Increasing D2 affinity results in the loss of clozapine's atypical antipsychotic action

Haloperidol-induced catalepsy as an animal model for parkinsonism: A systematic review of experimental studies

Several useful animal models for parkinsonism have been developed so far. Haloperidol-induced catalepsy is often used as a rodent model for the study of motor impairments observed in Parkinson's disease and related disorders and for the screening of potential antiparkinsonian compounds. The objective of this systematic review is to identify publications that used the haloperidol-induced catalepsy model for parkinsonism and to explore the methodological characteristics and the main questions addressed in these studies. A careful systematic search of the literature was carried out by accessing articles in three different databases: Web of Science, PubMed and SCOPUS. The selection and inclusion of studies were performed based on the abstract and, subsequently, on full-text analysis. Data extraction included the objective of the study, study design and outcome of interest. Two hundred and fifty-five articles were included in the review. Publication years ranged from 1981 to 2020. Most studies used the model to explore the effects of potential treatments for parkinsonism. Although the methodological characteristics used are quite varied, most studies used Wistar rats as experimental subjects. The most frequent dose of haloperidol used was 1.0 mg/kg, and the horizontal bar test was the most used to assess catalepsy. The data presented here provide a framework for an evidence-based approach to the design of preclinical research on parkinsonism using the haloperidol-induced catalepsy model. This model has been used routinely and successfully and is likely to continue to play a critical role in the ongoing search for the next generation of therapeutic interventions for parkinsonism.

Brain-imaging studies of treatment-resistant schizophrenia: a systematic review

Around 30% of patients with schizophrenia show an inadequate response to antipsychotics-ie, treatment resistance. Neuroimaging studies can help to uncover the underlying neurobiological reasons for such resistance and identify these patients earlier. Additionally, studies examining the effect of clozapine on the brain can help to identify aspects of clozapine that make it uniquely effective in patients with treatment resistance. We did a systematic search of PubMed between Jan 1, 1980, and April 13, 2015, to identify all neuroimaging studies that examined treatment-resistant patients or longitudinally assessed the effects of clozapine treatment. We identified 330 articles, of which 61 met the inclusion criteria. Replicated differences between treatment-resistant and treatment-responsive patients include reductions in grey matter and perfusion of frontotemporal regions, and increases in white matter and basal ganglia perfusion, with effect sizes ranging from 0·4 to greater than 1. Clozapine treatment led to reductions in caudate nucleus volume in three separate studies. The available evidence supports the hypothesis that some of the neurobiological changes seen in treatment-resistant schizophrenia lie along a continuum with treatment-responsive schizophrenia, whereas other differences are categorical in nature and have potential to be used as biomarkers. However, further replication is needed, and for neuroimaging findings to be clinically translatable, future studies need to focus on a-priori hypotheses and be adequately powered.

Subtyping Schizophrenia by Treatment Response: Antipsychotic Development and the Central Role of Positive Symptoms

We have recently proposed a model for subtyping schizophrenia based on antipsychotic (AP) treatment response. Evidence suggests that APs, both old and new, are comparable in terms of efficacy; however, one AP, clozapine, is uniquely effective in one subgroup of patients (that is, those with treatment-resistant schizophrenia [TRS]). This permits us to subdivide schizophrenia into 3 specific groups: AP responsive, clozapine responsive, and clozapine resistant. Here, we integrate this model with current criteria related to TRS and ultraresistant schizophrenia, the latter referred to in our model as clozapine resistant. We suggest several modifications to existing criteria, in line with current evidence and practice patterns, particularly emphasizing the need to focus on positive symptoms. While APs can favourably impact numerous dimensions related to schizophrenia, it is their effect on positive symptoms that distinguishes them from other psychotropics. Further, it is positive symptoms that are central to AP and clozapine resistance, and it is these people that place the greatest demands on acute and long-term inpatient resources. In moving AP development forward, we advocate specifically focusing on positive symptoms and capitalizing on the evidence we have of 3 subtypes of psychosis (that is, positive symptoms) based on treatment response, implicating 3 distinguishable forms of underlying pathophysiology. Conversely, pooling these groups risks obfuscating potentially identifiable differences. Such a position does not challenge the importance of dopamine D2 receptor blockade, but rather highlights the need to better isolate those other subgroups that require something more or entirely different.

Clozapine, a fast-off-D2 antipsychotic

Ever since clozapine was first synthesized and tested, it showed the unique property of having antipsychotic action but no Parkinson-like motor side effects. The antipsychotic basis of clozapine is to transiently occupy dopamine D2 receptors in the human striatum, in contrast to haloperidol and chlorpromazine, which have a prolonged occupation of D2 receptors. The chemical structure of clozapine facilitates a relatively rapid dissociation from D2 receptors. After short-term occupation of D2 receptors, peak neural activity raises synaptic dopamine, which then displaces clozapine. While clozapine also occupies other types of receptors, they may not have a significant role in preventing parkinsonism. Clozapine's transient occupation of D2 receptors permits patients to move easily and comfortably.

Antipsychotic medication and prefrontal cortex activation: a review of neuroimaging findings

Decreased prefrontal activation (hypofrontality) in schizophrenia is thought to underlie negative symptoms and cognitive impairments, and may contribute to poor social outcome. Hypofrontality does not always improve during treatment with antipsychotics. We hypothesized that antipsychotics, which share antagonism at dopamine receptors, with a relatively low dopamine receptor affinity and high serotonin receptor affinity may have a sparing effect on prefrontal function compared to strong dopamine receptor antagonists. We systematically investigated the relation between serotonin and dopamine antagonism of antipsychotics and prefrontal functioning by reviewing neuroimaging studies. The weight of the evidence was consistent with our hypothesis that antipsychotics with low dopaminergic receptor affinity and moderate to high serotonergic affinity were associated with higher activation of the prefrontal cortex. However, clozapine, a weak dopamine and strong serotonin antagonist, was associated with decrease in prefrontal activation. Future studies should further elucidate the link between prefrontal activation and negative symptoms using prospective designs and advanced neuroimaging techniques, which may ultimately benefit the development of treatments for disabling negative symptoms.

Slow dissociation of partial agonists from the D₂ receptor is linked to reduced prolactin release

In this study we investigated the correlation between affinity, efficacy, peripheral receptor occupancy, and kinetic properties of D₂ dopamine receptor ligands with time-course evaluations of prolactin release in rat blood. We profiled typical and atypical antipsychotic antagonists at D₂ receptors, the partial agonist aripiprazole, and four novel partial agonist compounds with different properties. Clozapine and quetiapine revealed lower prolactin release and fast dissociation kinetics, linking fast dissociation and prolactin-sparing properties. Surprisingly, haloperidol, a highly prolactin-releasing antagonist, shared intermediate dissociation properties. Factors other than kinetic properties may thus contribute to prolactin-releasing properties of antagonists. Partial agonists sharing similar efficacies and receptor occupancies differed markedly in their ability to induce hyperprolactinaemia. Aripiprazole moderately released prolactin even at high receptor occupancies, with slow dissociation from D₂ receptors. Other compounds displaying low affinities and fast dissociations released prolactin substantially, although less than haloperidol. The effect augmented after repeated administrations. Compounds with high affinities and slow dissociation rates stimulated moderate prolactin release at high receptor occupancies, reaching a ceiling effect at 50-60% occupancy. Moreover, the effect developed tolerance. In conclusion, we investigated the affinity and kinetic properties of D₂ partial agonists associated with their ability to induce prolactin release in blood. We propose that for D₂ partial agonists, at comparable intrinsic activities and peripheral occupancies, the prolactin-releasing properties are linked to their kinetic rate properties. Differently from D₂ antagonists, partial agonists display slow dissociation and high affinity associated with a low prolactin release profile.

Presynaptic regulation of dopamine transmission in schizophrenia

A role for dopamine (DA) release in the hallucinations and other positive symptoms associated with schizophrenia has long been inferred from the antipsychotic response to D2 DA receptor antagonists and because the DA releaser amphetamine can be psychotogenic. Recent studies suggest that patients with schizophrenia, including those never exposed to antipsychotic drugs, maintain high presynaptic DA accumulation in the striatum. New laboratory approaches are elucidating mechanisms that control the level of presynaptic DA stores, thus contributing to fundamental understanding of the basic pathophysiologic mechanism in schizophrenia.

The dopaminergic stabilizers pridopidine (ACR16) and (-)-OSU6162 display dopamine D(2) receptor antagonism and fast receptor dissociation properties

A new pharmacological class of CNS ligands with the unique ability to stimulate or suppress motor and behavioral symptoms depending on the prevailing dopaminergic tone has been suggested as "dopaminergic stabilizers". The molecular mode-of-action of dopaminergic stabilizers is not yet fully understood, but they are assumed to act via normalization of dopaminergic signaling, through interactions with the dopamine D(2) receptor. Here we have evaluated the dopaminergic stabilizers pridopidine (ACR16) and (-)-OSU6162, as well as the new compound N-{[(2S)-5-chloro-7-(methylsulfonyl)-2,3-dihydro-1,4-benzodioxin-2-yl]methyl}ethanamine (NS30678) in a series of cellular in vitro dopamine D(2) receptor functional and binding assays. Neither ACR16, (-)-OSU6162, nor NS30678 displayed detectable dopamine D(2) receptor-mediated intrinsic activity, whereas they concentration-dependently antagonized dopamine-induced responses with IC(50) values of 12.9microM, 5.8microM, and 7.0nM, respectively. In contrast to the high-affinity typical antipsychotics haloperidol and raclopride, the dopaminergic stabilizers ACR16 and (-)-OSU6162 both displayed fast dopamine D(2) receptor dissociation properties, a feature that has previously been suggested as a contributing factor to antipsychotic atypicality and attributed mainly to low receptor affinity. However, the finding that NS30678, which is equipotent to haloperidol and raclopride, also displays fast receptor dissociation, suggests that the agonist-like structural motif of the dopaminergic stabilizers tested is a critical dissociation rate determinant. The results demonstrate that dopaminergic stabilizers exhibit fast competitive dopamine D(2) receptor antagonism, possibly allowing for temporally variable and activity-dependent dopamine D(2) receptor occupancy that may partly account for their unique stabilization of dopamine dependent behaviors in vivo.

Clozapine treatment of childhood-onset schizophrenia: evaluation of effectiveness, adverse effects, and long-term outcome

OBJECTIVE

Clozapine is a unique atypical antipsychotic with superior efficacy in treatment-resistant schizophrenia. Plasma concentration of clozapine and its major metabolite N-desmethylclozapine (NDMC) as well as the ratio of NDMC to clozapine have been reported to be predictors of clozapine response. Here we evaluate these as well as other measures in an effort to find predictors of response to clozapine in our early-onset treatment-refractory population.

METHOD

Fifty-four children and adolescents participated in double-blind (n = 22) or open-label (n = 32) clozapine trials. Clinical evaluations took place at baseline, week 6 on clozapine, and at 2- to 6-year follow-up. The data were analyzed in relation to demographics, age at onset, IQ, clozapine dose, and plasma concentrations of prolactin, clozapine, NDMC, and NDMC/clozapine ratio. Stepwise regression and correlation analyses were performed to find predictors of treatment response.

RESULTS

Clinical improvement after 6 weeks of clozapine treatment, as measured by the percentage of improvement on the Brief Psychiatric Rating Scale and the Scale for the Assessment of Positive Symptoms, was strongly associated with the NDMC/clozapine ratio at the 6-week time point (Pearson correlation coefficient: r = 0.41; p < .01 for Brief Psychiatric Rating Scale and r = 0.43; p < .01 for Scale for the Assessment of Positive Symptoms). Although the rate of side effects was higher than that typically found in the adult population, it did not appear to be related to clozapine dose, clozapine or NDMC plasma concentrations, or NDMC/clozapine ratio. Outcome at long-term follow-up, as measured by Children's Global Assessment Scale, was associated with lesser illness severity at baseline and with greater improvement during the initial 6 weeks of clozapine treatment.

CONCLUSIONS

The NDMC/clozapine ratio may be a valuable predictor of response to clozapine and may suggest new approaches to clozapine treatment.

The effects of ziprasidone on regional c-Fos expression in the rat forebrain

RATIONALE

Typical and atypical antipsychotic drugs produce characteristic patterns of immediate early gene expression in rat forebrain that are considered to reflect their effects in schizophrenia subjects.

OBJECTIVE

To use c-Fos immunohistochemistry to investigate the functional neuroanatomical profile of the newly introduced atypical agent ziprasidone.

MATERIALS AND METHODS

c-Fos immunohistochemistry was performed on paraformaldehyde-fixed cryosections of rat brains obtained, initially, from animals 2, 4, or 6 h after oral administration of 10 mg/kg ziprasidone or vehicle and, subsequently, from animals 2 h after oral administration of 1, 3, or 10 mg/kg ziprasidone or vehicle. The density of immunoreactive nuclei was assessed in pre-determined forebrain regions.

RESULTS

Ziprasidone induced a time-dependent increase in the density of c-Fos-positive nuclei that was maximal at 2 h. At the 2 h time-point, c-Fos expression was significantly (p<0.05) elevated in the shell and core of the nucleus accumbens, lateral and medial caudate putamen, and lateral septum. At 4 h post-dose, c-Fos expression was also significantly increased in the cingulate gyrus. Ziprasidone-induced c-Fos expression was dose-dependent with significant (p<0.05) c-Fos expression observed in the nucleus accumbens (shell and core) and caudate putamen (lateral and medial) at 3 and 10 mg/kg and in the lateral septum at 10 mg/kg.

CONCLUSIONS

Increased c-Fos expression in the nucleus accumbens and lateral septum is considered to be predictive of activity against positive symptoms, in the caudate putamen of motor side effect liability, and in the cingulate gyrus of efficacy against negative symptoms. Thus, the observed pattern of c-Fos expression induced in rat brain by ziprasidone is consistent with its reported clinical effects, namely, efficacy against positive symptoms with a therapeutic window over motor side effects and with some activity against negative symptoms.

Structure-affinity relationships of halogenated predicentrine and glaucine derivatives at D1 and D2 dopaminergic receptors: halogenation and D1 receptor selectivity

Halogenation of the aporphine alkaloid boldine at the 3-position leads to increased affinity for rat brain D(1)-like dopaminergic receptors with some selectivity over D(2)-like receptors. A series of 3-halogenated and 3,8-dihalogenated (halogen=Cl, Br or I) derivatives of predicentrine (9-O-methylboldine) and glaucine (2,9-di-O-methylboldine) were prepared and assayed for binding at D(1) and D(2) sites. Halogenation of predicentrine led to strong increases in affinity for D(1)-like receptors, while the affinities for D(2)-like receptors were either practically unchanged or reduced three- to fourfold. Halogenated glaucine derivatives did not show any clear trend towards enhanced selectivity, and the affinities were poor and similar to or worse than the values previously recorded for glaucine itself. Together with earlier work on boldine derivatives, these results suggest that the 2-hydroxy group on the aporphine skeleton may determine a binding mode favoring D(1)-like over D(2)-like receptors, with enhanced affinity when the C-3 position is halogenated.

Contrasting loxapine to its isomer isoloxapine--the critical role of in vivo D2 blockade in determining atypicality

BACKGROUND

Loxapine is a typical antipsychotic while isoloxapine, its 8Cl-isomer, shows atypicality in some animal models. The basis for this difference is not well understood. The purpose of this study was to systematically compare the two drugs in in vitro and in vivo animal models, and to understand mechanisms underlying their differential typical/atypical profiles.

METHODS

The in vitro and in vivo receptor profiles as well as the action of loxapine and isoloxapine on rat conditioned avoidance response (CAR), catalepsy (CAT), striatal FOS expression and prolactin levels were determined. To understand loxapine's typical profile, we added MDL100,907, to provide loxapine+MDL the same in vivo 5-HT2/D2 ratio as isoloxapine, while holding its D2 component constant.

RESULTS

Isoloxapine behaved as an "atypical" antipsychotic demonstrating CAR inhibition, low CAT, no significant prolactin elevation, and minimal FOS expression in the dorsolateral striatum. Loxapine behaved like a typical antipsychotic, showing unexpectedly high in vivo D2 occupancy. Addition of MDL100,907, which resulted in a very high 5-HT2/D2 in vivo ratio, did not alter loxapine + MDL's typical profile.

CONCLUSIONS

Loxapine's behaviour as a typical antipsychotic is most likely due to its disproportionately high D2 occupancy. Appropriate action at D2 receptors in vivo, rather than the high 5-HT2/D2 ratio, seems to be critical in determining why isoloxapine behaves like an atypical antipsychotic.

[Leponex, 10 years after -- a clinical review]

Clozapine was one of the major advances in the treatment of schizophrenia since the introduction of the classic antipsychotic agent chlorpromazine in the 1950s. Over the past 10 years, clozapine has become the reference compound for the development of new antipsychotics, and new drugs have been developed which have also claimed atypical status. The indications of clozapine were recently extended to Psychosis in Parkinson's disease and harmonized in the European Union. This provides the opportunity to update the data on clozapine in the treatment of schizophrenia. In this article we review current clinical evidence in schizophrenia to address the following issues: 1) Efficacy in refractory/positive symptoms: a systematic and critical analysis of 14 double-blind clinical trials in comparison with both standard and novel antipsychotics show consistent findings in favour of clozapine, with all but three of the reports demonstrating superiority. The review of studies allow us to say little about the predictors of treatment response, time to clozapine response and about the impact of clozapine on the quality of patients'life and longer-term outcome. Treatment options for clozapine non-responders are reviewed. 2) Risk of EPS: clozapine is considered to have a minimal risk of EPS and in all studies where a valid methodology was used, a clear superiority over the other neuroleptics is demonstrated. It is pointed out that, if the prevalence and incidence of EPS with clozapine is low, it is not zero. All the studies assessing clozapine treatment for TD have major methodological limitations, so no final conclusion can be drawn. 3) Efficacy for primary and secondary negative symptoms and neurocognitive effects: the data of clinical studies where negative symptoms scales were used favour clozapine in terms of improvement. However most of the studies were carried out in populations with predominantly positive symptoms. With regard to the need to distinguish primary and secondary symptoms, data are conflicting regarding the benefit of clozapine. Due to the lack of studies with a valid methodology, no definitive conclusion can be drawn about the efficacy on clozapine on the deficit syndrome and on neurocognitive disorders. 4) Impact on suicide risk: 4 out of 6 retrospective studies provide evidence for the ability of clozapine therapy to reduce suicidal behaviour. The results of a recent randomized, parallel-group study designed to compare clozapine versus olanzapine in preventing suicide attempts seems to confirm this hypothesis. We also address the tolerability and safety data, especially haematologic, comitial, cardiovascular and metabolic side-effects. The effectiveness of blood monitoring for the management of neutropenia and agranulocytosis demands that the recommendations are strictly followed. The use of clozapine at doses higher than 600 mg daily should follow published recommendations, in order to minimize the risk of seizures; these include anticonvulsant regimens based on blood levels. With regard to the cardiovascular mortality, if clozapine therapy has negligible effects on QT interval, its association with potential fatal myocarditis cannot be excluded in young patients who should be investigated if they develop cardiac symptoms in the first weeks of treatment. Available data support the notion that the frequency of bodyweight gain is high with several new antipsychotics, including clozapine. Potential long term effects of bodyweight gain on mortality and morbidity have to be taken into consideration. The pharmacological mechanisms underlying the "unique clozapine profile" is discussed. Clozapine remains the only antipsychotic with efficacy at relatively low D2 receptor occupancy. The pharmacogenetic and pharmacokinetic aspects are also reviewed. Finally, the place of clozapine in the current treatment of schizophrenia is highlighted to inform the development of guidelines for clinical management.

Continuous but not intermittent olanzapine infusion induces vacuous chewing movements in rats

BACKGROUND

Continuous, but not intermittent, infusion with a conventional antipsychotic (haloperidol, HAL) can induce the vacuous chewing movement (VCM) syndrome in rats. The objective of this study was to determine whether continuous, versus intermittent, olanzapine (OLZ) infusion differently affects the development of VCMs.

METHODS

Experiment 1: Animals were treated with 7.5 mg/kg/day of OLZ or vehicle (VEH) via either minipump (MP) or daily subcutaneous (SC) injections for 8 weeks. Experiment 2: A separate group of rats were treated with 15 mg/kg/day of OLZ, or 1 mg/kg/day of HAL or VEH via MP for 8 weeks. Dopamine D2 receptor occupancy levels were measured, ex vivo, with [3H]-raclopride.

RESULTS

Experiment 1: Rats receiving 7.5 mg/kg/day of OLZ via MP (51% D2 occupancy), but not those receiving the same dose via daily SC injections (94% peak D2 occupancy), showed significant VCM levels compared with control animals (p = .02). Experiment 2: Both OLZ (67% D2 occupancy) and HAL (79% D2 occupancy) led to similar increases in VCMs compared with VEH (p = .005).

CONCLUSIONS

This study provides strong evidence that even an atypical antipsychotic like OLZ, which rarely gives rise to tardive dyskinesia in the clinic, can lead to the VCM syndrome in rats if the antipsychotic is administered in a method (via MP) that leads to continuous presence of the drug in the brain.

Combination treatments for schizophrenia

Combination treatments, especially combinations of antipsychotics, are used frequently for schizophrenia, despite a paucity of evidence regarding their safety and efficacy. Because the literature basis is weak and expert recommendations are largely lacking, providers should be vigilant in documenting improved outcomes for patients thought to benefit from combination treatments. Target symptoms that have been studied include psychosis, cognitive deficits, and negative symptoms. The strongest evidence is for augmentation of clozapine with another antipsychotic or with electroconvulsive therapy for persistent positive symptoms. Combination treatments for cognitive deficits and negative symptoms are being actively investigated, but current evidence is insufficient to recommend available agents for these components of schizophrenia. It is important that appropriate monotherapies be given adequate trials before resorting to combination therapies.

Subchronic phencyclidine treatment impairs performance of C57BL/6 mice in the attentional set-shifting task

Schizophrenic patients show deficits in their ability to shift attention to new stimuli during attentional set-shifting tasks. An equivalent task has been developed in rodents. This task was used to assess effects of phencyclidine (PCP), a drug widely used to model schizophrenia, in mice. Mice were administered PCP (0.63 or 1.3 mg/kg) daily, beginning 5 days prior to training and continuing throughout the test period. Mice were trained to discriminate stimuli to find a food reward in a series of discriminations. PCP increased the number of trials required to reach the criterion during simple and reversal discriminations, but not when the discriminations were repeated with a new set of stimuli (i.e. during the intradimensional shift and its reversal), suggesting an interaction between PCP effects and novelty. The 1.3 mg/kg of PCP also impaired performance of mice during the extradimensional shift. Similar results have been reported in schizophrenic patients. Although, in control mice, performance of the extradimensional shift was not impaired relative to the intradimensional shift, suggesting that the mouse may not be a suitable species for assessment in this model, specific discriminations within this task were sensitive to impairment by PCP, implying, therefore, that it may have some utility in modelling discrete aspects of schizophrenia in humans.

Pyrrolo[1,3]benzothiazepine-Based Serotonin and Dopamine Receptor Antagonists. Molecular Modeling, Further Structure−Activity Relationship Studies, and Identification of Novel Atypical Antipsychotic Agents

Recently we reported the pharmacological characterization of the 9,10-dihydropyrrolo[1,3]benzothiazepine derivative (S)-(+)-8 as a novel atypical antipsychotic agent. This compound had an optimum pK(i) 5-HT(2A)/D(2) ratio of 1.21 (pK(i) 5-HT(2A) = 8.83; pK(i) D(2) = 7.79). The lower D(2) receptor affinity of (S)-(+)-8 compared to its enantiomer was explained by the difficulty in reaching the conformation required to optimally fulfill the D(2) pharmacophore. With the aim of finding novel atypical antipsychotics we further investigated the core structure of (S)-(+)-8, synthesizing analogues with specific substituents; the structure-activity relationship (SAR) study was also expanded with the design and synthesis of other analogues characterized by a pyrrolo[2,1-b][1,3]benzothiazepine skeleton, substituted on the benzo-fused ring or on the pyrrole system. On the 9,10-dihydro analogues the substituents introduced on the pyrrole ring were detrimental to affinity for dopamine and for 5-HT(2A) receptors, but the introduction of a double bond at C-9/10 on the structure of (S)-(+)-8 led to a potent D(2)/5-HT(2A) receptor ligand with a typical binding profile (9f, pK(i) 5-HT(2A)/D(2) ratio of 1.01, log Y = 8.43). Then, to reduce D(2) receptor affinity and restore atypicality on unsaturated analogues, we exploited the effect of specific substitutions on the tricyclic system of 9f. Through a molecular modeling approach we generated a novel series of potential atypical antipsychotic agents, with optimized 5HT(2A)/D(2) receptor affinity ratios and that were easier to synthesize and purify than the reference compound (S)-(+)-8. A number of SAR trends were identified, and among the analogues synthesized and tested in binding assays, 9d and 9m were identified as the most interesting, giving atypical log Y scores respectively 4.98 and 3.18 (pK(i) 5-HT(2A)/D(2) ratios of 1.20 and 1.30, respectively). They had a multireceptor affinity profile and could be promising atypical agents. Compound 9d, whose synthesis is easier and whose binding profile is atypical (log Y score similar to that of olanzapine, 3.89), was selected for further biological investigation. Pharmacological and biochemical studies confirmed an atypical antipsychotic profile in vivo. The compound was active on conditioned avoidance response at 1.1 mg/kg, a dose 100-times lower than that required to cause catalepsy (ED(50) >90 mg/kg), it induced a negligible increase of prolactin serum levels after single and multiple doses, and antagonized the cognitive impairment induced by phencyclidine. In conclusion, the pharmacological profile of 9d proved better than clozapine and olanzapine, making this compound a potential clinical candidate.

Serotonin receptors: their key role in drugs to treat schizophrenia

Serotonin (5-HT)-receptor-based mechanisms have been postulated to play a critical role in the action of the new generation of antipsychotic drugs (APDs) that are usually referred to as atypical APDs because of their ability to achieve an antipsychotic effect with lower rates of extrapyramidal side effects (EPS) compared to first-generation APDs such as haloperidol. Specifically, it has been proposed by Meltzer et al. [J. Pharmacol. Exp. Ther. 251 (1989) 238] that potent 5-HT2A receptor antagonism together with weak dopamine (DA) D2 receptor antagonism are the principal pharmacologic features that differentiate clozapine and other apparent atypical APDs from first-generation typical APD. This hypothesis is consistent with the atypical features of quetiapine, olanzapine, risperidone, and ziprasidone, which are the most common treatments for schizophrenia in the United States and many other countries, as well as a large number of compounds in various stages of development. Subsequent research showed that 5-HT1A agonism may be an important consequence of 5-HT2A antagonism and that substitution of 5-HT1A agonism for 5-HT2A antagonism may also produce an atypical APD drug when coupled with weak D2 antagonism. Aripiprazole, the most recently introduced atypical APD, and a D2 receptor partial agonist, may also owe some of its atypical properties to its net effect of weak D2 antagonism, 5-HT2A antagonism and 5-HT1A agonism [Eur. J. Pharmacol. 441 (2002) 137]. By contrast, the alternative "fast-off" hypothesis of Kapur and Seeman [Am. J. Psychiatry 158 (2001) 360] applies only to clozapine and quetiapine and is inconsistent with the "slow" off rate of most atypical APDs, including olanzapine, risperidone and ziprasidone. 5-HT2A and 5-HT1A receptors located on glutamatergic pyramidal neurons in the cortex and hippocampus, 5-HT2A receptors on the cell bodies of DA neurons in the ventral tegmentum and substantia nigra and GABAergic interneurons in the cortex and hippocampus, and 5-HT1A receptors in the raphe nuclei are likely to be important sites of action of the atypical APDs. At the same time, evidence has accumulated for the important modulatory role of 5-HT2C and 5-HT6 receptors for some of the effects of some of the current APDs. Thus, 5-HT has joined DA as a critical target for developing effective APDs and led to the search for novel drugs with complex pharmacology, ending the exclusive search for single-receptor targets, e.g., the D3 or D4 receptor, and drugs that are selective for them.