Clinical pharmacokinetics of antipsychotics

The antipsychotics are a chemically diverse group of heterocyclic compounds, which ameliorate many symptoms of schizophrenia. Most of the antipsychotics are very lipophilic and cross lipoidal membranes freely. When administered orally, they are well absorbed and undergo substantial pre-systemic elimination (bioavailability: 10-70%), are highly bound to plasma proteins (75-99%) and tissues, and are extensively distributed (VD: 100-1000 L). Primary route of elimination for most of antipsychotics is hepatic metabolism and biotransformation produces active metabolites. There is no linear relationship between the concentration of parent compound and different metabolites, and clinical relevance of pharmacologically active metabolites is not well understood. There are wide interindividual variabilities in pharmacokinetics, which result in large differences in steady-state plasma concentrations on the same dose regimen. The existence of optimal therapeutic ranges for most antipsychotics remains controversial. One of the major problems is the lack of well-designed studies that involve sufficient numbers of patients to clearly establish a therapeutic range for these drugs and is further complicated by the presence of a large number of pharmacologically active metabolites. However, the pronounced interindividual kinetic variabilities, combined with problems of noncompliance and drug interactions, and the delayed onset of clinical response in relation to initiation of treatment with antipsychotics are reasons why drug monitoring in conjunction with clinical status of the patient can be useful. Indications for antipsychotic drug monitoring include lack of response, non-compliance, toxicity, and drug interactions when other drugs are coadministered.