Synthesis and Discovery of Pyrazine−Pyridine Biheteroaryl as a Novel Series of Potent Vascular Endothelial Growth Factor Receptor-2 Inhibitors

Pathological angiogenesis is associated with disease states such as cancer, diabetic retinopathy, rheumatoid arthritis, endometriosis, and psoriasis. There is much evidence that direct inhibition of the kinase activity of vascular endothelial growth factor receptor-2 (VEGFR-2) will result in the reduction of angiogenesis and the suppression of tumor growth. Attempts to optimize a cyclin-dependent kinase-1 (CDK1) inhibitor by using palladium-catalyzed C−C bond, C−N bond formation reactions to assemble diverse biheteroaryl molecules led to the unexpected discovery of a pyrazine−pyridine biheteroaryl as a novel series of potent VEGFR-2 inhibitors. Compound 15, which had IC₅₀ = 0.084 μM at VEGFR-2, showed very modest selectivity against fibroblast growth factor receptor-2 (IC₅₀ = 0.21 μM), platelet-derived growth factor receptor (IC₅₀ = 0.36 μM), and glycogen synthase kinase-3 (IC₅₀ = 0.478 μM), while it exhibited more than 10-fold selectivity against epidermal growth factor receptor (IC₅₀ = 1.36 μM) and insulin-R kinase (IC₅₀ = 1.69 μM). On the other hand, compound 15 exhibited more than 100-fold selectivity against calmodulin kinase 2; casein kinase-1 and -2; CDK1 and -4; mitogen-activated protein kinase; and protein kinase A, Cβ2, and Cγ (IC₅₀ >10 μM). Compound 15 also displayed high inhibitory potency on VEGF-stimulated human umbilical vein endothelial cell (HUVEC) proliferation (IC₅₀ = 0.005 μM) and good selectivity against cell lines such as HUVEC, human aortic smooth muscle cells, and MRC5 lung fibroblasts. Molecular docking studies were conducted in an attempt to rationalize the unexpected high VEGFR-2 selectivity of 15.