Clinical significance of drug binding, protein binding, and binding displacement drug interactions

In the course of treatment of psychiatric patients, it is often necessary to switch antipsychotic medications. In recent years, atypical antipsychotic agents have become the first-line therapeutic interventions for treating psychotic symptoms. Reasons for switching patients from the typical antipsychotics to the atypical agents can include enhanced efficacy against negative symptoms, improvement in cognitive capacity, and reduction of risk of extrapyramidal side effects. The presumed long-term benefits of switching to the new antipsychotic drug should be assessed. Pharmacokinetic and pharmacodynamic properties of antipsychotic agents and drug-drug interactions should be considered during the switch process. Two methods are employed for the switch process: crossover ("crosstitration") and an abrupt switch. With the crossover method, the patient's current medication is tapered over a period of several days to several months to prevent potential withdrawal symptoms, such as nausea, vomiting, insomnia, muscle aches, and diaphoresis. Due to withdrawal symptoms, clozapine is the only atypical agent recommended to proceed with a slow dose taper when switching to another atypical drug. Sudden cessation could also precipitate the emergence of motor symptoms, which can include pseudoparkinsonism, dystonia, akathisia, and dyskinesia. The initiation of the new antipychotic occurs concurrently with the tapering of the previous drug. In an abrupt switch, the previous antipsychotic is discontinued suddenly, independent of its dose, and the new antipsychotic is started on the next day. Both methods have been used in clinical practice and double-blind studies. To date, only two medications have been studied in large multicenter clinical trials. These are olanzapine and ziprasidone. The olanzapine study revealed optimal benefits when the previous agents were gradually withdrawn and olanzapine was initiated at 10 mg/day. The ziprasidone switch study demonstrated both reduced adverse side effects from the previous agents and improvements in clinical efficacy. Additional studies are needed to examine the optimal methods for switching patients from one atypical agent to another atypical antipsychotic.