Drug tissue concentration and STAT3 modulation as determinants of tofacitinib response in ulcerative colitis

Abstract Introduction Inflammatory bowel disease (IBD) management has advanced with therapies like Janus kinase inhibitors (JAKi). Despite their promise, JAKi pharmacokinetic-pharmacodynamic (PK-PD) profiles and tissue-level effects remain underexplored. This study investigates tissue and serum tofacitinib levels, their correlation with therapeutic efficacy, and molecular mechanisms underlying treatment response. Methods Thirty refractory UC patients receiving tofacitinib were prospectively studied. Tissue biopsies and serum samples were collected pre- and post-induction for PK analysis using liquid-chromatography mass spectrometry (LC-MS/MS). RNA sequencing and cytokine profiling were performed on tissue samples to explore molecular responses. Endoscopic improvement was defined as a Mayo endoscopic subscore of 0-1 by week 16. Results Tofacitinib tissue concentrations were 25-fold higher than serum levels and significantly correlated (ρ=0.92, p<0.001). Responders showed significantly higher tissue drug exposure (1047.5 ng/g vs. 467.1 ng/g, p=0.02) at time of endoscopic assessment. Tofacitinib treatment reduced phosphorylated STAT3 (pSTAT3) levels, particularly in responders (p=0.02). RNA sequencing revealed gene modules linked to tissue drug and pSTAT3 concentrations. Gene set enrichment analysis showed that these were more frequent in non-responders and associated with JAK-STAT pathways. Conclusion This study underscores the importance of tissue tofacitinib levels in UC treatment efficacy, with pSTAT3 reduction serving as a potential marker of drug efficacy. RNA sequencing identified molecular pathways for potential biomarkers and novel therapeutic targets in tofacitinib non-responders.