Substance P derivatives as versatile tools for specific delivery of various types of biomolecular cargo.

The use of proteins or nucleic acids as therapeutic agents has been severely hampered by their intrinsic inability to cross the cell membrane. Moreover, common techniques for driving the delivery of macromolecules lack the ability to distinguish between healthy and diseased tissue, precluding their clinical use. Recently, receptor-mediated delivery (RMD) has emerged as a technology with the potential to circumvent the obstacles associated with the delivery of drug targets by utilizing the natural endocytosis of a ligand upon binding to its receptor. Here, we describe the synthesis of variants of substance P (SP), an eleven amino acid neuropeptide ligand of the neurokinin type 1 receptor (NK1R), for the delivery of various types of cargo. The variants of SP were synthesized with an N-terminal maleimide moiety that allows conjugation to surface thiols, resulting in a nonreducible thioether. Cargos lacking an available thiol are conjugated to SP using commercially available cross-linkers. In addition to the delivery of proteins, we expand the use of SP to include nuclear delivery of DNA fragments that are actively expressed in the target cells. We also show that SP can be used to deliver whole bacteriophage particles as well as polystyrene beads up to 1 μm in diameter. The results show the ability of SP to deliver cargo of various sizes and chemical properties that retain their function within the cell. Furthermore, the overexpression of the NK1R in many tumors provides the potential for developing targeted delivery reagents that are specific toward diseased tissue.