Biomarkers for breast cancer immunotherapy: PD-L1, TILs, and beyond.

INTRODUCTION: Immune checkpoint inhibitors (ICIs) have recently entered into the therapeutic scenario of metastatic breast cancer. However, only a proportion of patients benefit from ICIs and immune-based combinations, so the identification of reliable predictors of response remains an unmet need. AREAS COVERED: We discuss potential predictors of response to ICIs in breast cancer, including PD-L1 expression, tumor-infiltrating lymphocytes (TILs), tumor mutational burden (TMB), and several other biomarkers and suggest future directions of research in this setting. A literature search was conducted in October 2021 of Pubmed/Medline, Cochrane library and Scopus databases; in addition, abstract of international cancer meetings were reviewed. EXPERT OPINION: In terms of predictors of response to immunotherapy in TNBC patients, several biomarkers are being evaluated. Valuable data on predictive biomarkers have recently emerged, including host-related factors, immune-related cells, and protein and genetic markers. Data supporting immunotherapy in the metastatic triple-negative breast cancer setting are not concordant, but there have been some positive phase III trials including IMpassion130 and KEYNOTE-355. Phase II and III (neo)adjuvant trials are supportive of this therapeutic strategy. Further investigations are warranted in this challenging area.