Bicyclic peptides are polypeptides forming two circular units. The cyclic structures often exhibit improved stability, higher potency and bioavailability. Therefore, they are considered as a novel therapeutic class, which lies between small molecules and monoclonal antibodies.
Bicyclic peptides can be prepared by both solution phase and solid phase synthesis; however, their synthesis remains a challenge. The multiple steps of synthesis, cyclization, and purification often result in low overall yield. Therefore, the synthesis strategy plays a critical role.
We recently synthesized a 13-mer bicyclic peptide, containing one disulfide bridge and one triazole bridge. Multiple synthetic routes were tested, including both on-resin cyclization and cyclization in solution. The original protocol using step-wise cyclization in solution required multiple steps of cyclization and purification, which gave an overall yield of 18%. Using a novel, one-pot synthesis strategy, we were able to carry out two, sequential cyclizations in one reaction solution and purify the final product with only one final purification step to give an increased yield of 30%. For the production of 100mg of final, bicyclic peptide product, the entire synthesis time was shortened by one week using the one-pot reaction protocol. We believe scale up of the reaction process to produce gram quantities of final pure, bicyclic compound is feasible, and no further process development will be needed.
This novel strategy is applicable to facilitate the synthesis of a broad range of bicyclic peptides when the preparation of a disulfide bridge and triazole bridge within a single sequence is required.