Preferential Inhibition of JAK1 Relative to JAK3 by Upadacitinib: Exposure‐Response Analyses of Ex Vivo Data From 2 Phase 1 Clinical Trials and Comparison to Tofacitinib

Upadacitinib is a selective Janus kinase (JAK) 1 inhibitor being developed for treatment of rheumatoid arthritis. This study characterizes the relationships between upadacitinib exposure and interleukin (IL)‐6–induced signal transducer and activator of transcription proteins 3 (STAT3) phosphorylation (pSTAT3) and IL‐7–induced STAT5 phosphorylation (pSTAT5) in the ex vivo setting as measures for JAK1 and JAK1/JAK3 inhibition, respectively, with comparison to tofacitinib. Drug plasma concentrations and ex vivo IL‐6–induced pSTAT3 and IL‐7–induced pSTAT5 in blood from subjects evaluated in 2 phase 1 studies who received immediate‐release 1 mg to 48 mg upadacitinib, 5 mg twice daily (BID) tofacitinib, or placebo were determined. Exposure‐response models were developed, and the effects of different upadacitinib doses on ex vivo biomarker responses were simulated and compared to tofacitinib. Upadacitinib (and tofacitinib) reversibly inhibited IL‐6–induced pSTAT3 and IL‐7–induced pSTAT5 in a concentration‐dependent manner. Model‐estimated values of 50% of the maximum effect were 60.7 nM for upadacitinib and 119 nM for tofacitinib for IL‐6–induced pSTAT3 inhibition, and 125 nM for upadacitinib and 79.1 nM for tofacitinib for IL‐7–induced pSTAT5 inhibition. Tofacitinib 5 mg BID is estimated to have a similar magnitude of effect on IL‐6–induced pSTAT3 to ∼3 mg BID of upadacitinib (immediate‐release formulation), whereas a 4‐fold higher dose of upadacitinib (∼12 mg BID), is estimated to show a similar magnitude of inhibition on IL‐7–induced pSTAT5 as tofacitinb 5 mg BID. This study confirms that in humans, upadacitinib has greater selectivity for JAK1 vs JAK3 relative to the rheumatoid arthritis approved dose of tofacitinib, and results from these analyses informed the selection of upadacitinib IR doses evaluated in phase 2.