Potent, selective inhibitor of Janus tyrosine kinase 3 (JAK3) which binds competitively to the JAK3 ATP site. pIC50 values are 6.8, 5.0, 4.7, and < 5.0 for JAK3, EGFR, JAK1 and CDK4 respectively. Inhibits STAT-5 phosphorylation and T-cell proliferation.