Deconvoluting Gut Versus Liver Cytochrome P450 3A4 Function: Drug Probes, Biomarkers, and Tissue Biopsy

Cytochrome P450 3A4 (CYP3A4) is an important drug‐metabolizing enzyme, whose function is impacted by age, sex, body weight, pregnancy, and disease. It is also the site of major drug interactions involving inhibition and induction. Because CYP3A4 is expressed in the intestine and liver, investigators have sought to study its function in both organs. As reviewed in the current narrative, approaches have included the profiling of tissue biopsy samples, use of novel plasma‐based liquid biopsies, studies with individuals undergoing liver transplant, intravenous (IV) probe pharmacokinetics (PK) to measure hepatic CYP3A4 activity (e.g., midazolam and erythromycin breath test, ERBT), oral probe PK to differentially capture gut and liver CYP3A4 activity (e.g., midazolam and alprazolam), biomarkers (e.g., 6β‐hydroxy cortisol/cortisol [6βHC/C] and 1β‐hydroxy deoxycholic acid/deoxycholic acid [1βHDCA/DCA] urine ratio, and 4β‐hydroxy cholesterol/cholesterol [4βHC/C] plasma ratio), modeling (e.g., static and physiologically based PK), and dosing of agents to inhibit intestinal CYP3A4 (e.g., grapefruit juice [GFJ]). Overall, there is also broad consensus that IV midazolam PK and ERBT are reflective of liver CYP3A4, that oral midazolam reports both gut and liver CYP3A4, and that one can selectively inhibit gut CYP3A4 using GFJ. Data for biomarkers are mixed, however, indicating that 4βHC/C plasma ratio is largely reflective of liver CYP3A4 in the basal state, unlike 6βHC/C and possibly 1βHDCA/DCA urine ratio. Literature reports also showcase challenges when correlating different CYP3A4 trait measures across subjects, impacted by genotype (e.g., CYP3A5, P‐glycoprotein), sex, body weight, and the contribution of gut (versus liver) to probe drug and biomarker disposition.