The dose of bevacizumab was not lowered for any of the side effects, with the exception of proteinuria. biologic response modifiers and does not account for its chemosensitizing effect. The E2100, AVADO, and RIBBON-1 trials differed in the type and dose of chemotherapy, the dose and frequency of bevacizumab, and in the trial design, making it difficult to effectively compare and evaluate the results. The efficacy of combining bevacizumab with a maximum tolerated dose (MTD) of chemotherapy is also discussed in view of the observation that Belotecan hydrochloride increased tumor response did not translate to an increase in survival. We suggest that even though an-giogenesis inhibitors are non-toxic as monotherapies, they increase the toxicity of standard chemotherapy, and consequently a re-design of the now classic clinical trial model should be considered. Modifying the existing clinical trial model will lead to a more accurate evaluation of the safety and efficacy of bevacizumab and other biological agents in treating metastatic cancer. strong class=”kwd-title” Keywords: Anti-angiogenic therapy, Angiogenesis, AVADO, Breast cancer, Avastin, Bevacizumab, E2100, RIBBON-1, Breast cancer treatment, VEGF, Vascular endothelial growth factor, Metronomic therapy Introduction Chemotherapy, as coined by Paul Ehrlich in the early 20th century, is the use of chemicals to treat diseases . Most traditional cancer chemotherapies are cytotoxic and either alter DNA synthesis or interfere with microtubule formation [see Fig. 1]. The number of these chemicals has been steadily increasing since the days Sidney Farber used folate antagonists to treat childhood leukemia, but the survival curves have plateaued. In contrast, targeted therapies inhibit specific physiological processes, and include tyrosine kinase inhibitors, immunomodulators, cytokines or cytokine inhibitors, protease inhibitors, anti-growth factor antibodies among others. Open in a separate window Fig. 1 Sites of action of traditional chemotherapeutic agents. The target of traditional chemotherapeutic agents is the DNA replication (cytarabine, methotrexate, 5-fluorouracil, 6 thioguanine), mature DNA (bleomycin, etoposide, teniposide, adriamycin and daunomycin), DNA alkylation (ifosfamide, cyclophosphamide, platin based drugs etc.), translation (L-asparginase) or the mitotic spindle (vincristine, vinblastine, taxanes). This is in direct contrast to the biologic agents such as bevacizumab. In this article, we use bevacizumab, a monoclonal antibody against Vascular Endothelial Growth Factor (VEGF), as a surrogate for targeted agents, and consider tumor angiogenesis host biological process supporting cancer progression [2C4]. The attractiveness of targeting angiogenesis was ensured by lower toxicity and the absence of physiological angiogenesis after birth . VEGF is an initiating signal for angiogenesis, and while it is haplotype lethal during embryogenesis Belotecan hydrochloride , it is only needed for initiation of a vascular sprout in the wound or tumor microenvironment postnatally. Once a sprout (tip cell) is formed, other angiogenesis stimulators such as bFGF and PDGF support the development of stalk cells, and recruitment of smooth muscle cell, rendering the vasculature quiescent [7,8]. VPF (VEGF) was discovered in Dr. Dvoraks laboratory in 1983 , and was re-named in 1989  after subsequent cDNA cloning of VPF  and VEGF  proved that VPF and VEGF were the same molecule . It proved to be an evolutionally well preserved protein Belotecan hydrochloride , and its secretion leads to a proliferative signal when bound to VEGFR2 on IB1 endothelial cells, and to a differentiation signal when it Belotecan hydrochloride binds to VEGFR1. Other functions of VEGF include recruitment, stimulation and differentiation of progenitor endothelial cells, promotion of monocyte chemotaxis in the bone marrow , induction of colony formation by mature subsets of granulocyteCmacrophage progenitor cells , and regulation of immune and anti-inflammatory cells . When in 1997 Ferrara et al. developed bevacizumab (Genentech: Avastin?), a neutralizing antibody to VEGF, it was the first of many angiogenesis inhibitors. Early safety and efficacy trials demonstrated that bevacizumab, similar to other monoclonal antibodies, lacked traditional toxicities when used as monotherapy , and that most bevacizumab-associated toxicities develop when one.