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.

Keywords: CYP2E1; CYP3A; drug interactions; microdosing; phenotyping.

Conflict of interest statement

N.H., J.B., Y.S., G.M. and W.E.H. have no competing interests to declare.

© 2019 The British Pharmacological Society.

Figures

Figure 1

Trial design of the 1:1 randomised, open‐label single ascending dose trial of oral chlorzoxazone (CZZ) followed by a drug interaction trial assessing chlorzoxazone's perpetrator characteristics with the oral CYP3A substrate midazolam (MDL)

Figure 2

Plasma concentration–time profiles (mean ± standard deviation) of (A) chlorzoxazone and (B) 6‐hydroxy‐chlorzoxazone after oral administration of ascending chlorzoxazone doses in 2 independent dose arms of 6 healthy volunteers (▲,▼); all 12 participants received the 0.1‐mg dose (O). LLOQ, lower limit of quantification

Figure 3

Plasma concentration–time profiles of chlorzoxazone and 6‐hydroxy‐chlorzoxazone dose‐normalised to 1 mg in healthy volunteers after oral doses ranging from 0.05 to 500 mg. All dose groups included 6 participants except the 0.1 mg dose group, which included 12 participants