Abstract B102: Mutational and copy number-based ctDNA profiles mirror high-grade serous cancer tumors and enable detection of genetic changes appearing at recurrence

Abstract Circulating tumor DNA (ctDNA) is the cancer-derived fraction of extracellular DNA fragments present in the bloodstream as cell-free DNA (cfDNA). Consequently, ctDNA harbors the distinct genomic information specific to the cancer from which it derives. Therefore, ctDNA from liquid biopsies provides a non-invasive, universally available alternative to traditional tumor biopsies. ctDNA amount may provide insight into tumor burden, as its concentration is typically highest during diagnosis and relapse. The non-invasive nature of this method facilitates frequent monitoring, enabling longitudinal tracking of the disease's trajectory and response to treatments. By focusing on the key genomic changes, either short mutations or copy number aberrations (CNA), liquid biopsy allows the investigation of cancer progression and evolution over time, providing invaluable insight into the behavior and adaptability of the cancer. We collected 152 longitudinal plasma samples and 92 fresh tissue samples from 29 patients enrolled in the DECIDER cohort (NCT04846933), after a diagnosis of high-grade serous carcinoma (HGSC), the most common and deadliest histological subtype of ovarian cancer. With ultra-deep targeted sequencing of a gene panel of more than 700 cancer-related genes we were able to comprehensively evaluate the cancer-related genomic changes. The concordance rate between mutations in plasma compared to tumor tissue was 83 % at diagnosis and 90 % at relapse, confirming the reliability of exploiting ctDNA in monitoring genomic alterations. Furthermore, by comparing plasma- and tissue-derived mutations, we demonstrated that comparable levels of ctDNA are released from adnexa, intra-abdominal lesions, and ascites, proposing that a liquid biopsy represents mutations from various locations. Analysis of CNA showed that our approach successfully detected concordant profiles in most plasma samples, even with tumor content as low as 3%. Additionally, highly amplified regions were identified in samples with approximately 1% tumor content. Longitudinal analysis of both mutations and CNA revealed genomic changes in plasma samples at relapse. Twenty-four new exonic mutations were identified in eight patients at relapse; two of these alterations were related to HRD status, supporting PARP-inhibitors as a valid treatment option. Furthermore, mutations in FGFR3, JAK2, FLT3 and CDKN2A could be investigated as target indications for extended use of currently available therapies. Eleven patients had high tumor content in their plasma samples at the time of relapse. We detected altered CNA profiles in seven of these eleven patients, so that five patients harbored only a few novel focal amplifications or losses, while two patients showed significant changes in their copy-number profile during the therapy. In conclusion, our study identified recurrence-specific genomic aberrations, proposing that ctDNA more accurately represents the metastatic tumor and provides possibilities for personalized therapy options. Citation Format: Giovanni Marchi, Mai T. N. Nguyen, Anna Rajavuori, Taru Muranen, Kaisa Huhtinen, Sinikka Oksa, Sakari Hietanen, Johanna Hynninen, Jaana Oikkonen, Sampsa Hautaniemi. Mutational and copy number-based ctDNA profiles mirror high-grade serous cancer tumors and enable detection of genetic changes appearing at recurrence abstract. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84 (5 Suppl₂): Abstract nr B102.