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Drug Conjugation Technology

In addition to the traditional chemotherapy, the main methods of treating tumors are usually platinum drugs, alkylating agents, paclitaxel and other cytotoxic molecules. Although these chemotherapeutics can effectively prevent or slow down the growth of malignant tumors and are still widely used as first-line treatments, a large part of chemotherapeutic drugs do not have the ability to target and recognize tumor cells, and are usually accompanied by systemic toxicity.

How to improve efficacy while reducing side effects? Drug conjugation technology may be a solution. Through coupling technology, on the basis of cytotoxic molecules, they are coupled with tumor-targeting components to produce a complex of multiple molecules linked together to make cytotoxic molecular targeted therapeutic drugs, which can be effective to solve the above problems.

Targeted therapy that selectively binds and exerts effects through molecules expressed on the surface of cancer cells is a major advancement in cancer treatment, because compared with traditional cytotoxic drugs, targeted therapy has higher efficacy and better tolerability.

In cancer treatment, there are three targeted methods for enhancing the specificity and anti-tumor activity of cancer treatment. Targeting agents can be designed to inhibit proteins expressed by tumor cells, such as receptors or enzymes; another method is to combine effector molecules (such as ADCs, double antibodies or CAR-T) with molecules overexpressed on the surface of tumor cells and cooperate with them. Inhibit tumor cell division, while providing cytotoxic payload or stimulating tumor-oriented immune response; the third method is to use peptide-conjugated drugs (PDC) to drive the enrichment of tumor cell toxic payload.

Antibody-drug Conjugate (ADC)

Antibody-Drug Conjugates (ADCs) are a new class of highly potent biological drugs built by attaching a small molecule anticancer drug or another therapeutic agent to an antibody, with either a permanent or a labile linker.

The first-generation ADC is represented by Pfizer’s Mylotarg, which has the disadvantages of low antigen specificity, strong toxicity, unstable linker, and short half-life. The second-generation ADC is represented by Seattle’s Adcetris and Roche’s Kadcyla. Compared with the first-generation ADC, the second-generation ADC has the advantages of strong antigen specificity, higher drug efficacy, and lower immunogenicity. However, it still has strong toxicity, drug resistance, and high DAR (drug-to- antbody ratio, antibody drug loading rate) value and other issues. The third-generation ADC is site-specifically coupled and adopts a new target to have a homogeneous and single ADC. The cytotoxic molecules are more effective, with higher accuracy and lower toxicity.

Peptide Drug Conjugates (PDC)

Many targeted drugs currently in clinical use are based on monoclonal antibodies. However, the therapeutic application of ADC is limited by its physicochemical and pharmacodynamic properties. PDC uses peptides as tumor targeting carriers with many advantages. Compared with ADCs, they are easy to synthesize, and structural modifications can be easily introduced, supporting reasonable drug design to improve bioavailability, affinity and stability. In addition, peptides have low immunogenicity.

There are three basic elements of PDC: peptide, cytotoxin and linker. The first is the choice of peptides. In recent years, with the rapid development of technologies such as proteomics, phage display, and peptide solid-phase synthesis, more and more new peptides have been discovered and rationally designed, which has greatly promoted the development of PDC. Peptide molecules used for PDC can generally be divided into cell penetrating peptides (CPPs) and cell targeting peptides (CTPs). The former can transport drugs across the cell membrane, and the latter can specifically interact with target cells. On the receptor binding. It has been reported in the literature that CPP-drug conjugates can enter cells through transport or receptor-mediated non-endocytic transport pathways that have nothing to do with energy.

Commonly used CPPs include trans-activator of transcription (TAT), transportan, penetratin and their derivatives or other peptides that have the ability to penetrate membranes. Commonly used CTPs include arginine-glycine-aspartate (RGD) constant series peptides, luteinizing hormone releasing hormone (LHRH)-like peptides, and new tumor-targeting peptides screened by phage display technology.

The second is the choice of cytotoxic drugs. The cytotoxic drugs used for PDC coupling are usually chemotherapeutic drugs, such as paclitaxel, DOX, CTP, etc., which exert anti-tumor effects by interfering or blocking the cell proliferation process. However, the classic and commonly used chemotherapeutic drugs tend to cause damage to normal cells and tissues due to their low selectivity and poor tumor targeting ability. The formation of PDC can improve the targeting of these drugs to tumor tissues and reduce their distribution in normal tissues, thereby reducing adverse reactions.

Then last is the choice of linker. The linker is an effective bridge connecting the peptide and the drug, and the linker will affect the function of the peptide or drug. Similar to ADC, PDC linkers are divided into non-breakable and breakable types. The ideal linker should have the characteristics of low molecular weight, appropriate length, appropriate stability and polarity.

Based on the above three elements, PDC has the following advantages:

In terms of technology, PDC is a new type of conjugated drug. Its design principle is partially similar to that of ADC. It is mainly used for drug delivery and tumor targeting. The difference is that the antibody component in ADC is used as a peptide that can be used as a targeting ligand. It is precisely because PDC replaces antibodies with peptides. Compared with ADC, PDC has a smaller molecular weight. Therefore, PDC drugs have better permeability of blood vessels, tissues and cells, and are easy to penetrate deep into the tumor, and do not cause an immunogenic reaction. In addition, PDC drugs can be quickly eliminated by the kidneys, and are less toxic to bone marrow and liver; on the other hand, unlike phages, adenoviruses or other microorganisms dedicated to transporting drugs, PDC drugs do not contain infectious substances.

In terms of technology, PDC targets tumor cells with a peptide chain of about 10 amino acids, and changes the conjugated hydrophobicity and ionization by controlling the amino acid sequence of the peptide chain, both of which affect the bioavailability in vitro and in vivo. Compared with the complex process of antibody production, PDC can be prepared on a large scale by solid-phase synthesis. The synthesis and purification, storage and quality control of PDC are relatively easy, and the stability in vivo and in vitro is better, which can effectively reduce large-scale production. the cost of.

FDA approved Peptide-Drug Conjugate

In January 2018, Lutathera developed by Advanced Accelerator Applications S.A, a subsidiary of Novartis, was approved by the FDA. Lutathera is the world’s first PDC drug. Lutathera (lutetium Lu 177 dotatate) is a peptide receptor radionuclide therapy (PRRT) indicated for the treatment of somatostatin receptor positive gastroenteropancreatic neuroendocrine tumours.

In February 2021, Oncopeptides announced that Pepaxto (melphalan flufenamide, also known as melflufen) has been approved by the FDA. Melflufen is an anti-cancer PDC drug targeting aminopeptidase for the treatment of multiple myeloma. It can couple an alkylating agent with a peptide targeting aminopeptidase. Aminopeptidase is present in all human cells and is overexpressed in many tumors, including multiple myeloma.

Small molecule-drug conjugates (SMDC)

Similar to the ADC structure, there are three key factors: small molecule targeting ligands, cytotoxic molecules and linkers. Its mechanism of action is also similar to that of ADC, but SMDC can disperse into tumor tissues more quickly and evenly, with low cost and no immunogenicity. At present, SMDC still has no marketed drugs. The main difficulty in its research and development is that small molecule ligands are difficult to obtain, which restricts its development.

Radionuclide Drug Conjugates (RDC)

Radionuclide drug conjugates (RDC) are mainly composed of targeting ligands, linkers, chelates and cytotoxic/imaging factors (radioisotopes). The biggest difference between RDC and ADC and SMDC is the drug load. RDC is no longer loaded with small molecules, but radionuclides. Using different medical nuclides, they can have different functions such as imaging or treatment, and some nuclides have both functions. Based on these functions, RDC will likely become a new technology for tumor diagnosis, imaging and treatment.

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