This podcast reviews the results of KEYNOTE 164 investigating the use of pembrolizumab for mismatch repair deficient metastatic colorectal cancer, the place of this agent in the current clinical paradigm, and future directions to identify which patients are most likely to benefit from this treatment strategy.

TRANSCRIPT

This JCO Podcast provides observations and commentary on the JCO article 'A Phase II, Open-Label Study of Pembrolizumab in Treatment-Refractory, Microsatellite Instability-High/Mismatch Repair-Deficient Metastatic Colorectal Cancer: KEYNOTE-164' by Le et al. My name is Dustin Deming, and I am an associate professor at the University of Wisconsin Carbone Cancer Center in Madison, Wisconsin. My oncologic specialty is gastrointestinal oncology.

Microsatellite instability high status or mismatch repair deficiency is found in approximately 15% of early stage colorectal cancers, but only 3-4% of metastatic colorectal cancer. The mechanisms by which these cancers acquire their DNA repair aberrations can vary, including germline mutations, somatic mutations and promoter methylation, which is often observed in the setting of the hypermethylation phenotype associated with BRAF mutations. This distinct colorectal cancer subtype is of particular interest for immunotherapy strategies as the lack of adequate mismatch repair can lead to 1000s of mutations and also fusions leading to the potential for expression of more neoantigens.

This world-wide phase 2, open-label study enrolled 124 patients with microsatellite instability high or metastatic mismatch repair deficient colorectal cancer following 2 or more lines of standard therapy in cohort A and following 1 or more lines of therapy in cohort B. Patients received pembrolizumab 200 mg every 3 weeks, up to 2 years, until progression, unacceptable toxicity, or withdrawal. The primary endpoint was objective response rate by Response Evaluation Criteria in Solid Tumors version 1.1 by independent central review and secondary endpoints included duration of response progression-free survival, overall survival, safety and tolerability.

At the time of this report the median follow-up for cohort A was 31.3 months and 24.2 months for cohort B. The objective response rate was 33% for both cohorts. This includes 7 patients who achieved a complete response. The median PFS was 2.3 months for cohort A and 4.1 months for cohort B. For those patients that developed an objective response the duration of response was quite prolonged with the median duration of response not reached in either cohort. The median overall survival was 31.4 months for cohort A and not reached in cohort B. This treatment was well-tolerated in this population with the most common toxicities being fatigue, pancreatitis, and increased alanine aminotransferase or lipase.

Overall pembrolizumab is an exciting addition to the treatment strategy for patients with metastatic mismatch repair deficient cancers. Based on these results, in part, this agent is now FDA approved for patients with previously treated microsatellite instability high or mismatch repair deficient metastatic colon cancers after fluoropyrimidine, oxaliplatin, and irinotecan, and for patients also for non-colorectal solid tumors following at least one prior therapy, regardless of tumor type or origin. This was the first FDA approval of a tumor histology agnostic anticancer therapy.

Long-term follow-up from this, and similar cohorts, is required to further define the duration of response for these patients, as there is hope that some of these patients could even be cured. Unfortunately, it is only a minority of patients that seem to benefit from this approach as demonstrated by the short median progression free survival in both cohorts. A better understanding of which patients are likely to benefit from immunotherapy approaches are clearly needed.

The presence of Lynch syndrome was not captured in this study to evaluate for differential response in this setting. The BRAF mutation status was collected and across both cohorts 14 patients had BRAF mutant cancers. The response rate for these patients was 43%. A similar benefit was also observed in KRAS or NRAS mutant and wild-type cancers. This study was limited in its ability to further assess those factors that could influence pembrolizumab response given the relatively small sample size and limited biospecimen collection.

Further clinical trials are investigating the use of anti-PD1 therapies for these patients in the first-line and adjuvant settings, in combination with chemotherapy and with other immune checkpoint agents, such as CTLA4 and LAG3, among others. This includes Checkmate 142, which is a phase II study that is examining nivolumab and ipilimumab in a cohort of 46 patients with microsatellite instability high or mismatch repair deficient colorectal cancer in the first-line setting. Preliminary results were presented at the 2018 European Society of Medical Oncology meeting demonstrating a 60% objective response rate and a 12 month progression free survival of 77%. These early results are promising, but further investigation is needed.

As we look forward to which factors could be leading to a lack of clinical benefit from these agents it is important to consider those factors that are intrinsic to the cancer cells, tumor microenvironment, and patient specific factors. Tumor cell intrinsic factors include important cell attributes for the immune response such as the tumor mutation burden, MHC class I expression, including beta-2-microglobulin expression, the mutation profile, including alterations in WNT signaling shown to be important for immunotherapy resistance in metastatic melanoma, and tumor heterogeneity. There is also a growing understanding of factors that are important within the tumor microenvironment for tumors to be permissive to immune cell infiltration. These factors include differences in the immune and fibroblast cell subtypes present and also the presence of certain matrix proteins. This includes a matrix proteoglycan called versican that my laboratory and others have demonstrated has immunosuppressive properties, but can be cleaved by ADAMTS proteases to an immunostimulatory fragment. Additionally, patient specific factors need to be considered such as the microbiome, immunosuppression and adverse event management.

In summary, the results of KEYNOTE 164 are a significant advance for patients with microsatellite instability-high and mismatch repair deficient cancers. Long-term follow-up from this study and further studies into the most efficacious clinical setting to use these agents will continue to advance the clinical use of immunotherapy options for these patients.

This concludes this JCO Podcast. Thank you for listening.

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Advancing Treatment Options for Mismatch Repair-Deficient Metastatic Colorectal Cancers