Background In the Phase 1/2 clinical study, E6011, a novel humanized anti-fractalkine (FKN) monoclonal antibody (mAb) demonstrated a promising efficacy in active rheumatoid arthritis (RA) patients who were inadequately controlled by MTX and/or TNF-a inhibitors. However, the effect of anti-FKN mAb on joint destruction remains to be elucidated. In RA, synovium-infiltrated monocytes/macrophages cause synovitis and cartilage damage. Osteoclasts are generated from osteoclast precursor cells (OCPs) and cause bone erosion. FKN is expressed on endothelial cells and fibroblast-like synoviocytes in synovium in both experimental arthritis model and RA patients. FKN is also expressed on osteoblasts in neonatal mouse calvariae. CX3CR1, the receptor for FKN, is expressed on monocytes/macrophages and OCPs. Therefore, the interaction of FKN and CX3CR1 might play important roles in migration, differentiation and activation of these cells, leading to cartilage damage and bone erosion.

Objectives We examined the efficacy of an anti-FKN mAb on collagen-induced arthritis (CIA) in mice, especially on joint destruction.

Methods For the induction of CIA, DBA/1J mice were immunized with bovine type II collagen. Anti-FKN mAb or control IgG was injected twice a week from the day of the 1st immunization (for the prophylactic treatment) or after the onset of CIA (for the therapeutic treatment). The clinical arthritis score was defined as the sum of the scores of four paws. Plasma concentrations of IL-6, TNF-a, serum amyloid A (SAA), Tartrate-Resistant Acid Phosphatase type 5b (TRAP-5b), Cartilage Oligomeric Matrix Protein (COMP) and Matrix Metalloproteinase 3 (MMP-3) were measured using ELISA. Radiological score was measured by the soft x-ray images of limb bones. Alcian blue/Alizarin red and Tartrate-Resistant Acid Phosphatase (TRAP) staining were performed for histopathological analysis of ankle joints.

Results In the prophylactic treatment, anti-FKN mAb markedly reduced the clinical arthritis score, soft x-ray score, and plasma levels of TRAP-5b, COMP and MMP-3, whereas plasma levels of IL-6, TNF-a and SAA were not significantly reduced compared with control IgG-treated mice. Histopathological analysis demonstrated almost complete suppression of joint destruction and dramatic reduction in the number of TRAP-positive cells by the treatment of anti-FKN mAb. Importantly, therapeutic treatment of anti-FKN mAb also significantly ameliorated clinical arthritis score and soft x-ray score. Synovitis, pannus formation, cartilage degradation and bone erosion were also strongly suppressed and the number of TRAP-positive cells in joint was also decreased by the therapeutic treated of anti-FKN mAb.

Conclusions Anti-FKN mAb demonstrated a remarkable efficacy in the arthritis score without affecting systemic inflammatory parameters and inhibited the joint destruction with the marked reduction of osteoclasts in CIA model. These results suggest that inhibition of FKN/CX3CR1 axis by a humanized anti-FKN mAb, E6011, is an attractive therapeutic strategy for the treatment of both inflammatory synovitis and joint destruction of RA.

Disclosure of Interest None declared