Stereoselective Determination of Omeprazole and 5‐Hydroxy‐Omeprazole in Dried Blood Spots for CYP2C19 Activity Assessment

We previously proposed stereoselective 5-hydroxy-omeprazole/omeprazole cut-offs for R- and S-omeprazole to differentiate between rapid, normal, and poor CYP2C19 metabolisers. However, the performance of these stereoselective CYP2C19 phenotyping approaches has never been compared with conventional racemic omeprazole phenotyping. Therefore, dried blood spots from healthy, genetically predicted rapid (n = 15), normal (n = 15), and intermediate (n = 14) CYP2C19 metabolisers were stereoselectively reanalysed after administration of omeprazole alone (10 mg) and following CYP2C19 inhibition with voriconazole (400 mg administered prior to omeprazole). Using stereoselective R-, S-, and racemic 5-hydroxy-omeprazole/omeprazole ratios, CYP2C19 phenotyping performance was assessed by genotype-phenotype concordance and phenoconversion patterns relative to prior reports. An adaptive phenotyping strategy using the 3-h metabolic ratio in individuals with delayed absorption and retaining the 2-h ratio otherwise improved agreement with AUC-based CYP2C19 activity measures (Spearman ρ = 0.99 vs. 0.93 and 0.88 for fixed 2- and 3-h ratios, respectively). Across methods, the racemic approach demonstrated the most favourable overall performance, closely followed by the S-enantiomer-based strategy. In conclusion, while racemic omeprazole phenotyping modestly remains the most accurate strategy for assessing CYP2C19 activity, stereoselective assessment of the S-enantiomer provides an alternative that enables CYP2C19 phenotyping in patients treated with esomeprazole without treatment interruption or substitution with omeprazole.