144P Development of a 6-color multiplex crystal digital PCR assay for the simultaneous detection of ESR1 and PIK3CA mutations in the plasma of metastatic breast cancer patients

Blood-based liquid biopsies have become a real asset for patient treatment management in the field of precision oncology over the past few years, has it allows quick and easy access to tumor genetic alterations of interest. Indeed, circulating tumor DNA (ctDNA) analysis is particularly well suited for longitudinal monitoring with the advantage of covering the potential genetic heterogeneity of metastatic lesions. For Hormone Receptor-positive (HR+) Metastatic Breast Cancer (MBC) patients, mutations on the ESR1 (encoding the estrogen receptor (ER) alpha), and PIK3CA (encoding the phosphatidylinositol-3-kinase (PI3K) catalytic subunit p110-alpha) genes are emerging clinical biomarkers that can both guide clinicians in their choice of treatment. Among the technologies currently used for liquid biopsies, digital PCR (dPCR) has the advantage of being among the fastest and least expensive, providing robust results with high sensitivities. Using the Naica® 3-color dPCR platform (Stilla Technologies), we previously developed two multiplexed assays that detects, in a single well, either 11 ESR1 (publication in preparation) or 21 PIK3CA pathogenic mutations (J. Corné et al., 2021, Scientific Reports). The recent availability of the 6-color version of the Naica® platform has allowed us to develop a single 6-color multiplexed assay that combines the two previously developed 3-color ESR1 and PIK3CA assays. This new screening assay, which combines two drop-off systems for the detection of 536-540 ESR1 and 542-546 PIK3CA hotspot mutations, has already shown great performances for the analysis of the 32 targeted ESR1/PIK3CA mutations, with results similar to those obtained with the 3-color assays currently used routinely in our laboratory. As ESR1 and PIK3CA analyses are often requested together, this new screening assay represents a significant saving of time and resources for the analysis of our MBC patients with a relative ease of implementation and analysis.