Selective Covalent Targeting of Anti‐apoptotic BFL‐1 by a Sulfonium‐Tethered Peptide

Anti‐apoptotic B cell lymphoma 2 (BCL‐2) family proteins are proven targets for human cancers. Targeting the BH3‐binding pockets of these anti‐apoptotic proteins could reactivate apoptosis in BCL‐2‐depedent cancers. BFL‐1 is a BCL‐2 family protein overexpressed in various chemoresistant cancers. A unique cysteine at the binding interface of the BH3 and BFL‐1 was previously proven to be an intriguing targeting site to irreversibly inhibit BFL‐1 functions with stabilized cyclic peptide bearing a covalent warhead. Recently, we developed a sulfonium‐tethered peptide cyclization strategy to construct peptide ligands that could selectively and efficiently react with the cysteine(s) of target proteins near the interacting interface. Using this method, we constructed a BFL‐1 peptide inhibitor, B4‐MC, that could selectively conjugate with BFL‐1 both in vitro and in cell. B4‐MC showed good cellular uptake, colocalized with BFL‐1 on mitochondria, and showed obvious growth inhibition of BFL‐1 over‐expressed cancer cell lines. Cysteine stabilizers: Sulfonium‐tethered peptide inhibitors, which were constructed by a facile bis‐alkylation strategy between cysteine and methionine, could increase peptides’ enzymatic stability, enhance cyclized peptide's cellular uptake, and selectively target anti‐apoptotic protein BFL‐1 to inhibit BFL‐1 over‐expressed cancer cell lines.