Multimodal liquid biopsy for early monitoring and outcome prediction of chemotherapy in metastatic breast cancer

Abstract

Circulating tumor cells (CTCs) and circulating tumor DNA (ctDNA) are two cancer-derived blood biomarkers that inform on patient prognosis and treatment efficacy in breast cancer. We prospectively evaluated the clinical validity of quantifying both CTCs (CellSearch) and ctDNA (targeted next-generation sequencing). Their combined value as prognostic and early monitoring markers was assessed in 198 HER2-negative metastatic breast cancer patients. All patients were included in the prospective multicenter UCBG study COMET (NCT01745757) and treated by first-line chemotherapy with weekly paclitaxel and bevacizumab. Blood samples were obtained at baseline and before the second cycle of chemotherapy. At baseline, CTCs and ctDNA were respectively detected in 72 and 74% of patients and were moderately correlated (Kendall's τ = 0.3). Only 26 (13%) patients had neither detectable ctDNA nor CTCs. Variants were most frequently observed in TP53 and PIK3CA genes. KMT2C/MLL3 variants detected in ctDNA were significantly associated with a lower CTC count, while the opposite trend was seen with GATA3 alterations. Both CTC and ctDNA levels at baseline and after four weeks of treatment were correlated with survival. For progression-free and overall survival, the best multivariate prognostic model included tumor subtype (triple negative vs other), grade (grade 3 vs other), ctDNA variant allele frequency (VAF) at baseline (per 10% increase), and CTC count at four weeks (≥5CTC/7.5 mL). Overall, this study demonstrates that CTCs and ctDNA have nonoverlapping detection profiles and complementary prognostic values in metastatic breast cancer patients. A comprehensive liquid-biopsy approach may involve simultaneous detection of ctDNA and CTCs.

Fig. 1. Variant-detection rate in cfDNA according to CTC numbers at baseline (n = 147).

Variant-detection rates are displayed for each gene at baseline (n = 147 patients). VAF is colored by quartiles with higher VAF (>14%) in red, middle VAF (4 < VAF ≤ 14%) in orange, and lower VAF in yellow (≤4%).

Fig. 2. Correlation between CTC counts and ctDNA levels (logarithmic scale).

Each dot represents ctDNA variant allelic fractions (VAF) as a function of CTCs counts (a at baseline, b at four weeks). ND ctDNA not detected. Dashed line highlights the 5 CTC thresholds.

Fig. 3. Overall survival and progression-free survival according to the combination of CTC counts and ctDNA levels.

OS curves by blood biomarker status at baseline (a) and four weeks (b). PFS curves by blood biomarker status at baseline (c) and four weeks (d).