Abstract PS5-12: Preliminary correlative analysis of clinical outcomes with PIK3CA mutation (mut) status from a phase I/Ib study of GDC-0077 in patients (pts) with hormone receptor-positive/HER2-negative metastatic breast cancer (HR+/HER2- mBC)

Abstract Background Mutations in p110α, encoded by PIK3CA, are present in ~40% of HR+/HER2- BCs. GDC-0077, a PI3Kα-selective inhibitor and mutant PI3Kα degrader, elicits antitumor activity in PIK3CAmut preclinical models as a single agent and when combined with endocrine therapy (ET). New evidence suggests BCs harboring multiple PIK3CAmut exhibit increased signaling through the PI3K/AKT pathway and are more sensitive to PI3Kα inhibitors compared with BCs with a single PIK3CAmut. We report a preliminary analysis of PIK3CAmut status with clinical outcomes from an ongoing study of GDC-0077 alone or with ET (letrozole/fulvestrant) ± palbociclib (palbo) in pts with PIK3CAmut HR+/HER2- mBC (NCT03006172). Methods Detectable PIK3CAmut from local tumor tissue/blood-based assay or tumor tissue by cobas PIK3CA assay were required to enroll. Plasma-derived circulating tumor (ct) DNA was collected at baseline (BL), cycle 1 day 15 (C1D15), and C2D1 (in the cohort where GDC-0077 starts at C1D15) to detect PIK3CAmut. Paired tumor samples were analyzed for Ki67 and pAKT/pS6 expression by immunohistochemistry. Single vs multiple PIK3CAmut was correlated with the percentage of pharmacodynamic (PD) inhibition of Ki67/pAKT/pS6 expression; with the PIK3CAmut allele frequency ratio between BL and C1D15 or C2D1 (MAFr15); with best overall response (BOR, RECIST v1.1); and with time on treatment (TOT) in days. Statistical analyses: Kruskal-Wallis and Mann-Whitney-Wilcoxon for group and pairwise comparisons, respectively, and two-sample proportion testing for categorical comparisons. Results Data cutoff was 03/20/2020. PIK3CAmut were detected in 87/103 (84.5%) pts with BL ctDNA available for sequencing. Multiple PIK3CAmut were detected in 21/87 (24.1%) BL ctDNA samples: 9 from pts treated with single-agent GDC-0077; 8 from pts treated with GDC-0077 + letrozole/fulvestrant; and 4 from pts treated with GDC-0077 + letrozole/fulvestrant + palbo. The median number of lines of prior therapy for metastatic disease was not different between pts with multiple (3.0 lines) vs single (2.5 lines) PIK3CAmut detected at BL (p = 0.205). Median percentage inhibition of Ki67/pAKT/pS6 expression was greater in pts with multiple (-65.8, -70.3, -66.8%, respectively) vs single (-42.1, -34.1, -29.5%) PIK3CAmut detected at BL (p = 0.095, 0.002, 0.056). Median MAFr15 was lower in pts with multiple (MAFr15 0.01) vs single PIK3CAmut (MAFr15 0.15) detected at BL (p = 0.004). Of 73 pts with both BL ctDNA-detected PIK3CAmut and BOR data, 16/16 (100%) with multiple PIK3CAmut experienced BOR of partial response (PR) or stable disease (SD) while 42/57 (73.7%) with single PIK3CAmut experienced BOR of PR or SD (p = 0.051). No pts with multiple PIK3CAmut detected experienced a BOR of progressive disease. Median TOT was greater in pts with multiple PIK3CAmut (196 days) vs single PIK3CAmut (140.5 days) detected at BL, but this was not significant (p = 0.1804). Conclusions The fraction of pts in which multiple PIK3CAmut were identified from BL ctDNA in this HR+/HER2- mBC dataset (24.1%) was slightly higher than reported elsewhere. This may be due to the method of detection (blood vs tissue) and/or the definition of multiple PIK3CAmut used. Pts in which multiple PIK3CAmut were detected by ctDNA exhibited greater depth of PD biomarker inhibition in tumors and experienced PR/SD more often compared with pts in which only one PIK3CAmut was detected. However, no significant associations were observed with the number of prior lines of therapy for metastatic disease or TOT. The dataset is currently too small to assess the impact of different treatment regimens in this study but will be re-evaluated as the data mature. Citation Format: Komal Jhaveri, Dejan Juric, Andrea Varga, Nicolas Turner, Peter Schmid, Cristina Saura, Mafalda Oliveira, Ian E Krop, Kevin Kalinsky, Antoine Italiano, Erika Hamilton, Valentina Gambardella, Andrés Cervantes, Philippe L Bedard, Bonnie P Liu, Jessica W Chen, Junko Aimi, Stephanie Royer-Joo, Jennifer L Schutzman, Katherine E Hutchinson. Preliminary correlative analysis of clinical outcomes with PIK3CA mutation (mut) status from a phase I/Ib study of GDC-0077 in patients (pts) with hormone receptor-positive/HER2-negative metastatic breast cancer (HR+/HER2- mBC) [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 PS5-12.