The present invention features a compound having the formula A-X-B, where A is peptide vector capable of enhancing transport of the compound across the blood-brain barrier or into particular cell types, X is a linker, and B is a leptin, a leptin analog, or OB receptor agonist. The compounds of the invention can be used to treat any disease in which increased amounts of leptin are desired, such as metabolic diseases including obesity and diabetes.