Design and Synthesis of Novel DFG-Out RAF/Vascular Endothelial Growth Factor Receptor 2 (VEGFR2) Inhibitors. 1. Exploration of [5,6]-Fused Bicyclic Scaffolds

To develop RAF/VEGFR2 inhibitors that bind to the inactive DFG-out conformation, we conducted structure-based drug design using the X-ray cocrystal structures of BRAF, starting from an imidazo­[1,2-<i>b</i>]­pyridazine derivative. We designed various [5,6]-fused bicyclic scaffolds (ring A, <b>1</b>–<b>6</b>) possessing an anilide group that forms two hydrogen bond interactions with Cys532. Stabilizing the planarity of this anilide and the nitrogen atom on the six-membered ring of the scaffold was critical for enhancing BRAF inhibition. The selected [1,3]­thiazolo­[5,4-<i>b</i>]­pyridine derivative <b>6d</b> showed potent inhibitory activity in both BRAF and VEGFR2. Solid dispersion formulation of <b>6d</b> (<b>6d-SD</b>) maximized its oral absorption in rats and showed significant suppression of ERK1/2 phosphorylation in an A375 melanoma xenograft model in rats by single administration. Tumor regression (<i>T</i>/<i>C</i> = −7.0%) in twice-daily repetitive studies at a dose of 50 mg/kg in rats confirmed that <b>6d</b> is a promising RAF/VEGFR2 inhibitor showing potent anticancer activity