# The Evolving Role for Repeat Molecular Testing in Metastatic Colorectal Cancer

## TL;DR

This review discusses the importance of repeat molecular testing in metastatic colorectal cancer to guide treatment and identify resistance mechanisms.

## Contribution

The paper provides a comprehensive review of the rationale, methods, and clinical implications of repeat molecular testing in metastatic colorectal cancer.

## Key findings

- Repeat molecular testing can identify resistance mechanisms and newly actionable alterations in metastatic colorectal cancer.
- Current guidelines for when and how to perform sequential testing remain limited despite increasing clinical adoption.
- Testing for biomarkers like RAS, BRAF, HER2, and MSI/MMR is essential for determining therapeutic eligibility.

## Abstract

Molecular profiling is central to the management of metastatic colorectal cancer (mCRC), in which genomic alterations and biomarker expression guide prognosis and the selection of targeted therapies. Advances in next-generation sequencing, including blood-based circulating tumor DNA (ctDNA) assays, have enabled repeat molecular testing throughout the disease course to identify resistance mechanisms and newly actionable alterations. Despite increasing clinical adoption, guidance on when and how to perform sequential testing remains limited. This review summarizes the biological rationale for repeat molecular assessment in mCRC, compares available testing modalities, reviews biomarker-specific patterns of evolution, and highlights ongoing clinical trials that may inform future standards of care.

Next-generation sequencing (NGS) has impacted the treatment landscape for mCRC, leading to improved outcomes through the use of molecularly targeted and immune checkpoint inhibitor therapies. The National Comprehensive Cancer Network (NCCN) and the American Society of Clinical Oncology (ASCO) recommend, at a minimum, initial testing to assess RAS, BRAF, HER2, and microsatellite instability (MSI)/mismatch repair (MMR) status, as these results determine therapeutic eligibility. Broader testing to identify the eligibility for tumor-agnostic therapy for a tumor mutation burden (TMB), NTRK gene fusions, and RET fusions is encouraged for all patients with advanced solid tumors. Patients with metastatic disease may develop progressive disease, often as a result of adaptive resistance mechanisms and selective therapeutic pressure on disease heterogeneity. Repeat biomarker testing at progression has the potential to define these resistance mechanisms and to guide the next therapy or clinical trial enrollment. While these practices have become more commonplace, unified guidelines have yet to be established. In this review of the literature, we evaluate the advantages and pitfalls of sequential biomarker testing during disease progression in patients with mCRC.