Dual TYK2/JAK1 Inhibition by Brepocitinib Reprograms Synoviocyte Pathobiology: Mechanistic Insights Into Targeted Therapy for Rheumatoid Arthritis.

Background: Rheumatoid arthritis (RA) is a chronic autoimmune disorder characterized by synovial hyperplasia, persistent inflammation, and joint destruction. Targeted inhibition of intracellular signaling pathways, such as JAK-STAT, has improved RA treatment outcomes, though safety and selectivity remain as concerns. Brepocitinib, a dual TYK2/JAK1 inhibitor, has shown clinical efficacy in the management of autoimmune diseases, yet its mechanistic impact on synoviocytes remains underexplored.

Objectives: To investigate the molecular and functional effects of brepocitinib on MH7A and RA-FLS synoviocytes, a key effector cell type in RA pathogenesis.

Methods: MH7A and RA-FLS cells were treated with brepocitinib (0.5 µM, 1 µM, and 5 µM) for 24 hours. Cell viability was assessed. Western blotting was used to examine phosphorylation of TYK2, JAK1, STAT1/3, and apoptotic markers (BAX, BCL-2, caspase-3). Quantitative PCR and ELISA were performed to evaluate mRNA and protein levels, respectively, of IL-6, TNF-α, and IFN-γ. Wound healing assays measured synoviocyte migration.

Results: Brepocitinib maintained ≥ 85% cell viability across all doses, compared with ~20% viability in doxorubicin-treated controls. At 5 µM, phosphorylation of JAK1 and STAT3 was suppressed by > 80%, while TYK2 and STAT1 inhibition reached ~70%. IL-6 and TNF-α transcripts were reduced by > 80% and IFN-γ by ~70%, with corresponding decreases in secreted cytokines (IL-6: 100 pg/mL to 20 pg/mL; TNF-α: 150 pg/mL to 15 pg/mL; IFN-γ: 41 pg/mL to 11 pg/mL). Brepocitinib shifted the BAX/BCL-2 ratio fourfold in favor of apoptosis and increased cleaved caspase-3 levels to ~80% of maximal response. Functionally, it reduced wound closure from ~75% in controls to ~20% at 5 µM, confirming potent inhibition of synoviocyte migration.

Conclusions: Brepocitinib exerts multi-faceted effects on RA synoviocytes by simultaneously inhibiting inflammatory signaling, suppressing cytokine expression, restoring apoptotic sensitivity, and reducing migratory potential. These findings provide mechanistic support for brepocitinib as a targeted therapeutic agent in RA.