Filgotinib (GLPG0634) is a selective JAK1 inhibitor. The IC50 values against JAK1, JAK2, JAK3, and TYK2 are 10 nM, 28 nM, 810 nM, and 116 nM, respectively.

JAK3:810 nM, JAK2:28 nM, TYK2:116 nM, JAK1:10 nM

Recombinant JAK1, TYK2, JAK2, and JAK3 are used to develop activity assays in 50 mM HEPES (pH 7.5), 1 mM EGTA, 10 mM MgCl2, 2 mM DTT, and 0.01% Tween 20. The amount of JAK protein is determined per aliquot, maintaining initial velocity and linearity over time. The ATP concentration is equivalent to 4× the experimental?Km?value and the substrate concentration (ULight-conjugated JAK-1(Tyr1023) peptide) corresponds to the experimentally determined?Km?value. After 90 min incubation at room temperature (RT), the amount of phosphorylated substrate is measured by addition of 2 nM europium-anti-phosphotyrosine Ab and 10 mM EDTA in Lance detection buffer. Compound IC50?values are determined by preincubating the enzyme with compound at RT for 60 min, prior to the addition of ATP.