Therapeutic Advances and New Directions for Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is a molecularly diverse grouping with poor prognosis for which chemotherapy remains the foundation of treatment. The molecular heterogeneity of the disease rationalizes its diverse biological behavior and differential response to treatment. Estimates of up to 20% of patients diagnosed have germline mutations in DNA-damage repair-pathway genes, namely BRCA1 and 2, and this can be used to select patients likely to respond to platinums and/or inhibitors of poly(ADP-ribose) polymerase (PARP). Similar strategies can be utilized in other subtypes of TNBC that have 'BRCA-like' tumor biology due to the presence of mutations in alternate DNA-damage repair genes. The diverse biological behavior of TNBC and its variable response to chemotherapy were largely decoded following genotyping studies that enabled the identification of distinct molecular subtypes, such that the biological and genetic heterogeneity of the disease could be understood. This subsequently enabled the identification of therapeutic 'vulnerabilities' for each subtype that encompass biological processes including proliferation, DNA repair, apoptosis, angiogenesis, immune modulation, and invasion and metastasis. To expedite the development of therapies for high-risk, early-stage breast cancer, we have adopted novel trial designs and re-defined endpoints as surrogates of clinical outcomes. The purpose of this review is to highlight the current standard and experimental treatment options for TNBC.