Linear scaffolds for multivalent targeting of melanocortin receptors

Abstract

Molecules bearing one, two, three, or four copies of the tetrapeptide His-dPhe-Arg-Trp were attached to scaffolds based on ethylene glycol, glycerol, and d-mannitol by means of the copper-assisted azide-alkyne cyclization. The abilities of these compounds to block binding of a probe at the melanocortin 4 receptor were evaluated using a competitive binding assay. All of the multivalent molecules studied exhibited 30- to 40-fold higher apparent affinites when compared to a monovalent control. These results are consistent with divalent binding to receptor dimers. No evidence for tri- or tetravalent binding was obtained. Differences in the interligand spacing required for divalent binding, as opposed to tri- or tetravalent binding, may be responsible for these results.

Bench-top to clinical therapies: A review of melanocortin ligands from 1954 to 2016

“…This finding is consistent with bivalent binding to putative melanocortin receptor dimers, without evidence for trivalent or tetravalent binding. They also reported the optimal distance between pharmacophores was between 17 and 23 Å, as previously reported [241, 242]. …”

“…Dehigaspitiya et al . reported similar results when comparing monovalent, bivalent, trivalent and tetravalent ligands featuring the tetrapeptide His-DPhe-Arg-Trp in different scaffolds [241]. All multivalent compounds possessed 30- to 40-fold higher binding affinities at the hMC4R compared to monovalent controls, although valencies beyond two did not result in further affinity gains.…”

“…The optimal linker length must be long enough to bridge or crosslink two receptors, but not too long to eliminate entropic gains. Various linker systems have been incorporated, including poly-lysine [233, 237], polyethylene glycol [134, 235, 237-239, 241], Ala-Gly [235], Pro-Gly [134, 235, 237, 239, 240], rigid amino acids [236], squalene [201], glycerol [241], D-mannitol [241], phloroglucinol [242], tripropargylamine [242], 1,4,7-triazacyclononane [242], others [205, 217, 243], and mixtures of these different linker systems together. Improper linker design may result in some increased binding affinity (<10-fold) that can be attributed to simply doubling the pharmacophore concentration [235-237].…”

“…Greater fold enhancements (>10-fold) are observed with linkers that appear to bridge two receptors resulting in cooperative synergistic binding (Figure 5A-C). An optimal linker length of approximately 23 ± 5 Å has been suggested by multiple studies at the hMC4R [217, 237, 239, 241, 242]. …”

An in Vitro and in Vivo Investigation of Bivalent Ligands That Display Preferential Binding and Functional Activity for Different Melanocortin Receptor Homodimers

“…Since Portoghese and coworkers pioneered bivalent ligands targeting GPCRs, 46 bivalent ligands have been developed for various GPCR systems including the opioid, 46-50 serotonin, 51-53 adenosine, 54 cannabinoid, 55-56 chemokine, 57 dopamine, 58 and melanocortin receptors. 25, 59-69 Bivalent ligands have been demonstrated to have a variety of different pharmacological effects as compared to their monovalent counterparts including: increasing or decreasing binding affinity, 52, 58, 64 positively or negatively changing functional responses, 53-55, 59, 70 altering receptor subtype selectivity, 47, 58 changing receptor trafficking, 71-73 and creating tissue selectivity. 50, 74 Due to these unique characteristics, bivalent ligands offer distinct advantages over the classical monomeric approach.…”