A Phase II Trial Assessing Nivolumab in Class II Expressing Microsatellite Stable Colorectal Cancer

Immuno-oncology is transforming the care of certain patients with cancer. Not all patients respond to these therapies however, and in some common cancers checkpoint blockade has failed to make any real impact. In 2014 there were over 41,000 new cases of colorectal cancer (CRC) in the UK and nearly 16,000 deaths from the disease, making it the second commonest cause of cancer death (Cancer Research UK Cancer Statistics Key Facts). 15% of patients with CRC develop it as a result of deficient mismatch repair (microsatellite instability - MSI): this cohort of patients respond well to PD-1/PD-L1 blockade as these tumours harbour a very high number of mutations thus increasing the likelihood of the presence of immunogenic neo-epitopes which elicit an immune response1. The majority of CRC patients, particularly those with metastatic disease (around 95%), do not display this hyper-mutator phenotype (microsatellite stable (MSS) CRC) and in these patients the results of PD-1/PD-L1 blockade have been disappointing. In summary, MSS CRC patients with a class II expression appear to represent immunologically a group of MSI-like MSS patients that may respond to usefully to the immunotherapy agent nivolumab as a single agent and thus a trial of nivolumab in patients with class II expression of their cancer cells appears to be highly justified.