Peptide drugs are generally derived from endogenous biologically active molecules, so they usually have the advantages of high affinity, good selectivity, clear target, obvious effect, high safety, etc., which can be truly realized "Molecular Targeting".But natural, endogenous peptides that meet physiological needs cannot be fully satisfied for the medicinal or clinical treatment:for example, peptides used as drugs have defects such as poor stability, easy aggregation, short half-life, high plasma clearance, and inconvenient administration.Therefore, the peptide drugs optimization has attracted the attention of pharmaceutical and clinical medicine.

Endogenous peptides are usually feasible and effective lead compounds for drug discovery. The needs of clinical applications hope that peptide drugs can be optimized to be superior to natural products.Precision medicine of major diseases puts forward higher requirements for peptide drugs.

Since the advent of genetically engineered drugs in the early 1980s, artificial design and optimization of peptide/protein drugs have emerged under the call of clinical precision treatment and the actual needs of patients. The artificial design and optimization of peptide drugs are first based on the relationship between the structure and function of peptide and its mechanism of action.Specifically, there are the following 8 aspects:

1.Multi-target drug design and development based on the mechanism of peptide

Complex diseases are usually multi-gene diseases, involving abnormalities in multiple signal pathways. Therefore, multi-target drugs which have a synergistic mechanism or target multiple signal pathways will have better efficacy.

This peptide drugs has the following modes of action: First, a drug molecule acts on multiple targets that have a synergistic effect.Second, combine multiple drugs targeting different drug targets with different mechanisms to produce synergistic therapeutic effects.The combination of the receptor agonist Exenatide and the GIP receptor agonist N-AcGIP can improve glucose tolerance, reduce food intake and weight.Thirdly, fusion of multiple peptides/proteins for different drug targets with different mechanisms into a new drug molecule, thereby producing better and synergistic therapeutic effects. For example, tirzepatide, which contains the GIP and exenatide active motif, has a very significant effect on reducing blood sugar and reducing body weight in patients with type 2 diabetes.

2. Long-acting strategies for peptide/protein drugs

The disadvantages of peptide/protein drugs are short half-life and high plasma clearance rate. Enhancing the ability of peptides to resist enzymatic hydrolysis to improve their stability in vivo, and increasing their hydrodynamic radius to reduce glomerular filtration are the main strategies to extend the half-life of such drugs in vivo. Among them, the stability of the peptide against enzymatic degradation can be obtained by modifying its amino acid sequence, or by methods such as acetylation modification and access to D amino acids. Increasing the hydrodynamic radius of peptide/protein drugs can be achieved by using genetic engineering methods or chemical methods to increase the molecular weight of peptides.

In addition, human plasma albumin is rich in plasma, has a large molecular weight and a long half-life in the body, which can be used as an effective carrier for the delivery of peptide drugs in the body. Therefore, peptide drugs can also be coupled to albumin affinity peptide or fatty acid modification to achieve the purpose of extending the half-life.

3. Peptides based on cyclization strategy to improve drug stability

In addition to short half-life and high plasma clearance, peptide/protein drugs also have poor stability, easy aggregation, and may have defects such as immunogenicity. Cyclization is an effective strategy to enhance the stability of peptide drugs. Cyclization include simple cyclization between amino acids, stapled peptide , proteins containing multiple pairs of disulfide bonds, and Lariat peptides carriers, etc.

The staple peptide can staple the two amino acids in the same plane in the α helix, thereby strengthening the α helix structure. Natural biotoxins are rich in disulfide bonds. This type of peptide/mini protein has a high-level structure due to the connection and fixation of multiple pairs of disulfide bonds, so it has a strong binding ability to target receptors and has better biological stability than linear peptides. Lariat peptides are a kind of natural products of peptides with lasso structure, which are highly stable to chemical, thermal and protease degradation. It can be used as a carrier for a variety of peptide epitopes, showing great potential in the development of peptide/protein drugs.

4. Targeted peptide/protein drug development

The clinical demand for precision treatment of major diseases expects that drugs can cross physiological barriers (such as blood-brain barrier, oral absorption barrier, etc.), can target lesions, and target multiple different targets. Peptide/protein drugs are characterized by their biological targets. Therefore, peptides can also be used as guiding molecules.

For example, tumor homing peptides can be used as tumor-related diagnostic reagents or targeted molecules to guide tumor drugs into tumor sites to exert anti-tumor effects. The penetrating peptide can enter the cell through receptor-mediated endocytosis and other mechanisms, so that the peptide can act on the target in the cell to function. Multifunctional integration is one of the development trends of peptide/protein drugs

5. Self-assembled peptide/protein drug development

Peptide/protein has the characteristics of good degradation performance and good biocompatibility, making it a good nano-material. Self-assembled peptides are a kind of peptides that use non-covalent bonds between amino acids or peptide chains to self-assemble into a highly ordered nanostructure. They can be used as drug carriers to improve drug properties, and play a sustained-release and targeted role.

Drug delivery systems based on peptide nanomaterials have bright application prospects in regenerative medicine and clinical applications due to their good biofunctionality and reproducibility. For example, the self-assembling peptide RADA-KLA with anti-tumor activity not only overcomes the shortcomings of KLA peptides, such as strong immunogenicity, poor cell penetration, and rapid degradation in vivo, but also inhibits the adhesion and migration of liver cancer cells and induces liver cancer cell death.

6. Chemical modification and transformation of peptide/protein drugs

Chemical methods can compensate for the limitations of biological methods. For example, replacing cysteine with selenocysteine in peptides/proteins to form more stable selenium-sulfur bonds or diselenium bonds can enhance the tolerance of peptides/proteins to reducing environments. Replace the disulfide bonds in insulin with selenium-sulfur bonds to obtain selenized insulin (Se-Ins) with a longer half-life. Its activity is similar to insulin, and its degradation rate is significantly slower (Se-Ins has a degradation half-life of about 8 h, while bovine insulin is only about 1 h)

The phage bicyclic peptide library introduces a linker that can react with sulfhydryl groups on a linear peptide library containing 3 cysteine groups, and covalently chemically connects with 3 sulfhydryl groups to form a bicyclic structure. Bicyclic peptides tend to have stronger target affinity and resistance to proteases. The target affinity of bicyclic peptide peptide ligands has reached picomolar level, which has good application prospects.

7. Development of dosage form promotes the clinical application of peptide/protein drugs

The main routes of administration of peptide/protein drugs are intravenous injection, intramuscular injection, subcutaneous injection, etc., and intravenous injection is the main method. Although the injection drug is rapidly distributed in the body and absorbed quickly, the bioavailability and patient compliance are both poor.

Compared with small molecule drugs, peptide/protein drugs have serious shortcomings of single dosage form and inconvenience to take. The injection drug delivery system can deliver drugs to specific tissues within a certain period of time by changing the formulation method, improving the targeting of drugs and reducing the toxic and side effects of peptide injection .

Exenatide microspheres are produced by encapsulating Exenatide with biodegradable material polylactic acid-glycolic acid copolymer (PLGA), which can greatly reduce the injection frequency and reduce the pain and irritation at the injection site. Compared with injection, oral method has always been the goal of peptide drug researchers, but the bioavailability of peptide oral is low, and finding a reasonable and effective oral delivery strategy has become a difficult problem for the development of peptide drugs.

The activity of peptide drugs is easily affected by factors such as temperature and pH. The way of oral peptides needs to overcome the enzymatic hydrolysis of gastrointestinal digestive enzymes and the low permeability of intestinal cells to water-soluble substances. Therefore, in the development of oral peptide drugs, people usually use protease inhibitors and absorption enhancers to increase the oral bioavailability of peptide drugs. In addition, wrapping peptide drugs in polymer nanospheres can not only prevent the acidic environment of the gastrointestinal tract and digestive enzymes from degrading the drugs, but also achieve the purpose of long-term drug release or targeted therapy.

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