A single 20 mg dose of dihydrexidine (DAR-0100), a full dopamine D1 agonist, is safe and tolerated in patients with schizophrenia

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.

The Signaling and Pharmacology of the Dopamine D1 Receptor

The dopamine D1 receptor (D1R) is a Gα_s/olf-coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα_s or Gα_olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.

Synthesis and dopamine receptor binding of dihydrexidine and SKF 38393 catecholamine-based analogues

Dopamine is an important neurotransmitter that regulates numerous essential functions, including cognition and voluntary movement. As such, it serves as an important scaffold for synthesis of novel analogues as part of drug development effort to obtain drugs for treatment of neurodegenerative diseases, such as Parkinson's disease. To that end, similarity search of the ZINC database based on two known dopamine-1 receptor (D1R) agonists, dihydrexidine (DHX) and SKF 38393, respectively, was used to predict novel chemical entities with potential binding to D1R. Three compounds that showed the highest similarity index were selected for synthesis and bioactivity profiling. All main synthesis products as well as the isolated intermediates, were properly characterized. The physico-chemical analyses were performed using HRESIMS, GC/MS, LC/MS with UV-Vis detection, and FTIR, ¹H NMR and ¹³C NMR spectroscopy. Binding to D1 and D2 receptors and inhibition of dopamine reuptake via dopamine transporter were measured for the synthesized analogues of DHX and SKF 38393.

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.

Dopamine D1R Receptor Stimulation as a Mechanistic Pro-cognitive Target for Schizophrenia

Decades of research have highlighted the importance of optimal stimulation of cortical dopaminergic receptors, particularly the D1R receptor (D1R), for prefrontal-mediated cognition. This mechanism is particularly relevant to the cognitive deficits in schizophrenia, given the abnormalities in cortical dopamine (DA) neurotransmission and in the expression of D1R. Despite the critical need for D1R-based therapeutics, many factors have complicated their development and prevented this important therapeutic target from being adequately interrogated. Challenges include determination of the optimal level of D1R stimulation needed to improve cognitive performance, especially when D1R expression levels, affinity states, DA levels, and the resulting D1R occupancy by DA, are not clearly known in schizophrenia, and may display great interindividual and intraindividual variability related to cognitive states and other physiological variables. These directly affect the selection of the level of stimulation necessary to correct the underlying neurobiology. The optimal mechanism for stimulation is also unknown and could include partial or full agonism, biased agonism, or positive allosteric modulation. Furthermore, the development of D1R targeting drugs has been complicated by complexities in extrapolating from in vitro affinity determinations to in vivo use. Prior D1R-targeted drugs have been unsuccessful due to poor bioavailability, pharmacokinetics, and insufficient target engagement at tolerable doses. Newer drugs have recently become available, and these must be tested in the context of carefully designed paradigms that address methodological challenges. In this paper, we discuss how a better understanding of these challenges has shaped our proposed experimental design for testing a new D1R/D5R partial agonist, PF-06412562, renamed CVL-562.

Selective YAP/TAZ inhibition in fibroblasts via dopamine receptor D1 agonism reverses fibrosis

Tissue fibrosis is characterized by uncontrolled deposition and diminished clearance of fibrous connective tissue proteins, ultimately leading to organ scarring. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) have recently emerged as pivotal drivers of mesenchymal cell activation in human fibrosis. Therapeutic strategies inhibiting YAP and TAZ have been hindered by the critical role that these proteins play in regeneration and homeostasis in different cell types. Here, we find that the Gα_s-coupled dopamine receptor D1 (DRD1) is preferentially expressed in lung and liver mesenchymal cells relative to other resident cells of these organs. Agonism of DRD1 selectively inhibits YAP/TAZ function in mesenchymal cells and shifts their phenotype from profibrotic to fibrosis resolving, reversing in vitro extracellular matrix stiffening and in vivo tissue fibrosis in mouse models. Aromatic l-amino acid decarboxylase [DOPA decarboxylase (DDC)], the enzyme responsible for the final step in biosynthesis of dopamine, is decreased in the lungs of subjects with idiopathic pulmonary fibrosis, and its expression inversely correlates with disease severity, consistent with an endogenous protective role for dopamine signaling that is lost in pulmonary fibrosis. Together, these findings establish a pharmacologically tractable and cell-selective approach to targeting YAP/TAZ via DRD1 that reverses fibrosis in mice.

Pharmacokinetics of ASP4345 from Single Ascending-Dose and Multiple Ascending-Dose Phase I Studies

Background

Cognitive impairment is a core feature of schizophrenia. While first- and second-generation antipsychotic drugs treat psychotic exacerbations, no treatment is approved for the cognitive dysfunction. We have identified ASP4345, a positive allosteric modulator of the dopamine type 1 (D₁) receptor that selectively binds to, and enhances the activity of, D₁ receptors. ASP4345 has the potential to be an effective and well-tolerated treatment option for cognitive impairment associated with schizophrenia.

Objective

The objective of this study was to determine the pharmacokinetics of ASP4345 in two phase I single ascending-dose and multiple ascending-dose studies.

Methods

Both phase I studies were randomized, double blind, and placebo controlled. The single dose-ascending study assessed pharmacokinetics of single oral doses of 3–900 mg of ASP4345 or placebo in the fasted state in healthy adult volunteers. This study also assessed cerebrospinal fluid pharmacokinetics, as well as the effects of food on pharmacokinetic parameters. The multiple ascending-dose study (NCT02720263) assessed the pharmacokinetics of multiple oral doses of 3–150 mg of ASP4345 in patients with schizophrenia or schizoaffective disorder receiving stable antipsychotic drug treatment. The pharmacokinetic data from both studies were summarized using descriptive statistics.

Results

The plasma concentration–time profile in both studies showed a rapid increase in concentrations of ASP4345. The median time to maximum concentration range was 1.00–2.26 h in the single ascending-dose study in the fasted state and 1.25–3.02 h in the multiple ascending-dose study at steady state. There were less than dose-proportional increases in maximum concentration and area under the curve in the single ascending-dose study, where doses had a range from 3 to 900 mg, and in the multiple ascending-dose study in patients with stabilized schizophrenia or schizoaffective disorder, where doses had a range from 3 to 150 mg. The mean terminal elimination half-life was dose independent and had a range from 9.12 to 14.3 h in the single ascending-dose study and from 11.1 to 26.8 h in the multiple ascending-dose study. Additionally, in the single ascending-dose study, absorption of 300 mg of ASP4345 was slightly delayed when administered in the fed state compared with the fasted state; median time to maximum concentration was 1.5 h under the fasting state and 4.0 h under fed states. All other pharmacokinetic parameters were comparable for both conditions. ASP4345 appeared in the cerebrospinal fluid with some delay; time to maximum concentration range was from 2.48 to 7.98 h in cerebrospinal fluid compared with 0.75 to 1.03 h in plasma (median cerebrospinal fluid/plasma = 0.188). The ratio of cerebrospinal fluid to total plasma for area under the curve from 0 to 24 h (0.157–0.573%) and maximum concentration (0.0899–0.311%) and the ratio of cerebrospinal fluid to unbound plasma for maximum concentration (25.0–86.4%) confirm the distribution of ASP4345 into the brain.

Conclusions

The pharmacokinetics of ASP4345 suggest that single daily dosing is appropriate for ASP4345. Furthermore, the concentration of ASP4345 in cerebrospinal fluid compared to free drug concentrations in plasma provides evidence of penetration of ASP4345 into the brain.

Subcortical Dopamine and Cognition in Schizophrenia: Looking Beyond Psychosis in Preclinical Models

Schizophrenia is characterized by positive, negative and cognitive symptoms. All current antipsychotic treatments feature dopamine-receptor antagonism that is relatively effective at addressing the psychotic (positive) symptoms of schizophrenia. However, there is no clear evidence that these medications improve the negative or cognitive symptoms, which are the greatest predictors of functional outcomes. One of the most robust pathophysiological observations in patients with schizophrenia is increased subcortical dopamine neurotransmission, primarily in the associative striatum. This brain area has an important role in a range of cognitive processes. Dopamine is also known to play a major part in regulating a number of cognitive functions impaired in schizophrenia but much of this research has been focused on cortical dopamine. Emerging research highlights the strong influence subcortical dopamine has on a range of cognitive domains, including attention, reward learning, goal-directed action and decision-making. Nonetheless, the precise role of the associative striatum in the cognitive impairments observed in schizophrenia remains poorly understood, presenting an opportunity to revisit its contribution to schizophrenia. Without a better understanding of the mechanisms underlying cognitive dysfunction, treatment development remains at a standstill. For this reason, improved preclinical animal models are needed if we are to understand the complex relationship between subcortical dopamine and cognition. A range of new techniques are facillitating the discrete manipulation of dopaminergic neurotransmission and measurements of cognitive performance, which can be investigated using a variety of sensitive translatable tasks. This has the potential to aid the successful incorporation of recent clinical research to address the lack of treatment strategies for cognitive symptoms in schizophrenia. This review will give an overview on the current state of research focused on subcortical dopamine and cognition in the context of schizophrenia research. We also discuss future strategies and approaches aimed at improving the translational outcomes for the treatment of cognitive deficits in schizophrenia.

The D1/D5 Dopamine Partial Agonist PF-06412562 in Advanced-Stage Parkinson's Disease: A Feasibility Study

BACKGROUND

Current drug treatments have little efficacy in advanced-to-end-stage Parkinson's disease (advPD), yet there are no reports of interventional trials in advPD. D1 dopamine agonists have the potential to provide benefit.

OBJECTIVE

To determine the feasibility and safety of the selective D1/D5 dopamine partial agonist PF 06412562 in advPD.

METHODS

A two-week, randomized, double blind, crossover phase Ib study in advPD patients compared standard-of-care (SoC) carbidopa/levodopa with PF 06412562. Each week, there was a Day 1 baseline evaluation with overnight levodopa washout, then treatment on Days 2 and 3 with either SoC or PF-06412562 (split dose 25 + 20 mg), followed by discharge on Day 4. Primary endpoints were safety and tolerability. Secondary endpoints were global clinical impression of change (GCI-C) rated by clinicians and caregivers.

RESULTS

Eight advPD patients and their caregivers consented to participate and six were randomized (average disease duration: 22 y). None withdrew voluntarily. One participant with baseline Day 1 dehydration, pre-renal kidney injury, and autonomic dysfunction experienced symptomatic and serious hypotension after receiving PF-06412562 in Week 1 and was discontinued from the study. All other adverse events were rated mild (PF-06412562: n = 1, SoC: n = 0), moderate (PF-06412562: n = 1, SoC: n = 1), or severe but non-serious (PF-06412562: n = 3, SoC: n = 2). No clinically meaningful laboratory changes were observed. Among the five participants who completed the study, GCI-C favored PF-06412562 in two per clinicians' and four participants per caregivers' rating.

CONCLUSION

PF-06412562 was tolerated in advPD patients. This study provides the feasibility for future safety and efficacy studies in this population with unmet needs.

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'.

Improving cognitive training for schizophrenia using neuroplasticity enhancers: Lessons from decades of basic and clinical research

Mounting evidence indicates that schizophrenia is a disorder that stems from maladaptive plasticity within neural circuits and produces broad cognitive deficits leading to loss of autonomy. A large number of studies have identified abnormalities spanning many neurotransmitter systems in schizophrenia, and as a result, a variety of drugs have been developed to attempt to treat these abnormalities and enhance cognition. Unfortunately, positive results have been limited so far. This may be in part because the scope of abnormalities in the schizophrenic brain requires a treatment capable of engaging many different neurotransmitter systems. One approach to achieving this kind of treatment has been to use neuroplasticity-based computerized cognitive training programs to stimulate the formation of more adaptive circuits. Although the number of studies implementing this approach has increased exponentially in recent years, effect sizes for cognitive gains have been modest and adherence to treatment remains an important challenge in many studies, as patients are often required to train for 40 h or more. In the present paper, we argue that cognitive training protocols will benefit from the addition of cognitive enhancers to produce more robust and longer lasting targeted neuroplasticity. Indeed, recent data from animal studies have provided support for combining plasticity-enhancing drugs with tailored behavioral training paradigms to restore normal function within dysfunctioning neural circuits. The advantages and challenges of applying this approach to patients with schizophrenia will be discussed.

A proof-of-principle study of the effect of combined haloperidol and levodopa administration on working memory-related brain activation in humans

OBJECTIVE

Cognitive deficits including impaired working memory are a hallmark feature of schizophrenia. Dopamine D1 receptor modulated changes in prefrontal cortex function play a potentially important role in the pathology underlying such deficits. However, pharmacological interventions that selectively engage the D1 receptor are severely restricted for research in humans. The present study is a proof-of-principle for enhancing cognitive performance and associated brain activation via indirect D1 stimulation, operationalised by combining the nonselective dopamine agonist L-dopa with the D2-antagonist haloperidol.

METHODS

Fourteen healthy volunteers received placebo or combined carbidopa (25 mg)/L-dopa (100 mg) plus haloperidol (2 mg) orally on two separate occasions according to a within-subjects crossover design. Drug-induced differences in brain activity were assessed during an N-back working memory task in a 3T magnetic resonance imaging environment.

RESULTS

Drug treatment was associated with greater functional connectivity between the dorsolateral prefrontal cortex and areas within the salience network during all N-back trials. Drug treatment was also associated with reduced activation, most prominently in the occipital/temporal brain areas during 2-back performance.

CONCLUSIONS

This preliminary study provides initial evidence for combined L-dopa/haloperidol modulation in cognition-related brain areas and networks, which is relevant for the treatment of cognitive impairments in mental illness.

Dopamine D1 receptor subtype mediates acute stress-induced dendritic growth in excitatory neurons of the medial prefrontal cortex and contributes to suppression of stress susceptibility in mice

Dopamine in prefrontal cortices is implicated in cognitive and emotional functions, and the dysfunction of prefrontal dopamine has been associated with cognitive and emotional deficits in mental illnesses. These findings have led to clinical trials of dopamine-targeting drugs and brain imaging of dopamine receptors in patients with mental illnesses. Rodent studies have suggested that dopaminergic pathway projecting to the medial prefrontal cortex (mPFC) suppresses stress susceptibility. Although various types of mPFC neurons express several dopamine receptor subtypes, previous studies neither isolated a role of dopamine receptor subtype nor identified the site of its action in mPFC. Using social defeat stress (SDS) in mice, here we identified a role of dopamine D1 receptor subtype in mPFC excitatory neurons in suppressing stress susceptibility. Repeated social defeat stress (R-SDS) reduces the expression of D1 receptor subtype in mPFC of mice susceptible to R-SDS. Knockdown of D1 receptor subtype in whole neuronal populations or excitatory neurons in mPFC facilitates the induction of social avoidance by SDS. Single social defeat stress (S-SDS) induces D1 receptor-mediated extracellular signal-regulated kinase phosphorylation and c-Fos expression in mPFC neurons. Whereas R-SDS reduces dendritic lengths of mPFC layer II/III pyramidal neurons, S-SDS increases arborization and spines of apical dendrites of these neurons in a D1 receptor-dependent manner. Collectively, our findings show that D1 receptor subtype and related signaling in mPFC excitatory neurons mediate acute stress-induced dendritic growth of these neurons and contribute to suppression of stress susceptibility. Therefore, we propose that D1 receptor-mediated dendritic growth in mPFC excitatory neurons suppresses stress susceptibility.

Interaction of childhood urbanicity and variation in dopamine genes alters adult prefrontal function as measured by functional magnetic resonance imaging (fMRI)

Brain phenotypes showing environmental influence may help clarify unexplained associations between urban exposure and psychiatric risk. Heritable prefrontal fMRI activation during working memory (WM) is such a phenotype. We hypothesized that urban upbringing (childhood urbanicity) would alter this phenotype and interact with dopamine genes that regulate prefrontal function during WM. Further, dopamine has been hypothesized to mediate urban-associated factors like social stress. WM-related prefrontal function was tested for main effects of urbanicity, main effects of three dopamine genes-catechol-O-methyltransferase (COMT), dopamine receptor D1 (DRD1), and dopamine receptor D2 (DRD2)-and, importantly, dopamine gene-by-urbanicity interactions. For COMT, three independent human samples were recruited (total n = 487). We also studied 253 subjects genotyped for DRD1 and DRD2. 3T fMRI activation during the N-back WM task was the dependent variable, while childhood urbanicity, dopamine genotype, and urbanicity-dopamine interactions were independent variables. Main effects of dopamine genes and of urbanicity were found. Individuals raised in an urban environment showed altered prefrontal activation relative to those raised in rural or town settings. For each gene, dopamine genotype-by-urbanicity interactions were shown in prefrontal cortex-COMT replicated twice in two independent samples. An urban childhood upbringing altered prefrontal function and interacted with each gene to alter genotype-phenotype relationships. Gene-environment interactions between multiple dopamine genes and urban upbringing suggest that neural effects of developmental environmental exposure could mediate, at least partially, increased risk for psychiatric illness in urban environments via dopamine genes expressed into adulthood.

Dopamine, psychosis and schizophrenia: the widening gap between basic and clinical neuroscience

The stagnation in drug development for schizophrenia highlights the need for better translation between basic and clinical research. Understanding the neurobiology of schizophrenia presents substantial challenges but a key feature continues to be the involvement of subcortical dopaminergic dysfunction in those with psychotic symptoms. Our contemporary knowledge regarding dopamine dysfunction has clarified where and when dopaminergic alterations may present in schizophrenia. For example, clinical studies have shown patients with schizophrenia show increased presynaptic dopamine function in the associative striatum, rather than the limbic striatum as previously presumed. Furthermore, subjects deemed at high risk of developing schizophrenia show similar presynaptic dopamine abnormalities in the associative striatum. Thus, our view of subcortical dopamine function in schizophrenia continues to evolve as we accommodate this newly acquired information. However, basic research in animal models has been slow to incorporate these clinical findings. For example, psychostimulant-induced locomotion, the commonly utilised phenotype for positive symptoms in rodents, is heavily associated with dopaminergic activation in the limbic striatum. This anatomical misalignment has brought into question how we assess positive symptoms in animal models and represents an opportunity for improved translation between basic and clinical research. The current review focuses on the role of subcortical dopamine dysfunction in psychosis and schizophrenia. We present and discuss alternative phenotypes that may provide a more translational approach to assess the neurobiology of positive symptoms in schizophrenia. Incorporation of recent clinical findings is essential if we are to develop meaningful translational animal models.

Pharmacological Augmentation of Psychosocial and Remediation Training Efforts in Schizophrenia

Pharmacological approaches to cognitive enhancement have received considerable attention but have not had considerable success in improving their cognitive and functional targets. Other intervention strategies, such as cognitive remediation therapy (CRT), have been shown to enhance cognitive performance but have not been found to improve functional outcomes without additional psychosocial interventions. Recently, several studies have attempted to enhance the effects of CRT by adding pharmacological interventions to the CRT treatments. In addition, as CRT has been shown to synergistically improve the effects of psychosocial interventions, the combination of pharmacological therapies aimed at cognition and psychosocial interventions may itself provide a promising strategy for improving functional outcomes. This review and commentary examines the current state of interventions combining CRT and psychosocial treatments with pharmacological augmentation. Our focus is on the specific level of effect of the pharmacological intervention, which could be enhancing motivation, training efficiency, or the consolidation of therapeutic gains. Different pharmacological strategies (e.g., stimulants, plasticity-inducing agents, or attentional or alertness enhancers) may have the potential to lead to different types of gains when combined with CRT or psychosocial interventions. The relative potential of these different mechanisms for immediate and durable effects is considered.

Novel Dopamine Therapeutics for Cognitive Deficits in Schizophrenia

Schizophrenia is characterized by profound cognitive deficits that are not alleviated by currently available medications. Many of these cognitive deficits involve dysfunction of the newly evolved, dorsolateral prefrontal cortex (dlPFC). The brains of patients with schizophrenia show evidence of dlPFC pyramidal cell dendritic atrophy, likely reductions in cortical dopamine, and possible changes in dopamine D1 receptors (D1R). It has been appreciated for decades that optimal levels of dopamine are essential for dlPFC working memory function, with many beneficial actions arising from D1R stimulation. D1R are concentrated on dendritic spines in the primate dlPFC, where their stimulation produces an inverted-U dose response on dlPFC neuronal firing and cognitive performance during working memory tasks. Research in both academia and the pharmaceutical industry has led to the development of selective D1 agonists, e.g., the first full D1 agonist, dihydrexidine, which at low doses improved working memory in monkeys. Dihydrexidine has begun to be tested in patients with schizophrenia or schizotypal disorder. Initial results are encouraging, but studies are limited by the pharmacokinetics of the drug. These data, however, have spurred efforts toward the discovery and development of improved or novel new compounds, including D1 agonists with better pharmacokinetics, functionally selective D1 ligands, and D1R positive allosteric modulators. One or several of these approaches should allow optimization of the beneficial effects of D1R stimulation in the dlPFC that can be translated into clinical practice.

A proof-of-concept, randomized controlled trial of DAR-0100A, a dopamine-1 receptor agonist, for cognitive enhancement in schizophrenia

BACKGROUND

Evidence from preclinical and human studies indicates the presence of reduced dopamine-1 receptor (D1R) signaling in the cortex, where D1Rs predominate, in patients with schizophrenia (SCZ), which may contribute to their cognitive deficits. Furthermore, studies in nonhuman primates (NHP) have suggested that intermittent administration of low doses of D1R agonists produce long-lasting reversals in cognitive deficits. The purpose of this trial was to test whether a similar design, involving subacute intermittent administration of low doses of a full, selective agonist at D1Rs, DAR-0100A, would improve cognitive deficits in SCZ.

METHODS

We randomized 49 clinically stable individuals with SCZ to three weeks of intermittent treatment with 0.5 mg or 15 mg of DAR-0100A, or placebo (normal saline). Functional magnetic resonance imaging (fMRI) BOLD was used to evaluate the effects of drug administration on brain activity during a working memory (WM) task. Effects on cognition were also assessed using the MATRICS and the N-back task as primary endpoints. The CogState battery was used as a secondary endpoint.

RESULTS

There were no observed treatment effects on either the BOLD fMRI signal during WM tasks or the WM domains of the MATRICS. Moderate improvement was detected on the CogState battery and on the attention domain of the MATRICS.

CONCLUSION

These results suggest that low doses of D1 agonists that do not result in measureable occupancy of the D1R do not reliably improve cognition in SCZ, unlike the observations in NHP. As this drug is limited by its pharmacokinetic profile, better D1R agonists that can achieve adequate levels of D1R occupancy are needed to test the efficacy of this mechanism for cognitive enhancement in SCZ.

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.

Cognitive enhancing agents in schizophrenia and bipolar disorder

Cognitive dysfunction is a core feature of schizophrenia and is also present in bipolar disorder (BD). Whereas decreased intelligence precedes the onset of psychosis in schizophrenia and remains relatively stable thereafter; high intelligence is a risk factor for bipolar illness but cognitive function decreases after onset of symptoms. While in schizophrenia, many studies have been conducted on the development of cognitive enhancing agents; in BD such studies are almost non-existent. This review focuses on the pharmacological agents with putative effects on cognition in both schizophrenia and bipolar illness; specifically agents targeting the dopaminergic, cholinergic and glutamatergic neurotransmitter pathways in schizophrenia and the cognitive effects of lithium, anticonvulsants and antipsychotics in BD. In the final analysis we conclude that cognitive enhancing agents have not yet been produced convincingly for schizophrenia and have hardly been studied in BD. Importantly, studies should focus on other phases of the illness. To be able to treat cognitive deficits effectively in schizophrenia, patients in the very early stages of the illness, or even before - in the ultra-high risk stages - should be targeted. In contrast, cognitive deficits occur later in BD, and therefore drugs should be tested in BD after the onset of illness. Hopefully, we will then find effective drugs for the incapacitating effects of cognitive deficits in these patients.

Effects of the D1 dopamine receptor agonist dihydrexidine (DAR-0100A) on working memory in schizotypal personality disorder

Pharmacological enhancement of prefrontal D1 dopamine receptor function remains a promising therapeutic approach to ameliorate schizophrenia-spectrum working memory deficits, but has yet to be rigorously evaluated clinically. This proof-of-principle study sought to determine whether the active enantiomer of the selective and full D1 receptor agonist dihydrexidine (DAR-0100A) could attenuate working memory impairments in unmedicated patients with schizotypal personality disorder (SPD). We performed a randomized, double-blind, placebo-controlled trial of DAR-0100A (15 mg/150 ml of normal saline administered intravenously over 30 min) in medication-free patients with SPD (n=16) who met the criteria for cognitive impairment (ie, scoring below the 25th percentile on tests of working memory). We employed two measures of verbal working memory that are salient to schizophrenia-spectrum cognitive deficits, and that clinical data implicate as being associated with prefrontal D1 availability: (1) the Paced Auditory Serial Addition Test (PASAT); and (2) the N-back test (ratio of 2-back:0-back scores). Study procedures occurred over four consecutive days, with working memory testing on Days 1 and 4, and DAR-0100A/placebo administration on Days 2-4. Treatment with DAR-0100A was associated with significantly improved PASAT performance relative to placebo, with a very large effect size (Cohen's d=1.14). Performance on the N-back ratio was also significantly improved; however, this effect rested on both a non-significant enhancement and diminution of 2-back and 0-back performance, respectively; therefore interpretation of this finding is more complicated. DAR-0100A was generally well tolerated, with no serious medical or psychiatric adverse events; common side effects were mild to moderate and transient, consisting mainly of sedation, lightheadedness, tachycardia, and hypotension; however, we were able to minimize these effects, without altering the dose, with supportive measures, eg, co-administered normal saline. Although preliminary, these findings lend further clinical support to the potential of D1 receptor agonists to treat schizophrenia-spectrum working memory impairments. These data suggest a need for further studies with larger group sizes, serum DAR-0100A levels, and a more comprehensive neuropsychological battery.

Update on the Mechanism of Action of Aripiprazole: Translational Insights into Antipsychotic Strategies Beyond Dopamine Receptor Antagonism

Dopamine partial agonism and functional selectivity have been innovative strategies in the pharmacological treatment of schizophrenia and mood disorders and have shifted the concept of dopamine modulation beyond the established approach of dopamine D2 receptor (D2R) antagonism. Despite the fact that aripiprazole was introduced in therapy more than 12 years ago, many questions are still unresolved regarding the complexity of the effects of this agent on signal transduction and intracellular pathways, in part linked to its pleiotropic receptor profile. The complexity of the mechanism of action has progressively shifted the conceptualization of this agent from partial agonism to functional selectivity. From the induction of early genes to modulation of scaffolding proteins and activation of transcription factors, aripiprazole has been shown to affect multiple cellular pathways and several cortical and subcortical neurotransmitter circuitries. Growing evidence shows that, beyond the consequences of D2R occupancy, aripiprazole has a unique neurobiology among available antipsychotics. The effect of chronic administration of aripiprazole on D2R affinity state and number has been especially highlighted, with relevant translational implications for long-term treatment of psychosis. The hypothesized effects of aripiprazole on cell-protective mechanisms and neurite growth, as well as the differential effects on intracellular pathways [i.e. extracellular signal-regulated kinase (ERK)] compared with full D2R antagonists, suggest further exploration of these targets by novel and future biased ligand compounds. This review aims to recapitulate the main neurobiological effects of aripiprazole and discuss the potential implications for upcoming improvements in schizophrenia therapy based on dopamine modulation beyond D2R antagonism.

Psychopharmacological treatment of neurocognitive deficits in people with schizophrenia: a review of old and new targets

Neurocognitive impairments significantly contribute to disability and the overall clinical picture in schizophrenia spectrum disorders. There has therefore been a concerted effort, guided by the discovery of neurotransmitter and synaptic systems in the central nervous system, to develop and test compounds that may ameliorate neurocognitive deficits. The current article summarizes the results of efforts to test neurocognitive-enhancing agents in schizophrenia. Overall, existing clinical trials provide little reason to be enthusiastic about the benefits of psychopharmacological agents at enhancing neurocognition in schizophrenia-a state of affairs that may reflect the inadequacy of single neurotransmitter or receptor models. The etiologic and phenomenological complexity of neurocognitive deficits in schizophrenia may be better served by psychopharmacological agents that (i) target neurotransmitter systems proximal in the causal chain to neurocognitive deficits; (ii) enhance distal survival processes in the central nervous system-neurogenesis, neuronal growth, synaptogenesis, and connectivity; and (iii) counteract the negative effects of aberrant neurodevelopment in schizophrenia, such as neuroinflammation and oxidative stress. Future efforts to develop psychopharmacological agents for neurocognitive impairment in schizophrenia should reflect the knowledge of its complex etiology by addressing aberrations along its causal chain. Clinical trials may benefit methodologically from (i) an appreciation of the phenomenological heterogeneity of neurocognitive deficits in schizophrenia; (ii) a characterization of the predictors of treatment response; and (iii) a recognition of issues of sample size, statistical power, treatment duration, and dosing.

Pharmacotherapy of cognitive deficits in schizophrenia

While second-generation antipsychotics treat negative as well as positive symptoms, recovery for persons with schizophrenia remains elusive, in part because there are no FDA-approved medications that treat the cognitive deficits of schizophrenia (CDS). Recent work has identified agents that, when added to antipsychotics, improve cognition in schizophrenia. This work and hypothesized mechanisms of action will be reviewed.

Treating working memory deficits in schizophrenia: a review of the neurobiology

Cognitive deficits are a core feature of schizophrenia. Among these deficits, working memory impairment is considered a central cognitive impairment in schizophrenia. The prefrontal cortex, a region critical for working memory performance, has been demonstrated as a critical liability region in schizophrenia. As yet, there are no standardized treatment options for working memory deficits in schizophrenia. In this review, we summarize the neuronal basis for working memory impairment in schizophrenia, including dysfunction in prefrontal signaling pathways (e.g., γ-aminobutyric acid transmission) and neural network synchrony (e.g., gamma/theta oscillations). We discuss therapeutic strategies for working memory dysfunction such as pharmacological agents, cognitive remediation therapy, and repetitive transcranial magnetic stimulation. Despite the drawbacks of current approaches, the advances in neurobiological and translational treatment strategies suggest that clinical application of these methods will occur in the near future.

Pharmacological Interventions for the MATRICS Cognitive Domains in Schizophrenia: What's the Evidence?

Schizophrenia is a disabling, chronic psychiatric disorder with a prevalence rate of 0.5-1% in the general population. Symptoms include positive (e.g., delusions, hallucinations), negative (e.g., blunted affect, social withdrawal), as well as cognitive symptoms (e.g., memory and attention problems). Although 75-85% of patients with schizophrenia report cognitive impairments, the underlying neuropharmacological mechanisms are not well understood and currently no effective treatment is available for these impairments. This has led to the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) initiative, which established seven cognitive domains that are fundamentally impaired in schizophrenia. These domains include verbal learning and memory, visual learning and memory, working memory, attention and vigilance, processing speed, reasoning and problem solving, and social cognition. Recently, a growing number of studies have been conducted trying to identify the underlying neuropharmacological mechanisms of cognitive impairments in schizophrenia patients. Specific cognitive impairments seem to arise from different underlying neuropharmacological mechanisms. However, most review articles describe cognition in general and an overview of the mechanisms involved in these seven separate cognitive domains is currently lacking. Therefore, we reviewed the underlying neuropharmacological mechanisms focusing on the domains as established by the MATRICS initiative which are considered most crucial in schizophrenia.

Schizotypal personality disorder

Early phenomenological descriptions of schizophrenia have acknowledged the existence of milder schizophrenia spectrum disorders characterized by the presence of attenuated symptoms typically present in chronic schizophrenia. The investigation of the schizophrenia spectrum disorders offers an opportunity to elucidate the pathophysiological mechanisms giving rise to schizophrenia. Differences and similarities between subjects with schizotypal personality disorder (SPD), the prototypical schizophrenia personality disorder, and chronic schizophrenia have been investigated with genetic, neurochemical, imaging, and pharmacological techniques. Patients with SPD and the more severely ill patients with chronic schizophrenia share cognitive, social, and attentional deficits hypothesized to result from common neurodevelopmentally based cortical temporal and prefrontal pathology. However, these deficits are milder in SPD patients due to their capacity to recruit other related brain regions to compensate for dysfunctional areas. Individuals with SPD are also less vulnerable to psychosis due to the presence of protective factors mitigating subcortical DA hyperactivity. Given the documented close relationship to other schizophrenic disorders, SPD will be included in the psychosis section of DSM-5 as a schizophrenia spectrum disorder as well as in the personality disorder section.

Neural substrates underlying effort computation in schizophrenia

The lack of initiative, drive or effort in patients with schizophrenia is linked to marked functional impairments. However, our assessment of effort and motivation is crude, relying on clinical rating scales based largely on patient recall. In order to better understand the neurobiology of effort in schizophrenia, we need more rigorous measurements of this construct. In the behavioural neuroscience literature, decades of work has been carried out developing various paradigms to examine the neural underpinnings of an animal's willingness to expend effort for a reward. Here, we shall review this literature on the nature of paradigms used in rodents to assess effort, as well as those used in humans. Next, the neurobiology of these effort-based decisions will be discussed. We shall then review what is known about effort in schizophrenia, and what might be inferred from experiments done in other human populations. Lastly, we shall discuss future directions of research that may assist in shedding light on the neurobiology of effort cost computations in schizophrenia.

PET neuroimaging of extrastriatal dopamine receptors and prefrontal cortex functions

The role of prefrontal dopamine D1 receptors in prefrontal cortex (PFC) functions, including working memory, is widely investigated. However, human (healthy volunteers and schizophrenia patients) positron emission tomography (PET) studies about the relationship between prefrontal D1 receptors and PFC functions are somewhat inconsistent. We argued that several factors including an inverted U-shaped relationship between prefrontal D1 receptors and PFC functions might be responsible for these inconsistencies. In contrast to D1 receptors, relatively less attention has been paid to the role of D2 receptors in PFC functions. Several animal and human pharmacological studies have reported that the systemic administration of D2 receptor agonist/antagonist modulates PFC functions, although those studies do not tell us which region(s) is responsible for the effect. Furthermore, while prefrontal D1 receptors are primarily involved in working memory, other PFC functions such as set-shifting seem to be differentially modulated by dopamine. PET studies of extrastriatal D2 receptors including ours suggested that orchestration of prefrontal dopamine transmission and hippocampal dopamine transmission might be necessary for a broad range of normal PFC functions. In order to understand the complex effects of dopamine signaling on PFC functions, measuring a single index related to basic dopamine tone is not sufficient. For a better understanding of the meanings of PET indices related to neurotransmitters, comprehensive information (presynaptic, postsynaptic, and beyond receptor signaling) will be required. Still, an interdisciplinary approach combining molecular imaging techniques with cognitive neuroscience and clinical psychiatry will provide new perspectives for understanding the neurobiology of neuropsychiatric disorders and their innovative drug developments.

Reward learning as a potential target for pharmacological augmentation of cognitive remediation for schizophrenia: a roadmap for preclinical development

Rationale: Impaired cognitive abilities are a key characteristic of schizophrenia. Although currently approved pharmacological treatments have demonstrated efficacy for positive symptoms, to date no pharmacological treatments successfully reverse cognitive dysfunction in these patients. Cognitively-based interventions such as cognitive remediation (CR) and other psychosocial interventions however, may improve some of the cognitive and functional deficits of schizophrenia. Given that these treatments are time-consuming and labor-intensive, maximizing their effectiveness is a priority. Augmenting psychosocial interventions with pharmacological treatments may be a viable strategy for reducing the impact of cognitive deficits in patients with schizophrenia.

Objective: We propose a strategy to develop pharmacological treatments that can enhance the reward-related learning processes underlying successful skill-learning in psychosocial interventions. Specifically, we review clinical and preclinical evidence and paradigms that can be utilized to develop these pharmacological augmentation strategies. Prototypes for this approach include dopamine D1 receptor and α7 nicotinic acetylcholine receptor agonists as attractive targets to specifically enhance reward-related learning during CR.

Conclusion: The approach outlined here could be used broadly to develop pharmacological augmentation strategies across a number of cognitive domains underlying successful psychosocial treatment.

Future perspectives on the treatment of cognitive deficits and negative symptoms in schizophrenia

Drug discovery based on classic models for cognitive impairment and negative symptoms of schizophrenia have met with only modest success. Because cognitive impairment and negative symptoms may result from disruptions in neurodevelopment, more complex developmental models that integrate environmental and genetic risk factors are needed. In addition, it has become clear that biochemical pathways involved in schizophrenia form complex, interconnected networks. Points at which risk factors converge, such as brain-derived neurotrophic factor (BDNF) and protein kinase B (AKT), and from which processes involved in neuroplasticity diverge, are of particular interest for pharmacologic interventions. This paper reviews elements of neurodevelopmental models for cognitive deficits and negative symptoms of schizophrenia with the aim of identifying potential targets for interventions.

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.

Neurobiological background for the development of new drugs in schizophrenia

Psychopharmacology of schizophrenia has remained static for many years because the mechanisms explored have been basically monoaminergics, primarily focused toward the modification of dopaminergic function and, later on, serotonergic. In fact, most of the antipsychotics introduced in clinical practice in the last years have been antagonists or selective agonists of these receptors (D(2)/5-HT(2)). The exploration of other receptor pathways, and in particular those additionally involved in the action of the paradigmatic "atypical" antipsychotic clozapine (ie, cholinergic and noradrenergic), has not been very significant. Besides, research in the antipsychotics field has developed also by exploring pathways that are beyond the spectrum of clozapine. Among the most promising mechanisms are those based on the glutamatergic hypothesis of schizophrenia (agonists at the glycine-binding modulatory site of the N-methyl-D-aspartate receptor, glycine transporter inhibitors, modulators of the AMPA [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid] receptor and selective agonists of the metabotropic receptor Glu(2)). Other less classic pathways are also under study and have led to some agents that are found in very early stages of development such as those acting on sigma receptors, cholecystokinin antagonists, neurotensin agonists, neurokinin receptor antagonists, GABAergic (+-aminobutyric acid [GABA]) enhancers, and cannabinoid(gamma-aminobutiric) receptor modulators.

Can genetics inform the management of cognitive deficits in schizophrenia?

There is no doubt that schizophrenia has a significant genetic component and a number of candidate genes have been identified for this debilitating disorder. Of note, several of these are implicated in cognition. Cognitive deficits constitute core symptoms of schizophrenia, and while current antipsychotic treatment strategies aim to help psychosis-related symptomatology, the cognitive symptom domain is largely inadequately treated. A number of other pharmacological approaches (e.g. using drugs that target specific neurotransmitter systems) have also been attempted for the amelioration of cognitive deficits in this population; however, these too have had limited success so far. Psychological interventions appear promising, though there has been speculation regarding whether or not these produce long-term functional improvements. Pharmacogenetic studies of the cognitive effects of currently available antipsychotics, although in relatively early stages, suggest that the treatment of cognitive deficits in schizophrenia may be advanced by focusing on genetic variants associated with specific cognitive dysfunctions in the general population and using this to match the most relevant pharmacological and/or psychological interventions with the genetic and cognitive profiles of the target population. Such a strategy would encourage bottom-up advances in drug development and provide a platform for individualised treatment of cognitive deficits in schizophrenia.

Schizophrenia: treatment targets beyond monoamine systems

We develop the proposal in this review that schizophrenia is a syndrome made up of component symptom complexes, each with distinctive clinical correlates, pathophysiology, and selective treatments. Psychosis is the necessary component of the syndrome; it has a young-adult onset and is sensitive to current antipsychotic drugs. Cognitive dysfunction often precedes psychosis onset, does not present an episodic course, and is poorly responsive to antipsychotic drugs. Treatments for cognition are being developed largely on the basis of animal pharmacology. Drugs for component symptom complexes will theoretically be coadministered to independent symptomatic end points. Animal models, some with genetic characteristics, can be more easily and directly developed to match an individual component than to match an illness definition as broad as schizophrenia.

Cognitive enhancers: focus on modulatory signaling influencing memory consolidation

Biological research has unraveled many of the molecular and cellular mechanisms involved in the formation of long-lasting memory, providing new opportunities for the development of cognitive-enhancing drugs. Studies of drug enhancement of cognition have benefited from the use of pharmacological treatments given after learning, allowing the investigation of mechanisms regulating the consolidation phase of memory. Modulatory systems influencing consolidation processes include stress hormones and several neurotransmitter and neuropeptide systems. Here, we review some of the findings on memory enhancement by drug administration in animal models, and discuss their implications for the development of cognitive enhancers.

Characterization of in vivo pharmacokinetic properties of the dopamine D1 receptor agonist DAR-0100A in nonhuman primates using PET with [11C] NNC112 and [11C] raclopride

DAR-0100A, the active enantiomer of dihydrexidine, is a potent dopamine D1 agonist under investigation for treatment of cognitive impairment and negative symptoms of schizophrenia. We measured the dose-occupancy relationship for DAR-0100A at D1 receptors using positron emission tomography (PET) imaging in baboons with [(11)C] NNC112 and its binding to D2 with [(11)C] raclopride. Two baboons were scanned with [(11)C] NNC112 at baseline and after three different doses of DAR-0100A. Two baboons were scanned with [(11)C] raclopride at baseline and after one dose of DAR-0100A. Occupancy (ΔBP(ND)) was computed in the striatum and cortex. A clear relationship was observed between plasma concentration of DAR-0100A and ΔBP(ND). ΔBP(ND) was larger in the striatum than in the cortex, consistent with reports showing that 25% of [(11)C] NNC112 BP(ND) in the cortex is attributed to 5-HT(2A). Plasma EC(50) estimates ranged from 150 to 550 ng/mL according to the constraints on the model. There was no detectable effect of DAR-0100A on [(11)C] raclopride BP(ND). These data suggest that at doses likely to be administered to patients, occupancy will not be detectable with [(11)C] NNC112 PET and binding of DAR-0100A to D2 will be negligible. This is the first demonstration with PET of a significant occupancy by a full D1 agonist in vivo.

Dopamine receptor homooligomers and heterooligomers in schizophrenia

Over the past two decades the dopamine D2 receptor has been undoubtedly the most widely studied dopamine receptor for the therapeutic treatment of schizophrenia, as the majority of antipsychotics exhibit antagonism at this receptor. However, the cognitive symptoms of the disorder are mostly resistant to the majority of available antipsychotic treatments and, as a result, there is a critical need to develop novel therapies that ameliorate all symptoms. The recognition that dopamine receptors, such as all G protein-coupled receptors (GPCRs), exist as oligomeric complexes has provided new avenues for drug design in the search for novel therapies. Furthermore, that it is now known that dopamine receptors can form heteromers, such as the dopamine D1-D2 receptor heteromer, with pharmacology and function distinct from its constituent receptors, has significantly expanded the range of potential drug targets. The aim of this review is to discuss the therapeutic relevance of these dopamine receptor oligomers to schizophrenia and to address the potential value of dopamine receptor heteromers in the search for new therapeutic strategies.

The treatment of cognitive impairment in schizophrenia

Cognitive deficits are major contributors to disability in schizophrenia. Many pharmacologic targets have been identified for cognitive enhancing agents, including receptors involved in dopaminergic, glutamatergic, GABAergic, serotonergic and cholinergic neurotransmission. In addition, new approaches to drug development have been directed towards neuroprotection and the facilitation of neuroplasticity. While several pharmacologic agents and cognitive remediation have shown promise in early trials, no treatment has yet demonstrated efficacy in large replication trials. The experience with different pharmacologic targets is reviewed and methodologic issues are discussed with recommendations for future research.

Polypharmacy in schizophrenia

PURPOSE OF REVIEW

Although most guidelines recommend monotherapy in schizophrenia, the combined application of multiple psychotropic agents is very common, especially in treatment-refractory cases. We review the empirical basis supporting these attempts and their relevance for clinical practice.

RECENT FINDINGS

Polypharmacy intends to address different aspects of treatment resistance, most importantly insufficient response of psychotic positive and negative symptoms, but also cognitive disturbances, affective comorbidity, obsessive-compulsive syndromes and side-effects of antipsychotic drugs. This review summarizes the current state of evidence of combined antipsychotic treatment strategies and the augmentation of antipsychotics with mood stabilizers, antidepressants and experimental substances.

SUMMARY

In general, rigorous data on combination therapy in schizophrenia are rare and further randomized controlled trials, naturalistic trials and head-to-head-trials are necessary. Some evidence supports a combination of antipsychotics and antidepressants for negative symptoms and comorbid major depressive episodes. The add-on of lithium and mood stabilizers lacks compelling evidence, but might be beneficial for specific subgroups. For treatment-resistant cognitive symptoms, antipsychotic medication should be combined with cognitive remediation, as no pharmacological add-on strategy has gained convincing evidence so far. Treatment-emergent positive and/or negative symptoms under clozapine monotherapy might benefit from adding a second atypical substance.

Pharmacological strategies for enhancing cognition in schizophrenia

Researchers have long recognized that individuals with schizophrenia experience challenges in a wide range of cognitive domains, and research on cognitive impairment in schizophrenia is not a recent phenomena. However, the past 10-20 years have seen an increasing recognition of the central importance of cognition to understanding function and outcome in this illness (Green et al. in Schizophr Bull 26:119-136, 2000), an awareness that has shifted the emphasis of at least some work on schizophrenia. More specifically, there has been a rapidly growing body of work on methods of enhancing cognition in schizophrenia, as a means to potentially facilitate improved outcome and quality of life for individuals with this debilitating illness. The current chapter reviews the results of a range of studies examining adjunctive pharmacological treatments to enhance cognition in schizophrenia using a range of designs, including single-dose studies, open-label repeated dosing studies, and double-blind parallel group and crossover designs with repeated dosing. Although many of the single-dose and open-label studies have suggested positive cognitive effects from a range of agents, few of the larger-scale double-blind studies have generated positive results. The current state of results may reflect the need to identify alternative molecular mechanisms for enhancing cognition in schizophrenia or the need to reconceptualize the ways in which pharmacological agents may improve cognition in this illness, with a concomitant change in the traditional clinical trial study design used in prior studies of cognitive enhancement in schizophrenia.

Modeling "psychosis" in vitro by inducing disordered neuronal network activity in cortical brain slices

INTRODUCTION

Dysregulation of neuronal networks has been suggested to underlie the cognitive and perceptual abnormalities observed schizophrenia.

DISCUSSIONS

An in vitro model of psychosis is proposed based on the two different approaches to cause aberrant network activity in layer V pyramidal cells of prefrontal brain slices: (1) psychedelic hallucinogens such as lysergic acid diethylamide and (2) minimal GABA(A) receptor antagonism, modeling the GABA interneuron deficit in schizophrenia. A test of this model would be to determine if drugs that normalize aberrant networks in brain slices have efficacy in the treatment of schizophrenia. Selective agonists of glutamate mGlu2/3 metabotropic receptors, which are highly effective in suppressing aberrant network activity in slices, are the most advanced toward reaching that clinical endpoint. In accord with the model, a recent phase II clinical trial shows that an mGlu2/3 receptor agonist is equivalent in efficacy to a standard antipsychotic drug for both negative and positive symptoms in schizophrenic patients, but without the usual side effects. D1/5 dopamine receptor agonists are also effective in normalizing aberrant network activity induced by both hallucinogens and minimal GABA(A) antagonism; clinical efficacy remains to be determined. A general model of network regulation is presented, involving astrocytes, GABA interneurons, and glutamatergic pyramidal cells, revealing a wide range of potential sites hitherto not considered as therapeutic targets.