The emergence of drug resistance limits the efficacy of targeted therapies in human tumors. The prevalent view is that resistance is a fait accompli: when treatment is initiated, cancers already contain drug-resistant mutant cells. Bacteria exposed to antibiotics transiently increase their mutation rates (adaptive mutability), thus improving the likelihood of survival. We investigated whether human colorectal cancer (CRC) cells likewise exploit adaptive mutability to evade therapeutic pressure. We found that epidermal growth factor receptor (EGFR)/BRAF inhibition down-regulates mismatch repair (MMR) and homologous recombination DNA-repair genes and concomitantly up-regulates error-prone polymerases in drug-tolerant (persister) cells. MMR proteins were also down-regulated in patient-derived xenografts and tumor specimens during therapy. EGFR/BRAF inhibition induced DNA damage, increased mutability, and triggered microsatellite instability. Thus, like unicellular organisms, tumor cells evade therapeutic pressures by enhancing mutability.

Entrectinib is a fi rst-in-class pan-TRK kinase inhibitor currently undergoing clinical testing in colorectal cancer and other tumor types. A patient with metastatic colorectal cancer harboring an LMNA-NTRK1 rearrangement displayed a remarkable response to treatment with entrectinib, which was followed by the emergence of resistance. To characterize the molecular bases of the patient's relapse, circulating tumor DNA (ctDNA) was collected longitudinally during treatment, and a tissue biopsy, obtained before entrectinib treatment, was transplanted in mice (xenopatient), which then received the same entrectinib regimen until resistance developed. Genetic profi ling of ctDNA and xenopatient samples showed acquisition of two point mutations in the catalytic domain of NTRK1 , p.G595R and p.G667C. Biochemical and pharmacologic analysis in multiple preclinical models confi rmed that either mutation renders the TRKA kinase insensitive to entrectinib. These fi ndings can be immediately exploited to design next-generation TRKA inhibitors. SIGNIFICANCE:We provide proof of principle that analyses of xenopatients (avatar) and liquid biopsies allow the identifi cation of drug resistance mechanisms in parallel with clinical treatment of an individual patient. We describe for the fi rst time that p.G595R and p.G667C TRKA mutations drive acquired resistance to entrectinib in colorectal cancers carrying NTRK1 rearrangements. Cancer Discov; 6(1);[36][37][38][39][40][41][42][43][44]

ALK, ROS1, and NTRK rearrangements define a new rare subtype of mCRC with extremely poor prognosis. Primary tumor site, MSI-high, and RAS and BRAF wild-type status may help to identify patients bearing these alterations. While sensitivity to available treatments is limited, targeted strategies inhibiting ALK, ROS, and TrkA-B-C provided encouraging results.

Colorectal cancer ( CRC ) is the third leading cause of cancer‐related deaths worldwide. Therapies that take advantage of defects in DNA repair pathways have been explored in the context of breast, ovarian, and other tumor types, but not yet systematically in CRC . At present, only immune checkpoint blockade therapies have been FDA approved for use in mismatch repair‐deficient colorectal tumors. Here, we discuss how systematic identification of alterations in DNA repair genes could provide new therapeutic opportunities for CRC s. Analysis of The Cancer Genome Atlas Colon Adenocarcinoma ( TCGA ‐ COAD ) and Rectal Adenocarcinoma ( TCGA ‐ READ ) PanCancer Atlas datasets identified 141 (out of 528) cases with putative driver mutations in 29 genes associated with DNA damage response and repair, including the mismatch repair and homologous recombination pathways. Genetic defects in these pathways might confer repair‐deficient characteristics, such as genomic instability in the absence of homologous recombination, which can be exploited. For example, inhibitors of poly( ADP )‐ribose polymerase are effectively used to treat cancers that carry mutations in BRCA 1 and/or BRCA 2 and have shown promising results in CRC preclinical studies. HR deficiency can also occur in cells with no detectable BRCA 1/ BRCA 2 mutations but exhibiting BRCA ‐ like phenotypes. DNA repair‐targeting therapies, such as ATR and CHK 1 inhibitors (which are most effective against cancers carrying ATM mutations), can be used in combination with current genotoxic chemotherapies in CRC s to further improve therapy response. Finally, therapies that target alternative DNA repair mechanisms, such as thiopurines, also have the potential to confer increased sensitivity to current chemotherapy regimens, thus expanding the spectrum of therapy options and potentially improving clinical outcomes for CRC patients.