Effects of fasting on biperiden pharmacokinetics in the rat

The effects of fasting on the pharmacokinetics of biperiden in rats were examined. Total clerance of biperiden was >90% ascribable to hepatic clearance and was essentially blood‐flow dependent. The number of compartments in the preferred phamcokinetic model of biperiden changed from three (for normal rats) to two (for fasted rats). The smaller mean residence time (MRT) values found for fasted rats were attributable to decreases in distribution volume. Biperiden showed much higher lipophilicity than haloperidol, thiopental, and hexobarbital, and its tissue‐to‐plasma partition coefficient in adipose tissue was 20‐fold higher than that in muscle. The influence of changes in volumes of adipose tissue and muscle on distribution volume (Vd_ss/BW) was evaluated from tissue‐to‐plasma partition coefficients. The value of Vd_ss/BW was predicted to decrease with decrease of adipose tissue, and to increase with decrease of muscle tissue. These results suggest that the observed decrease of Vd_ss/BW in fasted rats reflects reduced capacity to trap biperiden in the body, especially in adipose tissue. Possible clinical implications of these results are discussed.