Comparison of midazolam and simvastatin as cytochrome P450 3A probes

Objective Our objective was to compare simvastatin with the validated probe midazolam in the assessment of cytochrome P450 (CYP) 3A activity. Methods This study used an open-label, fixed-sequential, 3-way crossover study design. Nineteen subjects received oral doses of 0.075 mg/kg midazolam and 40 mg simvastatin during 3 phases (baseline, after inhibition with 400 mg ketoconazole for 10 days, and after induction with 600 mg rifampin [INN, rifampicin] for 9 days). Serial plasma concentrations of midazolam and simvastatin were obtained. Oral clearances of midazolam and simvastatin were compared. Results Oral midazolam clearance decreased after pretreatment with ketoconazole (from a geometric mean of 25 mL · min−1 · kg−1 [range, 12–57 mL · min−1 · kg−1] to 2.7 mL · min−1 · kg−1 [range, 1.2–8.5 mL · min−1 · kg−1], P < .001) and increased after pretreatment with rifampin (to a geometric mean of 203 mL · min−1 · kg−1 [range, 125–371 mL · min−1 · kg−1], P < .001). Oral simvastatin clearance decreased after ketoconazole (from a geometric mean of 312 mL · min−1 · kg−1 [range, 151–1478 mL · min−1 · kg−1] to 25 mL · min−1 · kg−1 [range, 8.0–147 mL · min−1 · kg−1], P < .001) and increased after rifampin (to a geometric mean of 3536 mL · min−1 · kg−1 [range, 413–10,329 mL · min−1 · kg−1], P < .001). The change in simvastatin clearance was highly variable from baseline to inhibition (6- to 33-fold decrease) and from baseline to induction (2- to 39-fold increase) compared with midazolam (7- to 18-fold decrease during inhibition and 4- to 12-fold increase during induction). Midazolam and simvastatin oral clearances were correlated for all study phases (r = 0.5 and P = .03 for baseline and r = 0.53 and P = .02 for inhibition) but were weakest for induction (r = −0.031, P = .22). The area under the concentration-time curve inhibitory ratio for midazolam was 9.4 versus 12.4 for simvastatin (r = 0.3, P = .03). Conclusions Compared with midazolam, simvastatin is a nonvalidated, suboptimal probe for studying CYP3A drug interactions because of its lack of CYP3A specificity. Clinical Pharmacology & Therapeutics (2006) 79, 350–361; doi: 10.1016/j.clpt.2005.11.016