A NOVEL INHIBITOR OF THE IMMUNOPROTEASOME INHIBITS IL-23 PRODUCTION IN VITRO AND ELICITS AN ANTI-ARTHRITIC EFFECT IN MULTIPLE MOUSE MODELS OF RHEUMATOID ARTHRITIS

Background: The immunoproteasome is a distinct class of proteasome found predominantly in monocytes, lymphocytes and cells exposed to inflammatory cytokines. Diseased tissues of several autoimmune disorders show overexpression of genes encoding the catalytic subunits of the immunoproteasome, LMP7, LMP2 and MECL1. Non-selective inhibitors of the proteasome have been shown to inhibit cytokine production in monocytes and lymphocytes and to inhibit inflammation in vivo. However, a specific role for the immunoproteasome in inflammatory pathways and the progression of rheumatoid arthritis (RA) has not been established.Objectives: To determine whether PR-957, a peptide epoxyketone that selectively inhibits the LMP7 subunit of the immunoproteasome, is capable of suppressing inflammatory cytokine production in vitro and disease progression in mouse models of rheumatoid arthritis (RA).Methods: Cytokine release from human PBMC was measured following stimulation with either lipopolysaccharide (LPS) or antibodies to CD3 and CD28. Pharmacodynamic activity was determined in mice one hour after administration of PR-957 by measuring proteasome active sites in lysates of blood and tissues. The therapeutic effect of PR-957 treatment was determined in two mouse models of RA: collagen antibody-induced arthritis (CAIA) in Balb/c mice and collagen-induced arthritis (CIA) in DBA/1 mice.Results: In PBMC, PR-957 concentrations resulting in selective and complete suppression of LMP7 inhibited the production of IL-23 (a key cytokine in RA pathogenesis) following LPS stimulation and IFN-γ following anti-CD3/CD28 stimulation. Concentrations of PR-957 resulting in inhibition of LMP7 and LMP2 blocked LPS-induced TNF-α and IL-6 production. In vivo, PR-957 administration selectively inhibited LMP7 activity in blood and tissues and inhibited clinical and histologic progression of disease in established CAIA. The minimally effective dose of PR-957 in this model was 10 – 15 fold lower than the maximum tolerated dose (MTD) in normal mice. In mice with established CIA, PR-957 treatment significantly inhibited the progression of disease, and the production of both anti-collagen antibodies and markers of bone remodeling. PR-957 therapy in mouse models of RA was associated with decreased expression of inflammatory cytokine genes in the paws of treated animals. Notably, the therapeutic response to PR-957 in these models was comparable to animals treated with etanercept.Conclusion: These results demonstrate that selective inhibition of the immunoproteasome with PR-957 can suppress the release of pro-inflammatory cytokines in vitro and can elicit an anti-arthritic response in vivo. Importantly, these outcomes are achieved at well tolerated doses, providing validation for the clinical development of PR-957 for treatment of autoimmune disorders.Citation: Ann Rheum Dis, volume 67, supplement II, year 2008, page 77Session: Abstract Session: Cytokines and inflammatory mediators