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.