Outcome and molecular landscape of patients with PIK3CA-mutated metastatic breast cancer

Background α-Selective phosphatidylinositol 3-kinase (PI3K) inhibitors improve outcome in patients with PIK3CA-mutated, hormone receptor-positive (HR+)/Her2− metastatic breast cancer (mBC). Nevertheless, it is still unclear how to integrate this new drug family in the treatment landscape. Patients and methods A total of 649 patients with mBC from the SAFIR02 trial (NCT02299999), with available mutational profiles were selected for outcome analysis. PIK3CA mutations were prospectively determined by next-generation sequencing on metastatic samples. The mutational landscape of PIK3CA-mutated mBC was assessed by whole-exome sequencing (n = 617). Finally, the prognostic value of PIK3CA mutations during chemotherapy was assessed in plasma samples (n = 44) by next-generation sequencing and digital PCR. Results Some 28% (104/364) of HR+/Her2− tumors and 10% (27/255) of triple-negative breast cancer (TNBC) presented a PIK3CA mutation (P Conclusion PIK3CA-mutated HR+/Her2− mBC patients present a poor outcome and resistance to chemotherapy. Patients with PIK3CA-mutated TNBC present a better OS. This could be explained by an enrichment of PIK3CA mutations in luminal BC which lost HR expression in the metastatic setting. Trial registration SAFIR02 trial: NCT02299999 .