Bivalent ACTH antagonists: influence of peptide and spacer components on potency enhancement

Synthesis and evaluation of bivalent ligands for binding to the human melanocortin-4 receptor

Membrane proteins, especially G-protein coupled receptors (GPCRs), are interesting and important theragnostic targets since many of them serve in intracellular signaling critical for all aspects of health and disease. The potential utility of designed bivalent ligands as targeting agents for cancer diagnosis and/or therapy can be evaluated by determining their binding to the corresponding receptors. As proof of concept, GPCR cell surface proteins are shown to be targeted specifically using multivalent ligands. We designed, synthesized, and tested a series of bivalent ligands targeting the over-expressed human melanocortin 4 receptor (hMC4R) in human embryonic kidney (HEK) 293 cells. Based on our data suggesting an optimal linker length of 25±10Å inferred from the bivalent melanocyte stimulating hormone (MSH) agonist, the truncated heptapeptide, referred to as MSH(7): Ac-Ser-Nle-Glu-His-D-Phe-Arg-Trp-NH2 was used to construct a set of bivalent ligands incorporating a hMC4R antagonist, SHU9119: Ac-Nle-c[Asp-His-2'-D-Nal-Arg-Trp-Lys]-NH2 and another set of bivalent ligands containing the SHU9119 antagonist pharmacophore on both side of the optimized linkers. These two binding motifs within the bivalent constructs were conjoined by semi-rigid (Pro-Gly)3 units with or without the flexible poly(ethylene glycol) (PEGO) moieties. Lanthanide-based competitive binding assays showed bivalent ligands binds to the hMC4R with up to 240-fold higher affinity than the corresponding linked monovalent ligands.

Tumor-targeted bioconjugate based delivery of camptothecin: design, synthesis and in vitro evaluation

Camptothecin (CPT) presents numerous challenges associated with optimal transport and delivery including variability in clinically observed effects, low target tissue concentrations and severe and unpredictable toxicity. The objective of the present study was to optimize the delivery of CPT by targeting it to cancer cells using an endogenous receptor system. A novel CPT bioconjugate was synthesized using carbodiimide chemistry with a linear poly(ethylene glycol) (PEG) and amino acid glycine as the spacer and linker respectively. Folic acid was used as the targeting ligand to take advantage of folate receptor mediated endocytosis. The bioconjugate was extensively characterized using MALDI, proton NMR, FT-IR and amino acid analysis. Furthermore, the bioconjugate was evaluated in vitro for specific targeting to folate receptor-expressing KB cells, a human nasopharyngeal carcinoma. Finally, the delivery system was evaluated for cytotoxicity using a MTT based assay. The results indicate significantly higher efficacy of the bioconjugate in comparison to CPT. A control conjugate without PEG demonstrated no improvement in efficacy over untargeted CPT emphasizing the importance of spacer between the anticancer compounds and targeting moiety. This bioconjugate represents the 'first-in-series' of targeted bioconjugates and serves as prototype for improving tumor cell concentration and efficacy.

Dimerization of the immunosuppressive peptide fragment of HLA-DR molecule enhances its potency

Our previous studies revealed that the nonapeptide fragment of HLA-DR molecule, located in the beta chain 164-172 with the VPRSGEVYT sequence, suppresses the immune responses. The sequence is located on the exposed molecule loop, therefore it may be involved in the interactions with other proteins. We suggested that the loop may serve as a functional epitope on the HLA class II surface for intermolecular binding, and that possible mechanism of biological action of the synthesized peptides is associated with interfering of adhesion of HLA class II molecules to their coreceptors. It has been postulated that oligomerization of the coreceptors is required for stable binding to class II HLA. Based on the crystal dimeric structure of HLA-DR molecules, we designed, and synthesized molecules able to induce the putative coreceptors dimerization. The synthesized series of compounds consisted of two VPRSGEVYT sequences linked through their C-termini by spacers of different length: (VPRSGEVYTGn)2K-NH2 ( n = 4-6). The results demonstrate that the dimerization of the nonapeptide fragment of HLA-DR results in enhanced immunosuppressory properties.