Evaluating the Evidence for CYP2C19 Inhibitor Classifications: A Scoping Review

The Food and Drug Administration (FDA) Table of Inhibitors and the Flockhart Table™ are important references for identifying CYP2C19 inhibitors. Accurate inhibitor classification is essential for evaluating phenoconversion and managing drug-drug interactions. The objective of this study was to assess the concordance between inhibitor classifications according to the FDA and Flockhart tables and pharmacokinetic data from the primary literature. A scoping review of PubMed, product labels, and the Drug Interactions Database (DIDB®) up to April 2024 was conducted. Inhibitor-substrate pairs (e.g., fluoxetine-omeprazole) were assigned literature-reported classifications (i.e., weak, moderate, or strong) based on pharmacokinetic data. Concordance between literature-reported and listed classifications was evaluated. Of 90 inhibitor-substrate pairs identified, 66% involved sensitive substrates, which were the focus of our primary analysis. Among sensitive inhibitor-substrate pairs, 36% of FDA-listed and 45% of Flockhart-listed classifications were concordant with the literature. CYP2C19 phenotype appeared to impact concordance, with greater concordance among normal metabolizers for both FDA-listed (42%) and Flockhart-listed (50%) classifications. Steady state status and dosing also appeared to affect concordance. Discrepancies between listed and literature-reported classifications led to the following recommendations: (1) upgrade fluoxetine from moderate to strong in the Flockhart Table™, (2) upgrade fluconazole from moderate to strong in the Flockhart Table™, (3) downgrade omeprazole (and esomeprazole) from moderate to weak in the Flockhart Table™, and (4) include footnotes describing dose-dependent inhibition for fluvoxamine and fluconazole in both tables. These recommendations aim to improve the accuracy of CYP2C19 inhibitor classifications and the clinical utility of these tables.