Cyclic peptide macrocycles

macrocyclic peptides, cyclic peptides, constrained peptides, peptide macrocycles, "beyond rule of five" peptides

Metabolic & weight

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Note on this profile. This page is a class overview, not a profile of a single compound. It explains why macrocyclic and cyclic peptides have become the main direction of peptide drug development and links to the individual profiles in this library. Individual members range from decades-old approved medicines to investigational compounds; each has its own page. No dose, formulation or use information on this page applies to any specific product.

Summary

Class: Macrocyclic and cyclic peptides — a drug class, not a single compound. Includes head-to-tail cyclic peptides, disulphide-cyclised peptides, lactam- and thioether-bridged macrocycles, stapled peptides, bicyclic peptides, lipopeptides with a cyclic core and natural-product-derived depsipeptides
Mechanism: Ring closure locks the peptide's shape, hides its backbone from proteases and reduces its polar surface — which gives protein-like target binding with drug-like stability and, in some cases, oral absorption
Routes studied: Every route, depending on the member: oral, intravenous, subcutaneous, topical, inhaled, intrathecal
Half-life: Not applicable at class level — minutes to weeks depending on the member and its modifications
Storage: Not applicable at class level; cyclic peptides are generally more stable than linear ones, which is part of the point
Regulatory status: Several members are approved medicines in all major regions; others are investigational. See individual profiles and the table below

Doses are as reported in studies or by the community — never recommendations.

1What it is

This page is an overview of a chemical strategy rather than a compound. The strategy is simple to state: take a peptide and close it into a ring.

Peptides make excellent binders. They are big enough to cover the broad, shallow surfaces where proteins talk to each other — the interactions that small molecules usually cannot block. But a linear peptide is also a terrible drug. Proteases in the gut and blood chop it up within minutes. It is too polar to cross membranes. It flops between many conformations, so most of its binding energy is spent freezing into shape rather than gripping its target. These are the reasons that, for most of pharmaceutical history, peptides were either injected hormones or nothing.

Cyclisation addresses all three problems at once. Closing the chain into a macrocycle removes the free ends that exopeptidases attack and constrains the backbone so endopeptidases cannot fit it into their active sites. It locks the molecule into one or a few conformations, so it pays a much smaller entropic penalty on binding — which raises affinity and selectivity. And the constrained ring can fold its polar amide groups inward, hydrogen-bonding to itself and presenting a less polar face to the world, which is what allows some macrocycles to cross membranes at all.

Nature discovered this long before medicinal chemists did. Cyclosporine, daptomycin, vancomycin, the echinocandins, polymyxins and bacitracin are all natural cyclic or lipo-cyclic peptides, and several are orally absorbed in a way no linear peptide of their size is. The modern field is the deliberate, designed version of that: screening vast libraries of synthetic macrocycles against a chosen target, then optimising the ring.

2How it's thought to work

The class advantages, stated precisely:

Protease resistance. No free N- or C-terminus for aminopeptidases and carboxypeptidases. A constrained backbone is a poor substrate for endopeptidases. Non-natural amino acids and N-methylation add further resistance.

Conformational restriction. Fewer accessible conformations means less entropy lost on binding, hence higher affinity and better selectivity. This is the single biggest medicinal-chemistry advantage.

Reduced effective polarity. Transannular hydrogen bonds — amide groups bonding to each other across the ring — shield polarity and let the molecule pass through membranes. This "chameleonic" behaviour is the mechanistic basis for oral macrocyclic peptides.

Large interaction surface. A macrocycle presents hundreds of square ångströms of binding surface, enough to cover protein-protein interfaces that small molecules cannot address and that previously required antibodies.

Tunable half-life. Added lipid chains give albumin binding and long duration; PEGylation does the same; plain macrocycles can be cleared in minutes when that is wanted.

What it does not fix. Oral bioavailability remains low in absolute terms even when it is useful, so oral doses are large and formulation often needs a permeation enhancer. Synthesis is harder and more expensive than for small molecules. Macrocycles cannot easily reach buried active sites that suit small molecules, and they still cross the blood-brain barrier poorly without a dedicated delivery strategy (see ANG1005 (#249)).

Not one mechanism but a shape. Closing a peptide into a ring leaves no loose ends for gut enzymes to cut, and the ring is still large enough to cover a flat protein surface that a small molecule cannot grip — the argument for the whole class.Schematic — not to scale

3What the research shows

Graded A, decades of evidence

Natural-product cyclic peptides

Cyclosporine (a cyclic undecapeptide, orally absorbed) transformed transplant medicine from the 1980s and remains in use. Daptomycin (a cyclic lipopeptide) is a mainstay intravenous antibiotic for resistant Gram-positive infection. These established that cyclic peptides can be real, durable medicines.

Designed and semi-synthetic cyclic peptide drugs

Eptifibatide, a cyclic heptapeptide glycoprotein IIb/IIIa inhibitor based on a snake venom motif, has randomised outcome evidence in acute coronary syndromes and has been in use since 1998. Pegcetacoplan, a PEGylated cyclic peptide dimer inhibiting complement C3, is approved for paroxysmal nocturnal haemoglobinuria and for geographic atrophy. Zilucoplan, a macrocyclic peptide inhibitor of complement C5, is approved for generalised myasthenia gravis on the basis of the randomised RAISE trial. Motixafortide, a cyclic CXCR4 antagonist, is approved for stem cell mobilisation in multiple myeloma. Each of these has its own profile in this library with the trial detail.

Graded A on surrogate endpoints, outcome data pending

Oral macrocyclic peptides

This is the frontier. Enlicitide (#247) is an orally dosed macrocyclic peptide PCSK9 inhibitor that reduced LDL-C by roughly 60% versus placebo across Phase 3 trials — doing orally what previously required an injected antibody. Its cardiovascular outcomes trial has not yet reported. Other oral macrocycles are in development against interleukin-23 receptor, integrins, and intracellular protein-protein interactions. If this group delivers, it is the most consequential development in peptide chemistry in thirty years; if it does not, the class remains what it already is — a successful but mostly injectable modality.

Graded B/C

Bicyclic and stapled peptides

Bicyclic peptides built on a small scaffold have entered oncology trials as tumour-targeting agents and conjugates. Stapled α-helical peptides, which lock a helix with a hydrocarbon bridge, have reached early clinical trials against intracellular targets such as the p53–MDM2 interaction with mixed results. Promising, unproven.

Graded D/C, enabling rather than therapeutic

Peptide display technologies

mRNA display and related methods can generate and screen libraries of trillions of macrocycles containing non-natural amino acids. This is why the pipeline has grown so quickly. It is laboratory technology, not clinical evidence. Community use: essentially none documented at class level. Individual members are prescription medicines or investigational drugs. Where "cyclic peptide" appears in community discussion it is usually as a marketing claim about stability rather than a meaningful statement about a specific compound — the term describes a chemistry, not a benefit.

4Reported side effects and interactions

Literature-reported

There is no class-wide safety profile, and claiming one would be wrong. Each member's adverse effects follow its target, not its chemistry: cyclosporine causes nephrotoxicity and immunosuppression because of calcineurin inhibition; daptomycin causes myopathy and raises creatine kinase; complement inhibitors like pegcetacoplan and zilucoplan carry meningococcal infection risk and require vaccination; eptifibatide causes bleeding and thrombocytopenia. Read the individual profiles.

Community-reported

nothing meaningful at class level.

Class-level patterns that do exist, cautiously stated.

Interactions. member-specific. The only general statement worth making is that the large, lipophilic natural cyclic peptides have a worse interaction record than the small designed ones.

Markers researchers commonly monitor

No class-wide marker set exists, and inventing one would be misleading — monitoring follows the target, not the ring. What is worth stating at class level:

Marker	Why it's watched
Markers follow the mechanism. Complement inhibitors need haemolysis markers (LDH, haemoglobin, reticulocytes) and meningococcal vaccination records; calcineurin inhibitors need creatinine, eGFR and drug trough levels; the antibiotic needs creatine kinase; the PCSK9 inhibitor needs a lipid panel. Each profile lists its own
Therapeutic drug monitoring is unusually relevant for the natural-product macrocycles. Cyclosporine trough levels are standard practice because absorption is variable and the therapeutic window is narrow. Variable oral absorption is a recurring property of orally absorbed macrocycles, so this question will recur with the new oral agents
Anti-drug antibodies	not routine, but the measurement to consider for any peptide drug with unexplained loss of effect.
Renal and hepatic function	relevant across most of the class because clearance route determines exposure, and because several members are themselves nephrotoxic or hepatically metabolised.
Gastrointestinal tolerability, for oral macrocycles	a formulation rather than a laboratory measure, but the practical limiting factor for the newest members.

Free for everyone. Safety information is never behind a paywall.

5Formulation and administration context

Nothing at class level translates to a specific product. Cyclic peptides are generally more stable in solution and at room temperature than linear peptides of comparable size, which is part of why they are attractive, but each approved product has its own storage and preparation requirements set by its label — from pre-filled syringes to hospital-prepared intravenous infusions to oral tablets with absorption enhancers. There is no self-preparation context for any approved member of this class and no general guidance that could responsibly be given here. See the individual profiles for each product's actual handling.

6Commonly studied alongside

The approved and late-stage members with their own profiles in this library: - Pegcetacoplan — PEGylated cyclic peptide, complement C3 inhibitor. - Zilucoplan — macrocyclic peptide, complement C5 inhibitor. - Motixafortide — cyclic peptide CXCR4 antagonist. - Eptifibatide — cyclic heptapeptide platelet GPIIb/IIIa inhibitor. - Cyclosporine — natural cyclic undecapeptide calcineurin inhibitor. - Daptomycin — natural cyclic lipopeptide antibiotic. - Enlicitide (#247) — oral macrocyclic peptide PCSK9 inhibitor, late-stage. Adjacent strategies covered elsewhere in the library: lipidation and albumin binding for half-life extension (see semaglutide (#3), insulin icodec (#241)), Fc fusion (insulin efsitora alfa (#242)), and peptide-drug conjugates for targeted delivery (melflufen (#248), ANG1005 (#249)).

Informational cross-links. Not stack recommendations.

7Regulatory status by region

Region	Status (checked —)
United States	Multiple approved members (cyclosporine, daptomycin, eptifibatide, pegcetacoplan, zilucoplan, motixafortide and others). Several investigational, including oral macrocyclic peptides in Phase 3
European Union	Same approved members authorised; investigational pipeline parallel to the US
United Kingdom	Same approved members licensed
Australia	Same approved members registered with the TGA †
South Africa	Several approved members registered with SAHPRA; coverage is narrower than US/EU †
Sport	Member-specific. Peptide hormones and growth factors are prohibited under WADA S2 regardless of whether they are cyclic; most cyclic peptide medicines here are not performance-relevant and are not listed. Any non-approved substance falls under S0. Check the current list for the specific compound

8Open questions

Whether oral macrocyclic peptides deliver clinical outcome benefit, not just surrogate-marker change — the enlicitide cardiovascular outcomes trial is the first real test.
Whether the oral bioavailability problem can be solved generally, or whether each successful oral macrocycle will remain a bespoke achievement.
Whether macrocycles can be made to reach intracellular targets reliably, which would open the large set of protein-protein interactions currently considered undruggable.
Whether CNS delivery can be solved for this class without a separate shuttle platform.
How manufacturing cost and scale compare with small molecules and antibodies once oral macrocycles reach population-scale indications.
Whether the display technologies that generate these libraries will find targets that neither small molecules nor antibodies can address, which is the strongest version of the class's promise.

9References

1Vinogradov AA, Yin Y, Suga H (2019). Macrocyclic peptides as drug candidates: recent progress and remaining challenges. J Am Chem Soc. PubMed · DOI
2Zorzi A, Deyle K, Heinis C (2017). Cyclic peptide therapeutics: past, present and future. Curr Opin Chem Biol. PubMed · DOI
3Driggers EM, Hale SP, Lee J, Terrett NK (2008). The exploration of macrocycles for drug discovery — an underexploited structural class. Nat Rev Drug Discov. PubMed · DOI
4Muttenthaler M, King GF, Adams DJ, Alewood PF (2021). Trends in peptide drug discovery. Nat Rev Drug Discov. PubMed · DOI
5Over B et al. (2016). Structural and conformational determinants of macrocycle cell permeability. Nat Chem Biol. PubMed · DOI
6Johns DG et al. (2023). Orally bioavailable macrocyclic peptide that inhibits binding of PCSK9 to the low-density lipoprotein receptor. Circulation. PubMed · DOI
7Huang Y, Wiedmann MM, Suga H (2019). RNA display methods for the discovery of bioactive macrocycles. Chem Rev. PubMed · DOI
8Walensky LD, Bird GH (2014). Hydrocarbon-stapled peptides: principles, practice, and progress. J Med Chem. PubMed · DOI

† Being confirmed by our reviewers against current sources.

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11Page history

31 Oct 2026Profile published — awaiting medical review

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