Stapled Peptides

Overview

Stapled peptides are alpha-helical peptides that have been chemically reinforced by a covalent hydrocarbon bridge — or "staple" — spanning one face of the helix. This structural modification locks the peptide into its bioactive helical conformation, conferring dramatically improved protease resistance, cell permeability, and in vivo half-life compared to the unmodified linear peptide.

Developed primarily through the work of Gregory Verdine at Harvard University, stapled peptide technology was designed to address one of the most challenging problems in drug discovery: targeting intracellular protein-protein interactions (PPIs). These interactions, which are estimated to number over 650,000 in the human proteome, are mediated by large, flat protein surfaces that are generally inaccessible to small molecules and to conventional peptides that cannot cross cell membranes.

Chemistry of Hydrocarbon Stapling

The Stapling Reaction

The core chemistry involves:

1. Non-natural amino acid incorporation — Alpha-methyl, alpha-alkenyl amino acids (typically S5 and R8 configurations) are incorporated at defined positions during solid-phase peptide synthesis. These residues bear olefin-terminated side chains of specific lengths.
2. Ring-closing olefin metathesis (RCM) — After chain assembly, a ruthenium carbene catalyst (typically Grubbs second-generation catalyst) promotes intramolecular olefin metathesis, forming a carbon-carbon double bond that bridges the two non-natural residues and creates the hydrocarbon staple.
3. Optional hydrogenation — The resulting double bond can be reduced to a fully saturated hydrocarbon bridge if desired.

Staple Positioning

The position and span of the staple are critical design parameters:

• i, i+4 staple — Connects residues separated by one helical turn (approximately 4 amino acids apart on the same face). Uses shorter olefin chains.
• i, i+7 staple — Connects residues separated by two helical turns. The longer span provides greater helical stabilization and is most commonly used.
• i, i+3 staple — Less common; connects residues on the same helical face separated by less than one turn.

The staple must be positioned on the solvent-exposed face of the helix — opposite the protein-binding interface — to avoid disrupting target engagement.

Double Stapling

For longer peptides or those requiring additional stabilization, two staples can be applied (a "stitched" peptide), further increasing helical content, protease resistance, and thermal stability.

Biophysical Properties

Hydrocarbon stapling transforms peptide pharmacology in several measurable ways:

Helical Stabilization

Unmodified peptides in solution typically exhibit low helical content (often less than 20% by circular dichroism). Stapling can increase alpha-helical content to 60-90%, pre-organizing the peptide into its binding-competent conformation and reducing the entropic penalty of binding.

Proteolytic Resistance

The hydrocarbon bridge physically blocks protease access to the peptide backbone. Stapled peptides typically show 10- to 100-fold improvement in serum half-life compared to their linear counterparts, directly addressing a major stability challenge.

Cell Permeability

Perhaps the most transformative property: stapled peptides can cross cell membranes to reach intracellular targets. The mechanism involves energy-dependent endosomal uptake followed by endosomal escape, facilitated by the amphipathic character of the stapled helix (hydrophobic staple on one face, charged residues on the other).

Cell permeability is influenced by:

• Overall charge (moderately cationic peptides show better uptake)
• Staple position and chemistry
• Amphipathicity of the helical surface
• Peptide length

Target Binding

When properly designed, stapling maintains or improves binding affinity for the target protein. The pre-organized helix reduces the conformational entropy loss upon binding, while the hydrocarbon staple can make additional hydrophobic contacts with the target surface.

Therapeutic Applications

Targeting p53-MDM2/MDMX

The p53 tumor suppressor pathway is the most extensively studied application of stapled peptides. In approximately 50% of human cancers, p53 is inactivated not by mutation but by overexpression of its negative regulators MDM2 and MDMX. The p53-MDM2 interaction is mediated by an alpha-helical segment of p53 binding into a hydrophobic cleft on MDM2.

ALRN-6924 (sulanemadlin), developed by Aileron Therapeutics, is a stapled peptide that mimics this p53 helix and blocks both MDM2 and MDMX, reactivating p53-mediated tumor suppression. ALRN-6924 entered clinical trials for multiple oncology indications:

• As a direct anti-tumor agent in cancers with wild-type p53
• As a chemoprotective agent, exploiting p53 activation to induce cell-cycle arrest in normal cells during chemotherapy (protecting them while tumors with mutant p53 remain vulnerable)

BCL-2 Family Interactions

The BCL-2 family of proteins regulates apoptosis through protein-protein interactions mediated by BH3 domain alpha-helices. Stapled BH3 peptides that mimic pro-apoptotic BH3-only proteins have been developed to directly activate apoptosis in cancer cells. These include stapled peptides targeting MCL-1, BCL-2, and BCL-XL — anti-apoptotic proteins frequently overexpressed in hematological malignancies and solid tumors.

Beta-Catenin/TCF

The Wnt/beta-catenin signaling pathway, frequently dysregulated in colorectal cancer, is mediated by the interaction between beta-catenin and TCF transcription factors. Stapled peptides disrupting this interaction are in preclinical development.

Estrogen Receptor Coactivator Interactions

Stapled peptides that block estrogen receptor interactions with coactivator proteins represent a novel approach to hormone receptor-positive breast cancer.

Beyond Oncology

While oncology dominates, stapled peptides are also being explored for:

• Infectious disease (disrupting viral protein-protein interactions)
• Metabolic disease (targeting intracellular signaling nodes)
• Inflammatory conditions (modulating NF-kB and other transcription factor pathways)

Aileron Therapeutics

Aileron Therapeutics, founded in 2005, is the most prominent company focused on stapled peptide drug development. The company has advanced ALRN-6924 through multiple clinical trials and maintains a pipeline of additional stapled peptide candidates targeting intracellular PPIs. The clinical experience with ALRN-6924 has provided valuable data on the pharmacokinetics, safety, and efficacy of stapled peptides as a drug class.

Design and Optimization

Developing a stapled peptide therapeutic involves:

1. Target validation — Confirming that the PPI is essential for disease biology and that the interaction is mediated by an alpha-helical segment
2. Lead identification — Determining the minimal helical segment that drives binding, often guided by structural biology (X-ray crystallography, cryo-EM)
3. Staple optimization — Systematic variation of staple position, length, and chemistry to maximize helical content, binding affinity, and cell permeability
4. Sequence optimization — Mutating non-contact residues to improve solubility, reduce aggregation, and enhance pharmacokinetic properties
5. Computational design — Molecular dynamics simulations and machine learning increasingly guide the optimization process

Limitations

• Manufacturing complexity — Non-natural amino acids and the metathesis reaction increase synthesis cost and complexity compared to standard peptides
• Delivery route — Most stapled peptides require intravenous or subcutaneous administration; oral delivery remains elusive due to size
• Cell permeability variability — Not all stapled peptides achieve efficient cellular uptake; the determinants of cell permeability are not fully predictable
• Off-target membrane activity — Highly amphipathic stapled peptides can exhibit non-specific membrane disruption at elevated concentrations
• Limited clinical data — The clinical track record remains thin compared to established drug modalities

Outlook

Stapled peptides represent a maturing technology that has established proof of concept for targeting intracellular protein-protein interactions with peptide-based drugs. As the clinical experience base grows, manufacturing processes improve, and design rules become better defined through computational approaches, stapled peptides are expected to become an increasingly important component of the peptide drug development pipeline.