Abstract PO4-13-12: PIK3CA mutational status in tissue & plasma as a prognostic tool in HR+/HER2- breast cancer (BC)

Abstract Background: Activating mutations of PIK3CA gene are described in about 30-40% of BC. They confer overall worse prognosis and resistance to endocrine and chemotherapeutic therapy. Concordance between testing methods (tissue p=0.08) and with collection of samples during disease progression (66.7% vs 47.4%; p=0.43). 80pts received treatment with CDK4/6 inhibitors + endocrine therapy. PFS was slightly shorter in PIK3CAm vs wt (24m vs 30m; HR=1.39 95%CI,0.7-2.4, p=0.26). A sub-analysis was performed based on exons 9 HR=2.84 95%CI,1.1-7.4, p=0.02). In addition, plasma detection of PIK3CAm was associated with worse PFS compared with PIK3CAm detected only in tissue (12.4m vs 29.3; HR=2.4 95%CI,0.8-6.5, p=0.08). Although we observed a trend towards a poorer PFS in pts with visceral involvement in PIK3CAm (21.9m vs 30.1m; HR=2.82 95%CI,0.6-12.3, p=0.14), in a multivariate analysis, mutations in exon 9 were an independent poor prognostic factor regardless visceral involvement and detection in plasma; p=0.05. Conclusions: Our results support the PIK3CA determination in tissue as the diagnostic method of choice, although further studies could better define the role of liquid biopsy in the detection of PIK3CAm. In our population, the presence of PIK3CAm confers poorer prognosis in luminal BC, being significantly worse in mutation carriers in exon 9 regardless visceral involvement and plasma mutation detection. Table. Tissue 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-13-12.