Abstract PO5-14-05: Circulating tumor DNA fraction correlates with residual cancer burden post-neoadjuvant chemotherapy in triple negative breast cancer patients

Abstract Background: Pathologic response after preoperative/neoadjuvant chemotherapy is a continuum that can range from no residual cancer to extensive disease. Viable cancers can shed tumor DNA into the blood and we hypothesized that plasma ctDNA levels can reflect pathologic response as continuum. Patients and methods: Post-neoadjuvant plasma samples were obtained after completion of chemotherapy but before surgery from patients with stage I to III TNBC who received treatment with durvalumab concurrent with weekly nab-paclitaxel followed by dose dense doxorubicin/cyclophosphamide in a single arm neoadjuvant clinical trial (NCT02489448). Up to 3 mL of blood was collected into EDTA- and heparin- containing vacutainer tubes, and plasma cfDNA was extracted and twice purified in the lab at Predicine Inc. Pathologic response was quantified on a continuous scale using the Residual Cancer Burden (RCB) score, and also categorized as RCB-0 (complete pathologic response, n=21), -I (minimal residual cancer, n=7), -II (moderate residual cancer, n=13), and -III (extensive residual cancer, n=3). Median follow-up was 35 months (range 25-70 months). Tumor whole exome sequencing (WES) data was previously generated using HiSeq 4000 at the Yale Center for Genome Analysis (dbGaP: PRJNA558949). A personalized MRD panel of up to 60 personalized variants per patient was designed based on the tumor WES data, and ultra-deep next-generation sequencing (100,000X) of plasma cfDNA was performed using the PredicineBEACON assay for MRD detection by Predicine. CtDNA positivity (MRD) was defined as a weighted score equivalent to 2 or more tumor variants detected in the plasma. cfDNA tumor fraction was measured based on mutant allele fraction of observed variants. Adjusted tumor fraction could be confidently measured for 37 out of 44 patients that had enough confidently called personalized ctDNA variants. We compared tumor fraction versus RCB score as continuous variables, and also compared tumor fraction between RCB categories as ordinal variables. Invasive disease-free survival was examined by Kaplan-Meier analysis. Bioinformatic data analysis was performed using Graphpad Prism and R. Results: 44 of 48 plasma samples yielded successful cfDNA generation and sequencing. Six patients (13.6%) were MRD positive. Among these 6 cases, 3 had RCB-0 response (14.3% of RCB 0 cases), 1 had RCB II response (7.7% of RCB II cases), and 2 had RCB III response (66.7% of RCB III cases). The median cfDNA tumor fraction of patients with pathological complete response to treatment (pCR) was significantly lower than patients with residual disease (RD) (0.06% vs. 0.3%, p=0.02). There was a positive correlation between RCB scores and tumor fraction (r= 0.45, Spearman). Tumor fraction was more strongly correlated with later recurrence than a binary ctDNA positivity call. Tumor fraction was significantly higher in patients that later experienced local or metastatic recurrence (n=8) compared to those with no evidence of disease (n=29) (Wilcoxon test, p=0.02). Both ctDNA positive RCB III cases had metastatic recurrence while the remaining 4 ctDNA positive cases currently remain recurrence-free. Mutational genomic landscape of both tumor WES and plasma ctDNA samples identified hotspot mutations in 19 cancer-associated genes. Novel hot-spot mutations were observed in the plasma of two patients following treatment. TP53 mutations were highly prevalent in tissue and plasma, however no significant association of mutation of any gene with treatment response was detected. Conclusions: PredicineBeacon MRD assay was successfully performed on 3 mL plasma samples collected using from EDTA and heparin-containing tubes. Tumor fraction correlated with residual disease, RCB score and disease recurrence. Citation Format: Naing Lin Shan, Billie Gould, Kim Blenman, Julia Foldi, Pan Du, Frank Zhang, Myles Walsh, Lajos Pusztai. Circulating tumor DNA fraction correlates with residual cancer burden post-neoadjuvant chemotherapy in triple negative breast cancer patients [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-14-05.