Biological Evaluation of Multiple Cyclotide Scaffolds and Other Mimetics at Human Melancortin Receptors

Because of their multiplicity and ubiquity in mammalian systems as central modulators of signaling, GPCRs have held a special interest in drug design. Specifically, ligands targeting the homeostatic class of GPCRs known as melanocortin receptors (MCRs) have the potential to remedy the symptoms of obesity, diabetes, and pain. Adenylate cyclase and binding assays were performed on HEK 293 cells expressing hMC1R, hMC3R, hMC4R, and hMC5R to assess a class of 39 multiple-disulfide, cyclized MSH analogues featuring strategic N-methylation and based on the protein topology of conotoxins found in the venoms of marine cone snails. Because of their potency and selectivity, conotoxins offer a promising avenue for drug design; furthermore, the cyclic backbone and cystine knot motifs of the analogues afforded specific conformational constraining in addition to enhanced proteolytic stability and bioavailability. Several selective compounds were identified from the library with IC₅₀ and EC₅₀ values in the low nanomolar to low micromolar range; these results provide important starting points for optimization. Additionally, two libraries of small molecule mimetics and cyclic peptide analogues synthesized by former group members James Cain and Joel Nyberg were assessed using the same methods; therapeutically relevant selectivity profiles were exhibited by several small molecules in these libraries