Abstract P2-01-17: Circulating tumor cell count and levels of circulating tumor DNA are complementary prognostic biomarkers in metastatic breast cancer - A pilot study

Abstract

Abstract Background and aim Minimally invasive liquid biopsies, usually referring to the analysis of tumor-derived material such as circulating tumor cells (CTC) and circulating tumor DNA (ctDNA), have already proven their individual prognostic value in metastatic breast cancer. Whereas CTCs can be quantified by enumeration, levels of ctDNA are often expressed as the variant allele frequency of the dominant mutation, determined by rather costly targeted next generation sequencing. The modified fast aneuploidy screening test-sequencing system (mFAST-SeqS) represents a fast and cost-effective assay to estimate the fraction of ctDNA by measuring chromosomal aneuploidy instead of mutations. Next to the low costs (EUR 50, USD 60) additional advantages of the mFAST-SeqS method include the independency of mutational status of the tumor and the comprehensive overview of the tumor genome. Our aim was to investigate the potential concordance and complementarity between CTC-count and the mFAST-SeqS-based genome-wide aneuploidy score (GWA score). Material and methods Consecutive blood samples obtained from ER positive, Her2 negative metastatic breast cancer patients, starting a new line of therapy, (n=23) were enumerated for CTCs using the CellSearch-system (Menarini Silicon Biosystems, Bologna, Italy) and screened for the percentage of ctDNA using mFAST-SeqS. This method provides a GWA-score by amplification of long interspaced nuclear elements (LINE-1 elements) throughout the genome and calculating the overall deviation from the average profile in normal controls. Spearman correlation between CTC-count and GWA-score was calculated. For survival analysis, CTC-count was divided into high (CTChigh) and low (CTClow) using 5 CTCs/7.5 ml blood as the cut-off as before; similarly GWA-score was divided in high (GWAhigh)and low (GWAlow) based on the previously described cut-off of 5. Kaplan-Meier analysis was performed to assess overall survival (OS) in subgroups. Results Mean patient age was 58 (range 42-78). Median follow up was 15 months (range 0-37). Median number of previous treatment lines of patients was 2 (range 0-11). Median CTC-count was 22 (range 0-4438) and median GWA-score was 3.94 (range 0.87-114.31). The Spearman correlation coefficient between CTC count and GWA-score was 0.44 (p=0.02). 3 out of 20 patients were alive at end of follow-up. OS was significantly different in the CTChigh group versus the CTClow group (p=0.01, log-rank test, median OS 31 and 7 months, respectively), as well as in the GWAhigh versus the GWAlow group (p =0.01, log-rank test, median OS 27 and 7 months). There was concordance in 12 out of 23 samples (52.2%). 4 samples had both scores low and 8 both high, 9 samples were CTChigh/GWAlow and 2 were CTClow/GWAhigh. OS was significantly different in these groups (p < 0.01, log-rank test), with a median OS of 6 months in the group with both scores high, a median OS of 18 months for the cases with a discordant score and median OS not reached in the group with both scores low.Conclusions Our pilot study confirmed that both CTC-count and mFAST-SeqS GWA-score are prognostic and indicate that a combination of these measurements may yield complementary prognostic value. It should be noted however that the sample size was limited thus far. Our current preliminary results will be extended to 98 cases, results of which will be presented at the meeting. Citation Format: Noraly Verschoor, Elisabeth M Jongbloed, Jaco Kraan, Vanja de Weerd, Ngoc M Van, Agnes Jager, Stefan Sleijfer, Saskia M Wilting, John WM Martens. Circulating tumor cell count and levels of circulating tumor DNA are complementary prognostic biomarkers in metastatic breast cancer - A pilot study [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 P2-01-17.