Cell-Permeable Peptides Containing Cycloalanine Residues

Cyclic Peptides as Drugs for Intracellular Targets: The Next Frontier in Peptide Therapeutic Development

Developing macrocyclic peptides that can reach intracellular targets is a significant challenge. This review discusses the most recent strategies used to develop cell permeable cyclic peptides that maintain binding to their biological target inside the cell. Macrocyclic peptides are unique from small molecules because traditional calculated physical properties are unsuccessful for predicting cell membrane permeability. Peptide synthesis and experimental membrane permeability is the only strategy that effectively differentiates between cell permeable and cell impermeable molecules. Discussed are chemical strategies, including backbone N-methylation and stereochemical changes, which have produced molecular scaffolds with improved cell permeability. However, these improvements often come at the expense of biological activity as chemical modifications alter the peptide conformation, frequently impacting the compound's ability to bind to the target. Highlighted is the most promising approach, which involves side-chain alterations that improve cell permeability without impact binding events.

Emerging Methods and Design Principles for Cell-Penetrant Peptides

Biomolecules such as antibodies, proteins, and peptides are important tools for chemical biology and leads for drug development. They have been used to inhibit a variety of extracellular proteins, but accessing intracellular proteins has been much more challenging. In this review, we discuss diverse chemical approaches that have yielded cell-penetrant peptides and identify three distinct strategies: masking backbone amides, guanidinium group patterning, and amphipathic patterning. We summarize a growing number of large data sets, which are starting to reveal more specific design guidelines for each strategy. We also discuss advantages and disadvantages of current methods for quantifying cell penetration. Finally, we provide an overview of best-odds approaches for applying these new methods and design principles to optimize cytosolic penetration for a given bioactive peptide.

Post-Translational Backbone Engineering through Selenomethionine-Mediated Incorporation of Freidinger Lactams

Amino-γ-lactam (Agl) bridged dipeptides, commonly known as Freidinger lactams, have been shown to constrain peptide backbone topology and stabilize type II' β-turns. The utility of these links as peptide constraints has inspired new approaches to their incorporation into complex peptides and peptoids, all of which require harsh reaction conditions or protecting groups that limit their use on unprotected peptides and proteins. Herein, we employ a mild and selective alkylation of selenomethionine in acidic aqueous solution, followed by immobilization of the alkylated peptide on to bulk reverse-phase C₁₈ silica and base-induced lactamization in DMSO. The utilization of selenomethionine, which is readily introduced by synthesis or expression, and the mild conditions enable selective backbone engineering in complex peptide and protein systems.