Poor and enantioselective bioavailability of naftopidil enantiomers is due to extensive and stereoselective metabolism in rat liver.

Racemic naftopidil (NAF) is used to treat benign prostatic hyperplasia (BPH) and prostatic cancer (PCa). It exhibits greater efficacy but requires higher dose than other ɑ1-adrenoceptor blockers because of its poor bioavailability. It was previously shown that bioavailability of S(-)-NAF (14.5%) was twice that of R(+)-NAF (6.8%). The present study aimed to elucidate the major factors contributing to the poor and enantioselective bioavailability of NAF. First, absorption of NAF enantiomers was examined using a perfusated intestinal model. NAF enantiomers were found to be equally and highly permeable in all segments of the intestine. Second, the metabolites formed in different parts of the intestine and in bile were investigated. Glucuronidation of NAF enantiomers was found to occur primarily in the liver. Third, a new method consisting of ultra performance liquid chromatography coupled with triple-quadruple mass spectrometry (UPLC-MS/MS) was employed to quantify and calculate the pharmacokinetic parameters of NAF enantiomers and their glucuronides after the enantiomers were intravenously injected into rats. The amounts of R(+)-NAF glucuronide (R(+)-NAF-G) and S(-)-NAF glucuronide (S(-)-NAF-G) were six-fold higher than that of R(+)-NAF, and three-fold higher than that of S(-)-NAF. Glucuronidation of S(-)-NAF was faster than that of R(+)-NAF, but the conjugated amount was half of that of R(+)-NAF. Thus, bioavailability of S(-)-NAF was twice that of R(+)-NAF. In conclusion, extensive phase II metabolism in the liver significantly contributes to the low bioavailability of NAF enantiomers. Glucuronidation is the most important metabolic pathway for NAF enantiomers. Glucuronidation of S(-)-NAF is faster but occurs to a lesser extent than that of R(+)-NAF.