Oral Presentation 8th Modern Solid Phase Peptide Synthesis & Its Applications Symposium 2022

Biocompatible peptide cyclisation and modification (#24)

Christoph Nitsche 1
  1. Australian National University, Canberra, ACT, Australia

Constrained and modified peptides fill an underexplored area of chemical space between small molecule therapies and larger antibodies. Noncanonical modifications, such as cyclisation or stapling, can (i) enhance metabolic stability by greater resistance towards proteolysis, (ii) promote biological uptake across cell membranes, and (iii) decrease the entropic penalty of binding by locking the peptide in the active conformation. Currently available chemical strategies for late-stage peptide modifications, however, are often laborious or incompatible with biological systems.

Macrocyclisation is a common strategy to constrain peptide molecules. We explored various amino acids bearing reactive nitriles and 1,2-aminothiol functional groups that allow direct incorporation into peptides by standard Fmoc solid-phase peptide synthesis. Peptide cyclisation can be facilitated between a 1,2-aminothiol group (e.g. N-terminal cysteine) and the unnatural amino acid 3-(2-cyano-4-pyridyl)alanine bearing the nitrile electrophile (refs. 1 and 3). In addition, we developed a two-component peptide stapling reaction between 1,2-aminothiol amino acids and the commercially available stapling reagent 2,6-dicyanopyridine (ref. 2). Cyclisation and stapling reactions proceed under biocompatible conditions in presence of protein drug targets.

Bicyclic peptides offer superior conformational rigidity, metabolic stability, and antibody-like affinity and specificity. The modification of three cysteine residues in a peptide has been exploited for bicycle formation in synthetic and genetically encoded peptide libraries employing alkylating scaffolds such as 1,3,5-tris(bromomethyl)benzene (TBMB). We introduced bismuth as a selective, stable, rigid and green center for bicyclic peptides, representing the smallest “scaffold” ever explored (ref. 4). We synthesised various peptides of the general sequence CXnCXnC and observed spontaneous formation of bicyclic peptides in aqueous solution at physiological pH. The strategy allows the in-situ generation of bicyclic peptides for biochemical screening assays, which was demonstrated for two screening campaigns targeting viral proteases, revealing compounds that were up to 130 times more active and almost 20 times more proteolytically stable than their linear analogues.

  1. Nitsche, C., Onagi, H., Quek, J.-P., Otting, G., Luo, D., Huber, T., Biocompatible macrocyclization between cysteine and 2-cyanopyridine generates stable peptide onhibitors, Org. Lett. 2019, 21 (12), 4709-4712.
  2. Morewood, R., Nitsche, C., A Biocompatible stapling reaction for in situ generation of constrained peptides, Chem. Sci. 2021, 12 (2), 669-674.
  3. Patil, N. A., Quek, J.-P., Schroeder, B., Morewood, R., Rademann, J., Luo, D., Nitsche, C., 2-Cyanoisonicotinamide conjugation: A facile approach to generate potent peptide inhibitors of the Zika virus orotease, ACS Med. Chem. Lett. 2021, 12 (5), 732-737.
  4. Voss, S., Rademann, J., Nitsche, C., Peptide-Bismuth Bicycles: In Situ Access to Stable Constrained Peptides with Superior Bioactivity, Angew. Chem. Int. Ed. 2022, 61 (4), e202113857.