Rational Design of a Dual-Targeting Natural Toxin-Like Bicyclic Peptide for Selective Bioenergetic Blockage in Cancer Cells

Stapled α-helical peptides emerge as one of the attractive peptidomimetics which can efficiently penetrate the cell membrane to access intracellular targets. However, the incorporation of a highly lipophilic cross-link may lead to nonspecific membrane toxicity in certain cases. Here, we report a new class of thioether-tethered bicyclic α-helical peptide to mimic the highly constrained loop-helix structure of natural toxins with the dual-targeting ability for both cell-surface receptors and intracellular targets. The thioether cross-links are introduced to replace the redox-sensitive disulfide bonds in natural toxins via a photoinduced thiol-yne reaction followed by macrolactamization. As a proof of concept, αVβ3 integrin targeting ligand was grafted into one of the macrocycles in the bicyclic scaffold, while a mitochondria-targeting proapoptotic motif was introduced into the other macrocycle stabilized by an i, i + 7 alkyl thioether cross-link to recapitulate its α-helical conformation. The obtained dual-targeting bicyclic α-helical BIRK peptides showed highly stable α-helical conformation in the presence of denaturants or under high temperature. Notably, BIRK peptides could induce selective cell death in αVβ3 integrin-positive B16F10 cells by interfering with the bioenergetic functions of mitochondria. This work provides a new avenue to design and stabilize α-helical peptides in a highly constrained bicyclic loop-helix scaffold with dual functionality.