ctDNA to Guide Adjuvant Therapy in Localized Colorectal Cancer (CRC)

Abstract

Currently, the standard treatment for patients with localized colorectal cancer (CRC) includes surgical resection followed by adjuvant chemotherapy based on clinicopathological features. Recurrence risk stratification in those patients is of utmost importance to guide clinicians to avoid both under- and overtreatment. Recently, the concept of minimal residual disease (MRD) has emerged as the detection of circulating tumor DNA (ctDNA) carrying tumor-specific genomic or epigenomic alterations in the bloodstream of patients after surgery. Emerging studies described how the detection of MRD is a powerful prognostic biomarker to identify patients at higher risk of recurrence and who will potentially benefit the most from a systemic adjuvant treatment. Based on that unprecedented finding, several clinical trials involving stage II and III CRC patients are ongoing evaluating the impact of ctDNA guided treatment by escalating or deescalating adjuvant chemotherapy based on ctDNA MRD detection. This review provides a critical overview of current perspectives of liquid biopsy in early-stage CRC including technical, biological, and clinical key points, as well as ongoing ctDNA-based clinical trials that ultimately aim to improve clinical outcomes of patients with CRC.

Keywords: cancer detection; circulating tumor DNA; colorectal cancer; liquid biopsy; minimal residual disease; next-generation sequencing.

Figure 1

Preparation of sequencing libraries using ID barcodes and unique molecular identifiers (UMI). After the area of interest is selected, the ID barcodes specific to each patient are added to enable to analyze multiple patients in the same assay. After that, UMIs are added to each molecule during library preparation so that sequencing reads originating from the same starting molecule of each patient included in the library can be identified. This approach increases the assay sensitivity in terms of detecting sequencing errors. In the right sequence corresponding to the blue UMI, the alteration is objectified in only 2 of the amplifications, so it is assumed a sequencing error. In the middle sequence corresponding to the orange UMI, the mutation is found in all the amplifications so it is assumed a real mutation. In the left sequence, there is no mutation founded in any of the reads, so it is a wt ctDNA molecule. cfDNA: cell free DNA; wt: wild type; ID: identification; * UMI: unique molecular identifier.

Figure 2

Circulating tumor (ct)DNA features. Description of genetic, epigenetic, and fragmentomic alterations that can be found in plasma cell free DNA analysis. *: The asterisks are placed as a way of listing the different techniques.

Figure 3

ctDNA detection of Minimal residual disease in colorectal cancer. (a) Design proposal for adjuvant clinical trials based on ctDNA analysis to detect MRD after surgery in localized colorectal cancer. (b) Graphical representation of early detection of MRD thanks to ctDNA compared to commonly surveillance used methods (CT scan, CEA). Sx: surgery; CRC: colorectal cancer; dPCR: digital PCR; NGS: next-generation sequencing; MRD: minimal residual disease; CT: computerized tomography; CEA: carcinoembryonic antigen.

Figure 4

Potential causes of false positive and false negative results in ctDNA analysis and proposed solutions. Empty dots represent false positive/negative results and filled dots represent true positive/negative ctDNA results. For each pre-analytical, analytical, or biological potential false positive/negative result, a possible solution is proposed. Ng: nanograms; cfDNA: cell free DNA; CH: clonal hematopoiesis.