Axitinib (AG-013736) is a potent and selective inhibitor of VEGFR 1 to 3. In transfected or endogenous RTK-expressing cells, Axitinib potently blocks growth factor-stimulated phosphorylation of VEGFR-2 and VEGFR-3 with average IC50 values of 0.2 and 0.1 to 0.3 nM, respectively. Cellular activity against VEGFR-1 is 1.2 nM (measured in the presence of 2.3% bovine serum albumin), equivalent to an absolute IC50 of ~0.1 nM, based on protein binding of Axitinib. The potency against murine VEGFR-2 (Flk-1) in Flk-1-transfected NIH-3T3 cells is 0.18 nM, similar to that of its human homologue. Axitinib shows ~8- to 25-fold higher IC50 against the closely related type III and V family RTKs, including PDGFR-β (1.6 nM), KIT (1.7 nM), and PDGFR-α (5 nM); nanomolar concentrations of Axitinib blocks PDGF BB-mediated human glioma U87MG cell (PDGFR-β-positive) migration but not proliferation.