Background: Inflammatory bowel disease 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 ulcerative colitis (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. 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 < .001). Responders showed significantly higher tissue drug exposure (1047.5 ng/g vs 467.1 ng/g, P = .02) at the time of endoscopic assessment. Tofacitinib treatment reduced phosphorylated STAT3 (pSTAT3) levels, particularly in responders (P = .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.

Conclusions: 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.