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

Abstract

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 & plasma) are not widely studied. We aim to correlate tissue & plasma assays and to analyze the discordant cases and prognostic value of PIK3CA mutations (PIK3CAm) in HR+/HER2- BC. Methods: We performed a retrospective and unicentric analysis of PIK3CA mutational status in tissue & plasma samples in patients (pts) with HR+/HER2- BC from February/21 to April/23. PIK3CA test: Cobas®PIK3CA Mutation Kit. We correlated both diagnostic methods. Kaplan-Meier and Cox models were used to analyze progression-free survival (PFS) and comparison outcomes in PIK3CAm vs wild-type (wt). Results: We analyzed 225 samples from 161pts with HR+/HER2- BC (149 in tissue & 76 in plasma). PIK3CA mutations were detected in 62pts (38.5%), of which 39.6% (59pts) were detected in tissue and 11.8% (9pts) in plasma. Hotspot mutations: H1047X (45.7%), E545X (20%) and E542K (12.8%). In advanced disease, metastatic BC (mBC), tissue & plasma concordance was conducted in 64 cases, with overall correlation rate of 70.3%. We found PIK3CAm in 28pts: by both methods in 9pts (32.1%) and exclusively tissue detection in 19 cases (67.8%). Plasma detection was correlated with the presence of ≥3 metastatic sites (66.7% vs 31.6%; 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 & 20, showing a significantly lower PFS in PIK3CAm exon 9 vs 20 population (9.7m vs 30.3m; 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 & plasma correlation in patients with PIK3CA mutational status tested in our population (mBC). Citation Format: Eduardo Terán Brage, Rebeca Lozano Mejorada, Aline Rodrigues Françoso, José Antonio Muñóz Leon, Álvaro López Gutiérrez, Luis Figuero-Pérez, Daniel Morchón Araujo, María Garijo Martínez, Jonnathan Roldán Ruiz, María Mar Abad Hernández, Magdalena Sancho de Salas, Emilio Fonseca Sánchez, César Augusto Rodríguez Sánchez. PIK3CA mutational status in tissue & plasma as a prognostic tool in HR+/HER2- breast cancer (BC) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-13-12.