Ritlecitinib tosylate, a tosylate compound of ritlecitinib, is a potent and selective JAK3 inhibitor. It is an effective and low clearance compound that has been proven to have in vivo effects. This JAK3 specific inhibitor has been evaluated in multiple human clinical studies due to its good efficacy and safety. JAK3 was among the first of the JAKs targeted for therapeutic intervention due to the strong validation provided by human SCID patients displaying JAK3 deficiencies.