The PI3K signaling pathway serves as a central node in regulating cell survival, proliferation, and metabolism. PIK3CA, the gene encoding the PI3K catalytic subunit p110-alpha, is commonly altered in
breast cancer resulting in the constitutive activation of the PI3K pathway. Using an unbiased cell line screening approach, we tested the sensitivity of
breast cancer cell lines to
taselisib, a potent PI3K inhibitor, and correlated sensitivity with key
biomarkers (PIK3CA, HER2, PTEN, and ESR1). We further assessed how
taselisib modulates downstream signaling in the different genomic backgrounds that occur within
breast cancer. We found that sensitivity to
taselisib correlated with the presence of PIK3CA mutations, but was independent of HER2 status. We further showed that HER2-amplified/PIK3CA wild-type cell lines are not as sensitive to
taselisib when compared with HER2-amplified/PIK3CA-mutant cell lines. In a PIK3CA-mutant/PTEN null background, PI3K downstream signaling rebounded in the presence of
taselisib correlating with decreased sensitivity at later time points. Finally, we observed that PIK3CA mutations cooccurred with mutations in the
estrogen receptor (ER; ESR1) in metastatic
tumors from patients with ER+
breast cancer. However, the cooccurrence of an ESR1 mutation with a PIK3CA mutation did not affect response to
taselisib in a single agent setting or in combination with
fulvestrant. In summary, these data suggest that development of
taselisib in
breast cancer should occur in a PIK3CA-mutant setting with cotreatments determined by the specific subtypes under investigation.