Reduced haloperidol/haloperidol ratios in plasma: polymorphism in Japanese psychiatric patients.

We measured plasma concentrations of haloperidol (HAL) and its metabolite, reduced haloperidol (RHAL), by high performance liquid chromatography (HPLC) in 45 Japanese psychiatric patients receiving HAL. Plasma levels of HAL had a highly positive correlation with daily dose per body weight. Plasma RHAL/HAL ratios had also a dose-dependent relationship, but their distribution was nonnormal and a bimodal pattern with an antimode at 0.7 was apparent by probit analysis. There were 8 subjects (18%) with high RHAL/HAL ratios (mean = 1.26, SD = 0.41) and 37 subjects (82%) with low RHAL/HAL ratios (mean = 0.42, SD = 0.13). RHAL/HAL ratios showed little intraindividual variability (+/- 10.6%), while interindividual variability was large. This may suggest that pharmacogenetic factors are involved in the metabolism of HAL and RHAL.