Summary Background Metastatic DNA mismatch repair–deficient/microsatellite instability–high (dMMR/MSI-H) colorectal cancer (mCRC) has a poor prognosis following conventional chemotherapy and exhibits high levels of tumour neoantigens, tumour-infiltrating lymphocytes, and checkpoint regulators, all features that correspond with response to programmed cell death receptor-1 (PD-1) blockade in other tumour types. Thus, nivolumab, a PD-1 immune checkpoint inhibitor, was evaluated in this population. Methods In this is ongoing, multicentre, open-label, nonrandomised, phase 2 trial, adult patients (aged ≥18 years) with histologically confirmed recurrent or mCRC locally assessed as dMMR/MSI-H who had progressed on/after or been intolerant of at least one prior line of treatment, including a fluoropyrimidine and oxaliplatin or irinotecan, were enrolled. Patients were given nivolumab 3 mg/kg every 2 weeks until disease progression, death, unacceptable toxicity, or withdrawal from study. The primary endpoint was investigator-assessed objective response rate (ORR) per Response Evaluation Criteria In Solid Tumors v1·1. All patients who received at least one dose of study drug were included in the primary and safety analysis. This trial is registered with ClinicalTrials.gov, number NCT02060188. Findings Among the 74 patients who were enrolled between March 12, 2014, and March 16, 2016, most (54·1%) had received ≥3 prior therapies. At a median follow-up of 12·0 months (interquartile range 8·57–18·00 months), 23 of 74 patients (31·1% [95% CI 20·8%–42·9%]) achieved an investigator-assessed objective response; 68·9% (95% CI 57·1%–79·2%) of patients had disease control for ≥12 weeks. Median duration of response was not yet reached; all responders were alive, and 8 (34·8%) had responses of ≥12 months. The most common (≥10% of patients) drug-related adverse events was fatigue (n=16 [21·6%]), diarrhoea (n=15 [20·3%]), pruritus (n=10 [13·5%]) and rash (n=8 [10·8%]). The most common grade 3 or 4 drug-related adverse events were increased lipase (n=6 [8·1%]) and amylase (n=2 [2·7%]) levels. Five patients (6·8%) discontinued treatment because of increased alanine aminotransferase, colitis, duodenal ulcer, acute kidney injury, and stomatitis (n=1 each). Twenty-three patients (31·1%) died during the study; none of these deaths was considered to be treatment related by the investigator. Interpretation Nivolumab provided durable responses and disease control, as well as long-term survival in pre-treated patients with dMMR/MSI-H mCRC, and is a new treatment option for these patients.

Inhibitory receptors on immune cells are pivotal regulators of immune escape in cancer. Among these inhibitory receptors, CTLA-4 (targeted clinically by ipilimumab) serves as a dominant off-switch while other receptors such as PD-1 and LAG-3 seem to serve more subtle rheostat functions. However, the extent of synergy and cooperative interactions between inhibitory pathways in cancer remain largely unexplored. Here we reveal extensive co-expression of PD-1 and LAG-3 on tumor-infiltrating CD4+ and CD8+ T cells in three distinct transplantable tumors. Dual anti-LAG-3/anti-PD-1 antibody treatment cured most mice of established tumors that were largely resistant to single antibody treatment. Despite minimal immunopathological sequelae in PD-1 and LAG-3 single knockout mice, dual knockout mice abrogated self-tolerance with resultant autoimmune infiltrates in multiple organs, leading to eventual lethality. However, Lag3−/−Pdcd1−/− mice demonstrated markedly increased survival from and clearance of multiple transplantable tumors. Together, these results define a strong synergy between the PD-1 and LAG-3 inhibitory pathways in tolerance to both self and tumor antigens. Additionally, they argue strongly that dual blockade of these molecules represents a promising combinatorial strategy for cancer.