In the past 2022, the FDA has approved only three peptide drugs: Tirzepatide, Lutetium 177Lu Vipivotide Tetraxetan and Terlipressin, of which terlipressin was previously available outside the United States as Glypressin®. There is also literature that classifies Gadopiclenol approved in 2022 as a peptide drug, but this classification is not rigorous. Gadopiclenol contains only an amide bond, but it is not a condensation of two amino acids and should not be considered a peptide.

Trofinetide

Trofinetide is a novel synthetic analog of glypromate, also known as glycine-proline-glutamate (GPE). GPE is an N-terminal tripeptide of a naturally occurring protein and insulin-like growth factor 1 (IGF-1) in the brain. Compared with the proline Cα of GPE, trofinetide has one more methyl group.

Trofinetide (Daybue^TM), developed by Acadia, is a peptide drug approved by the FDA for the treatment of Rett syndrome on March 10, 2023, making it the first FDA-approved Rett syndrome drug in history. Syndrome drugs. Rett syndrome is a rare disease with a spectrum of cognitive, motor, and autonomic symptoms. It is a complex, rare neurodevelopmental disorder usually caused by genetic mutations in the MECP2 gene. It is characterized by a period of normal development before 6 to 18 months of age, followed by significant developmental regression, with loss of acquired communication skills and use of the hands. Symptoms of Rett syndrome may also include hand rigidity, such as wringing and clapping, and abnormal gait. In the United States, Rett syndrome affects 6,000 to 9,000 patients, of whom approximately 4,500 have been diagnosed. The approval of Trofinetide is undoubtedly a milestone event in the history of human struggle against Rett syndrome.

• Mechanism of Action of Trofinetide

Most cases of Rett syndrome are associated with loss-of-function mutations in the gene encoding methyl CpG-binding protein 2 (MECP2). MECP2 is a DNA-binding protein that plays a role in the epigenetic regulation of gene expression. These mutations lead to immature synapses in the cerebral cortex, abnormal metabolism of cholesterol in the brain leading to abnormal neuronal development, and abnormal neuronal signaling. Rett syndrome is more common in girls than boys.

The exact mechanism of action of Trofinetide in Rett syndrome has not been fully elucidated. In mouse studies, GPE peptides improved exercise and cardiorespiratory function, increased brain weight, and extended the lifespan of MECP2-deficient mice. As a synthetic analogue of GPE, trofinetide similarly attenuates apoptosis and reduces infarct size in a dose-dependent manner in a rat model of hypoxic injury. Trofinetide plays a multifaceted role in reducing inflammation, excitotoxic-induced tissue damage and apoptosis, thereby protecting neurons and their surrounding underlying tissues.

Rezafungin

New Echinocandin Peptide Drugs

On March 22, 2023, the FDA approved Cidara and Melinta’s Rezzayo^TM (rezafungin for injection) for the treatment of candidemia and invasive candidiasis in patients 18 years of age or older. Rezafungin is a new generation of broad-spectrum, long-acting echinocandin. Rezafungin has shown effective antifungal activity against Candida, Aspergillus and pneumospora sp. Rezafungin also became the first new echinocandin to receive FDA approval in more than a decade. Unlike other echinocinins such as caspofungin and micafungin, rezafungin has long-acting pharmacokinetics and high stability, allowing it to be administered at long intervals and maintain high plasma exposure. Rezafungin has a half-life of more than 130 hours and can be administered once a week instead of daily.

• Candidemia and Invasive Candidiasis

Candidemia and invasive candidiasis are rare, serious and life-threatening infections. According to the U.S. Centers for Disease Control and Prevention, the average rate of new infections in the United States is about 9 per 100,000 people, or about 25,000 cases per year. These infections are prone to occur in patients who already have other diseases and can lead to high morbidity and mortality. Patients at risk for invasive fungal infections include severely ill patients, immunosuppressed patients, patients after surgery, and patients using central venous catheters. In this already vulnerable patient population, invasive infections with candida often lead to serious illness and death. Patients with these invasive infections may develop a range of comorbidities on top of their underlying disease, including fever and septic shock. An additional hospital stay of 3 to 13 days is estimated after diagnosis of candidemia and invasive candidiasis. In addition, the mortality rate of these infected patients exceeds 40%. Echinocandins are used as first-line antifungals in practice guidelines for candidemia and invasive candidiasis because of their proven efficacy and safety as well as their powerful fungicidal activity.

• Another Cyclic Peptide

Rezafugin was discovered during the development of an orally administered Echinocin. Its outstanding structural feature is its chemical modification, mainly the addition of the choline moiety to the cyclic hexapeptide ring. This significantly improves molecular stability and biological properties. Compared to other echinocinins, the reduced metabolism and degradation of rezafungin enhances its stability and prolongates the half-life of the drug. This made it possible to administer the drug intravenously once a week.

• Mechanism of Action of Rezafungin

Rezafungin is an echinocandin antifungal drug. It inhibits the 1,3-β-D-glucan synthase complex in the fungal cell wall, which is responsible for the formation of 1,3-β-D-glucan. Since 1,3-β-D-glucan is an important component of fungal cell walls, rezafungin disrupts the cell walls of fungal species including Candida and acts as a concentration-dependent fungicide in vitro. Mammalian cells do not express β-1-3-glucan synthase, therefore, the action of rezafungin is specific to fungi.

Posluma (flotufolastat F 18)

Posluma (flotufolastat F 18) is an 18F-labeled ligand and an imaging agent for the detection and localization of prostate cancer in PET (positron emission tomography). Posluma contains an imaging agent (18F) whose active ingredient is flotufolastat F 18 gallium bound to non-radioactive gallium. Flotufolastat F 18 binds to prostate-specific membrane antigen (PSMA) (IC50= 4.4 nM) expressed on cells, including prostate cancer cells. In patients with recurrent prostate cancer who require local treatment, positron emission tomography (PET) with an 18F-labeled ligand provides accurate diagnostic imaging. The 18F group in Flotufolastat is a beta + emitting radionuclide that can be detected using PET. Since prostate cancer cells overexpress PSMA and flotufolastat F-18 is internalized by the cells, this radioactive agent can be used for diagnostic imaging in prostate cancer patients. Posluma received FDA regulatory approval on May 25, 2023 for PET of PSMA-positive lesions in men with suspected metastatic prostate cancer. It is the first FDA-approved PSMA-targeted imaging agent developed using a proprietary radiohybridization technology.

Paxlovid (nirmatrelvir, ritonavir)

The two active substances in Paxlovid, nirmatrelvir and ritonavir, are both peptidomimetics. Nirmatrelvir acts as a protease inhibitor to block the activity of proteases required for virus reproduction. It is an inhibitor of 3C-like protease (3C-like protease, 3CLPRO, also known as Mpro), which plays a key role in the replication of SARS-CoV-2 virus. The second peptidomimetic active substance ritonavir in Paxlovid is used as a pharmacokinetic enhancer of nirmatrelvir, which increases the half-life and bioavailability of nirmatrelvir by inhibiting the degradation of nirmatrelvir by CYP3A4.

What is Cyclic Peptide?

Cyclic peptides are peptide chains with a cyclic structure, mostly composed of 5-14 amino acids, with a molecular weight of about 500-2000 Da. Cyclic peptides have several advantages over linear peptides. Linear peptides as drugs may be inherently unstable and have a high probability of intracellular proteolysis. Considering that the free acid and free amine are at both ends, they are also usually polar. Cyclization promotes intramolecular hydrogen bonding within the ring structure and reduces the external hydrogen bonding capacity of the molecule, which reduces the polarity of the molecule compared to the acyclic precursor and increases the membrane permeability of the compound. The two β-turn local secondary structures caused by cyclization also reduce polar surface area, which generally enhances cell permeability.

Cyclic peptides are usually produced by end-to-end, cephalic, or side-chain-to-side-chain cyclization reactions. Introducing cyclization can make the conformation of the peptide chain more stable, thereby increasing the binding affinity to the target protein and reducing nonspecific binding due to fewer alternative conformations. Reduced conformational flexibility reduces the chance of molecules fitting to protease catalytic sites and improves proteomic resistance. Intervention in protein-protein interaction (PPI) by forming larger interaction surfaces to improve peptide chain efficacy.

Chem. Eur. J. 2021, 27, 1487-1513

Cell Permeability of Cyclic Peptides

There are three pathways by which cyclic peptides enter cells: passive diffusion pathway, carrier-mediated transport pathway, and endocytosis pathway. The passive diffusion pathway usually requires good amphiphilicity and low molecular weight (<1000Da) of cyclic peptide molecules to enter cells. Some peptide-based drugs are recognized by transporters and subsequently introduced into cells, and some cyclic peptides enter cells through PepT1 and PepT2 transporters. However, these receptors are expressed primarily in the gut (absorption) and kidney (excretion), so they are important for oral bioavailability and elimination of peptide drugs but are unlikely to be involved in the cellular penetration of target tissues. Moreover, the membrane of the endocytic sac is similar to the cell membrane, and non-passive permeable molecules may not be able to effectively escape the endosome and lysosome into the cytoplasm. In order to enhance the cell permeability of peptide drugs, it is necessary to improve the molecular weight and amphiphilicity of drugs. Cyclic peptide drugs, however, are flexible enough to meet these needs. First, controlling the sequence length of the cyclic peptide molecule can control the molecular weight requirements. Secondly, by modifying the side chains of amino acids (introducing unnatural amino acids, forming disulfide bonds, etc.) or main chains (n-methylation, etc.), introducing unnatural hydrophobic groups can enhance the hydrophobicity of cyclic peptide molecules, thus improving the cellular permeability of cyclic peptide drug molecules. Furthermore, the hydrophilic and hydrophobic properties of cyclic peptides can be further optimized according to the symmetry of the framework (d-type amino acids).

Chem. Eur. J. 2021, 27, 1487-1513

FDA-Approved Cyclic Peptide Drugs

Cyclic peptide drugs targeting intracellular proteins

1. Romidepsin

Romidepsin is a natural bicyclic peptide discovered in Chromobacterium violaceum (1994), containing a head-to-tail lactone cyclization and a pair of disulfide bonds, mainly produced by fermentation. Romidepsin, a histone deacetylase inhibitor (HDACi), was approved by the FDA in 2009 for cutaneous T-cell lymphoma (CTCL) that has received at least one systemic therapy, primarily as an antineoplastic drug. It is a prodrug in which disulfide is reduced to two mercaptols in the intracellular matrix that chelates zinc ions at the zinc-dependent active site of histone deacetylase (HDAC), thereby inhibiting histone deacetylase and inducing apoptosis.

2. Voclosporin

Voclosporin has 11 residues, including 1 D-type amino acid, which is head and tail cyclization. The N-terminus of some amino acids is methylated by S-adenosine methionine. Volcyclosporine was obtained by optimizing cyclosporin A. Voclosporin has better efficacy and metabolic stability than cyclosporin A. As a novel best-in-class calcineurin inhibitor (CNI), voclosporin has a synergistic and dual mechanism of action: it stabilizes renal podocyte by inhibiting CALcineurin (CN), blocking IL-2 expression and T cell-mediated immune response.

Cyclic peptide drugs targeting extramembrane proteins

1. Ziconotide

Ziconotide is a synthetic omega-MVIIA toxin isolated from the venom of Conus Magus. It contains 25 residues, of which six amino acids are cysteine, linked by three pairs of disulfide bonds. As a potent selective blocker of N-type calcium channels, Ziconotide was approved in 2004 for the treatment of severe chronic pain.

2. Linaclotide

Linaclotide is an oral peptide composed of 14 amino acids developed in the 1990s and belongs to the Cyclic Guanosine Monophosphate (cGMP) regulatory peptide family. In 2012, linaclotide was approved by the FDA for the treatment of irritable bowel syndrome with constipation (IBS-C).

3. Plecanatide

Pukanatide is an oral guanylate cyclase C(GC-C) agonist developed by Synergy that regulates acid and base ions in the gastrointestinal tract, inducing fluid transport into the gastrointestinal tract and increasing peristalsis. Pukanatide, a 16-amino acid analog of uranyin, is produced by replacing Human uranyin Asp3 with Glu3 and binds to guanylate cyclase C(GC-C) receptors at pH 5.0 (duodenum and proximal jejunum pH) bound by 2 pairs of disulfide bonds. On 2017-01-19, Plcanatide was first approved by the FDA for the treatment of gastrointestinal disorders such as constipated irritable bowel syndrome (IBS-C) and chronic idiopathic constipation (CIC).

4. Pasireotide

Pasireotide is a second-generation analog of somatostatin, a naturally occurring inhibitory hormone that blocks the release of several other hormones by interacting with the somatostatin receptor (coupled to G protein), including growth hormone, thyroid-stimulating hormone (TSH), insulin, and glucagon. Compared with other somatostatin analogs, paretide has a 40-fold increase in affinity for somatostatin receptor 5. Pasireotide is a head-to-tail cyclized hexapeptide. Pasireotide has a higher binding affinity to the somatostatin receptors SSTR1, SSTR3, and SSTR5 than octreotide and lanreotide.

5. Lanreotide

Lanretide is the first sustained-release, long-acting somatostatin octapeptide analog, which is produced by cyclization of a disulfide bond between cysteine and cysteine, and binds stronger to somatostatin receptors on adenohypophysial cells than octreotide. Studies have shown that lanreotide mainly acts through somatostatin receptor-2 and somatostatin receptor-5, with a strong affinity for peripheral somatostatin receptors and a weaker affinity for central receptors.

6. Vasopressin

Vasopressin, discovered in 1928 and its sequence elucidated in 1951, is an antidiuretic hormone with nine amino acids, cyclized by a disulfide bond between Cys4 and Cys9.

7. Terlipressin

Terisopressin is a synthetic, long-acting vasopressin analog that acts on the vasopressin receptors V1a, V1b, and V2. It is a prodrug that is degraded in vivo by endopeptidase to form lysine-vasopressin by removing the trisglycine residue at the N-terminal of the cyclic peptide, which is the main active metabolite and is further degraded in the liver, kidney, and other tissues. As a result, teripressin acts slowly but has a longer duration, with a half-life of 6hr, which is longer than that of vasopressin (10min), and is safer with fewer side effects.

8. Bremelanotide

Bremelanotide is a synthetic peptide molecule consisting of seven amino acids cycled between side chains Asp2 and Lys7 to form the lactam analog (α-MSH) of the alpha-melanocyte-stimulating hormone.

9. Setmelanotide

Setmelanotide is a disulfide-cyclized octapeptide that preferentially agonizes the melanocortin 4 receptor (MC4R) with an EC50 of 0.2 nM.

Antimicrobial Cyclic Peptides

1. Daptomycin

Daptomycin is a cyclic lipopeptide antibiotic isolated from Streptomyces rosiana. It contains 13 residues, 2 of which are non-natural amino acids, and 10 of which form lactone rings through the side chain and C-terminal of Thr4.

2. Telavacin

Telavancin is a semi-synthetic vancomycin derivative, which is based on the vancomycin structure. It is obtained by chemical modification to introduce an aliphatic chain on the amino group of the sugar group and to introduce methylamine methyl phosphate on the seventh aromatic amino acid. Through modification, the binding force with the cell membrane and affinity with the antibacterial target is enhanced, and teravanin is a fast bactericidal antibiotic for injection.

3. Dalbavancin

Dalbavancin is a second-generation semi-synthetic lipoglycopeptide antibiotic that is more potent than vancomycin and inhibits bacterial cell wall synthesis. The vancomycin family of natural antimicrobial glycopeptides is highly modifiable heptapeptides in which 5 amino acids remain unchanged, while amino acids 1 and 3 are highly differentiated. The heptapeptide core is used to bind the C-terminal D-Ala-D-Ala of the peptidoglycan chain and the side chain functional groups are derived to improve the properties of the peptide molecule.

4. Oritavancin

Oritavancin, a second-generation, semi-synthetic lipoglycopeptide, is derived from chloroeremomycin, an analog of vancomycin. Similar to vancomycin, it contains a core heptapeptide.

5. Caspofungin, Micafungin and Anidulafungin

Capofungin, micafengin, and anifengin all belong to the cyclic hexaliphatic peptide family. They share the same cyclic peptide core structure and are cyclized by amidation between the n-end of the side chain of the first lysine residue and the c-end of the peptide chain, and all target the 1,3-β -glucan synthase in the cell wall for the synthesis of glucan. All three antifungal agents have clinical efficacy against invasive candidiasis and other forms of systemic mycosis.

References

Cyclic peptide drugs approved in the last two decades (2001-2021)