An interaction between the cytochrome P450 probe substrates chlorzoxazone (CYP2E1) and midazolam (CYP3A)

Aims The use of multiple probe substrates to evaluate the activity of drug metabolizing enzymes requires that there are no inter–substrate interactions. As part of a series of studies to develop a clinically useful collection of probe substrates that could be given alone or in any combination, we observed an interaction between midazolam (MDZ) and another component of the six‐drug cocktail. Published data indicated that the interacting component was likely to be chlorzoxazone. This was investigated as part of a second study. The data relating to the interaction from both studies are reported here. Methods Both studies were performed in 16 healthy subjects. All treatments were given orally after an overnight fast. In study 1, which was performed to a four‐period, open, crossover design, subjects received on separate occasions MDZ 5 mg, diclofenac 25 mg, a four drug cocktail (caffeine 100 mg, mephenytoin 100 mg, debrisoquine 10 mg and chlorzoxazone 250 mg) and a six drug cocktail (caffeine 100 mg, mephenytoin 100 mg, debrisoquine 10 mg, chlorzoxazone 250 mg, diclofenac 25 mg and MDZ 5 mg). In study 2, which was performed to a two‐period, open, crossover design, subjects received a five drug cocktail (as the six drug cocktail in the first study, but without chlorzoxazone and with diclofenac dose increased to 50 mg) and a six drug cocktail (as five drug cocktail, with chlorzoxazone 250 mg). In both studies, blood samples were taken for measurement of plasma MDZ and 1‐hydroxy MDZ (1‐OH MDZ) concentrations. In study 1, blood samples were taken up to 12 h post‐dose while in study 2 a single sample was taken 2 h after dosing. In study 1, the potential interaction between MDZ and the other components of the six drug cocktail was assessed by comparing AUClast ratios (1‐OH MDZ/MDZ) between the two treatments. Additionally, a single sampling timepoint of 2 h post‐dose for determination of concentration, rather than AUC, ratios was established. The 2 h plasma concentration ratios from studies 1 and 2 were combined and a pooled analysis performed to compare ratios within each study (to determine the change in ratio when MDZ was dosed with and without chlorzoxazone) and between studies (to determine the consistency of the ratios when MDZ was given either as part of the two six drug cocktails or when given alone and as part of the five drug cocktail). Results In study 1, both the AUClast ratio and the 2 h post‐dose plasma concentration ratio were reduced when MDZ was gi