Circulating endothelial progenitor cells (EPC), derived from bone marrow, can be used as a marker for optimizing and monitoring the anti-angiogenesis therapy including angiogenesis inhibitors and metronomic chemotherapy. Preclinical models indicated that the source of apoptotic circulating endothelial cells (CEC) was most likely the tumor vasculature. In breast cancer patients, apoptotic CEC were demonstrated to be a surrogate marker for efficacy of metronomic therapy.