In vitro activity: Tofacitinib (also known as tasocitinib or CP-690550, sold under the brand Xeljanz), is a novel and potent inhibitor of JAK3 (Janus-Associated kinase) with IC50 of 1 nM in cell-free assays. It is an FDA approved drug for the treatment of rheumatoid arthritis, psoriatic arthritis, and ulcerative colitis. The inhibition is JAK3 specific with a selectivity 1000-fold more than other non-JAK family kinases. Besides inhibiting JAK3 (with IC50 of 1 nM), tofacitinib also inhibits JAK2 and JAK1 with 20- and 100-fold less in potency respectively. However, in a recent study, the binding affinities (Ki) of tofacitinib towards JAK1, JAK2, and JAK3 were reported to be 1.6 nM, 21.7 nM, and 6.5 nM respectively. CP-690550 is a specific, orally inhibitor of JAK3, it is 20- to 100-fold less potent for JAK2 and JAK1 with IC50 of 20 nM and 112 nM, respectively. CP-690550 doesn't have potent activity against 30 other kinases (all median IC50> 3000 nM). CP-690,550 inhibits IL-2–induced proliferation with 30-fold greater potency than its effects on GM-CSF–induced proliferation. CP-690550 effectively inhibits a murine mixed lymphocyte reaction (MLR) (IC50 = 91 nM). CP-690550 potently inhibits IL-4 induced upregulation of CD23 (IC50=57 nM) and class II major histocompatibility complex (MHCII) expression (IC50=71 nM) on murine B cells. A recent research indicates low dose of CP-690550 accelerates the onset of experimental autoimmune encephalomyelitis by potentiating Th17 differentiation

Kinase Assay: A fragment encoding the catalytic domain of human JAK3 (785aa to 1125aa, JH1 catalytic domain) is amplified by PCR from the full length cDNA and cloned into the EcoRI site of the baculovirus transfer vector pVL1393. Recombinant baculovirus is used to infect Sf9 (Spodoptera frugipedra) cells and recombinant GSTJAK3 fusion protein is isolated on glutathione sepharose. The fusion protein is eluted with reduced glutathione and stored in buffer containing 50 mM Tris, pH 7.5, 10 mM DTT and 10% glycerol. JAK3 kinase activity is measured by ELISA as follows: Plates are coated overnight with a random L-glutamic acid and tyrosine co-polymer (4:1) (100 ug/mL). The plates are washed and recombinant JAK3 JH1:GST (100 ng/well) with or without inhibitors is incubated at room temperature for 30 minutes, after which HRP-conjugated PY20 anti-phosphotyrosine antibody (ICN) is added and developed by TMB (3,3',5,5'-tetramethylbenzidine). Other kinases (Table 1) are produced in E. coli or in insect cells, depending upon what is found to be optimal for the given kinase. The catalytic activity of tyrosine kinases is easured using the aftorementioned ELISA, whereas serine/threonine kinases are assayed using radioactive enzyme assays.

Cell Assay: To measure IL-2-dependent proliferation, isolated lymphocytes are resuspended to a cell density of 1-2 × 106/mL in complete RPMI medium (RPMI 1640 containing 10% (w/v) fetal calf serum (FCS), 1%(w/v) penicillin and treptomycin).Phytohemagluttinin (PHA) is added to a final concentration of 10mg/mL, and the culture incubated for 3 days at 37 °C in a humidified 5% (v/v) CO2 incubator to upregulate IL-2R and JAK3 expression. IL-2 (200U/mL), with or without CP-690,550 is then added and the cells are incubated for 72 hours at 37 °C in a humidified 5% (v/v) CO2 incubator, after which 50 mL of 3H-thymidine (5mCi/mL) is added. The plates are incubated for an additional 18 hours, harvested with a 96-well harvester, and counted on a scintillation counter. HUO3 cells are maintained in culture with granulocyte-macrophage colony stimulating factor and human foreskin fibroblasts are maintained in culture with 10% fetal calf serum. CP-690550 is added to freshly plated cells and cultured for 4 days. 3Hthymidine is added during the last 18 hours of the culture period.