AbstractAimsIn patients with rheumatoid arthritis (RA), interleukin (IL)‐6 affects the activity of cytochrome P450 (CYP) enzymes. Treatment with anti‐IL‐6 therapy can reverse the IL‐6‐mediated downregulation of CYP enzymes, resulting in changes in plasma levels of CYP substrates. The primary objective of this study was to evaluate the impact of the IL‐6 inhibitor olokizumab on the pharmacokinetics of CYP probe substrates in subjects with active RA.MethodsSeventeen patients with active RA were orally administered a phenotyping cocktail of midazolam (CYP3A4 substrate), omeprazole (CYP2C19 substrate), warfarin (CYP2C9 substrate) and caffeine (CYP1A2 substrate) alone and 2 weeks after a single subcutaneous injection of 128 mg olokizumab. The pharmacokinetic parameters of each substrate were calculated using noncompartmental analysis.ResultsSixteen of 17 enrolled patients received the complete doses of the cocktail drugs and olokizumab and were eligible for the pharmacokinetic evaluations. After single‐dose administration of olokizumab, the exposure of midazolam and omeprazole decreased by 30‐33% and 26‐32%, respectively, compared to when the substrates were administered along via cocktail. In the presence of olokizumab, caffeine exposure increased by 19‐23% compared to caffeine administration alone. There were no significant changes in S‐warfarin exposure.ConclusionIn patients with active RA, olokizumab potentially reverses the IL‐6‐mediated suppression of CYP3A4 and CYP2C19. According to FDA guidance, olokizumab is considered a weak inducer of CYP3A4 and CYP2C19.