Lck Inhibitor is a potent and selective inhibitor of lymphocyte-specific kinase (Lck) with IC50 values of 7 nM, 21 nM, 42 nM, 200 nM for Lck, Lyn, Src and Syk kinases, respectively It suppresses T-cell activation, inhibits T-cell proliferation, and demonstrates therapeutic potential in T-cell-mediated arthritis.