Occupancy of striatal and extrastriatal dopamine D2 receptors by clozapine and quetiapine

Optimisation of pharmacotherapy in psychiatry through therapeutic drug monitoring, molecular brain imaging and pharmacogenetic tests: focus on antipsychotics

BACKGROUND

For psychotic disorders (i.e. schizophrenia), pharmacotherapy plays a key role in controlling acute and long-term symptoms. To find the optimal individual dose and dosage strategy, specialized tools are used. Three tools have been proven useful to personalize drug treatments: therapeutic drug monitoring (TDM) of drug levels, pharmacogenetic testing (PG), and molecular neuroimaging.

METHODS

In these Guidelines, we provide an in-depth review of pharmacokinetics, pharmacodynamics, and pharmacogenetics for 50 antipsychotics. Over 30 international experts in psychiatry selected studies that have measured drug concentrations in the blood (TDM), gene polymorphisms of enzymes involved in drug metabolism, or receptor/transporter occupancies in the brain (positron emission tomography (PET)).

RESULTS

Study results strongly support the use of TDM and the cytochrome P450 (CYP) genotyping and/or phenotyping to guide drug therapies. Evidence-based target ranges are available for titrating drug doses that are often supported by PET findings.

CONCLUSION

All three tools discussed in these Guidelines are essential for drug treatment. TDM goes well beyond typical indications such as unclear compliance and polypharmacy. Despite its enormous potential to optimize treatment effects, minimize side effects and ultimately reduce the global burden of diseases, personalized drug treatment has not yet become the standard of care in psychiatry.

Update Lessons from Positron Emission Tomography Imaging Part I: A Systematic Critical Review on Therapeutic Plasma Concentrations of Antipsychotics

BACKGROUND

Positron emission tomography (PET) and single photon emission tomography (SPECT) of molecular drug targets (neuroreceptors and transporters) provide essential information for therapeutic drug monitoring-guided antipsychotic drug therapy. The optimal therapeutic windows for D 2 antagonists and partial agonists, as well as their proposed target ranges, are discussed based on an up-to-date literature search.

METHODS

This part I of II presents an overview of molecular neuroimaging studies in humans and primates involving the target engagement of amisulpride, haloperidol, clozapine, aripiprazole, olanzapine, quetiapine, risperidone, cariprazine, and ziprasidone. The systemic review particularly focused on dopamine D 2 -like and 5-HT 2A receptors. Target concentration ranges were estimated based on receptor occupancy ranges that relate to clinical effects or side effects (ie, extrapyramidal side effects). In addition, findings for other relevant receptor systems were included to further enrich the discussion.

RESULTS

The reported reference ranges for aripiprazole and clozapine align closely with findings from PET studies. Conversely, for haloperidol, risperidone, and olanzapine, the PET studies indicate that a lowering of the previously published upper limits would be necessary to decrease the risk of extrapyramidal side effect.

CONCLUSIONS

Molecular neuroimaging studies serve as a strong tool for defining target ranges for antipsychotic drug treatment and directing therapeutic drug monitoring.

Dopamine partial agonists: a discrete class of antipsychotics

Worldwide, there are now three marketed dopamine D2 partial agonists: aripiprazole, brexpiprazole and cariprazine. These three drugs share a number of properties other than their action at D2 receptors. Pharmacologically, they are 5HT2 antagonists and D3 and 5HT1A partial agonists but with little or no alpha-adrenergic, anticholinergic or antihistaminic activity. They also share a long duration of action. Clinically, D2 partial agonists are effective antipsychotics and generally have useful antimanic and antidepressant activity. They are usually well tolerated, causing akathisia and insomnia only at the start of treatment, and are non-sedating. These drugs also share a very low risk of increased prolactin and of weight gain and accompanying metabolic effects. They may also have a relatively low risk of tardive dyskinesia. There is some evidence that they are preferred by patients to dopamine antagonists. Individual dopamineD2 partial agonists have much in common and as a group they differ importantly from dopamine D2 antagonists. Dopamine D2 partial agonists should be considered a distinct class of antipsychotics.Key pointsD2 partial agonists share many pharmacological and clinical propertiesD2 partial agonists differ in several important respects from D2 antagonistsD2 partial agonists should be considered a discrete class of antipsychotics.

Neuroimaging glutamatergic mechanisms differentiating antipsychotic treatment-response

Glutamatergic dysfunction is associated with failure to respond to antipsychotic medication in individuals with schizophrenia. Our objective was to combine neurochemical and functional brain imaging methods to investigate glutamatergic dysfunction and reward processing in such individuals compared with those with treatment responsive schizophrenia, and healthy controls. 60 participants played a trust task, while undergoing functional magnetic resonance imaging: 21 classified as having treatment-resistant schizophrenia, 21 patients with treatment-responsive schizophrenia, and 18 healthy controls. Proton magnetic resonance spectroscopy was also acquired to measure glutamate in the anterior cingulate cortex. Compared to controls, treatment responsive and treatment-resistant participants showed reduced investments during the trust task. For treatment-resistant individuals, glutamate levels in the anterior cingulate cortex were associated with signal decreases in the right dorsolateral prefrontal cortex when compared to those treatment-responsive, and with bilateral dorsolateral prefrontal cortex and left parietal association cortex when compared to controls. Treatment-responsive participants showed significant signal decreases in the anterior caudate compared to the other two groups. Our results provide evidence that glutamatergic differences differentiate treatment resistant and responsive schizophrenia. The differentiation of cortical and sub-cortical reward learning substrates has potential diagnostic value. Future novel interventions might therapeutically target neurotransmitters affecting the cortical substrates of the reward network.

Informing individualized multi-scale neural signatures of clozapine response in patients with treatment-refractory schizophrenia

Clozapine is currently the only antipsychotic with demonstrated efficacy in treatment-refractory schizophrenia (TRS). However, response to clozapine differs widely between TRS patients, and there are no available clinical or neural predictive indicators that could be used to increase or accelerate the use of clozapine in patients who stand to benefit. Furthermore, it remains unclear how the neuropharmacology of clozapine contributes to its therapeutic effects. Identifying the mechanisms underlying clozapine's therapeutic effects across domains of symptomatology could be crucial for development of new optimized therapies for TRS. Here, we present results from a prospective neuroimaging study that quantitatively related heterogeneous patterns of clinical clozapine response to neural functional connectivity at baseline. We show that we can reliably capture specific dimensions of clozapine clinical response by quantifying the full variation across item-level clinical scales, and that these dimensions can be mapped to neural features that are sensitive to clozapine-induced symptom change. Thus, these features may act as "failure modes" that can provide an early indication of treatment (non-)responsiveness. Lastly, we related the response-relevant neural maps to spatial expression profiles of genes coding for receptors implicated in clozapine's pharmacology, demonstrating that distinct dimensions of clozapine symptom-informed neural features may be associated with specific receptor targets. Collectively, this study informs prognostic neuro-behavioral measures for clozapine as a more optimal treatment for selected patients with TRS. We provide support for the identification of neuro-behavioral targets linked to pharmacological efficacy that can be further developed to inform optimal early treatment decisions in schizophrenia.

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

Machine Learning algorithm unveils glutamatergic alterations in the post-mortem schizophrenia brain

Schizophrenia is a disorder of synaptic plasticity and aberrant connectivity in which a major dysfunction in glutamate synapse has been suggested. However, a multi-level approach tackling diverse clusters of interacting molecules of the glutamate signaling in schizophrenia is still lacking. We investigated in the post-mortem dorsolateral prefrontal cortex (DLPFC) and hippocampus of schizophrenia patients and non-psychiatric controls, the levels of neuroactive d- and l-amino acids (l-glutamate, d-serine, glycine, l-aspartate, d-aspartate) by HPLC. Moreover, by quantitative RT-PCR and western blotting we analyzed, respectively, the mRNA and protein levels of pre- and post-synaptic key molecules involved in the glutamatergic synapse functioning, including glutamate receptors (NMDA, AMPA, metabotropic), their interacting scaffolding proteins (PSD-95, Homer1b/c), plasma membrane and vesicular glutamate transporters (EAAT1, EAAT2, VGluT1, VGluT2), enzymes involved either in glutamate-dependent GABA neurotransmitter synthesis (GAD65 and 67), or in post-synaptic NMDA receptor-mediated signaling (CAMKIIα) and the pre-synaptic marker Synapsin-1. Univariable analyses revealed that none of the investigated molecules was differently represented in the post-mortem DLPFC and hippocampus of schizophrenia patients, compared with controls. Nonetheless, multivariable hypothesis-driven analyses revealed that the presence of schizophrenia was significantly affected by variations in neuroactive amino acid levels and glutamate-related synaptic elements. Furthermore, a Machine Learning hypothesis-free unveiled other discriminative clusters of molecules, one in the DLPFC and another in the hippocampus. Overall, while confirming a key role of glutamatergic synapse in the molecular pathophysiology of schizophrenia, we reported molecular signatures encompassing elements of the glutamate synapse able to discriminate patients with schizophrenia and normal individuals.

Pharmacotherapy of schizophrenia: Mechanisms of antipsychotic accumulation, therapeutic action and failure

Schizophrenia is a multi-dimensional disorder with a complex and mostly unknown etiology, leading to a severe decline in life quality. Antipsychotic drugs (APDs) remain beneficial interventions in the treatment of the disorder, but vary significantly in binding profile, clinical effects and adverse reactions. The present review summarizes the main principles of APD mechanisms of action with a particular focus on recent findings in APD accumulation and its role in the therapeutic efficacy and treatment failure. High and low doses of APDs were shown to be effective in different dimensions of antipsychotic-like behaviour in rodent models. Efficacy of the APDs correlates with high dopamine D2 receptor occupancy, which occurs quickly after drug administration. However, onset and peak of action are delayed up to several days or weeks. APD accumulation via acidic trapping in synaptic vesicles is considered to underlie the time course of APD action. Use-dependent exocytosis, co-release with dopamine and serotonin and inhibition of ion channels impact on the neuronal transmission and determine effects of APDs. Disruption in accumulating properties leads to diminished APD effects. In addition, long-term APD administration at therapeutic doses leads to treatment failure both in animal models and in humans. APD failure was associated with treatment induced neuroadaptations, including a decline in extracellular dopamine levels, dopamine transporter upregulation, and altered neuronal firing. However, enhanced synaptic vesicle release has also been reported. APD loss of efficacy may be reversed through inhibition of the dopamine transporter or switching the administration regimen from continuous to intermittent. Thus, manipulating the accumulation properties of APDs, changes in the administration regimen and doses, or co-administration with dopamine transporter inhibitors may be considered to yield benefits in the development of new effective strategies in the treatment of schizophrenia.

Rationale and neurobiological effects of treatment with antipsychotics in patients with chronic schizophrenia considering dopamine supersensitivity

The long-term treatment of patients with schizophrenia often involves the management of relapses for most patients and the development of treatment resistance in some patients. To stabilize the clinical course and allow as many patients as possible to recover, clinicians need to recognize dopamine supersensitivity, which can be provoked by administration of high dosages of antipsychotics, and deal with it properly. However, no treatment guidelines have addressed this issue. The present review summarized the characteristics of long-acting injectable antipsychotics, dopamine partial agonists, and clozapine in relation to dopamine supersensitivity from the viewpoints of receptor profiles and pharmacokinetics. The potential merits and limitations of these medicines are discussed, as well as the risks of treating patients with established dopamine supersensitivity with these classes of drugs. Finally, the review discussed the biological influence of antipsychotic treatment on the human brain based on findings regarding the relationship between the hippocampus and antipsychotics.

Quetiapine treatment for cannabis use disorder

BACKROUND

Pharmacotherapy for cannabis use disorder (CUD) is an important unmet public health need.

METHODS

In a 12-week randomized double-blind placebo-controlled trial, the efficacy of quetiapine (300 mg nightly) for the treatment of CUD was tested in 130 outpatients. Weekly cannabis use was categorized into three groups: heavy use (5-7 days), moderate use (2-4 days) and light use (0-1 days).

RESULTS

At baseline both groups were considered heavy users (using days per week: median = 7.0; interquartile range (IQR): 6.5-7.0; daily dollar value: median = $121.4; IQR: 73.8-206.3). The week-by-treatment interaction was marginally significant (χ²(2) = 5.56, P = .06). With each week, the odds of moderate compared to heavy use significantly increased in the quetiapine group (OR=1.17, P < .0001), but not significantly in the placebo group (OR=1.05, P = .16). The odds of light versus heavy use did not significantly differ over time (P = .12). Treatment was also associated with reduced cannabis withdrawal symptoms by 10.4% each week (95% CI: 8.9-11.8). No serious adverse events occurred during the study and no evidence of development of a movement disorder was detected. Adverse effects were not significantly different between the quetiapine and placebo treatment arms.

CONCLUSIONS

The use of quetiapine to treat CUD was associated with an increased likelihood of heavy frequency use transitioning to moderate use, but not light use. The clinical significance of reductions in cannabis use, short of abstinence warrants further study.

Ziprasidone, a second-generation antipsychotic, affects core clock gene mRNA expression in mice

Some psychiatric diseases are associated with disruptions in the circadian clock system. Ziprasidone (ZIP), a second-generation antipsychotic, is widely used for psychiatry-related pharmacotherapy but its mechanism has not been clearly elucidated. We measured clock gene fluctuation patterns in the hippocampus and the amygdala in ZIP-treated mice. ZIP significantly increased Per1, Per2, and Bmal1 mRNA 2 h after the lights were turned off (ZT14) in the hippocampus, but not in the amygdala. These results suggest that ZIP might affect clock gene regulation, which could represent the pathway underlying symptom amelioration.

A comparison of continuous subcutaneous paliperidone infusion and repeated subcutaneous injection of risperidone free-base in rats

It is proposed that to achieve a therapeutic effect in schizophrenia patients, dopamine D2-receptor occupancy by antipsychotics within the striatum must exceed 60−65%. However, at high levels of D2-receptor occupancy, the risk of extrapyramidal symptoms (EPS) is increased. Following oral dosing of antipsychotics, peaks and troughs in plasma drug concentrations may be mirrored by fluctuations in D2-receptor occupancy. Paliperidone, a novel antipsychotic available as extended-release tablets (paliperidone ER), is the major active metabolite of risperidone and exhibits a plasma pharmacokinetic profile with reduced peak−trough fluctuations and consistent D2-receptor occupancy compared with conventional oral antipsychotic formulations. Using formulations that resemble those in clinical practice, this study provides a preclinical evaluation of the pharmacological properties of paliperidone ER and risperidone immediate-release formulation in terms of consistent antipsychotic efficacy over time and extrapyramidal symptom liability. Significant fluctuations in inhibition of d-amphetamine-induced hyperlocomotion were observed for repeated subcutaneous (SC) risperidone injections, whereas stable inhibitory efficacy was demonstrated during continuous SC paliperidone infusion. Similarly, significant fluctuations in latency on-bar were observed with repeated SC risperidone injections, whereas significantly lower latency on-bar was demonstrated following continuous SC paliperidone infusion. These results in an animal model suggest that although risperidone and paliperidone demonstrate similar pharmacologic effects, continuous administration of paliperidone achieves more stable antipsychotic efficacy with reduced motor impairment, akin to the effects observed with paliperidone ER in clinical studies.

The Role of Dynorphin and the Kappa Opioid Receptor in the Symptomatology of Schizophrenia: A Review of the Evidence

Schizophrenia is a debilitating mental illness that affects approximately 1% of the world's population. Despite much research in its neurobiology to aid in developing new treatments, little progress has been made. One system that has not received adequate attention is the kappa opioid system and its potential role in the emergence of symptoms, as well as its therapeutic potential. Here we present an overview of the kappa system and review various lines of evidence derived from clinical studies for dynorphin and kappa opioid receptor involvement in the pathology of both the positive and negative symptoms of schizophrenia. This overview includes evidence for the psychotomimetic effects of kappa opioid receptor agonists in healthy volunteers and their reversal by the pan-opioid antagonists naloxone and naltrexone and evidence for a therapeutic benefit in schizophrenia for 4 pan-opioid antagonists. We describe the interactions between kappa opioid receptors and the dopaminergic pathways that are disrupted in schizophrenia and the histologic evidence suggesting abnormal kappa opioid receptor signaling in schizophrenia. We conclude by discussing future directions.

Delayed complications after severe clozapine intoxication: a case report. The pharmacokinetic profile of clozapine and it's important role in the course of symptoms

Clozapine intoxications have a varied clinical presentation and may have severe complications. Management and treatment guidelines rarely highlight the risks of delayed clinical presentations. We present the case of a 50-year-old man showing severe complications 15 hours after intoxication with 4200 mg clozapine. Treatment consisted of strict monitoring, including vital support and regular clozapine blood levels. Clinical presentations may be delayed up to 5 days after intoxication, for which strict monitoring of clinical symptoms and vital functions during this period is of major importance. We discuss the clinical course of clozapine intoxication, the value of sampling clozapine blood levels and provide an overview of the current treatment guidelines, which we suggest to update to include the management of delayed complications.

Hypofunctional Dopamine Uptake and Antipsychotic Treatment-Resistant Schizophrenia

Antipsychotic treatment resistance in schizophrenia remains a major issue in psychiatry. Nearly 30% of patients with schizophrenia do not respond to antipsychotic treatment, yet the underlying neurobiological causes are unknown. All effective antipsychotic medications are thought to achieve their efficacy by targeting the dopaminergic system. Here we review early literature describing the fundamental mechanisms of antipsychotic drug efficacy, highlighting mechanistic concepts that have persisted over time. We then reconsider the original framework for understanding antipsychotic efficacy in light of recent advances in our scientific understanding of the dopaminergic effects of antipsychotics. Based on these new insights, we describe a role for the dopamine transporter in the genesis of both antipsychotic therapeutic response and primary resistance. We believe that this discussion will help delineate the dopaminergic nature of antipsychotic treatment-resistant schizophrenia.

The relationship between excitement symptom severity and extrastriatal dopamine D2/3 receptor availability in patients with schizophrenia: a high-resolution PET study with [18F]fallypride

The purpose of this study was to investigate the relationship between specific symptom severity and D_2/3 receptor availability in extrastriatal regions in outpatients with schizophrenia to shed light on the role of extrastriatal dopaminergic neurotransmission in the pathophysiology of symptoms of schizophrenia. Sixteen schizophrenia patients receiving relatively low-dose maintenance atypical antipsychotics and seventeen healthy controls underwent 3-Tesla magnetic resonance imaging and high-resolution positron emission tomography with [¹⁸F]fallypride. For D_2/3 receptor availability, the binding potential with respect to non-displaceable compartment (BP_ND) was derived using the simplified reference tissue model. The BP_ND values were lower in patients on antipsychotic treatment than in controls across all regions with large effect sizes (1.03-1.42). The regions with the largest effect size were the substantia nigra, amygdala, and insula. Symptoms of schizophrenia were assessed using a five-factor model of the Positive and Negative Syndrome Scale (PANSS). The region of interest-based analysis showed that PANSS excitement factor score had a significant positive correlation with the [¹⁸F]fallypride BP_ND in the insula. The equivalent dose of antipsychotics was not significantly correlated with PANSS factor scores or regional BP_ND values. The voxel-based analysis also revealed a significant positive association between the PANSS excitement factor and the [¹⁸F]fallypride BP_ND in the insula. The present study revealed a significant association between excitement symptom severity and D_2/3 receptor availability in the insula in schizophrenia, suggesting a possible important role of D_2/3 receptor-mediated neurotransmission in the insula and related limbic system in the pathophysiology of this specific symptom cluster.

Therapeutic D2/3 receptor occupancies and response with low amisulpride blood concentrations in very late-onset schizophrenia-like psychosis (VLOSLP)

OBJECTIVE

Antipsychotic drug sensitivity in very late-onset schizophrenia-like psychosis (VLOSLP) is well documented, but poorly understood. This study aimed to investigate blood drug concentration, D2/3 receptor occupancy and outcome in VLOSLP during open amisulpride prescribing, and compare this with Alzheimer's disease (AD).

METHODS

Blood drug concentration, prolactin, symptoms and extrapyramidal side-effects (EPS) were serially assessed during dose titration. [¹⁸ F]fallypride imaging was used to quantify D2/3 receptor occupancy. Average steady-state amisulpride concentration (Caverage, ng/ml) was estimated by incorporating pharmacokinetic (PK) data into an existing population PK model (25 AD participants, 20 healthy older people).

RESULTS

Eight patients (target 20) were recruited (six women; 76 + - 6 years; six treatment compliant; five serially sampled; three with paired imaging data). Mean + - SD symptom reduction was 74 ± 12% (50-100 mg/day; 92.5 + -39.4 ng/ml). Mild EPS emerged at 96 ng/ml (in AD, severe EPS, 50 mg/day, 60 ng/ml). In three participants, imaged during optimal treatment (50 mg/day; 41-70 ng/ml), caudate occupancy was 44-59% (58-74% in AD across a comparable Caverage).

CONCLUSIONS

Despite the small sample size, our findings are highly relevant as they suggest that, as in AD, 50 mg/day amisulpride is associated with >40% occupancy and clinically relevant responses in VLOSLP. It was not possible to fully characterise concentration-occupancy relationships in VLOSLP, and it is thus unclear whether the greater susceptibility of those with AD to emergent EPS was accounted for by increased central drug access. Further investigation of age- and diagnosis-specific threshold sensitivities is warranted, to guide amisulpride prescribing in older people, and therapeutic drug monitoring studies offer a potentially informative future approach. Copyright © 2017 John Wiley & Sons, Ltd.

Psychosis in Parkinson's Disease: Epidemiology, Pathophysiology, and Management

Psychotic symptoms are common in Parkinson's disease (PD) and are associated with poorer quality of life and increased caregiver burden. PD psychosis is correlated with several factors, such as more advanced disease, cognitive impairment, depression, and sleep disorders. The underlying causes of psychosis in PD thus involve a complex interplay between exogenous (e.g., drugs, intercurrent illnesses) and endogenous (e.g., PD disease pathology) factors. Current theories of the pathophysiology of PD psychosis have come from several neuropathological and neuroimaging studies that implicate pathways involving visual processing and executive function, including temporo-limbic structures and neocortical gray matter with altered neurotransmitter functioning (e.g., dopamine, serotonin, and acetylcholine). Treatment of PD psychosis requires a step-wise process, including initial careful investigation of treatable triggering conditions and a comprehensive evaluation with adjustment of PD medications and/or initiation of specific antipsychotic therapies. Clozapine remains the only recommended drug for the treatment of PD psychosis; however, because of regular blood monitoring, quetiapine is usually first-line therapy, although less efficacious. Emerging studies have focused on agents involving other neurotransmitters, including the serotonin 5-HT2A receptor inverse agonist pimavanserin, cholinesterase inhibitors, and antidepressants and anxiolytics.

Therapeutic window of dopamine D2/3 receptor occupancy to treat psychosis in Alzheimer's disease

See Caravaggio and Graff-Guerrero (doi:10.1093/awx023) for a scientific commentary on this article.Antipsychotic drugs, originally developed to treat schizophrenia, are used to treat psychosis, agitation and aggression in Alzheimer's disease. In the absence of dopamine D2/3 receptor occupancy data to inform antipsychotic prescribing for psychosis in Alzheimer's disease, the mechanisms underpinning antipsychotic efficacy and side effects are poorly understood. This study used a population approach to investigate the relationship between amisulpride blood concentration and central D2/3 occupancy in older people with Alzheimer's disease by combining: (i) pharmacokinetic data (280 venous samples) from a phase I single (50 mg) dose study in healthy older people (n = 20, 65-79 years); (ii) pharmacokinetic, 18F-fallypride D2/3 receptor imaging and clinical outcome data on patients with Alzheimer's disease who were prescribed amisulpride (25-75 mg daily) to treat psychosis as part of an open study (n = 28; 69-92 years; 41 blood samples, five pretreatment scans, 19 post-treatment scans); and (iii) 18F-fallypride imaging of an antipsychotic free Alzheimer's disease control group (n = 10, 78-92 years), to provide additional pretreatment data. Non-linear mixed effects modelling was used to describe pharmacokinetic-occupancy curves in caudate, putamen and thalamus. Model outputs were used to estimate threshold steady state blood concentration and occupancy required to elicit a clinically relevant response (>25% reduction in scores on delusions, hallucinations and agitation domains of the Neuropsychiatric Inventory) and extrapyramidal side effects (Simpson Angus Scale scores > 3). Average steady state blood levels were low (71 ± 30 ng/ml), and associated with high D2/3 occupancies (65 ± 8%, caudate; 67 ± 11%, thalamus; 52 ± 11%, putamen). Antipsychotic clinical response occurred at a threshold concentration of 20 ng/ml and D2/3 occupancies of 43% (caudate), 25% (putamen), 43% (thalamus). Extrapyramidal side effects (n = 7) emerged at a threshold concentration of 60 ng/ml, and D2/3 occupancies of 61% (caudate), 49% (putamen) and 69% (thalamus). This study has established that, as in schizophrenia, there is a therapeutic window of D2/3 receptor occupancy for optimal treatment of psychosis in Alzheimer's disease. We have also shown that occupancies within and beyond this window are achieved at very low amisulpride doses in Alzheimer's disease due to higher than anticipated occupancies for a given blood drug concentration. Our findings support a central pharmacokinetic contribution to antipsychotic sensitivity in Alzheimer's disease and implicate the blood-brain barrier, which controls central drug access. Whether high D2/3 receptor occupancies are primarily accounted for by age- or disease-specific blood-brain barrier disruption is unclear, and this is an important future area of future investigation, as it has implications beyond antipsychotic prescribing.

Short-term Effects of Risperidone Monotherapy on Spontaneous Brain Activity in First-episode Treatment-naïve Schizophrenia Patients: A Longitudinal fMRI Study

It is unclear whether abnormal spontaneous neural activation patterns found in chronic schizophrenia patients (CSP) are part of the pathogenesis of disease, consequences of chronic illness, or effects of antipsychotic treatment. We performed a longitudinal resting-state functional magnetic resonance imaging (fMRI) study in 42 treatment-naïve first-episode schizophrenia patients (FESP) at baseline and then after 8-weeks of risperidone monotherapy, and compared the findings to 38 healthy volunteers. Spontaneous brain activity was quantified using the fractional amplitude of low-frequency fluctuations (fALFF) and regional homogeneity (ReHo) and compared between patients and controls. Pretreatment, patients exhibited higher fALFF in left caudate compared with controls. After treatment, patients had elevated fALFF in bilateral putamen and right caudate, and increased ReHo in right caudate and left putamen. Greater increase of fALFF in the left putamen correlated with less improvement in positive symptoms. Thus, abnormalities of spontaneous neural activity in chronic schizophrenia is at least partly due to a medication effect. The observed post-treatment increase in striatal intrinsic activity may reflect counter-therapeutic functional adaptation to dopamine D2 receptor occupancy required for medication effects on psychosis.

Brain Functional Effects of Psychopharmacological Treatments in Schizophrenia: A Network-based Functional Perspective Beyond Neurotransmitter Systems

Psychopharmacological treatments for schizophrenia have always been a matter of debate and a very important issue in public health given the chronic, relapsing and disabling nature of the disorder. A thorough understanding of the pros and cons of currently available pharmacological treatments for schizophrenia is critical to better capture the features of treatment-refractory clinical pictures and plan the developing of new treatment strategies. This review focuses on brain functional changes induced by antipsychotic drugs as assessed by modern functional neuroimaging techniques (i.e. fMRI, PET, SPECT, MRI spectroscopy). The most important papers on this topic are reviewed in order to draw an ideal map of the main functional changes occurring in the brain during antipsychotic treatment. This supports the hypothesis that a network-based perspective and a functional connectivity approach are needed to fill the currently existing gap of knowledge in the field of psychotropic drugs and their mechanisms of action beyond neurotransmitter systems.

First- and second-generation antipsychotic drug treatment and subcortical brain morphology in schizophrenia

Antipsychotic medication may influence brain structure, but to what extent effects of first-generation antipsychotics (FGAs) and second-generation antipsychotics (SGAs) differ is still not clear. Here we aimed to disentangle the effects of FGA and SGA on variation in volumes of subcortical structures in patients with long-term treated schizophrenia. Magnetic resonance images were obtained from 95 patients with schizophrenia and 106 healthy control subjects. Among the patients, 40 received only FGA and 42 received only SGA. FreeSurfer 5.3.0 was used to obtain volumes of 27 subcortical structures as well as total brain volume and estimated intracranial volume. Findings of reduced total brain volume, enlarged ventricular volume and reduced hippocampal volume bilaterally among patients were replicated, largely independent of medication class. In the basal ganglia, FGA users had larger putamen bilaterally and right caudate volume compared to healthy controls, and the right putamen was significantly larger than among SGA users. FGA and SGA users had similar and larger globus pallidus volumes compared to healthy controls. Post hoc analyses revealed that the difference between FGA and SGA could be attributed to smaller volumes in the clozapine users specifically. We therefore conclude that basal ganglia volume enlargements are not specific to FGA.

Baseline Perfusion Alterations Due to Acute Application of Quetiapine and Pramipexole in Healthy Adults

BACKGROUND

The dopaminergic system is implicated in many mental processes and neuropsychiatric disorders. Pharmacologically, drugs with dopamine receptor antagonistic and agonistic effects are used, but their effects on functional brain metabolism are not well known.

METHODS

In this randomized crossover, placebo-controlled, and rater-blinded study, 25 healthy adults received an acute dose placebo substance (starch), quetiapine (dopamine receptor antagonist), or pramipexole (dopamine agonist of the nonergoline class) 1 hour before the experiment. Background-suppressed 2D pseudo-continuous arterial spin labeling was used to examine whole-brain baseline cerebral blood flow differences induced by the 3 substances.

RESULTS

We found that quetiapine reduced perfusion in the occipital (early visual areas) and bilateral cerebellar cortex relative to placebo. In contrast, quetiapine enhanced cerebral blood flow (relative to placebo) in the striatal system (putamen and caudate nucleus) but also in the supplementary motor area, insular-, prefrontal- as well as in the pre- and postcentral cortex. Pramipexole increased cerebral blood flow compared with placebo in the caudate nucleus, putamen, middle frontal, supplementary motor area, and brainstem (substantia nigra), but reduced cerebral blood flow in the posterior thalamus, cerebellum, and visual areas. Pramipexole administration resulted in stronger cerebral blood flow relative to quetiapine in the hypothalamus, cerebellum, and substantia nigra.

CONCLUSIONS

Our results indicate that quetiapine and pramipexole differentially modulate regional baseline cerebral blood flow. Both substances act on the dopaminergic system, although they affect distinct regions. Quetiapine altered dopaminergic function in frontal, striatal, and motor regions. In contrast, pramipexole affected cerebral blood flow of the nigrostriatal (striatum and substantia nigra) dopaminergic, but less the fronto-insular system.

Dopamine Targeting Drugs for the Treatment of Schizophrenia: Past, Present and Future

Schizophrenia is a chronic and debilitating neuropsychiatric disorder affecting approximately 1% of the world's population. This disease is associated with considerable morbidity placing a major financial burden on society. Antipsychotics have been the mainstay of the pharmacological treatment of schizophrenia for decades. The traditional typical and atypical antipsychotics demonstrate clinical efficacy in treating positive symptoms, such as hallucinations and delusions, while are largely ineffective and may worsen negative symptoms, such as blunted affect and social withdrawal, as well as cognitive function. The inability to treat these latter symptoms may contribute to social function impairment associated with schizophrenia. The dysfunction of multiple neurotransmitter systems in schizophrenia suggests that drugs selectively targeting one neurotransmission pathway are unlikely to meet all the therapeutic needs of this heterogeneous disorder. Often, however, the unintentional engagement of multiple pharmacological targets or even the excessive engagement of intended pharmacological targets can lead to undesired consequences and poor tolerability. In this article, we will review marketed typical and atypical antipsychotics and new therapeutic agents targeting dopamine receptors and other neurotransmitters for the treatment of schizophrenia. Representative typical and atypical antipsychotic drugs and new investigational drug candidates will be systematically reviewed and compared by reviewing structure-activity relationships, pharmacokinetic properties, drug metabolism and safety, pharmacological properties, preclinical data in animal models, clinical outcomes and associated side effects.

Histamine H1 receptor occupancy by the new-generation antipsychotics olanzapine and quetiapine: a positron emission tomography study in healthy volunteers

RATIONALE

Histamine H1 antagonists have hypnotic, appetite-promoting, and sedative side effects. Most second-generation antipsychotics have potent antagonistic effects on histamine H1 receptor (H1R). Positron emission tomography (PET) can measure the H1R occupancy (H1RO) in vivo, although there are no reports regarding antipsychotics.

OBJECTIVES

We studied the H1RO of olanzapine and quetiapine in vivo with respect to their plasma concentrations and subjective drowsiness by performing human PET imaging studies with [(11)C]doxepin, a potent PET ligand of H1R.

METHODS

Six healthy Japanese male volunteers were enrolled. Cross-randomized PET imaging was performed after a single oral administration of olanzapine (2.5 mg), quetiapine (25 mg), or placebo. PET data were analyzed by region of interest and voxel-by-voxel analysis. We concurrently measured plasma drug concentrations by liquid chromatography/tandem mass spectrometry and evaluated subjective sleepiness.

RESULTS

The binding potential ratios of olanzapine and quetiapine in the cerebral cortex were significantly lower than that of the placebo. The H1RO values of olanzapine and quetiapine in the cortex were approximately 61-80 and 56-81%, respectively. The binding potential ratios of the drugs were significantly lower than that of the placebo in the dorsolateral prefrontal and lateral temporal cortices, and anterior and posterior cingulate gyri. The H1RO values in the cortex were significantly correlated with subjective sleepiness but not plasma drug concentrations.

CONCLUSIONS

Olanzapine and quetiapine have high H1RO values in the human brain under their clinical minimum doses. This study provides a foundation of the properties by which new-generation antipsychotics block the central histaminergic system in humans.

ITI-007 demonstrates brain occupancy at serotonin 5-HT₂A and dopamine D₂ receptors and serotonin transporters using positron emission tomography in healthy volunteers

RATIONALE

Central modulation of serotonin and dopamine underlies efficacy for a variety of psychiatric therapeutics. ITI-007 is an investigational new drug in development for treatment of schizophrenia, mood disorders, and other neuropsychiatric disorders.

OBJECTIVES

The purpose of this study was to determine brain occupancy of ITI-007 at serotonin 5-HT2A receptors, dopamine D2 receptors, and serotonin transporters using positron emission tomography (PET) in 16 healthy volunteers.

METHODS

Carbon-11-MDL100907, carbon-11-raclopride, and carbon-11-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile) (carbon-11-DASB) were used as the radiotracers for imaging 5-HT2A receptors, D2 receptors, and serotonin transporters, respectively. Brain regions of interest were outlined using magnetic resonance tomography (MRT) with cerebellum as the reference region. Binding potentials were estimated by fitting a simplified reference tissue model to the measured tissue-time activity curves. Target occupancy was expressed as percent change in the binding potentials before and after ITI-007 administration.

RESULTS

Oral ITI-007 (10-40 mg) was safe and well tolerated. ITI-007 rapidly entered the brain with long-lasting and dose-related occupancy. ITI-007 (10 mg) demonstrated high occupancy (>80 %) of cortical 5-HT2A receptors and low occupancy of striatal D2 receptors (~12 %). D2 receptor occupancy increased with dose and significantly correlated with plasma concentrations (r (2) = 0.68, p = 0.002). ITI-007 (40 mg) resulted in peak occupancy up to 39 % of striatal D2 receptors and 33 % of striatal serotonin transporters.

CONCLUSIONS

The results provide evidence for a central mechanism of action via dopaminergic and serotonergic pathways for ITI-007 in living human brain and valuable information to aid dose selection for future clinical trials.

Imaging dopaminergic dysfunction as a surrogate marker of neuropathology in a small-animal model of HIV

The dopaminergic system is especially vulnerable to the effects of human immunodeficiency virus (HIV) infection, rendering dopaminergic deficits early surrogate markers of HIV-associated neuropathology. We quantified dopamine D2/3 receptors in young HIV-1 transgenic (Tg) (n  =  6) and age-matched control rats (n  =  7) and adult Tg (n  =  5) and age-matched control rats (n  =  5) using [18F]fallypride positron emission tomography (PET). Regional uptake was quantified as binding potential (BPND) using the two-tissue reference model with the cerebellum as the reference. Time-activity curves were generated for the ventral striatum, dorsal striatum, thalamus, and cerebellum. Whereas BPND values were significantly lower in the ventral striatum (p < .001) and dorsal striatum (p  =  .001) in the adult Tg rats compared to controls rats, they were significantly lower only in the dorsal striatum (p < .05) in the young rats. Tg rats had smaller striatal volumes on magnetic resonance imaging. We also found lower expression levels of tyrosine hydroxylase on immunohistochemistry in the Tg animals. Our findings suggest that progressive striatal D2/3 receptor deficits occur in Tg rats as they age and can be detected using small-animal PET imaging. The effectiveness of various approaches in preventing or halting this dopaminergic loss in the Tg rat can thus be measured preclinically using [18F]fallypride PET as a molecular imaging biomarker of HIV-associated neuropathology.

Open-label pilot study of quetiapine treatment for cannabis dependence

BACKGROUND

There are no efficacious pharmacotherapies for cannabis dependence. The effects of quetiapine are well matched to the symptoms of cannabis withdrawal and could be useful in the treatment of cannabis dependence.

OBJECTIVES

To evaluate quetiapine for the treatment of cannabis dependence and determine the optimal dosing.

METHODS

In an eight-week open-label outpatient pilot trial, we evaluated the feasibility of quetiapine treatment for cannabis dependence in 15 outpatients. Quetiapine was gradually titrated to 600 mg or the maximum tolerated dose.

RESULTS

The mean study retention was 6.5 weeks (±2.3), with 67% of participants completing all eight weeks of the trial. The mean maximum dose achieved was 197 mg/day (range: 25-600 mg/day). Only two of the 15 participants were able to achieve the target dose of 600 mg daily. There were no serious adverse events and no participants were discontinued from the trial due to adverse effects. The most common reported adverse effects were fatigue (80% of participants) and somnolence (47%). From baseline to week 8, the modeled overall decrease in daily dollar value of marijuana was 76.3% (CI: 63.4%, 84.7%). Over the eight weeks of the study, there was a 46.9% (CI: 11%, 68.3%) decrease in urine tetrahydrocannabinol-9-carboxylic acid (THCOOH) levels.

CONCLUSIONS

These preliminary results are promising in that quetiapine treatment was tolerated by cannabis-dependent patients and associated with decreased cannabis use. The recommended maximum target dose for cannabis-dependent patients is 300 mg daily. These preliminary data support further evaluation of quetiapine as a treatment for cannabis dependence.

Neocortical serotonin2A receptor binding predicts quetiapine associated weight gain in antipsychotic-naive first-episode schizophrenia patients

Antipsychotic-induced weight gain is of major clinical importance since it is associated with severe metabolic complications and increased mortality. The serotonin2A receptor system has been suggested to be implicated in weight gain and obesity. However, no previous in vivo imaging data have related serotonin2A receptor binding to weight gain before and after antipsychotic monotherapy. Fifteen antipsychotic-naive first-episode schizophrenia patients were included and investigated before and after six months of quetiapine treatment. We examined the relationship between serotonin2A receptor binding as measured with positron emission tomography (PET) and [18F]altanserin and change in body mass index (BMI). Quetiapine was chosen because it is characterized by a moderately high affinity for the serotonin2A receptor and a fast dissociation rate from the dopamine D2 receptor. At baseline the mean BMI was 24.2 kg/m2, range 18-36 kg/m2. After six months of quetiapine treatment (mean dose: 383 mg/day) the BMI had, on average, increased by 6.7%, corresponding to an average weight gain of 5.0 kg. We found a significant positive correlation both between neocortical serotonin2A receptor binding prior to treatment and subsequent increase in BMI (rho=0.59, p=0.022). At follow-up, the serotonin2A receptor occupancy was positively correlated with BMI increase (rho=0.54, p=0.038). To our knowledge, these are the first in vivo receptor imaging data in initially antipsychotic-naive first-episode schizophrenia patients to show that the cerebral serotonin2A receptor is associated with antipsychotic-induced weight gain.

Comparison of the biological effects of (18)F at different intracellular levels

We herein examined the biological effects of cells treated with (18)F labeled drugs for positron emission tomography (PET). The relationship between the intracellular distribution of (18)F and levels of damaged DNA has yet to be clarified in detail. We used culture cells (Chinese Hamster Ovary cells) treated with two types of (18)F labeled drugs, fluorodeoxyglucose (FDG) and fluorine ion (HF). FDG efficiently accumulated in cells, whereas HF did not. To examine the induction of DNA double strand breaks (DSB), we measured the number of foci for 53BP1 that formed at the site of DNA DSB. The results revealed that although radioactivity levels were the same, the induction of 53BP1 foci was stronger in cells treated with (18)F-FDG than in those treated with (18)F-HF. The clonogenic survival of cells was significantly lower with (18)F-FDG than with (18)F-HF. We concluded that the efficient accumulation of (18)F in cells led to stronger biological effects due to more severe cellular lethality via the induction of DNA DSB.

Towards a therapeutic window of D2/3 occupancy for treatment of psychosis in Alzheimer's disease, with [18F]fallypride positron emission tomography

OBJECTIVE

Dopamine D2/3 receptor positron emission tomography tracers have guided antipsychotic prescribing in young people with schizophrenia by establishing a 'therapeutic window' of striatal D2/3 receptor occupancy. Older people, particularly those with dementia, are highly susceptible to motor side effects and may benefit from the appropriate application of imaging techniques. The study aimed to adapt [18F]fallypride imaging for use in occupancy studies in Alzheimer's disease (AD) and to investigate whether data acquisition could be made more tolerable by piloting the protocol in a small sample.

METHODS

Six participants with AD (three men; 85.0 ± 5.6 years old; MMSE = 16.0 ± 2.4) were recruited prior to commencing amisulpride for the treatment of psychosis and associated agitation. [18F]fallypride binding potential (BPND ) was determined using an interrupted scanning protocol at baseline (n = 6) and after 27.0 ± 6.1 days of amisulpride (25-50 mg) treatment (n = 4). D2/3 occupancy was calculated by percentage reduction in BPND between scanning sessions. Image data were re-analysed after reducing individual sampling times to 20 min.

RESULTS

The protocol was tolerated well, apart from the final (40 min) session of the post-treatment scan in one participant. Higher occupancies were achieved in the striatum (caudate 47-70%, putamen 31-58%) and thalamus (54-76%) than in the inferior temporal gyrus (27-43%). There was high agreement between occupancy values derived using longer and shorter sampling times (mean absolute difference 6.1% in the inferior temporal gyrus; <2% all other regions).

CONCLUSIONS

The protocol is feasible for use in AD and represents the first step towards establishing dose-occupancy relationships across older clinical populations.

Effect of quetiapine on Per1, Per2, and Bmal1 clock gene expression in the mouse amygdala and hippocampus

Circadian rhythms are related to various psychiatric disorders. Recently, antipsychotics, including quetiapine (QTP), have been accepted as potential therapeutic agents for the treatment of depression, but its mechanism remains poorly understood. In this study, we examined clock gene fluctuation patterns in QTP-treated mice. QTP significantly increased Per2 mRNA at ZT12 and Per1 and Per2 expression at ZT18 in the amygdala. There were significant differences between the control and QTP groups in the cross-time effects of Per2 mRNA expression in the amygdala. Our findings suggest that QTP possibly acts on the circadian system, which then induces changes in mood symptoms.

Establishing test-retest reliability of an adapted [(18)F]fallypride imaging protocol in older people

[(18)F]fallypride is a high-affinity dopamine D2/3 receptor tracer with the ability to reliably quantify D2/3 receptor sites in both striatal and corticolimbic regions. The translational potential of [(18)F]fallypride imaging is, however, limited by the lengthy scanning sessions (60-80 minutes duration over a total of 3-4 hours) required by current protocols. The aims of our study were to adapt [(18)F]fallypride imaging for use in clinical populations with neurological and neuropsychiatric disorders, by reducing the duration of individual scanning sessions; and to establish the reproducibility and reliability of our adapted protocol in healthy older people. Eight participants (five male and three female; mean age=75.87±4.39 years) were scanned twice, 4-6 weeks apart. [(18)F]fallypride binding potential was determined from image data collected during three sampling times: 0-30; 60-90; and 210-240 minutes post injection. High reproducibility and reliability (test-retest variability <8%; intraclass correlation coefficient >0.8) were observed in all but the prefrontal regions, and remained so when sampling times were reduced to 20 minutes (0-20; 70-90; 220-240 minutes). The adapted protocol is feasible for use across neuropsychiatric disorders in which dopamine has been implicated and is sufficiently sensitive to detect within-subject changes between 2.7% and 5.5% in striatal and limbic regions.

Role of concomitant inhibition of the norepinephrine transporter for the antipsychotic effect of quetiapine

Quetiapine alleviates both positive and negative symptoms as well as certain cognitive impairments in schizophrenia despite a low D2 receptor occupancy and may also be used as monotherapy in bipolar and major depressive disorder. The mechanisms underlying the broad clinical utility of quetiapine remain to be clarified, but may be related to the potent inhibition of the norepinephrine transporter (NET) by norquetiapine, the major metabolite of quetiapine in humans. Since norquetiapine is not formed in rodents we here investigated in rats whether NET-inhibition may, in principle, contribute to the clinical effectiveness of quetiapine and allow for its low D2 receptor occupancy, by combining quetiapine with the selective NET-inhibitor reboxetine. Antipsychotic-like activity was assessed using the conditioned avoidance response (CAR) test, dopamine output in the medial prefrontal cortex (mPFC) and the nucleus accumbens was measured using in vivo microdialysis, and NMDA receptor-mediated transmission was measured using intracellular electrophysiological recordings in pyramidal cells of the mPFC in vitro. Adjunct reboxetine potentiated the suppression of CAR by quetiapine. Moreover, concomitant administration of quetiapine and reboxetine resulted in a synergistic increase in cortical, but not accumbal, dopamine output. The combination of low, clinically relevant concentrations of quetiapine (60 nM) and reboxetine (20 nM) markedly facilitated cortical NMDA receptor-mediated transmission in contrast to either drug alone, an effect that could be inhibited by the D₁ receptor antagonist SCH23390. We conclude that concomitant NET-inhibition by norquetiapine may contribute to the overall antipsychotic effectiveness of quetiapine in spite of its relatively low level of D₂ occupancy.

Striatal and extrastriatal dopamine D2 receptor occupancy by a novel antipsychotic, blonanserin: a PET study with [11C]raclopride and [11C]FLB 457 in schizophrenia

Blonanserin is a novel antipsychotic with high affinities for dopamine D(2) and 5-HT(2A) receptors, and it was recently approved for the treatment of schizophrenia in Japan and Korea. Although double-blind clinical trials have demonstrated that blonanserin has equal efficacy to risperidone, and with a better profile especially with respect to prolactin elevation, its profile of in vivo receptor binding has not been investigated in patients with schizophrenia. Using positron emission tomography (PET), we measured striatal and extrastriatal dopamine D(2) receptor occupancy by blonanserin in 15 patients with schizophrenia treated with fixed doses of blonanserin (ie, 8, 16, and 24 mg/d) for at least 4 weeks before PET scans, and in 15 healthy volunteers. Two PET scans, 1 with [(11)C]raclopride for the striatum and 1 with [(11)C]FLB 457 for the temporal cortex and pituitary, were performed on the same day. Striatal dopamine D(2) receptor occupancy by blonanserin was 60.8% (3.0%) [mean (SD)] at 8 mg, 73.4% (4.9%) at 16 mg, and 79.7% (2.3%) at 24 mg. The brain/plasma concentration ratio calculated from D(2) receptor occupancy in the temporal cortex and pituitary was 3.38, indicating good blood-brain barrier permeability. This was the first study to show clinical daily dose amounts of blonanserin occupying dopamine D(2) receptors in patients with schizophrenia. The clinical implications obtained in this study were the optimal therapeutic dose range of 12.9 to 22.1 mg/d of blonanserin required for 70% to 80% dopamine D(2) receptor occupancy in the striatum, and the good blood-brain barrier permeability that suggested a relatively lower risk of hyperprolactinemia.

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.

In vivo quantification of striatal dopamine D2 receptor occupancy by JNJ-37822681 using [11C]raclopride and positron emission tomography

JNJ-37822681 is a novel, fast-dissociating dopamine D(2) receptor antagonist, currently in development as an antipsychotic drug candidate. A previous first-in-human study demonstrated mild central nervous system effects of JNJ-37822681 in healthy male volunteers. Significant but transient serum prolactin elevations were demonstrated, whereas other neurophysiological effects were relatively small. To investigate striatal dopamine D(2) receptor occupancy by variable single doses of JNJ-37822681, an open-label [(11)C]raclopride positron emission tomography study was performed in 12 healthy male volunteers, using the simplified reference tissue model with cerebellum as reference tissue. Oral administration of JNJ-37822681 resulted in dose-dependent dopamine D(2) receptor occupancy. Receptor occupancy increased from 9-19% at 2 mg doses to 60-74% at 20 mg doses of JNJ-37822681. Therefore, single oral doses of JNJ-37822681 can produce occupancy levels that are generally associated with clinical efficacy for registered antipsychotic drugs.

Striatal and extrastriatal dopamine D₂ receptor occupancy by the partial agonist antipsychotic drug aripiprazole in the human brain: a positron emission tomography study with [¹¹C]raclopride and [¹¹C]FLB457

RATIONALE

Second-generation antipsychotics demonstrate clinical efficacy with fewer extrapyramidal side effects compared with first-generation antipsychotics. One of the proposed explanations is the hypothesis of preferential extrastriatal dopamine D₂ receptor occupancy (limbic selectivity) by antipsychotics. In the present study, we focused on aripiprazole, which has a unique pharmacological profile with partial agonism at dopamine D₂ receptors and the minimal risk of extrapyramidal side effects. Previous positron emission tomography (PET) studies using high-affinity radioligands for dopamine D₂ receptors have reported inconsistent results regarding regional differences of dopamine D₂ receptor occupancy by aripiprazole.

OBJECTIVE

To test the hypothesis of preferential binding to extrastriatal dopamine D₂ receptors by aripiprazole, we investigated its regional dopamine D₂ receptor occupancies in healthy young subjects.

MATERIALS AND METHODS

Using PET and two radioligands with different affinities for dopamine D₂ receptors, [¹¹C]raclopride and [¹¹C]FLB457, striatal and extrastriatal dopamine D₂ receptor bindings at baseline and after oral administration of 6 mg aripiprazole were measured in 11 male healthy subjects.

RESULTS

Our data showed that dopamine D₂ receptor occupancies in the striatum measured with [¹¹C]raclopride were 70.1% and 74.1%, with the corresponding values for the extrastriatal regions measured with [¹¹C]FLB457 ranging from 46.6% to 58.4%.

CONCLUSIONS

In the present study, preferential extrastriatal dopamine D₂ receptor occupancy by aripiprazole was not observed. Our data suggest partial agonism at dopamine D₂ receptors is the most likely explanation for the minimal risk of extrapyramidal side effects in the treatment by aripiprazole.

Quetiapine enhances oligodendrocyte regeneration and myelin repair after cuprizone-induced demyelination

Myelin and oligodendrocyte dysfunctions have been consistently found in patients with schizophrenia. The effect of antipsychotics on myelin disturbances is unknown. The present study examined the effects of quetiapine on oligodendrocyte regeneration and myelin repair in a demyelination animal model. C57BL/6 mice were fed with cuprizone (0.2% w/w) for 12 weeks to induce chronic demyelination and oligodendrocyte degeneration, after which cuprizone was withdrawn to allow recovery. Quetiapine (10mg/kg/day) or vehicle (water) was administrated orally to mice for 0, 2, 3, or 4 weeks after cuprizone withdrawal. Locomotor activity and Y-maze tests were used to evaluate behavioral changes in the mice. Immunohistochemical staining was used to detect morphological and biological changes in the brains. Cuprizone administration for 12 weeks resulted in severe demyelination, locomotor hyperactivity, and working memory impairment in mice. Remyelination occurred when cuprizone was withdrawn. Quetiapine treatment during the recovery period significantly improved the spatial working memory and increased myelin restoration. Quetiapine treatment also enhanced the repopulation of mature oligodendrocytes in the demyelinated lesions, which was associated with down-regulation of transcription factor olig2 in the process of cell maturation. The results of this study demonstrated that quetiapine treatment during the recovery period improves spatial working memory and promotes oligodendrocyte development and remyelination. This study supports the role of oligodendrocyte dysfunction in memory deficits in a schizophrenia mouse model and suggests that quetiapine may target oligodendrocytes and improve cognitive function.

Receptor targets for antidepressant therapy in bipolar disorder: an overview

The treatment of bipolar depression is one of the most challenging issues in contemporary psychiatry. Currently only quetiapine and the olanzapine-fluoxetine combination are officially approved by the FDA against this condition. The neurobiology of bipolar depression and the possible targets of bipolar antidepressant therapy remain relatively elusive. We performed a complete and systematic review to identify agents with definite positive or negative results concerning efficacy followed by a second systematic review to identify the pharmacodynamic properties of these agents. The comparison of properties suggests that the stronger predictors for antidepressant efficacy in bipolar depression were norepinephrine alpha-1, dopamine D1 and histamine antagonism, followed by 5-HT2A, muscarinic and dopamine D2 and D3 antagonism and eventually by norepinephrine reuptake inhibition and 5HT-1A agonism. Serotonin reuptake which constitutes the cornerstone in unipolar depression treatment does not seem to play a significant role for bipolar depression. Our exhaustive review is compatible with a complex model with multiple levels of interaction between the major neurotransmitter systems without a single target being either necessary or sufficient to elicit the antidepressant effect in bipolar depression.

Augmenting clozapine with sertindole: a double-blind, randomized, placebo-controlled study

Clozapine augmentation with antipsychotic drugs is widely used despite sparse evidence supporting this strategy. Sertindole is a nonsedating atypical antipsychotic drug with low affinity for cholinergic receptors, which makes it potentially suitable for augmentation of clozapine. The study design was a 12-week, double-blind, randomized, placebo-controlled study including patients with International Statistical Classification of Diseases, 10th Revision schizophrenia (F20.0-F20.3) and treated with clozapine for at least 6 months who had not achieved sufficient response. Patients were randomized 1:1 to either sertindole 16 mg or placebo, and assessment was done at baseline and after 6 and 12 weeks. Assessment included the Positive and Negative Syndrome Scale, Clinical Global Impression, Udvalg for Kliniske Undersøgelser, World Health Organization Quality of Life Brief, Drug Attitude Inventory, fasting glucose, lipids, and electrocardiogram. Clozapine augmentation with sertindole was not superior to placebo regarding total score or subscale score of the Positive and Negative Syndrome Scale, Clinical Global Impression, World Health Organization Quality of Life Brief, or Drug Attitude Inventory. No increased adverse effects compared with placebo were found. Four patients randomized to sertindole experienced a significant worsening of psychosis, and 2 of them required psychiatric admission. Metabolic parameters were unchanged during the study, but augmentation of clozapine with sertindole was associated with a 12-millisecond (SD, 20-millisecond) QTc prolongation compared with 0 millisecond (SD, 20 milliseconds) in the placebo group (P < 0.03). Augmentation with sertindole showed no benefits compared with placebo. Psychiatrists should be aware that augmentation might not add any benefits for the patients and in some cases worsen psychosis.

Clozapine, atypical antipsychotics, and the benefits of fast-off D2 dopamine receptor antagonism

Drug-receptor interactions are traditionally quantified in terms of affinity and efficacy, but there is increasing awareness that the drug-on-receptor residence time also affects clinical performance. While most interest has hitherto been focused on slow-dissociating drugs, D(2) dopamine receptor antagonists show less extrapyramidal side effects but still have excellent antipsychotic activity when they dissociate swiftly. Fast dissociation of clozapine, the prototype of the "atypical antipsychotics", has been evidenced by distinct radioligand binding approaches both on cell membranes and intact cells. The surmountable nature of clozapine in functional assays with fast-emerging responses like calcium transients is confirmatory. Potential advantages and pitfalls of the hitherto used techniques are discussed, and recommendations are given to obtain more precise dissociation rates for such drugs. Surmountable antagonism is necessary to allow sufficient D(2) receptor stimulation by endogenous dopamine in the striatum. Simulations are presented to find out whether this can be achieved during sub-second bursts in dopamine concentration or rather during much slower, activity-related increases thereof. While the antagonist's dissociation rate is important to distinguish between both mechanisms, this becomes much less so when contemplating time intervals between successive drug intakes, i.e., when pharmacokinetic considerations prevail. Attention is also drawn to the divergent residence times of hydrophobic antagonists like haloperidol when comparing radioligand binding data on cell membranes with those on intact cells and clinical data.

Relationship between dose, drug levels, and D2 receptor occupancy for the atypical antipsychotics risperidone and paliperidone

Blockade of D2 family dopamine receptors (D2Rs) is a fundamental property of antipsychotics, and the degree of striatal D2R occupancy has been related to antipsychotic and motor effects of these drugs. Recent studies suggest the D2R occupancy of antipsychotics may differ in extrastriatal regions compared with the dorsal striatum. We studied this issue in macaque monkeys by using a within-subjects design. [(18)F]fallypride positron emission tomography scans were obtained on four different doses of risperidone and paliperidone (the 9-OH metabolite of risperidone) and compared with multiple off-drug scans in each animal. The half-life of the two drugs in these monkeys was determined to be between 3 and 4 h, and drug was administered by a constant infusion through an intragastric catheter. The D2R occupancy of antipsychotic was determined in the caudate, putamen, ventral striatum, and four prefrontal and temporal cortical regions and was related to serum and cerebrospinal fluid drug levels. Repeated 2-week treatment with risperidone or paliperidone did not produce lasting changes in D2R binding potential in any region examined. As expected, D2R binding potential was highest in the caudate and putamen and was approximately one-third that level in the ventral striatum and 2% of that level in the cortical regions. We found dose-dependent D2R occupancy for both risperidone and paliperidone in both basal ganglia and cortical regions of interest. We could not find evidence of regional variation in D2R occupancy of either drug. Comparison of D2R occupancy and serum drug levels supports a target of 40 to 80 ng/ml active drug for these two atypical antipsychotics.

Role of dopamine D(2) receptors for antipsychotic activity

This review summarizes the current state of knowledge regarding the proposed mechanisms by which antipsychotic agents reduce the symptoms of schizophrenia while giving rise to adverse side effects. The first part summarizes the contribution of neuroimaging studies to our understanding of the neurochemical substrates of schizophrenia, putting emphasis on direct evidence suggestive of a presynaptic rather than a postsynaptic dysregulation of dopaminergic neurotransmission in this disorder. The second part addresses the role of D(2) and non-D(2) receptor blockade in the treatment of schizophrenia and highlights a preponderant role of D(2) receptors in the mechanism of antipsychotic action. Neuroimaging studies have defined a narrow, but optimal, therapeutic window of 65-78 % D(2) receptor blockade within which most antipsychotics achieve optimal clinical efficacy with minimal side effects. Some antipsychotics though do not conform to that therapeutic window, notably clozapine. The reasons for its unexcelled clinical efficacy despite subthreshold levels of D(2) blockade are unclear and current theories on clozapine's mechanisms of action are discussed, including transiency of its D(2) receptor blocking effects or preferential blockade of limbic D(2) receptors. Evidence is also highlighted to consider the use of extended antipsychotic dosing to achieve transiency of D(2) blockade as a way to optimize functional outcomes in patients. We also present some critical clinical considerations regarding the mechanisms linking dopamine disturbance to the expression of psychosis and its blockade to the progressive resolution of psychosis, keeping in perspective the speed and onset of antipsychotic action. Finally, we discuss potential novel therapeutic strategies for schizophrenia.