Two oral macrocyclic peptides are closest to patients: Merck's MK-0616 met Phase III goals in hypercholesterolemia, and JNJ-2113 reported positive Phase III results in plaque psoriasis, while Chugai's oral KRAS inhibitor LUNA18 is in Phase I. This reference article covers the structural…
Macrocyclic peptides are ring-shaped molecules of roughly 5 to 17 amino acids, weighing 500 to 2000 Da. Two oral candidates in this class sit at the final stage of clinical testing, and a third is in early human trials. Merck's MK-0616, an oral PCSK9 inhibitor for lowering LDL cholesterol, met the primary endpoints of both its Phase III trials, with the announcement made on June 9, 2025. JNJ-2113, an oral IL-23 receptor antagonist developed through a 2017 collaboration between Protagonist Therapeutics and Johnson & Johnson, reported positive Phase III topline results in plaque psoriasis in November 2024. Chugai Pharmaceutical's LUNA18, a KRAS inhibitor, is in Phase I in Japan and the United States. None has been approved. The direct answer to how close macrocyclic peptide drugs are to patients is that two candidates sit at the last clinical hurdle, with regulatory decisions still pending.
The clinical pipeline includes 26 named candidates across Phase I to Phase III, delivered by oral, injectable, ophthalmic, and inhaled routes. The stakes are simple. Cyclic peptides have long been proposed as a way to reach disease targets that linear peptides and conventional small molecules cannot engage. MK-0616 and JNJ-2113 are the first late-stage tests of whether the oral versions of such molecules deliver their pharmacology in patients.
Cyclization is the defining structural event. A linear peptide presents free amino and carboxyl termini, the natural attack points for exopeptidases, and its backbone can adopt many conformations, most of them not competent for binding. Closing the ring removes both termini and restricts the backbone to a smaller set of conformations. The molecule is simultaneously more stable and more pre-organized for target engagement. Because less conformational entropy is lost when the peptide binds, a macrocycle can reach high target-binding affinity within a relatively small structure.
Stability, specificity, and half-life follow from the same design. The rigid ring resists gastric acid and proteolytic enzymes, which prolongs in vivo half-life compared with linear peptides. The constrained surface reduces the number of off-target contacts available. The field's claim is that this yields higher target specificity and biological activity, permitting lower doses and fewer off-target effects. Those are plausible design consequences, but for much of the class they remain claims rather than measured clinical outcomes. The same caution applies to immunogenicity. Macrocyclic peptides are often described as having low immunogenicity, strong target selectivity, and minimal impact on normal cells. Across a class of molecules with varied rings, linkers, and sources, that statement rests on a thinner patient dataset than the confident phrasing suggests.
The chemical range is wide. Disulfide-constrained natural products, synthetic stapled peptides, and N-methylated rings can all fall inside the 5 to 17 amino acid, 500 to 2000 Da envelope. What they share is a continuous ring backbone. That architecture is what allows macrocyclic peptides to modulate protein-protein interactions and intrinsically disordered proteins, target classes that are flat, extended, and poorly suited to the binding pockets of typical small molecules. Access to so-called undruggable targets is the core scientific rationale for the class.
The 500 to 2000 Da window places macrocycles between conventional small molecules and biologic proteins. They are large enough to present extended, multipoint contact surfaces and small enough that oral absorption remains conceivable for a subset of the class. Discovery platforms differ, and the pipeline reflects that variety. mRNA display, phage display, stapled peptide design, and natural product derivation or sourcing have all produced candidates. These methods solve complementary problems. Display technologies generate large libraries and select high-affinity rings. Stapled design and natural product chemistry supply the ring-forming reactions and the membrane-permeability features that many display hits lack.
Oral delivery is the class's bottleneck. A peptide that survives the stomach must still cross the intestinal epithelium, and passive transcellular permeability is hard for molecules whose backbone amides must shed bound water before crossing a lipid membrane. Cyclization removes the termini but does not by itself remove those amides. This is why the oral macrocycle field has so few reference points.
Cyclosporine A is the benchmark. The FDA approved it in 1983, and since then only a small number of orally bioavailable macrocyclic peptides have reached the clinic. The practical stakes of oral delivery are highest in chronic disease. Hypercholesterolemia and plaque psoriasis both require long-term treatment, and a patient who can take a pill rather than receive an injection avoids the cost, the clinic visit, and the adherence burden of an injectable. That is the practical logic behind the two oral Phase III programs, and it is also a reason to read their results carefully. The Phase III programs matter because they represent two different answers to the permeability problem.
The first answer is formulation. MK-0616 has approximately 2% oral bioavailability, and its intrinsic permeability is limited; it reaches that level only because an absorption enhancer is built into the formulation. The enhancer transiently increases epithelial permeability, and the potency of the PCSK9 inhibitor does the rest. A 2% oral bioavailability sounds negligible, but for a high-affinity molecule the absorbed fraction can be pharmacologically sufficient. The costs are formulation complexity and a permanent dependence on the enhancer for every dose.
The second answer is intrinsic permeability. LUNA18 demonstrated 21% to 47% oral bioavailability across mice, rats, monkeys, and dogs without permeability enhancers. If those numbers hold in humans, they would show that the ring itself can be designed to cross membranes through backbone modification and side-chain placement, rather than relying on a formulation additive. The preclinical range, spanning four species, is a stronger permeability signal than a single-species result, but it is not human evidence.
MK-0616 works through PCSK9 inhibition. PCSK9 normally targets LDL receptors for degradation; when MK-0616 binds PCSK9, the peptide prevents that degradation, leaving more LDL receptors on hepatocyte surfaces to clear LDL cholesterol from circulation. The Phase III program consisted of the CORALreef HeFH and CORALreef AddOn trials, both in patients with hypercholesterolemia, including those with heterozygous familial hypercholesterolemia. Merck announced on June 9, 2025, that both trials met their primary endpoints. These are sponsor-reported results; no peer-reviewed publication is described.
JNJ-2113 targets a different biology. It antagonizes the IL-23 receptor, interrupting the IL-23-driven inflammatory signaling that sustains plaque psoriasis. The program traces to a 2017 collaboration between Protagonist Therapeutics and Johnson & Johnson. In November 2024, the ICONIC-LEAD Phase III trial reported positive topline results in moderate-to-severe plaque psoriasis, with the study population including adolescents aged 12 years and older. If approved, JNJ-2113 would become the first oral peptide drug targeting the IL-23 receptor. That status is conditional on regulatory review, which has not concluded.
LUNA18 is the early-stage program with the most distinctive pharmacology. It inhibits KRAS, a GTPase mutated across a large share of solid tumors, and it is in Phase I trials in Japan and the United States. Preclinically, it showed dose-dependent antitumor activity in animal models alongside the 21% to 47% oral bioavailability already described. The open question is whether the animal efficacy and the intrinsic permeability both translate to humans.
The rest of the pipeline broadens the picture. The 26-candidate clinical list includes ALRN-6924, BT1718, BT8009, BT5528, BT7480, AMY-101, ORMD-0801, rusfertide, balixafortide, plitidepsin, FOG-001, and others, spanning Phase I through Phase III and covering oral, injectable, ophthalmic, and inhaled routes of administration. Discovery origins vary accordingly, from mRNA display and phage display to stapled peptide design and natural product derivation.
The framing that macrocyclic peptide development has entered an oral therapeutics era rests on this selected set of successes, not on the pipeline as a whole. Most of the 26 candidates are not oral. MK-0616 and JNJ-2113 demonstrate that oral delivery can work in late-stage trials; they do not demonstrate that it is routine.
Researchers who need cyclic peptide reference materials will find a commercial catalog from Creative Peptides that lists products with catalog numbers, molecular weights, and molecular formulas. The catalog's reported molecular weights include:
The catalog also includes polymyxin B, capreomycin, zilucoplan, and cyclosporine A alongside those entries. The list is useful as a sourcing reference and should not be treated as a structural classification. Semaglutide, listed at 4113.57, is a GLP-1 receptor agonist that is not generally classified as a macrocyclic peptide. Its presence in a cyclic peptide catalog is a reminder that vendor categories are commercial conveniences. Buyers should verify compound identity, class, and purity independently rather than accepting a catalog label.
The Peptide Atlas dataset shows how far catalog presence and clinical evidence can diverge. Semaglutide has one of the largest clinical footprints of any peptide. Its file holds 668 registered clinical trials, with 10 listed as recruiting. The trial phase breakdown lists Phase 2: 4, Phase 4: 4, Phase 3: 1. Notable registered trials include NCT07586150 LIFETRAIN, personalized pharmaco-lifestyle interventions for severe mental illnesses , NCT07430332 a Phase 2 study of a GLP-1 receptor agonist for stage 1 type 1 diabetes , NCT07614412 SHIELD-T1D, Shingrix and a GLP-1 agonist for beta-cell preservation in recent-onset type 1 diabetes , NCT07462663 SHAPE-ENDO, a Phase 4 pilot trial of multimodal pre-surgical optimization in obesity and early-stage endometrial cancer , NCT07027969 metabolic surgery for atrial fibrillation elimination , and NCT06977438 a Phase 4 study of GLP-1 plus lifestyle for childhood obesity .
The file indexes 197 PubMed papers, including a JAMA Psychiatry randomized clinical trial of semaglutide and effort-based decision-making in major depressive disorder, the STRIDE trial of semaglutide in peripheral artery disease and diabetes in the European Heart Journal, and a systematic review of long-term safety and renal outcomes in non-diabetic obesity with chronic kidney disease or hypertension. Eight third-party lab purity tests are on file, with the highest observed purity at 99.979%. For a buyer, that kind of third-party measurement is the check that matters more than the catalog listing itself.
Lanreotide, a genuine cyclic somatostatin analog listed at 1096.33, sits at the other extreme in the same database: 0 registered clinical trials on file and 1 indexed PubMed paper, the CLARINET trial of lanreotide in metastatic enteropancreatic neuroendocrine tumors, published in the New England Journal of Medicine in 2014. The contrast is instructive. A molecule can carry an enormous catalog footprint and an unmatched trial record without being a macrocycle, while a true macrocycle with an approved indication can rest on a single registration trial. Catalog presence and evidence depth are independent variables.
Three operational lessons follow from the record so far. For researchers evaluating oral macrocyclic candidates, the first question to ask is whether permeability is intrinsic or formulation-dependent. MK-0616 shows that a low single-digit oral bioavailability can support a late-stage program when the molecule is potent and the enhancer is consistent. It also shows that 2% is a number with real costs. A program that depends on an enhancer inherits questions about food effects, dose size, and interpatient variability. LUNA18's preclinical range is the counterexample that matters: intrinsic permeability, if confirmed in humans, would remove those formulation variables.
For clinicians and payers tracking the space, JNJ-2113 defines the anti-cytokine frontier. The IL-23 pathway is dominated by injectable antibodies. An oral peptide with positive Phase III psoriasis results, including an adolescent population, would be a new delivery format for a validated target. The approval decision and the market availability timeline will determine whether that happens.
For buyers of reference materials, the practical rule is verification. Use vendor catalogs as a starting point, cross-check the molecular weight and formula against the catalog entry, and treat structural classification with suspicion when a listed product does not fit the class. Peptide Atlas data files, with registered trial counts and third-party purity results, are one independent check. The semaglutide entry in a macrocyclic catalog is the clearest example of why that check matters.
Approval is the first open question. MK-0616 and JNJ-2113 have positive late-stage results and no regulatory decisions. Whether they reach patients, and when, will set the template for the next wave of oral macrocycles. The deeper bioavailability question feeds into the same timeline. Can oral bioavailability be improved substantially beyond roughly 2% without absorption enhancers? LUNA18 suggests yes in four preclinical species, but whether that result generalizes across chemical matter and across humans is unknown.
Translation and platform competition are the next open items. LUNA18's dose-dependent antitumor activity and its oral bioavailability are both preclinical findings; human Phase I and II results will determine whether an oral KRAS inhibitor built on a macrocycle is a viable therapeutic hypothesis. Separately, mRNA display, phage display, stapled peptide design, and natural product derivation have all produced candidates, and which platform yields the largest number of approved drugs cannot be answered from the pipeline so far, because most of its members have not reached approval.
Finally, the evidence base itself is limited. All clinical results described here are sponsor-reported; no peer-reviewed publications of the late-stage trials are described, and no regulatory approvals have been granted. The low-immunogenicity claim attached to the class is stated widely and tested thinly. The oral therapeutics era framing rests on a handful of successful trials, not the whole pipeline. The ceiling for macrocyclic peptides will be set by whether the next round of candidates converts positive trials into approved medicines, and by whether the oral successes can be repeated without the crutch of an absorption enhancer.
Peptides referenced: Semaglutide, PT-141, Octreotide, Lanreotide, Pasireotide, Somatostatin, Bremelanotide, Ziconotide.
Related reading: Representative Peptides for In Vivo Tumor Imaging, Six biocatalytic strategies for enzymatic oligopeptide synthesis, Enzymatic Routes to Oligopeptide Synthesis: A Technical Overview, Solid-Phase Peptide Synthesis: Resins and Working Protocols.