The present invention is directed to compositions and methods of treating cancer by gene therapy using a therapeutic gene formulated in a buffer comprising a delivery-enhancing agent. The delivery-enhancing agents of the invention can be used to formulate therapeutic or diagnostic agents, such as proteins, nucleic acids, antisense RNA, small molecules, etc., for administration to any tissue or organ having an epithelial membrane. The delivery-enhancing agents include detergents, alcohols, surfactants and other molecules.