Stapled Peptides in Cancer: Alpha-Helix Drugs

Anticancer Peptides

59% DCR

Disease control rate in 41 evaluable patients with wild-type TP53 tumors treated with ALRN-6924, the first stapled peptide to reach Phase 1 clinical trials for cancer.

Saleh et al., Clinical Cancer Research, 2021

Saleh et al., Clinical Cancer Research, 2021

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Most protein-protein interactions in cancer cells happen through flat, extended surfaces that small-molecule drugs cannot grip. Antibodies can bind these surfaces but cannot enter cells. Peptides can mimic one side of the interaction, but unmodified peptides are floppy, quickly degraded, and usually cannot cross cell membranes. Stapled peptides solve all three problems at once. By inserting a hydrocarbon bridge across one turn of an alpha-helix, chemists lock the peptide into its bioactive shape, protect it from proteases, and give it enough lipophilicity to cross cell membranes. This article covers the cancer applications of stapled peptides within the anticancer peptides cluster.

Why Peptides Need Stapling

Proteins interact through structured surfaces. When an alpha-helical segment of one protein binds into a groove on another, the helix must maintain its shape. A synthetic peptide with the same amino acid sequence as that helix will usually unravel in solution, losing the 3D geometry required for binding.

The stapling solution, developed by Gregory Verdine and colleagues at Harvard, involves replacing two amino acids on the same face of the helix (separated by one or two turns) with non-natural amino acids bearing olefin side chains. Ring-closing metathesis then forms a covalent hydrocarbon bridge across the helix, locking it into shape.^[1]

This single modification produces three simultaneous effects. First, the helix stays folded: alpha-helical content jumps from 20-30% to 80-90%. Second, the hydrocarbon bridge shields the peptide backbone from proteolytic enzymes, extending half-life from minutes to hours. Third, the added lipophilicity from the hydrocarbon allows the peptide to cross cell membranes without requiring transporters or endocytic uptake. For more on peptide stapling chemistry and methods, see the dedicated article.

The BH3 Proof of Concept

The landmark experiment came in 2004. Walensky et al. created a stapled version of the BH3 domain from the pro-apoptotic protein BID. The BH3 domain is a short alpha-helix that binds anti-apoptotic BCL-2 family proteins (BCL-2, BCL-XL, MCL-1), neutralizing their survival function and triggering cell death.^[1]

The unstapled BH3 peptide had no cellular activity: it could not enter cells and was rapidly degraded. The stapled version (SAHB, Stabilized Alpha-Helix of BCL-2 domains) crossed cell membranes, bound BCL-XL with higher affinity than the unstapled peptide, activated the pro-apoptotic effector BAX at the mitochondrial outer membrane, and suppressed the growth of human leukemia xenografts in mice. This was the first demonstration that a stapled peptide could function as an anticancer agent in a living animal. The study, published in Science, established the core premise: chemical stabilization of a natural protein-protein interaction surface can create a drug-like molecule from a peptide sequence that would otherwise have no pharmacological utility.

Joseph et al. (2012) extended the approach to MCL-1, a BCL-2 family member that confers resistance to many cancer therapies. Using atomistic simulations, they designed stapled BH3 peptides optimized for MCL-1 binding and demonstrated structure-activity relationships between staple position, helix stability, and binding affinity.^[2] This work established that staple placement is not arbitrary: the bridge must reinforce the binding face without sterically clashing with the target protein.

p53-MDM2: The Primary Cancer Target

The most advanced stapled peptide program targets the interaction between the tumor suppressor p53 and its negative regulators MDM2 and MDMX. In roughly half of all human cancers, TP53 is mutated and non-functional. In the other half, p53 is genetically intact (wild-type) but held inactive by MDM2 and MDMX proteins that bind its transactivation domain and mark it for degradation. Releasing p53 from this inhibition reactivates its tumor-suppressive functions: cell cycle arrest, DNA repair, and apoptosis.

The p53-MDM2 interaction occurs through an alpha-helical surface on p53 that fits into a hydrophobic cleft on MDM2. This makes it an ideal stapled peptide target.

Chang et al. (2013) developed ATSP-7041, a stapled alpha-helical peptide that binds both MDM2 and MDMX with nanomolar affinity. The dual-targeting is critical because cancer cells that overexpress MDMX can escape MDM2-only inhibitors. ATSP-7041 demonstrated submicromolar cellular activity in cancer cell lines in the presence of serum proteins, showed intracellular localization by fluorescence microscopy, and suppressed tumor growth in xenograft models.^[3]

The clinical version, ALRN-6924 (sulanemadlin), became the first cell-permeating stapled peptide to enter human clinical trials. The Phase 1 dose-escalation trial enrolled 71 patients with solid tumors and lymphomas bearing wild-type TP53 across two dosing schedules: once weekly for 3 weeks every 28 days (arm A, 41 patients, doses from 0.16 to 4.4 mg/kg) and twice weekly for 2 weeks every 21 days (arm B, 30 patients, doses from 0.32 to 2.7 mg/kg). Among 41 efficacy-evaluable patients, the disease control rate was 59%, with 2 complete responses, 2 partial responses, and 20 patients with stable disease. The most common treatment-related adverse events were gastrointestinal effects (nausea, vomiting, diarrhea), fatigue, anemia, and headache.

The trial demonstrated proof of mechanism: serum levels of MIC-1 (a p53-regulated protein) increased in a dose-dependent manner, confirming that ALRN-6924 was reactivating p53 in patients. This pharmacodynamic biomarker provided evidence that the stapled peptide was reaching its intracellular target in human tumors, not just in cell culture or mouse models.

Design Rules: What 350+ Molecules Taught Researchers

Chandramohan et al. (2024) published a systematic study that synthesized over 350 stapled peptide variants targeting MDM2/MDMX and analyzed the relationship between chemical properties and biological activity.^[4] The key findings form a practical design manual:

Lipophilicity drives permeability. There is a clear correlation between a stapled peptide's lipophilicity (logP) and its ability to cross cell membranes. Peptides below a threshold lipophilicity failed to penetrate cells regardless of their binding affinity.

Positive charge causes off-target toxicity. Stapled peptides with net positive charge showed membrane-disruptive activity against healthy cells, mimicking the behavior of amphipathic anticancer peptides. Removing positive charge eliminated off-target toxicity without sacrificing target binding.

Staple type and number matter. Single versus double staples, i,i+4 versus i,i+7 spacing, and all-hydrocarbon versus lactam bridges each produced different pharmacological profiles. There is no universal optimal staple: the best configuration depends on the target protein's binding groove geometry.

In vivo activity requires all properties simultaneously. High binding affinity alone did not predict in vivo efficacy. Peptides needed adequate permeability, metabolic stability, selectivity, and binding potency together to show tumor suppression in mouse models.

Beyond p53: Expanding the Target Space

The stapled peptide platform has expanded well beyond the p53-MDM2 axis.

BCL-2 family. Following the original BH3 proof-of-concept, multiple groups have developed stapled peptides targeting specific BCL-2 family members (BCL-XL, MCL-1, BFL-1) that drive resistance to chemotherapy. A 2016 review by Iyer et al. cataloged the growing landscape of stapled peptide and small molecule approaches to cancer-relevant protein-protein interactions.^[5]

RAS. Han et al. (2025) designed stapled peptides targeting RAS-effector interactions, one of the most sought-after targets in oncology. KRAS mutations drive approximately 25% of all human cancers, and RAS proteins were considered "undruggable" until recently. The stapled peptide approach offers a new angle on this target by disrupting protein-protein interactions that small molecules struggle to block.^[6]

Transcription factors. Stapled peptides have been developed against transcription factor interactions (WASF3-CYFIP1, VGLL4-TEAD4) that regulate cancer cell invasion and proliferation. These targets are effectively inaccessible to conventional drug modalities because transcription factors lack the enzymatic pockets that small molecules typically bind. The alpha-helical interfaces that mediate transcription factor dimerization and co-activator recruitment are, however, well suited to stapled peptide mimicry.

Viral targets. Beyond cancer, stapled peptides have been applied to viral entry mechanisms. A 2025 study by Wang et al. developed a short double-stapled peptide mimicking the HR2 domain that potently inhibited human betacoronaviruses, demonstrating the versatility of the platform beyond oncology.

For how stapled peptides compare to macrocyclic peptides and cell-penetrating peptide delivery strategies, see the respective articles.

Limitations and Open Questions

Manufacturing complexity. Stapled peptides require non-natural amino acids and ring-closing metathesis, making synthesis more expensive and technically demanding than standard peptide production. Scale-up for clinical supply remains a challenge, contributing to the slow clinical development timeline.

Permeability prediction. Despite the Chandramohan design rules, predicting which stapled peptide will achieve adequate cell permeability remains imperfect. The relationship between lipophilicity and membrane crossing is correlative but not fully mechanistic. Some peptides with favorable physical properties still fail to enter cells, and the reasons are not always clear.

Limited clinical data. ALRN-6924 is the only stapled peptide to have completed a Phase 1 cancer trial. The 59% disease control rate is encouraging for a first-in-class agent, but the trial was small (71 patients), uncontrolled, and included a heterogeneous mix of tumor types. Whether stapled peptides can produce durable responses in larger, randomized trials remains to be seen.

Selectivity in vivo. The same cell permeability that lets stapled peptides enter tumor cells also lets them enter healthy cells. For p53-reactivating peptides, this means normal cells experience p53 activation too. In the ALRN-6924 trial, hematologic toxicity (neutropenia, thrombocytopenia) was observed, consistent with p53-mediated effects on rapidly dividing bone marrow cells.

Competition from small molecules. Several small-molecule MDM2 inhibitors (navtemadlin, milademetan, brigimadlin) are in clinical development, some in Phase 2 and Phase 3 trials. If small molecules can disrupt the p53-MDM2 interaction adequately, the added complexity of stapled peptide manufacturing may not be justified for this particular target. The dual MDM2/MDMX inhibition that stapled peptides offer is a potential differentiator, since most small molecules only target MDM2 and leave MDMX-mediated p53 suppression intact. Cancer cells can upregulate MDMX to escape MDM2-only inhibitors, making dual inhibition a clinically meaningful advantage if it translates to better patient outcomes.

An innovative review by Dongrui et al. (2024) placed stapled peptides alongside foldamers and other constrained peptide architectures, noting that the field is rapidly evolving toward hybrid designs that combine stapling with other stabilization strategies.^[7] For the broader context of pro-apoptotic peptides in cancer therapy, stapled peptides represent one of several approaches to forcing tumor cells to die.

The Bottom Line

Stapled peptides use hydrocarbon bridges to lock alpha-helices into their bioactive shape, producing molecules that cross cell membranes, resist degradation, and disrupt intracellular protein-protein interactions in cancer cells. The first in vivo anticancer demonstration came in 2004 with a BH3-domain stapled peptide. The most advanced clinical candidate, ALRN-6924, achieved a 59% disease control rate in a Phase 1 trial targeting wild-type p53 tumors. Design rules from 350+ synthetic variants show that lipophilicity, charge, and staple geometry all determine clinical viability. The technology faces challenges in manufacturing cost, permeability prediction, and competition from small-molecule alternatives.

Frequently Asked Questions

What is a stapled peptide?

A stapled peptide is a short protein fragment with a chemical cross-link (typically a hydrocarbon bridge) inserted across one turn of its alpha-helix. This "staple" locks the peptide into its bioactive 3D shape, protects it from enzymatic degradation, and increases its ability to cross cell membranes. The technology was developed to enable peptides to disrupt intracellular protein-protein interactions that drive cancer.

How do stapled peptides work against cancer?

Stapled peptides mimic the alpha-helical surface of one protein and bind to the groove of another, blocking their interaction. The most studied application reactivates the p53 tumor suppressor by disrupting its inhibition by MDM2 and MDMX proteins. ATSP-7041 binds both MDM2 and MDMX with nanomolar affinity, freeing p53 to trigger cell cycle arrest and apoptosis in tumor cells (Chang et al., 2013).

Has any stapled peptide been tested in humans for cancer?

Yes. ALRN-6924 (sulanemadlin), a stapled peptide dual inhibitor of MDM2 and MDMX, completed a Phase 1 trial in 71 patients with solid tumors and lymphomas bearing wild-type TP53. The disease control rate was 59% among 41 evaluable patients, with 2 complete responses, 2 partial responses, and 20 patients with stable disease.

What makes stapled peptides different from regular peptides?

Regular peptides are flexible, rapidly degraded by enzymes, and cannot cross cell membranes. Stapled peptides have a hydrocarbon bridge that increases alpha-helical content from 20-30% to 80-90%, extends half-life from minutes to hours, and provides enough lipophilicity for membrane crossing. This combination allows them to reach intracellular targets that regular peptides cannot access.

What cancers can stapled peptides target?

Stapled peptides can target any cancer driven by an alpha-helical protein-protein interaction. Current programs include p53-MDM2/MDMX (active in ~50% of cancers with wild-type p53), BCL-2 family proteins (chemotherapy resistance), RAS-effector interactions (~25% of cancers), and transcription factor complexes. The platform is adaptable to new targets by redesigning the peptide sequence.

Are stapled peptides better than small molecule drugs for cancer?

Stapled peptides access a different target space. Small molecules work best on enzymes with deep binding pockets. Stapled peptides work best on flat protein-protein interaction surfaces that small molecules cannot grip. For the p53-MDM2 target, stapled peptides offer dual MDM2/MDMX inhibition while most small molecules target only MDM2. The tradeoff is higher manufacturing complexity and cost.