Evaluation of On-Resin Head-to-Tail Cyclization Protocols of Peptides Using Automated Peptide Synthesizers

Peptide drug discovery programs have advanced to the clinic as the rules for small molecule-like transport and bioavailability have become more thoroughly understood. Macrocyclic peptides, in particular, have emerged as an extension to current peptide-based drug modalities of interest to the pharmacological community. Because macrocycles are structurally more rigid, improvements in specificity, stability and bioavailability are observed when compared to traditional linear peptide therapeutics.

Methods enabling cyclization via side chain functionalities like lactam bridges or disulfide bonds are readily completed while the peptide is on-resin and can therefore easily be incorporated into the automated synthesis protocol. Head-to-tail cyclization has proven difficult to perform on solid supports due to the C-terminus being bound to the resin. Some techniques have been developed that cleave and cyclize in a single reaction step however these reactions typically run overnight and result in a lower crude purity1. Here, we evaluate conditions to automate head-to-tail cyclization reactions on-resin to improve recovery, potentially purity and decrease manual intervention requirements.