Effect of benzodiazepines on the metabolism of buprenorphine in human liver microsomes

Objective: To determine whether enzyme inhibition explains the clinical adverse interaction of benzodiazepines and buprenorphine. Methods: Buprenorphine was incubated in the presence of benzodiazepines (or metabolites) with human liver microsomes (HLMs). A number of benzodiazepines were screened at therapeutic concentrations after 0-min and 15-min preincubation times. For tentative metabolically activated inhibitors, the kinetics of inhibition was studied in a secondary incubation system. Buprenorphine and norbuprenorphine were quantified by means of liquid chromatography-mass spectrometry. Results: Buprenorphine elimination and norbuprenorphine formation were at most reduced by 26% (i.e., weak or negligible inhibition). Evidence of metabolically activated inhibition suggested the need for further studies on the inhibitory kinetics. Midazolam caused time- and concentration-dependent inhibition of norbuprenorphine formation with pseudo-first-order kinetics, and K-I and k(inact) values of 10.5 muM and 0.045 min(-1), respectively. Mixed-type inhibition of buprenorphine elimination (K-i=30-35 muM) and a noncompetitive inhibition of norbuprenorphine formation were also observed. For clonazepam (up to 10 muM), 3-hydroxy-7-acetamidoclonazepam (up to 10 muM), and alpha-hydroxy-triazolam (up to 1.0 muM), no time- or concentration-dependent inhibition of buprenorphine metabolism was found. Conclusion: A single benzodiazepine, midazolam, is a moderate mechanism-based inactivator of buprenorphine N-dealkylation. It is anticipated that repeated exposures to midazolam might alter the in vivo metabolism of buprenorphine.