Standard of Care and Promising New Agents for the Treatment of Mesenchymal Triple-Negative Breast Cancer

Simple Summary Mesenchymal triple negative breast cancer subtype expresses genes involved in proliferation, epithelial to mesenchymal transition, stromal interaction and cell motility. Moreover, this subgroup is characterized by an immunosuppressive microenvironment. This review focuses on the intracellular pathways involved in tumorigenesis and cancer progression, as well as in the immune evasion mechanisms. Furthermore, we provide an overview of current clinical trials investigating the efficacy and safety of different therapeutic molecules for this aggressive subtype of triple negative breast cancer. The challenge is to restore immunocompetence by overcoming the chemo and immune-resistance profile of mesenchymal triple negative breast cancer to achieve a lasting response to therapy. The pathologic definition of triple negative breast cancer (TNBC) relies on the absence of expression of estrogen, progesterone and HER2 receptors. However, this BC subgroup is distinguished by a wide biological, molecular and clinical heterogeneity. Among the intrinsic TNBC subtypes, the mesenchymal type is defined by the expression of genes involved in the epithelial to mesenchymal transition, stromal interaction and cell motility. Moreover, it shows a high expression of genes involved in proliferation and an immune-suppressive microenvironment. Several molecular alterations along different pathways activated during carcinogenesis and tumor progression have been outlined and could be involved in immune evasion mechanisms. Furthermore, reverting epithelial to mesenchymal transition process could lead to the overcoming of immune-resistance. This paper reviews the current knowledge regarding the mesenchymal TNBC subtype and its response to conventional therapeutic strategies, as well as to some promising molecular target agents and immunotherapy. The final goal is a tailored combination of cytotoxic drugs, target agents and immunotherapy in order to restore immunocompetence in mesenchymal breast cancer patients.