Colorectal cancer is the third most common cancer worldwide, with about 15% of these tumours related with microsatellite instability, which confers distinct characteristics to these tumours, both clinicopathological and in the response to treatments. In fact, the poor response to chemotherapy in these tumours has led to the investigation for new treatments, with immunotherapy being the most successful one to date. The focus of this review is to assess the response of microsatellite unstable colorectal cancer to PD-1 blockade, and the mechanisms behind that response. A PubMed research was conducted, resulting in the inclusion of 47 articles in this review. Microsatellite instability results in a high neoantigen load, leading to a highly active immune microenvironment of the tumour, mainly due to T-cells. To counteract this, there is an upregulation of PD-1, acting as a “brake” for immune cells, facilitating tumour growth and metastasis. This upregulation makes these tumours great candidates for treatment with PD-1 blockade, as seen in many clinical trials, where the overall responses and progression free survival rates were higher than those observed in microsatellite stable tumours. With the importance of colorectal cancer with microsatellite instability new treatments are necessary. Therefore, PD-1 blockade is a promising treatment for colorectal cancer with microsatellite instability, with improvement in survival rates and a better prognosis for these patients.

Born in Porto in 1996. Attended the Faculty of Medicine of University of Porto with a Master’s Degree in Medicine, graduating in 2020. My interest in the Oncology field begun early in Medical School and I have been keen on learning about cancer, mainly how patients can be treated and what can be done to improve the patients’ quality of life. Furthermore, microsatellite instability and its role in cancer development and treatment modalities has been important in my life for many years, making this the object of my interest.