5-Hydroxypyrroloindoline Affords Tryptathionine and 2,2'-bis-Indole Peptide Staples: Application to Melanotan-II.

This study explores novel peptide stapling strategies inspired by natural product motifs found in fungal toxins (amatoxins, phallotoxins) and the alkaloid staurosporine. The researchers developed a chemical method using a 5-hydroxypyrroloindoline building block that can react with either a cysteine thiol (forming a tryptathionine staple) or a tryptophan indole (forming a 2,2'-bis-indole staple) to create constrained macrocyclic peptide structures. The authors applied these two stapling approaches to α-melanocyte-stimulating hormone (α-MSH), using the Melanotan-II scaffold as a model, with careful protecting group strategies to achieve chemoselectivity between the two staple types. Both classes of stapled peptides were evaluated for binding affinity at the melanocortin receptor, and the study reports that both series achieved nanomolar inhibition constants (Ki values), with at least one compound reaching sub-nanomolar Ki. Limitations include that all work is conducted in vitro (binding assays and synthetic chemistry), with no cell-based functional data, animal studies, or human data reported. The study is primarily a proof-of-concept for expanding the chemical toolbox of peptide stapling using underexplored natural product-derived cross-links.

Why this grade: All experiments are synthetic chemistry and in vitro receptor binding assays; no animal models or human subjects were involved.