Cysteine protease activity is up‐regulated in inflamed ankle joints of rats with adjuvant‐induced arthritis and decreases with in vivo administration of a vinyl sulfone cysteine protease inhibitor

Objective Cysteine proteases are postulated to play a role in tissue destruction in the joints of animals with arthritis. The purpose of the present study was to confirm the concept that cysteine proteases are enzymes involved in the pathology of rheumatoid arthritis (RA). Methods Arthritis was induced in Lewis rats by adjuvant injection (adjuvant-induced arthritis [AIA] model) and scored for inflammation. At necropsy, the rear paws were either fixed in formalin and assigned a histologic score (based on synovial cell proliferation, cartilage erosion, bone erosion, and fibroproliferative pannus) or frozen, cryosectioned, and assayed for enzyme activity either by in situ cytochemical staining with a post-azo-coupling method using a chromogenic substrate (Z-arg-arg-MNA) or by a novel assay placing the tissue section directly in a cuvette using the fluorogenic substrate Z-arg-arg-AMC. Results Enzymatic activity, measured either in frozen sections in situ or in the cuvette assay, was positively correlated with joint destruction (r = 0.7) and inflammation (r = 0.8). Activity was not inhibited significantly by Pefabloc (a serine protease inhibitor), EDTA (a metalloprotease inhibitor), or pepstatin A (an aspartyl protease inhibitor) but was inhibited by E-64 and vinyl sulfone irreversible inhibitors of cysteine proteases. The effect of one of the vinyl sulfone cysteine protease inhibitors, Mu-Leu-HomoPhe-vinylsulfone, was tested in vivo by dietary administration at 2.2 mg/kg/day in the AIA model; this resulted in a significant decrease in inflammation and in the amount of cysteine protease activity measured in the joint tissue. Conclusion Cysteine protease activity levels increase in the diseased state and may be an important target for designing small molecule inhibitors to reduce the inflammation and tissue destruction associated with RA.