Tc-99m-labeled neuropeptide Y analogues as potential tumor imaging agents

The possible use of neuropeptide Y (NPY) as a novel radiopeptide has been i nvestigated. NPY is a 36-amino acid peptide of the pancreatic polypeptide f amily, 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. A s structure-activity relationships of NPY are well-known, we could assume w here 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 pepti des 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, bu t presumably as well to the His in position 26. Since the replacement of Hi s(26) by Ala only slightly decreased binding affinity, [K-4(PADA),A(26)]-NP Y was specifically postlabeled, and the Re-185 surrogate maintained high bi nding 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)-PAD A)]-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 hi gh binding affinity of Ac-[Ahx(5-24),K-4(Re-185(CO)(3)-PADA),A(26)]-NPY. In ternalization. studies with the corresponding Tc-99m-labeled analogue revea led receptor-mediated internalization. Furthermore, biodistribution studies were performed in mice, and stability was tested in human plasma. Our cent rally 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 i n nuclear medicine.