Impact, Mechanisms, and Novel Chemotherapy Strategies for Overcoming Resistance to Anthracyclines and Taxanes in Metastatic Breast Cancer

Despite advances in treatment of patients with metastatic breast cancer (MBC), prognosis remains poor and median survival is 2-3 years. Resistance to antineoplastics mediated by many factors, potentially including overexpression of drug efflux proteins or altered beta-tubulin isotype expression limits the effectiveness of MBC chemotherapy. Capecitabine, approved for anthracycline- and taxane-resistant MBC, has produced modest responses, highlighting the need for more effective treatments for MBC resistant to multiple chemotherapeutic agents. Agents with potential to reverse drug resistance have not proven effective. Albumin-bound paclitaxel is a formulation that may enhance delivery to tumor tissues. Conversely, ixabepilone, an epothilone analog, has been reported to have lower susceptibility to at least some mechanisms of tumor resistance and clinical activity in resistant/refractory MBC. The topoisomerase-I inhibitor irinotecan also has low cross-resistance with other antineoplastics, and has shown some activity against refractory MBC. Development of new agents and identification of genetic biomarkers in translational studies promise to improve management of patients with resistant/refractory breast cancer.