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

Abstract

To determine whether enzyme inhibition explains the clinical adverse interaction of benzodiazepines and buprenorphine. 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. 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 microM and 0.045 min(-1), respectively. Mixed-type inhibition of buprenorphine elimination (K(i) = 30-35 microM) and a noncompetitive inhibition of norbuprenorphine formation were also observed. For clonazepam (up to 10 microM), 3-hydroxy-7-acetamidoclonazepam (up to 10 microM), and alpha-hydroxy-triazolam (up to 1.0 microM), no time- or concentration-dependent inhibition of buprenorphine metabolism was found. 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.