Neuroleptic plasma concentrations and clinical response: in search of a therapeutic window

Plasma Antipsychotic Drug Concentrations: Practical Clinical Considerations

The practical application of plasma antipsychotic moni toring to achieve dose optimization remains problematic. Despite numerous studies attempting to establish antipsychotic dosing guidelines based on target plasma concentration ranges, a therapeutic window for most neuroleptics remains elusive and drug individualiza tion is still most often empirically determined. Drug concentration testing for all antipsychotics is commercially available, but at present, only haloperidol and clozapine appear to provide clinically useful plasma values. Reasons for obtaining levels include suspected noncompliance, pharmacokinetic drug interactions, toxicity, and cases in which difficulty arises in distinguishing between drug-induced toxicity and underlying pathology. Accordingly, antipsychotic plasma concentration monitoring offers potential to improve therapeutic outcomes only when interpreted carefully in conjunction with the patient's overall clinical status. Copyright © 1996 by W.B. Saunders Company

Mechanisms of action of antipsychotic drugs of different classes, refractoriness to therapeutic effects of classical neuroleptics, and individual variation in sensitivity to their actions: Part II

Rapid-onset psychotic rebound is uncommon on discontinuation of most antipsychotic drugs, as might be expected for antipsychotic drugs with (hypothetically) indirect actions at their final target receptors. Rapid-onset psychosis is more common on withdrawal of clozapine, which might be expected if its action is direct. Drugs other than clozapine (notably thioridazine) may have hitherto unrecognised similarities to clozapine (but without danger of agranulocytosis), and may be useful in treatment of refractory psychosis. Quetiapine fulfils only some criteria for a clozapine-like drug. Clinical response to neuroleptics varies widely at any given plasma level. Haase's "neuroleptic threshold" concept suggests that the dose producing the slightest motor side effects produces most or all of the therapeutic benefit, but analyses presented here suggest that antipsychotic actions are not subject to a sharp "all-or-none" threshold but increase over a small dose range. This concept could provide a method for quantitative determination of individualized optimal doses.

Smoking and body weight influence the clearance of chlorpromazine

The population pharmacokinetic parameters of chlorpromazine (CPZ) in chronic schizophrenic patients were evaluated using 189 plasma concentration measurements from 31 patients. A NONMEM analysis demonstrated that the clearance of CPZ depended on the patient's body weight. Cigarette smoking and cannabis smoking increased the clearance of CPZ. Chronic alcohol consumption and the concurrent use of anticholinergics did not appear to influence the clearance of CPZ significantly.

Reduced haloperidol plasma concentration and clinical response in acute exacerbations of schizophrenia

Twenty-nine hospitalized patients suffering acute exacerbations of schizophrenia were treated for 2 weeks with fixed daily oral doses of haloperidol prospectively calculated to achieve a haloperidol plasma concentration of either 8-18 ng/ml or 25-35 ng/ml. Reduced haloperidol as well as haloperidol concentrations were assayed to determine if the former enhanced the predictability of response. Wee 2 haloperidol plasma concentrations were negatively correlated to clinical response as measured by the percentage change in the BPRS score from baseline (r = -0.43, P less than 0.05). In contrast, week 2 plasma concentrations of reduced haloperidol, total haloperidol (haloperidol + reduced haloperidol), and reduced haloperidol/haloperidol ratio did not correlate with the change in the BPRS score. Chi-square analysis concluded that patients with ratios greater than one were no less likely to be treatment responders (less than 25% improvement in BPRS from baseline and week 2 BPRS less than 55) than those with ratios less than one. Although these data lend additional support to reports of a curvilinear relationship between haloperidol plasma concentration and clinical response, they also suggest that reduced haloperidol plasma concentrations are of no value in predicting treatment response.