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INTRODUCTION

Cancer remains a leading cause of morbidity and mortality worldwide, and the need for therapies capable of addressing complex molecular networks is more urgent than ever. Classical small-molecule drugs often fail to modulate protein-protein interactions (PPIs). In contrast, macrocyclic peptides offer a unique combination of high specificity, structural rigidity, and the ability to penetrate cells. A macrocyclic peptide is a short chain of amino acids (a peptide) that has been chemically “closed” into a ring structure, usually by linking the ends of the chain or connecting side chains of the amino acids. This ring (macrocycle) makes the peptide more rigid and stable compared to a regular, flexible linear peptide. Their size and surface characteristics allow them to engage large or flat interfaces with nanomolar to sub-nanomolar affinity, while chemical modifications enhance stability and bioavailability. Over the last three years, research has demonstrated the capacity of macrocyclic peptides to modulate both intracellular oncogenic pathways (e.g., MDM2–p53, BCL-2 family, FAK scaffolds) and extracellular immune checkpoints (e.g., PD-L1), positioning them as versatile therapeutic platforms.  Despite these advances, challenges remain: achieving intracellular delivery, controlling pharmacokinetics, minimizing off-target toxicity, and ensuring manufacturability at scale. This review describes recent advances, designed to address these challenges through rational chemical design, mechanistic studies, and innovative delivery strategies.

TARGET MOLECULES AND MECHANISMS

Macrocyclic peptide drugs represent an emerging and highly versatile class of therapeutics in cancer therapy, leveraging their structural rigidity and specificity to target complex oncogenic pathways that are often inaccessible to traditional small molecules or antibodies. These compounds demonstrate a wide range of mechanisms, including immune checkpoint inhibition, disruption of protein–protein interactions, targeted cytotoxic delivery, inhibition of cell cycle regulators, and blockade of angiogenesis or hypoxia-driven signaling. Many of these macrocyclic peptides have progressed to clinical trials, with at least ten distinct candidates currently being evaluated in phases ranging from preclinical studies to Phase 2/3 trials. Early results suggest that macrocyclic peptides can achieve high specificity and potency, potentially reducing off-target effects while effectively modulating cancer-relevant pathways. While still an evolving field, the diversity of molecular targets and mechanisms, coupled with favorable pharmacological properties, positions macrocyclic peptides as promising agents in the development of next-generation cancer therapeutics.

Intracellular Oncogenic Pathways

Dysregulated intracellular signaling is a central driver of cancer progression, yet many of the critical protein-protein interactions that sustain survival, proliferation, and metastasis remain inaccessible to conventional small molecules and biologics. Over the past decade, macrocyclic peptides have emerged as a powerful therapeutic class uniquely suited to address this challenge. Three examples of macrocyclic peptides that regulate intracellular pathways are discussed below.  By selectively intervening in these fundamental oncogenic pathways, macrocyclic peptides not only expand the available therapeutic landscape but also offer new opportunities to achieve durable and selective tumor control.

MDM2/X–p53 Modulation

The interaction between MDM2/X and p53 is a canonical oncogenic axis, where MDM2/X-mediated degradation of p53 suppresses apoptosis and tumor suppression. Stapled α-helical peptides have emerged as potent modulators of this pathway. Optimal designs achieved low-nanomolar binding to MDM2 and robust stabilization of p53 in TP53 wild-type cancer cell lines. Importantly, in vivo xenograft models demonstrated significant tumor growth inhibition with minimal systemic toxicity. The drug ALRN-6924, exemplifies clinical translation of stapled peptides. Iterative medicinal chemistry optimization, including staple placement, side-chain hydrophobicity tuning, and sequence refinement, enabled high-affinity MDM2/MDMX binding, favorable PK/PD profiles, and robust p53 activation in tumor-bearing animals. These studies illustrate that careful molecular design and biophysical optimization can yield macrocyclic peptides capable of both high intracellular engagement and therapeutic effect.

BCL-2 Family Modulation and Molecular Glue Approaches

The BCL-2 family of proteins regulates intrinsic apoptosis, and MCL1, an anti-apoptotic member, is frequently upregulated in hematologic malignancies. Traditional inhibitors struggle to achieve selective intracellular targeting. Recent work identified a de novo macrocyclic peptide that acts as a molecular glue, inducing MCL1 homodimerization, thereby preventing interactions with pro-apoptotic BH3-only proteins. Structural studies revealed that the macrocyclic peptide bridges two MCL1 molecules, triggering intrinsic apoptosis in vitro and in vivo, demonstrating that rationally designed molecular glue macrocycles can be potent anti-cancer agents.

Targeting FAK Scaffolds and Cell Adhesion Pathways

Intracellular scaffolding proteins such as FAK mediate integrin signaling, survival, and migration in solid tumors. Structure-guided stapled peptides mimicking the paxillin FAK-binding motif effectively displaced FAK from focal adhesions, reducing downstream ERK and Src phosphorylation and inducing apoptosis under anchorage-independent conditions. These results highlight that macrocyclic peptides can function as precise inhibitors of scaffolding interactions, which are traditionally challenging to modulate with small molecules.

Modulating Immune Checkpoints

PD-L1 / PD-1

PD-1 is an inhibitory receptor on T cells, and PD-L1 is its ligand often expressed on tumor cells. Their interaction suppresses immune responses, allowing cancer cells to evade detection. Blocking PD-1/PD-L1 with antibodies releases this “immune brake,” restoring T cell activity and enabling effective anti-tumor immunity. Macrocyclic peptides can recapitulate antibody-like epitope coverage to inhibit immune checkpoints. The macrocyclic binder pAC65 blocks PD-L1 interactions with both PD-1 and CD80, restoring T-cell activation in vitro. Mechanistic studies show enhanced cytokine release and proliferation of cytotoxic T lymphocytes, suggesting that macrocycles can serve as smaller, more penetrant alternatives to monoclonal antibodies.

 

 Examples of Macrocyclic Peptides in Development

Drug NameDeveloperClinical StageMolecular MechanismTumor Target / Indication
BT8009Bicycle TherapeuticsPhase 2/3Bicycle toxin conjugate targeting nectin-4, delivering cytotoxic payloads to tumor cellsNectin-4–expressing solid tumors, including urothelial carcinoma and breast cancer
CID-078Circle PharmaPhase 1Oral macrocycle inhibiting cyclin A/B–E2F and cyclin B–Myt1 interactions, disrupting cell cycle progressionBreast cancer (preclinical studies), potential broader solid tumor applicability
PD-29875PeptiDreamPreclinicalMacrocyclic peptide-radioisotope conjugate targeting Claudin 18.2Gastric, pancreatic, and other Claudin 18.2–positive solid tumors
LUNA18Luna InnovationsPreclinicalOral macrocyclic peptide disrupting KRAS–SOS1 interaction, inhibiting oncogenic KRAS signalingKRAS-driven solid tumors such as pancreatic and colorectal cancers
pAC65Bristol Myers SquibbPreclinicalMacrocyclic peptide immune checkpoint inhibitor targeting PD-L1/CD80 and PD-L1/PD-1 interactionsMultiple solid tumors expressing PD-L1
MK-0616MerckPhase 1Oral macrocyclic peptide inhibiting PCSK9, modulating cholesterol metabolism (may affect tumor metabolic pathways)Indirectly cancer-related; primarily developed for lipid metabolism disorders, potential in metabolic oncology

 

CHEMICAL STRATEGIES FOR MACROCYCLE OPTIMIZATION

Chemical innovation underpins the recent advances in macrocyclic peptide therapeutics, enabling both improved pharmacokinetics and enhanced intracellular targeting relevant to cancer therapy. Several approaches are being taken to overcome limitations of macrocyclic peptides as therapeutic drugs.

Hydrocarbon stapling introduces covalent crosslinks across helical segments, locking the peptide into its biologically active α-helical conformation. This structural constraint increases binding affinity to intracellular oncogenic targets, such as transcription factors or protein–protein interaction domains, and protects against proteolytic degradation, potentially enabling direct modulation of cancer-driving pathways.

Backbone N-methylation reduces hydrogen-bond donor availability, increasing membrane permeability and metabolic stability, which enhances intracellular accumulation in tumor cells and improves efficacy against intracellular targets like kinases or epigenetic regulators. Guanidinium-stapling leverages cationic guanidinium groups to facilitate active transport across the cell membrane, increasing the concentration of macrocyclic inhibitors within malignant cells and potentially overcoming drug resistance mechanisms.

The FTDR (fluoro-thiol displacement reaction) approach enables chemoselective cyclization on unprotected peptides, producing protease-resistant macrocycles capable of accessing intracellular oncogenic proteins, including transcription factors and signal transduction mediators.

These platforms facilitate the selection of peptides that not only bind tightly to tumor-associated proteins but also exhibit favorable pharmacokinetics and intracellular stability, thereby improving the likelihood of therapeutic efficacy in cancer models.

 

 Selected Macrocyclic Modifications and Functional Impact

ModificationStructural ImpactFunctional Outcome
Hydrocarbon staplesHelix stabilizationProtease resistance, nanomolar binding
Cyclotide scaffoldsRigid cystine knotProtease stability, endosomal escape
FTDRMild cyclizationEnhanced stability, uptake
Guanidinium staplesCationic, stabilized helixCPP-free cell penetration
DELIVERY STRATEGIES  Several innovative strategies have been developed to enhance the in vivo delivery of macrocyclic peptides, addressing challenges such as cellular uptake, stability, and targeted release. These approaches leverage advances in nanotechnology, peptide engineering, and chemical conjugation to improve therapeutic outcomes. Below are five mechanistically distinct strategies: 1. PEGylated Polymersome Nanoparticles for Targeted Delivery Polymersomes, amphiphilic block copolymer vesicles, have been utilized to encapsulate macrocyclic peptides, enhancing their stability and bioavailability. For instance, a study demonstrated the in vivo delivery of a stapled p53 peptide using antigen-targeted polymersomes, leading to significant tumor regression in lymphoma models. The PEGylation of polymersomes improves their plasma half-life and reduces renal clearance, facilitating enhanced tumor accumulation. Upon reaching the tumor microenvironment, these polymersomes can release the encapsulated peptide, which then reactivates p53 to induce apoptosis in cancer cells 2. Cell-Penetrating Peptide (CPP) Conjugation for Cytosolic Delivery CPPs, particularly those rich in arginine, have been conjugated to macrocyclic peptides to facilitate cellular entry. These conjugates exploit the inherent ability of CPPs to traverse cellular membranes, delivering the therapeutic peptide directly into the cytosol. Recent advancements have focused on optimizing CPP sequences and conjugation strategies to enhance the efficiency and specificity of delivery. For example, cyclic CPPs have shown improved stability and reduced immunogenicity compared to their linear counterparts, making them more suitable for therapeutic applications. 3. Tumor-Homing Ligand-Mediated Targeting Tumor-homing peptides (THPs), such as RGD and NGR, have been conjugated to macrocyclic peptides to direct them specifically to tumor sites. These ligands bind to overexpressed receptors on tumor cells, facilitating targeted delivery and uptake. By conjugating THPs to macrocyclic peptides, researchers have achieved enhanced tumor localization and therapeutic efficacy. This strategy minimizes off-target effects and reduces systemic toxicity, improving the overall therapeutic index of the peptide. 4. pH-Responsive Release Systems Macrocyclic peptides have been incorporated into pH-responsive delivery systems, such as hydrogels or nanoparticles, that release their payloads in response to the acidic environment characteristic of tumors or inflamed tissues. This approach ensures that the therapeutic peptide is released precisely at the site of action, enhancing its efficacy while minimizing systemic exposure. For instance, pH-sensitive linkers have been used to conjugate macrocyclic peptides to carriers, allowing for controlled release upon encountering acidic conditions. 5. Endosomal Escape Mechanisms To achieve effective intracellular delivery, macrocyclic peptides must escape from endosomes after cellular uptake. Strategies have been developed to enhance endosomal escape, such as the use of endosomolytic peptides or fusogenic peptides that disrupt endosomal membranes. These peptides facilitate the release of the therapeutic cargo into the cytosol, improving the bioavailability and activity of the macrocyclic peptide. For example, cyclic CPPs with endosomolytic properties have been engineered to promote endosomal escape, thereby enhancing the therapeutic potential of the delivered peptide. In summary, these strategies, PEGylated polymersome delivery, CPP conjugation, tumor-homing ligand targeting, pH-responsive release, and endosomal escape mechanisms, represent significant advancements in the field of macrocyclic peptide drug delivery. By addressing key barriers to intracellular delivery, these approaches hold promise for the effective treatment of a variety of diseases, including cancer and genetic disorders.

CONCLUSION

Macrocyclic peptides have emerged as a versatile and potent therapeutic class for oncology. By integrating chemical design, structural optimization, and delivery innovations, researchers have demonstrated their capacity to modulate both intracellular and extracellular targets with high specificity and potency. Mechanistic studies elucidate how macrocycles induce apoptosis, inhibit scaffold-mediated signaling, and restore immune function. While challenges in delivery and systemic stability remain, recent advances in stapling chemistries, cyclotide scaffolds, encoded-library discovery, and nanocarrier-mediated delivery are rapidly expanding the therapeutic horizon. With ongoing innovations, macrocyclic peptides are poised to become a central modality in next-generation anticancer therapy.

Disclaimer: This blog post is intended solely for educational and scientific informational purposes. Any mention of therapeutic drug names, including FDA-approved medications, is for the purpose of accurate reporting and discussion of biomedical research and does not constitute medical advice, endorsement, or promotion. Readers should not interpret the content as a recommendation for any specific treatment. Always consult a qualified healthcare professional for medical advice or treatment decisions.

 

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1. Development of VNX-101, an Adeno-Associated Virus with Less Immunogenicity and Efficient Long-Term Expression of a CD19 T-Cell Engager. Molecular Therapy Methods & Clinical Development, published online July 24, 2025.

2. Development of a Pharmacokinetic (PK) Mouse Serum GLP ELISA for an Anti–CD19–AntiCD3 Diabody bioRxiv 2025.03.19.644217; doi: https://doi.org/10.1101/2025.03.19.644217.

3. Cell-Based Potency Assay for Anti-CD3-Anti-CD19 Diabody. bioRxiv 2025.04.15.648836v1 https://www.biorxiv.org/content/10.1101/2025.04.15.648836v1.

4.  American Society of Hematology (ASH) Annual Meeting 2024. Abstract link: Using Gene Therapy to Solve Challenges with CAR-T Cell Immunotherapy: Lead Selection and Preclinical Development of an Adeno-Associated Virus with Reduced Immunogenicity  Exhibiting Efficient and Long-Term Expression of an Anti-CD19 T-Cell Engager.

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