Molecular characterization and monitoring of patient ctDNA in phase I study of H3B-6545 in ER+ MBC.

Abstract

1052 Background: Because of lack of effective treatment in endocrine resistant metastatic breast cancer (MBC), we developed H3B-6545, a novel selective ERa covalent antagonist, capable of irreversibly inactivating both wild-type and mutant ERa. The aims of this study are to 1) characterize hotspot mutation profiles in heavily pretreated MBC and correlate ESR1, PIK3CA and AKT1 mutations in plasma vs tumor tissue 2) determine if mutations in ESR1 or PIK3CA predict response to H3B-6545 and 3) evaluate if longitudinal tracking of ctDNA correlates with response to H3B-6545. Methods: Fresh plasma samples were collected at baseline (predose), cycle 1 day 15 (C1D15), C2D1, C3D1 and every 8 weeks thereafter with a final sample collection at disease progression. At baseline, BEAMing digital PCR was used to evaluate hotspot mutations in ESR1, PIK3CA and AKT1. Patient specific ctDNA mutations were subsequently monitored by ddPCR. Baseline tumor biopsies were subjected to a targeted Next Generation Sequencing (NGS) panel to identify hotspot mutations. Results: In 77% of patients (30/39), mutations were detected at baseline by the BEAMing assay and of those, 21/39, 16/39 and 3/39 had mutations in ESR1, PIK3CA and AKT1, respectively. 20% (9/39) of patients exhibited co-mutations in ESR1 and PIK3CA. In 60% (9/15) of patients, DNA mutations identified by the plasma BEAMing assay were also detected in the tumor biopsy whereas; DNA mutations found in tissue were also detected in plasma in 86% (12/14) of cases. Serial ctDNA monitoring revealed that in patients with confirmed partial responses (3/3), ctDNA levels were undetectable by C2D1. In contrast, ctDNA levels increased from baseline in 3/4 patients with progressive disease. Exploration of ctDNA ratios (day 15/baseline and day 30/baseline) and correlations of PIK3CA and ESR1 mutations with response to H3B-6545 will be presented. Conclusions: ctDNA is a reliable sample type for assessing ESR1, PIK3CA, and AKT1 mutations in MBC, overcoming the challenges of obtaining biopsies in the metastatic setting. In addition, ctDNA dynamics may be a useful tool to monitor the efficacy of H3B-6545. Clinical trial information: NCT03250676.