Therapeutic applications of peptides are currently limited by their proteolytic instability and impermeability to the cell membrane. Here, we report a general, reversible bicyclization strategy to increase both the proteolytic stability and cell permeability of peptidyl drugs. A peptide drug is fused with a short cell-penetrating motif and converted into a conformationally constrained bicyclic structure through the formation of a pair of disulfide bonds. The resulting bicyclic peptide has greatly enhanced proteolytic stability as well as cell-permeability. Once inside the cell, the disulfide bonds are reduced to produce a linear, biologically active peptide. This strategy was applied to generate a cell-permeable bicyclic peptidyl inhibitor against the NEMO-IKK interaction.

Therapeutic applications of peptides are currently limited by their proteolytic instability and impermeability to the cell membrane. Here, we report a general, reversible bicyclization strategy to increase both the proteolytic stability and cell permeability of peptidyl drugs. A peptide drug is fused with a short cell-penetrating motif and converted into a conformationally constrained bicyclic structure through the formation of a pair of disulfide bonds. The resulting bicyclic peptide has greatly enhanced proteolytic stability as well as cell-permeability. Once inside the cell, the disulfide bonds are reduced to produce a linear, biologically active peptide. This strategy was Supporting information for this article is given via a link at the end of the document HHS Public Access Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript applied to generate a cell-permeable bicyclic peptidyl inhibitor against the NEMO-IKK interaction. Drug deliveryPeptide bicyclization via a pair of disulphide bonds increases its proteolytic stability and cell permeability and yet allows for regeneration of the functional linear peptide once inside the cytosol of the cell. KeywordsCell-penetrating peptide; cyclic peptide; NEMO inhibitor; bicyclization; protein-protein interactionCompared to small-molecule drugs, peptides are highly selective and efficacious and, at the same time, relatively safe and well tolerated. A particularly exciting application of peptides is the inhibition of protein-protein interactions (PPIs), which remain challenging targets for small molecules. [1] Consequently, there is an increased interest in peptides in pharmaceutical research and development, and ~140 peptide therapeutics are currently being evaluated in clinical trials. [2] However, peptides are inherently susceptible to proteolytic degradation. Additionally, peptides are generally impermeable to the cell membrane, largely limiting their applications to extracellular targets. Although N-methylation of the peptide backbone and formation of intramolecular hydrogen bonds have been shown to improve the proteolytic stability and membrane permeability of certain cyclic peptides, [3,4] alternative strategies to increase both the metabolic stability and cell permeability of peptide drugs are clearly needed.NF-κB is a transcription factor that controls the expression of numerous gene products involved in immune, stress, inflammatory responses, cell proliferation, and apoptosis. [5] Aberrant activation of NF-κB signaling has been implicated in a number of autoimmune diseases (e.g., rheumatoid arthritis) and cancer (e.g., diffuse large B-cell lymphoma), among others. [6] Canonical NF-κB signaling is mediated by the interaction between the inhibitor of κB (IκB)-kinase (IKK) complex and regulatory protein NF-κB essential modifier (NEMO). [7] Binding to NEMO activates IKK, which in turn phosphorylates IκB, promoting the proteasomal degradation of IκB and release of active NF-κB. Modulators targeting various steps of the NF-κB signaling pa...