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

Abstract

Pharmacological probes for the melanocortin receptors have been utilized for studying various disease states including cancer, sexual function disorders, Alzheimer's disease, social disorders, cachexia, and obesity. This study focused on the design and synthesis of bivalent ligands to target melanocortin receptor homodimers. Lead ligands increased binding affinity by 14- to 25-fold and increased cAMP signaling potency by 3- to 5-fold compared to their monovalent counterparts. Unexpectedly, different bivalent ligands showed preferences for particular melanocortin receptor subtypes depending on the linker that connected the binding scaffolds suggesting structural differences between the various dimer subtypes. Homobivalent compound 12 (CJL-1-140) possessed a functional profile that was unique from its monovalent counterparts providing evidence of the discrete effects of bivalent ligands. Lead compound 7 (CJL-1-87) significantly decreased feeding in mice after intracerebroventricular administration. To the best of our knowledge, this is the first report of a melanocortin bivalent ligand's in vivo physiological effects.

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

“…From these search parameters, three studies reported selective pA 2 values at two melanocortin receptors (mMC3R and mMC4R) and reported functional agonist EC 50 values for the other two receptors (mMC1R and mMC5R, Table 3) [132-134]. For the sake of clarity, antagonists in Table 3 were divided into two sections: ligands with antagonist activity at the mMC4R but no agonist or antagonist activity observed at the mMC3R at concentrations up to 10 μM (pA 2 < 5) and antagonists that were active at the mMC3R and mMC4R.…”

“…After the above report, homobivalent ligand development primarily focused on increasing the binding affinity at the hMC4R through various bivalent ligand design strategies [134, 201, 205, 217, 235-242], with some reports focusing on the MC1R and in vivo imaging [134, 243-245]. While high affinity ligands are desirable for biological responses, the use of low affinity pharmacophores in bivalent ligand design allow the greatest detection of synergistic binding [201, 204, 206, 217, 235, 236, 239, 243].…”

“…These included the tetrapeptide His-DPhe-Arg-Trp [134, 201, 205, 217, 235, 236, 238, 240-242], six residue analogs[235, 244, 245], seven residue analogs [237, 239, 243], and full length NDP-MSH [201, 236, 238]. Antagonist analogs in which the DPhe was replaced with DNal(2′) have been utilized to produce antagonist analogs with increased binding affinity [134, 239]. There has also been one report in which a melanocortin agonist pharmacophore was attached to an antagonist pharmacophore via a linker that also resulted in increased binding affinity [239].…”

“…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].…”

“…In contrast, Lensing et al . reported the linker systems, independent of the binding pharmacophore, displayed preferential patterns for different melanocortin receptor subtypes [134]. A 36 atom (Pro-Gly) 6 linker system was optimal for mMC1R binding (14-fold enhancement compared to monovalent counterpart), suggesting a cooperative bivalent binding mode (Figure 1 A-C).…”