Acta Pharmacologica Sinica (2009) 30: 1323–1329; doi: 10.1038/aps.2009.116; published online 24 August 2009

Aim: Currently, there is considerable debate as to which method is more accurate for measuring the activity of CYP 3A in vivo: cortisol 6β-hydroxylation clearance (Cl_m(6β)) or the urinary ratio of 6β-OHF to F (6β-OHF/F).  Furthermore, the value of measuring endogenous levels of cortisol over a 24 h period (AUC_F) needs to be confirmed.  The aim of the present study was to determine which method was most effective at measuring changes in the in vivo activity of CYP 3A : AUC_F, Cl_m(6β), or 6β-OHF/F.

Methods: A two phase, cross-over design was adopted in this study.  A total of 24 subjects (12 males and 12 females) were randomly assigned to one of two groups: the test group subjects were given 250 mg clarithromycin tablets twice a day for a period of 4 d, whereas the control group received a placebo twice daily for a similar period.  On d 5 of the study, the last dose of either clarithromycin or placebo was supplemented with an oral dose of 7.5 mg midazolam (MDZ); blood and urine samples were then collected at various times.  All samples collected at the same sampling times on d 4 were used to evaluate the effects of MDZ administration on cortisol levels and metabolism.  The ratio of 1-hydroxymidazolam (1-OHMDZ) concentration to MDZ concentration at 1 h (MR) was taken as a measure of the in vivo CYP 3A activity.  AUC_F, Cl_m(6β), and 6β-OHF/F were also used as biomarkers for CYP 3A activity. 

Results: No correlations were found (either before or after inhibition) between CYP 3A activity and any of the following measures: AUC_F, Cl_m(6β), or 6β-OHF/F (r<0.4, P>0.05).  After 4 d of clarithromycin administration, CYP 3A activity (MR) decreased by 75% (P=0.000), whereas AUC_F increased by 19% (P=0.040), and Cl_m(6β) and 6β-OHF/F decreased by 54.2% (P=0.000) and 50% (P=0.003), respectively.  No significant changes in AUC_F (P=0.178), or in the amount of urinary 6β-OHF (P=0.169), or in F (P=0.391) were found over a 24 h time period, either with or without MDZ administration.

Conclusion: Although Cl_m(6β) and 6β-OHF/F can reflect the decline in CYP 3A activity, the impression they provide is neither accurate nor complete.  AUC_F is completely ineffective for evaluating variations in CYP 3A activity.  MDZ administration had no evident effects on either cortisol metabolism or excretion over a period of 24 h.

Keywords: CYP 3A activity; cortisol; 6β-hydroxycortisol; midazolam