Bifunctional Peptides: Overview and Prospect
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Introduction
Peptides are compounds of α-amino acids linked together by peptide bonds with a molecular weight of less than 10 kDa. It is usually composed of 10-100 amino acid molecules. Peptide drug molecules are between macromolecules and small molecular drugs, with the following characteristics:
-High safety and high target affinity
-Widely used as the template with a small risk of missing targets
-Mature peptide synthesis technology and less cost
-Easy to modify and separate from impurities or byproducts
-Relatively low immunogenicity
-Two times higher approval rate of clinical trials than that of small molecule drugs
Classifications of peptide drugs
At present, peptide drugs are mainly divided into several categories including binding peptides, conjugated peptides, oral peptides, cell-penetrating peptides (CPPs), peptide gene carriers, peptide vaccines/anticancer peptides, and bi/multifunctional peptides.
Due to the large elasticity, the peptide chain can be twisted and turned arbitrarily. By introducing the ring structure, the stability of the peptide can be improved, and some side effects can be reduced as well, thereby enhancing the medicinal properties. Binding peptides come in many forms, including monocyclic peptides (amide bond, disulfide bond), stapled peptides, and dicyclic peptides.
Conjugated peptides include those conjugates with antibodies (PAC), oligonucleotides (POC), multifunctional peptides, radionuclides (RDC), and other peptide drug conjugates (PDC).
There are only four approved oral peptide drugs, namely Linaclotide and Plecanatide for constipation, Semaglutide for diabetes, and Octreotide for acromegaly. However, Linaclotide and Plecanatide work in the gastrointestinal tract and can't be absorbed. Strictly speaking, oral peptide drugs include Semaglutide and Octreotide.
This class of polypeptides contains multiple lysine or arginine residues that are positively charged at physiological pH, bind to nucleic acids through electrostatic interactions, then interact with cell membranes, and enter the cell. So far, there are no commercially available drugs.
-Tumor therapeutic peptide vaccine: To identify specific neoantigens in tumor cells and develop cancer vaccines based on the amino acid sequence, which can activate related immune cells to kill tumor cells with the same antigen.
-Preventive peptide vaccine: CoVac-1, a peptide vaccine designed by the University of Tubingen in Germany, has shown good safety, reactivity, and immunogenicity in phase I clinical trials, as well as good protection against various current mutant strains. Phase II clinical trials of the vaccine are currently underway.
- Bifunctional/Multifunctional peptides
Bifunctional or multifunctional peptides refer to the combination of two or more peptide sequences through different conjugation forms, which have multiple pharmacological activities and improved safety and can target several targets and form synergies between different mechanisms.
Development and prospect of bifunctional peptides
Due to the complex pathogenesis, some complex diseases such as cardiovascular and cerebrovascular diseases, metabolic diseases, central nervous system and immune diseases usually fail to achieve the ideal results if applying the conventional mode of "one disease, one target, one therapeutic drug".
In contrast, bifunctional peptide drugs have the following advantages:
- Simultaneous action of different signal transduction mechanisms can significantly improve the effectiveness of druggability and indications, maximize the biological activity effect, and present a more balanced PK behavior while reducing side effects.
- Combining with existing targets, many peptides have been clinically validated, which provides insightful combinations to develop better drugs.
- In principle, there is no difference in clinical development between multifunctional peptides and other single moleculars in terms of the risks and costs involved.
- Compared with multi-component drugs, multifunctional peptides do not have complex drug-drug interactions and can avoid complex PK/PD behaviors caused by single drug combinations, with lower risk and better compliance by patients and doctors.
Combined forms of bifunctional peptides
Conjugation: A linker is introduced to release two ligands in metabolism and interact with each target independently.
Fusion : As the linker decreases in size until it can be conjugated directly, it is considered "fusion".
Merge : mTo merge by utilizing the common structure of two target compounds.
Double cyclic peptide: Bicycle Therapeutics uses a phage screening platform to screen out targeted peptides. The Bicycle is a short peptide with two rings (9~15 amino acids, 1.5~2KDa molecular weight) formed by two linear peptides under the action of "scaffold". It has good tissue permeability and renal clearance, high affinity, and selectivity. Researchers can design more combinations based on this kind of peptide.
Examples of bifunctional peptides
1. Pegcetacoplan-the first two-target peptide drug on the market
Pegcetacoplan (APL-2), a PEGylated circular bifunctional ligand peptide developed by Apellis targeting the C3 family of proximal complement proteins, specifically binds to C3 and C3b complement and inhibits intravascular and extravascular hemolysis. The clinical data was even better than Alexion's SolirisC5 inhibitor.
Apellis is also evaluating the therapeutic potential of APL-2 in various areas, including hematology, ophthalmology, nephrology, and the nervous system. APL-2 was screened by phage display and optimized to form the earliest lead compound APL-1.
APL-2 was formed by linking two APL-1 peptides via PEG to prolong the drug's half-life to about one week. Patients only need a subcutaneous injection every 2 weeks, significantly improving their compliance.
2. Tirzepatide-the second two-target peptide drug on the market
Tirzepatide is the first and currently only FDA-approved GIP/GLP-1 dual receptor agonist. It integrates the effects of two types of incretins into one molecule, achieving a double glucose-dependent hypoglycemic effect once a week.
Tirzepatide has shown an advantage over Smeiglutide in lowering blood glucose (A1C) and body weight in 40 weeks of head-to-head 3 clinical trials for type 2 diabetes. Indications under investigation include obesity, cardiovascular risk-related diseases, heart failure, and non-alcoholic steatohepatitis.
3. Molecular structure analysis of GLP-1/GIP/GCG
More and more studies have shown that the double agonist of GIPR/GLP-1R and the triple agonist of GIPR/GLP-1R/GCGR show better efficacy than the single agonist in vivo and in vitro.
In terms of molecular structure, GLP-1/GIP/GCG has a certain same sequence, which creates the structural basis for designing double/triple functions in one peptide molecule. Peptide 20 (GIPR/GLP-1R/GCGR triple agonist) was reported to significantly reduce body weight and glycosylated hemoglobin in diabetic rat models. Peptide 20 and Tirzepatide also altered the ECL1 conformation of GIPR without affecting the ECL1 conformation of GLP-1R or GCGR, suggesting that GIPR may be a key factor in the efficacy of multiple agonists.
Glucagon is no longer considered a single glycemic agent because its insulin-promoting effect on β cells can lower blood sugar. Given this, GIPR in α cells induces greater insulin secretion through α-β cell communication promoted by glucagon secretion, and not only through GIPR in β cells. While some early theories of GIP biology are being upended, the contribution of GIP activity to body weight remains controversial. There is growing evidence on both sides of the debate that GIPR activation or suppression can lead to weight loss.
Summary and prospect of bi/multifunctional peptide drugs
Technology of delivery: According to statistics, >90% of peptides in clinical development target extracellular targets, while most peptide membranes are impermeable and unable to target intracellular targets, thus limiting their application in drug development. Lipid nanoparticles with membrane-penetrating function or cell-penetrating peptides are likely to be paid more attention to.
Molecular Design: The increasingly complex molecular structure of peptides is an important development trend, which is not only manifested in the introduction of unnatural amino acids, or other more functional groups (proteins, small molecules). It also includes more combined forms, such as cyclic peptides, bicyclic peptides, and β folds. Due to the limitations of amino acids, peptides often have a short half-life/poor stability in vivo, and solubility and long-acting modification are often considered.
Peptide screening techniques and computational biology: Peptide screening techniques, such as phage display, mRNA display, and other new technologies, provide a reliable way to develop highly efficient and selective lead peptides in a short time. Meanwhile, computational tools can be used to quickly search for the commonalities of binding sites, which can help to predict/develop those multifunctional peptides with similar targets and support the discovery of new molecular entities of peptides.
Conclusion
Rather than simply incorporating target molecules from the framework, bifunctional peptides require continuous design and optimization at the pharmacophore level. Because the combination of different ligands often leads to the complexity of the overall molecular structure, it is necessary to modify the molecular structure reasonably.
It should be noted that for highly different targets, achieving multiple activities in a single compact molecule has proved impossible or hard to produce a drug. Because it is much more difficult to adjust the rate of activity for highly different targets, and because of the differences in disease severity and relative metabolic rate between patients, such drug candidates are inferior to monotherapy-based combinations, which have more flexible dosing depending on the actual situation. Therefore, bifunctional peptides are easier to achieve better effects only when drugs are reasonably designed for similar targets or different targets for the same indication.
References
- Nature,Vol 580,16 April 2020, 329-338; 10.1038/s41586-020-2168-1
- Signal Transduction and Targeted Therapy (2022) 7:48; 10.1038/s41392-022-00904-4
- Nature Reviews volume 20, April 2021, 309-325; 10.1038/s41573-020-00135-8
- NATURE COMMUNICATIONS | 8:16092; 10.1038/ncomms16092
