STX-721 is an irreversible EGFR inhibitor targeting the exon 20 insertion (ex20ins) mutation. It specifically inhibits the proliferation of Ba/F3 cells with EGFR exon20 A769_V770 insASV and EGFR exon20 D770_N771 insSVD mutations, demonstrating IC50 values of 5.4 and 5.8 nM, respectively, while showing much less inhibition of cells expressing wild-type EGFR (IC50 = 127.365 nM). Additionally, STX-721, administered at 50 mg/kg once daily, induces tumor regression in a mouse xenograft model utilizing NCI H2073 non-small cell lung cancer (NSCLC) cells with the EGFR exon20 D770_N771 insSVD mutation.