The Symmetric Tetravalent Sulfhydryl-Specific Linker NATBA Facilitates a Combinatorial “Tool Kit” Strategy for Phage Display-Based Selection of Functionalized Bicyclic Peptides

[Image: see text] The rigid conformation of constrained bicyclic peptides provides a number of advantages over larger protein-based ligands, including better chemical stability, enhanced tissue penetration, and a wider field of possible applications. Selective chemical modification strategies are able to extend the scope of applications not only in a therapeutic manner but also for the development of novel tools for protein capturing, bioimaging, and targeted drug delivery. Herein, we report the synthesis of an adamantane-based, symmetrical, tetravalent, sulfhydryl-specific peptide linker. We have developed an in vitro two-step modification strategy that allows the generation of differently functionalized bicyclic peptides. This “tool kit” strategy was applied to cyclize and functionalize a phage-encoded peptide library bearing the sequence CX(6)CX(6)C. After phage display against a model target, isolated peptides show strong consensus sequences, indicating target-specific binding. The newly developed symmetric tetravalent linker opens new avenues for the combinatorial selection and functionalization of bicyclic peptide ligands with affinity to virtually any target.