Abstract 1700: Serial monitoring of single-cell circulating tumor cell genomics in metastatic lobular breast cancer to identify precision and immuno-oncology biomarkers with therapeutic implications (2022)

Abstract Clinical decisions for precision and immuno-oncology therapies are based on predictive biomarkers commonly obtained from a single metastatic biopsy, or from archived primary tumor material. Circulating genomic biomarkers present a minimally invasive way to monitor the intra-patient tumor heterogeneity and its fluctuations in order to provide a real-time evaluation of the changing clonal architecture with potential therapeutic implications. Single-cell DNA next generation sequencing (scNGS) of circulating tumor cells (CTC) is a particularly well-suited method of unraveling and monitoring that heterogeneity to complement biomarker information obtained from tissue and cell-free circulating tumor DNA (ctDNA). In this proof-of-concept study we analyzed 123 CTC, 15 white blood cells (WBC), and ctDNA from 15 CTC-positive lobular breast cancer patients, five of whom had CTC available at both metastatic baseline and after progression on a variety of therapies chosen at their physician’s discretion. CTC were enriched with the CellSearch® system and isolated as single cells with the DEPArray™ system. Whole genome amplified CTC DNA underwent scNGS with the Oncomine Comprehensive Assay covering ~500 genes and 1.1Mb of genomic space to detect mutations, copy number alterations, tumor mutation burden (TMB) and microsatellite instability (MSI). 99.1% of cells were informative, with a mean sequencing depth of 664x. Using our previously developed, CTC-based precision medicine reporting platform, MI-CTCSeq, multiple CTC in seven of 15 patients (47%) had mutations that were actionable by FDA-approved targeted therapies including in the oncogenes PIK3CA (alpelisib) and FGFR2 (erdafitinib). 13 patients (87%) displayed intra-patient, inter-CTC genomic heterogeneity of putative driver mutations. Two of five (40%) patients with CTC at both baseline and progression displayed fluctuations in their CTC subclonal makeup between timepoints. One of the two harbored a baseline ESR1 (estrogen receptor α) p.D538G activating mutation that largely disappeared at progression and was replaced by a CTC subclone with a different ESR1 activating mutation, p.Y537S. Intriguingly, this patient’s CTC also harbored an FGFR2 p.K659M mutation in an actionable “hotspot” at progression, which was absent at baseline, suggesting potential utility of serial monitoring by CTC scNGS. TMB scores and MSI status in CTC were highly concordant with those measured in clinical tissue biopsies. Taken together, these data suggest the non-invasive interrogation of the CTC genomic landscape and its serial monitoring to inform precision and immuno-oncology treatments in real time. Citation Format: Andi K. Cani, Emily M. Dolce, Elizabeth P. Darga, Kevin Hu, Chia-Jen Liu, James M. Rae, Daffyd G. Thomas, Scott A. Tomlins, Arul M. Chinnaiyan, Aaron M. Udager, Costanza Paoletti, Erin F. Cobain, Daniel F. Hayes. Serial monitoring of single-cell circulating tumor cell genomics in metastatic lobular breast cancer to identify precision and immuno-oncology biomarkers with therapeutic implications [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1700.