Tofacitinib-d₃(citrate) is intended for use as an internal standard for the quantification of tofacitinib by GC- or LC-MS. Tofacitinib is a potent, cell-permeable inhibitor of all JAK isoforms (IC₅₀s = 6.1, 12, and 8 nM for JAK1, JAK2, and JAK3, respectively).¹It is selective for JAK1-3 over ROCK-2 and Lck (IC₅₀s = 3,400 and 3,870 nM, respectively) as well as 28 additional kinases in enzyme assays (IC₅₀s = >10,000 nM). It inhibits IL-2-mediated phosphorylation of JAK3 and STAT5 when used at a concentration of 30 ng/ml.²Tofacitinib prevents rejection and prolongs survival in murine and cynomolgus monkey models of heterotopic heart and kidney transplantation, respectively. Formulations containing tofacitinib have been used in the prevention of organ allograft rejection as well as in the treatment of the inflammatory or autoimmune components of a host of diseases, including rheumatoid arthritis and ulcerative colitis.^2,3,4,5

1.Haan, C., Rolvering, C., Raulf, F., et al.Jak1 has a dominant role over Jak3 in signal transduction through γc-containing cytokine receptorsChem. Biol.18(3)314-323(2011) 2.Changelian, P.S., Flanagan, M.E., Ball, D.J., et al.Prevention of organ allograft rejection by a specific Janus kinase 3 inhibitorScience302(5646)875-878(2003) 3.Flanagan, M.E., Blumenkopf, T.A., Brissette, W.H., et al.Discovery of CP-690,550: A potent and selective Janus kinase (JAK) inhibitor for the treatment of autoimmune diseases and organ transplant rejectionJ. Med. Chem.53(24)8468-8484(2010) 4.Cutolo, M.The kinase inhibitor tofacitinib in patients with rheumatoid arthritis: Latest findings and clinical potentialTher. Adv. Musculoskelet. Dis.5(1)3-11(2013) 5.Sandborn, W.J., Ghosh, S., Panes, J., et al.Tofacitinib, an oral Janus kinase inhibitor, in active ulcerative colitisN. Engl. J. Med.367(7)616-624(2012)