A double-blind, randomized, placebo-controlled study of the dopamine D₃ receptor antagonist ABT-925 in patients with acute schizophrenia

Association between treatment response and dose of blonanserin transdermal patch in patients with acute schizophrenia: A post hoc cluster analysis based on baseline psychiatric symptoms

AIM

To explore the optimal dose of blonanserin transdermal patch (BNS-P) based on baseline psychiatric symptomatic characteristics during acute schizophrenia.

METHODS

A post hoc cluster analysis was conducted using data from a 6-week randomized, double-blind, placebo-controlled study of BNS-P (40 or 80 mg/day) in acute schizophrenia. We classified patients into three clusters based on baseline psychiatric symptoms. Efficacy was assessed using the change from baseline to week 6 in the PANSS total score. Safety was assessed by the incidence of adverse events.

RESULTS

Among 577 patients, three clusters were identified, characterized by severe psychiatric (Cluster-S; n = 122), predominant negative (Cluster-N; n = 191), and predominant positive (Cluster-P; n = 264) symptoms. In Cluster-P, both BNS-P 40 and 80 mg/day reduced PANSS total score significantly more than placebo (p = 0.036, effect size = 0.342; p < 0.001, effect size = 0.687, respectively). In Cluster-S and -N, only BNS-P 80 mg/day reduced PANSS total score significantly more than placebo (p = 0.045, effect size = 0.497; p = 0.034, effect size = 0.393, respectively). The effect size was greater at 80 mg/day than at 40 mg/day across all clusters. The most common treatment-emergent adverse events were akathisia and skin-related adverse events in all clusters.

CONCLUSION

BNS-P exhibited a dose-dependent antipsychotic effect in all clusters, particularly highlighting its efficacy in patients with predominant positive symptoms, even at lower doses. These findings provide novel and valuable insights for determining BNS-P dose tailoring to individual symptomatic characteristics in real-world practice.

Carbazole and tetrahydro-carboline derivatives as dopamine D3 receptor antagonists with the multiple antipsychotic-like properties

Dopamine D3 receptor (D3R) is implicated in multiple psychotic symptoms. Increasing the D3R selectivity over dopamine D2 receptor (D2R) would facilitate the antipsychotic treatments. Herein, novel carbazole and tetrahydro-carboline derivatives were reported as D3R selective ligands. Through a structure-based virtual screen, ZLG-25 (D3R Ki = 685 nmol/L; D2R Ki > 10,000 nmol/L) was identified as a novel D3R selective bitopic ligand with a carbazole scaffold. Scaffolds hopping led to the discovery of novel D3R-selective analogs with tetrahydro-β-carboline or tetrahydro-γ-carboline core. Further functional studies showed that most derivatives acted as hD3R-selective antagonists. Several lead compounds could dose-dependently inhibit the MK-801-induced hyperactivity. Additional investigation revealed that 23j and 36b could decrease the apomorphine-induced climbing without cataleptic reaction. Furthermore, 36b demonstrated unusual antidepressant-like activity in the forced swimming tests and the tail suspension tests, and alleviated the MK-801-induced disruption of novel object recognition in mice. Additionally, preliminary studies confirmed the favorable PK/PD profiles, no weight gain and limited serum prolactin levels in mice. These results revealed that 36b provided potential opportunities to new antipsychotic drugs with the multiple antipsychotic-like properties.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

Potential Mechanisms for Why Not All Antipsychotics Are Able to Occupy Dopamine D3 Receptors in the Brain in vivo

Dysfunctions of the dopaminergic system are believed to play a major role in the core symptoms of schizophrenia such as positive, negative, and cognitive symptoms. The first line of treatment of schizophrenia are antipsychotics, a class of medications that targets several neurotransmitter receptors in the brain, including dopaminergic, serotonergic, adrenergic and/or muscarinic receptors, depending on the given agent. Although the currently used antipsychotics display in vitro activity at several receptors, majority of them share the common property of having high/moderate in vitro affinity for dopamine D₂ receptors (D₂Rs) and D₃ receptors (D₃Rs). In terms of mode of action, these antipsychotics are either antagonist or partial agonist at the above-mentioned receptors. Although D₂Rs and D₃Rs possess high degree of homology in their molecular structure, have common signaling pathways and similar in vitro pharmacology, they have different in vivo pharmacology and therefore behavioral roles. The aim of this review, with summarizing preclinical and clinical evidence is to demonstrate that while currently used antipsychotics display substantial in vitro affinity for both D₃Rs and D₂Rs, only very few can significantly occupy D₃Rs in vivo. The relative importance of the level of endogenous extracellular dopamine in the brain and the degree of in vitro D₃Rs receptor affinity and selectivity as determinant factors for in vivo D₃Rs occupancy by antipsychotics, are also discussed.

A Historical Perspective on the Dopamine D3 Receptor

Before 1990, the multiplicity of dopamine receptors beyond D1 and D2 had remained a controversial concept, despite its substantial clinical implications, at a time when it was widely accepted that dopamine interacted with only two receptor subtypes, termed D1 and D2, differing one from the other by their pharmacological specificity and opposite effects on adenylyl cyclase. It was also generally admitted that the therapeutic efficacy of antipsychotics resulted from blockade of D2 receptors. Thanks to molecular biology techniques, the D3 receptor could be characterized as a distinct molecular entity having a restricted anatomical gene expression and different signaling, which could imply peculiar functions in controlling cognitive and emotional behaviors. Due to the structural similarities of D2 and D3 receptors, the search for D3-selective compounds proved to be difficult, but nevertheless led to the identification of fairly potent and in vitro and in vivo selective compounds. The latter permitted to confirm a role of D3 receptors in motor functions, addiction, cognition, and schizophrenia, which paved the way for the development of new drugs for the treatment of psychiatric disorders.

Synthesis and In Vitro Evaluation of Novel Dopamine Receptor D2 3,4-dihydroquinolin-2(1H)-one Derivatives Related to Aripiprazole

In this pilot study, a series of new 3,4-dihydroquinolin-2(1H)-one derivatives as potential dopamine receptor D₂ (D₂R) modulators were synthesized and evaluated in vitro. The preliminary structure-activity relationship disclosed that compound 5e exhibited the highest D₂R affinity among the newly synthesized compounds. In addition, 5e showed a very low cytotoxic profile and a high probability to cross the blood-brain barrier, which is important considering the observed affinity. However, molecular modelling simulation revealed completely different binding mode of 5e compared to USC-D301, which might be the culprit of the reduced affinity of 5e toward D₂R in comparison with USC-D301.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Binding of the D3-preferring antipsychotic candidate F17464 to dopamine D3 and D2 receptors: a PET study in healthy subjects with [11C]-(+)-PHNO

RATIONALE

F17464, a dopamine D3 receptor antagonist with relatively high D3 selectivity (70 fold vs D2 in vitro), exhibits an antipsychotic profile in preclinical studies, and therapeutic efficacy was demonstrated in a randomized placebo-controlled clinical trial in patients with schizophrenia (Bitter et al. Neuropsychopharmacology 44(11):1917-1924, 2019).

OBJECTIVE

This open-label study in healthy male subjects aimed at characterizing F17464 binding to D3/D2 receptors and the time course of receptor occupancy using positron emission tomography (PET) imaging with a D3-preferring tracer, [¹¹C]-(+)-PHNO.

METHODS

PET scans were performed at baseline and following a single 30 mg or 15 mg dose of F17464 (3 subjects/dose), and blood samples were collected for pharmacokinetic analysis. Receptor occupancy was calculated based upon reduction in binding potential of the tracer following F17464 administration. The relationship between plasma F17464 concentration and D3/D2 receptor occupancy was modeled and the plasma concentration corresponding to 50% receptor occupancy (EC₅₀) calculated.

RESULTS

Both doses of F17464 robustly blocked [¹¹C]-(+)-PHNO D3 receptor binding, with substantial occupancy from 1 h post-administration, which increased at 6-9 h (89-98% and 79-87% for the 30 mg and 15 mg groups, respectively) and remained detectable at 22 h. In contrast, D2 binding was only modestly blocked at all time points (< 18%). F17464 exhibited a combination of rapid peripheral kinetics and hysteresis (persistence of binding 22 h post-dose despite low plasma concentration). The best estimate of the EC₅₀ was 19 ng ml^-1 (~ 40 nM).

CONCLUSION

Overall, F17464 was strongly D3-selective in healthy volunteers, a unique profile for an antipsychotic candidate drug.

Antipsychotics: Mechanisms underlying clinical response and side-effects and novel treatment approaches based on pathophysiology

Antipsychotic drugs are central to the treatment of schizophrenia and other psychotic disorders but are ineffective for some patients and associated with side-effects and nonadherence in others. We review the in vitro, pre-clinical, clinical and molecular imaging evidence on the mode of action of antipsychotics and their side-effects. This identifies the key role of striatal dopamine D2 receptor blockade for clinical response, but also for endocrine and motor side-effects, indicating a therapeutic window for D2 blockade. We consider how partial D2/3 receptor agonists fit within this framework, and the role of off-target effects of antipsychotics, particularly at serotonergic, histaminergic, cholinergic, and adrenergic receptors for efficacy and side-effects such as weight gain, sedation and dysphoria. We review the neurobiology of schizophrenia relevant to the mode of action of antipsychotics, and for the identification of new treatment targets. This shows elevated striatal dopamine synthesis and release capacity in dorsal regions of the striatum underlies the positive symptoms of psychosis and suggests reduced dopamine release in cortical regions contributes to cognitive and negative symptoms. Current drugs act downstream of the major dopamine abnormalities in schizophrenia, and potentially worsen cortical dopamine function. We consider new approaches including targeting dopamine synthesis and storage, autoreceptors, and trace amine receptors, and the cannabinoid, muscarinic, GABAergic and glutamatergic regulation of dopamine neurons, as well as post-synaptic modulation through phosphodiesterase inhibitors. Finally, we consider treatments for cognitive and negative symptoms such dopamine agonists, nicotinic agents and AMPA modulators before discussing immunological approaches which may be disease modifying. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Meta-analysis of Dropout Rates in Placebo-Controlled Randomized Clinical Trials of Atypical Antipsychotics Assessed by PANSS

BACKGROUND AND OBJECTIVES

Subject dropout rates in placebo-controlled randomized clinical trials (RCTs) of antipsychotics are high. The missing values due to dropout represent a potential source of bias in clinical trials. We aimed to identify the potential factors affecting subject dropout in atypical antipsychotics RCTs by conducting a meta-analysis.

METHODS

Placebo-controlled RCTs for atypical antipsychotics using positive and negative syndrome scale (PANSS) as a psychiatric assessment scale were selected by database search. The potential factors affecting subject dropout, such as publication year, study design, and operational factors, were analyzed by meta-regression.

RESULTS

Forty-seven placebo controlled RCTs of atypical antipsychotics of which results were published between 1993 and 2018 were identified through the database search. In the multivariate meta-regression analysis, earlier publication year, older age of subjects, and longer study duration were significantly associated with high subject dropout rates in placebo-controlled clinical trials of atypical antipsychotics.

CONCLUSION

Subject dropout rates in clinical trials of atypical antipsychotics published between 1993 and 2018 year decreased over time. Study duration should be taken into consideration when designing future studies, where short study periods yet appropriate for evaluating the efficacy of new atypical antipsychotics are preferable. Additionally, previous medications and the degree of subject satisfaction with antipsychotics might affect subject dropout rate.

The past and future of novel, non-dopamine-2 receptor therapeutics for schizophrenia: A critical and comprehensive review

Since the discovery of chlorpromazine in the 1950's, antipsychotic drugs have been the cornerstone of treatment of schizophrenia, and all attenuate dopamine transmission at the dopamine-2 receptor. Drug development for schizophrenia since that time has led to improvements in side effects and tolerability, and limited improvements in efficacy, with the exception of clozapine. However, the reasons for clozapine's greater efficacy remain unclear, despite the great efforts and resources invested therewith. We performed a comprehensive review of the literature to determine the fate of previously tested, non-dopamine-2 receptor experimental treatments. Overall we included 250 studies in the review from the period 1970 to 2017 including treatments with glutamatergic, serotonergic, cholinergic, neuropeptidergic, hormone-based, dopaminergic, metabolic, vitamin/naturopathic, histaminergic, infection/inflammation-based, and miscellaneous mechanisms. Despite there being several promising targets, such as allosteric modulation of the NMDA and α7 nicotinic receptors, we cannot confidently state that any of the mechanistically novel experimental treatments covered in this review are definitely effective for the treatment of schizophrenia and ready for clinical use. We discuss potential reasons for the relative lack of progress in developing non-dopamine-2 receptor treatments for schizophrenia and provide recommendations for future efforts pursuing novel drug development for schizophrenia.

Meta-analysis of Placebo Response in Randomized Clinical Trials of Antipsychotic Drugs Using PANSS Focusing on Different Approaches to the Handling of Missing Data

BACKGROUND AND OBJECTIVE

Schizophrenia treatment has been shifting to resocialization by efficacious antipsychotic drugs. However, even some of the pivotal studies of approved new antipsychotic drugs with proven efficacy had failed due to high placebo response. The aim of this study was to identify the potential factors affecting placebo response by meta-analysis for randomized clinical trials for antipsychotic drugs using Positive and Negative Syndrome Scale (PANSS) focusing on the current methodological change in the handling of missing data [from last observation carried forward (LOCF) to mixed-effect models for repeated measures (MMRM)] for successful future clinical trials.

METHODS

Recent trends in the degree of placebo response were investigated between publication year (1993 to 2016) and the mean change of PANSS total score in the placebo arm. The potential factors affecting the degree of placebo response, such as study design and operational factors, were analyzed separately by meta-regression for LOCF- and MMRM-based data.

RESULTS

There was no correlation between publication year and the mean change of PANSS score in the placebo arm in schizophrenia studies of 10 years applying MMRM. The number of countries and treatment setting in MMRM-based data and study duration in LOCF-based data were significantly associated with placebo response.

CONCLUSION

Placebo response in schizophrenia clinical trials published between 2007 and 2016 has not increased over time. Differences in the healthcare environment among countries were suggested to affect the evaluation of antipsychotic drugs. Further analyses on the potential factors of placebo response for MMRM-based data are required.

The impact of genetics on future drug discovery in schizophrenia

INTRODUCTION

Failures of investigational new drugs (INDs) for schizophrenia have left huge unmet medical needs for patients. Given the recent lackluster results, it is imperative that new drug discovery approaches (and resultant drug candidates) target pathophysiological alterations that are shared in specific, stratified patient populations that are selected based on pre-identified biological signatures. One path to implementing this paradigm is achievable by leveraging recent advances in genetic information and technologies. Genome-wide exome sequencing and meta-analysis of single nucleotide polymorphism (SNP)-based association studies have already revealed rare deleterious variants and SNPs in patient populations. Areas covered: Herein, the authors review the impact that genetics have on the future of schizophrenia drug discovery. The high polygenicity of schizophrenia strongly indicates that this disease is biologically heterogeneous so the identification of unique subgroups (by patient stratification) is becoming increasingly necessary for future investigational new drugs. Expert opinion: The authors propose a pathophysiology-based stratification of genetically-defined subgroups that share deficits in particular biological pathways. Existing tools, including lower-cost genomic sequencing and advanced gene-editing technology render this strategy ever more feasible. Genetically complex psychiatric disorders such as schizophrenia may also benefit from synergistic research with simpler monogenic disorders that share perturbations in similar biological pathways.

Investigational dopamine antagonists for the treatment of schizophrenia

INTRODUCTION

Schizophrenia is a debilitating illness with a chronic impact on social function and daily living. Although various antipsychotics are available, there are still many challenges and unmet needs. Thus, many compounds with diverse mechanisms have been investigated, but all approved antipsychotics still require interactions with dopamine D₂ receptors. Areas covered: We searched for investigational drugs using the key words 'dopamine' and 'schizophrenia' in American and European clinical trial registers (clinicaltrials.gov; clinicaltrialsregister.eu). Published articles were searched in PubMed, Embase, Medline, PsycINFO, Cumulative Index to Nursing and Allied Health Literature (CINAHL), the Web of Science and the Cochrane Central Register of Controlled Trials Library. Expert opinion: The prospect of developing a dopamine antagonist is hopeful. Brexpiprazole and cariprazine, which were agents listed as 'investigational dopamine antagonists,' just received FDA approval. Novel agents such as BL 1020, ITI-007, and JNJ-37822681 have solid published data available, and agents such as L-THP, Lu AF35700, S33138, and SB-773812 are under vigorous investigation. However, the expected benefits of the newly developed antagonists may not be great because they offer little enhanced efficacy for negative symptoms, cognition and functional outcomes.

Preferential binding to dopamine D3 over D2 receptors by cariprazine in patients with schizophrenia using PET with the D3/D2 receptor ligand [(11)C]-(+)-PHNO

RATIONALE

Second-generation antipsychotics occupy dopamine D2 receptors and act as antagonists or partial agonists at these receptors. While these drugs alleviate positive symptoms in patients with schizophrenia, they are less effective for treating cognitive deficits and negative symptoms. Dopamine D3 receptors are highly expressed in areas of the brain thought to play a role in the regulation of motivation and reward-related behavior. Consequently, the dopamine D3 receptor has become a target for treating negative symptoms in combination with D2 antagonism to treat positive symptoms in patients with schizophrenia.

OBJECTIVE

The purpose of this study was to determine the cariprazine receptor occupancies in brain for D2 and D3 receptors in patients with schizophrenia.

METHODS

Using [(11)C]-(+)-PHNO as a radioligand, positron emission tomography (PET) scans were performed in eight patients at baseline and postdose on days 1, 4, and 15. Plasma and cerebrospinal fluid (CSF) samples were analyzed for cariprazine concentrations.

RESULTS

A monotonic dose-occupancy relationship was observed for both receptor types. After 2 weeks of treatment, near complete (∼100 %) occupancies were observed for both receptors at a dose of 12 mg/day. At the lowest cariprazine dose (1 mg/day), mean D3 and D2 receptor occupancies were 76 and 45 %, respectively, suggesting selectivity for D3 over D2 receptors at low doses. An exposure-response analysis found a ∼3-fold difference in EC50 (D3 = 3.84 nM and D2 = 13.03 nM) in plasma after 2 weeks of dosing.

CONCLUSION

This PET imaging study in patients with schizophrenia demonstrated that cariprazine is a D3-preferring dual D3/D2 receptor partial agonist.

Targeting the dopamine D3 receptor: an overview of drug design strategies

INTRODUCTION

Dopamine is a neurotransmitter widely distributed in both the periphery and the central nervous system (CNS). Its physiological effects are mediated by five closely related G protein-coupled receptors (GPCRs) that are divided into two major subclasses: the D1-like (D1, D5) and the D2-like (D2, D3, D4) receptors. D3 receptors (D3Rs) have the highest density in the limbic areas of the brain, which are associated with cognitive and emotional functions. These receptors are therefore attractive targets for therapeutic management.

AREAS COVERED

This review summarizes the functional and pharmacological characteristics of D3Rs, including the design and clinical relevance of full agonists, partial agonists and antagonists, as well as the capacity of these receptors to form active homodimers, heterodimers or higher order receptor complexes as pharmacological targets in several neurological and neurodegenerative disorders.

EXPERT OPINION

The high sequence homology between D3R and the D2-type challenges the development of D3R-selective compounds. The design of new D3R-preferential ligands with improved physicochemical properties should provide a better pharmacokinetic/bioavailability profile and lesser toxicity than is found with existing D3R ligands. It is also essential to optimize D3R affinity and, especially, D3R vs. D2-type binding and functional selectivity ratios. Developing allosteric and bitopic ligands should help to improve the D3R selectivity of these drugs. As most evidence points to the ability of GPCRs to form homomers and heteromers, the most promising therapeutic strategy in the future is likely to involve the application of heteromer-selective drugs. These selective ligands would display different affinities for a given receptor depending on the receptor partners within the heteromer. Therefore, designing novel compounds that specifically target and modulate D1R-D3R heteromers would be an interesting approach for the treatment of levodopa (L-DOPA)-induced dyskinesias.

Pharmacological targeting of dopamine D3 receptors: Possible clinical applications of selective drugs

Dopamine D3 receptors have been pharmacologically engaged in humans since the development of the first antipsychotics and ergot-derivative dopamine (DA) agonists, even without knowing it. These agents were generally non-selective, developed primarily to target D2 receptors. In the last 10 years the understanding of the clinical implication of D3 receptors has been progressing also due to the identification of D3 gene polymorphisms, the use of more selective PET ligands such as [(11)C]-(+)-PHNO and the learning regarding the clinical use of the D3-preferential D2/D3 agonists ropinirole and pramipexole. A new specific neuroplasticity role of D3 receptor regarding dendrite arborisation outgrowth in dopaminergic neurons was also proposed to support, at least in part, the slowing of disease observed in subjects with Parkinson׳s Disease treated with DA agonists. Similar mechanisms could be at the basis of the antidepressant-like effects observed with DA agonists when co-administered with standard of care. Severe adverse event occurring with the use of anti-parkinsonian DA agonists in predisposed subjects, i.e., impulse control disorders, are now suggested to be putatively related to overactive D3 receptors. Not surprisingly, blockade of D3 receptors was proposed as treatment for addictive disorders, a goal that could be potentially achieved by repositioning buspirone, an anxiolytic drug with D3-preferential antagonistic features, or with novel selective D3 antagonists or partial agonists currently in development for schizophrenia. At the moment ABT-925 is the only selective D3 antagonist tested in schizophrenic patients in Phase II, showing an intriguing cognitive enhancing effects supported by preclinical data. Finally, exploratory pharmacogenetic analysis suggested that ABT-925 could be effective in a subpopulation of patients with a polymorphism on the D3 receptor, opening to a possible personalised medicine approach.

Multitarget drug discovery projects in CNS diseases: quantitative systems pharmacology as a possible path forward

Clinical development in brain diseases has one of the lowest success rates in the pharmaceutical industry, and many promising rationally designed single-target R&D projects fail in expensive Phase III trials. By contrast, successful older CNS drugs do have a rich pharmacology. This article will provide arguments suggesting that highly selective single-target drugs are not sufficiently powerful to restore complex neuronal circuit homeostasis. A rationally designed multitarget project can be derisked by dialing in an additional symptomatic treatment effect on top of a disease modification target. Alternatively, we expand upon a hypothetical workflow example using a humanized computer-based quantitative systems pharmacology platform. The hope is that incorporating rationally multipharmacology drug discovery could potentially lead to more impactful polypharmacy drugs.

Synthesis and evaluation of amide, sulfonamide and urea – benzisoxazole derivatives as potential atypical antipsychotics

A series of amide derivatives of benzisoxazole has been synthesized and the target compounds evaluated for atypical antipsychotic activity in vitro and vivo.

Fluorine-Containing Diazines in Medicinal Chemistry and Agrochemistry

The combination of a fluorine atom and a diazine ring, which both possess unique structural and chemical features, can generate new relevant building blocks for the discovery of efficient fluorinated biologically active agents. Herein we give a comprehensive review on the biological activity and synthesis of fluorine containing, pyrimidine, pyrazine and pyridazine derivatives with relevance to medicinal and agrochemistry.

Association of dopamine-related genetic loci to dopamine D3 receptor antagonist ABT-925 clinical response

ABT-925, a selective dopamine D3 receptor (DRD3) antagonist, was tested in schizophrenia. A DRD3 gene polymorphism results in an S9G amino-acid change that has been associated with lower risk of schizophrenia, higher affinity for dopamine and some antipsychotics, and differential response to some antipsychotics. The effect of S9G genotype on response to ABT-925 was examined. DNA samples (N=117) were collected in a proof-of-concept, double-blind, randomized, placebo-controlled study of ABT-925 (50 or 150 mg QD) in acute exacerbation of schizophrenia. A pre-specified analysis assessed impact of genotype (SS versus SG+GG) on change from baseline to final evaluation for the Positive and Negative Syndrome Scale (PANSS) total score using analysis of covariance with genotype, treatment and genotype-by-treatment interaction as factors, and baseline score as covariate. Significant genotype-by-treatment interaction (P=0.015) was observed for change from baseline to final evaluation for the PANSS total score. Within subgroup analyses showed significant improvement from placebo in the SG+GG group treated with ABT-925 150 mg. More favorable clinical outcomes were observed in patients treated with ABT-925 150 mg who carried the DRD3 G allele than in those who carried the DRD3 SS genotype.

Quantitative systems pharmacology as an extension of PK/PD modeling in CNS research and development

Quantitative systems pharmacology (QSP) is a recent addition to the modeling and simulation toolbox for drug discovery and development and is based upon mathematical modeling of biophysical realistic biological processes in the disease area of interest. The combination of preclinical neurophysiology information with clinical data on pathology, imaging and clinical scales makes it a real translational tool. We will discuss the specific characteristics of QSP and where it differs from PK/PD modeling, such as the ability to provide support in target validation, clinical candidate selection and multi-target MedChem projects. In clinical development the approach can provide additional and unique evaluation of the effect of comedications, genotypes and disease states (patient populations) even before the initiation of actual trials. A powerful property is the ability to perform failure analysis. By giving examples from the CNS R&D field in schizophrenia and Alzheimer's disease, we will illustrate how this approach can make a difference for CNS R&D projects.

The role of dopamine D(3) receptors in antipsychotic activity and cognitive functions

Dopamine D(3) receptors have a pre- and postsynaptic localization in brain stem nuclei, limbic parts of the striatum, and cortex. Their widespread influence on dopamine release, on dopaminergic function, and on several other neurotransmitters makes them attractive targets for therapeutic intervention. The signaling pathways of D(3) receptors are distinct from those of other members of the D(2)-like receptor family. There is increasing evidence that D(3) receptors can form heteromers with dopamine D(1), D(2), and probably other G-protein-coupled receptors. The functional consequences remain to be characterized in more detail but might open new interesting pharmacological insight and opportunities. In terms of behavioral function, D(3) receptors are involved in cognitive, social, and motor functions, as well as in filtering and sensitization processes. Although the role of D(3) receptor blockade for alleviating positive symptoms is still unsettled, selective D(3) receptor antagonism has therapeutic features for schizophrenia and beyond as demonstrated by several animal models: improved cognitive function, emotional processing, executive function, flexibility, and social behavior. D(3) receptor antagonism seems to contribute to atypicality of clinically used antipsychotics by reducing extrapyramidal motor symptoms; has no direct influence on prolactin release; and does not cause anhedonia, weight gain, or metabolic dysfunctions. Unfortunately, clinical data with new, selective D(3) antagonists are still incomplete; their cognitive effects have only been communicated in part. In vitro, virtually all clinically used antipsychotics are not D(2)-selective but also have affinity for D(3) receptors. The exact D(3) receptor occupancies achieved in patients, particularly in cortical areas, are largely unknown, mainly because only nonselective or agonist PET tracers are currently available. It is unlikely that a degree of D(3) receptor antagonism optimal for antipsychotic and cognitive function can be achieved with existing antipsychotics. Therefore, selective D(3) antagonism represents a promising mechanism still to be fully exploited for the treatment of schizophrenia, cognitive deficits in schizophrenia, and comorbid conditions such as substance abuse.

Pharmacological treatment of schizophrenia: a critical review of the pharmacology and clinical effects of current and future therapeutic agents

Since the introduction of chlorpromazine and throughout the development of the new-generation antipsychotic drugs (APDs) beginning with clozapine, the D(2) receptor has been the target for the development of APDs. Pharmacologic actions to reduce neurotransmission through the D(2) receptor have been the only proven therapeutic mechanism for psychoses. A number of novel non-D(2) mechanisms of action of APDs have been explored over the past 40 years but none has definitively been proven effective. At the same time, the effectiveness of treatments and range of outcomes for patients are far from satisfactory. The relative success of antipsychotics in treating positive symptoms is limited by the fact that a substantial number of patients are refractory to current medications and by their lack of efficacy for negative and cognitive symptoms, which often determine the level of functional impairment. In addition, while the newer antipsychotics produce fewer motor side effects, safety and tolerability concerns about weight gain and endocrinopathies have emerged. Consequently, there is an urgent need for more effective and better-tolerated antipsychotic agents, and to identify new molecular targets and develop mechanistically novel compounds that can address the various symptom dimensions of schizophrenia. In recent years, a variety of new experimental pharmacological approaches have emerged, including compounds acting on targets other than the dopamine D(2) receptor. However, there is still an ongoing debate as to whether drugs selective for singe molecular targets (that is, 'magic bullets') or drugs selectively non-selective for several molecular targets (that is, 'magic shotguns', 'multifunctional drugs' or 'intramolecular polypharmacy') will lead to more effective new medications for schizophrenia. In this context, current and future drug development strategies can be seen to fall into three categories: (1) refinement of precedented mechanisms of action to provide drugs of comparable or superior efficacy and side-effect profiles to existing APDs; (2) development of novel (and presumably non-D(2)) mechanism APDs; (3) development of compounds to be used as adjuncts to APDs to augment efficacy by targeting specific symptom dimensions of schizophrenia and particularly those not responsive to traditional APD treatment. In addition, efforts are being made to determine if the products of susceptibility genes in schizophrenia, identified by genetic linkage and association studies, may be viable targets for drug development. Finally, a focus on early detection and early intervention aimed at halting or reversing progressive pathophysiological processes in schizophrenia has gained great influence. This has encouraged future drug development and therapeutic strategies that are neuroprotective. This article provides an update and critical review of the pharmacology and clinical profiles of current APDs and drugs acting on novel targets with potential to be therapeutic agents in the future.

Direct and indirect interactions of the dopamine D₃ receptor with glutamate pathways: implications for the treatment of schizophrenia

This article, based on original data as well as on previously reported preclinical and clinical data that are reviewed, describes direct and indirect interactions of the D(3) receptor with N-methyl-D-aspartate receptor (NMDA) signaling and their functional consequences and therapeutic implications for schizophrenia. D(3) receptor immunoreactivity at ultrastructural level with electron microscopy was identified at presumably glutamatergic, asymmetric synapses of the medium-sized spiny neurons of the nucleus accumbens. This finding supports the existence of a direct interaction of the D(3) receptor with glutamate, in line with previously described interactions with NMDA signaling involving Ca(2+)/calmodulin-dependent protein kinase II at post-synaptic densities (Liu et al. 2009). Indirect interactions of the D(3) receptor with glutamate could involve a negative control exerted by the D(3) receptor on mesocortical dopamine neurons and the complex regulation of the glutamatergic pyramidal cells by dopamine in the prefrontal cortex. This could be exemplified here by the regulation of pyramidal cell activity in conditions of chronic NMDA receptor blockade with dizocilpine (MK-801). BP897, a D(3) receptor-selective partial agonist, reversed the dysregulation of cortical c-fos mRNA expression and pyramidal cell hyperexcitability, as measured by paired-pulse electrophysiology. At the behavioral level, blockade of the D(3) receptor, by known D(3) receptor antagonists or the novel D(3) receptor-selective antagonist F17141, produces antipsychotic-like effects in reversing hyperactivity and social interaction deficits induced by NMDA receptor blockade by MK-801 in mice. The glutamate-D(3) receptor interactions described here offer a conceptual framework for developing new D(3) receptor-selective drugs, which may appear as an original, efficacious, and safe way to potentially indirectly target glutamate in schizophrenia.