Pembrolizumab in microsatellite-stable CRC

### Explanation of Key Terms:

#### **Microsatellite-Stable (MSS) Colorectal Cancer (CRC):**

Microsatellite stability refers to the genetic stability of certain DNA sequences called microsatellites, which are short, repetitive sequences scattered throughout the genome. In microsatellite-stable (MSS) colorectal cancer (CRC), these sequences remain intact, indicating that the tumor lacks defects in the mismatch repair (MMR) system. MSS tumors are characterized by low levels of mutations and are typically less responsive to immunotherapy compared to microsatellite instability-high (MSI-H) tumors, which have defective MMR and high mutation rates.

#### **Tumor Mutational Burden-High (TMB-H):**

Tumor mutational burden (TMB) measures the number of mutations per megabase (mut/Mb) of DNA within a tumor genome. TMB-H refers to tumors with a high mutation load, often defined as ≥10 mutations per megabase. High TMB is thought to generate more neoantigens, making tumors potentially more recognizable to the immune system. In certain cancers, TMB-H predicts better responses to immune checkpoint inhibitors (ICIs) like pembrolizumab. However, in MSS CRC, high TMB alone does not reliably predict immunotherapy efficacy.

#### **Metastatic Colorectal Cancer (mCRC):**

Metastatic colorectal cancer (mCRC) is an advanced stage of CRC where cancer has spread from the colon or rectum to other parts of the body, such as the liver, lungs, or lymph nodes. Treatment for mCRC often includes chemotherapy, targeted therapy, and, in some cases, immunotherapy depending on the tumor's genetic profile.

#### **Pembrolizumab:**

Pembrolizumab is an immune checkpoint inhibitor targeting the PD-1 receptor on T-cells. By blocking PD-1, pembrolizumab prevents cancer cells from evading immune detection, allowing the immune system to attack the tumor. It is FDA-approved for various cancers, including MSI-H CRC, but its effectiveness in MSS CRC is limited due to the immune-resistant nature of these tumors.

---

### **Summary of the Study:**

This study focused on evaluating the real-world efficacy of pembrolizumab in patients with **microsatellite-stable (MSS), tumor mutational burden-high (TMB-H) metastatic colorectal cancer (mCRC)**. The researchers used data from Japan’s national C-CAT genomic database, which included 127 patients with TMB-H mCRC treated with pembrolizumab. Of these, 77 had MSS tumors, and 50 had MSI-H tumors.

#### **Key Findings:**

1. **Poor Outcomes in MSS-TMB-H Subgroup:**

• Patients with MSS-TMB-H mCRC had significantly worse outcomes compared to MSI-H-TMB-H patients.
• Median overall survival (OS) for MSS-TMB-H was **4.5 months**, whereas MSI-H-TMB-H patients had a median OS of **33.6 months**.
• Median time to treatment failure (TTF) was **2.0 months** for MSS-TMB-H versus **10.6 months** for MSI-H-TMB-H.

2. **Comparative Efficacy:**

• Pembrolizumab demonstrated **shorter overall survival (OS)** and **time to treatment failure (TTF)** compared to standard later-line treatments like **trifluridine/tipiracil (FTD/TPI)** ± bevacizumab and regorafenib.
• For MSS-TMB-H patients, pembrolizumab showed no survival advantage over regorafenib and performed worse than FTD/TPI ± bevacizumab.

3. **Limited Predictive Power of TMB:**

• High TMB alone (≥10 muts/Mb) was not a reliable predictor of pembrolizumab response in MSS tumors.
• MSI status was a stronger determinant of benefit, highlighting the importance of combining MSI and TMB profiling in treatment decisions.

4. **Genomic Insights:**

• MSS-TMB-H tumors resembled MSS-TMB-L tumors genomically, explaining their poor response to pembrolizumab despite high TMB.
• MSS-TMB-H CRCs were found to be "immune cold," with low T-cell infiltration and reduced immune activation.

5. **Rare Exceptions:**

• Rare MSS-TMB-H cases with pathogenic **POLE mutations** (hypermutated subtype) showed exceptionally high TMB (>100 muts/Mb) and better outcomes with pembrolizumab.

6. **Predictive Biomarkers:**

• Amplifications in **BCL2L1** and **SRC** (Chr20q11 region) correlated with longer progression-free survival (PFS) and overall survival (OS) under pembrolizumab treatment, suggesting these may serve as prognostic markers.

---

### **Clinical Implications:**

1. **Pembrolizumab Limitations:**

• Pembrolizumab should **not be considered a standard later-line therapy** for MSS-TMB-H mCRC due to minimal efficacy in this subgroup.
• MSI status remains the key biomarker for pembrolizumab eligibility.

2. **Alternative Therapies:**

• Standard treatments like **FTD/TPI ± bevacizumab** and **regorafenib** demonstrated better survival outcomes in MSS-TMB-H mCRC and should remain the preferred options for later-line therapy.

3. **Biomarker Refinement:**

• The study emphasizes the need for **refined biomarker-based therapy approvals**, recommending combined MSI and TMB profiling to guide immunotherapy decisions.

4. **Research Direction:**

• Future studies should explore alternative strategies for treating MSS-TMB-H mCRC, including therapies targeting the tumor microenvironment or novel combinations of immunotherapy and other modalities.

---

### **Core Conclusion:**

Pembrolizumab demonstrates **limited efficacy** in MSS-TMB-H metastatic colorectal cancer due to genomic and immunologic similarities to MSS-TMB-L disease, which is inherently resistant to immune checkpoint inhibitors. The study underscores the importance of alternative, non-immunotherapy strategies for this subgroup and highlights the need for improved biomarker-based treatment approaches.