both have the proper structure and physical chemical properties to behave as pH shift agents,8 the final brain-to-blood ratios observed for both compounds ( - 20) far exceed that predicted by theory. 1~At this point it is reasonable to conclude that no proposed single mechanism accounts for the high brain retention of these iodine-labeled amines. The biodistribution in rats studied by paired label experiments (1231 HIPDM and 1251IMP) is remarkably similar.8 However, some controversy exists on the brain uptake in humans. Comparative studies show a significantly higher uptake of IMP, 11 but on the other hand quantitative data from a number of laboratories indicate essentially equal uptake. 12 The choice between IMP and HIPDM will depend more on factors like the
~H3 /CHs CH=CHNHCH= ~~i ~CHs IMP OH
H3C~I)I~N ICH~ ~1~ HIC:3DM "cH3 Fig 1.
(A) Structure of IMP, (B) structure of HIPOM.
RADIOTRACERS FOR BRAIN IMAGING
purity of the mI label or availability rather than any inherent difference between the compounds. Other 1-labeled amines. A variety of Ilabeled amines have been proposed as brain imaging agents. In general, their structure is similar to IMP or HIPDM and in no case has a regional brain distribution or a mechanism of localization been established which would indicate significantly different behavior. These include a catecholamine analog, 13 piperazines, ]4 and phentermine analogs] 5 99mTc-labeled brain perfusion agents. The search for 99mTc-labeled compounds suitable for brain imaging has been pursued in several laboratories over the past 5 to 10 years. Lipophilic compounds have been reported with ligands such as aminoethanethiol]6 long chain alkyl derivatives of carbamoylmethyliminodiacetate,~7 oxines, 17 N,N diesters of EDTA and DTPA, ~8 tropolone] 9 2,4-pentanedione, 2~ and bis-aminoethanol.2~ None of these 99mTc-labeled chelates were reported to show significant brain uptake after intravenous (IV) injection. This may be due to protein binding 17or poor in vivo stability. More recently, at least two types of ligands have been shown to form 99mTccomplexes which cross the BBB. Tetradentate chelates based on the bis-aminoethanethiol (BAT) ring (Fig 2) had been prepared, but no biodistributions were reported. 22-24 Kung et a125 synthesized three derivatives of the BAT ring (tetramethyl, tetraethyl, and hexamethyl), and showed that these complexes were neutral and lipid soluble, and readily crossed the BBB following an IV injection in rats and monkeys. A second ring system, propyleneamine oxine (PnAO) (Fig 3), developed by Troutner et a126
\ / BAT Fig 2.
Structure of BAT ring.
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H(O HOi N
Fig 3.
N
Structure of PnAO ring.
also forms 99raTC complexes which cross the BBB. To provide a microsphere-like tracer for brain perfusion imaging with 99mTc it is necessary to alter the BAT or PnAO ring with a functional group which will provide retention in brain without interfering with the ability of the 99mTc complex to enter brain. One obvious approach is to add pH shift amine side chains similar to those in IMP or HIPDM. This has been done for the BAT ring27'28and the resulting complexes indeed show brain retention adequate (or nearly adequate) for single photon emission computed tomography (SPECT) imaging. Similar progress has been made in preparing suitable derivatives of the PnAO ring. 29 It is clear that 99mTc-labeled brain perfusion imaging will be a clinical reality in the very near future.
Positron-labeled agents for brain perfusion. Positron studies of local cerebral blood flow are also carried out with either a freely diffusible tracer (usually 150 labeled water 3~or a microsphere-like agent, 13N ammonia) ~Neutral ammonia is lipid soluble, crosses the BBB, and once inside brain cells is rapidly incorporated into amino acids. Miscellaneous perfusion agents. In all likelihood the mI-labeled amines and diamines will be the SPECT brain perfusion imaging agents for the next few years until they are supplanted by the 99raTC compounds. A number of other agents have been proposed, and while they may not have any advantages over the present 123Ior the coming 99mTcagents, the mechanism of brain retention is of some interest. Bodor et a123 have proposed a general scheme for delivery of drugs to brain based on a reduced
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carrier form that is lipid soluble and freely crosses the BBB. Once inside brain the carrier is oxidized to a charged form which traps the drug. This system has been applied by Knapp 33 to I-labeled brain perfusion agents with moderate success. A similar redox derivative of one of the neutral 99raTc ligands might have some promise as a mechanism for keeping these agents in brain. Fukushi et a134 found that 18F-labeled 6-F9-benzylpurine was lipid soluble enough to have a high brain uptake. In addition, they observed a very long retention (Tl/z > 1 hour). This was due to the rapid enzymatic dehalogenation and the inability of the free F- ions to recross the BBB. Similar behavior was found for the iodo- and bromo-benzylpurines. Enzymatic or spontaneous decomposition of a compound or complex which results in a charged fragment containing the radioactive label provides a very useful general mechanism for brain retention. The relatively unstable 2~ chelate T1-DDC (diethyldithiocarbamate) which has high brain uptake and retention35 may function by this self-destruct mechanism. The retention of the new 99mTc-PnAO derivatives 29 in brain must be the result of similar in vivo instability. METABOLIC INDICATORS Although local cerebral blood flow and local metabolism are usually strongly coupled in normal brain, in the presence of pathology blood flow may not reflect metabolism. For positron imaging, several agents are in use which accurately measure metabolism. Unfortunately, no SPECT radiopharmaceuticals are available that have distributions related to metabolism. Because of the strict molecular structure requirements for metabolic agents, it is difficult to predict that such agents will become available in the near future.
ISFFluorodeoxyglucose Glucose is the principal energy substrate for brain metabolism. Fluorodeoxyglucose (FDG), like glucose itself, is transported into brain by a specific facilitated transport system. Once inside brain, FDG is phosphorylated by brain hexokinase, but the metabolic product FDG-6-PO4 cannot be further metabolized. Passage back across the BBB is not possible so the FDG is
"metabolically trapped." Using plasma measurements and brain levels it is possible to calculate local glucose metabolism from a knowledge of the rate constants of a simple glucose metabolism model. 36 Where it is desireable to make repeated measurements, l~C-labeled deoxyglucose has been used because of the convenient 20-minute halflife. 37
150 Oxygen Using nonequilibrium (bolus injection)38 or equilibrium (continuous inhalation)39methods, it is possible to measure oxygen metabolism and oxygen extraction fraction instead of glucose metabolism. RECEPTOR IMAGING AGENTS
Although it is too early to predict ultimate clinical utility, there is currently great interest in the visualization of the distribution of neuroreceptors. Many compounds have been proposed for receptor site imaging, but very few have met the rather stringent requirements to prove that the regional brain distribution reflects receptor sites. 40,41 Since there are very few transport mechanisms for neuroreceptor agonists or antagonists, initial uptake into brain is almost always related to lipid solubility. Therefore, the early pattern of brain distribution for the neuroreceptor imaging agents is essentially one of perfusion blood flow. ~ With time, the bulk of the agent is washed out and only the fraction bound to receptor sites remains. It is only following the washout of the unbound agent that the distribution of activity indicates receptor site distribution. The strict requirements for structural specificity in receptor binding strongly favor the use of positron-emitting radionuclides, either HClabeled drug analogs or molecules with an 18F for H substitution. However, at least one I-labeled compound has been developed. Possible use of 99mTC for these purposes seems remote at the present time.
Muscarinic-AcetylcholineReceptors The only authentic SPECT receptor imaging agent is 123I-labeled (R)-3-quinuclidinyl-4-iodobenzilate (I-123IQNB) developed by Eckelman et a142 for the visualization of cerebral muscar-
RADIOTRACERS FOR BRAIN IMAGING
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inic-acetylcholine receptors. This compound is stereospecific, can be blocked by excess QNB, and shows a brain distribution pattern quite different from perfusion agents. 42
Doparnine Receptors A variety of ~SF- and "C-labeled agents have been developed as possible dopamine-receptor imaging agents. The widest clinical experience has been with HC-labeled 3-N-methyl-spiperone developed by Wagner et al. 43'44This neuroleptic ligand binds preferentially to D2 dopamine and $2 serotonin receptors. Other dopamine-receptor scanning agents which have been tested clinically and show some specifity include tSF fluorodopa 45 and HC pimozide. 46 A variety of 18F and HC derivatives of haloperidol, spiroperidol, and other neuroleptics have been synthesized and studied in vitro and in animal systems, but little if any human data exists for these compounds.
Other Receptors While the dopamine receptor has been the focus of much attention, a variety of agents have been or are being developed for the imaging of other important brain receptors. These include ~C flunitrozepam for benzodiazepine receptors, 47 =lC carbomethoxyfentany148 for opiate receptors,"C mesulergin for serotonin-2 receptors, 49and HC dimethyltryptamine for serotonin1 receptors. 5~ Not strictly receptor binding, but related to neurotransmitter amine studies, are a new class of agents designed to visualize the distribution of monoamine oxidase (MAO) activity. N1Ndimethylphenylethylamine labeled with HC at the N-methyl position is metabolically trapped in brain when deaminated because it becomes positively charged and cannot recross the BBB. The local concentration of radioactivity is proportional to the regional MAO activity. 51
REFERENCES
1. Oldendorf WH: Lipid solubility and drug penetration of the blood brain barrier. Proc Soc Exp Biol Med 147:813816, 1974 2. Levin VA: Relationship of octanol/water partition coefficient and molecular weight to rat brain capillary permeability. J Med Chem 23:682-684, 1980 3. Lassen NA, Ingvar D, Skinhoj E: Brain function and blood flow. Sci Am 239:62-71, 1978 4. Stokely EM, Sveinsdottir E, Lassen NA, et al: A single photon dynamic computer assisted tomograph (DCAT) for imaging brain function in multiple cross-sections. J Comput Assist Tomogr 4:230-240, 1980 5. Winchell HS, Baldwin RM, Lin TH: Development of I-123 labeled amines for brain studies: Localization of I-123 iodophenylalkylamines in rat brain. J Nucl Med 21:940-946, 19go
6. Kuhl DE, Barrio JR, Huang SC, et al: Quantifying local cerebral blood flow by N-isopropyl-p-(1-123)-iodoamphetamine (IMP) tomography. J Nucl Med 23:196-203, 1982 7. Lassen NA, Henriksen L, Holm S, et al: Cerebral blood flow tomography: Xenon-133 compared with isopropylamphetamine-l-123. J Nucl Med 24:17-21, 1983 8. Kung HF, Tramposch KM, Blau M: A new brain perfusion imaging agent: [I-123] HIPDM: N,N,N-trimethyl-N- [2-hydroxy-3-methyl-5-iodobenzyl]- 1,3-propanediamine. J Nucl Med 24:66-72, 1983 9. Kung HF, Blau M: Regional intracellular pH shift: A proposed new mechanism for radiopharmaceutical uptake in brain and other tissues. J Nucl Med 21:147-152, 1980 10. Loberg MD: Radiotracers for cerebral functional imaging-A new class. J Nucl Med 21:183-185, 1980 11. Holman BL, Lee RGL, Hill TC, et al: A comparison
of two cerebral perfusion tracers, N-isopropyl 1-123 piodoamphetamine and 1-123 HIPDM, in the human. J Nucl Med 25:25-30, 1984 12. Kung HF, Blau M: Re: A comparison of two cerebral perfusion tracers, N-isopropyl I- 123 p-iodoamphetamine and 1-123 HIPDM in the human. J Nucl Med 25:945, 1984 13. Sargent T, Budinger TF, Braun G, et al: An iodinated catecholamine congener for brain imaging and metabolic studies. J Nucl Med 19:71-76, 1978 14. Hanson RN, E1-Shourbagy T, Hassan M: Radioiodinated 1-substituted-4-phenyl piperazines as potential brain imaging agents. Fifth International Symposium on Radiopharmaceutical Chemistry, Tokyo, 1984, p 92 15. Elmaleh DR, Kizuka H, Garneau J, et al: Synthesis and evaluation of p-iodophentermine (IP) as a brain perfusion imaging agent. J Nucl Med 25:32, 1984 16. Burns HD, Manspeaker H, Miller R, et al: Preparation and biodistribution of neutral, lipid soluble Tc-99m complexes of bis (2-mercaptoethyl) amine ligands. J Nud Med 20:654, 1979 17. Loberg MD, Corder EH, Fields AT, et al: Membrane transport of Tc-99m-labeled radiopharmaceuticals. I. Brain uptake by passive transport. J Nucl Med 20:1181-1188, 1979 18. Karesh SM, Eckelman WC, Reba RC: Biological distribution of chemical analogs of fatty acid and long chain hydrocarbons containing a strong chelating agent. J Pharmacol Sci 66:225-228, 1977 19. Spitznagle LA, Marino CA, Kasina S: Lipophilic chelates of technetium-99m:tropolone. J Nucl Med 22:13, 1981 20. Packard AB, Srivastava SC, Richards P, et al: Tech-
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netium-99 complexes of oxygen and nitrogen donor ligands. J Nucl Med 22:70, 1981 21. Kramer AV, Epps LA, Ranganathan N, et al: Synthesis and characterization of neutral aminoethanol complexes of technetium. Fourth International Symposium on Radiopharmaceutical Chemistry, Julich, Germany, 1982, pp 323324 22. Burns HD, Dannals RF, Dannals TE, et al: Synthesis of tetradentate aminothiol ligands and their technetium complexes. J Label Comp 18:54-55, 1981 23. Corbin JL, Work DE: 1-Akyl- (or aryl-) amino2-methylpropane-2-thiols. Some bi- and tetradentate nitrogen-sulfur ligands from Schiff's base disulfides. J Org Chem 41:489-491, 1976 24. Dannals RF: The preparation and characterization of nitrogen-sulfur donor ligands and their technetium complexes. PhD Thesis, Johns Hopkins University, Baltimore, 1981, pp 98-205 25. Kung HF, Molnar M, Billings J, et al: Synthesis and biodistribution of neutral lipid-soluble Tc-99m complexes that cross the blood-brain barrier. J Nucl Med 25:326-332, 1984 26. Troutner DE, Volkert WA, Hoffman T J, et al: A tetra-dentate amine oxine complex of Tc-99m. J Nucl Med 24:10, 1984 27. Kung HF, Efange S, Yu CC, et al: Synthesis and biodistribution of Tc-99m bisaminoethanethiol (BAT) complexes with amine sidechains. J Nucl Med 26:18, 1985 (abstr) 28. Lever SZ, Burns HD, Kervitsky TM, et al: The preparation and biodistribution of a technetium-99m triaminodithiol complex designed to reflect regional cerebral blood flow. J Nucl Med 26:18, 1985 (abstr) 29. Chaplin SB, Oberle PO, Hoffman T J, et al: Regional brain uptake and retention of Tc99m-propylene amine oxine derivatives. J Nucl Med 26:18, 1985 (abstr) 30. Ter-Pogossian MM, Eichling JO, Davis DO, et al: The determination of regional blood flow by means of water labeled with radioactive oxygen-15. Radiology 93:31-40, 1969 31. Phelps ME, Hoffman E J: Factors which affect cerebral uptake and retention of NH 3. Stroke 8:694-702, 1977 32. Bodor N, Farag HH, Brewster ME: Site-specific sustained release of drugs to the brain. Science 214:13701372, 1981 33. Knapp FF: Redox based brain perfusion imaging agents. Presented at Amphetamines and pH-shift Agents for Brain Imaging Workshop, Rheinisch-Westfalische Gesellschaft fur Nuklearmedizin, Bonn, 1984 34. Fukushi K, Irie T, Inoue O, et al: 6-halo-9-benzylpurines as carriers of various radiohalogens into brain. Fifth International Symposium on Radiopharmaceutical Chemistry. Tokyo, 1984, pp 90-92 35. Vyth A, Fennema PJ, Van der School JB: T1-201diethyl-dithiocarbamate: A possible radiopharmaceutical for brain imaging. Pharm Weekbl (Sci) 5:213-216, 1983 36. Reivich M, Kuhl D, Wolf AP, et al: The F-18fluorodeoxyglucose method for the measurement of local
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cerebral glucose utilization in man. Circ Res 44:127-137, 1979 37. Reivich M, Alavi A, Wolf AP, et al: The use of 2-deoxy-D[1-C-11] glucose for the determination of local cerebral glucose metabolism in humans. J Cereb Blood Flow Metab 2:307-320, 1982 38. Raichle ME, Grubb RL, Eichling JO, et al: Measurement of brain oxygen utilization with radiotracer oxygen- 15. Experimental verification. J Appl Physio140:638~40, 1976 39. Frakowiak RSJ, Lenzi G-L, Jones T, et al: Quantitative measurement of regional cerebral blood flow and oxygen metabolism in man using O-15 and positron emission tomography: Theory, procedure, and normal values. J Comput Assist Tomogr 4:727-736, 1980 40. Eckelman WC (ed): Receptor-Binding Radiotracers, Vol 1. Boca Raton, Fla, CRC, 1982 41. Katzenellenbogen JA, Carlson KE, Heiman DF, et al: Receptor binding as a basis for radiopharmaceutical design, Spencer RP (ed): Radiopharmaeeuticals. Structure-Activity Relationships. Orlando, Fla, Grune and Stratton, 1981, p 23 42. Eckleman WC, Reba RC, Rzeszotarski, et al: External imaging of cerebral muscarinic acetylcholine receptors. Science 223:291-292, 1984 43. Wagner HN, Burns HD, Dannals RF, et al: Imaging dopamine receptors in the human brain by positron tomography. Science 221:1264-1266, 1983 44. Wong DF, Wagner HN, Dannals RF, et al: Effects of age on dopamine and serotonin receptors measured by positron tomography in the living human brain. Science 226:1393-1396, 1984 45. Garnett RS, Firnau G, Nahmias C: Dopamine visualized in the basal ganglia of living man. Nature 305:137-138, 1983 46. Baron JC, Comar D, Zarifian E, et al: An in vivo study of the dopaminergic receptors in the brain of man using C-t l-pimozide and positron emision tomography, in Magistretti PL (ed): Functional Radionuclide Imaging of the Brain. New York, Raven, 1983, pp 337-345 47. Comar DF, Maziere M, Cepeda C, et al: The kinetics and displacement of C-11 flunitrazepam in the brain of the living baboon. Eur J Pharmacol 75:21-26, 1981 48. Frost J J, Dannals RF, Ravert HT, et al: Imaging opiate receptors with positron emission tomography. J Nucl Med 25:73, 1984 49. Kloster G, Hanus J, Voges R, et al: C-I l-mesulergin, a potential agent for mapping the serotonin receptor: Synthesis and animal experiments. Fifth International Symposium on Radiopharmaceutical Chemistry, Tokyo, 1984, pp 173174 50. Yanai K, Takahashi K, Ishiwata K, et al: C-11labeled indolealkylamines as potential serotonin-1 receptor mapping agents: Synthesis and biodistribution Fifth International Symposium on Radiopharmaceutical Chemistry, Tokyo, 1984, pp 170-173 5I. Inoue O, Tominaga T, Suzuki K, et al: Development of new type of radiotracers for in vivo estimation of brain M A t activity: N-methyl labeled phenethylamine derivatives. Fifth International Symposium on Radiopbarmaceutical Chemistry, Tokyo, 1984, pp 197-199
The rapid growth of nuclear medicine 25 years ago was in large part related to the success of brain tumor imaging using radiopharmaceuticals designed to detect changes in the blood-brain barrier (BBB). The success of computed tomography, and more recently nuclear magnetic resonance, in imaging these lesions has all but eliminated the use of radioactive agents for brain tumor detection. But, in recent years there has been a new wave of interest in isotope studies of the brain. The recent emphasis
has been on agents which enter the brain across the BBB and are designed to provide functional data ranging from regional perfusion and metabolism to the distribution of binding sites for neuroactive compounds. While none of these new radiopharmaceuticals has yet come into widespread clinical application, the research results already achieved clearly indicate that brain imaging will again be an important aspect of nuclear medicine practice.
HE MOST BASIC functional brain study is
the brain washout curves need not be corrected for recirculated xenon. Although it is usual to use multimillicurie amounts of xenon, it is still not possible to make adequate measurements with ordinary imaging devices. Xenon washout curves are determined either with a battery of probes 3 or a multicrystal imaging system with only moderate resolution. 4 Other freely diffuseable tracers have been proposed and have found limited use for regional brain blood flow imaging, but the xenon inhalation technique is by far the most widely used clinically. These agents include [~23I] iodoantipyrene, labeled nicotine, [~SF] fluoromethane, ~C butanol, and others.
T regional blood perfusion, and a number of agents have been found which provide measurements more or less directly related to regional perfusion. These fall into two general categories; freely diffuseable materials which cross the blood-brain barrier (BBB) in both directions, and agents which readily cross the BBB, but once in brain are trapped by one mechanism or another and maintain a fixed distribution. REGIONAL PERFUSION
Freely Diffuseable Tracers With the exception of a restricted number of materials which enter brain by specific facilitated transport systems (eg, glucose), entry into brain is restricted to neutral lipid-soluble molecules I of moderate molecular weight (mol wt) (< 500). 2 If these molecules do not undergo any metabolic or other changes once in brain, they leave the brain as easily as they entered. With freely diffuseable tracers, regional blood flow can be deduced either from the initial distribution or from the rate of disappearance, since both wash-in and washout are determined by the local blood flow. For practical reasons it is usually more convenient to measure the washout rates. The freely diffuseable agent in most common use for measuring regional brain perfusion by this technique is xenon gas labeled with ~33Xe or ~27Xe. Originally injected intraarterially, xenon gas is now usually administered by inhalation. In addition to providing a convenient route of administration, the lungs also serve to prevent recirculation of xenon back into brain once it has been washed out. Since xenon is essentially quantitatively removed from the blood by the lungs,
Seminars in Nuclear Medicine, Vol XV, No 4 (October), 1985
9 1985 b y G r une & S t r a t t o n , Inc.
Microsphere-Like Tracers To avoid the difficulties of measuring or imaging the rapidly changing local concentration of the freely diffuseable tracers, a series of agents have been developed that are intended to behave like microspheres. Ideally, these tracers have a 100% first-pass brain extraction with no subsequent washout. This provides fixed distribution in brain proportional to regional blood flow. Imaging this distribution is substantially less difficult than measuring washout rates of diffuseable tracers. Since none of the available agents is ideal, the quantitative determination of regional perfusion with microsphere-like agents depends on knowlFrom the Department of Radiology, Harvard Medical School Boston. Address reprint requests to Monte Blau, PhD, Department of Radiology, Harvard Medical School, 25 Shattuck St, Boston, MA 02115. 9 1985 by Grune & Stratton, Inc. 0001-2998/85/1504-0002505.00/0
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edge of (1) the first pass extraction fraction for all regions of brain at all blood flow rates from 0 to well above normal, (2) washout rates at all blood flows for all brain regions, (3) recirculation values for all flow rates and regions, and (4) counting efficiency and selfabsorption corrections to convert observed count rates to absolute tissue radioactivity. While these factors have been reasonably well studied for some of the positron-labeled agents, for the most part they are not well established for the single photon agents described below. It seems likely that for some time to come brain-blood flow determinations with these agents will be qualitative or semiquantitative, at best. The design of radiopharamaceuticals for these studies must permit ready passage across the BBB and, once inside brain, provide some mechanism which prevents washout. These two requirements are quite independent and to some extent, mutually exclusive. As with the freely diffuseable tracers, all of the microsphere-like brain perfusion agents cross the BBB because they are lipid soluble. Iodoarnphetamine. Of the single photon agents, N-isopropyl-p- [lZ3I]-iodoamphetamine (123I IMP) has had the widest application and is the best studied. First proposed by Winchell et al 5 in 1980, it was selected from a group of iodine-labeled amphetamine derivatives on the basis of its high uptake and slow washout from brain. There is evidence that the regional distribution of IMP is reasonably well correlated with blood flow.6"7 Although IMP was developed as an amphetamine analog, it is not totally clear that retention in brain is related to binding at amphetamine receptor sites. The usual criteria for receptor binding, including stereospecificity and blocking, have not been well established. Widely used in the United States and Europe for experimental studies, 123I IMP should soon be available for clinical use. A new drug application has been filed by Medi-Physics (Richmond, Calif), and this will no doubt be approved in due course. HIPDM. The second most studied (after IMP) single photon brain perfusion radiopharmaceutical is the "pH shift" agent developed by Kung, Tramposch, and BlauS: [I-123]N,N,N'trimethyl-N'- [2-hydroxy-3-methyl-5-iodoben-
MONTE BLAU
zyl]-l,3-propanediamine (HIPDM). HIPDM is one of a series of compounds designed to take advantage of the low pH in brain (pH = 7.0) compared to blood (pH = 7.4). 9 These compounds are lipid soluble and neutral at blood pH, and freely cross the BBB. Once inside brain the molecule picks up a H + and becomes charged and therefore unable to leave the brain. In spite of the totally dissimilar rationale behind the development of IMP and HIPDM, the molecular structures are remarkably similar (Fig 1). Neither compound has precisely the biological behavior predicted. Considered as amphetamine analogs, both fail to be blocked by excess carrier. 8 On the other hand, although both have the proper structure and physical chemical properties to behave as pH shift agents,8 the final brain-to-blood ratios observed for both compounds ( - 20) far exceed that predicted by theory. 1~At this point it is reasonable to conclude that no proposed single mechanism accounts for the high brain retention of these iodine-labeled amines. The biodistribution in rats studied by paired label experiments (1231 HIPDM and 1251IMP) is remarkably similar.8 However, some controversy exists on the brain uptake in humans. Comparative studies show a significantly higher uptake of IMP, 11 but on the other hand quantitative data from a number of laboratories indicate essentially equal uptake. 12 The choice between IMP and HIPDM will depend more on factors like the
~H3 /CHs CH=CHNHCH= ~~i ~CHs IMP OH
H3C~I)I~N ICH~ ~1~ HIC:3DM "cH3 Fig 1.
(A) Structure of IMP, (B) structure of HIPOM.
RADIOTRACERS FOR BRAIN IMAGING
purity of the mI label or availability rather than any inherent difference between the compounds. Other 1-labeled amines. A variety of Ilabeled amines have been proposed as brain imaging agents. In general, their structure is similar to IMP or HIPDM and in no case has a regional brain distribution or a mechanism of localization been established which would indicate significantly different behavior. These include a catecholamine analog, 13 piperazines, ]4 and phentermine analogs] 5 99mTc-labeled brain perfusion agents. The search for 99mTc-labeled compounds suitable for brain imaging has been pursued in several laboratories over the past 5 to 10 years. Lipophilic compounds have been reported with ligands such as aminoethanethiol]6 long chain alkyl derivatives of carbamoylmethyliminodiacetate,~7 oxines, 17 N,N diesters of EDTA and DTPA, ~8 tropolone] 9 2,4-pentanedione, 2~ and bis-aminoethanol.2~ None of these 99mTc-labeled chelates were reported to show significant brain uptake after intravenous (IV) injection. This may be due to protein binding 17or poor in vivo stability. More recently, at least two types of ligands have been shown to form 99mTccomplexes which cross the BBB. Tetradentate chelates based on the bis-aminoethanethiol (BAT) ring (Fig 2) had been prepared, but no biodistributions were reported. 22-24 Kung et a125 synthesized three derivatives of the BAT ring (tetramethyl, tetraethyl, and hexamethyl), and showed that these complexes were neutral and lipid soluble, and readily crossed the BBB following an IV injection in rats and monkeys. A second ring system, propyleneamine oxine (PnAO) (Fig 3), developed by Troutner et a126
\ / BAT Fig 2.
Structure of BAT ring.
331
H(O HOi N
Fig 3.
N
Structure of PnAO ring.
also forms 99raTC complexes which cross the BBB. To provide a microsphere-like tracer for brain perfusion imaging with 99mTc it is necessary to alter the BAT or PnAO ring with a functional group which will provide retention in brain without interfering with the ability of the 99mTc complex to enter brain. One obvious approach is to add pH shift amine side chains similar to those in IMP or HIPDM. This has been done for the BAT ring27'28and the resulting complexes indeed show brain retention adequate (or nearly adequate) for single photon emission computed tomography (SPECT) imaging. Similar progress has been made in preparing suitable derivatives of the PnAO ring. 29 It is clear that 99mTc-labeled brain perfusion imaging will be a clinical reality in the very near future.
Positron-labeled agents for brain perfusion. Positron studies of local cerebral blood flow are also carried out with either a freely diffusible tracer (usually 150 labeled water 3~or a microsphere-like agent, 13N ammonia) ~Neutral ammonia is lipid soluble, crosses the BBB, and once inside brain cells is rapidly incorporated into amino acids. Miscellaneous perfusion agents. In all likelihood the mI-labeled amines and diamines will be the SPECT brain perfusion imaging agents for the next few years until they are supplanted by the 99raTC compounds. A number of other agents have been proposed, and while they may not have any advantages over the present 123Ior the coming 99mTcagents, the mechanism of brain retention is of some interest. Bodor et a123 have proposed a general scheme for delivery of drugs to brain based on a reduced
332
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carrier form that is lipid soluble and freely crosses the BBB. Once inside brain the carrier is oxidized to a charged form which traps the drug. This system has been applied by Knapp 33 to I-labeled brain perfusion agents with moderate success. A similar redox derivative of one of the neutral 99raTc ligands might have some promise as a mechanism for keeping these agents in brain. Fukushi et a134 found that 18F-labeled 6-F9-benzylpurine was lipid soluble enough to have a high brain uptake. In addition, they observed a very long retention (Tl/z > 1 hour). This was due to the rapid enzymatic dehalogenation and the inability of the free F- ions to recross the BBB. Similar behavior was found for the iodo- and bromo-benzylpurines. Enzymatic or spontaneous decomposition of a compound or complex which results in a charged fragment containing the radioactive label provides a very useful general mechanism for brain retention. The relatively unstable 2~ chelate T1-DDC (diethyldithiocarbamate) which has high brain uptake and retention35 may function by this self-destruct mechanism. The retention of the new 99mTc-PnAO derivatives 29 in brain must be the result of similar in vivo instability. METABOLIC INDICATORS Although local cerebral blood flow and local metabolism are usually strongly coupled in normal brain, in the presence of pathology blood flow may not reflect metabolism. For positron imaging, several agents are in use which accurately measure metabolism. Unfortunately, no SPECT radiopharmaceuticals are available that have distributions related to metabolism. Because of the strict molecular structure requirements for metabolic agents, it is difficult to predict that such agents will become available in the near future.
ISFFluorodeoxyglucose Glucose is the principal energy substrate for brain metabolism. Fluorodeoxyglucose (FDG), like glucose itself, is transported into brain by a specific facilitated transport system. Once inside brain, FDG is phosphorylated by brain hexokinase, but the metabolic product FDG-6-PO4 cannot be further metabolized. Passage back across the BBB is not possible so the FDG is
"metabolically trapped." Using plasma measurements and brain levels it is possible to calculate local glucose metabolism from a knowledge of the rate constants of a simple glucose metabolism model. 36 Where it is desireable to make repeated measurements, l~C-labeled deoxyglucose has been used because of the convenient 20-minute halflife. 37
150 Oxygen Using nonequilibrium (bolus injection)38 or equilibrium (continuous inhalation)39methods, it is possible to measure oxygen metabolism and oxygen extraction fraction instead of glucose metabolism. RECEPTOR IMAGING AGENTS
Although it is too early to predict ultimate clinical utility, there is currently great interest in the visualization of the distribution of neuroreceptors. Many compounds have been proposed for receptor site imaging, but very few have met the rather stringent requirements to prove that the regional brain distribution reflects receptor sites. 40,41 Since there are very few transport mechanisms for neuroreceptor agonists or antagonists, initial uptake into brain is almost always related to lipid solubility. Therefore, the early pattern of brain distribution for the neuroreceptor imaging agents is essentially one of perfusion blood flow. ~ With time, the bulk of the agent is washed out and only the fraction bound to receptor sites remains. It is only following the washout of the unbound agent that the distribution of activity indicates receptor site distribution. The strict requirements for structural specificity in receptor binding strongly favor the use of positron-emitting radionuclides, either HClabeled drug analogs or molecules with an 18F for H substitution. However, at least one I-labeled compound has been developed. Possible use of 99mTC for these purposes seems remote at the present time.
Muscarinic-AcetylcholineReceptors The only authentic SPECT receptor imaging agent is 123I-labeled (R)-3-quinuclidinyl-4-iodobenzilate (I-123IQNB) developed by Eckelman et a142 for the visualization of cerebral muscar-
RADIOTRACERS FOR BRAIN IMAGING
333
inic-acetylcholine receptors. This compound is stereospecific, can be blocked by excess QNB, and shows a brain distribution pattern quite different from perfusion agents. 42
Doparnine Receptors A variety of ~SF- and "C-labeled agents have been developed as possible dopamine-receptor imaging agents. The widest clinical experience has been with HC-labeled 3-N-methyl-spiperone developed by Wagner et al. 43'44This neuroleptic ligand binds preferentially to D2 dopamine and $2 serotonin receptors. Other dopamine-receptor scanning agents which have been tested clinically and show some specifity include tSF fluorodopa 45 and HC pimozide. 46 A variety of 18F and HC derivatives of haloperidol, spiroperidol, and other neuroleptics have been synthesized and studied in vitro and in animal systems, but little if any human data exists for these compounds.
Other Receptors While the dopamine receptor has been the focus of much attention, a variety of agents have been or are being developed for the imaging of other important brain receptors. These include ~C flunitrozepam for benzodiazepine receptors, 47 =lC carbomethoxyfentany148 for opiate receptors,"C mesulergin for serotonin-2 receptors, 49and HC dimethyltryptamine for serotonin1 receptors. 5~ Not strictly receptor binding, but related to neurotransmitter amine studies, are a new class of agents designed to visualize the distribution of monoamine oxidase (MAO) activity. N1Ndimethylphenylethylamine labeled with HC at the N-methyl position is metabolically trapped in brain when deaminated because it becomes positively charged and cannot recross the BBB. The local concentration of radioactivity is proportional to the regional MAO activity. 51
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