Biomarkers of Immune Checkpoint Blockade Response in Triple-Negative Breast Cancer.

OPINION STATEMENT: Immune checkpoint blockade (ICB) has revolutionized the treatment landscape across multiple solid tumor types. In triple-negative breast cancer (TNBC), clinical benefit for the addition of ICB to chemotherapy has been shown in both the metastatic and early stage disease settings. A minority of patients with TNBC will truly benefit from ICB, with many tumors unlikely to respond, and ICB can cause additional toxicities for patients to incur. In clinical practice, ICB-based regimens are emerging as standard-of-care treatment options in TNBC subpopulations. Atezolizumab in combination with nab-paclitaxel is recommended as first-line treatment for patients with PD-L1-positive metastatic TNBC. Clinical trials are needed to confirm this benefit and evaluate if additional biomarkers may allow for improved patient selection. Trials investigating ICB in early-stage breast cancer show promise for the benefit of combination ICB with neoadjuvant chemotherapy. However, longer follow-up is required before ICB can be considered as standard-of-care treatment in the early stage setting. In both metastatic and early stage TNBC, novel biomarkers to improve patient selection are now under investigation. These include multiplex IHC to profile immune cell subtypes (such as tumor infiltrating lymphocytes) or RNA gene expression profiling to detect signatures suggestive of a T-cell-inflamed microenvironment. Detecting somatic mutations through tumor next-generation DNA sequencing may predict resistance mechanisms or suggest sensitivity to ICB monotherapy or in combination with other forms of systemic therapy. These biomarker platforms may allow for a more granular analysis of immune activity and should be further investigated in prospective studies with the aim of personalizing ICB-focused therapies in TNBC.