Structural optimization and structure-activity relationship studies of N-phenyl-7,8-dihydro-6H-pyrimido[5,4-b][1,4]oxazin-4-amine derivatives as a new class of inhibitors of RET and its drug resistance mutants.

The RET tyrosine kinase is an important therapeutic target for medullary thyroid cancer (MTC), and drug resistance mutations of RET, particularly V804M and V804L, are a main challenge for the current targeted therapy of MTC based on RET inhibitors. In this investigation, we report the structural optimization and structure-activity relationship studies of N-phenyl-7,8-dihydro-6H-pyrimido[5,4-b][1,4]oxazin-4-amine derivatives as a new class of RET inhibitors. Among all the obtained kinase inhibitors, 1-(5-(tert-butyl)isoxazol-3-yl)-3-(4-((6,7,8,9-tetrahydropyrimido[5,4-b][1,4]oxazepin-4-yl)amino)phenyl)urea (17d) is a multi-kinase inhibitor and potently inhibits RET and its drug resistance mutants. It showed IC50 (half maximal inhibitory concentration) values of 0.010 μM, 0.015 μM, and 0.009 μM against RET-wild-type, RET-V804M, and RET-V804L, respectively. 17d displayed significant anti-viability potencies against various RET-driving tumor cell lines. In a xenograft mouse model of NIH3T3-RET-C634Y, 17d exhibited potent in vivo anti-tumor activity, and no obvious toxicity was observed. Mechanisms of action were also investigated by Western blot and immunohistochemical assays. Collectively, 17d could be a promising agent for the treatment of MTC, hence deserving a further investigation.