Immunotherapy for Colorectal Cancer: Who Benefits

Immune checkpoint inhibitors have transformed treatment for several cancers, but in colorectal cancer their benefit is sharply divided: for one biological subgroup the effect is dramatic and sometimes durable, while for the majority it is absent. The dividing line is mismatch repair status, and testing for it is the first and most important step. The Chinese expert consensus on immunotherapy for colorectal cancer sets out how to test, how to treat, and where the boundaries are.

The test that decides

Colorectal cancers are classified as mismatch repair deficient with high microsatellite instability (dMMR/MSI-H), or mismatch repair proficient and microsatellite stable (pMMR/MSS). The consensus recommends that MMR status be determined by immunohistochemistry in all patients, with PCR-based microsatellite instability testing performed routinely where available; next-generation sequencing is a further option. MSI-H tumours account for roughly 5% of metastatic disease but a higher proportion of early-onset and right-sided cancers, and about 15% of them are associated with Lynch syndrome — which is why a young patient with MSI-H disease should be referred for genetic counselling.

Metastatic disease: where immunotherapy has become standard

For dMMR/MSI-H metastatic colorectal cancer, PD-1 inhibitor therapy is the recommended first-line standard rather than a later option, and the consensus states it is better used early than reserved for second or later lines. Approved agents include pembrolizumab and nivolumab, alone or in combination with a CTLA-4 inhibitor where available. Two nuances matter: if the immediate goal is rapid control of a very high tumour burden, chemotherapy with or without a targeted agent may be the better initial choice; and if the goal is converting borderline disease to resectable, immunotherapy is preferred. Patients treated with immunotherapy should be discussed in a multidisciplinary meeting as part of a broader plan that may include surgery or ablation. Combination therapy is generally avoided outside trials when toxicity is a concern.

Where immunotherapy does not work

For pMMR/MSS metastatic disease — the large majority — anti-PD-1 therapy alone or in combination outside a clinical trial is not recommended. This is a firm statement and it protects patients from treatment with no benefit and real toxicity. Research continues on combinations that might convert "cold" tumours to responsive ones; the answer, for now, is clinical trials rather than off-protocol prescribing.

Early-stage disease

For non-metastatic MSI-H colorectal cancer, participation in neoadjuvant or adjuvant anti-PD-1 trials is encouraged, and neoadjuvant immunotherapy has produced remarkable complete-response rates in early rectal cancer series — raising the possibility of organ preservation in selected patients. For non-metastatic pMMR/MSS disease, anti-PD-1 therapy is not recommended.

Practical guidance

Ask two questions early: what is my MMR or MSI status, and was it tested by immunohistochemistry or PCR? Every patient with metastatic disease should know this result, because it determines eligibility for the most effective treatment available. If you have dMMR/MSI-H disease, ask whether first-line immunotherapy is appropriate for you, whether a trial is available, and whether Lynch syndrome testing is indicated for you and your family. If you have MSS disease, ask what trial options exist rather than seeking immunotherapy off-protocol.