Tersolisib (STX-478) is a highly potent, mutant-selective, allosteric inhibitor of PI3Kα, that selectively targets prevalent mutant forms of PI3Kα. It demonstrates strong efficacy against common PI3Kα helical- and kinase-domain mutations, including the H1047R variant, with an IC50 value of 9.4 nmol/L. STX-478 exhibits 14-fold greater selectivity for mutant PI3Kα over the wild-type form. It spares metabolic dysfunction and improves therapeutic response in PI3Kα-Mutant Xenografts.