Abstract P5-13-29: Analytical and clinical validation of a ctDNA assay for detecting copy number loss and structural rearrangement variants contributing to homologous recombination and repair (HRR) deficiency

Abstract

Abstract Background: Inactivating HRR gene mutations can lead to HRR deficiency (HRD) and predict response to PARPi therapy in patients with breast cancer. Copy number loss and large genomic rearrangements (LGR) can result in HRD but are challenging to detect in ctDNA. Here, we present the analytical validation of homozygous deletions, loss of heterozygosity (LoH) and LGR detection on the Guardant360 (G360) liquid biopsy panel, previously validated for detection of small variants, copy number amplifications, and fusions. We present real-world outcomes of BRCA1/2-mutant PARPi-treated patients to demonstrate the clinical validity of the detected variants. Methods: Analytical validation was performed using the G360 83-gene ctDNA panel. Cell line DNA and clinical patient cfDNA were titrated into matched normal cell line DNA or healthy donor cfDNA to establish the limit of detection (LoD) and precision for copy number loss and LGRs, respectively. Accuracy results were compared to those from an orthogonal, externally validated tissue and ctDNA panel. De-identified, longitudinal, claims data were linked to the cancer genomic profiles in Guardant INFORM, a clinical-genomics database. Advanced PARPi treated breast cancer patients with an inactivating or reversion BRCA1/2 mutation detected by G360 were assessed. Results: The analytical sensitivity (95% LoD) for detecting homozygous and LoH deletions for deletion sizes >10MB was established at tumor fractions (TF) of 12.5% and 25%, respectively. The 95% LoD for LGRs was 0.2% variant allele fraction (VAF). The per-sample false positive rate for copy number loss and LGRs was <0.5%. Prevalence of BRCA1/2 homozygous deletions, LoH and LGRs in >1000 advanced breast cancer patients was 1.8%, 16.6% and 0.25% respectively, compared to 2.4%, 56.7% and 0.3% in TCGA. To verify the clinical impact of cfDNA-detected HRR alterations, overall survival was determined for PARPi-treated patients with >1 BRCA1/2 germline or somatic SNV, indel or LGR reversion mutation to be 23.2 months [16.4, 30, CI, n=75] compared to 54.4 [28, NA, CI, n=14] months for BRCA1/2-mutant patients without a reversion (p-value=0.049). Conclusion:. This analytical validation demonstrates that G360 detection of inactivating mutations, including copy number loss and LGRs, is highly sensitive, reliable and robust. Real-world evidence analysis confirmed worse survival outcomes in PARPi treated patients harboring a BRCA1/2 reversion compared to BRCA1/2-mutant patients with no reversion. This data further supports ctDNA as a compelling non-invasive means to identify potential PARPi sensitizing and resistance mutations in patients with advanced breast cancer. Citation Format: Jennifer Yen, Leylah Drusbosky, Caroline Weipert, Nicole Zhang, David Hanna, Catalin Barbacioru, Hao Wang, Alex Artyomenko, Arielle Yablonovitch, Yu Fu, Aaron Hardin, Nagesh Alla, Robert Foley, Max Maligska, Bhargavi Panchangam, Phil Yen, Jane Meisel, Neelima Vidula, Massimo Cristofanilli, Jeremy Force, Michael Dorschner, Martina Lefterova, Elena Helman, Becky Nagy, Darya Chudova, AmirAli Talasaz. Analytical and clinical validation of a ctDNA assay for detecting copy number loss and structural rearrangement variants contributing to homologous recombination and repair (HRR) deficiency [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-13-29.