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

Abstract

Aims: Chlorzoxazone is the paradigm marker substrate for CYP2E1 phenotyping in vivo. Because at the commonly used milligram doses (250–750 mg) 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. Method: 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 (0.05–500 mg) and assessed the effect of 0.1 and 500 mg of chlorzoxazone on oral midazolam pharmacokinetics (0.003 mg). Results: Chlorzoxazone area under the concentration–time curve was dose-linear in the dose range between 0.05 and 5 mg. A nonlinear increase occurred with doses ≥50 mg, probably due to saturated presystemic metabolic elimination. While midazolam area under the concentration–time curve increased 2-fold when coadministered with 500 mg of chlorzoxazone, there was no pharmacokinetic interaction between chlorzoxazone and midazolam microdoses. Conclusion: 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.