In vitro activity: GS-9973 shows good bidirectional permeability across Caco-2 cell monolayers in vitro. In cells, GS-9973 also shows excellent selectivity for Syk, and potently inhibits BCR-mediated activation and proliferation of B-cells as well as immune-complex-stimulated cytokine production in monocytes. The combination of idelalisib and GS-9973 synergistically inhibits CLL cell viability and further disrupts chemokine signaling.

Kinase Assay: Full-length baculovirus-expressed Syk kinase activity is measured in a Lance-based assay format in a final volume of 25 μL containing 25 mM Tris–HCl, pH 7.5, 5 mM β-glycerophosphate, 2 mM DTT, 0.1 mM Na3VO4, 10 mM MgCl2, 0.5 μM Promega PTK biotinylated peptide substrate 1, 0.01% casein, 0.01% Triton X-100, 0.25% glycerol, and 40 mM ATP (Km for ATP) incubated at room temperature for 60 min. Reactions are stopped with the addition of 30 mM EDTA containing 30 μL of SA-APC and 4 nM PT-66 antibody and the plates measured on a Perkin-Elmer Envision. IC50 values for test compounds are determined using a four-parameter linear regression algorithm.

Cell Assay: Functional impact on cellular Flt3 activity is determined by measuring compound inhibition of MV-4-11 cell proliferation. A total of 104 cells are diluted in RPMI medium containing 10% FBS in 96-well flat-bottomed tissue culture plates and incubated with compound dilutions for 72 h at 37 °C. Alamar blue (10%) is added to the cells, which were incubated for an additional 12-18 h at 37 °C, and inhibition of the relative cell numbers is determined by spectrophotometer readings at 570/600 nm.