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

## TL;DR

Brepocitinib, a drug for rheumatoid arthritis, reduces inflammation and cell migration in joint cells while keeping them alive.

## Contribution

This study reveals how brepocitinib affects synoviocytes at the molecular level, offering new insights into its therapeutic mechanism.

## Key findings

- Brepocitinib significantly reduces cytokine production and inhibits cell migration in synoviocytes.
- The drug restores apoptotic sensitivity by altering the BAX/BCL-2 ratio and increasing caspase-3 levels.
- Phosphorylation of JAK1 and STAT3 is suppressed by over 80% at high doses of brepocitinib.

## Abstract

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.

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

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.

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.

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.