Toshiro Yamasaki

AGE-RELATED CHANGES IN HUMAN D1 DOPAMINE RECEPTORS MEASURED BY POSITRON EMISSION TOMOGRAPHY

The effects of age on the binding parameters of11C-SCH23390, the highly selective ligand for central D1 dopamine receptors, at specific binding sites in the brain were studied. Seventeen healthy male volunteers (20–72 years old) participated. Regional radioactivity in the brain was followed for 40 min by positron emission tomography (PET). A high accumulation of radioactivity was observed in the striatum and there was a conspicuous accumulation in the neocortex. A two-compartment model was used to obtain quantitative estimates of rate constants of association (k3) and dissociation (k4). The binding potential (k3/k4) of the dopamine D1 receptors in the striatum and frontal cortex decreased by 35% and 39%, respectively, with age. The value of k3 decreased by 58% in the striatum and 83% in the frontal cortex, whereas the value of k4 decreased by 35% in the striatum and 72% in the frontal cortex with age.

Computer-controlled large scale production of high specific activity [11C]RO 15-1788 for PET studies of benzodiazepine receptors.

Ethyl 8-fluoro-5,6-dihydro-5-[11C]methyl-6-oxo-4H-imidazo [1,5-a] [1,4]benzodiazepine-3-carboxylate ([11C]RO 15-1788) has been prepared automatically with high specific activity for in vivo visualization or quantitative analysis of brain benzodiazepine receptors. The yield, radiochemical yield, radiochemical purity and specific activity of the product ready for an i.v. injection were 276 +/- 76 mCi, 50.8 +/- 7.8%, 99.3 +/- 0.3% and 2.9 +/- 0.5 Ci/mumol, respectively, taking an average of the latest 3 runs. The time required was about 25 min. Each product was sufficient to carry out three successive clinical studies by positron emission tomography (PET). All the procedures other than evaporation and filtration at the final stage were carried out with specially designed equipment connected to a central control system for radioisotope production.

Synthesis and evaluation of11C-PK 11195 forin vivo study of peripheral-type benzodiazepine receptors using position emission tomography

The biodistribution of3H-PK 11195, an antagonist of the peripheral-type benzodiazepine receptors, was studied in mice. High accumulations of radioactivity in the heart, lung, spleen, kidney and adrenal were observed after intravenous injection of tracer amounts of3H-PK 11195 into the mice. The radioactivity in the heart, lung, spleen, kidney and adrenal was significantly decreased by the coadministration of carrier PK 11195, which indicated that PK 11195 specifically binds to the receptors. No radioactive metabolites were observed in the heart, lung and brain 20 min after intravenous administration of3H-PK 11195. The accumulation of3H-PK 11195 in the lung was not affected by pretreatment with either α-methyl benzylamine or imipramine, suggesting that3H-PK 11195 specifically binds to the receptors. The ratios of radioactivity of the kidney, adrenal and spleen to blood increased as a function of time, whereas that of the lung and heart rapidly reached to a steady state.11C-PK 11195 was synthesized by the N-methylation of desmethyl precursor yielding more than 100 mCi with high specific activity (more than 1.4 Ci/μmol). The labeling and purification procedure was completed within 23 min after the end of bombardment (EOB). The11C-PK 11195 solution for injection seems to have a high potential for thein vivo study of the peripheral-type benzodiazepine receptors in the living human by means of positron emission tomography (PET).

Detection of benzodiazepine receptor occupancy in the human brain by positron emission tomography

Benzodiazepine receptor occupancy in the brain following oral administration of clonazepam (CZP) with a dose of 30 μg/kg in six healthy young men and a further dose of 50 μg/kg in one of the subjects was estimated by carbon-11 labeled Ro15-1788 and positron emission tomography (PET). The effects of CZP on the latency of auditory event-related potentials (P300) were also studied. Overall brain 11C uptake was depressed and the % inhibition of 11C uptake in the gray matter of the brain at 30 min after [11C]Ro15-1788 injection was 15.3–23.5% (mean, n=6) following 30 μg/kg CZP when compared with that in the control experiment without any previous treatment. The 11C uptake in the cerebral cortex in the subject who received both doses decreased in a dose-related manner after 30 μg/kg and 50 μg/kg CZP. The P300 latency was prolonged significantly by 30 μg/kg CZP [31.6±16.3 ms (mean±SD, n=6), P<0.05]. The P300 latency in the same subject was prolonged in a dose-related manner by 30 μg/kg and 50 μg/kg CZP. The technique using [11C]Ro15-1788 and PET permits comparison of the pharmacological effects with the percentage of receptor sites which benzodiazepines occupy in the human brain. P300 also seems to be useful to investigate the pharmacological effects of benzodiazepines.

The detection of age-related decrease of dopamine D1, D2 and serotonin 5-HT2 receptors in living human brain

1. Using positron emission tomography and 11C-N-methylspiperone and 11C- SCH23390, dopamine D1 and D2 receptors, and serotonin 5-HT2 receptors in human brain were evaluated in living healthy age different subjects. 2. Dynamic analysis of PET data indicated that these receptor binding abilities decreased with aging in healthy humans.

Alterations in biodistribution of [3H]ro 15-1788 in mice by acute stress: Possible changes in in vivo binding availability of brain benzodiazepine receptor*

The biodistribution of [3H]Ro 15-1788 in control and stress-loaded mice (forced swimming) was compared. In control mice, carrier-free [3H]Ro 15-1788 was selectively and highly distributed in Bz receptor rich brain regions, while radioactivity in the brain was very low following administration of carrier-added tracer, which suggested that in vivo non-specific binding of this tracer was very low. Significant changes in biodistribution of carrier-free [3H]Ro 15-1788 were observed in stress-loaded mice, which strongly indicated that in vivo binding availability of Bz receptor in the brain was rapidly and reversibly reduced by acute stress. The degree of these changes was very dependent upon the stressful conditions, such as swimming duration and water temperature, and a significant alteration in biodistribution of [3H]Ro 15-1788 was particularly observed in the cerebral cortex. Simplified Scatchard analysis of in vivo binding of this tracer was performed, and results suggested that these alterations were mainly caused by changes in the Kd value rather than the Bmax value.

Quantitative in vivo analysis of benzodiazepine binding sites in the human brain using positron emission tomography

Central-type benzodiazepine binding sites were characterized in a single normal human subject, using positron emission tomography (PET) and the radiolabelled benzodiazepine antagonist, carbon-11 labelled flumazenil ([11C] Ro 15-1788). The subject was scanned using tracer alone and tracer plus 4 different concentrations of unlabelled Ro 15-1788, including one concentration of unlabelled Ro 15-1788, chosen to produce maximum displacement of [11C] Ro 15-1788 from specific binding sites. Concentrations of free, unmetabolized [11C] Ro 15-1788 in plasma were estimated using a simple extraction and ultrafiltration method. Radioactivity in the regional exchangeable pool in brain was estimated under non-saturation conditions from the ratio of radioactivity in brain to plasma, under saturation conditions and the kinetics of free ligand in plasma. The specific binding was, then, estimated by the difference between the total radioactivity in brain and exchangeable pool radioactivity. Scatchard analyses were performed to yield Bmax and Kd values under pseudo-equilibrium conditions, which was observed as an increase of specific binding/free with reduction in specific binding. In cerebral cortex and cerebellum, the Bmax values were about 62-73 nmol/l and the Kd values were 3.6-6 nM in the estimation of free ligand in plasma and 12-15 nM in the estimation of exchangeable pool in brain, as free in brain.

Dopamine D2 and serotonin S2 receptors in susceptibility to methamphetamine psychosis detected by positron emission tomography

Positron emission tomography (PET) was used to assess the role of dopamine D2 receptors in the striatum and serotonin S2 receptors in the frontal cortex in the susceptibility to methamphetamine-induced psychosis. Subjects were six men who had previously experienced methamphetamine psychosis (methamphetamine subjects) and 10 age- and sex-matched control subjects. The radiotracer used was 11C-N-methylspiperone. Although binding availability, assessed by dynamic analysis, in the two regions did not differ between the two groups, the ratio of binding availability in the striatum to that in the frontal cortex significantly decreased in the methamphetamine subjects as compared with the control subjects. These findings suggest that an imbalance in the activity of these two receptors may be related to the susceptibility to methamphetamine psychosis.

Synthesis and evaluation of [11C]cyanoimipramine.

[11C]Cyanoimipramine has been prepared by methylation of the desmethyl cyanoimipramine with [11C]methyl iodide. The chemically and radiochemically pure labelled product was obtained with a high specific activity (greater than 300 mCi/mumol). When 11C (or 3H)-cyanoimipramine was intravenously administered in mice, high accumulations were shown in brain and lung. Thirty minutes after injection of the tracer, differences were found in the radioactivity between the cerebral cortex and the cerebellum. The regional distribution of radioactivity in the rat brain 30 min after i.v. injection of [11C]cyanoimipramine was also examined, and the radioactivity was high in receptor rich areas (striatum, cerebral cortex etc.) but low in receptor poor area (cerebellum). The in vivo stability of [3H]cyanoimipramine was quite stable in the mouse brain for at least 30 min. Thirty minutes after injection, the radioactivity in the cerebral cortex of the carrier-added state was reduced as compared with the carrier-free state. Taken together, the in vivo specific binding of [3H]cyanoimipramine in the cerebral cortex was estimated at about 40-50% of the total radioactivity. Furthermore, the distribution of [3H]cyanoimipramine in the mice forced to swim was examined. Significant changes in the distribution of [3H]cyanoimipramine were observed in the cerebral cortex.

Measurement of Arterial Tracer Concentrations from Cardiac Pet Images

Objective Cardiac PET imaging offers the potential capability for the noninvasive measurement of arterial input function. This capability is somewhat limited, however, because of partial volume and spillover effects. The purpose of this study was to validate arterial tracer concentration measurements using PET images by assigning a region of interest (ROI) to the left ventricle, left atrium (LA), and ascending aorta. Materials and Methods We selected [11C]CO and [13N]ammonia as tracers, because the activity of [11C]CO is primarily in the blood pool and that of [13N]amtnonia primarily in myocardial tissue. Six [11C]CO and 11 [13N]ammonia PET scans were obtained in 17 subjects. Arterial tracer concentrations determined by PET were compared with those measured by well counting of arterial blood sampling obtained at the midpoint of each image acquisition. Results Arterial tracer concentrations as determined by LA-PET imaging correlated closely with those measured by blood sampling (r = 0.996, slope = 1.00 for [11C]CO; r = 0.841, slope = 0.974 for [13N]ammonia). Partial volume and spillover effects were minimized by assigning an ROI to the LA. Conclusion Arterial tracer concentrations can be measured noninvasively with cardiac PET. Index Terms Emission computed tomography—Heart—Tracers—Emission computed tomography, techniques.