Brain 5-HT2A receptor occupancy of deramciclane in humans after a single oral administration – a positron emission tomography study

Rationale: Deramciclane fumarate is a new 5-HT_2A and 5-HT_2C receptor antagonist with putative anxiolytic effects. In the present study the binding of deramciclane to serotonin 5-HT_2A receptors in frontal cortex of healthy male volunteers was studied using [¹¹C]-N-methyl spiperone ([¹¹C]-NMSP) and positron emission tomography. Methods: The receptor occupancy percentage was assessed by the means of inhibition of [¹¹C]-NMSP from the 5-HT_2A receptors in the frontal cortex. Single oral doses of 20, 50 and 150 mg deramciclane were given to three subjects at each dose level (total n = 9). The receptor occupancy was measured before deramciclane and at 3 and 6 h post-dosing except at the 20 mg dose level where only the 3-h measurement was done. The occupancy percentage was calculated with the ratio method using cerebellum as a reference area. Results: Deramciclane inhibited [¹¹C]-NMSP binding dose and concentration dependently. However, deramasciclane inhibited maximally only 52% of the [¹¹C]-NMSP binding in the frontal cortex, indicating a non-5-HT_2A receptor binding component of this radioligand in frontal cortex. On average, specific [¹¹C]-NMSP binding cerebellum ratios below 0.355 were not possible to achieve in this population. The 52% inhibition was regarded to represent near 100% 5-HT_2A receptor occupancy. The 50 and 90% receptor occupancies were reached at deramciclane plasma concentrations of 21 ng/ml and 70 ng/ml, respectively. Conclusions: Deramciclane penetrates the blood-brain barrier in humans. Deramciclane binds to the 5-HT_2A receptors in the frontal cortex in a saturable manner in vivo. Consequently, the increase in deramciclane concentration in plasma above 70 ng/ml will not result in major increase in the 5-HT_2A receptor occupancy in the brain. Received: 7 September 1998/Final version: 11 January 1999     

A positron emission tomography (PET) study of cerebral dopamine D<Subscript>2</Subscript> and serotonine 5-HT<Subscript>2A</Subscript> receptor occupancy in patients treated with cyamemazine (Tercian)

Rationale Cyamemazine (Tercian) is an antipsychotic drug with anxiolytic properties. Recently, an in vitro study showed that cyamemazine possesses high affinity for serotonin 5-HT_2A receptors, which was fourfold higher than its affinity for dopamine D₂ receptors (Hameg et al. 2003).Objectives The aim of this study is to confirm these previous data in vivo in patients treated with clinically relevant doses of Tercian.Methods Eight patients received 37.5, 75, 150 or 300 mg/day of Tercian depending on their symptomatology. Dopamine D₂ and serotonin 5-HT_2A receptor occupancies (RO) were assessed at steady-state plasma levels of cyamemazine with positron emission tomography (PET), using [¹¹C]raclopride and [¹¹C]N-methyl-spiperone, respectively. The effective plasma level of the drug leading to 50% of receptor occupancy was estimated by fitting RO with plasma levels of cyamemazine at the time of the PET scan.Results Cyamemazine induced near saturation of 5-HT_2A receptors (RO=62.1–98.2%) in the frontal cortex even at low plasma levels of the drug. On the contrary, occupancy of striatal D₂ receptors increased with plasma levels, and no saturation was obtained even at high plasma levels (RO=25.2–74.9%). The effective plasma level of cyamemazine leading to 50% of D₂ receptor occupancy was fourfold higher than that for 5-HT_2A receptors. Accordingly, individual 5-HT_2A/D₂ RO ratios ranged from 1.26 to 2.68. No patients presented relevant increased prolactin levels, and only mild extrapyramidal side effects were noticed on Simpson and Angus Scale.Conclusion This in vivo binding study conducted in patients confirms previous in vitro findings indicating that cyamemazine has a higher affinity for serotonin 5-HT_2A receptors compared to dopamine D₂ receptors. In the dose range 37.5–300 mg, levels of dopamine D₂ occupancy remained below the level for motor side effects observed with typical antipsychotics and is likely to explain the low propensity of the drug to induce extrapyramidal side effects.     

Occupancy of dopamine D<Subscript>1</Subscript>, D<Subscript>2</Subscript> and serotonin<Subscript>2A</Subscript> receptors in schizophrenic patients treated with flupentixol in comparison with risperidone and haloperidol

Rationale Flupentixol (FLX) has been used as a neuroleptic for nearly 4 decades. In vitro data show comparable affinity to dopamine D₂, D₁ and 5-HT_2A receptors and recently, FLX showed to be not inferior to risperidone in schizophrenic patients with predominant negative symptomatology, which was implicated with flupentixol’s interaction with 5-HT_2A and/or D₁ receptors. Objectives To assess in vivo receptor occupancy (RO) in patients clinically treated with FLX (n = 13, 5.7 ± 1.4 mg/day) in comparison with risperidone (RIS, n = 11, 3.6 ± 1.3 mg/day) and haloperidol (HAL, n = 11, 8.5 ± 5.5 mg/day). Materials and methods Each patient underwent two PET scans with 3-N-[¹¹C]methylspiperone (target: frontal 5-HT_2A), [¹¹C]SCH23390 (striatal D₁) or [¹¹C]raclopride (striatal D₂). RO was calculated as the percentage reduction of specific binding in comparison with healthy controls. Results D₂-RO under FLX was between 50% and 70%, indicating an ED₅₀ of about 0.7 ng/ml serum. 5-HT_2A and D₁-RO was 20 ± 10% and 20 ± 5% (mean, SEM). Under HAL, D₁-RO was 14 ± 6% and under RIS not significantly different from zero. Conclusions We were able to demonstrate a moderate 5-HT_2A and D₁ occupancy under clinically relevant doses of flupentixol, albeit lower than expected from in vitro data and clearly below saturation. Therefore, if flupentixol’s efficacy on negative symptoms is based on its interaction with 5-HT_2A and/or D₁ receptors, it should be highly dependent on serum concentration and thus on dosage and metabolism. However, these data suggest that mechanisms other than D₁ or 5-HT_2A antagonism may contribute to flupentixol’s efficacy on negative symptoms.     

Occupancy of 5-HT<Subscript>1A</Subscript> receptors by clozapine in the primate brain: a PET study

Abstract Rationale. The pharmacological mechanism underlying the atypical properties of the antipsychotic drug clozapine remains to be identified. The serotonin 5-hydroxytryptamine-1A (5-HT_1A) receptor subtype has been suggested to play a role in the pathophysiology of schizophrenia and is one among several central neuroreceptors for which clozapine has moderate affinity in vitro. Objective. The aim of this positron emission tomography (PET) study was to determine 5-HT_1A receptor occupancy in the monkey brain after IV injection of clozapine in doses that previously have been shown to give plasma concentrations representative of the 200 to 800 mg oral dose range recommended for clinical management of patients. Methods. Each of four cynomolgus monkeys was examined three times on the same day with PET and the radioligand [carbonyl-¹¹C]WAY-100635. The first measurement was performed at baseline conditions, the second after clozapine 1.5 mg/kg and the third after 6 mg/kg. Two additional monkeys were examined at baseline and after 15 mg/kg IV. Central 5-HT_1A receptor occupancy was calculated using an equilibrium-ratio analysis. Results. The occupancy ranged from 23 to 34% after 1.5 mg/kg clozapine and from 36 to 49% after 6 mg/kg in different brain regions of the four monkeys. The regional receptor occupancy values after 15 mg/kg were between 39 and 51% in the two monkeys. There was no evident difference between the frontal cortex, the temporal cortex and the raphe nucleus. Conclusion. The study shows that clozapine occupies 5-HT_1A receptors in the primate brain at clinically representative plasma concentrations. The results support that the 5-HT_1A receptor is a candidate target for the atypical drug actions of clozapine. Electronic Publication     

A PET study of D2 and 5-HT2 receptor occupancy induced by risperidone in poor metabolizers of debrisoquin and risperidone

The hydroxylation of the new antipsychotic drug risperidone to its main, active metabolite 9-hydroxyrisperidone is catalyzed by the hepatic cytochrome P450 enzyme CYP2D6, and cosegregates with the polymorphic hydroxylation of debrisoquin. We have previously examined central D₂ dopamine and 5-HT₂ receptor occupancy after 1 mg risperidone orally in three healthy subjects who were extensive metabolizers (EM) of debrisoquin, using positron emission tomography and the radioligands [¹¹C]raclopride and [¹¹C]NMSP. In this study, the same experimental design was repeated in two healthy poor metabolizers (PM) of debrisoquin to compare the D₂ and 5-HT₂ receptor occupancy induced by risperidone in EM and PM. The two PM had much higher plasma concentrations and longer elimination half-lives of risperidone than the three EM. Plasma concentrations of the sum of risperidone and 9-hydroxyrisperidone partly overlapped among the EM and PM. D₂ receptor occupancy was 50% and 54% in the two PM, as compared to 40%, 43% and 55% in the EM. 5-HT₂ receptor occupancy was 63% and 73%, as compared to 45%, 56% and 68% in the EM. These findings support the view that the active 9-hydroxyl metabolite of risperidone contributes to the in vivo effects of risperidone in humans, and thus partly counterbalances the marked variability in the disposition of risperidone.     

5-HT2 and D2 dopamine receptor occupancy in the living human brain

It has been suggested that a combined blockade of 5-HT₂ and D₂ dopamine receptors may be superior to D₂ dopamine antagonists alone in the treatment of schizophrenia. Risperidone, which has a high affinity for 5-HT₂ and D₂ dopamine receptors in vitro, is a new antipsychotic drug that has been developed according to this hypothesis. The aim of this study was to examine if risperidone indeed induces 5-HT₂ and D₂ dopamine receptor occupancy in vivo in humans. Central receptor occupancy was examined by positron emission tomography (PET) in three healthy men after oral administration of 1 mg risperidone. [¹¹C]N-methylspiperone ([¹¹C]NMSP) was used as a radioligand for determination of 5-HT₂ receptor occupancy in the neocortex. Both an equilibrium ratio analysis and a kinetic three-compartmental analysis indicated a 5-HT₂ receptor occupancy about 60%. [¹¹C]raclopride was used as a radioligand for determination of D₂ dopamine receptor occupancy in the striatum and the calculated occupancy was about 50%. This is the first quantitative determination of 5-HT₂ receptor occupancy induced by an antipsychotic drug in the living human brain. The results indicate that 5-HT₂ receptor occupancy should be very high at the dose level of 4–10 mg risperidone daily, as suggested for clinical use. Risperidone is thus an appropriate compound for clinical evaluation of the benefit of combined 5-HT₂ and D₂ dopamine receptor blockade in the treatment of schizophrenia.This paper was presented in part at the XVIIIth C.I.N.P. Congress, Nice, France, 28th June-2nd July 1992.     

In vivo occupancy of dopamine D2 receptors by antipsychotic drugs and novel compounds in the mouse striatum and olfactory tubercles

Interaction with dopamine D₂-like receptors plays a major role in the therapeutic effects of antipsychotic drugs. We examined in vivo dopamine D₂ receptor occupancy of various established and potential antipsychotics in mouse striatum and olfactory tubercles 1 h after administration of the compound, using [³H]nemonapride as a ligand. All the compounds reduced in vivo binding of [³H]nemonapride in the striatum. When administered systemically, conventional antipsychotics, D₂ antagonists, nemonapride (ID₅₀: 0.034 mg/kg), eticlopride (0.047), haloperidol (0.11) and raclopride (0.11) potently inhibited [³H]nemonapride binding. The ‘atypical’ antipsychotics, risperidone (0.18), ziprasidone (0.38), aripiprazole (1.6), olanzapine (0.99), and clozapine (11.1) were less potent for occupying D₂-like receptors. New compounds, displaying marked agonism at 5-HT_1A receptors in addition to D₂ receptor affinity, exhibited varying D₂ receptor occupancy: bifeprunox (0.25), SLV313 (0.78), SSR181507 (1.6) and sarizotan (6.7). ID₅₀ values for inhibition of [³H]nemonapride binding in the striatum correlated with those in the olfactory tubercles (r=0.95, P<0.0001). These values also correlated with previously-reported in vitro affinity of the compounds at rat D₂ receptors (r=0.85, P=0.0001) and with inhibition of apomorphine-induced climbing in mice (r=0.79 P=0.0005). In contrast, there was no significant correlation between ID₅₀ values herein and previously-reported ED₅₀ values for catalepsy in mice. These data indicate that: (1) there is no difference in D₂ receptor occupancy in limbic versus striatal regions between most classical and atypical or potential antipsychotics; and (2) high occupancy of D₂ receptors can be dissociated from catalepsy, if the drugs also activate 5-HT_1A receptors. Taken together, these data support the strategy of simultaneously targeting D₂ receptor blockade and 5-HT_1A receptor activation for new antipsychotics.     

Clozapine binds preferentially to cortical D1-like dopamine receptors in the primate brain: a PET study

Rationale The D₁-like dopamine receptors have been suggested to play a role in the pathophysiology and treatment of schizophrenia. Previous positron emission tomography studies have demonstrated that the atypical antipsychotic clozapine occupies D₁-like dopamine receptors in the striatum in clozapine-treated patients. Objectives The aim of the present study was to compare striatal and cortical D₁-like dopamine receptor occupancy by clozapine in the primate brain. Methods Three monkeys were each examined three times at the same day with the radioligand (+)−[¹¹C]NNC 112. The first measurement was at baseline conditions, the second after 1.5 mg/kg and the third after 6 mg/kg clozapine IV. To compare regional levels of nonspecific binding in brain regions, an additional monkey was examined using the inactive enantiomer (−)−[¹¹C]NNC 112. Receptor occupancy was calculated using both the equilibrium–ratio analysis and the simplified reference tissue model. Results After 1.5 mg/kg the D₁-like dopamine receptor occupancy ranged from 30 to 38% in the striatum, whereas the range was 51 to 57% in the frontal cortex. After 6.0 mg/kg the occupancy was 53 to 64% in the striatum and 63 to 83% in the frontal cortex. The differences between striatal and cortical D₁-like receptors occupancy were between 12 and 25%. The study with (−)−[¹¹C]NNC 112 did not show regional differences in nonspecific binding that might explain the regional differences in occupancy. Conclusions The higher D₁-like dopamine receptor occupancy in the frontal cortex may reflect a different distribution of the D₁ and D₅ dopamine receptor subtypes among brain regions and different affinity of clozapine for the two subtypes. The finding supports the suggestion that binding to D₁-like dopamine receptors may explain clozapine’s atypical drug actions.     

Central 5-HT2A and D2 dopamine receptor occupancy after sublingual administration of ORG 5222 in healthy men

It has been suggested that a combined blockade of 5-HT_2A and D₂-dopamine receptors improves efficacy and reduces the risk for extrapyramidal symptoms when treating schizophrenic patients with antipsychotic drugs. ORG 5222 is a new potential anti-psychotic drug which has high affinity for 5-HT_2A, D₂-dopamine and α₁ adrenergic receptors in vitro. The objective of this study was to examine if ORG 5222 occupies 5-HT_2A and D₂-dopamine receptors in human subjects in vivo. Central receptor occupancy was measured by positron emission tomography (PET) in three healthy subjects after sublingual administration of 100 μg ORG 5222. [¹¹C]N-methylspiperone ([¹¹C] NMSP) was the radioligand used to measure 5-HT_2A receptor binding in the neocortex and [¹¹C]raclopride to measure D₂-dopamine receptor binding in the striatum. The 5-HT_2A occupancy was 15–30% and the D₂-dopamine receptor occupancy was 12–23%. The study confirms that ORG 5222 binds to 5-HT_2A and D₂-dopamine receptors in human brain. Since receptor occupancy of ORG 5222 is rather low, doses higher than 100 μg are suggested in future clinical trials to evaluate the antipsychotic drug effect of ORG 5222. Received: 9 September 1996 / Final version: 2 January 1997     

Comparison of the effects of clozapine and 8-hydroxy-2-(di-<Emphasis Type="Italic">n</Emphasis>-propylamino)tetralin (8-OH-DPAT) on progressive ratio schedule performance: evidence against the involvement of 5-HT<Subscript>1A</Subscript> receptors in the behavioural effects of clozapine

Rationale Performance on progressive ratio schedules has been proposed as a means of assessing the effects of drugs on the efficacy of reinforcers. A mathematical model (Killeen PR (1994) Mathematical principles of reinforcement. Behav Brain Sci 17:105–172) affords a basis for quantifying the effects of drugs on progressive ratio schedule performance. The model postulates a bitonic function relating response rate and ratio size. One parameter of the function, a, expresses the motivational effect of the reinforcer, whereas another parameter, δ, expresses the minimum time needed to execute a response, and is regarded as an index of ‘motor capacity’. Previously we found that the atypical antipsychotic clozapine increased a, indicating an increase in reinforcer efficacy; a similar effect was observed with the 5-hydroxytryptamine (5-HT)_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). It has been suggested that some of clozapine’s behavioural effects are mediated by agonistic action at 5-HT_1A receptors. Objective This study was conducted to compare the effects of clozapine and 8-OH-DPAT on progressive ratio schedule performance. Methods Rats were trained under a time-constrained progressive ratio schedule (50-min sessions). In experiment 1, they received acute doses of clozapine (4 mg kg⁻¹) and 8-OH-DPAT (100 μg kg⁻¹), alone and in combination with the 5-HT_1A receptor antagonist N-[2-(4-[2-methoxyphenyl]-1-piperazinyl)ethyl]-N-2-yridinylcyclohexanecarboxamide (WAY-100635; 30 μg kg⁻¹). In experiment 2, the effects of clozapine (2, 4 and 8 mg kg⁻¹) and 8-OH-DPAT (25, 50 and 100 μg kg⁻¹) were compared between intact rats and rats whose 5-HTergic pathways had been ablated by 5,7-dihydroxytryptamine (5,7-DHT). Results In both experiments, clozapine and 8-OH-DPAT increased a and δ. In experiment 1, WAY-100635 abolished the effect of 8-OH-DPAT on a and δ, but did not alter clozapine’s effects on these parameters. In experiment 2, the effects of clozapine and 8-OH-DPAT did not differ between sham-lesioned and 5,7-DHT-lesioned rats. Conclusions The results confirm previous findings on the effects of clozapine and 8-OH-DPAT on progressive ratio schedule performance. 8-OH-DPAT’s effects are probably mediated by post-synaptic 5-HT_1A receptors; clozapine’s effects are mediated by a different mechanism, which does not appear to involve 5-HT_1A receptors and which does not depend upon an intact 5-HTergic pathway. Jonathan Francis Rickard (1977–2003), a gifted and dedicated Ph.D. student, made a major contribution to this work     

Deramciclane, a putative anxiolytic drug, is a serotonin 5-HT2C receptor inverse agonist but fails to induce 5-HT2C receptor down-regulation

 Deramciclane (EGIS-3886) is a novel anxiolytic agent that binds with high affinity to 5-HT_2A/2C receptors. The interactions of deramciclane with the serotonin 5-HT_2C receptor were characterized further using receptor phosphoinositide hydrolysis assays and receptor autoradiography. Deramciclane antagonized 5-HT_2C receptor mediated 5-HT-stimulated phosphoinositide hydrolysis with an IC₅₀ value of 168 nM. Deramciclane also decreased basal phosphoinositide hydrolysis by up to 33% (EC₅₀ = 93 nM) in a physiological system in the choroid plexus, suggesting that deramciclane possesses inverse agonist properties at this receptor. Administration of single doses of 0.5 mg/kg and 10 mg/kg resulted in a maximal 5-HT_2C receptor occupancy of up to 45% and 79%, respectively, in the choroid plexus. Chronic (14 days) treatment with 0.5 mg/kg or 10 mg/kg deramciclane did not alter [¹²⁵I]DOI (agonist) or [³H]mesulergine (antagonist) binding to 5-HT_2C receptors in the choroid plexus compared to saline-treated controls, as determined by quantitative receptor autoradiography. In comparison, the effects of deramciclane on 5-HT_2A binding characteristics and receptor occupancy were also studied. Deramciclane treatment resulted in 5-HT_2A receptor occupancy of up to 78%, but no significant effect of chronic treatment on 5-HT_2A receptor agonist binding levels was found. In conclusion, these data indicate that deramciclane is a 5-HT_2C receptor inverse agonist and occupies 5-HT_2C receptors during treatment, and that chronic treatment with deramciclane does not lead to 5-HT_2C receptor down-regulation. Received: 24 October 1996/Final version: 8 August 1997     

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-HT_1A 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 pindolol, induced a marked inhibition of [carbonyl-¹¹C]WAY-100635 binding in the neocortex and the raphe nuclei. A preliminary Scatchard analysis yielded 5-HT_1A 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-HT_1A receptors in the primate brain and has potential for determination of 5-HT_1A receptor occupancy and density in psychiatric patients. Received: 24 December 1996/Final version: 7 March 1997     

Antipsychotics possessing antidepressive efficacy increase G<Subscript>olf</Subscript> protein in rat striatum

Rationale Paliperidone ER is a novel antipsychotic drug in an extended-release (ER) formulation. As with all antipsychotics, careful dose setting is necessary to avoid side effects. Objectives In this study, we measured striatal and extrastriatal dopamine D₂ receptor occupancy during paliperidone ER treatment in patients with schizophrenia using positron emission tomography (PET) to compare regional occupancy and to estimate the optimal dose. Materials and methods Thirteen male patients with schizophrenia participated in this 6-week multiple-dose study. Six of them took 3 mg of paliperidone ER per day, four took 9 mg, and three took 15 mg. Two to 6 weeks after first drug intake, two PET scans, one with [¹¹C]raclopride and one with [¹¹C]FLB 457, were performed in each patient on the same day. The relationship between the dose or plasma concentration of paliperidone and dopamine D₂ receptor occupancy was calculated. Results The dopamine D₂ receptor occupancies in the striatum measured with [¹¹C]raclopride and the temporal cortex measured with [¹¹C]FLB 457 were 54.2–85.5% and 34.5–87.3%, respectively. ED₅₀ values of the striatum and temporal cortex were 2.38 and 2.84 mg/day, respectively. There was no significant difference in dopamine D₂ receptor occupancy between the striatum and the temporal cortex. Conclusions The data from this study suggest that paliperidone ER at 6–9 mg provides an estimated level of dopamine D₂ receptor occupancy between 70–80% and that the magnitude of dopamine D₂ receptor occupancy is similar between the striatum and temporal cortex.     

Differential regulation of rat peripheral 5-HT<Subscript>2A</Subscript> and 5-HT<Subscript>2B</Subscript> receptor systems: influence of drug treatment

Most studies of 5-HT₂ receptor regulation have been carried out on the central nervous system (CNS) (which expresses 5-HT_2A and 5-HT_2C receptors); very few in vitro studies have addressed the peripheral receptors 5-HT_2A and 5-HT_2B. The aim of this investigation was to compare the possible short- and long-term processes regulating these peripheral receptors in the rat.The in vitro contractile response elicited by serotonin (5-HT, 10 µM) in the rat gastric fundus (5-HT_2B receptor system) was rapid and followed by a partial fade to a steady state, in contrast with the rat thoracic aorta response (5-HT_2A receptor system), which was more stable, slower and sustained. To characterize drug-receptor interactions, cumulative concentration/response curves (CCRCs) for 5-HT were constructed ex vivo for rat tissues treated with drugs acting at these receptors. Rats were examined 4 or 24 h after a single, i.p. administration of (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)DOI, 1 or 2.5 mg/kg], clozapine, cyproheptadine or rauwolscine (10 mg/kg), 48 h after a single i.p. administration of (±)DOI (2.5 mg/kg), clozapine or cyproheptadine (10 mg/kg) or 24 h after the last of with 15 daily i.p. administrations of (±)DOI (1 or 2.5 mg/kg), clozapine, cyproheptadine or rauwolscine (10 mg/kg). In the aorta, E_max (the maximum response elicited by 5-HT) was unchanged 4 h after a single dose of any of the drugs tested. However, 24 h after a single dose, E_max was lower in animals treated with (±)DOI (2.5 mg/kg), clozapine or cyproheptadine than in controls, whilst 48 h after a single dose of (±)DOI (2.5 mg/kg), clozapine or cyproheptadine there was no difference in E_max between experimental and control animals. After chronic treatment with (±)DOI (2.5 mg/kg), clozapine and cyproheptadine, E_max was lower than in controls. In the gastric fundus, E_max 4 h after a single dose of each drug was lower than in controls, and the response recovered by 24 or 48 h. Following chronic treatment, E_max was significantly lower than in controls for each drug used.These findings suggest first, that regulation of peripheral 5-HT₂ receptors (5-HT_2A and 5-HT_2B) is a functionally significant phenomenon in vivo, and occurs after administration of both agonists and antagonists. Second, the kinetics of peripheral 5-HT₂ receptor regulation were similar in both in vivo and ex vivo experiments. The 5-HT_2B receptors in rat gastric fundus are more sensitive to drug-induced regulation than the 5-HT_2A rat aortic receptors. Finally, long-term regulation of both receptors stabilizes short-term desensitization for longer.     

[<Superscript>3</Superscript>H]LY334370, a novel radioligand for the 5-HT<Subscript>1F</Subscript> receptor. I. In vitro characterization of binding properties

[³H]LY334370 was developed as a radioligand to study the characteristics of this compounds interaction with the 5-HT_1F receptor. Monovalent or divalent cations did not enhance the binding of [³H]LY334370 to the cloned human 5-HT_1F receptor. In the presence of MgCl₂, the time to reach equilibrium was approximately 2 h, while in its absence equilibrium was reached in less than 1 h. [³H]LY334370 had high affinity for the cloned human 5-HT_1F receptor (K_d=0.446 nM) and the 5-HT_1F receptor in rat brain (K_d=0.388 nM). The expression density of 5-HT_1F receptors, as determined by binding to homogenates of cortical regions from rat, was low (B_max=79.1 fmol/mg protein). There was a statistically significant correlation between the apparent pK_i for inhibition of [³H]LY334370 binding and the pEC₅₀ for stimulation of [³⁵S]GTPS binding to homogenates of cells expressing the cloned human 5-HT_1F receptor. In addition, there was a statistically significant correlation between the apparent pK_i for inhibition of [³H]LY334370 binding to the cloned human 5-HT_1F receptor and the pID₅₀ for inhibition of trigeminal nerve stimulated dural plasma protein extravasation in the guinea pig. The conclusion from these studies is that [³H]LY334370 is a high affinity radioligand which can be used for the study of the 5-HT_1F receptor in rat brain or in cells transformed with the human 5-HT_1F receptor.     

Dose-dependent binding of AZD3783 to brain 5-HT1B receptors in non-human primates and human subjects: a positron emission tomography study with [11C]AZ10419369

Rationale

The serotonin 5-HT_1B receptor is a potential target for the pharmacologic treatment of depression. Positron emission tomography (PET) determination of 5-HT_1B receptor occupancy with drug candidates targeting this receptor in non-human primate and human subjects may facilitate translation of research from animal models and guide dose selection for clinical studies. AZD3783 is a recently developed, orally bioavailable 5-HT_1B receptor antagonist with potential antidepressant properties.

Objectives

To determine the relationship between plasma concentration of AZD3783 and occupancy at primate brain 5-HT_1B receptors using PET and the radioligand [¹¹C]AZ10419369.

Methods

PET studies with [¹¹C]AZ10419369 were performed in three non-human primates at baseline and after intravenous injection of AZD3783. Subsequently, PET measurements were undertaken in six human subjects at baseline and after administration of different single oral doses of AZD3783 (1?C40?mg).

Results

After administration in non-human primates and human subjects, AZD3783 reduced regional [¹¹C]AZ10419369 binding in a dose-dependent and saturable manner. The AZD3783 plasma concentration required for 50% receptor occupancy (K _i,plasma) for monkeys was 25 and 27?nmol/L in occipital cortex and striatum, respectively. Corresponding estimates for human occipital cortex and ventral striatum were 24 and 18?nmol/L, respectively.

Conclusions

The potential antidepressant AZD3783 binds in a saturable manner to brain 5-HT_1B receptors with a similar in vivo affinity for human and monkey receptors. [¹¹C]AZ10419369 can be successfully used to determine occupancy at brain 5-HT_1B receptors in vivo and constitutes a useful tool for dose selection in clinical studies with 5-HT_1B receptor compounds.     

IBZM SPECT imaging of striatal dopamine-2 receptors in psychotic patients treated with the novel antipsychotic substance quetiapine in comparison to clozapine and haloperidol

We investigated the striatal dopamine-2 (D₂) receptor occupancy caused by different antipsychotic substances in 18 psychotic patients (16 with schizophrenic and two with schizoaffective disorder according to DSM-IV) with single photon emission computed tomography (SPECT) using ¹²³I-iodobenzamide (IBZM) as tracer substance. Four patients were treated with the novel antipsychotic compound quetiapine (300–700 mg/day), six with clozapine (300–600 mg/ day) and eight with haloperidol (10–20 mg/day). They were compared with eight healthy controls. Measurement of S/F ratios and consecutive calculation of D₂ receptor occupancy revealed a significantly lower striatal D₂ occupancy rate with quetiapine and clozapine in comparison to haloperidol. In correspondence with the low striatal D₂ receptor occupancy rates and again in contrast to the haloperidol treatment group, there were no extrapyramidal motor side-effects (EPS) in the quetiapine and clozapine treatment groups. Therefore, the reported data support the position that quetiapine can be considered to be an atypical antipsychotic substance due to its relatively weak striatal D₂ receptor blocking property and therefore its low propensity to induce EPS. Received: 12 August 1996/Final version: 11 May 1997     

D1- and D2-dopamine receptor occupancy during treatment with conventional and atypical neuroleptics

Using positron emission tomography and the selective ligands ¹¹C-SCH23390 and ¹¹C-raclopride, central D1- and D2-dopamine receptor occupancy was determined in schizophrenic patients treated with clinical doses of classical and atypical neuroleptics. Treatment with ten chemically distinct classical neuroleptics resulted in a 65–89% occupancy of D2-dopamine receptors. This finding represents strong support for the hypothesis that the mechanism of action of antipsychotic drugs is indeed related to a substantial degree of D2-dopamine receptor occupancy. In two patients treated with teh atypical neuroleptic clozapine, 300 mg b.i.d. and 150 mg b.i.d., the D2-dopamine receptor occupancy was 65 and 40%, respectively. D1-dopamine receptor occupancy was determined in six antipsychotic drugtreated patients. No D1-dopamine receptor occupancy was found in patients treated with sulpiride and perphenazine, compounds known to be selective D2-dopamine receptor antagonists. The highest D1-dopamine receptor occupancy, 42%, was found in the patient treated with clozapine 150 mg b.i.d. The effects of the atypical neuroleptic clozapine may be related to a combined effect on both D1- and D2-dopamine receptors.     

Positron emission tomographic analysis of dose-dependent NAD-299 binding to 5-hydroxytryptamine-1A receptors in the human brain

Rationale. The serotonin 5-HT_1A receptor has been ascribed a putative role in the pathophysiology and drug treatment of depression. NAD-299 (generic name robalzotan) is a new potential antidepressant with high affinity and selectivity for the 5-HT_1A receptor. Objectives. The aim of this positron emission tomography (PET) study was to examine the extent and time-course of 5-HT_1A occupancy by NAD-299 in the human brain, in relation to plasma concentration after escalating single oral doses. Methods. Five healthy male subjects received one or more single oral doses of NAD-299 (0.5, 2.5 and 10 mg) in aqueous solution under fasting conditions. Total and unbound (after ultrafiltration) plasma concentrations of NAD-299 were determined by liquid chromatography-mass spectrometry (LC-MC), over a tentative dosage interval of 8 h. Regional 5-HT_1A receptor occupancy in brain was calculated by the simplified reference tissue model using the radioligand [carbonyl-¹¹C]WAY-100635. Results. After the 10 mg dose, occupancy was high in the raphe (62–85%) and neocortical regions (68–75%) at time for C_max, but had declined considerably (17–44%) at 7 h after dose intake. Conclusions. This study confirmed that the new selective 5-HT_1A antagonist NAD-299 occupies 5-HT_1A receptors in the living human brain in a dose-dependent manner following oral dosage. The curvilinear relationship between NAD-299 drug concentration and 5-HT_1A receptor occupancy was established and can be used for dose selection in subsequent clinical patient studies. Electronic Publication     

Role of serotonin 5-HT<Subscript>1A</Subscript> and opioid receptors in the antiallodynic effect of tramadol in the chronic constriction injury model of neuropathic pain in rats

Rationale Tramadol (1RS, 2RS)-2-[(dimethylamino)-methyl]-1-(3-methoxyphenyl)-cyclohexanol) is an atypical centrally acting analgesic agent with weak opioid receptor affinity that, like some antidepressants, enhances the extraneuronal concentrations of the monoamine neurotransmitters, noradrenaline and serotonin, by interfering with their re-uptake and release mechanisms. Objectives The present study was undertaken to evaluate the potential role of 5-HT_1A receptors and opioids receptors in the analgesic effect of tramadol in neuropathic pain. With this aim, the effect of either a selective 5-HT_1A receptor antagonist (WAY-100635, N-2-[4-(2-methoxyphenyl-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane carboxamide) or a selective 5-HT_1A receptor agonist (8-OH-DPAT, 8-hydroxy-2-(di-n-propylamine) tetralin hydrobromide) or an opioid receptor antagonist (naloxone; naloxone hydrochloride dihydrate) was investigated in combination with tramadol by means of the cold-plate test in the chronic constriction injury model in rats. Results The results showed that WAY-100635 (0.8 mg/kg) significantly enhanced the antiallodynic effect of non-effective doses of tramadol (5–10 mg/kg). In contrast, 8-OH-DPAT (0.5 mg/kg) counteracted the antiallodynic effect of an effective dose of tramadol (22 mg/kg). Naloxone (0.5 mg/kg) partially counteracted the antiallodynic effect of tramadol (22 mg/kg). Conclusions These findings suggest the involvement of opioid and 5-HT_1A receptors in the antinociceptive effect of tramadol and support the idea that the combination of tramadol with compounds having 5-HT_1A antagonist properties could be a new strategy to improve tramadol-induced analgesia in neuropathic pain.