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Table 1. Summary of peptide synthesis and receptor binding assay resultsPeptidePeptideCuSO 4 (1 eq)Sodium ascorbate (1 eq)TBTA (1eq)DMF:H 2 O (3:1)Fig. 2. Attachment of afluorophore via the click reaction.Peptide Mol. Wt. IC 50 Ref.1 Des-AA11-18,Cys7,21,D-Lys(Ac)9,D-His26, Pro34]-NPY3480.0 0.16 nM Mullins et al.2 1992U91 (GR231118) 2352.7 0.084 nM Daniels et al.3 1911U90 (BVD15) 1206.4 16 nM Daniels et al.4 DOTA(Cu)-Lys4-BVD15 1669.4 7.9 nM Guerin et al.5 Fluorophore-triazole-(2) 2623.0 31 nM6 Fluorophore-A-triazole-(1) 3621.1 3.2 nM7 Fluorophore-B-triazole-(1) 3783.3 9.1 nMUtilizing brain tissue homogenates from Y2,Y4-receptor knockout mice, each of thesepeptides has been assayed as Y1 receptor ligands in competition binding assays versus125 I-PYY (25 pM). The IC 50 values obtained were in accord with previously reported datafrom other cell types.With these peptides in hand we have beenpreparing and assessing a range of conjugatespossessing potential imaging moieties. We have beeninvestigating the DOTA-style chelating groups andfound that incorporation into the BVD15 structure, asdescribed by Guerin, [7] is well tolerated by thereceptor.Our other synthetic focus has been to utilize clickchemistry to allow a variety of peptide ligands andimaging moieties to be interchanged readily. We havesynthesized known and novel alkyne and azide aminoacids, and incorporated them efficiently into syntheticpeptides. For click coupling, we have been utilizing arange of potential fluorophores as exemplified inFigure 2. Though successful, the incorporation of afluorophore (not shown) into GR231118, resulted in apeptide 5 that is approximately 370-fold less potent atthe Y1 receptor. Introduction of fluorophores intopeptide 1, has been more successful with product 6only 20-fold less potent than the parent peptide.A number of other highly potent peptideconjugates have been identified in this process thatalso reveal novel structure-activity relationships forY1 receptor ligands. Further studies on the selectivityand metabolic stability of these peptides is currentlyunderway.AcknowledgmentsWe thank Oscar Liu for technical support. Supported by the Co-operative Research Centre forBiomedical Imaging and Development.References1. Wettstein, J.G., et al. Pharmacol. Ther. 65, 397-414 (1995).2. Sheikh, S.P. Am. J. Physiol. 261, G701-715 (1991).3. Michel, M.C., et al. Pharmacol. Rev. 50, 143-150 (1998).4. Reubi, J.C., et al. Cancer Res. 61, 4636-4641 (2001).5. Daniels, A.J., et al. PNAS 92, 9067-9071 (1995).6. Mullins, D., et al. Mol. Pharmacol. 60, 534-540 (2001).7. Guérin, B., et al. Bioorg. Med. Chem. Lett. 20, 950-953 (2010).213