Progress and pitfalls in the use of immunotherapy for patients with triple negative breast cancer

Triple negative breast cancer (TNBC) is an area of high unmet medical need in terms of new effective treatment strategies. Although breast cancer is traditionally considered a 'cold' tumor type, TNBC is the most appropriate subtype for immunotherapeutic strategies; this is due to the high level of tumor infiltrating lymphocytes, PD-L1 expression, and tumor mutational burden compared to other breast cancer subtypes.This review examines the available evidence on the use of immunotherapeutic strategies in early and advanced TNBC, discusses the pitfalls and limitations often encountered in clinical research, and summarizes data on novel promising immunomodulatory approaches that have been explored in early-phase trials.PD-1-blockade is approved for stage II/III TNBC and for first-line treatment of PD-L1-positive TNBC patients with metastatic disease and should be considered standard of care. However, question marks and difficulties remain; these include the identification of predictive biomarkers to select patients who benefit from the addition of PD1-blockade and the balance between efficacy and long-term toxicity for an individual patient. Numerous treatment combinations and new immunotherapeutic strategies beyond PD1 blockade are being evaluated, thus reflecting a promising evolution towards a more personalized approach, and extended clinical benefit in TNBC.Abbreviations:Triple-negative breast cancer (TNBC); breast cancers (BCs); estrogen receptor (ER); progesterone receptor (PgR); human epidermal growth factor-2 (HER-2); basal-like 1 (BL1), basal-like 2 (BL2); mesenchymal (MES); mesenchymal stem-like (MSL); immunomodulatory (IM); luminal androgen receptor (LAR); basal-like immunosuppressed (BLIS); basal-like immune-activated (BLIA); tumor-infiltrating lymphocytes (TILs); tumor mutational burden (TMB); immune cells (ICs); immunohistochemistry (IHC); overall response rate (ORR); overall survival (OS); progression-free survival (PFS); intention-to-treat (ITT); hazard ratio (HR); confidence interval (CI); Food and Drug Administration (FDA); European Medicines Agency (EMA); immune checkpoint inhibitors (ICI); Combined Positive Score (CPS); disease control rate (DCR); neoadjuvant chemotherapy (NACT); pathological complete response (pCR); event-free survival (EFS); disease-free survival (DFS); residual cancer burden (RCB); San Antonio Breast Cancer Symposium (SABCS); antibody-drug conjugates (ADCs); PARP inhibitors (PARPi); clinical benefit rate (CBR); Histone deacetylase inhibitors (HDACi); Dendritic cell (DC); talimogene laherparepvec (TVEC); granulocyte-macrophage colony-stimulating factor (GM-CSF); mismatch repair deficiency (dMMR).