Phenothiazine levels in plasma and red blood cells. Their relationship to clinical improvement in schizophrenia.

A controlled study investigated the relationship between steady-state plasma and RBC concentrations of the phenothiazine derivative butaperazine maleate and the therapeutic response in 24 hospitalized schizophrenic patients who received constant maintenance doses of butaperazine during the first two weeks of treatment. Butaperazine concentrations in RBCs correlated significantly with clinical improvement in an inverted U-shaped pattern, whereas plasma levels of butaperazine were not significantly related to clinical response. Both plasma and RBC levels of butaperazine showed large interpatient variations. The level of RBC-bound drug might be a better peripheral correlate of drug levels in the brain than are drug levels in plasma. Thus, monitoring drug levels in RBCs might have an advantage over measured drug levels in plasma. These findings might not allow generalization to other antipsychotic agents.