Extrastriatal dopamine D(2) receptor occupancy in olanzapine-treated patients with schizophrenia

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.

Acute anxiety disorder, major depressive disorder, bipolar disorder and schizophrenia are related to different patterns of nigrostriatal and mesolimbic dopamine dysfunction

Dopamine (DA) receptor and transporter dysfunctions play a major role in the pathophysiology of neuropsychiatric diseases including anxiety disorder (AD), major depressive disorder (MDD), bipolar disorder (BD) in the manic (BDman) or depressive (BDdep) state and schizophrenia (SZ). We performed a PUBMED search, which provided a total of 239 in vivo imaging studies with either positron emission tomography (PET) or single-proton emission computed tomography (SPECT). In these studies, DA transporter binding, D1 receptor (R) binding, D2R binding, DA synthesis and/or DA release in patients with the primary diagnosis of acute AD (n=310), MDD (n=754), BDman (n=15), BDdep (n=49) or SZ (n=1532) were compared to healthy individuals. A retrospective analysis revealed that AD, MDD, BDman, BDdep and SZ differed as to affected brain region(s), affected synaptic constituent(s) and extent as well as direction of dysfunction in terms of either sensitization or desensitization of transporter and/or receptor binding sites. In contrast to AD and SZ, in MDD, BDman and BDdep, neostriatal DA function was normal, whereas MDD, BDman, and BDdep were characterized by the increased availability of prefrontal and frontal DA. In contrast to AD, MDD, BDman and BDdep, DA function in SZ was impaired throughout the nigrostriatal and mesolimbocortical system with an increased availability of DA in the striatothalamocortical and a decreased availability in the mesolimbocortical pathway.

Do atypical antipsychotics really enhance smoking reduction more than typical ones?: the effects of antipsychotics on smoking reduction in patients with schizophrenia

Whether atypical antipsychotics (AAs) can enhance smoking reduction in schizophrenic patients remains controversial because of methodological limitations in existing studies. This study explored whether certain types of antipsychotics predict smoking reduction in schizophrenic patients. Three hundred eight smoking, predominantly male schizophrenic patients (271/308 [88.9%]) participated in an 8-week open-label study with antismoking medications (high-dose, low-dose nicotine transdermal patch and bupropion). Antipsychotics were classified into (1) typical antipsychotics (TAs) and (2) AAs, including multiacting receptor-targeted antipsychotics (clozapine, olanzapine, and quetiapine), serotonin-dopamine antagonists (risperidone), D2/D3 receptor antagonists (amisulpride), and partial dopamine receptor agonists (aripiprazole). A general linear model was used to explore whether types of antipsychotic predict changes in the number of cigarettes smoked per day (CPD) and the score of the Fagerstrom Test for Nicotine Dependence (FTND) while controlling for confounding factors. The type of antipsychotic (TAs or AAs) was not significantly associated with smoking cessation (n = 21; χ = 1.8; df = 4; P = 0.77). Regarding smoking reduction, the type of antipsychotic was significantly predictive of a change in the CPD (P = 0.027; partial eta square = 0.055) and FTND scores (P = 0.002; partial eta square = 0.073). The 95% confidence intervals of the estimated means of change in the CPD and FTND scores did not contain zero only among subjects on TAs or clozapine.These findings suggest that TAs and clozapine enhance smoking reduction compared with nonclozapine atypical antipsychotics in schizophrenic patients. The mechanisms underlying the effects of various antipsychotics on smoking reduction remain unclear and warrant future study.

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.

Central nervous system drug evaluation using positron emission tomography

In conventional pharmacological research in the field of mental disorders, pharmacological effect and dose have been estimated by ethological approach and in vitro data of affinity to the site of action. In addition, the frequency of administration has been estimated from drug kinetics in blood. However, there is a problem regarding an objective index of drug effects in the living body. Furthermore, the possibility that the concentration of drug in blood does not necessarily reflect the drug kinetics in target organs has been pointed out. Positron emission tomography (PET) techniques have made progress for more than 20 years, and made it possible to measure the distribution and kinetics of small molecule components in living brain. In this article, we focused on rational drug dosing using receptor occupancy and proof-of-concept of drugs in the drug development process using PET.