Monitoring treatment response in metastatic breast cancer currently consists mainly of radiological and clinical assessments. These methods have high inter-observer variation, suboptimal sensitivity to determine response to treatment and give little insight into the biological characteristics of the tumor. Assessing circulating tumor DNA (ctDNA) over time could be employed to address these limitations. Several ways to quantify and characterize ctDNA exist, based on somatic mutations, copy number variations, methylation, and global circulating cell-free DNA (cfDNA) fragment sizes and concentrations. These methods are being explored and technically validated, but to date none of these methods are applied clinically. We systematically reviewed the literature on the use of quantitative ctDNA measurements over time to monitor response to systemic therapy in patients with metastatic breast cancer. Cochrane, Embase, PubMed and Google Scholar databases were searched to find studies focusing on the use of cfDNA to longitudinally monitor treatment response in advanced breast cancer patients until October 2020. This resulted in a total of 33 studies which met the inclusion criteria. These studies were heterogeneous in (pre-)processing procedures, applied techniques and design. An association between ctDNA and treatment response was found in most of the included studies, independent of the applied assay. To implement ctDNA-based response monitoring into daily clinical practice for metastatic breast cancer patients, sample (pre-) processing procedures need to be standardized and large prospectively collected sample cohorts with well annotated clinical follow-up are required to establish its clinical validity.

Purpose: The addition of two years of abemaciclib treatment to standard adjuvant endocrine therapy in all patients with high risk ER+, HER2- early breast cancer (EBC) has been approved by the US Food and Drug Administration (FDA). Pre-selection of patients with an immediate risk of recurrence within the group of clinically high risk patients using detection of minimal residual disease (MRD) using patient-informed circulating tumor DNA assays during follow-up could enhance efficacy. Here, we investigate the cost-effectiveness of the addition of two years abemaciclib in all high risk HR+, HER2- patients and in MRD-guided high risk patients only. Methods: Two semi-Markov models were developed to evaluate the cost-effectiveness of adding two years of abemaciclib compared to “standard treatment”: 1) “abemaciclib all” and 2) “MRD-guided abemaciclib” using MRD-guidance. Data of the MonarchE trial were used to model the invasive disease-free survival (iDFS). Since iDFS and overall survival (OS) data of abemaciclib were currently limited, abemaciclib effects were extrapolated using a favorable, intermediate and unfavorable effect scenario. Results: The addition of abemaciclib in all high-risk EBC patients prolonged iDFS slightly (0.04 additional quality adjusted life years (QALYs)) and led to higher costs compared to standard ET, leading to a high incremental cost effectiveness ratio (ICER) of € 1,551,876/QALY. Neither the favorable effect scenario (additional 1.09 QALYs) was cost-effective (ICER € 62,935/QALY), using a willingness-to-pay threshold of € 50,000/QALY. The “MRD-guided abemaciclib” strategy resulted in lower costs and an increase in QALYs (1.27) compared to “standard treatment” in the unfavorable effect scenario. Conclusion: The addition of abemaciclib to adjuvant endocrine therapy in all high-risk ER+, HER2- EBC patients is not cost-effective. However, using MRD detection to justify the addition of abemaciclib treatment dominates standard treatment in this cost-effectiveness analysis. Further evaluation of MRD detection in EBC by means of prospective clinical trials assessing clinical utility is recommended and promising in terms of cost-effectiveness.