| Summary: | Dynamics of mRNA from circulating tumor cells (CTCs), mRNA from extracellular vesicles (EVs), and cell-free DNA (cfDNA) were assessed to examine the relevance of a longitudinal multi-parametric liquid biopsy strategy. Eighteen milliliters of blood was drawn from 27 hormone receptor-positive and human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer (MBC) patients at disease progression and at two subsequent radiologic staging time points. CTC mRNA and EV mRNA were analyzed using multi-marker qPCR, and cfDNA was analyzed using targeted next-generation sequencing (NGS). The presence of <i>ERBB2</i> or <i>ERBB3</i> overexpression signals in CTCs significantly correlated with disease progression (87% specificity, 36% sensitivity, <i>p</i>-value = 0.023), and the presence of either <i>ERBB3</i> signals in CTCs or EVs or cfDNA variants in <i>ERBB3</i> also showed a significant association with progressive MBC. Fluctuations during treatment were detected in the EV fraction with the appearance of hitherto undetected <i>ERCC1</i> signals correlating with progressive disease (97% specificity, 18% sensitivity, <i>p</i>-value = 0.030). Allele frequency development of <i>ESR1</i> and <i>PIK3CA</i> variants detected at subsequent staging time points could be used as a predictor for therapy success and, importantly, might help guide therapy decisions. The three analytes, each with their own unique features for disease monitoring, were shown to be complementary, underlining the usefulness of the longitudinal multi-parametric liquid biopsy approach.
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