CYP2C19 and rheumatoid arthritis: In a cocktail DDDI study of sirukumab, another anti‐IL‐6 mAb, exposure of the CYP probe substrates midazolam (CYP3A), omeprazole (CYP2C19), and S‐warfarin (CYP2C9) was reduced by 30% to 35%, 37% to 45%, and 18% to 19%, respectively, and caffeine (CYP1A2) was increased by 20% to 34%, after sirukumab administration in patients with active RA.35