Further Stratifying Patients With High Microsatellite Instability (MSI-H) In Colorectal Carcinoma with Tumor Mutation...

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Alexa Schrock, PhD, of Foundation Medicine Discusses High Microsatellite Instability (MSI-H) in Colorectal Carcinoma Study: Tumor mutational burden is predictive of response to immune checkpoint inhibitors in MSI-high metastatic colorectal cancer.

This study, published in Annals of Oncology, shows that tumor mutational burden (TMB) may help inform treatment decisions for MSI-H metastatic colorectal cancer patients. In the study, researchers found that TMB appears to be an important biomarker within this subset of patients and may be a predictor for whether these patients will respond to immunotherapy. The study supports the idea that further stratifying MSI-H metastatic colorectal cancer patients based on the level of TMB present could be an important part of treatment planning.

The study used genomic insights from Foundation Medicines comprehensive genomic profiling (CGP) assays (FoundationOne® and FoundationOne®CDx), as well as real-world outcome data from 22 patients with advanced colorectal cancer from five oncology institutions throughout the U.S., including City of Hope (Duarte, CA), The Angeles Clinic (Los Angeles, CA), Comprehensive Cancer Centers of Nevada, UC Davis, and University of Chicago.

Microsatellite instability (MSI) is a biomarker for response to immune checkpoint inhibitors (ICPIs). PD-1 inhibitors in metastatic colorectal carcinoma (mCRC) with MSI-high (MSI-H) have demonstrated a high disease control rate and favorable progression-free survival (PFS); however, reported response rates to pembrolizumab and nivolumab are variable and often <50%, suggesting that additional predictive biomarkers are needed.

Clinicopathologic data were collected from patients with MSI-H mCRC confirmed by hybrid capture-based next-generation sequencing (NGS) treated with PD-1/L1 inhibitors at five institutes. Tumor mutational burden (TMB) was determined on 0.81.1 Mb of sequenced DNA and reported as mutations/Mb. Potential biomarkers of response and time to progression were analyzed by univariate and multivariate analyses. Once TMB was confirmed as a predictive biomarker, a larger dataset of 18 140 unique CRC patients was analyzed to define the relevance of the identified TMB cut-point.

A total of 22 patients were treated with PD-1/L1 inhibitors including 19 with pembrolizumab monotherapy. Among tested variables, TMB showed the strongest association with objective response (OR; P?<?0.001) and PFS, by univariate (P?<?0.001) and multivariate analysis (P?<?0.01). Using log-rank statistics, the optimal predictive cut-point for TMB was estimated between 37 and 41 mutations/Mb. All 13 TMBhigh cases responded, while 6/9 TMBlow cases had progressive disease. The median PFS for TMBhigh has not been reached (median follow-up >18?months) while the median PFS for TMBlow was 2?months. A TMB of 37.4 mutations/Mb in a large MSI-H mCRC population (821/18, 140 cases; 4.5%) evaluated by NGS corresponded to the 35th percentile cut-point.

TMB appears to be an important independent biomarker within MSI-H mCRC to stratify patients for likelihood of response to ICPIs. If validated in prospective studies, TMB may play an important role in guiding the sequencing and/or combinations of ICPIs in MSI-H mCRC.