CtDNA from ascites as an alternative to tumor sampling for HRD (homologous recombination deficiency) testing in ovarian cancer (OC).

Abstract

6066 Background: Knowledge regarding HRD status is becoming crucial to guide maintenance strategies for patients with newly diagnosed OC. Unfortunately, for patients (pts) treated with neoadjuvant chemotherapy (NACT), HRD testing on small biopsies from diagnostic laparoscopies (Dx Lap) or interval debulking has a high failure rate. At relapse, biopsies may not be feasible. Aim: Evaluate the feasibility and usefulness of HRD testing on cfDNA from ascites Methods: Pts enrolled in a prospective biological study (OvBIOMark) consented to analysis of biological samples obtained as part of routine diagnosis. cfDNA was extracted from 1-2ml of double-centrifuged fresh ascites and subjected to 1) targeted NGS including the most common somatic mutations in high grade ovarian cancer ( TP53) to confirm presence of tumor cfDNA and 2) SNParray for copy number (CN) analyses to calculate a genomic instability score (GIS) for HRD. Results: Thirty four ascites samples were collected from 25 pts with suspected or confirmed OC. For 15/25 pts samples were obtained at Dx Lap, and for 10 pts samples were obtained at relapse. Seven pts underwent repeat ascitic drains during treatment or at relapse. 97% (33/34) of ascitic samples had detectable cfDNA (median = 980ng, range:80-5730ng) even when obtained during chemotherapy. A deleterious mutation was identified in 87% (29/33) of samples with high allelic frequencies (median allelic frequency, AF = 60%; 3.3-87%), confirming that most of detected cfDNA was tumoral. The most common mutation was a TP53m (86%; 25/29). We have performed CN analysis on cfDNA from ascites on 17 of these patients to evaluate their HRD status. Ten pts had a high GIS (HRD+), and 7 pts a low GIS (HRD-). The 4 pts with confirmed BRCAm included in this study had a high GIS on ascites. When available from the same patient, the CN profiles derived from ascites cfDNA and tumor sampling were superimposable. Conclusions: Ascites yields large amounts of cfDNA, which can be confirmed as tumoral based on TP53 mutation detection. CN analysis on ascitic cfDNA is feasible and can be used to detect the same HRD scar as tumor testing. Ascites is frequent at diagnosis, especially in pts with inoperable disease planned for NACT and could provide a useful alternative to tumor for HRD and BRCA testing.