A JAK inhibitor (IC50s = 3.9, 5, and 0.71 nM for JAK1-3, respectively); 14-fold selective for JAK1 and JAK3 over JAK2 in erythropoietin-induced leukemia cell proliferation assays; inhibits IL-2-induced proliferation of rat splenocytes (IC50 = 10 nM) and phosphorylation of STAT5 in rat and human whole blood (IC50s = 124 and 127 nM, respectively); reduces paw swelling (ED50 = 5.6 mg/kg) and bone destruction in a rat model of adjuvant-induced arthritis at 30 mg/kg