Neoadjuvant Immunotherapy-Based Systemic Treatment in MMR-Deficient or MSI-High Rectal Cancer: Case Series.

Treatment options for locally advanced rectal cancer have continued to consist largely of chemotherapy, chemoradiation, and/or surgical resection. For patients who are unable to undergo these therapeutic modalities or who do not to experience a response to them, treatment options are limited. We report 3 cases of mismatch repair-deficient (dMMR) locally advanced adenocarcinoma of the rectum that showed significant response with neoadjuvant immunotherapy-based systemic treatment. The first patient was not eligible for standard therapy because of a history of radiotherapy to the prostate with concurrent comorbidities and therefore received single-agent pembrolizumab. The second patient did not respond to total neoadjuvant chemoradiation and subsequently received combined nivolumab and ipilimumab. The third patient had a known family history of Lynch syndrome and presented with locally advanced rectal cancer and a baseline carcinoembryonic antigen level of 1,566 ng/mL. She was treated using neoadjuvant pembrolizumab and FOLFOX (folinic acid, fluorouracil, oxaliplatin). In this small series, we suggest that single-agent and combined-modality neoadjuvant immunotherapy/chemotherapy appear to be safe and effective treatment options for patients with (dMMR) locally advanced rectal cancer. Our findings encourage further studies to investigate the role of neoadjuvant immunotherapy as a viable treatment strategy in this population.