What molecular tests, such as KRAS (Kirsten rat sarcoma viral oncogene homolog) and NRAS (Neuroblastoma RAS viral oncogene homolog), are needed for a patient with metastatic rectal cancer to guide treatment decisions?

Molecular Testing for Metastatic Rectal Cancer

All patients with metastatic rectal cancer must undergo comprehensive molecular testing including KRAS (exons 2,3,4), NRAS (exons 2,3,4), BRAF V600E, and MMR/MSI status at the time of diagnosis to guide anti-EGFR therapy decisions and identify candidates for immunotherapy. 1, 2, 3

Mandatory Molecular Tests

KRAS Testing (Essential)

• Test KRAS mutations in codons 12,13,61,117, and 146 (exons 2,3, and 4) in all metastatic rectal cancer patients who are candidates for anti-EGFR antibody therapy. 1
• KRAS mutations occur in approximately 49% of metastatic colorectal cancers and predict complete non-responsiveness to cetuximab and panitumumab. 4, 5
• The American Society of Clinical Oncology and FDA mandate that patients with KRAS codon 12 or 13 mutations should NOT receive anti-EGFR antibody therapy. 1
• Testing must be performed in a CLIA-accredited laboratory on tumor tissue from either the primary tumor or metastasis. 1

NRAS Testing (Essential)

• Test NRAS mutations in codons 12,13, and 61 (exons 2,3, and 4) in all metastatic rectal cancer patients being considered for anti-EGFR therapy. 2, 3, 6
• NRAS mutations occur in approximately 3-4% of metastatic colorectal cancers and also predict resistance to anti-EGFR therapy. 5, 7
• The FDA drug label for panitumumab explicitly states that RAS mutations (including NRAS) render EGFR inhibition ineffective because mutant RAS proteins remain continuously active independent of EGFR regulation. 6

BRAF V600E Testing (Strongly Recommended)

• Test BRAF V600E mutation status in all KRAS wild-type metastatic rectal cancer patients at diagnosis. 1, 2
• BRAF V600E mutations occur in approximately 5-10% of metastatic colorectal cancers and confer poor prognosis with median overall survival of only 9.2 months. 4, 5
• The National Comprehensive Cancer Network guidelines include BRAF genotyping as an option for KRAS wild-type stage IV disease. 1
• For BRAF V600E-mutated tumors, targeted combination therapy with BRAF and EGFR inhibitors extends overall survival to 9.3 months compared to 5.9 months with standard chemotherapy. 4

MMR/MSI Status Testing (Essential)

• Test all metastatic rectal cancers for mismatch repair (MMR) protein expression by immunohistochemistry (MLH1, MSH2, MSH6, PMS2) or microsatellite instability (MSI) by PCR. 2, 3
• This testing has Level I, Grade A recommendation from the European Society for Medical Oncology. 2
• Approximately 5% of metastatic colorectal cancers are MSI-H/dMMR, which completely changes the treatment paradigm. 2, 4
• For MSI-H/dMMR metastatic rectal cancer, first-line treatment is immunotherapy (dostarlimab, pembrolizumab, or nivolumab), NOT chemotherapy, with objective response rates of 40-43.5% and median overall survival of 31.4 months. 2, 4

Testing Algorithm

Step 1: Order Comprehensive Panel at Diagnosis

• Perform all molecular testing (KRAS, NRAS, BRAF, MMR/MSI) at the time of stage IV diagnosis, not in a time-sensitive manner before starting treatment. 1
• Next-generation sequencing panels are preferred over individual gene testing as they can simultaneously detect rare actionable mutations (NTRK, RET fusions, HER2 amplifications). 2

Step 2: Interpret Results for Treatment Selection

If MSI-H/dMMR detected:

• Offer immunotherapy (dostarlimab, pembrolizumab, or nivolumab) as first-line treatment regardless of RAS status. 2, 4
• Anti-EGFR therapy and standard chemotherapy are inferior options. 2

If MSS/pMMR and RAS wild-type (KRAS and NRAS):

• Patient is eligible for anti-EGFR therapy (cetuximab or panitumumab) in combination with chemotherapy. 1, 6
• This represents approximately 50% of metastatic colorectal cancer patients. 4
• Median survival extends by 2-4 months compared to chemotherapy alone. 4

If MSS/pMMR and RAS mutant (KRAS or NRAS):

• Anti-EGFR therapy is contraindicated and will not provide benefit. 1, 6
• Standard chemotherapy regimens (FOLFOX, FOLFIRI, CAPOX) with or without bevacizumab are appropriate. 2, 3
• This represents approximately 53% of metastatic colorectal cancer patients. 5

If BRAF V600E mutant:

• Consider targeted combination therapy with BRAF and EGFR inhibitors. 4
• Prognosis is poor with median overall survival of 9.2 months. 5

Additional Considerations

Testing Quality and Tissue Requirements

• Test at least two tumor sample blocks when possible, as testing a single sample may misclassify 10% of patients as wild-type due to tumor heterogeneity. 1
• For patients with limited tissue from endoscopic treatment, prioritize immunohistochemistry for MMR over PCR-based MSI testing as IHC requires less tissue. 2

Markers with Insufficient Evidence

• Evidence for PIK3CA, PTEN, and AKT testing remains insufficient to make definitive recommendations for or against their use in guiding anti-EGFR therapy. 1
• No US-based guidelines specifically recommend testing these markers for anti-EGFR therapy decisions. 1

Common Pitfalls to Avoid

• Do not start anti-EGFR therapy without confirming both KRAS and NRAS wild-type status, as approximately 4% of patients have NRAS mutations that would be missed by KRAS testing alone. 5, 7
• Do not delay molecular testing until after first-line therapy fails; test at diagnosis to plan the entire treatment continuum. 1
• Do not assume KRAS wild-type status guarantees response to anti-EGFR therapy; 40-60% of KRAS wild-type patients still fail to respond. 1