In vitro activity: GSK583 possesses comparable binding affinity for RIP3 kinase as demonstrated by an in-house FP binding assay configured similarly to the RIP2 FP assay (RIP2 FP IC50 = 5 nM; RIP3 FP IC50 = 16 nM). Despite this potent biochemical activity against RIP3 kinase, GSK583 shows little or no inhibition of RIP3-dependent necroptotic cell death in a cellular assay up to 10 μM concentration. GSK583 potently and dose dependently inhibits MDP-stimulated tumor necrosis factor-alpha (TNFα) production with an IC50 = 8 nM in primary human monocytes. Following treatment with GSK583 at 1 μM, little inhibition of pro-inflammatory signaling is observed upon activation of Toll-like receptors (TLR2, TLR4, TLR7) or cytokine receptors (IL-1R, TNFR) but complete inhibition is observed upon activation of both NOD1 and NOD2 receptors, which signal in a RIP2-dependent manner. Although GSK583 has excellent kinase selectivity, it does inhibit both the hERG channel and Cyp3A4, which precludes it from further progression as a drug candidate.

Kinase Assay: A fluorescent polarization based binding assay is developed to quantitate interaction of novel test compounds at the ATP binding pocket of RIP2K by competition with a fluorescently labeled ATP competitive ligand. Full length FLAG His tagged RIP2K is purified from a baculovirus expression system and is used at a final assay concentration of twice the KD apparent. A fluorescent labeled ligand that is reversible and competitive with the inhibitors is used at a final assay concentration of 5 nM. Both the enzyme and ligand are prepared in solutions in 50 mM HEPES pH 7.5, 150 mM NaCl, 10 mM MgCl2, 1 mM DTT, and 1 mM CHAPS. Test compounds are prepared in 100% DMSO, and 100 nL is dispensed to individual wells of a multiwell plate. Next, 5 μL of RIP2K is added to the test compounds at twice the final assay concentration and incubated at room temperature for 10 min. Following the incubation, 5 μL of the fluorescent labeled ligand solution is added to each reaction at twice the final assay concentration and incubated at room temperature for at least 10 min. Finally, samples are read on an instrument capable of measuring fluorescent polarization. Test compound inhibition is expressed as percent (%) inhibition of internal assay controls. For concentration response experiments, normalized data are fit using the following four parameter logistic equation: y = A + ((B-C))/(1+(10x)/(10C)D), where y is the % activity (% inhibition) at a specified compound concentration, A is the minimum % activity, B is the maximum % activity, C = log10(IC50), D = Hill slope, x = log10(compound concentration [M]), and pIC50 = (–C).

Cell Assay: To assess cellular selectivity, monocytes are pretreated with inhibitor for 30 min, then stimulated for 6 h with ligands which selectively agonize NLRs NOD1, NOD2; Toll-like receptors TLR, TLR4, TLR7, or cytokine receptors IL-1R, TNFR. Release of pro-inflammatory cytokines, either TNFα (NOD2, TLR2, TLR4, IL1R) or IL-8 (NOD1, TLR7, TNFR), is measured by immunoassay. Percent inhibition and/or IC50 values are calculated.

J Med Chem. 2016 May 26;59(10):4867-80.