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

D(1)-like receptor activation improves PCP-induced cognitive deficits in animal models: Implications for mechanisms of improved cognitive function in schizophrenia

Phencyclidine (PCP) produces cognitive deficits of relevance to schizophrenia in animal models. The aim was to investigate the efficacy of the D(1)-like receptor agonist, SKF-38393, to improve PCP-induced deficits in the novel object recognition (NOR) and operant reversal learning (RL) tasks. Rats received either sub-chronic PCP (2 mg/kg) or vehicle for 7 days, followed by a 7-day washout. Rats were either tested in NOR or the RL tasks. In NOR, vehicle rats successfully discriminated between novel and familiar objects, an effect abolished in PCP-treated rats. SKF-38393 (6 mg/kg) significantly ameliorated the PCP-induced deficit (P<0.01) an effect significantly antagonised by SCH-23390 (0.05 mg/kg), a D(1)-like receptor antagonist (P<0.01). In the RL task sub-chronic PCP significantly reduced performance in the reversal phase (P<0.001); SKF-38393 (6.0 mg/kg) improved this PCP-induced deficit, an effect antagonised by SCH-23390 (P<0.05). These results suggest a role for D(1)-like receptors in improvement of cognitive function in paradigms of relevance to schizophrenia.

Antipsychotics in the early stage of development

PURPOSE OF REVIEW

In this article, we will summarize some of the most recent pharmacological approaches by briefly reviewing the latest research results related to different neurotransmitter receptor systems but also alternative approaches that do not affect central nervous system receptors directly.

RECENT FINDINGS

Although the field still struggles with the lack of major breakthroughs, some novel pharmacological leads have provided encouraging findings in proof of concept or early phase II clinical trials. Although some of these approaches are based on the modified dopamine/glutamate hypothesis of schizophrenia, others appear to have been driven by more serendipitous observations. Apart from this, more traditional routes such as compounds targeting D2/HT2 receptors are also still being pursued. Attempts to advance treatment response of negative symptoms and cognitive impairment, two areas of considerable unmet need have been an overriding theme in many studies.

SUMMARY

Clinical psychopharmacologists increasingly realize that it is highly unlikely that we have a magic bullet to treat adequately all aspects of this complex disorder in the foreseeable future. Rational polypharmacy may be the trend for the next decade.

Recent advances in treating cognitive impairment in schizophrenia

INTRODUCTION

Schizophrenia is often associated with chronic disability and poor outcome. In addition to positive symptoms, such as hallucinations and delusions, and negative symptoms including poverty of speech and blunted affect, schizophrenia is also associated with deficits in cognitive function. It has been increasingly recognized that the severity of cognitive impairment is a major determinant of outcome. Therefore, interventions to improve cognitive function also have the capacity to improve quality of life and social and occupational outcomes. Whilst some of the antipsychotic drugs have shown some selective benefits, there is some controversy about the extent of these benefits.

OBJECTIVES

This article provides an overview of research into drugs that might enhance cognition in schizophrenia.

CONCLUSION

Drugs such as modafanil and galantamine are being evaluated, and a number of new drugs are currently in development. Standardized cognitive assessment measures are being developed so studies can be compared more easily. This field is advancing rapidly, but as yet, no widely applicable, evidence-based treatments are available to the clinician.

Better pharmacotherapy for schizophrenia: what does the future hold?

Despite the expansion of available antipsychotic drugs over the past 50 years, functional outcomes for individuals with schizophrenia have not markedly improved. These agents are efficacious for psychosis but do not adequately address other core domains of schizophrenia psychopathology, namely negative symptoms and cognitive impairment, which have a greater impact on functional outcomes, including vocational or academic performance and interpersonal relationships. In addition, treatment-refractory psychosis still precludes functional improvement in many patients. Schizophrenia is a clinical syndrome consisting of these domains, which likely have some disparities in their respective pathophysiologies. This suggests that drug development should look to other molecular targets besides the D2 receptor, which characterizes the mechanism of available medications for schizophrenia. In this report, we review novel pharmacologic approaches that aim to specifically address each individual domain of schizophrenia. The goal of this future pharmacotherapy strategy is to advance outcomes beyond psychosis remission and toward functional recovery.

From vice to virtue: insights from sensitization in the nonhuman primate

Repeated, intermittent administration of psychomotor stimulants, or D1 agonists in dopamine-deficient states, induces behavioral sensitization, characterized by an enhanced response to a subsequent acute low dose challenge, which may be manifested in form of altered behavior or cognitive function. Amphetamine sensitization in the nonhuman primate encompasses profound and enduring changes to similar neuronal and neurochemical substrates that occur in rodents. The process of sensitization in the monkey also results in a long-lasting depression in baseline behavioral responding, as well as emergence of hallucinatory-like behaviors reminiscent of human psychosis in response to an acute challenge. Nonhuman primates show a reduction in spine density and dendritic length in prefrontal neurons and a marked reduction in basal dopamine turnover in both prefrontal cortex and striatum. A major hallmark of amphetamine sensitization in both nonhuman primates and rodents is the manifestation of deficits in executive function and working memory which rely upon the integrity of prefrontal cortex and thereby, may yield significant insights into the cognitive dysfunction associated with addiction. Together with evidence from human and rodent studies, it can be concluded that repeated exposure to psychomotor stimulants can lead to a corruption of neuroadaptive systems in the brain by an extraordinary influence on synaptic plasticity, learning, and memory. Actively harnessing this same process by repeated, intermittent D1 agonist administration may be the key to improved working memory and decision making in addiction and other dopamine dysfunctional states, such as schizophrenia.

Recent advances in the development of novel pharmacological agents for the treatment of cognitive impairments in schizophrenia

Wayne Fenton was a major driving force behind the establishment of the Measurement and Treatment Research to Improve Cognition in Schizophrenia (MATRICS) and Treatment Units for Research on Neurocognition and Schizophrenia (TURNS) project mechanisms. These projects were designed to facilitate the development of new drugs for the treatment of cognitive impairments in people with schizophrenia. The MATRICS project identified 3 drug mechanisms of particular interest: cholinergic, dopaminergic, and glutamatergic. The TURNS project is designed to select potential cognitive-enhancing agents and evaluate their potential efficacy in the context of proof of concept or clinical efficacy trials. This article reviews the rationale for these 3 approaches and provides an update on the development of therapeutic agents, which act through one of these 3 mechanisms.

A single 20 mg dose of the full D1 dopamine agonist dihydrexidine (DAR-0100) increases prefrontal perfusion in schizophrenia

Dopamine D1 receptors play an important role in memory and cognition in non-human primates. Dopamine D1 agonists have been shown to reverse performance deficits in both aged non-human primates and in primates with lesions to dopamine systems. This study explored whether a single dose of the first full D1 agonist dihydrexidine (DAR-0100) would cause changes in brain activity (perfusion) in dopamine-rich brain regions. We used a new gadolinium-contrast magnetic resonance perfusion scanning technique to measure brain activity. A within-subject cross-over double-blind randomized design was used in 20 adults with SCID-diagnosed schizophrenia. Each morning at 0800 h, they were scanned on a 3.0 T MRI scanner for perfusion. They then received either 20 mg of dihydrexidine, or placebo, subcutaneously over 15 min. Over the next 45 min, they had intermittent MRI scans. Two days later, they had a repeat of the Day 1 schedule, but received the opposite treatment from that given on the first day. Within-day, as well as between-day, comparisons were made to test for perfusion effects of dihydrexidine. Analysis revealed that dihydrexidine induced a significant increase in both prefrontal and non-prefrontal perfusion compared to placebo. The greatest increases occurred approximately 20 min after dihydrexidine infusion, consistent with the short pharmacokinetic half-life of dihydrexidine. These data are consistent with the hypothesis formulated from studies of non-human primates that dihydrexidine and other D1 agonists may be able to modulate prefrontal dopaminergic function.