99mTc-labeled neuropeptide Y analogues as potential tumor imaging agents

The possible use of neuropeptide Y (NPY) as a novel radiopeptide has been investigated. NPY is a 36-amino acid peptide of the pancreatic polypeptide family, which is expressed in the peripheral and central nervous system, and is one of the most abundant neuropeptides in the brain. Its receptors are produced in a number of neuroblastoma and the thereof derived cell lines. As structure-activity relationships of NPY are well-known, we could assume where a radionuclide might be introduced without affecting receptor affinity. We applied the. novel [Tc-99m(OH2)(3)(CO)(3)](+) aqua complex and PADA (2-picolylamine-N,N-diacetic acid) as bifunctional chelating agent. The peptides were synthesized by solid-phase peptide synthesis, and PADA was coupled to the side chain of Lys(4) of the resin-bound peptide. Upon postlabeling of [K-4(PADA)]-NPY, Tc-99m(CO)(3) did not only bind to the desired PADA, but presumably as well to the His in position 26. Since the replacement of His(26) by Ala only slightly decreased binding affinity, [K-4(PADA),A(26)]-NPY was specifically postlabeled, and the Re-185 surrogate maintained high binding affinity. Furthermore, the prelabeling approach has been applied for the centrally truncated analogue [Ahx(5-24)]-NPY, which is highly selective for the Y2 receptor. The resulting Ac-[Ahx(5-24),K-4((TC)-T-99m(CO)(3)-PADA)]-NPY was produced with a yield of only 16%. Therefore, postlabeling was applied for the short analogue as well, again substituting His(26) by Ala. Competitive binding assays using Re-185 as a surrogate for Tc-99m showed high binding affinity of Ac-[Ahx(5-24),K-4(Re-185(CO)(3)-PADA),A(26)]-NPY. Internalization. studies with the corresponding Tc-99m-labeled analogue revealed receptor-mediated internalization. Furthermore, biodistribution studies were performed in mice, and stability was tested in human plasma. Our centrally truncated analogue revealed a 6-fold increased stability compared to the natural peptide NPY. We conclude that Ac-[Ahx(5-24),K-4((TC)-T-99m(CO)(3)-PADA),A(26)]-NPY has promising characteristics for future applications in nuclear medicine.