The functional role of CYP2B6 in human drug metabolism: substrates and inhibitors in vitro, in vivo and in silico.

CYP2B6 metabolizes a number of drug substrates, that are usually non-planar, neutral or weakly basic, fairly lipophilic with one or two hydrogen bond acceptors, on which it catalyses various oxidative reactions. For bupropion, cyclophosphamide, ifosfamide, pethidine, ketamine and propofol, these reactions represent major metabolic or activation pathways and for their kinetics CYP2B6 function is of considerable importance. For the rest of the substrates found, CYP2B6 contributes to overall metabolism or to a single pathway, but probably not to a materially significant extent. Among inhibitors, thiotepa, ticlopidine and clopidogrel have been characterised extensively in terms of selectivity and potency. Thiotepa is the most selective of the inhibitors, but is not useful as an in vivo inhibitor, whereas ticlopidine and clopidogrel can be used as CYP2B6-selective probes in human clinical studies. Bupropion hydroxylation is a selective, and consequently useful, in vivo probe for CYP2B6. Computational approaches are being developed to the extent that predictions on affinity of chemicals to CYP2B6 are becoming reliable enough as a first screen of new drug molecules and other chemicals. With validated in vitro and in vivo substrates (e.g. bupropion) and inhibitors (e.g. ticlopidine), it is expected that pharmacological (including pharmacogenetic) and clinical significance of CYP2B6 will be delineated more fully in the near future.