Achievements and challenges in the use of metronomics for the treatment of breast cancer.

Two interesting therapeutic proposals for cancer treatment emerged at the beginning of the 21st century. The first one was metronomic chemotherapy, which refers to the chronic administration of chemotherapeutic agents, in low doses, without extended drug-free periods. Then, the idea of drug repositioning in oncology, the use of well-known drugs that were created for other uses to be utilized in oncology, gained strength. Shortly after, the two strategies were merged in one, named metronomics. Both approaches share several features which make metronomics an appealing choice for cancer treatment: use of known and approved drugs, thus diminishing the time necessary to enter to the clinic, therapeutic effect, low toxicity, oral administration, better life quality, low costs because of the use of, generally, out of patent drugs, possibility of use, even in countries with very low economic resources. Many chemotherapy and repurposed drugs were tested with metronomics approaches for the treatment of mammary cancer, the most common malignancy in women worldwide, leading to high rates of mortality. The wide range of therapeutic models studied, paralleled the wide range of responses obtained, like tumor growth and metastasis inhibition, overall survival increase, lack of toxicity, better life quality, among others. The accomplishments reached, and the challenges faced by researchers, are discussed.