Resistance mechanisms to drug therapy in breast cancer and other solid tumors: An opinion.

Cancer is an important contributor to mortality worldwide. Breast cancer is the most common solid tumor in women. Despite numerous drug combinations and regimens, all patients with advanced breast cancer, similarly to other solid tumors, inevitably develop resistance to treatment. Identified mechanisms of resistance could be classified into intra- and extracellular mechanisms. Intracellular mechanisms include drug metabolism and efflux, target modulations and damage restoration. Extracellular mechanisms might be attributed to the crosstalk between tumor cells and environmental factors. However, current knowledge concerning resistance mechanisms cannot completely explain the phenomenon of multi-drug resistance, which occurs in the vast majority of patients treated with chemotherapy. In this opinion article, we investigate the role of these factors in the development of drug-resistance.