In vitro and preclinical in vivo data suggest that leptin acts as a mitogenic agent to promote prostate, breast, and ovarian cancer cell EX 527 growth and/or enhances cancer angiogenesis and migration. Thus leptin antagonists hold potential for future therapeutic use in cancer. A few anti-LepR antibodies have been generated and tested in models of heart failure, multiple sclerosis, and autoimmune encephalomyelitis. An anti-rat LepR mAb reduced the growth of bone marrow leukemic cells with concomitant decrease in angiogenesis, and prolonged survival. A pegylated leptin peptide antagonist significantly inhibited breast cancer xenografts hosted by immunodeficient mice without affecting energy balance. In this study we assessed the effects of a neutralizing anti-LepR nanobody in a mouse model of melanoma. Local subcutaneous administration of low-dose 2.17-mAlb significantly inhibited melanoma growth associated with decreased angiogenesis in the tumor. The absence of effects on weight and food intake suggested that the central actions of leptin were not disrupted by low-dose 2.17-mAlb although the low-dose nanobody administered adjacent to the tumor was sufficient to decrease the growth of a highly aggressive melanoma by 33%. These results further support our finding that the EE-induced anti-cancer effect was mediated, at least in part, by leptin. The effects of high dose 2.17-mAlb are more complex. The intraperitoneal injection of 2.17-mAlb at high-dose resulted in weight gain, hyperphagia, increased adiposity, hyperleptinemia, and hyperinsulinemia indicating efficient blockade of leptin signaling in CNS. On the other hand, lowdose 2.17-mAlb showed neither significant metabolic effects nor anticancer effect suggesting that the antagonist availability and activity were insufficient at the respective sites of action. Therefore the overall impact of 2.17-mAlb on tumor growth was determined not only by the direct effects of LepR antagonist on tumor cells and/or other cells supporting tumor growth, but also by other systemic factors such as insulin metabolism that are regulated by leptin. In the context of cancer, insulin signaling and thus the role of leptin in the regulation of pancreatic b-cell functions are of importance. Our previous data have shown that obesity increases B16 melanoma growth in obese leptin-deficient ob/ob mice consistent with other reports. Prevention of the obesity by pair feeding ob/ob mice dramatically reduces tumor weight to a level significantly lower than wild-type mice of the same weight. Our leptin replacement data also showed that exogenous leptin increased melanoma mass in ob/ob mice by 140% compared to pair-fed saline-infused mice with identical body weight and fat mass. These data all support the role of leptin in promoting melanoma growth.