The Impact of CYP2C9*11 Allelic Variant on the Pharmacokinetics of Phenytoin and (S)‐Warfarin

Cytochrome P450 2C9 (CYP2C9) is responsible for the oxidative metabolism of about 15% of commonly used drugs, some of which are characterized by a narrow therapeutic window. CYP2C9 is highly polymorphic, and over 60 alleles have been described. CYP2C9*2 and CYP2C9*3 are the most common polymorphisms among White patients and both are associated with decreased activity. The evidence concerning the functional importance of less frequent variant alleles is scarce. The objective of the current study was to characterize the in vivo activity of CYP2C9 among carriers of CYP2C9*11, one of the "African" alleles and the fourth most common CYP2C9 variant allele among White patients by using two prototype substrates, phenytoin and (S)‐warfarin. Single 300‐mg phenytoin and 20‐mg warfarin doses were given to 150 healthy Ethiopian Jewish participants who were nonsmokers, at least one week apart. (S)‐warfarin oral clearance and phenytoin metabolic ratio (PMR) derived from the ratio of 5‐(4‐hydroxyphenyl)‐5‐phenylhydantoin in 24‐hour urine collection to plasma phenytoin 12 hours (PMR 24/12) or 24 hours (PMR 24/24) post dosing, were used as markers of CYP2C9 activity. PMR 24/12 and PMR 24/24 were reduced by 50% and 62.2%, respectively, among carriers of CYP2C9*1/*11 (n = 13) as compared with carriers of CYP2C9*1/*1 (n = 127) (false discovery rate (FDR) q < 0.001). The respective decrease in (S)‐warfarin oral clearance was 52.6% (FDR q < 0.001). In conclusion, the enzyme encoded by CYP2C9*11 is characterized by a more than 50% decrease in the enzymatic activity, resembling the extent of decrease associated with CYP2C9*3 ("no‐function allele"). Among patients of African ancestry, CYP2C9*11 genetic analysis should be considered prior to prescribing of narrow therapeutic window drugs such as phenytoin, warfarin, nonsteroidal anti‐inflammatory drugs, or siponimod.