Peptides are increasingly recognized as mid-sized therapeutic agents capable of meeting various medical needs. In contrast to small-molecule drugs, peptides can more accurately target intricate biological mechanisms and are typically simpler and more cost-efficient than large biological drugs like antibodies. Since the development of the first peptide hormone, insulin, in 1923, over 100 peptide drugs have received FDA approval; among these, roughly 40 drugs aimed at treating diverse ailments (including cancer, heart disease, and metabolic disorders) contain at least one tryptophan (Trp) residue, which is an essential amino acid.
Altering Trp residues in peptide molecules can enhance drug-target interactions and boost the drug’s stability, bioavailability, and pharmacokinetic properties. However, performing these changes on such complex molecules requires high levels of selectivity, including chemoselectivity, regioselectivity, and stereoselectivity. Furthermore, the nucleophilic characteristics of peptides make them vulnerable to redox conditions, which adds to the complexity of modifications. The limited number of solvents that can dissolve unprotected peptides also presents challenges. Consequently, achieving site-specific late-stage modifications to peptides is quite challenging.
Recently, a research team led by Professor Xuechen Li from the Department of Chemistry at The University of Hong Kong (HKU) developed a new strategy for clickable tryptophan modification. This method facilitates the straightforward alteration of specific regions of a peptide molecule, even during the later stages of drug development. This strategy enables researchers to fine-tune peptides after establishing the core structure. Their research has been published in Science Advances.
The novel approach involves a catalyst-free late-stage C2-sulfenylation reaction utilizing S-modified quinoline-containing thiosulfonate reagents. Through this technique, researchers can efficiently introduce various functional groups to the tryptophan (Trp) residues within the natural peptide structures. The functional groups added include trifluoromethylthio, difluoromethylthio, (ethoxycarbonyl) difluoromethylthio, alkylthio, and arylthiol.
In this process, trifluoroacetic acid (TFA) served as the ideal solvent and was crucial in activating the reagents through hydrogen bonding. Additionally, TFA’s excellent dissolving ability for hydrophobic and aggregation-prone peptides proves this method suitable for complex molecules, such as lipopeptides and self-assembling peptides, even at relatively high concentrations.
This strategy has shown success in modifying several commercially available peptide drugs, including somatostatin, octreotide, lanreotide, setmelanotide, daptomycin, and semaglutide, as well as the bioactive glycopeptide hAdn-WM6877. This demonstrates the method’s versatility in enhancing the diversity of peptide-based active pharmaceutical ingredients.
Professor Li’s team observed enhanced bioactivity and serum stability in modified melittin analogs, indicating the significant potential of this approach for drug development. Since Trp is a common component in RiPP natural products like darobactin and chloropeptin I, as well as drug candidates identified through phage display and mRNA display methods, this technique will also be valuable for late-stage diversification of natural products to create molecular libraries and functional probes.
The team believes that this single-step clickable late-stage Trp modification method will offer a robust platform for generating structural analogues efficiently and cost-effectively. This will meet the demand for optimizing drug functions and pharmacokinetics, making it an invaluable tool for medicinal chemists, peptide chemists, and chemical biologists.
