Stapled Peptides: Conformational Constraints

The Concept of Peptide Stapling

Stapled peptides represent an innovative class of constrained peptide therapeutics in which a synthetic hydrocarbon bridge is introduced across one or two turns of an alpha-helix, locking the peptide into its bioactive helical conformation. The technology was pioneered by Gregory Verdine and colleagues at Harvard University, who demonstrated that incorporation of olefin-bearing alpha-methylated amino acids at positions i and i+4 or i and i+7 on the same helical face, followed by ruthenium-catalyzed ring-closing olefin metathesis, produced all-hydrocarbon stapled peptides with dramatically enhanced helical content. This conformational constraint simultaneously addresses multiple limitations of conventional peptide therapeutics by improving target binding affinity through preorganization, increasing protease resistance through backbone shielding, and enhancing cellular uptake through increased hydrophobicity and amphipathic character.

Design Principles and Optimization Strategies

The successful design of stapled peptides requires careful consideration of staple placement, length, stereochemistry, and the physicochemical properties of the resulting macrocycle. The staple must be positioned on the solvent-exposed face of the helix to avoid disrupting target-binding interactions, and the stereochemistry of the alpha-methylated amino acid building blocks must be matched to the desired helical geometry. Single-turn staples spanning positions i to i+4 employ shorter olefin tethers and are suitable for stabilizing short helical segments, while double-turn i to i+7 staples provide more extensive stabilization of longer helical regions. Stitched peptides bearing two adjacent staples connected through a shared central residue offer even greater conformational control and have shown enhanced cell permeability compared to singly stapled counterparts.

Therapeutic Applications and Clinical Progress

Stapled peptides have shown particular promise in targeting intracellular protein-protein interactions that are traditionally considered undruggable by small molecules. The BH3 domain of pro-apoptotic BCL-2 family members was among the first targets addressed by stapled peptide technology, with stapled BH3 mimetics demonstrating direct activation of apoptosis in cancer cells through engagement of BAX and BAK. ATSP-7041 and its clinical successors targeting the p53-MDM2 and p53-MDMX interactions represent the most advanced stapled peptide programs, aiming to reactivate the tumor suppressor p53 pathway in cancers retaining wild-type p53. Additional therapeutic applications under investigation include stapled peptides targeting estrogen receptor coactivator interactions, beta-catenin-TCF signaling, and Ras-effector protein complexes.