In colon adenocarcinoma, when is immunotherapy indicated and what regimens are recommended?

Immunotherapy for Colon Adenocarcinoma

Immunotherapy is indicated as first-line treatment for metastatic colon adenocarcinoma with deficient mismatch repair (dMMR) or microsatellite instability-high (MSI-H) status, with pembrolizumab monotherapy being the preferred regimen based on superior safety profile and durable responses compared to chemotherapy. 1

Primary Indication: dMMR/MSI-H Metastatic Disease

First-Line Treatment

• Pembrolizumab monotherapy is the standard of care for unresectable or metastatic dMMR/MSI-H colon cancer 1, 2
• This recommendation is based on the KEYNOTE-177 trial, which demonstrated progression-free survival advantage over chemotherapy with or without bevacizumab or cetuximab 1
• Pembrolizumab achieved 83% ongoing response at 24 months versus 35% for chemotherapy, despite similar overall response rates 1
• Clinically meaningful improvements in health-related quality of life were observed compared to chemotherapy 1
• Grade 3-4 immune-related adverse events occur in only 4-5% of patients 1, 3

Alternative First-Line Option

• Nivolumab plus ipilimumab is recommended as an alternative first-line regimen for dMMR/MSI-H metastatic colon cancer 4, 2
• The CheckMate 142 study showed promising results with this combination 1
• This regimen is particularly useful when pembrolizumab is unavailable or contraindicated 4

Second-Line and Beyond

For dMMR/MSI-H Tumors Progressing After First-Line Chemotherapy

• Ipilimumab-nivolumab combination is recommended for patients who received chemotherapy first-line 2
• Single-agent PD-1 inhibitors (pembrolizumab or nivolumab) are options in second- or third-line settings 5
• Do not use one PD-1 inhibitor after progression on another—there is no evidence of benefit from sequential PD-1 blockade 5

Locally Advanced Non-Metastatic Disease

Neoadjuvant Setting

• Neoadjuvant nivolumab plus ipilimumab is recommended for locally advanced dMMR colon cancer 3, 6
• The NICHE-2 trial demonstrated 98% pathological response rate, including 68% complete pathological response 3
• 98% of patients underwent timely surgery with acceptable safety profile 3
• This approach was endorsed by NCCN in April 2023 for select patients with nonmetastatic dMMR colon cancer 6
• Dostarlimab monotherapy achieved 100% complete clinical response in stage II-III dMMR rectal cancer and may be considered for colon cancer 7

Critical Exclusions and Testing Requirements

Who Should NOT Receive Immunotherapy

• Proficient mismatch repair (pMMR) or microsatellite stable (MSS) tumors do not benefit from immunotherapy 8, 1
• High tumor mutational burden (TMB ≥10 mutations/megabase) alone is NOT sufficient—pembrolizumab showed no benefit in pMMR colon cancer with high TMB 1
• PD-L1 testing has no role in colon cancer and should not be performed 9

Required Testing

• All metastatic colon adenocarcinoma must be tested for MMR/MSI status before treatment decisions 9
• Testing can be performed via immunohistochemistry for MMR proteins (MLH1, MSH2, MSH6, PMS2) or PCR-based MSI testing 9
• dMMR/MSI-H occurs in approximately 5% of metastatic colon cancer and 10-15% of nonmetastatic disease 8, 3

Response Patterns and Management

Expected Outcomes

• Approximately 20-30% of dMMR patients do not respond to immunotherapy 8
• Progressive disease occurs in 29% with pembrolizumab versus 12% with chemotherapy 1
• For responders, responses are highly durable with 83% maintaining response at 24 months 1

Non-Responders to Immunotherapy

• Patients with dMMR tumors who progress on immunotherapy should receive standard chemotherapy regimens (FOLFOX, FOLFIRI) with or without targeted agents 8
• The biological rationale for chemotherapy efficacy in dMMR tumors remains under investigation 8

Special Molecular Considerations

BRAF V600E Mutations

• If tumor is both dMMR/MSI-H and BRAF V600E mutated, prioritize checkpoint inhibitor first-line 9
• BRAF inhibitor regimens (encorafenib-cetuximab) can be reserved for later lines 9

POLE/POLD1 Mutations

• Pathogenic POLE mutations in pMMR colon cancer may predict response to PD-1/PD-L1 inhibitors 9
• Clinical benefit rates of 82.4% and median PFS of 15.1 months were observed in POLE pathogenic variant tumors treated with checkpoint inhibitors 9

Common Pitfalls to Avoid

• Never use immunotherapy in pMMR/MSS colon cancer outside clinical trials—this is ineffective and exposes patients to unnecessary toxicity 1
• Do not delay MMR/MSI testing until after first-line chemotherapy failure—test at diagnosis to optimize treatment sequencing 9
• Avoid sequential use of different PD-1 inhibitors after progression on one agent 5
• Do not combine immunotherapy with anti-EGFR antibodies in routine practice outside of specific clinical scenarios 4