What is the role of Microsatellite Instability (MSI) testing in determining suitability for immunotherapy, specifically with pembrolizumab (pembrolizumab)?

The Role of Microsatellite Instability (MSI) Testing in Determining Suitability for Immunotherapy with Pembrolizumab

MSI testing is essential for identifying patients who will benefit from pembrolizumab immunotherapy across multiple tumor types, with MSI-high status serving as a tissue-agnostic biomarker that predicts favorable response regardless of cancer origin.

Understanding MSI and Its Significance

Microsatellite Instability (MSI) is a genomic marker that indicates deficiency in DNA mismatch repair (dMMR). This condition results in:

• Accumulation of mutations in short tandem repeat sequences (microsatellites)
• Hypermutation phenotype with thousands of somatic mutations
• Increased neoantigen production that stimulates immune response

MSI/dMMR status has emerged as a critical predictive biomarker for response to immune checkpoint inhibitors, particularly pembrolizumab, across multiple tumor types 1.

Recommended Testing Approaches

First-Line Testing Method

• Immunohistochemistry (IHC) is the recommended first test for assessing MSI/dMMR status 1
  • Requires a four-antibody panel detecting MLH1, MSH2, MSH6, and PMS2 proteins
  • Loss of nuclear staining indicates dMMR
  • VENTANA MMR RxDx Panel is an FDA-approved companion diagnostic for pembrolizumab and dostarlimab 1

Confirmatory Testing Methods

• PCR-based testing should be used when IHC results are ambiguous 1

  • Recommended panel includes five poly-A mononucleotide repeats (BAT-25, BAT-26, NR-21, NR-24, NR-27)
  • MSI is present when two or more markers show repeat length alterations

• Next-Generation Sequencing (NGS) is an emerging alternative 1

  • Can simultaneously assess MSI and tumor mutational burden (TMB)
  • FoundationOne CDx is FDA-approved as a companion diagnostic for MSI-H determination 1
  • Particularly valuable for rare cancers not associated with Lynch syndrome

Important Testing Considerations

• For colorectal cancer, ESMO recommends using both MMR-IHC and MSI-PCR to avoid false positives (which occur in ~10% of cases) 1
• IHC for dMMR is preferred over MSI testing for non-colorectal cancers due to validation issues 1
• MSH6 mutations in endometrial cancers may cause minimal microsatellite shifts that standard MSI testing might miss 1

Patient Selection for MSI Testing

Recommended for Testing

• All patients with advanced (unresectable or metastatic) solid tumors with high incidence of MSI/dMMR 1

  • Colorectal, endometrial, gastric cancers
  • Small intestine, urothelial, central nervous system tumors
  • Sebaceous gland neoplasms

• Consider testing in advanced solid tumors with low incidence of MSI/dMMR 1

Not Recommended for Testing

• Patients with localized resectable non-colorectal tumors unless Lynch syndrome is clinically suspected 1

Clinical Benefits of Pembrolizumab in MSI-H/dMMR Tumors

• High objective response rates (ORR) of 39.6-55.9% across tumor types 1, 2
• Durable responses with median duration of response not reached in many studies, with ≥78% of patients maintaining response at 6 months 1
• Improved progression-free survival compared to chemotherapy (24.2 months vs. 6.75 months) 2
• Preserved or improved quality of life during treatment 3
  • Patients with complete or partial response showed significant improvements in physical, role, emotional, and social functioning

Relationship Between MSI and Other Biomarkers

MSI and Tumor Mutational Burden (TMB)

• MSI-high tumors typically have high TMB, though the relationship varies by tumor type 1
• It's possible to have TMB-high without MSI-high, but MSI-high with TMB-low is rare (except in endometrial cancer) 1
• For MSI-high tumors, further analysis for TMB or PD-1/PD-L1 expression is not required to receive immunotherapy 1

MSI and PD-L1 Expression

• PD-L1 expression shows great variability across cancer types 1
• PD-L1 can be expressed in MSI-negative and TMB-low cases 1
• For MSI-high tumors, PD-L1 testing is not necessary for immunotherapy eligibility 1

Emerging Applications

• Neoadjuvant setting: Pembrolizumab shows promise in localized MSI-H/dMMR tumors with high pathologic complete response rates (65%) 4
• Organ-sparing strategies: High radiographic and endoscopic response rates may allow for non-surgical management in select patients 4

Pitfalls and Caveats

• Testing turnaround time: Real-world NGS testing may take a median of 73 days, which could delay treatment decisions 2
• Discordance between testing methods: A small fraction of tumors may lose MMR protein expression while remaining microsatellite stable, and vice versa 1
• Non-response in MSI-H tumors: Not all MSI-H tumors respond to immunotherapy, suggesting additional factors may influence response 5
• Different approval criteria: Pembrolizumab is approved for both MSI-H and dMMR tumors, while dostarlimab's tissue-agnostic indication includes only dMMR tumors 1

MSI testing has transformed cancer treatment by enabling a tissue-agnostic approach to immunotherapy with pembrolizumab, making it a critical component of precision oncology.