Potent Pan-Raf and Receptor Tyrosine Kinase Inhibitors Based on a Cyclopropyl Formamide Fragment Overcome Resistance.

While selective BRafV600E inhibitors have been proven effective clinically, acquired resistance rapidly develops through reactivation of the mitogen-activated protein kinase (MAPK) pathway. Simultaneous targeting of multiple nodes in the pathway offers the prospect of enhanced efficacy as well as reduced potential for acquired resistance. Replacement pyridine group of Y-1 by a cyclopropyl formamide group afforded I-01 as a novel multitargeted kinase inhibitor template. I-01 displayed enzyme potency against Pan-Raf and receptor tyrosine kinases (RTKs). Based on the binding mode of I-01, analogues I-02-I-18 were designed and synthesized. The most promising compound I-16 potently inhibits all subtypes of Rafs with IC50 values of 3.49 (BRafV600E ), 8.86 (ARaf), 5.78 (BRafWT ), and 1.65 nM (CRaf), respectively. I-16 not only exhibit comparable antiproliferative activities with positive control compounds against HepG2, SW579, MV4-11, and COLO205 cell lines, but also suppress the proliferation of melanoma SK-MEL-2 harboring overexpressed BRafWT with IC50 values of 0.93 μM. The Western blot results for the ERK inhibition in human melanoma SK-MEL-2 cell lines show that I-16 inhibits the proliferation of SK-MEL-2 cell lines without paradoxical activation of ERK, which support the hypothesis that the inhibition of Pan-Raf and RTKs might be a tractable strategy to overcome the resistance of melanoma induced by the therapy with the current selective BRafV600E inhibitors.