How many cyclic peptide drugs are approved?

As of 2024, 66 cyclic peptide drugs have been approved globally, with 39 gaining approval since 2000. Approximately one new cyclic peptide drug enters the market each year. In 2023 alone, three of six approved peptide drugs were cyclic: rezafungin (antifungal), motixafortide (CXCR4 antagonist), and zilucoplan (complement C5 inhibitor).[1]

Cyclic Peptides

Macrocyclic Peptides: Between Small Molecules and Biologics

12 min read|March 22, 2026

Cyclic Peptides

66 approved drugs

As of 2024, 66 cyclic peptide drugs have been approved globally, with 39 gaining approval since 2000 and roughly one new approval per year.

Zorzi et al., Current Opinion in Chemical Biology, 2017

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Small molecule drugs can enter cells and be swallowed as pills, but they struggle to target the large, flat protein surfaces involved in most diseases. Antibody biologics can bind these surfaces with exquisite precision, but they cannot cross cell membranes and must be injected. Macrocyclic peptides sit in the gap between these two worlds. By constraining a peptide chain into a ring, medicinal chemists create molecules large enough to grip protein surfaces like an antibody, yet small enough to cross membranes and, in some cases, survive oral delivery like a small molecule drug. For broader context on how cyclic structures enhance peptide stability, see Cyclotides: The Ultra-Stable Plant Peptides with Drug Potential.

Key Takeaways

Over 66 cyclic peptide drugs have been approved globally, with approximately one new approval per year and 39 entering the market since 2000 (Zorzi et al., 2017)
Cyclization improves peptide drug properties in three ways: it increases protease resistance, enhances membrane permeability through intramolecular hydrogen bonding, and locks the molecule into a bioactive conformation (Wang & Craik, 2016)
A combinatorial library approach identified multiple cell-permeable cyclic peptide scaffolds whose permeability could be maintained while varying functional groups for different targets (Hewitt et al., 2015)
The oral RAS inhibitor LUNA18 achieved 21-47% bioavailability without special formulation, and Merck's MK-0616 (oral PCSK9 inhibitor) entered Phase III trials in 2023
Macrocyclic peptides typically weigh 500-2,000 daltons, violating Lipinski's Rule of Five but achieving drug-like properties through conformational flexibility and intramolecular bonding
Cyclosporine A remains the paradigm: a cyclic peptide that achieves oral bioavailability through N-methylation of backbone amides and conformational switching between polar and nonpolar states

Why Rings Change Everything

Linear peptides face three problems as drugs: they are rapidly degraded by proteases, they cannot cross cell membranes, and they adopt too many conformations in solution to bind targets efficiently. Cyclization addresses all three.

Wang and Craik (2016) reviewed the structural features that make cyclic peptides orally bioavailable, identifying key principles from approved drugs like cyclosporine A.^[2]

Protease resistance. Exopeptidases require a free N-terminus or C-terminus to initiate degradation. Cyclization eliminates both termini, making the peptide invisible to these enzymes. Endoproteases also struggle because the constrained ring limits access to internal cleavage sites.^[2]

Membrane permeability. The ring constraint reduces the conformational flexibility of the peptide backbone. In an aqueous environment, polar amide bonds are exposed and solvated. At a lipid membrane, cyclic peptides can undergo conformational switching: intramolecular hydrogen bonds form between backbone amides, shielding them from the hydrophobic membrane interior and allowing passive diffusion across the bilayer. Cyclosporine A exemplifies this: its N-methylated backbone amides reduce hydrogen bond donors, and the ring topology allows it to adopt a compact, lipophilic conformation during membrane transit.^[2]

Conformational preorganization. By locking the peptide into a defined shape, cyclization reduces the entropic cost of binding to the target. A linear peptide must pay an energetic penalty to adopt the binding conformation from among many possible states. A macrocyclic peptide arrives at the target already shaped for binding, improving affinity and selectivity.

For a deeper look at how cyclic structures compare to linear peptides and how disulfide bridges contribute to stability, see our dedicated articles.

The Approved Drug Landscape

Zorzi et al. (2017) catalogued over 40 clinically approved cyclic peptide drugs, a number that has since grown to 66 by 2024.^[1] Roughly one new cyclic peptide drug enters the market each year. The vast majority are derived from natural products:

Antimicrobial cyclic peptides. Vancomycin (glycopeptide antibiotic), daptomycin (lipopeptide), polymyxins (colistin), and caspofungin (echinocandin antifungal) are all macrocyclic or contain macrocyclic elements. Their ring structures are critical for binding to bacterial cell wall or membrane targets.

Immunosuppressants. Cyclosporine A, discovered in a soil fungus in 1971, remains the most commercially successful cyclic peptide drug. Its 11-residue macrocycle with seven N-methylated amides creates the membrane permeability and oral bioavailability that made it a transplant medicine breakthrough.

Hormones and analogs. Oxytocin, vasopressin, and their synthetic analogs (desmopressin, octreotide) contain disulfide-mediated cyclization that stabilizes their bioactive conformations.

Recent approvals (2023). Three of the six peptide drugs approved in 2023 were cyclic: rezafungin (antifungal), motixafortide (CXCR4 antagonist for stem cell mobilization), and zilucoplan (complement C5 inhibitor for myasthenia gravis).^[1]

The shift from natural product-derived to rationally designed cyclic peptides is accelerating. Zorzi et al. noted that new techniques based on phage display, mRNA display, and chemical synthesis now enable de novo development of cyclic peptide ligands for previously untargetable proteins.^[1]

Breaking the Cell Permeability Barrier

The central challenge for macrocyclic peptide drugs is cell permeability. Most protein-protein interactions that drive disease occur inside cells, but cyclic peptides larger than about 500 daltons typically cannot cross the plasma membrane by passive diffusion.

Hewitt et al. (2015) attacked this problem systematically. They synthesized a combinatorial library of cyclic peptides inspired by the structural features of cell-permeable natural products, then identified scaffolds with good to excellent passive membrane permeability.^[3]

The key finding: multiple geometrically diverse cyclic peptide scaffolds achieved passive cell permeability, and single-point side-chain modifications could introduce new bioactive functional groups without losing that permeability.^[3] This demonstrated that the chemical space of cell-permeable cyclic peptides extends far beyond known natural products like cyclosporine.

Computational and experimental analysis revealed specific structure-permeability relationships among cyclic peptide diastereomers (identical amino acid sequences in different stereochemical configurations). Small changes in backbone stereochemistry (replacing L-amino acids with D-amino acids at specific positions) dramatically altered membrane permeability without changing the amino acid composition.^[3] For more on D-amino acid strategies, see D-Amino Acid Peptides: Mirror-Image Molecules That Resist Degradation.

Oral Macrocyclic Peptides: Two Clinical Frontrunners

The ultimate goal is a macrocyclic peptide that can be swallowed as a pill and reach its intracellular target. Two programs are approaching this benchmark:

MK-0616 (Merck). This macrocyclic peptide inhibits PCSK9, a protein that raises LDL cholesterol by promoting degradation of LDL receptors. Injectable PCSK9 antibodies (evolocumab, alirocumab) are effective but inconvenient. MK-0616 entered Phase III trials in August 2023 with the potential to become the first oral PCSK9 inhibitor. Key modifications included replacing oxidation-sensitive thiols, switching from alkene to amide cross-linkers for improved solubility, and optimizing the macrocycle size for oral absorption.

LUNA18 (Chugai). This cyclic peptide directly binds and inhibits RAS proteins, which are mutated in approximately 25% of all cancers and have been considered "undruggable" for decades. LUNA18 inhibits protein-protein interaction between inactive RAS and its activating partners (GEFs). In preclinical studies, it achieved 21-47% oral bioavailability without any special formulation, an achievement that would have seemed impossible for a peptide targeting an intracellular protein just a decade ago.

Both programs represent the convergence of macrocyclic chemistry with previously intractable targets. The pipeline also includes cyclic peptide programs for oral delivery of insulin and other hormones.

Beyond Rule of Five: Why Standard Drug Rules Don't Apply

Lipinski's Rule of Five, developed in 1997, predicts that oral drugs will have molecular weights under 500 Da, fewer than 5 hydrogen bond donors, fewer than 10 hydrogen bond acceptors, and calculated logP under 5. Macrocyclic peptides violate most of these criteria. They typically weigh 500-2,000 Da, have numerous hydrogen bond donors and acceptors, and are highly polar by small molecule standards.

They get away with it through mechanisms unavailable to small molecules:

Conformational chameleoning. Macrocyclic peptides can adopt different shapes in different environments. In water, polar groups are exposed for solvation. At a membrane, intramolecular hydrogen bonds form to hide polar groups, creating a transiently lipophilic conformation that crosses the bilayer. Small molecules have fixed conformations; macrocycles are shape-shifters.

Backbone modifications. N-methylation, D-amino acid substitution, and non-natural amino acid incorporation reduce the number of exposed hydrogen bond donors while maintaining the ring size needed for target engagement. Each modification pushes the peptide further from "peptide-like" and closer to "small molecule-like" in terms of oral drug properties.

Ring size optimization. Smaller rings (5-8 residues) are more conformationally constrained and more likely to achieve oral bioavailability. Larger rings (10-20 residues) have more surface area for target binding but are harder to make orally available. The optimal size depends on the specific target.

What Remains Challenging

Synthesis cost. Macrocyclic peptides require solid-phase peptide synthesis followed by cyclization chemistry. Both steps are more expensive than small molecule synthesis and harder to scale. Manufacturing costs limit the commercial viability of macrocyclic peptides for chronic conditions requiring daily dosing.

Design complexity. Predicting which cyclic peptide sequences will be cell-permeable remains difficult despite advances in computational modeling. The structure-permeability relationships are nonlinear and highly sensitive to small changes in stereochemistry, ring size, and backbone modification.^[3]

Limited oral examples. Despite the excitement around MK-0616 and LUNA18, the vast majority of approved cyclic peptide drugs are administered by injection. Cyclosporine remains almost unique in achieving robust oral bioavailability. Generalizing its success to a broader range of macrocyclic peptide targets has proven slow.

Target limitations. Macrocyclic peptides excel at disrupting protein-protein interactions, which are too large and flat for small molecules but accessible to the extended surfaces of a macrocycle. They are less suited for traditional enzyme active sites, where small molecules already perform well, or for extracellular targets, where antibodies are superior.

AI-driven design is early-stage. New generative AI tools (like 1910's PEGASUS) can design macrocyclic peptides computationally, but the field is in its infancy. Most AI-designed macrocyclic peptides have not advanced past in vitro testing. The gap between computational predictions of permeability and actual in vivo bioavailability remains substantial.

The Bottom Line

Macrocyclic peptides occupy a unique therapeutic space between small molecules and antibodies. By constraining peptide chains into rings, chemists achieve protease resistance, membrane permeability, and conformational preorganization that linear peptides lack. Over 66 cyclic peptide drugs are approved, with the field shifting from natural product-derived to rationally designed molecules. Oral macrocyclic peptides targeting previously "undruggable" intracellular proteins (RAS, PCSK9) are advancing through clinical trials. The main barriers remain synthesis cost, difficulty predicting permeability from structure, and the limited number of oral examples beyond cyclosporine A.

Frequently Asked Questions

Macrocyclic peptides are peptide chains constrained into a ring structure, typically containing 5-20 amino acid residues with molecular weights of 500-2,000 daltons. The ring constraint improves protease resistance (no free termini for enzymes to grip), membrane permeability (intramolecular hydrogen bonding during membrane transit), and target binding (preorganized bioactive conformation). Over 66 macrocyclic peptide drugs are approved globally.^[1]

Some macrocyclic peptides achieve oral bioavailability, though most approved cyclic peptide drugs are still injected. Cyclosporine A is the classic example of an orally bioavailable macrocyclic peptide. Newer examples include LUNA18 (21-47% oral bioavailability targeting RAS) and MK-0616 (oral PCSK9 inhibitor in Phase III trials). Key design strategies include N-methylation, D-amino acid substitution, and ring size optimization.^[2]

Macrocyclic peptides cross membranes through "conformational chameleoning." In water, polar backbone amides are exposed. At a membrane, intramolecular hydrogen bonds form between these amides, hiding them from the hydrophobic lipid interior and creating a transiently lipophilic shape that diffuses across the bilayer. Specific stereochemical configurations and backbone modifications (N-methylation) enhance this ability.^[3]

Macrocyclic peptides can cross cell membranes to reach intracellular targets that antibodies cannot access. They are also smaller (500-2,000 Da vs. 150,000 Da for antibodies), cheaper to manufacture, and potentially orally bioavailable. However, antibodies have longer half-lives, greater binding affinity for extracellular targets, and a more established regulatory pathway. The choice depends on the target location and required drug properties.

Cyclosporine A, discovered in 1971 from a soil fungus, is the most commercially successful macrocyclic peptide drug. This 11-residue cyclic peptide immunosuppressant revolutionized organ transplantation. Its oral bioavailability comes from seven N-methylated backbone amides that reduce hydrogen bond donors and enable conformational switching between polar and nonpolar states during intestinal absorption.^[2]