Stapled Peptides

Stapled peptides are synthetic peptides that incorporate a chemical crosslink — typically an all-hydrocarbon bridge — between two non-natural amino acids positioned along one face of an alpha helix. This "staple" locks the peptide into its bioactive helical conformation, overcoming the three major limitations of conventional peptides: proteolytic degradation, poor membrane permeability, and conformational entropy penalties upon target binding.

The technology was pioneered by Gregory Verdine and colleagues at Harvard in the early 2000s, building on earlier work in helix-stabilizing crosslinks by Grubbs and others. It represents one of the most significant advances in peptide drug design of the past two decades.

The alpha-helix problem

Approximately 60% of protein-protein interactions (PPIs) are mediated by alpha-helical interfaces. These interactions — a helix from one protein binding into a groove on another — are critical in cancer biology (p53/MDM2), apoptosis (BH3/BCL-2 family), and transcription factor signaling.

Short peptides derived from these helical interfaces should, in principle, mimic the interaction and serve as PPI inhibitors. In practice, isolated peptides shorter than about 15-20 residues rarely maintain stable alpha-helical structure in solution. They exist as disordered ensembles, losing binding affinity (due to the entropic cost of folding upon binding) and becoming easy targets for proteases that preferentially cleave extended conformations.

Stapling solves this by pre-organizing the helix.

Chemistry of hydrocarbon stapling

The most common approach uses olefin metathesis. Two alpha-methyl, alpha-alkenyl amino acids are incorporated at positions i and i+4 (one helical turn) or i and i+7 (two helical turns) on the same face of the helix. Ruthenium-catalyzed ring-closing metathesis then forms a covalent hydrocarbon bridge across the helix surface.

Key design considerations:

• Staple position — the crosslink must be placed on the solvent-exposed face of the helix, away from the binding interface. Placing the staple on the binding face blocks target engagement.
• i, i+4 vs. i, i+7 — single-turn staples (i, i+4) use shorter linkers and provide moderate stabilization. Two-turn staples (i, i+7) provide greater helical stabilization and more hydrophobic surface area for membrane interaction.
• Stereochemistry — the alpha-methyl substitution (S or R configuration) influences helix propensity and staple geometry. S5/S5 for i, i+4 and R8/S5 for i, i+7 are the most common configurations.

Properties gained through stapling

Protease resistance

The alpha-methyl substitution at staple positions sterically blocks protease access to the backbone. Combined with the rigid helical conformation (which prevents the extended conformations proteases require), stapled peptides show dramatically improved serum stability — often 10-100x longer half-life compared to their unstapled counterparts.

Cell permeability

Hydrocarbon staples add hydrophobic surface area to the peptide, and the amphipathic helix structure (hydrophobic staple face, hydrophilic binding face) resembles the architecture of cell-penetrating peptides. Stapled peptides enter cells predominantly through endocytic pathways, with cationic residues facilitating endosomal escape. Not all stapled peptides are cell-permeable — permeability depends on overall charge, hydrophobicity balance, and staple length.

Binding affinity

Pre-organizing the alpha helix eliminates the conformational entropy penalty of folding upon binding. This typically improves binding affinity 2-50 fold compared to the unstapled linear peptide, since the free energy of binding no longer needs to pay for helix formation.

Clinical and preclinical examples

ALRN-6924 (sulanemadlin)

The most advanced stapled peptide therapeutic. ALRN-6924 mimics the p53 transactivation domain helix that binds MDM2 and MDMX — the two primary negative regulators of p53. By disrupting p53/MDM2 and p53/MDMX interactions, it reactivates wild-type p53 tumor suppressor activity. It has been evaluated in clinical trials for advanced solid tumors and hematologic malignancies harboring wild-type p53, and as a chemoprotectant for normal cells during chemotherapy in p53-mutant tumors.

BH3 mimetics (SAHBs)

Stabilized Alpha-Helix of BCL-2 domains (SAHBs) were among the earliest stapled peptides developed. They mimic pro-apoptotic BH3 domain helices, binding anti-apoptotic BCL-2 family members (BCL-2, BCL-xL, MCL-1) and triggering apoptosis in cancer cells. While small-molecule BH3 mimetics (venetoclax) reached the market first, stapled BH3 peptides provided critical proof-of-concept for the technology.

Stapled peptide inhibitors of beta-catenin, estrogen receptor, and RAS

Research-stage stapled peptides target intracellular PPIs previously considered "undruggable" — including the RAS/effector interaction, one of the most sought-after targets in oncology. These programs demonstrate the unique ability of stapled peptides to access the intracellular PPI target space that small molecules struggle to address due to the large, flat binding surfaces involved.

Limitations

Stapled peptides are expensive to synthesize (non-natural amino acids, metathesis catalysts, HPLC purification). Oral bioavailability remains limited despite improved protease resistance — most are administered by injection. Cell permeability is not guaranteed and requires careful optimization of charge, hydrophobicity, and staple placement. Manufacturing scale-up for clinical supply has been achieved but remains more complex than standard peptide synthesis.

Despite these challenges, stapled peptides occupy a valuable niche: they can drug intracellular protein-protein interactions — a target class largely inaccessible to both conventional peptides (which cannot enter cells) and small molecules (which lack the surface area to disrupt PPIs).