4β-Hydroxycholesterol as an Endogenous Biomarker for CYP3A Activity: Literature Review and Critical Evaluation

A number of cytochrome P450 (CYP)3A phenotyping probes have been used to characterize the drug interaction potential of new molecular entities; of these, midazolam has emerged as the gold standard. Recently, plasma 4 beta-hydroxycholesterol (4 beta-OHC), the metabolite of CYP3A-mediated cholesterol metabolism, has been championed as an endogenous biomarker for CYP3A, particularly during chronic conditions where CYP3A activity is altered by disease and in long-term treatment studies where midazolam administration is not optimal. Multiple studies in humans have shown that 4 beta-OHC can qualitatively differentiate among weak, moderate, and potent CYP3A induction when an inducer, typically rifampin, is administered for up to 2 weeks. Conversely, longer durations of CYP3A inhibitor administration (>= 1 month) appear to be necessary to differentiate among weak, moderate, and potent CYP3A inhibitors. A number of studies have reported statistically significant linear relationships between 4 beta-OHC plasma concentrations (and 4 beta-OHC:cholesterol ratios) and midazolam clearance. However, sufficiently powered studies assessing the ability of 4 beta-OHC or 4 beta-OHC:cholesterol ratios to measure CYP3A activity (ie, predictive performance) have not been conducted to date. Additional limitations associated with 4 beta-OHC phenotyping include inability to detect acute changes in CYP3A activity, uncertainty with regard to its intestinal formation, ambiguity surrounding the role of CYP3A5 in its metabolism, and lack of clarity regarding the role of transporters in its disposition. As such, the data do not support the use of 4 beta-OHC or 4 beta-OHC:cholesterol ratios as an endogenous biomarker for CYP3A activity.