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Summary
Colorectal cancer (CRC) is the second leading cause of cancer death in the US, with approximately 153,000 new cases diagnosed annually. Molecular profiling has revealed that CRC is not one disease — MSI-H, KRAS, BRAF, HER2, and NTRK alterations each define distinct subtypes with different prognoses and treatment strategies. In 2026, immunotherapy has become standard for MSI-H disease, KRAS G12C inhibitors are reshaping one previously undruggable subgroup, and liquid biopsy (ctDNA) is revolutionizing how we monitor treatment response and guide adjuvant therapy decisions.
Immunotherapy for MSI-H/dMMR Colorectal Cancer
Mismatch repair deficiency (dMMR) or microsatellite instability-high (MSI-H) tumors — about 5% of metastatic CRC — have high mutational burden and are highly responsive to PD-1 checkpoint inhibitors. The KEYNOTE-177 trial established pembrolizumab as first-line therapy for MSI-H metastatic CRC, showing superior progression-free survival (16.5 vs. 8.2 months) versus FOLFOX/FOLFIRI ± bevacizumab. Nivolumab ± ipilimumab (CheckMate 142) is also approved for this subgroup.
The most remarkable development in this space has been the KEYNOTE-158 and CMT116 trials in locally advanced rectal cancer: pembrolizumab given as neoadjuvant (pre-surgery) therapy produced complete clinical responses in up to 100% of MSI-H rectal cancer patients in some cohorts — allowing patients to avoid surgery entirely ("watch and wait" approach). The MRC PROSPECT trial and Andrea Cercek's Memorial Sloan Kettering data are driving a paradigm shift in how MSI-H rectal cancer is managed. Phase 3 trials formalizing organ-preservation approaches are now enrolling.
KRAS G12C Inhibitors: Cracking an "Undruggable" Target
KRAS mutations are present in ~45% of colorectal cancers. For decades, KRAS was considered undruggable — its smooth GTP-binding surface offered no obvious drug-binding pocket. KRAS G12C mutations create a cysteine residue that allows covalent inhibition when KRAS is in its GDP-bound (inactive) state.
Sotorasib (Lumakras) and adagrasib (Krazati) are both FDA-approved KRAS G12C inhibitors in CRC, but single-agent response rates are modest (approximately 9–19%) compared to their activity in lung cancer. This appears to be due to rapid feedback reactivation of EGFR signaling in CRC. Combination strategies pairing KRAS G12C inhibitors with EGFR inhibitors (cetuximab, panitumumab) are in Phase 2/3 trials and showing substantially higher response rates: sotorasib + panitumumab (CODEBREAK 300) showed 49% ORR vs. 2% for sotorasib alone. This combination received FDA approval in 2024 and is now standard of care for KRAS G12C-mutated CRC after prior therapy.
HER2-Targeted Therapy in CRC
HER2 amplification or overexpression is present in approximately 3–5% of metastatic CRC — a small but clinically relevant subgroup that has historically been undertreated. Unlike breast cancer where trastuzumab is transformative, HER2-targeted therapy alone in CRC showed modest results, but combinations are more effective.
The MOUNTAINEER trial established tucatinib + trastuzumab as a standard option for HER2-positive CRC, showing a 38.1% ORR and median OS of 24.1 months in heavily pre-treated patients. This led to FDA approval in 2023. Trastuzumab deruxtecan (T-DXd) showed a 45.3% ORR in HER2-positive CRC in DESTINY-CRC02, establishing it as another option. Ongoing trials are evaluating T-DXd in earlier lines of therapy and in combination with chemotherapy backbones for HER2-positive CRC.
ctDNA-Guided Adjuvant Therapy: The Liquid Biopsy Revolution
Circulating tumor DNA (ctDNA) detected in blood can identify residual microscopic disease after surgery — so-called molecular residual disease (MRD) — before it manifests as clinical recurrence. This has profound implications for adjuvant chemotherapy decisions: patients who are ctDNA-negative after surgery may not need chemotherapy, while ctDNA-positive patients face high recurrence risk and may benefit most from aggressive treatment.
The DYNAMIC trial (Australia) demonstrated that ctDNA-guided decisions in Stage II colon cancer were non-inferior to standard clinicopathological decision-making — and reduced the proportion of patients receiving chemotherapy by 50%, sparing toxicity in low-risk patients. Multiple confirmatory trials are ongoing:
The COBRA trial (NCI-sponsored) and CIRCULATE-US are evaluating ctDNA-guided escalation and de-escalation in Stage II and III colon cancer. The ALTAIR trial in Japan showed ctDNA-positive patients who received immunotherapy showed improved outcomes in MSI-H ctDNA-positive CRC. The FDA has not yet approved ctDNA for routine adjuvant treatment guidance, but regulatory submissions are anticipated in 2026 based on accumulating data.
Key Takeaways
- Pembrolizumab is first-line standard of care for MSI-H/dMMR metastatic CRC, and complete responses in MSI-H rectal cancer are enabling organ-preservation "watch and wait" strategies.
- Sotorasib + panitumumab is FDA-approved for KRAS G12C-mutated CRC, showing ~49% ORR — the combination approach overcame the EGFR feedback resistance that limited monotherapy.
- HER2-amplified CRC (~3–5% of cases) is now targetable with tucatinib + trastuzumab (FDA-approved) and trastuzumab deruxtecan.
- ctDNA (liquid biopsy) after surgery can identify patients at high recurrence risk and may guide decisions to add or withhold adjuvant chemotherapy; regulatory submissions expected in 2026.
- Molecular profiling (MSI, KRAS, BRAF, HER2, NTRK) is now essential before treatment decisions in metastatic CRC.