Design and synthesis of highly selective Janus kinase 3 covalent inhibitors for the treatment of rheumatoid arthritis

Selective inhibition of Janus kinase 3 (JAK3) is a promising strategy for the treatment of autoimmune diseases. Based on the discovery of a hydrophobic pocket unutilized between the lead compound RB1 and the JAK3 protein, a series of covalent JAK3 inhibitors were prepared by introducing various aromatic fragments to RB1 . Among them, J1b (JAK3 IC 50 = 7.2 nM, other JAKs IC 50 > 1000 nM) stood out because of its low toxicity (MTD > 2 g/kg) and superior anti‐inflammatory activity in Institute of Cancer Research mice. Moreover, the acceptable bioavailability (F% = 31.69%) ensured that J1b displayed excellent immune regulation in collagen‐induced arthritis mice, whose joints in the high‐dose group were almost recovered to a normal state. Given its clear kinase selectivity (Bmx IC 50 = 539.9 nM, other Cys909 kinases IC 50 > 1000 nM), J1b was nominated as a highly selective JAK3 covalent inhibitor, which could be used to safely treat arthritis and other autoimmune diseases.