Among the six peptide drugs approved in 2023, three are macrocyclic peptides (Rezafungin, Motixafortide, and Zilucoplan). Macrocyclic peptides have emerged as a significant modality in peptide drug development, indicating a promising future trend, particularly for oral peptide drugs.

Figure 1. FDA Approved macrocyclic peptides in 2023

Small-molecule drugs are convenient but struggle to affect large interface ligand-receptor interactions. In contrast, large-molecule drugs are effective but expensive and have delivery limitations. Peptides, as an intermediate modality between small molecules and proteins, combine the advantages of both. Macrocyclic peptides, with their balanced properties and unique ring structures, have become an important drug modality capable of effectively interfering with protein-protein interactions.

Figure 2. A peptide (2000 Da) compared in size to a small molecule (180 Da) and an antibody (150,000 Da). Source: Merck official website

The advantages of macrocyclic peptide drugs can be highlighted in the following aspects:

• ▶ Target Specificity: Macrocyclic peptides precisely recognize and bind to specific biomolecules like proteins, enzymes, and receptors. This allows them to effectively target specific biological processes without affecting other molecules.
• ▶ High Potency: Macrocyclic peptides are highly active and can produce strong biological effects at low concentrations, reducing the required dosage and potential side effects.
• ▶ Biostability: Macrocyclic peptides resist degradation by stomach acid and proteases, increasing their half-life and prolonging their efficacy.
• ▶ Diversity: The structure of macrocyclic peptides can be tailored through synthesis to treat various diseases, making them versatile in addressing different medical needs.
• ▶ Low Immunogenicity: Macrocyclic peptides generally have lower immunogenicity compared to other drugs, resulting in fewer adverse immune reactions.
• ▶ Fewer Side Effects: Due to their high target specificity, macrocyclic peptides typically have fewer adverse effects on normal cells, reducing overall treatment side effects.
• ▶​ Improved Solubility and Delivery: Advances have improved the solubility and delivery of macrocyclic peptides, making them easier to administer via oral, injectable, and other routes.

Macrocyclic Peptide Pipelines

MK-0616

Merck's pipeline includes a promising oral macrocyclic peptide, MK-0616, which targets PCSK9 (Proprotein Convertase Subtilisin/Kexin type 9), a protein that plays a key role in cholesterol metabolism. Researchers have developed PCSK9 inhibitors to treat high cholesterol, especially when other treatments fail.

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Figure 4. Structure of MK-0616

PCSK9 plays a key role in cholesterol homeostasis by regulating levels of LDL receptors, which are responsible for the uptake of cholesterol into cells. Inhibition of PCSK9 prevents the interaction of PCSK9 with LDL receptors. This results in greater numbers of LDL receptors available on the cell surface to remove LDL cholesterol from the blood.

The FDA approved two PCSK9 inhibitors in 2015: alirocumab (Praluent) and evolocumab (Repatha). In 2022, the  FDA has approved inclisiran, a small interfering mRNA, that inhibits intracellular PCSK9 synthesis. 

MK-0616 is currently in Phase III for Atherosclerosis. The oral delivery method of MK-0616 is one reason for the high expectations. To improve its oral bioavailability, Merck uses sodium caprate as a permeation enhancer, which temporarily opens tight junctions between intestinal epithelial cells, helping the drug pass into the bloodstream. Unlike small molecules that are absorbed across cells after oral intake, MK-0616's large, polar structure makes this difficult. Permeation enhancers are particularly suitable for extracellular targets like PCSK9, reducing off-target effects and protecting intracellular targets from interference.

Zosurabalpin  (RG6006)

Zosurabalpin is a new cyclic peptide antibiotic candidate targeting drug-resistant Acinetobacter baumannii. Its development marks a significant breakthrough in the field of antibiotics, offering new possibilities for treating this difficult-to-tackle bacterium.

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Figure 4. Structure of Zosurabalpin

Zosurabalpin stands out due to its unique mechanism of action, which differs from widely used antibiotics. It was meticulously selected from a library of 45,000 cyclic peptides using advanced solid-phase parallel synthesis technology, specifically designed against a range of human pathogens. Notably, Zosurabalpin shows strong inhibitory effects against CRAB (Carbapenem-resistant Acinetobacter baumannii). CRAB poses a major challenge in the medical field due to its multiple drug resistances, threatening public health, and has been classified as a high-priority pathogen by the World Health Organization.

Zosurabalpin works by blocking the transport of lipopolysaccharides (LPS), a critical component of the bacterial outer membrane. LPS helps protect Gram-negative bacteria by preventing many antibiotics from entering. Zosurabalpin inhibits the transport proteins between the bacterial inner and outer membranes, preventing normal LPS flow and causing it to accumulate to toxic levels inside the bacteria, thereby killing them.

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Figure 5. Zosurabalpin mechanism of action

The emergence of new classes of antibiotics is rare. It has been over half a century since a new antibiotic effective against Acinetobacter baumannii was last approved. Gram-negative bacteria are particularly hard to kill due to their unique cell structure, especially their LPS-protected outer membrane, which traditional antibiotics struggle to penetrate. A lack of incentives for pharmaceutical companies to develop new antibiotics also contributes to the growing problem of drug-resistant bacteria.

Fortunately, Zosurabalpin has shown great potential in multiple experiments. It performed excellently in vitro and successfully combated highly resistant CRAB in various mouse infection models, including sepsis. Zosurabalpin has completed two Phase I clinical trials, showing that single intravenous doses ranging from 10mg to 2000mg are safe and well-tolerated. This success has attracted the attention of major pharmaceutical company Roche, indicating that Zosurabalpin could become a crucial weapon in the fight against drug-resistant bacterial infections.

BT8009

Bicycle Toxin Conjugates (BTCs) are an innovative class of cyclic peptide molecules designed to deliver toxins directly to tumor cells with high specificity. BT8009, developed by Bicycle Therapeutics, is a BTC candidate targeting Nectin-4 and is currently in clinical trials.

Figure 6. Structure of BT8009

Nectin-4 is an adhesion molecule that is overexpressed in several cancers and is recognized as a promising target for cancer therapy. Using phage display technology, scientists selected highly specific bicyclic peptides that bind to Nectin-4 and linked them to a powerful cytotoxic agent, monomethyl auristatin E, to create BT8009. This drug has demonstrated strong anti-cancer effects in animal models, especially against tumors like urothelial carcinoma. Thanks to its small size and short half-life, BT8009 can quickly penetrate tumors and minimize damage to normal tissues, which could reduce treatment-related toxicity.

Current Phase 1/2 clinical trials (NCT04561362) show that BT8009 exhibits preliminary anti-tumor activity and acceptable safety in patients with advanced cancers, including urothelial carcinoma. The upcoming Phase 2/3 multicenter adaptive study will evaluate the safety and efficacy of BT8009 as a standalone treatment or in combination with pembrolizumab (pembro) for locally advanced or metastatic urothelial carcinoma.

LUNA18

RAS gene alterations, found in approximately 15% of all cancers and known oncogenes, are recognized as promising therapeutic targets. KRAS G12C inhibitors were the first FDA-approved RAS targeting agent, but their efficacy of monotherapy is limited. 

Figure 7. Breakthrough in class-drug cyclic peptide synthesis method and successful synthesis of LUNA 18. Source: https://synapse.patsnap.com/

LUNA18 is the cyclic peptide that binds to RAS mutants and wildtype, including KRAS, NRAS and HRAS, thereby inhibiting protein-protein interaction (PPI) between the inactive form of RAS and GEFs (guanine nucleotide exchange factors). It achieves a bioavailability of (21-47%) when taken orally without a special formulation. It is currently in Phase I of clinical trials, recruiting 195 patients and expected to complete the phase by March 31, 2025. The indications are locally advanced or metastatic solid tumors. 

Future Prospects of Macrocyclic Peptides

Macrocyclic peptides hold significant promise and offer several exciting possibilities:

• ▶​ New Drug Development: Macrocyclic peptides have vast potential for treating a range of diseases, such as cancer, autoimmune disorders, infections, and neurodegenerative conditions. Their precise targeting and stability make them strong candidates for developing new treatments.
• ▶​ Biotechnology Applications: These peptides can be designed for specific functions and have broad applications in biotechnology. They can be used for biomarker discovery, protein research, and drug delivery systems.
• ▶​ Cancer Treatment: Macrocyclic peptides show great potential in oncology by accurately targeting and attacking cancer cells while minimizing harm to healthy cells. Some macrocyclic peptides are currently being tested in clinical trials for cancer treatment.
• ▶​ Immunotherapy: Macrocyclic peptides can enhance immune responses in treatments for cancer and infections. They can be tailored to stimulate the immune system more effectively.
• ▶​ Drug Delivery Systems: They can improve drug delivery by targeting specific tissues or cells, which may increase drug effectiveness and reduce side effects. For example, the cyclic peptide BT8009 is an innovative approach in this area.
• ▶​ Personalized Medicine: The specificity and versatility of macrocyclic peptides make them suitable for personalized medicine. They can be customized to match individual genetic profiles and disease characteristics, leading to more tailored and effective treatments.

Conclusion

Despite the many potential advantages of macrocyclic peptide drugs, several challenges remain, including production costs, delivery technology, drug stability, and immunogenicity. However, with ongoing efforts from scientists and pharmaceutical companies, the research and application of cyclic peptide drugs will continue to advance the field of medicine, offering safer and more effective treatment options for patients.

Biopharma PEG, as a professional PEG supplier, offers a variety of PEG linkers to facilitate your drug development, such as peptides, antibody-drug conjugate (ADC), peptide drug conjugates (PDC), etc. All PEG linkers are of >95% purity and they are the basic building blocks for a successful drug.

References:
[1] Mudd, G. E. et al. Discovery of BT8009: A Nectin-4 Targeting Bicycle Toxin Conjugate for the Treatment of Cancer. J. Med. Chem. 2022, 65, 14337–14347.
[2] Tanada, M. et al. Development of Orally Bioavailable Peptides Targeting an Intracellular Protein: From a Hit to a Clinical KRAS Inhibitor. J. Am. Chem. Soc. 2023, 145, 16610–16620.
[3] Kingwell, K. Macrocycle drugs serve up new opportunities. Nature Review Drug Discovery. 11. 09. 2023
[4] Macrocyclic peptides: the next wave of drug discovery. Merck & Co. 14. 09. 2023.
[5] Loriot, Y. et al. A phase 2/3 study of Bicycle toxin conjugate BT8009 targeting nectin-4 in patients with locally advanced or metastatic urothelial cancer (la/mUC): Duravelo-2. Journal of Clinical Oncology. 2024. 42. https://doi.org/10.1200/JCO.2024.42.16_suppl.TPS4619
[6] Merck Initiates Phase 3 Clinical Program for Oral PCSK9 Inhibitor Candidate MK-0616. Merck Press Release. 25. 08. 2023.