RT Journal Article SR Electronic T1 Characterization of T-5 N-Oxide Formation as the First Highly Selective Measure of CYP3A5 Activity JF Drug Metabolism and Disposition JO Drug Metab Dispos FD American Society for Pharmacology and Experimental Therapeutics SP 334 OP 342 DO 10.1124/dmd.113.054726 VO 42 IS 3 A1 Li, Xiaohai A1 Jeso, Valer A1 Heyward, Scott A1 Walker, Gregory S. A1 Sharma, Raman A1 Micalizio, Glenn C. A1 Cameron, Michael D. YR 2014 UL http://dmd.aspetjournals.org/content/42/3/334.abstract AB Almost half of prescription medications are metabolized by cytochrome P450 3A4 and 3A5. CYP3A4 and 3A5 have significant substrate overlap, and there is currently no way to selectively monitor the activity of these two enzymes, which has led to the erroneous habit of attributing the cumulative activity to CYP3A4. While CYP3A4 expression is ubiquitous, CYP3A5 expression is polymorphic, with large individual differences in CYP3A5 expression level. The CYP3A5 genotype has been shown to alter the pharmacokinetics of drugs in clinical trials. We report the first tool compound capable of determining CYP3A5 activity in biologic samples containing both enzymes. Oxidation of T-5 by CYP3A5 yields an N-oxide metabolite that is over 100-fold selective over CYP3A4. Formation of T-5 N-oxide highly correlates with the CYP3A5 genotype and CYP3A5 expression levels in human liver microsomes and human hepatocytes.