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Abstract Peptide therapeutics have gained great interest due to their multiple advantages over small molecule and antibody‐based drugs. Peptide drugs are easier to synthesize, have the potential for oral bioavailability, and are large enough to target protein‐protein interactions that are undruggable by small molecules. However, two major limitations have made it difficult to develop novel peptide therapeutics not derived from natural products, including the metabolic instability of peptides and the difficulty of reaching antibody‐like potencies and specificities. Compared to linear and disulfide‐monocyclized peptides, multicyclic peptides can provide increased conformational rigidity, enhanced metabolic stability, and higher potency in inhibiting protein‐protein interactions. The identification of novel multicyclic peptide binders can be difficult, however, recent advancements in the construction of multicyclic phage libraries have greatly advanced the process of identifying novel multicyclic peptide binders for therapeutically relevant protein targets. This review will describe the current approaches used to create multicyclic peptide libraries, highlighting the novel chemistries developed and the proof‐of‐concept work done on validating these libraries against different protein targets. 

Abstract Efficient and site‐specific modification of native peptides and proteins is desirable for synthesizing antibody‐drug conjugates as well as for constructing chemically modified peptide libraries using genetically encoded platforms such as phage display. In particular, there is much interest in efficient multicyclization of native peptides due to the appeals of multicyclic peptides as therapeutics. However, conventional approaches for multicyclic peptide synthesis require orthogonal protecting groups or non‐proteinogenic clickable handles. Herein, we report a cysteine‐directed proximity‐driven strategy for the constructing bicyclic peptides from simple natural peptide precursors. This linear to bicycle transformation initiates with rapid cysteine labeling, which then triggers proximity‐driven amine‐selective cyclization. This bicyclization proceeds rapidly under physiologic conditions, yielding bicyclic peptides with a Cys‐Lys‐Cys, Lys‐Cys‐Lys or N‐terminus‐Cys‐Cys stapling pattern. We demonstrate the utility and power of this strategy by constructing bicyclic peptides fused to proteins as well as to the M13 phage, paving the way to phage display of novel bicyclic peptide libraries.