Human dopamine receptor subtypes--in vitro binding analysis using 3H-SCH 23390 and 3H-raclopride

Affinities and regional densities of the D1- and D2-dopamine receptor subtypes were studied in the human post-mortem brain in vitro using the two selective radioligands 3H-SCH 23390 and 3H-raclopride. 3H-Raclopride binding was confined to the caudate nucleus, the putamen and the substantia nigra, while 3H-SCH 23390 bound to cortical regions as well. The binding of 3H-SCH 23390 was reduced by a low concentration of ketanserin, indicating binding to 5-HT2 receptors in addition to the D1-dopamine receptors. The endogenous neurotransmitter dopamine interacted potently both with the D1-dopamine receptor and the D2-dopamine receptor, displaying two affinity states for each subtype. The distribution of the dopamine receptor subtypes obtained in the present in vitro investigation is in agreement with data obtained with 11C-SCH 23390 and 11C-raclopride in positron emission tomographic studies in human volunteers.

An open label trial of raclopride in acute schizophrenia. Confirmation of D2-dopamine receptor occupancy by PET

Raclopride, a highly selective D2-dopamine receptor antagonist, was administered in doses up to 4 mg b.i.d. to ten schizophrenic patients in an open label non-comparative study lasting 4 weeks. Safety, tolerability, potential antipsychotic effect, prolactin response and drug effect on plasma homovanillic acid were evaluated. Central D2-dopamine receptor occupancy was determined by positron emission tomography (PET). No major deviations were found in biochemical and physiological safety parameters. Raclopride was well tolerated. The mean BPRS score was reduced by 55% at endpoint. In the global evaluation seven patients were "very much" or "much" improved. Extrapyramidal side effects were recorded in four patients and disappeared after dose reduction or single doses of biperiden. An increase in plasma prolactin of short duration was observed in both sexes. A significant decrease of plasma HVA was obtained after 4 weeks of treatment. In two of the patients the central D2-dopamine receptors occupancy was measured using PET. The receptor occupancy was 68 and 72% which is the same as that found in patients treated with conventional neuroleptics.

Stereoselective binding of 11C-raclopride in living human brain--a search for extrastriatal central D2-dopamine receptors by PET

The selective D2-dopamine receptor antagonist raclopride and its pharmacologically inactive (R)-enantiomer FLB472 were labelled with 11C and used in a study with positron emission tomography to examine the stereoselectivity of 11C-raclopride binding to central D2-dopamine receptors in three healthy men. After the injection of 11C-raclopride, there was a high accumulation of radioactivity in the dopamine-rich basal ganglia, whereas after the injection of 11C-FLB472 there was no such accumulation of radioactivity. Thus, the binding of 11C-raclopride is stereoselective. Distribution ratios [radioactivity in a brain region/"free" (not protein-bound) radioactivity in plasma] were calculated for the two enantiomers to study regional differences in the accumulation of radioactivity. The distribution ratios in white matter were similar for the two enantiomers. In the putamen, a three to four-fold higher distribution ratio was found for 11C-raclopride than for 11C-FLB472, reflecting the presence of specific binding of 11C-raclopride binding to D2-dopamine receptors in the basal ganglia. In the temporal and frontal cortices the distribution ratios were, however, only a few per cent higher for 11C-raclopride than for 11C-FLB472, indicating that if D2-dopamine receptors are present in the human neocortex, then their density is indeed very low.

PET scanning--a new tool in clinical psychopharmacology

Quantitative methods were developed for the determination of dopamine and benzodiazepine receptor characteristics in the living human brain by positron emission tomography (PET). As ligands, the 11C-labelled analogues of the selective antagonists of dopamine receptor subtypes, SCH 23390 and raclopride, and the benzodiazepine antagonist, Ro 15-1788, were used. Tracer amounts of the ligands were injected intravenously into healthy volunteers and schizophrenic patients. The distribution of ligand indicated high densities of D1 as well as D2 dopamine receptors in the basal ganglia. Binding of [11C]-SCH 23390 was also significant in the neocortex where it was shown to represent binding to D1 as well as to 5-HT2 serotonin receptors. High densities of specific benzodiazepine receptor binding were obtained in most neocortical brain areas and in the cerebellum. Using saturation procedures, Bmax and Kd values could be obtained for D2 and benzodiazepine receptors. A comparison of D2 receptor densities in drug-naive schizophrenic patients and healthy volunteers demonstrated similar receptor characteristics in the major basal ganglia in these groups of subjects. Different chemical classes of conventional and unconventional antipsychotic drugs produced a 65%-85% occupancy of D2 receptors when given in clinical doses to schizophrenic patients. High does of diazepam produced a marked occupancy of benzodiazepine receptors during the first hours after oral administration to healthy volunteers. These in vivo methods should be valuable tools for the further analysis of the effects of drug on neuroreceptors in the living brain of neuropsychiatric patients.

Central D2-dopamine receptor occupancy in schizophrenic patients treated with antipsychotic drugs

Using positron emission tomography and the carbon 11-labeled ligand raclopride, central D2-dopamine receptor occupancy in the putamen was determined in psychiatric patients treated with clinical doses of psychoactive drugs. Receptor occupancy in drug-treated patients was defined as the percent reduction of specific carbon 11-raclopride binding in relation to the expected binding in the absence of drug treatment. Clinical treatment of schizophrenic patients with 11 chemically distinct antipsychotic drugs (including both classic and atypical neuroleptics such as clozapine) resulted in a 65% to 85% occupancy of D2-dopamine receptors. In a depressed patient treated with the tricyclic antidepressant nortriptyline, no occupancy was found. The time course for receptor occupancy and drug levels was followed after withdrawal of sulpiride or haloperidol. D2-dopamine receptor occupancy remained above 65% for many hours despite a substantial reduction of serum drug concentrations. In a sulpiride-treated patient, the dosage was reduced in four steps over a nine-week period and a curvilinear relationship was demonstrated between central D2-dopamine receptor occupancy and serum drug concentrations. The results demonstrate that clinical doses of all the currently used classes of antipsychotic drugs cause a substantial blockade of central D2-dopamine receptors in humans. This effect appears to be selective for the antipsychotics, since it was not induced by the antidepressant nortriptyline.

Raclopride, a new selective ligand for the dopamine-D2 receptors

1. The use of raclopride, a new compound of the salicylamide series, as a ligand for the labelling of dopamine-D2 receptors in vitro and in vivo is described. 2. 3H-Raclopride has a high affinity for the dopamine-D2 receptors (Kd = 1 nM in rat striatum) with much less affinity for any other receptor. 3. 3H-Raclopride enters the brain easily and has therefore also been used in in vivo binding and autoradiography. The nonspecific binding is very low both in vitro and in vivo. 4. Raclopride has been labelled with 11C, and is used as a marker for dopamine-D2 receptors in the living human brain using positron emission tomography.

Workshop on schizophrenia, PET, and dopamine D2 receptors in the human neostriatum

Recently, two research groups published numbers for D2 receptor sites in the neostriatum of drug-naive schizophrenic patients, obtained in vivo by positron emission tomography (PET). One study appeared to confirm the increase of D2 receptor numbers, while the other study did not. A workshop was convened in Montreal to examine the reasons for the discrepancy between the results obtained by the two groups. The workshop considered patient populations, PET instrumentation and scanning methods, pharmacology, and modeling. The workshop identified differences between the approaches of the two groups that could contribute to the divergent results, including age and chronicity of the patient samples, brain region selected for study, metabolism of the different radioligands in blood and brain, reversibility of binding, PET instrumentation, and complexity of data analysis. The workshop concluded that these initial efforts had made considerable progress in establishing the role of PET in the understanding of the biochemical processes underlying mental illness. In particular, the unique ability to quantify regional neuroreceptor density at different stages in the evolution of the disease has been implemented. At the same time, the work so far and this conference served to identify the main sources contributing to the different findings from the two centers. This information will be important in designing the next phase of the research which will build upon and reconcile these apparent discrepancies.

Remoxipride--a new potential antipsychotic drug. Pharmacological effects and pharmacokinetics following repeated oral administration in male volunteers

Remoxipride, a new potential antipsychotic drug, was administered over 4 days at two dose levels, 70 and 140 mg t.i.d., to eight healthy male volunteers. Pharmacokinetics, safety, tolerability, and effect on plasma prolactin levels were evaluated. Remoxipride exhibited essentially linear pharmacokinetics. Only minor deviations in biochemical and physiological safety parameters were found. The drug was well tolerated by all subjects at the 70 mg dose level. At 140 mg akathisia appeared in seven subjects. The drug induced a rapid and transient increase in plasma prolactin concentrations at both dose levels after single doses. During steady state, a significant reduction in the prolactin response was observed as compared to after the first dose.

Muscle biopsy findings, conduction velocity and refractory period of single motor nerve fibres in schizophrenia

Eight untreated and eight neuroleptic treated male schizophrenic patients were studied. Light and electron microscopical analysis of muscle biopsies from the anterior tibial muscle showed a spectrum of pathological changes without significant quantitative or qualitative differences between the two groups. The changes included atrophic fibres, central nuclei, "moth-eaten fibres", "ring fibres", fibre splitting and subsarcolemmal and intermyofibrillar glycogen droplets. Electrophysiological investigation of single motor unit properties showed impaired peripheral impulse propagation in both patient groups while the conduction velocity and the refractory period of single motor nerve fibres were within the same range as in healthy subjects. In conclusion there are neuromuscular abnormalities in schizophrenic patients which cannot be attributed to medication or drug abuse.

Transplantation in Parkinson's disease: two cases of adrenal medullary grafts to the putamen

Autologous adrenal medullary tissue was transplanted unilaterally to the putamen in two patients with severe Parkinson's disease. The first patient exhibited a transient, two-day improvement of motor performance in the limbs contralateral to the implantation site. He also had significantly longer episodes of normal function for about two months. The second patient reported a minor improvement of balance and gait, again lasting for two months. Electrophysiological studies of the motor readiness and auditory evoked potentials were consistent with increased catecholaminergic activity in the basal ganglia after transplantation in both patients. Positron emission tomography showed no postoperative alteration of receptor density in the putamen. No significant adverse effects of the transplantation were observed in the patients' performance on neurological and psychological tests or in their quantitative electroencephalogram and cerebral blood flow recordings. Immediately after the grafting, one patient showed transient signs of sympathetic hyperactivity, probably caused by release of catecholamines from the implanted tissue into the peripheral circulation. We conclude that catecholamine-rich cellular implants in the basal ganglia have transient beneficial effects in patients with severe Parkinson's disease.

Use of graphical techniques for error evaluation

Anatomic localization in functional (such as PET) imaging often requires a structural (such as CT or NMRI) study of the brain in the same plane. Some neuropsychiatric conditions make it difficult for a patient to hold his/her head immobile even when a rigid head holder is used. We studied the effect of tilt of the anatomic reference image, in this instance CT, relative to the functional image, here PET, on the quantification of receptor ligand concentration in anatomically defined regions of the brain.

PET analysis of human dopamine receptor subtypes using 11C-SCH 23390 and 11C-raclopride

Tracer doses of 11C-SCH 23390 and 11C-raclopride, selective D1-dopamine and D2-dopamine receptor antagonists, respectively, were injected intravenously into three healthy male volunteers and two drug-treated schizophrenic patients. Regional radioactivity in brain and plasma was followed during 1 h by positron emission tomography (PET). After injection of both ligands a high accumulation of radioactivity was observed in the dopamine-rich caudate putamen. Experiments with 11C-SCH 23390, but not 11C-raclopride, showed a conspicuous accumulation of radioactivity also in the neocortex. None of the ligands accumulated in the dopamine-poor cerebellum. Specific binding of 11C-raclopride in the putamen was reduced by more than 80% in schizophrenic patients treated with antipsychotic doses of sulpiride or cis(Z)-flupentixol decanoate. 11C-SCH 23390 binding was slightly reduced in both the cortex and the putamen after treatment with cis(Z)-flupentixol decanoate but not after sulpiride. The results indicate that clinical antipsychotic drug treatment with sulpiride and cis(Z)-flupentixol decanoate causes a substantial blockade of D2-dopamine receptors in the basal ganglia but has only a minor effect on D1-dopamine receptors.

Quantitative analysis of D2 dopamine receptor binding in the living human brain by PET

D2 dopamine receptors in the putamen of living human subjects were characterized by using the selective, high-affinity D2 dopamine receptor antagonist carbon-11-labeled raclopride and positron emission tomography. Experiments in four healthy men demonstrated saturability of [11C]raclopride binding to an apparently homogeneous population of sites with Hill coefficients close to unity. In the normal putamen, maximum binding ranged from 12 to 17 picomoles per cubic centimeter and dissociation constants from 3.4 to 4.7 nanomolar. Maximum binding for human putamen at autopsy was 15 picomoles per cubic centimeter. Studies of [11C]raclopride binding indicate that clinically effective doses of chemically distinct neuroleptic drugs result in 85 to 90 percent occupancy of D2 dopamine receptors in the putamen of schizophrenic patients.

Preparation of 11C-labelled SCH 23390 for the in vivo study of dopamine D-1 receptors using positron emission tomography

The dopamine D-1 receptor antagonist, SCH 23390 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepin-7-ol), was labelled by alkylation of the desmethyl compound SCH 24518 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-5-phenyl-1H-3-benzazepin-7- ol) with [11C]methyl iodide. A multivariate optimization method, Simplex, was employed to obtain the optimal radiochemical yield. Both straight-phase and reversed-phase preparative HPLC were investigated in the purification of [11C]SCH 23390. Reaction in acetone with subsequent straight-phase LC separation resulted in 80% radiochemical yield, based on [11C]methyl iodide, with a total synthesis time of 35-40 min and a radiochemical purity greater than 99%. The average specific activity was on the order of 11.1 GBq/mmol. The 11C-labelled SCH 23390 was used to visualize the dopamine D-1 receptor-rich areas of a monkey brain by positron emission tomography. The data obtained showed a rapid distribution of radioactivity into the brain and a conspicuous accumulation of [11C]SCH 23390 in the striatum.

Substituted benzamides as ligands for visualization of dopamine receptor binding in the human brain by positron emission tomography

Two substituted benzamides, FLB 524 and raclopride, were labeled with 11C and examined for their possible use as ligands for positron emission tomography (PET)-scan studies on dopamine-2 (D-2) receptors in the brains of monkeys and healthy human subjects. Both ligands allowed the in vivo visualization of D-2 receptor binding in the corpus striatum caudate nucleus/putamen complex in PET-scan images. [11C]Raclopride showed a high ratio of specific striatal to nonspecific cerebellar binding, and the kinetics of binding of this ligand made it optimal for PET studies. The in vivo binding of [11C]raclopride in the striatum of cynomolgus monkeys was markedly reduced by displacement with haloperidol. This and previous in vitro data indicate that [11C]raclopride binds selectively to striatal D-2 dopamine receptors. In healthy human subjects, [11C]raclopride binding in the caudate nucleus/putamen was 4- to 5-fold greater than nonspecific binding in the cerebellum. In comparison with previously available ligands for PET-scan studies on central dopamine receptors in man, [11C]raclopride appears to be advantageous with regard to (i) specificity of binding to D-2 receptors, (ii) the high ratio between binding in dopamine-rich (caudate, putamen) and dopamine-poor (cerebellum) human brain regions, and (iii) rapid association and reversibility of specific binding. [11C]Raclopride should be a valuable tool for characterizing D-2 receptors in the brains of patients with neuropsychiatric disorders.

Preparation of 11C-labelled Raclopride, a new potent dopamine receptor antagonist: preliminary PET studies of cerebral dopamine receptors in the monkey

A new dopamine receptor antagonist, Raclopride (S-(-)-3,5-dichloro-N-[(1-ethyl-2-pyrrolidinyl)]methyl-2-hydroxy- 6-methoxybenzamide, FLA 870) (1), has been labelled using [11C]ethyl iodide for alkylation of the nitrogen of the pyrrolidine ring in the corresponding secondary amine (5). The synthesis of 5 and an efficient method for the preparation of [11C]ethyl iodide are described. The 11C-labelled FLA 870 (1) was purified by HPLC and then used in positron emission tomography to visualize the dopamine receptor-rich areas of the monkey brain. The images obtained show selective accumulation of FLA 870 in striatum and a 10-fold separation between the binding to caudate vs cerebellum.

Imaging of [11C]-labelled Ro 15-1788 binding to benzodiazepine receptors in the human brain by positron emission tomography

The benzodiazepine antagonist Ro 15-1788 was labelled with [11C] and examined for possible use as ligand for PET scan studies on benzodiazepine receptors in the brain of cynomolgus monkeys and human subjects. [11C] Ro 15-1788 allowed the in vivo visualization of benzodiazepine receptor binding in cerebral and cerebellar cortical areas as well as in basal brain nuclei in PET scan images. [11C] Ro 15-1788 exhibited a high ratio of specific benzodiazepine receptor binding (cerebral cortex) to non-specific binding (pons) and the kinetics of binding should be satisfactory for quantitative clinical PET scan studies using [11C]. The in vivo binding of [11C] Ro 15-1788 in the cerebral cortex of cynomolgus monkeys and healthy human subjects was reduced by approximately 90% within 10 min after the intravenous injection of a high dose of unlabelled Ro 15-1788 (0.5 mg/kg i.v.). Different areas of the healthy human brain showed an approximately 10-fold variation in maximal [11C] Ro 15-1788 binding that corresponded to the previously known distribution of benzodiazepine receptors in these regions. The highest degree of binding was obtained in the medial occipital cerebral cortex followed by frontal cortex, cerebellum, thalamus, striatum and pons. Two psychiatric patients with anxiety syndromes who had been treated for a long time with high doses of benzodiazepines had roughly the same degree of maximal [11C] Ro 15-1788 binding in brain regions as the healthy subjects but the rate of decline of [11C] Ro 15-1788 in the brain was higher. This indicates that there is measurable competition between [11C] Ro 15-1788 binding and clinical benzodiazepine concentrations in the body fluids of psychiatric patients. The results demonstrate that [11C] Ro 15-1788 should be a valuable tool for quantitative analyses of benzodiazepine receptor characteristics and receptor occupancy in the brain of patients with neuropsychiatric disorders.