What is the role of PD-L1 (Programmed Death-Ligand 1) and MMR (Mismatch Repair) testing in a patient with metastatic breast cancer and liver and ovarian involvement?

PD-L1 and MMR Testing in Metastatic Breast Cancer

In metastatic breast cancer with liver and ovarian involvement, PD-L1 testing is mandatory only for triple-negative disease using the FDA-approved 22C3 assay (CPS ≥10 threshold for pembrolizumab) or SP142 assay (IC ≥1% for atezolizumab), while dMMR/MSI-H testing should be performed to identify rare candidates for tissue-agnostic immunotherapy with pembrolizumab or dostarlimab, though MMR deficiency occurs in only 0.04% of breast cancers. 1, 2

PD-L1 Testing: When and How

Indication for Testing

• PD-L1 testing is required only for locally recurrent unresectable or metastatic hormone receptor-negative, HER2-negative (triple-negative) breast cancer when immune checkpoint inhibitor therapy is being considered. 1, 3
• Testing is NOT indicated for hormone receptor-positive or HER2-positive breast cancer outside clinical trials, as immunotherapy has no established role in these subtypes. 1, 3
• Immunotherapy eligibility requires that metastatic disease developed de novo OR at least 12 months after completion of (neo)adjuvant chemotherapy. 3

Assay-Specific Requirements

The specific assay used MUST match the immunotherapy agent being considered—these are NOT interchangeable: 1, 3, 4

• For pembrolizumab plus chemotherapy: Use the 22C3 pharmDx assay on Dako Autostainer Link 48 platform, with positivity defined as Combined Positive Score (CPS) ≥10. 1, 3

  • CPS = (number of PD-L1 staining tumor cells + lymphocytes + macrophages) ÷ total viable tumor cells × 100. 1
  • This assay evaluates both tumor cells AND immune cells in tumor and stroma. 1, 3

• For atezolizumab plus nab-paclitaxel: Use the SP142 assay on Ventana BenchMark Ultra platform, with positivity defined as Immune Cell (IC) score ≥1%. 3, 5

  • IC score = percentage of tumor area occupied by PD-L1-positive immune cells (lymphocytes, dendritic cells, macrophages, granulocytes). 5
  • This assay evaluates ONLY immune cells, not tumor cells. 5

• Alternative validated assay: The SP263 assay on Ventana BenchMark Ultra can assess CPS with almost perfect interobserver agreement (κ=0.972) and may be used where country-specific approvals permit, but must use the same platform and CPS ≥10 threshold. 4

Critical Testing Pitfalls to Avoid

• Never use different assays interchangeably—CPS and IC scores identify different patient cohorts and have different clinical validation. 4, 5
• Avoid biopsying bone metastases for PD-L1 testing due to technical limitations with decalcified tissue; select soft tissue metastases when possible. 1
• PD-L1 expression shows substantial intratumor heterogeneity, so adequate tissue sampling is essential. 6
• The 22C3 and SP263 assays show excellent concordance for CPS scoring (ICC=0.909), but each must be used on its designated platform. 4

MMR/MSI-H Testing: Tissue-Agnostic Immunotherapy Eligibility

When to Test

Patients with metastatic breast cancer who are candidates for immune checkpoint inhibitor therapy should undergo dMMR/MSI-H testing to determine eligibility for pembrolizumab or dostarlimab monotherapy, despite the extremely low prevalence in breast cancer. 1

Testing Methodology

• IHC for dMMR is PREFERRED over MSI testing for breast cancer (and all non-colorectal cancers) per College of American Pathologists recommendations. 1
• Use four-antibody IHC panel (MLH1, MSH2, MSH6, PMS2) or two-antibody panel; the VENTANA MMR RxDx Panel is the FDA-approved companion diagnostic. 1
• MSI testing via PCR or NGS (FoundationOne CDx) may miss minimal microsatellite shifts common in breast cancer, particularly MSH6 mutations. 1

Clinical Context and Expectations

• MMR deficiency occurs in only 0.04% of breast cancers, making this an extremely rare finding. 2
• The single documented MMR-deficient breast cancer case was a high-grade triple-negative tumor with dual MLH1/PMS2 loss that also expressed PD-L1. 2
• In the KEYNOTE-158 trial of pembrolizumab in MSI-H/dMMR solid tumors, only 6.4% of enrolled patients had ovarian cancer (relevant to your patient's ovarian involvement), with overall response rate of 34.3% and median OS of 23.5 months. 1

Important Distinction Between Tests

• Pembrolizumab is FDA-approved for both MSI-H AND dMMR solid tumors, while dostarlimab is approved only for dMMR tumors. 1
• MSI and MMR status are largely concordant, but a small fraction of tumors may lose MMR protein expression while remaining microsatellite stable (MSS), and vice versa. 1

Practical Algorithm for Your Patient

Step 1: Confirm Tumor Biology

• Biopsy metastatic site (preferably liver over bone) to confirm histology and reassess ER, PR, HER2 status. 1
• If discordant results between primary and metastatic sites, multidisciplinary discussion required; use most recent biology to guide treatment when feasible. 1

Step 2: Determine PD-L1 Testing Need

• If triple-negative (ER/PR/HER2-negative): Proceed with PD-L1 testing using 22C3 assay (for pembrolizumab) or SP142 assay (for atezolizumab). 1, 3
• If hormone receptor-positive or HER2-positive: PD-L1 testing NOT indicated; immunotherapy has no role outside clinical trials. 1, 3

Step 3: MMR/MSI-H Testing

• Perform IHC for dMMR using four-antibody panel regardless of subtype, given tissue-agnostic approval of pembrolizumab/dostarlimab. 1
• Recognize this will be positive in <1% of cases but identifies patients eligible for monotherapy with durable responses. 1, 2

Step 4: Treatment Selection Based on Results

For triple-negative disease:

• If PD-L1 CPS ≥10: Pembrolizumab plus chemotherapy (nab-paclitaxel, paclitaxel, or gemcitabine-carboplatin) improves PFS from 5.6 to 9.7 months (HR 0.65). 1, 3
• If PD-L1 IC ≥1%: Atezolizumab plus nab-paclitaxel improves OS from 15.1 to 25 months. 3
• If dMMR/MSI-H (regardless of PD-L1): Pembrolizumab or dostarlimab monotherapy with 34.3% response rate and median OS 23.5 months. 1

For non-triple-negative disease:

• If dMMR/MSI-H: Pembrolizumab or dostarlimab monotherapy is the only immunotherapy option. 1
• If MMR-proficient: No role for immunotherapy; proceed with standard endocrine therapy or HER2-directed therapy. 1, 3

Emerging Data on Combined Biomarkers

Recent research suggests that 45-59% of PD-L1-negative triple-negative breast cancers show heterogeneous MMR expression (hMMR) or high tumor-infiltrating lymphocytes (TILs), potentially identifying additional immunotherapy candidates. 6 However, this remains investigational and should not guide current clinical practice outside trials. 6