Characterisation of the antinociceptive effect of baricitinib in the collagen antibody-induced arthritis mouse model

Many rheumatoid arthritis (RA) patients continue to experience persistent pain even after successful management of joint inflammation. Clinical data indicate that RA patients treated with the JAK inhibitor baricitinib consistently achieve pain relief that cannot be entirely attributed to its anti-inflammatory effects. In this study, we investigated the antinociceptive properties of baricitinib using the collagen antibody-induced arthritis (CAIA) model in which mechanical hypersensitivity persists long after resolution of joint inflammation. The effects of baricitinib, etanercept (tumour necrosis factor inhibitor), and LP-922761 (adaptor protein-2 (AP2) associated kinase 1 (AAK1) inhibitor) on pain-like behaviour in CAIA mice were examined. Tissue samples from the late, low-grade inflammatory phase were examined for the effect of the treatments. Additionally, in vitro experiments using dorsal root ganglion (DRG) cells were conducted to assess baricitinib's influence on neuronal excitability and cell morphology. Baricitinib reduced CAIA-induced joint inflammation, but its antinociceptive effects were most pronounced during the late phase when etanercept was ineffective. Administering baricitinib both early and late significantly decreased CAIA-induced bone loss, synovial innervation, and baseline STAT3 phosphorylation in ankle joints and DRGs. Unlike etanercept, baricitinib effectively reduced pain-like behaviour and synovial hyperinnervation when administered exclusively in the late phase. Additionally, baricitinib modulated glial cell morphology and neuronal excitability in vitro. Notably, it inhibited AAK1 signalling in DRGs, with AAK1 kinase activity blockade providing an antinociceptive effect in the CAIA model. Our data suggests that baricitinib has antinociceptive effects by targeting not only immune cells but also neurons and glia cells via inhibition of 2 signalling pathways linked to chronic pain.