Combining LOXO-783 with either fulvestrant (FUL; CI at 50% inhibition = 0.28) or imlunestrant (CI at 50% inhibition = 0.43) showed increased efficacy in cell proliferation assays using the HR+, HER2-, PI3Kα H1047R- mutant T47D model. LOXO-783 also demonstrated an additive effect in combination with these endocrine therapies in vivo. Similar results were observed in a T47D model engineered to express ESR1 D538G, as well as in an HR+, HER2- PI3Kα double in-cis mutant model (H1047R/D350G) also harboring ESR1 D538G and derived from a patient who had progressed on prior letrozole plus taselisib....LOXO-783 shows additive effects when combined with SOC in breast cancers harboring the PI3Kα H1047R-mutation (as single or double in-cis mutations) in both HR+ and triple negative settings.