11543 Background: Within metastatic breast cancer (mBC), tumor heterogeneity limited efficacy and duration of response to treatment. In this study, circulating tumor DNA (ctDNA) was used to evaluate tumor heterogeneity as a prognostic factor and monitor therapeutic response in patients with mBC. Methods: We collected plasma samples from 37 HER2-positive mBC patients treated with pyrotinib. Target-capture deep sequencing was performed to detect somatic mutations in plasma ctDNA. Clonal population structures were identified based on variations from ctDNA using Bayesian cluster with PyClone. Molecular tumor burden index (mTBI) was calculated with the mean variant allele frequency of mutations in trunk clonal population. Results: Mutations in TP53 and genes of PI3K/Akt/mTOR pathway were associated with drug resistance for pyrotinib. The result showed that patients with resistant mutations occurring as a truncal event, who receiving monotherapy of pyrotinib, presented worse therapeutic effect (HR, 4.52; P = 0.03). The median PFS of patients with versus without resistant mutations in trunk clonal population was 7.8 weeks (95% CI 7.4 to 26.8 weeks) versus 31.6 weeks (95% CI 15.7 to 60 weeks), respectively. Patients with high heterogeneity (clonal population ≥3) had a significantly worse PFS (HR, 2.79; 95% CI 1.23 to 6.34; P = 0.014). The median PFS among patients with high versus low heterogeneity was 30.0 weeks (95% CI 13.9 to 53.5 weeks) versus 60.0 weeks (95% CI 31.4 to 84 weeks), respectively. Longitudinal monitoring of 21 patients during treatment showed positive correlation between mTBI in ctDNA and tumor size evaluated by CT imaging (P < 0.0001). Monitoring the mTBI in serial ctDNA increased sensitivity for prediction of progressive disease in 6 of 21 patients, with a mean time of 12.7 weeks earlier than using CT scan. ROC curve analysis showed an area under the curve value was 0.97 (p < 0.0001). Conclusions: Assessing tumor heterogeneity in ctDNA provides genetic predictors of treatment outcome. Molecular tumor burden in ctDNA is a potential indicator of therapeutic response. These observations might be supplements for the current therapeutic response evaluation.
Abstract 2929: Comparative analysis of circulating human tumor DNA and bioluminescent imaging in monitoring tumor burden and therapeutic response of ionizing radiation in intratibial human prostate tumor xenografts
