Structural Optimization and Structure–Activity Relationship Studies of 6,6-Dimethyl-4-(phenylamino)‑6<i>H</i>‑pyrimido[5,4‑<i>b</i>][1,4]oxazin-7(8<i>H</i>)‑one Derivatives as A New Class of Potent Inhibitors of Pan-Trk and Their Drug-Resistant Mutants

Tropomyosin receptor kinases (TrkA, TrkB, and TrkC) are attractive therapeutic targets for multiple cancers. Two first-generation small-molecule Trks inhibitors, larotrectinib and entrectinib, have just been approved to use clinically. However, the drug-resistance mutations of Trks have already emerged, which calls for new-generation Trks inhibitors. Herein, we report the structural optimization and structure–activity relationship studies of 6,6-dimethyl-4-(phenylamino)-6H-pyrimido­[5,4-b]­[1,4]­oxazin-7­(8H)-one derivatives as a new class of pan-Trk inhibitors. The prioritized compound 11g exhibited low nanomolar IC50 values against TrkA, TrkB, and TrkC and various drug-resistant mutants. It also showed good kinase selectivity. 11g displayed excellent in vitro antitumor activity and strongly suppressed Trk-mediated signaling pathways in intact cells. In in vivo studies, compound 11g exhibited good antitumor activity in BaF3-TEL-TrkA and BaF3-TEL-TrkCG623R allograft mouse models without exhibiting apparent toxicity. Collectively, 11g could be a promising lead compound for drug discovery targeting Trks and deserves further investigation.