Temozolomide Treatment Alters Mismatch Repair and Boosts Mutational Burden in Tumor and Blood of Colorectal Cancer Patients

The majority of metastatic colorectal cancers (mCRC) are mismatch repair (MMR) profi cient and unresponsive to immunotherapy, whereas MMR-defi cient (MMRd) tumors often respond to immune-checkpoint blockade. We previously reported that the treatment of colorectal cancer preclinical models with temozolomide (TMZ) leads to MMR defi ciency, increased tumor mutational burden (TMB), and sensitization to immunotherapy. To clinically translate these find-ings, we designed the ARETHUSA clinical trial whereby O6-methylguanine -DNA-methyltransferase (MGMT)-defi cient, MMR-profi cient, RAS-mutant mCRC patients received priming therapy with TMZ. Analysis of tissue biopsies and circulating tumor DNA (ctDNA) revealed the emergence of a distinct mutational signature and increased TMB after TMZ treatment. Multiple alterations in the nucleotide context favored by the TMZ signature emerged in MMR genes, and the p.T1219I MSH6 variant was detected in ctDNA and tissue of 94% (16/17) of the cases. A subset of patients whose tumors dis-played the MSH6 mutation, the TMZ mutational signature, and increased TMB achieved disease stabi-lization upon pembrolizumab treatment.SIGNIFICANCE: MMR-profi cient mCRCs are unresponsive to immunotherapy. We provide the proof of concept that inactivation of MMR genes can be achieved pharmacologically with TMZ and molecularly monitored in the tissue and blood of patients with mCRC. This strategy deserves additional evaluation in mCRC patients whose tumors are no longer responsive to standard-of-care treatments. See related commentary by Willis and Overman, p. 1612.

Automated summary

The use of TMZ therapy in mCRC patients can induce MMR gene defects and TMB increase, leading to disease stabilization in a subset of patients.