Midazolam Single Time Point Concentrations to Estimate Exposure and Cytochrome P450 (CYP) 3A Constitutive Activity...

Midazolam is the preferred probe to phenotype cytochrome P450 (CYP) 3A activity. This study evaluated a single-concentration, midazolam limited sampling strategy utilizing a population pharmacokinetic (PK) approach to estimate area under the curve, and thus CYP3A activity. Midazolam concentrations from adults during CYP3A constitutive conditions were obtained from previous studies after single, oral or intravenous administration. Population PK modeling was conducted by nonlinear mixed-effects modeling. Potential covariates of clearance, volume of distribution, and bioavailability were evaluated. A limited sampling model at 1, 2, 4, or 6 hours was selected and fitted with post hoc estimation with the final population PK model. Preset criterion for the limited sampling model selection was a coefficient of determination ≥0.9. Bias and precision were also evaluated. The studies provided 2122 observations from 152 healthy adults. Midazolam concentrations were adequately described by a two-compartment model with first order absorption. Age and sex were significant covariates of central volume (V₂ ) and were retained in the final model. An estimate (interindividual variability) of midazolam clearance was 32.5 L/hr (52.9%), covariate of central volume was 67 L (39.1%), and oral bioavailability was 0.33 (45.5%). The final population parameter estimates were within the 95% confidence intervals and were similar to the median bootstrap estimates. Upon comparison to the population PK model, the 4-hour limited sampling model estimated area under the curve had an acceptable coefficient of determination and acceptable bias and precision limits. A 4-hour, but not the 1-, 2-, and 6-hour, single concentration accurately estimated midazolam area under the curve during constitutive CYP3A conditions in healthy adults.