Release date:2023/8/31 15:37:51

Peptide drugs, with their high purity, low toxicity and good biological activity, have become a hotspot for new drug development worldwide. The therapeutic potential of peptides has been exploited in the fields of antibacterial, antihypertensive, antioxidant, anticancer, antidiabetic and anti-inflammatory, etc. However, there are many problems in the research and development of peptide drugs, such as the lack of stability of peptide drugs, the difficulty in finding targets, the short half-life, and the poor cell permeability.

Researchers have made unremitting efforts to optimize peptides in order to improve the bioavailability of peptide drugs. Cyclization of peptides is one of the methods to optimize peptides. Cyclic peptides combine several favorable properties such as good binding affinity, target selectivity and low toxicity that make them an attractive modality for the development of therapeutics.

What Are Cyclic Peptides?

Cyclic peptides are polypeptide chains that are formed by a cyclic sequence of 5 to 14 amino acids with a molecular weight of about 500 to 2000 Da. Cyclic peptide molecules are present in almost all living organisms, from small bacteria to large plants and mammals. These cyclic peptides have a variety of novel structures, including not only peptide bonds, but also ether, thioether, lactone, and disulfide bonds in the skeleton. Moreover, it has been found that many cyclic peptides have excellent biological activities, including antibacterial, antitumor and immunosuppression. Therefore, the research on cyclic peptides is receiving more and more attention.

Classification of Cyclic Peptides

According to the type of bond between the two amino acids that furnishes the cycle ring, cyclic peptides can be classified into two major categories: homodetic and heterodetic. Homodetic cyclic peptides are those in which the ring is composed exclusively of normal peptide bonds (i.e. between the alpha carboxyl of one residue to the alpha amine of another), such as cyclosporine A. Heterodetic cyclic peptides contain diverse functional groups (at least one non-alpha amide linkage) used to connect the amino acids.

On the basis of the site of the two reactive groups within a peptide, the cyclic peptide can be generally categorized into four types: head-to-tail, head-to-side chain, side chain-to-tail, and side chain-to-side chain cyclization.


Figure 1. Four types of cyclic peptides, source: reference [1]

Cyclic Peptides vs. Linear Peptides

Compared with linear peptides, cyclic peptides have the following three major advantages.

1. Greater Metabolic Stability

Peptides are susceptible to deamidation reactions on the main chain and conformational changes of amino acid residues on the side chain, making them unstable in the organism. Cyclic peptides are one of the methods to reduce the reactivity of peptide structures involved in degradation reactions. Cyclic peptides have a stable and homogeneous conformation due to their rigid structure and are resistant to degradation by peptide hydrolases. In addition, cyclic peptides can constrain the conformation of peptide substrates, thereby protecting them from attack by exopeptidases and endopeptidases.

2. Higher Binding Affinity and Selectivity

Cyclic peptides have moderate size and diverse functional groups, ensuring the large contact area to provide high selectivity. Cyclic peptides have the potential to form multiple hydrogen bonds which can lead to strong binding affinity.

3. Improved Membrane Permeability

The cyclization of peptides contributes to the formation of intramolecular hydrogen bonds and the orientation of side chains, which in turn helps to protect polar atoms from solvent mediators, reduces intestinal, blood, and tissue degradation, decreases flexibility, reduces polar surface area, and facilitates the permeability of cell membranes.

Approved Cyclic Peptides

Cyclic peptide drugs hold a significant position in the peptide drugs market. As of May 2023, more than 100 peptides have been approved by regulatory agencies as therapeutic agents and diagnostic agents. To date, more than 50 cyclic peptides have been approved by different regulatory authorities, and many others are in clinical trials for a wide diversity of conditions. [2]

In the past 20 years, 22 cyclic peptides have been marketed, mainly from the United States, mostly formed through disulfide and amide bonding. Most of the approved cyclic peptides are modified from natural analogs to maintain affinity for the target protein by maintaining the overall cyclic peptide structure.

No. Drug Name Trade Name Company Indication Cyclization type FDA Approval Date EMA Approval Date
1 Caspofungin Cancidas MERCK & CO., Inc. Antifungal Amidation 26 Jan. 2001 24 Oct. 2001
2 Daptomycin Cubicin Lilly/Cubist Pharma/Merck Antibiotic Ether 12 Sep. 2003 19 Jan. 2006
3 Ziconotide Prialt Elan Pharms Severe and chronic pain Disulfide 28 Dec. 2004 21 Feb. 2205
4 Micafungin Mycamine Fujisawa Pharma/Astellas Inc. Antifungal Amidation 16 Mar. 2005 25 Apr. 2008
5 Anidulafungin Eraxis Lilly/Vicuron Antifungal Amidation 17 Feb. 2006 20 Sep. 2007
6 Lanreotide Somatuline Ipsen  Acromegaly and symptoms caused by neuroendocrine tumors Disulfide 30 Aug. 2007 11 Oct. 1996
7 Telavancin Vibativ Theravance Antibiotic Ether 11 Sep. 2009 2 Sep. 2011
8 Romidepsin Istodax Gloucester/Celgene/BMS CTCL Lactone and disulfide 5 Nov. 2009 Declined
10 Linaclotide Linzess AbbVie & IronWood Inc Irritable bowel syndrome/chronic constipation Disulfide 30 Aug. 2012 26 Nov. 2012
11 Pasireotide Signifor Novartis Cushing's disease Head–tail amidation 14 Dec. 2012 24 Apr. 2012
12 Vasopressin Vasostrict Par sterile products LLC Anti-diuretic hormone deficiency Disulfide 17 Apr. 2014 Declined
13 Dalbavancin Dalvance Durata Antibiotic Ether 23 May 2014 19 Feb. 2015
14 Oritavancin Orbactiv Lilly/Novartis Antibiotic Ether 6 Aug. 2014 18 Mar. 2015
15 Plecanatide Trulance Synergy Pharmaceuticals Chronic idiopathic constipation/irritable bowel syndrome Disulfide 19 Jan. 2017 Declined
16 Bremelanotide Vyleesi Palatin Technologies Inc. Hypoactive sexual desire disorder Asp–Lys 21 Jun. 2019 Declined
17 Setmelanotide Imcivree Rhythm Pharmaceuticals Obesity Disulfide 25 Nov. 2020 16 Jul. 2021
18 Voclosporin Lupkynis Aurinia Inc. Lupus nephritis Head–tail amidation 22 Jan. 2021 15 Sep. 2022
19 Pegcetacoplan Empaveli/Syfovre Apellis Pharmaceuticals PNH/GA Disulfide 19 May 2021/17 Feb, 2023 13 Dec. 2021
20 Vosoritide Voxzogo BioMarin promote bone growth in pediatric patients with achondroplasia Disulfide 19 Nov. 2021 26 Aug. 2021
21 Terlipressin Terlivaz Ferring Pharma Management of low blood pressure Head–tail, disulfide  14 Sep. 2022 2012
22 Rezafungin Rezafugin Cidara Therapeutics/Melinta Therapeutics candidemia and invasive candidiasis   27 Mar. 2023 /

Table 1. Approved cyclic peptides from 2000 to 2023

It is worth noting that among the approved cyclic peptide drugs, Pegcetacoplan is a PEGylated bicyclic peptide therapy targeting C3 complement proteins. Bicyclic peptide molecules combine the properties of antibodies, small molecule drugs, and peptides, with similar affinity and precise targeting specificity as antibodies; at the same time, their small molecular weight allows them to penetrate tissues quickly and thoroughly. In May 2021, pegcetacoplan was approved by the FDA for the treatment of paroxysmal sleep hemoglobinuria (PNH). On 17 Feb 2023, FDA approved syfovre™ (pegcetacoplan injection) as the first and only treatment for geographic atrophy (ga).


Figure 3. Pegcetacoplan structure

Cyclic Peptides in Pipeline

The number of cyclic peptide drugs under research worldwide has reached hundreds, and some of the drugs have entered the late clinical stage. With the deepening of research, more cyclic peptide drugs will be approved to enter the market in the future.

Name Generic Name Company Target Indication Highest Phase
RA-101495 Zilucoplan  Ra Pharmaceuticals C5 Paroxysmal nocturnal hemoglobinuria, generalized myasthenia gravis IND
MK-0616   MSD PCRK9 hypercholesterolemia III
BT8009   Bicycle Therapeutics NECTIN-4 Cancer types, where Nectin-4 is expressed II
BT1718   Bicycle Therapeutics MMP14+Tubulin Cancer with MT1-MMP expression  II
BT5528   Bicycle Therapeutics EphA2 Advanced solid tumors associated with EphA2 expression II
POL7080 Inhaled murepavadin Spexis AG —— Antibiotic to treat Pseudomonas infections in patients with cystic fibrosis  III
POL6326 Balixafortide Spexis AG —— Advanced breast cancers III
POL6014 Lonodelestat Spexis AG —— Cystic fibrosis II
ALRN-6924   Aileron MDM2+MDMX Chemoprotective agent II
CEND-1   Lisata Therapeutics CD51+NRP-1 Enhance the efficacy of chemotherapy II
NP-213   Taro Pharmaceutical
—— onychomycosis II
THR-149   Thrombogenics KLKB1 Diabetic macular edema II

Table 2. Cyclic peptides in late-stage

Prospects of Cyclic Peptides

Cyclic peptides have gained increasing attention as a unique class of molecules, with natural peptides being the main source of approved cyclic peptides in the last century. In the last two decades, the major trend in cyclic peptide drug discovery has been the optimization of naturally isolated cyclic peptides to improve the potency, stability and pharmacokinetic properties of cyclic peptide drugs.

1. Modern technology for cyclic peptide drug development

From a technological point of view, cyclic peptide drug development has benefited from rapid lead compound discovery, tunable and scalable optimization by chemical synthesis, which is a milestone in peptide drug development. In particular, recombinant technology combined with peptide chemistry has addressed major issues such as cyclic peptide production, mutagenesis and primary sequence optimization. However, as the need to target multiple proteins expands, the scope for optimization of natural cyclic peptides is limited. The emergence of multiple screening technologies, such as mRNA display, DNA display and phage display, has generated higher orders of magnitude of combinatorial libraries, enabling the development of novel cyclic peptides. Meanwhile, the directed evolution of gene-displayed cyclic peptide recombinant libraries plays an increasingly important role in next-generation cyclic peptide drug discovery. In addition, the introduction of various cyclization strategies, non-natural amino acids and even functional building blocks can further improve the functionality of recombinant libraries, such as chemical stability, metabolic stability and conformational stability. The development of selection strategies also enables the discovery of cyclic peptides with specific properties (e.g., with cell membrane permeability or high oral bioavailability).

2. PDC as a popular development direction

A second trend in cyclic peptide drug discovery is the development of peptide drug conjugates(PDCs) to enable selective delivery of different effector molecules to target tissues. The bicyclic peptide drug conjugates are under investigation, which offer several advantages, such as tumor deeper penetration, less immunogenicity, and faster renal clearance. Three investigational bicyclic peptide drug conjugates (BT1718, BT5528, and BT8009) are in phase I/II clinical development. BT1718 is a novel bicyclic peptide anticancer drug targeting membrane type I matrix metalloproteinase to release its toxic payload DM1. BT5528 has shown preliminary anti-tumor activity as a drug targeting EphA2. BT8009, as a nectin-4 targeting drug, has demonstrated anti-tumor activity.

[1] Chow HY, Zhang Y, Matheson E, Li X. Ligation Technologies for the Synthesis of Cyclic Peptides. Chem Rev. 2019;119(17):9971-10001. doi:10.1021/acs.chemrev.8b00657
[2] PepTherDia. Available online:
[3] Huiya Zhanga,Shiyu Chen.Cyclic peptide drugs approved in the last two decades (2001–2021).RSC Chem Biol 2021 Nov 5,3(1):18-31.
[4] Costa, L.; Sousa, E.; Fernandes, C. Cyclic Peptides in Pipeline: What Future for These Great Molecules? Pharmaceuticals 2023, 16, 996.
[5]Joon-Seok Choi , Sang Hoon Joo.Recent Trends in Cyclic Peptides as Therapeutic Agents and Biochemical Tools.Biomol Ther (Seoul) 2020 Jan 1,28(1):18-24.

Related Articles:
Complement Inhibitors as Therapeutic Agents
Peptide-Drug Conjugates (PDCs): Development Status & Research Progres
Peptide Therapeutics: Current Status And Future Directions
​FDA Approved PEGylated Drugs By 2023


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