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.
Footnotes
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↵1 Abbreviations used are: P450, cytochrome P450; F, cortisol; 6β-OHF, 6β-hydroxycortisol; 6β-OHF-d5, 6β-[1,1,19,19,19-2H5]hydroxycortisol; cortisol-d5, [1,1,19,19,19-2H5]cortisol; cortisol-13C4, [1,2,4,19-13C4]cortisol; CLm(6β)-exo, 6β-hydroxylation clearance of exogenous (labeled) cortisol; X(6β)-exo, urinary excretion amount of exogenous (labeled) 6β-hydroxycortisol; AUC(F)-exo, area under the curve of exogenous (labeled) cortisol; X(F)-exo, urinary excretion amount of exogenous (labeled) cortisol; CLr(F), renal clearance of cortisol; CLr(F)-exo, renal clearance of exogenous (labeled) cortisol; CLm(6β)-endo, 6β-hydroxylation clearance of endogenous cortisol; X(6β)-endo, urinary excretion amount of endogenous 6β-hydroxycortisol; AUC(F)-endo, area under the curve of endogenous cortisol; X(F)-endo, urinary excretion amount of endogenous cortisol; CLr(F)-endo, renal clearance of endogenous cortisol.
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This work was supported in part by a Grant for Private Universities provided by the Promotion and Mutual Aid Corporation for Private Schools of Japan. This study was presented in part at the 16th Annual Meeting of Japanese Society for the Study of Xenobiotics, Kobe, Japan, October 17-19, 2001.
- Received March 24, 2003.
- Accepted July 23, 2003.
- The American Society for Pharmacology and Experimental Therapeutics
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