Abstract

This study uses stable isotope methodology to evaluate the validity of 6β-hydroxylation clearance of endogenous cortisol as a new index for in vivo CYP3A phenotyping in humans. Important factors contradictory to the use of a conventional index of urinary ratio of 6β-hydroxycortisol to cortisol (6β-OHF/F) to evaluate in vivo CYP3A activity are also discussed. Stable isotopically labeled cortisol (3-5 mg) was orally administered to three healthy adult subjects to accurately determine the fractional metabolic clearance specific for the 6β-hydroxylation of cortisol. Plasma concentrations of labeled cortisol and urinary excreted amounts of labeled cortisol and 6β-OHF were analyzed by gas chromatography-mass spectrometry simultaneously with their endogenous counterparts. There was a good correlation between endogenous and exogenous 6β-hydroxylation clearances in the three subjects tested (r = 0.7733, 0.9112, and 0.9534 for 2-, 4-, and 6- to 8-h urine collection periods, respectively). This strongly suggests that the endogenous 6β-hydroxylation clearance can be used as an appropriate index for phenotyping the in vivo CYP3A activity. Furthermore, observed intra- (2.1- to 4.6-fold) and interindividual variabilities (ca. 5-fold) in the labeled cortisol renal clearance suggest that the urinary ratio 6β-OHF/F, a function of 6β-hydroxylation clearance and renal clearance of cortisol, does not always reflect the in vivo CYP3A activity. When a macrolide antibiotic, clarithromycin, was administered to a healthy volunteer in a dose of 200 mg every 12 h for 6 days, the inhibitory effects of clarithromycin on the in vivo CYP3A activity were clearly seen by the 6β-hydroxylation clearance of endogenous cortisol but not by the urinary ratio 6β-OHF/F.