Abstract PS4-06: Comparison of liquid biopsy and tissue based detection ofPIK3CAmutations in HR positive metastatic breast cancer patients

Abstract The treatment of hormone receptor positive metastatic breast cancer patients has changed dramatically over the last few years and combination strategies attempting to overcome resistance of the disease are gaining importance. After introducing CDK 4/6 inhibitors in the treatment one of the subsequent strategies is definitely targeting PI3 kinase pathway. Several drugs have been tested, but only recently, SOLAR-1 phase III trial demonstrated the benefit of adding alpelisib to fulvestrant, with acceptable tolerability. With this trial the importance of PIK3CA, testing was postulated. In the present study a comparison of liquid biopsy and tissue-based detection of PIK3CA mutations in HR positive metastatic breast cancer patients was performed. Materials and Methods: Plasma and the most recent tumor tissue were screened for PIK3CA hotspot mutations from 58 patients with metastatic hormone-receptor positive breast cancer. For plasma samples, SiMSen-Seq NGS sequencing was used covering 11 frequent PIK3CA mutations, which were applied for stratification in SOLAR-1. Only plasma samples with variant allele frequency (VAF) >1% were considered as positive. For tumor tissue samples, targeted Ion Torrent NGS was used. Matched tissue plasma samples were available from 40 patients.Results: PIK3CA mutations were detected in 25/50 (50.0%) tissue samples and in 18/48 (37.5%) plasma samples. VAF ranged from 4.4 to 72.9% with an average of 28.7% in tissue samples, and from 1.1% to 49.9% with an average of 8.2% in plasma samples. In tumor samples, the most frequent variant was H1047R (11/25, 44.0%), followed by E545K (8/25, 32.0%), E542K (4/25, 16.0) and H1047L (2/25, 8.0%). In plasma samples, the most frequent PIK3CA variant was H1047R (12/18, 66.7%), followed by E542K (6/18, 33.3%), and E545K (2/18, 11.1%). Double mutations in plasma samples occurred in two patients (both with H1047R + E542K). In 15/48 (31.3%) plasma samples, 19 low-level mutations (AF between 0.1% and 1.0 %) were detected, with an average of 0.49% (range 0.18-0.98).Matched tissue and plasma samples were available from 40/58 (68.9%) patients with a median time between matched tissue and plasma collection of 285 days (25th-75th percentile: 24-1833 days). The overall concordance rate between plasma and tissue samples was 62.5% (25/40 patients). In detail, 8/40 patients (20.0%) had the same PIK3CA mutation in plasma and tumor tissue, while in 17/40 patients (42.5%) were tested negative in both sample types Discordant results occurred in 15/40 samples (37.5%). Specifically, nine patients had a PIK3CA mutation in the tissue sample only, while four patients had a PIK3CA mutation in the plasma sample only. In addition, in two patients a different PIK3CA mutation was detected in the plasma sample compared to the tissue sample.Conclusion: SiMSen-Seq based detection of PIK3CA mutations from plasma demonstrated promising results for selection of candidate patients for Alpelisib treatment. However, these results will be validated in a larger cohort of patients. Our data align with FDA recommendation to initially carry out the mutation testing in ctDNA. Only if the test is negative for PIK3CA mutations in plasma, patients should undergo testing for PIK3CA mutations in tumor tissue. Citation Format: Christoph Suppan, Qing Zhou, Stephan Jahn, Ricarda Graf, Eva Valentina Klocker, Angelika Terbuch, Karl Kashofer, Florian Posch, Hanno Gerritsmann, Nadia Dandachi, Ellen Heitzer, Marija Balic. Comparison of liquid biopsy and tissue based detection of PIK3CA mutations in HR positive metastatic breast cancer patients [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS4-06.