Suggested minimal effective dose of risperidone based on PET-measured D2 and 5-HT2A receptor occupancy in schizophrenic patients.

OBJECTIVE: Multicenter trials with the novel antipsychotic risperidone have suggested a standard dose of 6 mg/day. However, a dose producing the highest response rate in fixed-dose studies is likely to exceed the minimal effective dose in most patients. The aim of this positron emission tomography (PET) study was to suggest a minimal effective dose of risperidone based on measurements of dopamine D2 and serotonin 5-HT2A receptor occupancy. METHOD: Eight first-episode or drug-free schizophrenic patients were treated with risperidone, 6 mg/day, for 4 weeks and then 3 mg/day for 2 weeks. PET was performed after 4 and 6 weeks, with [11C]raclopride to measure D2 receptor occupancy and [11C]N-methylspiperone to measure 5-HT2A receptor occupancy. RESULTS: Seven patients completed the study and responded to treatment with risperidone. No patient had extrapyramidal side effects at the time of inclusion in the study. At the 6-mg/day dose, mean D2 receptor occupancy was 82% (range = 79%-85%), 5-HT2A receptor occupancy was 95% (range = 86%-109%), and six patients had developed extrapyramidal side effects. After dose reduction to 3 mg/day, D2 receptor occupancy was 72% (range = 53%-78%), and 5-HT2A receptor occupancy was 83% (range = 65%-112%). Three patients had extrapyramidal side effects at this time. CONCLUSIONS: Treatment with risperidone, 6 mg/day, is likely to induce unnecessarily high D2 receptor occupancy, with a consequent risk of extrapyramidal side effects. High 5-HT2A receptor occupancy did not prevent extrapyramidal side effects completely. The authors previously suggested an optimal interval for D2 receptor occupancy of 70%-80%. To achieve this, resperidone, 4 mg/day, should be a suitable initial dose for antipsychotic effect with a minimal risk of extrapyramidal side effects in most patients.     

Prediction and assessment of extrapyramidal side effects induced by risperidone based on dopamine D(2) receptor occupancy

In the present study, we attempted to predict the risk of extrapyramidal side effects of a selective monoaminergic antagonist, risperidone, by analyzing the correlation between the dopamine D(2) receptor occupancy and the degree of extrapyramidal side effects of the drug. The occupancies of D(2) and 5-HT(2) receptors at various doses of risperidone were calculated by means of a receptor occupancy theory. The extrapyramidal side effects after administration of risperidone were attempted to predict by theoretical analysis of the correlation between the calculated occupancies and the evidence of extrapyramidal symptoms using a ternary complex model. The pharmacokinetic/pharmacodynamic analysis utilized the data concerning the pharmacokinetics of risperidone and 9-hydroxyrisperidone (active metabolite), their binding affinities with D(2) and 5-HT(2) receptors, and the clinical evidence of extrapyramidal symptoms (Extrapyramidal Symptom Rating Scale: ESRS), gathered from the literature. The mean occupancy of 5-HT(2) receptors after the administration of regular doses of risperidone was suggested to be more than 90%, whereas the mean occupancy of D(2) receptors varied between 50-80%, depending on the dose. The correlation between the occupancy of D(2) receptors and the extrapyramidal symptoms could be successfully analyzed with a ternary complex model, showing the predictability of the model for the extrapyramidal side effects of risperidone. Since the estimated risk of the extrapyramidal side effects varied with the dose, the present method of predicting the extrapyramidal side effects of risperidone may provide a basis for developing a rational dosing regimen for the drug.     

Is amoxapine an atypical antipsychotic? Positron-emission tomography investigation of its dopamine2 and serotonin2 occupancy.

BACKGROUND: All currently available atypical antipsychotics have, at clinically relevant doses: i) high serotonin (5-HT)2 occupancy; ii) greater 5-HT2 than dopamine (D)2 occupancy; and iii) a higher incidence of extrapyramidal side effects when their D2 occupancy exceeds 80%. A review of pharmacologic and behavioral data suggested that amoxapine should also conform to this profile; therefore, we undertook a positron-emission tomography (PET) study of its 5-HT2 and D2 occupancy. METHODS: Seven healthy volunteers received 50-250 mg/day of amoxapine for 5 days and then had [11C]-raclopride and [18F]-setoperone PET scans. RESULTS: 5-HT2 receptors showed near saturation at doses of 100 mg/day and above. The D2 receptor occupancies showed a dose-dependent increase, never exceeding 80%; at all doses 5-HT2 occupancy exceeded D2 occupancy. CONCLUSIONS: PET data show that amoxapine's profile is very similar to that of the established atypical antipsychotics. These data, together with amoxapine's in vitro pharmacologic profile, effectiveness in animal models, and efficacy in psychotic depression raise the possibility of amoxapine as an "atypical" antipsychotic agent in the treatment of schizophrenia.     

A PET study of dopamine D2 and serotonin 5-HT2 receptor occupancy in patients with schizophrenia treated with therapeutic doses of ziprasidone

OBJECTIVE: Ziprasidone is an atypical antipsychotic drug that shows a higher affinity for serotonin 5-HT(2) receptors compared with dopamine D(2) receptors in vitro. The affinity of ziprasidone for these receptors in vivo in patients was examined in a positron emission tomography (PET) study. METHOD: The authors conducted a PET study to evaluate D(2) occupancy (using [(11)C]raclopride) and 5-HT(2) occupancy (using [(18)F]setoperone) in brain regions of interest in 16 patients with schizophrenia or schizoaffective disorder randomly assigned to receive 40, 80, 120, or 160 mg/day of ziprasidone, which reflected the recommended dose range. PET scanning was done after 3 weeks of administration and at trough plasma levels, i.e., 12-16 hours after the last dose. RESULTS: The mean 5-HT(2) receptor occupancy was significantly higher than the mean D(2) receptor occupancy (mean=76%, SD=15%, and mean=56%, SD=18%, respectively). The estimated plasma ziprasidone concentration associated with 50% maximal 5-HT(2) receptor occupancy was almost four times lower than that for D(2) receptor occupancy. CONCLUSIONS: These data affirm that ziprasidone is similar to other novel antipsychotics in having greater 5-HT(2) than D(2) receptor occupancy at therapeutic doses and suggest that the optimal effective dose of ziprasidone is closer to 120 mg/day than to the lower doses suggested by previous PET studies. The relatively high D(2) receptor occupancy, even at trough plasma levels, suggests that ziprasidone is more similar to risperidone and olanzapine in receptor occupancy profile than to clozapine and quetiapine. Since ziprasidone plasma levels show significant (more than twofold) variation within a single dose cycle, studies that are aimed at peak plasma levels (6 hours after the last dose) and that examine extrastriatal regions are required to fully characterize the in vivo occupancy profile of ziprasidone.     

In vivo occupation of dopamine D1, D2 and serotonin (5-HT)2A receptors by sertindole in the rat brain.

OBJECTIVE: To determine the in vivo occupation of dopamine D1, D2 and serotonin (5-HT)2A receptors by novel antipsychotic agent sertindole using N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), an irreversible antagonist at these receptor sites. DESIGN: Animal study. INTERVENTIONS: Intraperitoneal administration to Wistar rats of 1 of 4 test compounds: a control compound of 0.15% tartaric acid, or a compound of either sertindole (0.5 mg/kg or 2.0 mg/kg) or clozapine (20 mg/kg) dissolved in 0.15% tartaric acid 1 hour before intraperitoneal administration of EEDQ (8 mg/kg) or ethanol/water solution. RESULTS: Sertindole exhibited little or no effect on D1 and D2 binding sites in vivo. On the other hand, sertindole occupied 5-HT2A receptors more extensively and firmly than EEDQ. This study indicates that sertindole is characterized by high occupancy of 5-HT2A receptors and by low or minimum occupancy of D1 and D2 receptors. CONCLUSIONS: These characteristics are very similar to atypical antipsychotic agents such as clozapine. Sertindole's low liability to cause extrapyramidal side effects (EPS) may be related to greater long-term binding for 5-HT2A receptors relative to D2 receptors.     

The next generation of "atypical" antipsychotics: the role of positron emission tomography

Almost fifteen years of research with Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) have led to a profound understanding of the relationships between antipsychotic doses and plasma levels on the one hand and occupancy of (striatal) D2 -like dopamine receptors on the other hand as well as with the associated clinical effects and side effects. Furthermore, with the development of clinically "atypical" antipsychotics PET studies helped to generate hypotheses regarding the essential pharmacological properties of this heterogeneous class of drugs. Possible mechanisms of action include combined D2 -/5-HT2 antagonism, preferential mesolimbic binding, and fast dissociation from the D2 -receptor. Our recently published PET study on the in vivo characterization of the partial dopamine receptor agonist, aripiprazole, suggests a novel mechanism of action, which leads to clinically "atypical" properties of an antipsychotic. Aripiprazole, of which the antipsychotic efficacy has been proven in various multicenter clinical trials, leads to almost complete saturation of D2 -like dopamine receptors at clinically used doses; however, the incidence of extrapyramidal side effects under aripiprazole is not higher than under placebo. PET like no other method is suitable to display in vivo a novel mechanism of "atypicality" of a new class of antipsychotics.     

Mechanism of new antipsychotic medications: occupancy is not just antagonism

Antagonism of D2-like dopamine receptors is the putative mechanism underlying the antipsychotic efficacy of psychotropic drugs. Positron emission tomographic studies suggest that the antipsychotic effect of dopamine receptor antagonists occurs within a therapeutic window between 60% and 80% (striatal) D2 receptor occupancy. The incidence of extrapyramidal side effects increases above the 80% threshold. However, the novel atypical antipsychotic drug, aripiprazole, occupies up to 95% of striatal D2-like dopamine receptors at clinical doses, and the incidence of extrapyramidal side effects with aripiprazole is no higher than with placebo. The most likely explanation for this finding is aripiprazole's weak partial agonism at D2-like dopamine receptors. This particular pharmacologic feature characterizes a new class of atypical antipsychotics that does not match the original concept of a therapeutic occupancy window for antagonist antipsychotics. When not involving pure antagonists, it implies a need to adjust the expected receptor occupancy (measured using positron emission tomography) for the therapeutic window.     

In vitro characterization of SLV308 (7-[4-methyl-1-piperazinyl]-2(3H)-benzoxazolone, monohydrochloride): a novel partial dopamine D2 and D3 receptor agonist and serotonin 5-HT1A receptor agonist

Present Parkinson's disease treatment strategies are far from ideal for a variety of reasons; it has therefore been suggested that partial dopamine receptor agonism might be a potential therapeutic approach with potentially fewer side effects. In the present study, we describe the in vitro characterization of the nonergot ligand SLV308 (7-[4-methyl-1-piperazinyl]-2(3H)-benzoxazolonemonohydrochloride). SLV308 binds to dopamine D(2), D(3), and D(4) receptors and 5-HT(1) (A) receptors and is a partial agonist at dopamine D(2) and D(3) receptors and a full agonist at serotonin 5-HT(1) (A) receptors. At cloned human dopamine D(2,L) receptors, SLV308 acted as a potent but partial D(2) receptor agonist (pEC(50) = 8.0 and pA(2) = 8.4) with an efficacy of 50% on forskolin stimulated cAMP accumulation. At human recombinant dopamine D(3) receptors, SLV308 acted as a partial agonist in the induction of [(35)S]GTPgammaS binding (intrinsic activity of 67%; pEC(50) = 9.2) and antagonized the dopamine induction of [(35)S]GTPgammaS binding (pA(2) = 9.0). SLV308 acted as a full 5-HT(1) (A) receptor agonist on forskolin induced cAMP accumulation at cloned human 5-HT(1) (A) receptors but with low potency (pEC(50) = 6.3). In rat striatal slices SLV308 concentration-dependently attenuated forskolin stimulated accumulation of cAMP, as expected for a dopamine D(2) and D(3) receptor agonist. SLV308 antagonized the inhibitory effect of quinpirole on K(+)-stimulated [(3)H]-dopamine release from rat striatal slices (pA(2) = 8.5). In the same paradigm, SLV308 had antagonist properties in the presence of quinpirole (pA(2) = 8.5), but the partial D(2) agonist terguride had much stronger antagonistic properties. In conclusion, SLV308 combines high potency partial agonism at dopamine D(2) and D(3) receptors with full efficacy low potency serotonin 5-HT(1) (A) receptor agonism and is worthy of profiling in in vivo models of Parkinson's disease.     

In vivo determination of striatal dopamine D2 receptor occupancy in patients treated with olanzapine.

In vivo studies of dopamine D2 receptor occupancy with atypical antipsychotics have suggested good clinical efficacy at occupancy rates less than those observed with typical neuroleptics, and few extrapyramidal side effects (EPS), possibly even at high levels of D2 occupancy. We used [123I]IBZM-SPECT to investigate striatal D2 receptor occupancy in 10 schizophrenic patients who were treated with both a low (5 mg) and a high dose (20 mg) of the novel antipsychotic olanzapine without concomitant medications. The mean D2 occupancy at 5 mg was 59.8% (range 33-81%); the mean D2 occupancy at 20 mg was 82.8% (range 56-97%). Although the D2 occupancy rates on 5 and 20 mg olanzapine were significantly different (P < 0.001), there were no significant differences in clinical ratings for psychiatric symptoms or extrapyramidal side effects between the two doses of olanzapine. These data suggest that: (1) olanzapine doses below those used routinely occupy D2 receptors at levels approaching those associated with therapeutic response; (2) higher doses produce relatively high levels of D2 occupancy rates; and (3) EPS are mild even at relatively high levels of D2 occupancy.     

Aripiprazole for drug-induced psychosis in Parkinson disease: preliminary experience

Aripiprazole is the newest atypical antipsychotic (AA) drug to be released in the US. It is the only AA that is a partial agonist at the D2 and 5HT1a receptors and an antagonist at 5HT2a receptors. It also has a high 5HT2/D2 ratio and may therefore carry a low risk of extrapyramidal side effects and alleviate psychosis in Parkinson-vulnerable populations. We report our preliminary experience in 8 patients with probable Parkinson disease (PD) treated with aripiprazole for drug-induced psychosis. Two patients were neuroleptic-naive, 5 patients were "quetiapine failures", and 1 patient was switched from olanzapine to aripiprazole. Aripiprazole was started at 5 mg to 10 mg a day and slowly increased over 3 to 7 days until side effects or improvement of psychosis occurred. Only 2 out of 8 patients experienced near complete resolution of their psychosis using aripiprazole. The other six patients discontinued aripiprazole within 40 days, 2 of whom discontinued due to motor worsening. Our preliminary experience with aripiprazole is mixed but not very encouraging. Controlled studies are needed to evaluate aripiprazole in parkinsonian patients.     

Current evidence for aripiprazole as augmentation therapy in major depressive disorder

Aripiprazole (Abilify) is the first atypical antipsychotic approved as adjunctive therapy for the treatment of major depressive disorder. The pharmacological basis of the action of aripiprazole in major depressive disorder remains unclear, but it may be related to its potent partial agonism of the dopamine D(2)/D(3) receptors, partial agonism of the 5-hydroxytryptamine (5-HT)(1A) receptor and antagonism of the 5-HT(2A) receptor. This article reviews findings on the efficacy and tolerability of aripiprazole from two identical placebo-controlled trials and from smaller open-label and retrospective studies. At doses of 2-15 mg/day aripiprazole was efficacious and well tolerated as adjunctive therapy to antidepressants in patients who had not responded to monotherapy.     

Increased dopamine d(2) receptor occupancy and elevated prolactin level associated with addition of haloperidol to clozapine

OBJECTIVE: The authors added haloperidol, a potent D(2) blocker, to ongoing treatment with clozapine in patients with schizophrenia to determine the effects of this combination on dopamine D(2) receptor blockade, prolactin level, and extrapyramidal side effects. METHOD: At baseline and 4-8 weeks after the addition of haloperidol (4 mg/day) to ongoing clozapine treatment, five patients were examined for prolactin elevation, extrapyramidal side effects, drug plasma levels, and D(2) receptor occupancy measured with [(11)C]raclopride and positron emission tomography imaging. RESULTS: Adding haloperidol significantly increased D(2) receptor occupancy, from a mean of 55% to 79%, and significantly increased the prolactin level. One patient developed akathisia, and another manifested mild extrapyramidal side effects. CONCLUSIONS: Adding a modest dose of haloperidol to clozapine results in the high D(2) receptor occupancy and sustained prolactin elevation usually associated with typical antipsychotics. These findings suggest that the lack of prolactin elevation associated with clozapine derives mainly from low D(2) receptor occupancy and not from the medication's effects on other receptors.     

Validation of a tracer kinetic model for the quantification of 5-HT(2A) receptors in human brain with [(11)C]MDL 100,907.

The positron emission tomography (PET) ligand [(11)C]MDL 100,907 has previously been introduced to image the serotonin 2A (5-HT(2A)) receptor in human brain. The aim of this work was to contribute to the verification of the tracer kinetic modelling in human studies. Five healthy volunteers were scanned twice after intravenous bolus injection of approximately 370 MBq [(11)C]MDL 100,907 using dynamic PET. One scan was performed under baseline condition, the other scan commenced 90 mins after a single oral dose of 30 mg of the antidepressant mirtazapine, which binds to the 5-HT(2A) receptor. There did not appear to be radiolabelled metabolites of [(11)C]MDL 100,907 in human plasma, which are likely to cross the blood-brain barrier. Total volumes of distribution VD in 11 different brain regions were estimated using a reversible, two tissue, four rate constants compartment model with a variable fractional blood volume term and the metabolite-corrected plasma input function. There were no significant changes of the VD in the cerebellum between the baseline and the blocked scans confirming the cerebellum as a region devoid of displaceable binding. Regional estimates of binding potential were then obtained indirectly using the cerebellar VD and occupancies calculated. The mean occupancy with this clinically effective dose of mirtazapine was 60% without significant regional differences. This study confirmed the use of an arterial input kinetic model for the quantification of 5-HT(2A) receptor binding with [(11)C]MDL 100,907 and the use of the cerebellum as a reference region for the free and nonspecific binding.     

Relationship between dopamine D(2) occupancy, clinical response, and side effects: a double-blind PET study of first-episode schizophrenia

OBJECTIVE: Since all antipsychotics block dopamine D(2) receptors, the authors investigated how well D(2) receptor occupancy in vivo predicts clinical response, extrapyramidal side effects, and hyperprolactinemia. METHOD: In a double-blind study, 22 patients with first-episode schizophrenia were randomly assigned to 1.0 or 2. 5 mg/day of haloperidol. After 2 weeks of treatment, D(2) receptor occupancy was determined with [(11)C]raclopride and positron emission tomography, and clinical response, extrapyramidal side effects, and prolactin levels were measured. Patients who showed adequate responses continued taking their initial doses, those who did not respond had their doses increased to 5.0 mg/day, and evaluations were repeated at 4 weeks for all patients. RESULTS: The patients showed a wide range of D(2) occupancy (38%-87%). The degree of receptor occupancy predicted clinical improvement, hyperprolactinemia, and extrapyramidal side effects. The likelihood of clinical response, hyperprolactinemia, and extrapyramidal side effects increased significantly as D(2) occupancy exceeded 65%, 72%, and 78%, respectively. CONCLUSIONS: The study confirms that D(2) occupancy is an important mediator of response and side effects in antipsychotic treatment. The data are consistent with a "target and trigger" hypothesis of antipsychotic action, i.e., that the D(2) receptor specificity of antipsychotics permits them to target discrete neurons and that their antagonist properties trigger within those neurons intracellular changes that ultimately beget antipsychotic response. While limited to haloperidol, the relationship between D(2) occupancy and side effects in this study helps explain many of the observed clinical differences between typical and atypical antipsychotics.     

5-HT(1A) receptor activation contributes to ziprasidone-induced dopamine release in the rat prefrontal cortex.

BACKGROUND: Ziprasidone (Zeldox) is a novel antipsychotic with a unique combination of antagonist activities at monoaminergic receptors and transporters and potent agonist activity at serotonin 5-HT(1A) receptors. 5-HT(1A) receptor agonism may be an important feature in ziprasidone's clinical actions because 5-HT(1A) agonists increase cortical dopamine release, which may underlie efficacy against negative symptoms and reduce dopamine D(2) antagonist-induced extrapyramidal side effects. This study investigated the in vivo 5-HT(1A) agonist activity of ziprasidone by measuring the contribution of 5-HT(1A) receptor activation to the ziprasidone-induced cortical dopamine release in rats. METHODS: Effects on dopamine release were measured by microdialysis in prefrontal cortex and striatum. The role of 5-HT(1A) receptor activation was estimated by assessing the sensitivity of the response to pretreatment with the 5-HT(1A) antagonist, WAY-100635. For comparison, the D(2)/5-HT(2A) antagonists clozapine and olanzapine, the D(2) antagonist haloperidol, the 5-HT(2A) antagonist MDL 100,907 and the 5-HT(1A) agonist 8-OHDPAT were included. RESULTS: Low doses (<3.2 mg/kg) of ziprasidone, clozapine, and olanzapine increased dopamine release to approximately the same extent in prefrontal cortex as in striatum, but higher doses (> or =3.2 mg/kg) resulted in an increasingly preferential effect on cortical dopamine release. The 5-HT(1A) agonist 8-OHDPAT produced a robust increase in cortical dopamine (DA) release without affecting striatal DA release. In contrast, the D(2) antagonist haloperidol selectively increased striatal DA release, whereas the 5-HT(2A) antagonist MDL 100,907 had no effect on cortical or striatal DA release. Prior administration of WAY-100635 completely blocked the cortical DA increase produced by 8-OHDPAT and significantly attenuated the ziprasidone- and clozapine-induced cortical DA increase. WAY-100635 pretreatment had no effect on the olanzapine-induced DA increase. CONCLUSIONS: The preferential increase in DA release in rat prefrontal cortex produced by ziprasidone is mediated by 5-HT(1A) receptor activation. This result extends and confirms other in vitro and in vivo data suggesting that ziprasidone, like clozapine, acts as a 5-HT(1A) receptor agonist in vivo, which may contribute to its activity as an antipsychotic with efficacy against negative symptoms and a low extrapyramidal side effect liability.     

Equivalent occupancy of dopamine D1 and D2 receptors with clozapine: differentiation from other atypical antipsychotics

OBJECTIVE: Clozapine, the prototype of atypical antipsychotics, remains unique in its efficacy in the treatment of refractory schizophrenia. Its affinity for dopamine D(4) receptors, serotonin 5-HT(2A) receptor antagonism, effects on the noradrenergic system, and its relatively moderate occupancy of D(2) receptors are unlikely to be the critical mechanism underlying its efficacy. In an attempt to elucidate the molecular/synaptic mechanism underlying clozapine's distinctiveness in refractory schizophrenia, the authors studied the in vivo D(1) and D(2) receptor profile of clozapine compared with other atypical antipsychotics. METHOD: Positron emission tomography with the radioligands [(11)C]SCH23390 and [(11)C]raclopride was used to investigate D(1) and D(2) receptor occupancy in vivo in 25 schizophrenia patients receiving atypical antipsychotic treatment with clozapine, olanzapine, quetiapine, or risperidone. RESULTS: Mean striatal D(1) occupancies ranged from 55% with clozapine to 12% with quetiapine (rank order: clozapine > olanzapine > risperidone > quetiapine). The striatal D(2) occupancy ranged from 81% with risperidone to 30% with quetiapine (rank order: risperidone > olanzapine > clozapine > quetiapine). The ratio of striatal D(1)/D(2) occupancy was significantly higher for clozapine (0.88) relative to olanzapine (0.54), quetiapine (0.41), or risperidone (0.31). CONCLUSIONS: Among the atypical antipsychotics, clozapine appears to have a simultaneous and equivalent occupancy of dopamine D(1) and D(2) receptors. Whether its effect on D(1) receptors represents agonism or antagonism is not yet clear, as this issue is still unresolved in the preclinical arena. This distinctive effect on D(1)/D(2) receptors may be responsible for clozapine's unique effectiveness in patients with schizophrenia refractory to other typical and atypical antipsychotics.     

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.     

Effect of 5-HT on binding of [(11)C] WAY 100635 to 5-HT(IA) receptors in rat brain, assessed using in vivo microdialysis nd PET after fenfluramine

By using a combination of positron emission tomography (PET) and postmortem tissue dissection, the effect of increased endogenous serotonin on specific binding of [(11)C]WAY 100635 to the 5-HT(1A) receptor was investigated in rat brain in vivo. The binding studies were complemented by in vivo microdialysis to monitor 5-HT levels in similarly treated isoflurane-anaesthetised rats, with the dialysis probe locations corresponding to two of the tissues sampled for specific binding of the radioligand. Fenfluramine treatment (10 mg/kg i.p.) resulted in a approximately 5-fold increase in extracellular 5-HT in medial prefrontal cortex and a approximately 15-fold increase in lateral hippocampus, maximal at approximately 40 min after injection. PET scan duration was either 60 or 90 min, beginning 30 min after fenfluramine injection. The specific binding of [(11)C]WAY 100635 was reduced by 10-20% in hippocampus, which showed highest binding in control animals. Specific binding, however, was unaffected in both prefrontal cortex and midbrain raphe, each additional high binding regions. The minimal effects are consistent with a low baseline occupancy of the 5-HT(1A) receptor by 5-HT in vivo, so that only a large change in endogenous agonist concentration will affect radioligand binding. This implies that utilisation of [(11)C]WAY 100635 in human PET to quantify 5-HT(1A) receptor expression can be extended to pathology where synaptic 5-HT levels are altered as a consequence of the disease state.     

Are animal studies of antipsychotics appropriately dosed? Lessons from the bedside to the bench.

Animal models are crucial for understanding the mechanism of action of antipsychotics. However, the dose of an antipsychotic in animal studies is often arbitrarily chosen, with haloperidol 1 mg/kg being a rather common standard. Recent clinical positron emission tomography (PET) studies in patients show all antipsychotics to block dopamine D2 receptors, and most are effective at doses that lead to 60% to 80% D2 occupancy. When occupancy exceeds 80%, the incidence of side effects rises sharply. To use this "bedside" information to inform the "bench," we measured D2 occupancy in rats using a method similar in principle to the [11C]-raclopride PET method in humans. We found that: 1) as in humans, haloperidol is effective in animal models of antipsychotic action when D2 occupancy > 70% and leads to effects in models of extrapyramidal side effects when D2 occupancy is > 80%; 2) very low doses, 0.06 mg/kg/sc, cause acute D2 occupancy of 75%; 3) and even doses that acutely saturate D2 receptors give little D2 occupancy after 24 hours due to the very short half-life of haloperidol in rats (2.5 hours versus 24 hours in humans). We conclude that most previous animal studies of antipsychotics have used doses giving rise to inappropriately high acute D2 occupancy and inappropriately low D2 occupancy between doses. We exemplify how this dosing confounder could lead to inappropriate conclusions. Data from the bedside translated to the bench--using D2 occupancy as a mediating variable--will lead to more valid animal models.     

Binding characteristics of the 5-HT2A receptor antagonists altanserin and MDL 100907

To study the 5-HT(2A) receptors in the living human brain, using positron emission tomography (PET), two selective radiotracers are currently in use: [(18)F]altanserin and [(11)C]MDL 100907. It is, however, currently unknown to what extent data obtained with either tracer are directly comparable. The aim of this study was to compare binding characteristics of these two radiotracers in rat brain with respect to affinity (K(d)), receptor binding density (B(max)), binding potential (BP), and nonspecific binding. Further, binding kinetics, sensitivity towards competition with the endogenous transmitter serotonin, and the competitive/noncompetitive interaction between the two radioligands were evaluated. In addition, the selectivity of [(18)F]altanserin for the 5-HT(2A) receptor was assessed.The K(d) value of [(18)F]altanserin and [(3)H]MDL 100907 was in the order of 0.3 nM. B(max) in frontal cortex was 523 and 527 fmol/mg protein, respectively. The binding of [(18)F]altanserin was not influenced by blocking either the 5-HT(2B/2C) or the alpha(1)-adrenergic receptors. At 37 degrees C the association t(1/2) was 2.8 and 2.7 min and the dissociation t(1/2) was 11 and 13.5 min for [(18)F]altanserin and [(3)H]MDL 100907, respectively.Both radioligands were displaced by 5-HT, only at high concentrations; the K(i) value of 5-HT ranging between 650 and 3,300 nM. This indicates that binding of both radioligands in PET studies is not directly influenced by changes in endogenous 5-HT.Overall, the binding of [(18)F]altanserin and [(3)H]MDL 100907 to the 5-HT(2A) receptor was very comparable, showing selective high affinity binding in the subnanomolar range.