Simultaneous phenotyping of CYP2E1 and CYP3A using oral chlorzoxazone and midazolam microdoses.

Abstract

Chlorzoxazone is the paradigm marker substrate for CYP2E1 phenotyping in vivo. Because at the commonly used milligram doses (250-750mg) chlorzoxazone acts as an inhibitor of the CYP3A4/5 marker substrate midazolam, previous attempts failed to combine both drugs in a common phenotyping cocktail. Microdosing chlorzoxazone could circumvent this problem. We enrolled 12 healthy volunteers in a trial investigating the dose-exposure relationship of single ascending chlorzoxazone oral doses over a 10,000-fold range (0.05-500mg) and assessed the effect of 0.1and 500mg of chlorzoxazone on oral midazolam pharmacokinetics (0.003mg). Chlorzoxazone area under the concentration-time curve was dose-linear in the dose range between 0.05and 5mg. A nonlinear increase occurred with doses ≥50mg, probably due to saturated presystemic metabolic elimination. While midazolam area under the concentration-time curve increased 2-fold when coadministered with 500mg of chlorzoxazone, there was no pharmacokinetic interaction between chlorzoxazone and midazolam microdoses. The chlorzoxazone microdose did not interact with the CYP3A marker substrate midazolam, enabling the simultaneous administration in a phenotyping cocktail. This microdose assay is now ready to be further validated and tested as a phenotyping procedure assessing the impact of induction and inhibition of CYP2E1 on chlorzoxazone microdose pharmacokinetics.