Autoradiographic localization of 5-HT(2A) receptors in the human brain using [(3)H]M100907 and [(11)C]M100907.

M100907 (MDL 100907, R-(+)-alpha-(2, 3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol++ +) is a new selective antagonist of 5-HT(2A) receptors. The compound has been labeled with (11)C and proved useful for in vivo studies of 5-HT(2A) receptors using positron emission tomography (PET). In the present study the distribution of 5-HT(2A) receptors was examined in the postmortem human brain using whole hemisphere autoradiography and [(3)H]M100907 and [(11)C]M100907. The autoradiograms showed very dense binding to all neocortical regions, whereas the hippocampus was only weakly labeled with [(3)H]M100907. Other central brain regions, such as the basal ganglia and thalamus, showed low [(3)H]M100907 binding, reflecting low densities of 5-HT(2A) receptors. The cerebellum or structures of the brain stem were virtually devoid of 5-HT(2A) receptors. [(11)C]M100907 gave images qualitatively similar to those of [(3)H]M100907, although with lower spatial resolution. The labeling of human 5-HT(2A) receptors with [(3)H]M100907 was inhibited by the addition of the 5-HT(2A) receptor blockers ketanserin or SCH 23390 (10 microM), leaving a very low background of nonspecific binding. The 5-HT(1A) receptor antagonist WAY-100635 and the D(2)-dopamine receptor antagonist raclopride had no effect on the binding of [(3)H]M100907. The selective labeling of 5-HT(2A) receptors with [(3)H]M100907 clearly shows that this compound is suitable for further studies of the human 5-HT(2A) receptor subtype in vitro. The in vitro autoradiography of the distribution of 5-HT(2A) receptors obtained with radiolabeled M100907 provides detailed qualitative and quantitative information on the distribution of 5-HT(2A)-receptors in the human brain as well as reference information for the interpretation of previous initial results at much lower resolution in humans in vivo with PET and [(11)C]M100907.

Characterization of bromine-76-labelled 5-bromo-6-nitroquipazine for PET studies of the serotonin transporter.

The development of suitable radioligands for brain imaging of the serotonin transporter is of great importance for the study of depression and other affective disorders. The potent and selective serotonin transporter ligand, 5-iodo-6-nitro-2-piperazinylquinoline, has been labelled with iodine-123 and used as a radioligand for single photon emission computerized tomography. To evaluate the potential of the bromine-76-labelled analogue, 5-bromo-6-nitroquipazine, as a radioligand for positron emission tomography (PET), its brain distribution and binding characteristics were examined in rats. In vivo brain distribution and ex vivo autoradiography demonstrated that [76Br]5-bromo-6-nitroquipazine enters the brain rapidly. The regional brain distribution of [76Br]5-bromo-6-nitroquipazine was consistent with the known distribution of serotonin transporters in the midbrain, pons, thalamus, striatum, and neocortex. Specific binding was inhibited by the selective serotonin reuptake inhibitor citalopram. The peripheral metabolism in plasma was rapid, but more than 90% of the radioactivity in brain represented unchanged radioligand 2 h postinjection (p.i.). A preliminary PET study was also performed in a baboon. Following the intravenous injection of [76Br]5-bromo-6-nitroquipazine in a baboon, there was a conspicuous accumulation of radioactivity in thalamus, striatum, and pons. The radioactivity in these brain regions was 1.5 times higher than in the cerebellum at 3 h and 2.5-4 times higher at 24 h. A rapid metabolism of the radioligand in plasma was observed (38% unchanged after 5 min). The results indicate that [76Br]5-bromo-6-nitroquipazine has potential for PET imaging of the serotonin transporter.

Carbon-11 epidepride: a suitable radioligand for PET investigation of striatal and extrastriatal dopamine D2 receptors.

Epidepride [(S)-(-)-N-([1-ethyl-2-pyrrolidinyl]methyl)-5-iodo-2,3-dimethoxybenza mide] binds with a picomolar affinity (Ki = 24 pM) to the dopamine D2 receptor. Iodine-123-labeled epidepride has been used previously to study striatal and extrastriatal dopamine D2 receptors with single photon emission computed tomography (SPECT). Our aim was to label epidepride with carbon-11 for comparative quantitative studies between positron emission tomography (PET) and SPECT. Epidepride was synthesized from its bromo-analogue FLB 457 via the corresponding trimethyl-tin derivative. In an alternative synthetic pathway, the corresponding substituted benzoic acid was reacted with the optically pure aminomethylpyrrolidine-derivative. Demethylation of epidepride gave the desmethyl-derivative, which was reacted with [11C]methyl triflate. Total radiochemical yield was 40-50% within a total synthesis time of 30 min. The specific radioactivity at the end of synthesis was 37-111 GBq/micromol (1,000-3,000 Ci/mmol). Human postmortem whole-hemisphere autoradiography demonstrated dense binding in the caudate putamen, and also in extrastriatal areas such as the thalamus and the neocortex. The binding was inhibited by unlabeled raclopride. PET studies in a cynomolgus monkey demonstrated high uptake in the striatum and in several extrastriatal regions. At 90 min after injection, uptake in the striatum, thalamus and neocortex was about 11, 4, and 2 times higher than in the cerebellum, respectively. Pretreatment experiment with unlabeled raclopride (1 mg/kg) inhibited 50-70% of [11C]epidepride binding. The fraction of unchanged [11C]epidepride in monkey plasma determined by a gradient high performance liquid chromatography (HPLC) method was about 30% of the total radioactivity at 30 min after injection of [11C]epidepride. The availability of [11C]epidepride allows the PET-verification of the data obtained from quantitation studies with SPECT.

Different brain radioactivity curves in a PET study with [11C]beta-CIT labelled in two different positions.

The cocaine congener 2beta-carbomethoxy-3beta-(4'-iodophenyl)tropane (beta-CIT) has a chemical structure that enables labelling with carbon-11 either by N-methylation or by O-methylation. The regional brain uptake of [N-methyl-11C]beta-CIT and [O-methyl-11C]beta-CIT was compared in cynomolgus monkeys using positron emission tomography (PET). The striatal uptake of radioactivity after intravenous injection of [O-methyl-11C]beta-CIT reached a plateau at 30-40 min, whereas the uptake of [N-methyl-11C]beta-CIT increased continuously during the time of the PET measurement. Two of the putative labelled metabolites, [N-methyl-11C]beta-CIT-acid and [O-methyl-11C]nor-beta-CIT were prepared and examined with PET to investigate if they may enter the brain and thus add to the radioactivity uptake obtained with [11C]beta-CIT. Less than 0.4% of injected [N-methyl-11C]beta-CIT-acid entered the brain whereas 5-6% of [O-methyl-11C]nor-beta-CIT entered the brain and accumulated in the striatum and in the thalamus. The fraction of [O-methyl-11C]nor-beta-CIT obtained in plasma after intravenous injection of [O-methyl-11C]nor-beta-CIT, however, never exceeded 3%. Consequently, the formation of [N-methyl-11C]beta-CIT-acid and [O-methyl-11C]nor-beta-CIT cannot be the explanation for the different time-activity curves in the monkey brain demonstrated with [11C]beta-CIT labelled in two different positions. An unidentified labelled lipophilic metabolite, detected in monkey plasma after injection of [O-methyl-11C]beta-CIT, remains as the only possible explanation for the differences between [N-methyl-11C]beta-CIT and [O-methyl-11C]beta-CIT.

[11C]NNC 22-0215, a metabolically stable dopamine D1 radioligand for PET.

NNC 22-0215 has been found to be a metabolically stable dopamine D1 antagonist with high affinity and selectivity for D1 receptors in vitro. We prepared [11C]NNC 22-0215 with a specific radioactivity of about 50 GBq/micromol at time of administration. In PET experiments with [11C]NNC 22-0215 there was a rapid uptake of radioactivity in the cynomolgus monkey brain (1.8% of total radioactivity injected). Radioactivity accumulated most markedly in the striatum and the neocortex. The striatum to cerebellum ratio was about 4, with specific binding that remained at a plateau level from 50 min to 100 min after injection. Binding in the striatum and neocortex was markedly displaced by SCH 23390, whereas binding in the cerebellum was not reduced. Metabolite studies showed that about 80% of the radioactivity in the monkey plasma represented unchanged radioligand 30 min after injection. The rate of metabolism in monkey plasma in vivo was also determined for a series of structurally related 11C-labelled benzazepines, previously used as dopamine D1 receptor ligands for PET. Results indicate a significantly slower rate of metabolism for [11C]NNC 22-0215 than for any of the previously labelled benzazepines. Thus [11C]NNC 22-0215 has potential for imaging of selective binding to the dopamine D1 receptors in the human brain with high count rates at time of equilibrium.

Characterisation of the appearance of radioactive metabolites in monkey and human plasma from the 5-HT1A receptor radioligand, [carbonyl-11C]WAY-100635--explanation of high signal contrast in PET and an aid to biomathematical modelling.

N-(2-(4-(2-Methoxy-phenyl)-1-piperazin-1-yl)ethyl)-N-(2-pyridyl)++ +cyclohexanecarboxamide (WAY-100635), labelled in its amido carbonyl group with 11C (t1/2 = 20.4 min), is a promising radioligand for the study of brain 5-HT1A receptors with positron emission tomography (PET). Thus, in PET experiments in six cynomolgus monkeys and seven healthy male volunteers, [carbonyl-11C]WAY-100635 was taken up avidly by brain. Radioactivity was retained in regions rich in 5-HT1A receptors, such as occipital cortex, temporal cortex and raphe nuclei, but cleared rapidly from cerebellum, a region almost devoid of 5-HT1A receptors. [Carbonyl-11C]WAY-100635 provides about 3- and 10-fold higher signal contrast (receptor-specific to nonspecific binding) than [O-methyl-11C]WAY-100635 in receptor-rich areas of monkey and human brain, respectively. To elucidate the effect of label position on radioligand behaviour and to aid in the future biomathematical interpretation of the kinetics of regional cerebral radioactivity uptake in terms of receptor-binding parameters, HPLC was used to measure [carbonyl-11C]WAY-100635 and its radioactive metabolites in plasma at various times after intravenous injection. Radioactivity cleared rapidly from monkey and human plasma. Parent radioligand represented 19% of the radioactivity in monkey plasma at 47 min and 8% of the radioactivity in human plasma at 40 min. [Carbonyl-11C]desmethyl-WAY-100635 was below detectable limits in monkey plasma and at most a very minor radioactive metabolite in human plasma. [11C]Cyclohexanecarboxylic acid was identified as a significant radioactive metabolite. In human plasma this maximally represented 21% of the radioactivity at 10 min after radioligand injection. All other major radioactive metabolites in monkey and human plasma were even more polar. No-carrier-added [carbonyl-11C]cyclohexanecarboxylic acid was prepared in the laboratory and after intravenous administration into cynomolgus monkey was shown with PET to give only a low uptake of radioactivity into brain tissue. The acid rapidly gave rise to several radioactive metabolites of higher polarity in plasma. The observed lack of any significant metabolism of [carbonyl-11C]WAY-100635 to highly lipophilic or pharmacologically potent radioactive compounds is consistent with its high signal contrast in primate brain.

[carbonyl-11C]Desmethyl-WAY-100635 (DWAY) is a potent and selective radioligand for central 5-HT1A receptors in vitro and in vivo.

[carbonyl-11C]Desmethyl-WAY-100635 (DWAY) is possibly a low-level metabolite appearing in plasma after intravenous administration of [carbonyl-11C]WAY-100635 to human subjects for positron emission tomographic (PET) imaging of brain 5-HT1A receptors. In this study we set out to assess the ability of DWAY to enter brain in vivo and to elucidate its possible interaction with 5-HT1A receptors. Desmethyl-WAY-100635 was labelled efficiently with carbon-11 (t1/2 = 20.4 min) in high specific radioactivity by reaction of its descyclohexanecarbonyl analogue with [carbonyl-11C]cyclohexanecarbonyl chloride. The product was separated in high radiochemical purity by high-performance liquid chromatography (HPLC) and formulated for intravenous injection. Rats were injected intravenously with DWAY, sacrificed at known times and dissected to establish radioactivity content in brain tissues. At 60 min after injection, the ratios of radioactivity concentration in each brain region to that in cerebellum correlated with previous in vitro and in vivo measures of 5-HT1A receptor density. The highest ratio was about 22 in hippocampus. Radioactivity cleared rapidly from plasma; HPLC analysis revealed that DWAY represented 55% of the radioactivity in plasma at 5 min and 33% at 30 min. Only polar radioactive metabolites were detected. Subsequently, a cynomolgus monkey was injected intravenously with DWAY and examined by PET. Maximal whole brain uptake of radioactivity was 5.7% of the administered dose at 5 min after injection. The image acquired between 9 and 90 min showed high radioactivity uptake in brain regions rich in 5-HT1A receptors (e.g. frontal cortex and neocortex), moderate uptake in raphe nuclei and low uptake in cerebellum. A transient equilibrium was achieved in cortical regions at about 60 min, when the ratio of radioactivity concentration in frontal cortex to that in cerebellum reached 6. The corresponding ratio for raphe nuclei was about 3. Radioactive metabolites appeared rapidly in plasma, but these were all more polar than DWAY, which represented 52% of the radioactivity in plasma at 4 min and 20% at 55 min. In a second PET experiment, in which a cynomolgus monkey was pretreated with the selective 5-HT1A receptor antagonist, WAY-100635, at 25 min before DWAY injection, radioactivity in all brain regions was reduced to that in cerebellum. Autoradiography of post mortem human brain cryosections after incubation with DWAY successfully delineated 5-HT1A receptor distribution. Receptor-specific binding was eliminated in the presence of the selective 5-HT1A receptor agonist, 8-OH-DPAT [(+/-)-8-hydroxy-2-dipropylaminotetralin]. These findings show that: (a) intravenously administered DWAY is well able to penetrate brain in rat and monkey, (b) DWAY is a highly effective radioligand for brain 5-HT1A receptors in rat and monkey in vivo and for human brain in vitro, and (c) the metabolism and kinetics of DWAY appear favourable to successful biomathematical modelling of acquired PET data. Thus, DWAY warrants further evaluation as a radioligand for PET studies of 5-HT1A receptors in human brain.

PET imaging of central 5-HT2A receptors with carbon-11-MDL 100,907.

UNLABELLED: Serotonergic 5-HT2A receptors are of central interest in the complex pathophysiology of schizophrenia. These receptors have also been proposed as putative targets for atypical antipsychotic drugs. Suitable radioligands for 5-HT2A receptors are required to evaluate this hypothesis in vivo with PET. MDL 100,907 is a highly selective 5-HT2A receptor antagonist that is currently being developed as a potential antipsychotic drug. We have previously reported on the preparation of [11C]MDL 100,907 and initial characterization of [11C]MDL 100,907 binding in the monkey brain. In this preliminary PET study, the regional distribution and binding kinetics of [11C]MDL 100,907 were examined in healthy men. METHODS: A PET examination was performed in each of three subjects after intravenous injection of [11C]MDL 100,907. The metabolite-corrected arterial input function was used in a kinetic analysis according to the standard three-compartment model. RESULTS: The highest radioactivity concentration was observed in the neocortex, whereas radioactivity was lower in the cerebellum, pons, thalamus, striatum and white matter. The binding potential (BP) in the neocortical regions was 4-6 times higher, whereas BP in the striatum was slightly higher than that in the cerebellum, demonstrating a regional distribution in good agreement with 5-HT2A receptor densities measured in vitro. The BP in the cerebellum was small but not negligible. CONCLUSION: This preliminary study suggests that [11C]MDL 100,907 is a suitable PET radioligand for studies on 5-HT2A receptors in man. The high selectivity of MDL 100,907 represents a major advantage as compared to presently available radioligands with poor selectivity. Thus, [11C]MDL 100,907 is recommended in the future for PET studies in healthy subjects and schizophrenic patients, including the determination of drug-induced 5-HT2A receptor occupancy.

[18F] beta-CIT-FP is superior to [11C] beta-CIT-FP for quantitation of the dopamine transporter.

beta-CIT-FP [N-(3-fluoropropyl)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)nortropane] is a cocaine analogue with high affinity for the dopamine transporter. Positron emission tomography (PET) studies with [O-methyl-11C] beta-CIT-FP ([11C] beta-CIT-FP) has shown that equilibrium conditions were approached but, however, not reached at the end of measurement. Moreover, metabolite studies of [11C] beta-CIT-FP in monkey plasma demonstrated a lipophilic-labelled metabolite that may enter the brain. We therefore labelled beta-CIT-FP with fluorine-18 in a position that may avoid the formation of labelled lipophilic metabolites. The more long-lived radionuclide (18F) was used to allow for measurements over longer time. [N-fluoropropyl- 18F] beta-CIT-FP ([18F] beta-CIT-FP) was prepared by N-alkylation of nor-beta-CIT with [18F]fluoropropyl bromide. PET studies were performed in cynomolgus monkeys. [18F] beta-CIT-FP entered the brain rapidly. There was a high concentration of radioactivity in the striatum and much lower in the thalamus, neocortex, and cerebellum. The striatum-to-cerebellum ratio was about 5 at time of transient equilibrium, which occurred after 60 to 100 min. After pretreatment with GBR 12909, radioactivity in the striatum was markedly reduced, thus indicating specific [18F] beta-CIT-FP binding to the dopamine transporter. The fraction of unchanged [18F] beta-CIT-FP determined by HPLC was 10-15% after 140 min. No lipophilic labelled metabolites were detected. The absence of measurable lipophilic labelled metabolites and the occurrence of transient equilibrium within the time of the PET measurement indicate that [18F] beta-CIT-FP is superior to [11C] beta-CIT-FP as a PET radioligand for quantification of the dopamine transporter in the human brain.

PET-characterization of [carbonyl-11C]WAY-100635 binding to 5-HT1A receptors in the primate brain.

[carbonyl-11C]WAY-100635 is a new radioligand which can be used with positron emission tomography (PET) to provide high contrast delineation of human brain regions that are rich in 5-HT1A receptors. In the present PET study, the binding of [carbonyl-11C]WAY-100635 was characterized in the cynomolgus monkey brain. Pretreatment with each of the two reference compounds, WAY-100635 and 8-OH-DPAT, as well as the drugs buspirone and pindolo, induced a marked inhibition of [carbonyl-11C]WAY-100635 binding in the neocortex and the raphe nuclei. A preliminary Scatchard analysis yielded 5-HT1A receptor density values of the same order as those that have been reported in vitro. The study shows that [carbonyl-11C]WAY-100635 binds specifically to 5-HT1A receptors in the primate brain and has potential for determination of 5-HT1A receptor occupancy and density in psychiatric patients.

In vitro and in vivo characterisation of nor-beta-CIT: a potential radioligand for visualisation of the serotonin transporter in the brain.

Radiolabelled 2beta-carbomethoxy-3beta-(4-iodophenyl)tropane (beta-CIT) has been used in clinical studies for the imaging of dopamine and serotonin transporters with single-photon emission tomography (SPET). 2beta-Carbomethoxy-3beta-(4-iodophenyl)nortropane (nor-beta-CIT) is a des-methyl analogue of beta-CIT, which in vitro has tenfold higher affinity (IC50=0.36 nM) to the serotonin transporter than beta-CIT (IC50=4.2 nM). Nor-beta-CIT may thus be a useful radioligand for imaging of the serotonin transporter. In the present study iodine-125 and carbon-11 labelled nor-beta-CIT were prepared for in vitro autoradiographic studies on post-mortem human brain cryosections and for in vivo positron emission tomography (PET) studies in Cynomolgus monkeys. Whole hemisphere autoradiography with [125I]nor-beta-CIT demonstrated high binding in the striatum, the thalamus and cortical regions of the human brain. Addition of a high concentration (1 microM) of citalopram inhibited binding in the thalamus and the neocortex, but not in the striatum. In PET studies with [11C]nor-beta-CIT there was rapid uptake of radioactivity in the monkey brain (6% of injected dose at 15 min) and high accumulation of radioactivity in the striatum, thalamus and neocortex. Thalamus to cerebellum and cortex to cerebellum ratios were 2.5 and 1.8 at 60 min, respectively. The ratios obtained with [11C]nor-beta-CIT were 20%-40% higher than those previously obtained with [11C]beta-CIT. Radioactivity in the thalamus and the neocortex but not in the striatum was displaceable with citalopram (5 mg/kg). In conclusion, nor-beta-CIT binds to the serotonin transporter in the primate brain in vitro and in vivo and has potential for PET and SPET imaging of the serotonin transporter in human brain.

Autoradiographic localization of 5-HT1A receptors in the post-mortem human brain using [3H]WAY-100635 and [11C]way-100635.

The distribution of 5-HT1A receptors was examined in the post-mortem human brain using whole hemisphere autoradiography and the selective 5-HT1A receptor antagonist [3H]WAY-100635 ([O-methyl-3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridinyl)cyclohexanecarboxamide trihydrochloride). The autoradiograms showed very dense binding to hippocampus, raphe nuclei and neocortex. The labeling in neocortex was slightly lower than in the hippocampus and was mainly at superficial layers, although a faintly labeled band could be seen in deeper neocortical layers. Other regions, such as the amygdala, septum and claustrum, showed low densities caudatus and putamen, in cerebellum or in structures of the brain stem except in the raphe nuclei. The labeling of human 5-HT1A receptors with [3H]WAY-100635 was antagonised by the addition of 5-HT1A receptor ligands, 5-HT, buspirone, pindolol or 8-OH-DPAT (10 microM), leaving a very low background of non-specific binding. Saturation analysis of semiquantitative data from several human regions indicated that [3H]WAY-100635 has a Kd of approximately 2.5 nM. The selective labeling of 5-HT1A receptors with [3H]WAY-100635 clearly show that this compound is useful for further studies of the human 5-HT1a receptor subtype in vitro [11C]WAY-100635 is used for the characterization of 5-HT1A receptors with positron emission tomography (PET). WAY-100635 was also radiolabeled with the short-lived positron-emitting radionuclide carbon-11 (t1/2 = 20 min) and used for in vitro autoradiography on human whole hemisphere cryosections. [11C]WAY-100635 gave images qualitatively similar to those of [3H]WAY-100635, although with a lower resolution. Thus, the hippocampal formation was densely labeled, with lower density in the neocortex. Buspirone, pindolol or 8-OH-DPAT (10 microM), blocked all binding of [11C]WAY-100635. The in vitro autoradiography of the distribution of 5-HT1A receptors obtained with radiolabeled WAY-100635 provide detailed qualitative and quantitative information on the distribution of 5-HT1A-receptors in the human brain. Moreover, the studies give reference information for the interpretation of previous initial results at much lower resolution in humans with PET and [11C]Way-100635. These data provide a strong basis for expecting [11C]WAY-100635 to behave as a highly selective radioligand in vivo.

Characterization of the radioactive metabolites of the 5-HT1A receptor radioligand, [O-methyl-11C]WAY-100635, in monkey and human plasma by HPLC: comparison of the behaviour of an identified radioactive metabolite with parent radioligand in monkey using PET.

N-(2-(4-(2-Methoxy-phenyl)-1-piperazin-1-yl)ethyl)-N-(2-pyridyl) cyclohexanecarboxamide (WAY-100635), labelled in the O-methyl group with carbon-11 (t1/2 = 20.4 min), is a promising radioligand for application with positron emission tomography (PET) to the study of 5-HT1A receptors in living human brain. An understanding of the metabolism of this new radioligand is crucial to the development of a biomathematical model for the interpretation of the kinetics of radioactivity uptake in brain in terms of receptor-binding parameters. After intravenous injection of [O-methyl-11C]WAY-100635 into humans, radioactivity was found to clear rapidly from blood and plasma. By using established methods for the analysis of radioactivity in plasma, it was found that intravenously injected [O-methyl-11C]WAY-100635 is rapidly metabolised to more polar radioactive compounds in a cynomolgus monkey and in humans. Thus, at 60 min postinjection, parent radioligand represented 40% and 5% of the radioactivity in monkey and human plasma, respectively. In monkey and human, one of the radioactive metabolites was identified as the descyclohexanecarbonyl analogue of the parent radioligand, namely [O-methyl-11C]WAY-100634. This compound is known to have high affinity for 5-HT1A receptors and alpha 1-adrenoceptors. In a PET experiment it was demonstrated that, after IV injection of [O-methyl-11C]WAY-100634 into a cynomolgus monkey, radioactivity was avidly taken up by brain. Uptake of radioactivity was higher in 5-HT1A receptor-rich frontal cortex than in cerebellum, which is devoid of 5-HT1A receptors. Polar radioactive metabolites appeared in plasma. The results suggest that the use of WAY-100635 labelled with carbon-11 in its cyclohexanecarbonyl moiety may provide enhanced signal contrast in PET studies and a possibility to develop a simple biomathematical model for regional brain radioactivity uptake.

[11C]beta-CIT-FE, a radioligand for quantitation of the dopamine transporter in the living brain using positron emission tomography.

The cocaine analogue beta-CIT-FE (N-(2-fluoroethyl)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)nortropane) was labeled with 11C for positron emission tomography (PET) studies of the dopamine transporter. After intravenous administration to a cynomolgus monkey, [11C]beta-CIT-FE accumulated in the striatum with a striatum-to-cerebellum ratio of about 9 after 60 min. Pseudoequilibrium of specific [11C]beta-CIT-FE binding in the striatum was obtained within 30-50 min. The radioactivity ratios of the thalamus to the cerebellum and the neocortex to the cerebellum were about 2 and 1.5, respectively. In displacement and pretreatment experiments, radioactivity in the striatum but not in the cerebellum was reduced after injection of beta-CIT or the dopamine transporter inhibitor GBR 12909, indicating that striatal radioactivity following injection of [11C]beta-CIT-FE represents reversible binding to dopamine transporter sites. After displacement or pretreatment with cocaine there was a marked effect not only in the striatum but also in the thalamus and neocortex. [11C]beta-CIT-FE has potential as a useful PET radioligand for quantitation of the dopamine transporter in the primate brain in vivo.

[11C]MDL 100907, a radioligland for selective imaging of 5-HT(2A) receptors with positron emission tomography.

The highly selective 5-HT2A receptor antagonist, MDL 100907 ((R)-(+)-4 -(l-hydroxy-1-(2,3-dimethoxyphenyl)methyl)-N -2-(4-fluorophenylethyl)piperidine), was labeled with 11C for Positron Emission Tomography (PET) studies. After i.v. injection of (R)-(+)-[3-OCH3-11C]MDL 100907 [11C]MDL 100907) in Cynomolgus monkeys a marked accumulation in the 5-HT2A receptor rich neocortical regions was obtained with a neocortex to cerebellum ratio of 3.5-4.5 after 60-80 minutes. In the neocortical regions a transient equilibrium occurred within 40-60 minutes. Radioactivity in the neocortex, but not in the cerebellum, was reduced after injection of ketanserin, indicating that neocortical radioactivity following injection of [11C]MDL 100907 represents specific binding to 5-HT2A receptors. There was no evident effect on neocortical binding after pretreatment with raclopride or SCH 23390. [11C]MDL 100907 has potential to become the first selective radioligand for PET-quantitation of 5-HT2A receptors in the human brain in vivo.

[O-methyl-11C]beta-CIT-FP, a potential radioligand for quantitation of the dopamine transporter: preparation, autoradiography, metabolite studies, and positron emission tomography examinations.

beta-CIT-FP [N-(3-fluoropropyl)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)nortropane] is a cocaine analogue with a high affinity for the dopamine transporter. [O-methyl-11C]beta-CIT-FP ([11C]beta-CIT-FP) was prepared by O-alkylation of the free acid with [11C]methyl iodide. The total radiochemical yield of [11C]beta-CIT-FP was 50 to 60% with an overall synthesis time of 30 min. The radiochemical purity was > 99%, and the specific radioactivity at time of injection was about 37 GBq/mumol (1000 Ci/mmol). Autoradiographic examination of [11C]beta-CIT-FP binding in human brain postmortem demonstrated specific binding in the caudate nucleus and putamen. Positron emission tomography (PET) examination of [11C]beta-CIT-FP in a Cynomolgus monkey demonstrated accumulation in the striatum with a striatum-to-cerebellum ratio of about 8 after 60 min. Equilibrium in the striatum was attained within 70 to 90 min. The radioactivity ratios of thalamus/cerebellum and neocortex/cerebellum were about 2 and 1.5, respectively. In a displacement experiment, radioactivity in the striatum but not in the cerebellum was reduced after injection of beta-CIT, indicating that striatal radioactivity following injection of [11C]beta-CIT-FP is associated with dopamine transporter sites and that the binding is reversible. The fraction of the total radioactivity in plasma representing [11C]beta-CIT-FP determined by high-performance liquid chromatography (HPLC) was 84% at 15 min and 50% at 95 min. [11C]beta-CIT-FP should be a useful PET radioligand for the quantitation of dopamine transporters in the human brain in vivo.