The Molecular and Functional Landscape of Resistance to FOLFIRI Chemotherapy in Metastatic Colorectal Cancer.

The combination of 5-fluorouracil and irinotecan (FOLFIRI) remains a standard-of-care treatment for metastatic colorectal cancer (mCRC) yet benefits only about half of patients. Using patient-derived xenografts, we investigated the biological underpinnings of this heterogeneous response. FOLFIRI-resistant models showed transcriptional upregulation of innate immunity and mitochondrial metabolism genes, together with reduced expression of the DNA polymerase POLD1. Sensitive counterparts exhibited a BRCAness-like phenotype with genomic scars of homologous recombination (HR) deficiency, not caused by genetic or epigenetic loss of HR genes but by low abundance of the RAD51 recombinase. In tumoroids, forced RAD51 overexpression attenuated HR deficiency-related scars and chemotherapy-induced damage, whereas HR inhibition through ATM blockade enhanced drug sensitivity. The predictive relevance of key response determinants was validated in clinical samples. This work illuminates functional, nongenetic facets of BRCAness in mCRC and introduces actionable biomarkers and targets, offering prospects to improve clinical decision-making and broaden therapeutic options for chemorefractory patients. SIGNIFICANCE: FOLFIRI response biomarkers in mCRC are lacking. Evidence in patient-derived xenografts, tumoroids, and patients shows that chemosensitivity arises from functional relaxation, rather than (epi)genetic inactivation, of the HR DNA repair pathway. Integrative analyses yield a chemopredictive algorithm centered on the expression of the RAD51 recombinase, with potential to refine patient stratification.