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. 2015 Sep 30;5:14538. doi: 10.1038/srep14538

Figure 8. Mutagenesis of residues in the substrate-binding pocket confers moderate resistance to fedratinib.

Figure 8

(a) Surface depiction of JAK2 kinase in active conformation, showing the activation loop (green) and binding of fedratinib (yellow) in the substrate-binding pocket. (b) Substrate-binding pocket is comprised of an activation loop (green) and helices F (blue), FG (purple) and G (green). Amino-acid residues IFWY in the activation loop is a highly conserved kinase motif that serves as a hydrophobic platform for inhibitor and substrate-binding (green surface sandwiched between inhibitor and helix F). (c) Surface depiction of the IFWY motif (upper panel); an Ile 1018-to-tryptophan substitution results in steric clash with fedratinib (lower panel). (d) Surface depiction of Leu 1026 (upper panel); a Leu 1026-to-phenylalanine substitution results in steric clash with the benzene-sulfonamide group of fedratinib. (e) Dose-response cell proliferations showing moderate increase in IC50 values by JAK2 variants JAK2V617F-Y931C/I1018W and JAK2V617F-Y931C/L1026F; on the other hand, mutations at F1019L and W1020C resulted in inactive kinase (i.e., no change in IC50 values). (f) Mutations identified from drug resistant screen at low dose of fedratinib (2.5 μM) using BaF3 cells expressing JAK2-V617F/Y931C. The α-carbon of each resistant mutation is shown as a circle. (g) Surface depiction of Tyr 1045 showing the π-stacking with residues Val 1042 (helix-F), Leu1086 (helix H), Phe1061 (helix FG), and the IFWY activation-loop motif. A bulkier substitution at Y1045 would push the IFWY motif towards the inhibitor, thereby possibly conferring resistance. (h) A bulkier substitution at Pro 1058 would push the Phe 1019 of IFWY, which might affect fedratinib binding. (i) A bulkier substitution at Val1075 (e.g., to phenylalanine) would cause steric hindrance to fedratinib. (j) Dose-response growth curves showing higher IC50 values for S1025C, Y1045W, F1061W, and V1075F. Variants W1038C, S1039F and Y1045* encode weekend kinase (Fig. S14), but displayed subtle increases in IC50. (k) Immunoblot analysis of phospho STAT5 (upper panel) and total STAT5 (lower panel) in BaF3 cells expressing JAK2-V617/Y931C variants. Variants S1025C, Y1045W and V1075F are resistant to 5 μM fedratinib, and also conferred highest resistance in cell-proliferation assays.