Use of midazolam as a human cytochrome P450 3A probe: I. In vitro-in vivo correlations in liver transplant patients.

The clearance of midazolam (MDZ) in humans is principally due to metabolic biotransformation catalyzed by CYP3A isoforms. A study was conducted in patients who had undergone liver transplants that provides evidence that MDZ can be used as an in vivo probe of interindividual hepatic CYP3A variability. The clearance of MDZ and cyclosporine after i.v. administration were determined in 10 patients approximately 10 days after transplant surgery. Liver biopsy specimens were obtained within 24 hr of the pharmacokinetic study and CYP3A content and MDZ 1'-hydroxylation activity were measured in 13,000 x g tissue supernatants (S-13). The in vitro rate of 1'-hydroxy-MDZ formation was found to correlate significantly with the total CYP3A content in hepatic S-13 fractions (r = .84, P < .01). The total MDZ clearance measured in vivo was highly correlated with the hepatic CYP3A content measured in vitro (r = .93, P < .001) and with in vivo cyclosporine clearance (r = .81, P < .001). For five of the patients, the intrinsic clearance of midazolam to 1'-hydroxy-MDZ (Vmax/Km) in vitro measured in S-13 preparations was scaled for total liver mass and applied to the well stirred model of hepatic clearance to yield a prediction of MDZ clearance in vivo. The mean MDZ clearance predicted from in vitro 1'-hydroxylation data was identical to the mean clearance observed in vivo (0.60 +/- 0.24 versus 0.59 +/- 0.25 liter/min). Together, the results suggest that variability in hepatic CYP3A expression in liver transplant recipients, and possibly in other populations, can be determined by the measurement of MDZ metabolic clearance.