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

Aims To determine the kinetics of the formation of 6 beta -naltrexol from n altrexone in human liver cytosol, and to investigate the role of potential inhibitors. Methods The kinetics of the formation of 6 beta -naltrexol from naltrexone were examined in eight human liver cytosol preparations using h.p.l.c. to q uantify 6 beta -naltrexol and, the extent of inhibition of 6 beta -naltrexo l formation was determined using chemical inhibitors. The formation of 6 be ta -naltrexol and the back reaction of 6 beta -naltrexol to naltrexone were also examined in a microsomal preparation. Results The V-max, K-m and CLint values for the formation of 6 beta -naltre xol from naltrexone were in the ranges of 16-45 nmol mg(-1) protein h(-1), 17-53 mum and 0.3-2.2 ml h(-1) mg(-1) protein, respectively. The steroid ho rmones testosterone (K-i = 0.3 +/- 0.1 mum) and dihydrotestosterone (K-i = 0.7 +/- 0.4 mum) were the most potent competitive inhibitors of 6 beta -nal trexol formation, with naloxone, menadione and corticosterone also producin g > 50% inhibition at a concentration of 100 mum. The opioid agonists morph ine, oxycodone, oxymorphone and hydromorphone, and a range of benzodiazepin es showed < 20% inhibition at 100 mum. In the microsomal preparation, there was no formation of naltrexone from 6 beta -naltrexol nor any formation of 6 beta -naltrexol from naltrexone. Conclusion The intersubject variability in the kinetic parameters of 6 beta -naltrexol formation could play a role in the efficacy of and patient comp liance with naltrexone treatment. This variability could be due in part to a genetic polymorphism of the dihydrodiol dehydrogenase DD4, one of the enz ymes reported to be responsible for the formation of 6 beta -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 remai ns to be established.