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Cathepsin C (CatC) is a dipeptidyl-exopeptidase which activates neutrophil serine protease precursors (elastase, proteinase 3, cathepsin G and NSP4) by removing their N-terminal propeptide in bone marrow cells at the promyelocytic stage of neutrophil differentiation. The resulting active proteases are implicated in chronic inflammatory and autoimmune diseases. Hence, inhibition of CatC represents a therapeutic strategy to suppress excessive protease activities in various neutrophil mediated diseases. We designed and synthesized a series of dipeptidyl cyclopropyl nitrile compounds as putative CatC inhibitors. One compound, IcatC XPZ-01 ((S)-2-amino-N-((1 R ,2 R)-1-cyano-2-(4′-(4-methylpiperazin-1-ylsulfonyl)biphenyl-4-yl)cyclopropyl)butanamide)) was identified as a potent inhibitor of both human and rodent CatC. In mice, pharmacokinetic studies revealed that IcatC XPZ-01 accumulated in the bone marrow reaching levels suitable for CatC inhibition. Subcutaneous administration of IcatC XPZ-01 in a monoclonal anti-collagen antibody induced mouse model of rheumatoid arthritis resulted in statistically significant anti-arthritic activity with persistent decrease in arthritis scores and paw thickness. [ABSTRACT FROM AUTHOR]

The function of neutrophil protease 3 (PR3) is poorly understood despite of its role in autoimmune vasculitides and its possible involvement in cell apoptosis. This makes it different from its structural homologue neutrophil elastase (HNE). Endogenous inhibitors of human neutrophil serine proteases preferentially inhibit HNE and to a lesser extent, PR3. We constructed a single-residue mutant PR3 (I217R) to investigate the S4 subsite preferences of PR3 andHNEand used the best peptide substrate sequences to develop selective phosphonate inhibitors with the structure Ac-peptidylP(O-C6H4-4-Cl)2. The combination of a prolyl residue at P4 and an aspartyl residue at P2 was totally selective for PR3. We then synthesized N-terminally biotinylated peptidyl phosphonates to identify the PR3 in complex biological samples. These inhibitors resisted proteolytic degradation and rapidly inactivated PR3 in biological fluids such as inflammatory lung secretions and the urine of patients with bladder cancer. One of these inhibitors revealed intracellular PR3 in permeabilized neutrophils and on the surface of activated cells. They hardly inhibited PR3 bound to the surface of stimulated neutrophils despite their low molecular mass, suggesting that the conformation and reactivity of membrane-bound PR3 is altered. This finding is relevant for autoantibody binding and the subsequent activation of neutrophils in granulomatosis with polyangiitis (formerly Wegener disease). These are the first inhibitors that can be used as probes to monitor, detect, and control PR3 activity in a variety of inflammatory diseases. [ABSTRACT FROM AUTHOR]