ANTAGONISTIC PROPERTIES OF CENTRALLY TRUNCATED ANALOGS OF [D-TRP(32)]NPY

We have previously shown that [D-Trp(32)]NPY can competitively antagon ize NPY-induced feeding in rats (Balasubramaniam et al. J. Med. Chem. 1994, 37, 811-815). This peptide, however, did not bind to SK-N-MC cel ls with Y-1 receptors. Since centrally truncated NPY analogs have been shown to bind Y-1 receptors, we synthesized similar analogs of [D-Trp (32)]NPY and investigated their Y-1 (SK-N-MC) and Y-2 (SK-N-BE2) recep tor affinities and their properties in human erythroleukemia (HEL) cel ls. None of the analogs with D-Trp(32) mobilized intracellular calcium , [Ca2+](i), in HEL cells. Although Des-AA(6-24)[Aoc(6)]NPY and the co rresponding D-Trp(32) analog exhibited no affinity to Y-1 receptors, D es-AA(7-24)[AoC(6),D-Trp(32)]Npy (6) exhibited weak binding. Replacing pro(5) in 6 with D-Ala to stabilize the central chain reversal, and h ence the antiparallel alignment of the N- and C-terminal regions known to be important for Y-1 binding, resulted in an analog, Des-AA(7-24)[ D-Ala(5),Aoc(6),D-Trp(32)]NPY (7), which exhibited moderate antagonist potency in attenuating NPY effects on cAMP and [Ca2+](i) in SK-N-MC a nd HEL cells, respectively. This analog also shifted the dose-response curve of NPY on blood pressure in anesthetized rats. Deletion of only the 7-17 and/or the incorporation of N-Me-Ala(5), a superior p-turn s tabilizer, in 7 did not improve the Y-1 receptor affinity. Des-AA(7-24 )[D-Ala(5),Gly(6),D-Trp(32)]NPY exhibited an affinity similar to that of 7, suggesting that a long spacer arm is not necessary for efficient Y-1 receptor interaction. Locking the antiparallel alignment via a 2/ 26 or 2/27 lactam bridge did not improve the binding. Finally, replace ment of D-Ala(5) in 7 with D-Trp dramatically increased both the bindi ng and the antagonistic potencies. Modeling based on the avian pancrea tic polypeptide X-ray structure suggested that analogs which have the Nand C-terminal regions in close proximity might exhibit good binding, and that the D-Trp(32) substitution may induce a beta-turn that could be important for exhibiting antagonism. A systematic investigation ha s resulted in the development of relatively potent Y-1 receptor antago nists. Further structure-activity studies with these compounds and tho se previously reported by us and other investigators should result in the development of long-acting and receptor selective antagonists.