Filgotinib-d4 is intended for use as an internal standard for the quantification of filgotinib by GC- or LC-MS. Filgotinib is a JAK1 inhibitor (IC50 = 10 nM).1 It is selective for JAK1 over JAK3 (IC50 = 810 nM) but also inhibits JAK2 and tyrosine kinase 2 (Tyk2; IC50s = 28 and 116 nM, respectively), as well as Abl, FLT1, -3 and -4, FMS, Mer, and TBK1 activity by greater than 35% in a panel of 177 tyrosine kinases at 1 μM. Filgotinib inhibits IL-6-induced phosphorylation of STAT1 in CD4+ T cells with an IC50 value of 629 nM in isolated human whole blood. It reduces hind paw macrophage and T cell infiltration and bone erosion in a rat model of collagen-induced arthritis when administered at doses ranging from 0.1 to 30 mg/kg per day for 15 days.

|1. Van Rompaey, L., Galien, R., van der Aar, E.M., et al. Preclinical characterization of GLPG0634, a selective inhibitor of JAK1, for the treatment of inflammatory diseases. J. Immunol. 191(7), 3568-3577 (2013).