Background: Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease characterized by joint inflammation and damage. The aim of this study was to evaluate the therapeutic effects of Loxo-305, a novel treatment, on RA in a mouse model.

Methods: Through intravenous injection of collagen-Complete Freund's Adjuvant (CFA) emulsion (100 μL), Dilute Brown Non-Agouti 1 (DBA/1) mice were induced with RA. Two weeks after immunization, the severity of joint swelling was assessed. Mice in the low-dose and high-dose Loxo-305 groups were treated with 10 mg/kg/day and 20 mg/kg/day of Loxo-305, respectively, for 2 weeks, while animals in the control and model groups were administered saline. The mice were euthanized after the treatment period, and their peripheral blood, spleen, and joint tissues were collected for analysis. Histological analysis of joint tissues was performed using hematoxylin and eosin (HE) staining, and osteoclastogenesis was evaluated using tartrate-resistant acid phosphatase (TRAP) staining. The expression of Cluster of Differentiation 19 (CD19), Receptor Activator of Nuclear Factor Kappa-B Ligand (RANKL), and Cluster of Differentiation 11b (CD11b) in tissues was detected by means of immunohistochemistry (IHC). Spleen cells were isolated and stimulated with anti-immunoglobulin M (IgM) antibodies to induce B cell differentiation. Cell proliferation was assessed using 5-ethynyl-2′-deoxyuridine staining (EdU) staining, and B cell activation was analyzed by flow cytometry. Serum immunoglobulin and inflammatory cytokine levels were measured through enzyme-linked immunosorbent assay (ELISA), and quantitative real-time polymerase chain reaction (qRT-PCR) was used to quantify the expression of osteoclast-related genes in joint tissues.

Results: Loxo-305 effectively inhibited the progression of RA in mice, significantly reducing paw swelling, synovial hyperplasia, inflammatory cell infiltration, cartilage erosion, and bone destruction in a dose-dependent manner (p < 0.05). It attenuated osteoclast formation, inhibited B cell activation, and reduced plasma cell numbers and serum immunoglobulin G (IgG) levels (p < 0.05). Loxo-305 also decreased the expression of TRAP, Dendritic Cell-Specific Transmembrane Protein (DC-STAMP), Cathepsin K (CTSK), and RANKL while increasing Osteoprotegerin (OPG) levels in joint tissues (p < 0.05) and significantly suppressed joint inflammation by reducing interleukin (IL)-6, tumor necrosis factor alpha (TNF-α), IL-1β, and CD11b expression (p < 0.05).

Conclusions: Loxo-305 effectively inhibits the progression of RA by suppressing osteoclastogenesis, B cell and plasma cell activation, and reducing the expression of inflammatory cytokines and RANKL, demonstrating significant therapeutic potential. Loxo-305 effectively controls the progression of RA through a dual mechanism of “immune regulation and bone protection”. Compared to existing Bruton's tyrosine kinase (BTK) inhibitors, it offers superior target specificity, safety, and breadth of action, highlighting its potential and promising clinical translational value in the treatment of RA.