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 
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. 
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|
|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|
|CEND-1||Lisata Therapeutics||CD51+NRP-1||Enhance the efficacy of chemotherapy||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.
 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
 PepTherDia. Available online: http://peptherdia.herokuapp.com/list
 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.
 Costa, L.; Sousa, E.; Fernandes, C. Cyclic Peptides in Pipeline: What Future for These Great Molecules? Pharmaceuticals 2023, 16, 996. https://doi.org/10.3390/ph16070996
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.
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