Introducing the researchers:
Eric A. Appel is an Assistant Professor of Materials Science & Engineering at Stanford University. He received a BS in Chemistry (2008) and MS in Polymer Science (2008) from Cal Poly San Luis Obispo, and a PhD in Chemistry (2013) from Cambridge University. His research group at Stanford integrates concepts and approaches from supramolecular chemistry and polymer science to develop (bio)materials that can be used as tools to better understand fundamental biological processes and to engineer advanced healthcare solutions.
Anton A. A. Smith holds a PhD from Aarhus University, Denmark. He will soon be joining the Technical University of Denmark (DTU) where he will be continuing research at the interface of chemistry and biology.
Caitlin L. Maikawa received her BASc (2016) in Chemical Engineering from the University of Toronto. She is currently working on her PhD in Bioengineering at Stanford University with Prof. Eric Appel. Her PhD research focuses on using supramolecular biomaterials to create improved insulin formulations for the treatment of diabetes.
Gillie A. Roth received her B.S. (2015) in Bioengineering from UC San Diego and then completed her PhD in Bioengineering at Stanford University with Eric Appel. Her PhD research focused on designing biomaterials to modulate the pharmacokinetics of therapeutics across diverse disease indications.
What inspired your research in this area?
We were already using a conjugate of cucurbituril with poly(ethylene glycol) (CB-PEG) as a “designer” excipient in insulin formulations (https://www.nature.com/articles/s41551-020-0555-4, https://www.pnas.org/content/113/50/14189.short) to improve insulin stability and alter pharmacokinetics. In this work we figured we could exploit the affinity of CB for the N-terminal phenylalanine on insulin as a tool for insulin modification. Covalent PEG conjugates of insulin had already been examined on numerous occasions in the literature, with insulin bioactivity being heavily dependent on the site of conjugation (steric repulsion from modification in the wrong spot can completely remove activity). Conjugation to the A chain N-terminal glycine significantly reduced activity, making it an interested target for stimulus responsive activation of insulin. Unfortunately, the existing means of selectively functionalizing this site are cumbersome because the preferred site of modification on insulin is the B chain N-terminal phenylalanine. The strong, selective non-covalent binding of CB to this N-terminal phenylalanine presented itself as a practical shortcut to block the nucleophilicity of this site, thereby acting as a non-covalent protecting group, to allow for selective modification of the A-chain N-terminal glycine by simple acylations.
What do you personally feel is the most interesting outcome of your study?
For insulin specifically, this work enables a simple approach to selectively functionalize the A chain, which has traditionally been very challenging to modify on account of its poor nucleophilicity compared to the B chain. Our approach shortens the synthetic route towards selective conjugation at this site as the self-assembly provides direct blocking of the site in the reaction flask with simple mixing, but the dynamic non-covalent binding allows for removal of the CB protection directly during purification without need for a deprotection step.
For a broader perspective, non-covalent protection groups in protein and peptide conjugation chemistry are virtually unexplored, and we show that CB-based host-guest complexation provides a simple and effective approach to blocking of aromatic amino acids to drive selective modification elsewhere on the peptide/protein. This approach is potentially applicable to selective conjugation with an array of proteins or peptides.
What directions are you planning to take with your research in future?
Insulin conjugates, and possibly stimulus responsive activation, along with excipients in formulation are areas we are very excited about. We imagine there is ample room for developing both the chemistry and clinically relevant translational research using this new approaches to non-covalent protection.
See the other articles showcased in this month’s Editor’s Collection