A Look at Established, Emerging Biomarkers for I-O Therapy in Breast Cancer

During the San Antonio Breast Cancer Symposium 2020, Lajos Pusztai, MD, DPhil, of Yale Cancer Center, reviewed recently established and emerging biomarkers in both the neoadjuvant and metastatic breast cancer settings.

Research focused on leveraging immuno-oncology (I-O) in breast cancer has been rampant in recent years, with 17,000 related manuscripts having been published in the last 2 years alone, explained Lajos Pusztai, MD, DPhil, professor of medicine, Yale Cancer Center, during a session at the San Antonio Breast Cancer Symposium 2020.

Throughout the session, Pusztai gave an overview of newly established and emerging biomarkers in both the neoadjuvant and metastatic breast cancer setting.

A Focus on PD-L1

Notable differences in the treatment strategy of neoadjuvant versus metastatic disease can be highlighted from the use of PD-1/PD-L1 immune checkpoint inhibitors, explained Pusztai, noting that PD-L1 expression does not hold as much weight in the neoadjuvant setting as it does in the metastatic setting.

For the neoadjuvant setting, he gave the examples of the KEYNOTE-522 and IMpassion031 trials, which both showed the benefit of (I-O) therapy in patients, regardless of PD-L1 expression. In the phase 3 KEYNOTE-522 study, which assessed pembrolizumab with chemotherapy in patients with triple negative breast cancer (TNBC), the combination was favorable over chemotherapy alone for both PD-L1+ (68.9% vs 54.9%) and PD-L1- (45.3% vs 30.3%) patients.1

In the Impassion031 study of atezolizumab with chemotherapy, the combination elicited improved complete response rates over chemotherapy alone for both PD-L1+ (69% vs 49%) and PD-L1- patients (48% vs 34%).2

Meanwhile, when administered in the metastatic setting, PD-L1 expression is all but required for response to I-O treatment, said Pusztai. For example, in the KEYNOTE-119 trial, while pembrolizumab was not found to be advantageous over chemotherapy, the study did show a “remarkably close” correlation between benefit with pembrolizumab and PD-L1 expression.3 In fact, in the group with the highest expression (CPS>20), response rates were 26.3% compared with 11.5% for chemotherapy alone. These patients accounted for 18% of the study population.

Similarly, the KEYNOTE-355 trial confirmed that benefit of adding pembrolizumab to chemotherapy as first-line treatment of patients with metastatic TNBC,4 leading to the FDA’s approval of the immunotherapy for patients with CPS>10.

Atezolizumab—the first FDA-approved immune checkpoint inhibitor in breast cancer—showed its benefit against PD-L1 expression in the IMpassion130 trial, in which the treatment in combination with nab-paclitaxel led to improved response rates in metastatic TNBC for the subset of patients who were PD-L1+.5

Pusztai noted that in this study, it became evident that choice of PD-L1 immunohistochemistry assay matters. Using the threshold of CPS>1, the researchers found that the SP142 assay identified the fewest positive patients but had the highest accuracy, confirming the notion that high PD-L1 expression is needed for benefit of an I-O.

“It is clear that in the neoadjuvant treatment setting PD-L1 expression is not required for benefit from anti-PD1/PD-L1 therapy with chemo whereas in the metastatic setting, PD-L1 expression is required,” said Pusztai, who left the audience with a question of, “Why?”

To answer just that, Pusztai and a group of researchers a few years back analyzed the microenvironment of primary and metastatic tissues. Their research showed that in a “fit” immune microenvironment of a primary tumor, with a few scattered immune cells (that could be missed by a PD-L1 immunohistochemistry in a biopsy) that can ignite an immune response, whereas with an immune attenuated metastatic lesion, a massive immune presence (high levels of PD-L1 expression serve as a surrogate for immune presence) is needed to mount effective immune response.

Other Novel Marker Concepts Gain Steam

Pusztai explained that there are several immunophenotypic markers being explored in research. Some research has focused on the potential use of imaging analysis techniques to query whether the proximity of tumor and immune cells or if the semi-quantitative analysis of different immune cell types and their ratios could serve as response markers. For example, in the I-SPY trial consortium, researchers found several correlations, including that greater T cell to tumor cell density is associated with a pathologic complete response (pCR), as is greater PD-1 tumor cell to PD-L1 immune cell proximity.

Another line of investigation has been focused on asking the question of what researchers can learn from exceptionally resistant breast cancer cases.

In ER+ early stage disease, Pusztai says researchers may have found a way to identify which patients need an immune checkpoint inhibitor added to their neoadjuvant chemotherapy to maximize response rated. In the I-SPY 2 trial, both pembrolizumab and durvalumab plus olaparib increased the pCR rates when combined with paclitaxel in ER+/HER2- patients.6,7

Among the ER+, MammaPrint—a prognostic and predictive diagnostic test for breast cancer that assesses the risk that a tumor will metastasize to other parts of the body—high patients in the study, only the MammaPrint ultra-high (or MP2) subpopulation had increased pCR rates, while the MP1 group had the same pCR rate seen with chemotherapy alone. Researchers looked further to see what molecular features characterized this MP2 subset, finding that low sensitivity to endocrine therapy gene signature and high expression of proliferation genes are the hallmarks of the group.

Other biomarkers being explored for identifying patients with metastatic breast cancer that can benefit from immune checkpoint therapy include: microsatellite instability, high tumor mutation burden, and tumor PD-L1 gene copy number gain/amplification.

A Future for Targeting Myeloid Cells?

Following Pusztai’s presentation, Xiang “Shawn” Zhang, assistant professor of molecular and cellular biology, Baylor College of Medicine, gave an overview of research being done to determine if there is a future for therapeutic targets in myeloid cells.

It’s being increasingly realized that myeloid cells are not only in normal tissue, but they also infiltrate tumors, and myeloid cells—including macrophages and neutrophils—are important constituents of the immunosuppressive microenvironment in breast cancer, said Zhang. While research is somewhat recent, there have been several strategies undertaken to target myeloid cells, including:

  • Blockade of recruitment: Myeloid cells typically use chemokine gradient, and through chemokine receptor signaling migrate into the tumor microenvironment, so blocking the relevant chemokine receptors has emerged as a promising strategy. For example, for granulocytic myeloid-derived suppressor cells, CXCR2 is a key chemokine receptor and disruption of chemokine 2 signaling has been pursued and tested
  • Depletion: For myeloid-derived suppressor cells, S100 calcium-binding protein A8 has been thought to be a target marker
  • Epigenomic reprogramming: Several clinical trials have been initiated to test the synergistic effect between drugs that are modifying genomic profiles with check 1 blockade therapies
  • Targeting the immunosuppressive pathways within myeloid cells: These pathways include the IL6-STAT3 pathway, PI3Kγ pathway, and ERa pathway


  1. Schmid P, Cortes J, Pusztai L, et al. Pembrolizumab for early triple-negative breast cancer. New Engl J Med. 2020;382:810-821.
  2. Mittendorf E, Zhang H, Barrios C, et al. Neoadjuvant atezolizumab in combination with sequential nab-paclitaxel and anthracycline-based chemotherapy versus placebo and chemotherapy in patients with early-stage triple-negative breast cancer (IMpassion031): a randomised, double-blind, phase 3 trial. Lancet. 2020;396(10257): P1090-1100.
  3. Cortes J, Lipatov O, Goncalves A, et al. KEYNOTE-119: Phase III study of pembrolizumab (pembro) versus single-agent chemotherapy (chemo) for metastatic triple negative breast cancer (mTNBC). Ann Oncol. 2019;30(5):v859-860.
  4. Cortes J, Cescon D, Rugo H, et al. KEYNOTE-355: Randomized, double-blind, phase III study of pembrolizumab + chemotherapy versus placebo + chemotherapy for previously untreated locally recurrent inoperable or metastatic triple-negative breast cancer. J Clin Oncol. 2020;38(15):1000.
  5. Schmid P, Rugo H, Adams S, et al. Atezolizumab plus nab-paclitaxel as first-line treatment for unresectable, locally advanced or metastatic triple-negative breast cancer (IMpassion130): updated efficacy results from a randomised, double-blind, placebo-controlled, phase 3 trial. Lancet Oncol. 2020;21(1):p44-59.
  6. Nanda R, Liu M, Yau, et al. Effect of pembrolizumab plus neoadjuvant chemotherapy on pathologic complete response in women with early-stage breast cancer: An analysis of the ongoing phase 2 adaptively randomized I-SPY2 trial. JAMA Oncol. 2020;6(5):676–684.
  7. I-SPY2 TRIAL Demonstrates Significant Improvement in pCR with Durvalumab and Olaparib with Paclitaxel, (Compared to Chemotherapy Alone) in Women with Stage II/III High-Risk, HER2-Negative Breast Cancer, in HR+ and TNBC Subsets. https://www.prnewswire.com/news-releases/i-spy2-trial-demonstrates-significant-improvement-in-pcr-with-durvalumab-and-olaparib-with-paclitaxel-compared-to-chemotherapy-alone-in-women-with-stage-iiiii-high-risk-her2-negative-breast-cancer-in-hr-and-tnbc-subsets-301047042.html. Published online April 27, 2020. Accessed December 9, 2020.