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.

Our research uses proteases from pathogenic RNA viruses as model targets to develop peptide-based inhibitors. Many viral protease inhibitors in clinical use are modified peptide substrate analogues. Examples are approved drugs against HIV, hepatitis C virus (HCV), and the recently approved SARS-CoV-2 main protease inhibitor nirmatrelvir.

We developed various unnatural amino acids functionalized with cyanopyridine and 1,2-aminothiol groups which can be directly incorporated into peptides by standard Fmoc solid-phase peptide synthesis. Cyclisation and stapling reactions proceed under biocompatible conditions in presence of protein drug targets. Using these approaches in small screening campaigns, we were able to identify various macrocyclic peptide inhibitors of viral proteases. [1,2]

Bicyclic peptides offer even greater conformational rigidity, metabolic stability, and antibody-like affinity and specificity. We explored the reaction between 1,2-aminothiols and 2,6-dicyanopyridine to establish a biocompatible, selective, and catalyst-free pathway to access bicyclic peptides. The bicycles displayed plasma stability, conformational preorganization, and high target affinity. [3]  

In addition, we introduced a fundamentally new method to generate bicyclic peptides that uses bismuth as a selective, stable, rigid and green reagent for peptide modification. Bismuth represents the smallest “scaffold” ever explored and allows in situ access to bicyclic peptides for biochemical screening assays. [4]

Literature:

[1] R. Morewood and C. Nitsche, Chem. Sci. 2021, 12, 669. [2] R. Morewood and C. Nitsche, Chem. Commun. 2022, 58, 10817. [3] S. Ullrich, J. George, A. E. Coram, R. Morewood, C. Nitsche, Angew. Chem. Int. Ed. 2022, 61, e202208400. [4] S. Voss, J. Rademann, C. Nitsche, Angew. Chem. Int. Ed. 2022, 61, e202113857.