Current therapeutic approaches to rheumatoid arthritis (RA) (corticosteroids, methotrexate, neutralization of TNF-?) rely upon broad-spectrum immunosuppression, an approach that is not uniformly effective and is frequently associated with significant side-effects. To address this market gap, Radikal Therapeutics (RTX) is developing a novel soluble receptor decoy receptor fusion protein (hR-421) that induces immune tolerance in antigen (Ag)-specific activated T-cells. Constructed from Ig-Fc and the second ecto domain of the CCR5 receptor, hR-421 binds and neutralizes al 3 CCR5-binding chemokine ligands (MIP-?, MIP-?, RANTES) and prevents their binding and activation of the CCR1, CCR3, and CCR5 membrane receptors. In contrast to CCR5 inhibitors, mR-421 (the murine homologue of hR-421) blocks CCR5-independent pro-inflammatory pathways and is effective in CCR5 knock-out mice. Therapy with mR- 421 profoundly suppresses experimental allergic encephalomyelitis (EAE), a classic autoimmune model system, even when treatment is initiated after disease onset. Moreover, Ag-specific effector Th1 cells isolated from EAE donors treated in vivo with mR-421 produce substantially less pro-inflammatory cytokines and suppress EAE in adoptive transfer experiments. This latter effect is driven by the ability of mR-421 to prevent the co-stimulatory signal on Th1 CD4+ T cells. Based on these findings, we hypothesize that R-421 will prevent co-stimulation and thus the downstream proliferation and activation of Th1 CD4+ cells in RA. We now propose to extend the protective effects of the R-421 technology beyond the published findings in neuroinflammation and establish its potential benefit in a clinically-relevant model of RA. Male DBA mice inoculated with collagen in CFA and rechallenged on Day 21 to induce autoimmune arthritis, will be treated with mR-421 (2, 4, 8 mg/kg), an irrelevant IgG1 control, or etanercept (6.25 mg/kg) beginning at the time of significant disease (Day 21), as characterized by an arthritic score of 2 (scale of 0-16). Additionally, we will include a sham control group not exposed to collagen/CFA or treatment. Animals will be monitored for clinical evidence of arthritis over a period of 6 weeks, a timepoint characterized in controls by erythema and edema of the entire paw including digits (score=12). Plasma m-R421 concentrations will be related to clinical outcome, so as to construct a pharmacodynamic profile that will guide future clinical dosing. We expect mR-421 to reduce joint injury, in a dose and plasma concentration dependent fashion, as defined by a reduction in the mean arthritic score that is equivalent or superior to the treatment effect elicited by the gold-standard clinically-approved agent etanercept. These findings are expected to parallel the effects of mR-421 on synovial lipid peroxidation, neutrophil infiltration, pro-inflammatory gene expression peroxynitrite production, poly(ADP-ribose)polymerase formation, and gross and histologic injury scores, and on draining lymph node populations of Th1 and Treg lymphocytes. PUBLIC HEALTH RELEVANCE: Rheumatoid arthritis is a devastating autoimmune disease characterized by lifelong disability and inexorable progression. Currently approved agents are partially effective and nearly one-third of patients do not obtain adequate relief. We are developing a novel drug that specifically blocks the specific immune disturbance that underlies this condition. We will test this agent in a clinically-relevant small animal model and determine the most effective dose, in anticipation of definitive large animal studies to confirm its efficacy and safety.