Identification of Marrubiin as a Cathepsin C Inhibitor for Treating Rheumatoid Arthritis

Cathepsin C (CTSC) mediates neutrophil serine protease (NSP) maturation, contributing to inflammatory cascades, making it a key therapeutic target. In this study, through large-scale screening of a natural product library, marrubiin, a diterpenoid lactone compound, was identified as a potent CTSC inhibitor, which holds potential value in the treatment of inflammatory diseases. It inhibited human recombinant CTSC (IC₅₀ = 57.5 nM) and intracellular CTSC (IC₅₀ = 51.6 nM) with acceptable cytotoxicity, and reduced the activity and protein levels of downstream NSPs in vitro. Functionally, marrubiin inhibited lipopolysaccharide-induced nitric oxide release and regulated the levels of cytokines and chemokines. Docking result predicted marrubiin may achieve CTSC activity inhibition by using lactone structure as a covalent unit to target Cys234. In vivo study indicated that high-dose marrubiin (IC₅₀ = 30 mg/kg) reduced CTSC and NSPs activities in blood and bone marrow in mice without toxicity, and its efficacy was comparable to that of positive compound AZD7986. In the adjuvant-induced arthritis model, high-dose marrubiin (IC₅₀ = 60 mg/kg) exerted a therapeutic effect by reducing the activities of CTSC and NSPs. These findings indicated marrubiin is a promising natural CTSC inhibitor, which can be used for the treatment of neutrophil-related inflammatory diseases.