Kinetics and inhibition of the formation of 6β-naltrexol from naltrexone in human liver cytosol

Abstract

Aims: To determine the kinetics of the formation of 6β-naltrexol from naltrexone in human liver cytosol, and to investigate the role of potential inhibitors. Methods: The kinetics of the formation of 6β-naltrexol from naltrexone were examined in eight human liver cytosol preparations using h.p.l.c. to quantify 6β-naltrexol and, the extent of inhibition of 6β-naltrexol formation was determined using chemical inhibitors. The formation of 6β-naltrexol and the back reaction of 6β-naltrexol to naltrexone were also examined in a microsomal preparation. Results: The Vmax, Km and CLint values for the formation of 6β-naltrexol from naltrexone were in the ranges of 16–45 nmol mg−1 protein h−1, 17–53 µm and 0.3–2.2 ml h−1 mg−1 protein, respectively. The steroid hormones testosterone (Ki = 0.3 ± 0.1 µm) and dihydrotestosterone (Ki = 0.7 ± 0.4 µm) were the most potent competitive inhibitors of 6β-naltrexol formation, with naloxone, menadione and corticosterone also producing > 50% inhibition at a concentration of 100 µm. The opioid agonists morphine, oxycodone, oxymorphone and hydromorphone, and a range of benzodiazepines showed < 20% inhibition at 100 µm. In the microsomal preparation, there was no formation of naltrexone from 6β-naltrexol nor any formation of 6β-naltrexol from naltrexone. Conclusions: The intersubject variability in the kinetic parameters of 6β-naltrexol formation could play a role in the efficacy of and patient compliance with naltrexone treatment. This variability could be due in part to a genetic polymorphism of the dihydrodiol dehydrogenase DD4, one of the enzymes reported to be responsible for the formation of 6β-naltrexol from naltrexone. DD4 also has hydroxysteroid dehydrogenase activity which could account for the potent inhibition by the steroid hormones testosterone and dihydrotestosterone. The clinical significance of the latter finding remains to be established.