ADC: The "Magic Bullet" Road to Innovation

The two major treatment strategies for clinical cancer treatment are antibody-based immunotherapy and chemical drug-based chemotherapy.
After years of research, both therapies have mature treatment systems
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Chemotherapy includes antitumor antibiotics such as doxorubicin that interfere with transcription by directly destroying DNA or intercalating DNA, alkylating agents such as nitrogen mustard that inhibit tumor proliferation, and antibody metabolizers such as methotrexate; immunotherapy includes targeted antibodies, adoptive cells There are five categories of therapies, including oncolytic viruses, cancer vaccines, and immunomodulators.
Among them, CAR-T and other promising therapies have also emerged
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However, both types of mainstream therapies have their own defects.
.
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In the chemotherapy treatment system, although small molecule chemical drugs have a high killing effect on cancer cells, their selectivity is poor, and they attack normal cells and tumor cells almost indiscriminately.
The negative effects are serious, and it can damage important tissues and organs such as the liver, kidney, nerve, and heart
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When immunotherapy is used in clinical practice, although therapeutic antibodies are highly targeted, their therapeutic effects on solid tumors are limited due to their large molecular weight
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Therefore, in order to solve the shortcomings of the two mainstream cancer therapies, the researchers imagined whether the high-efficiency killing of chemotherapy could be combined with the precise targeting of immunotherapy
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The initial inspiration for ADC antibody-drug conjugates came from the magic bullet idea proposed by Nobel Prize winner Paul Ehrlich: according to the special structural characteristics of pathogens, drugs that only kill pathogens and do not affect normal tissues and cells can be found and developed, and if chemotherapeutic drugs are used Attached to an antibody, the selectivity and efficacy of the treatment can be improved
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Under the guidance of this idea, the antibody-drug conjugate (ADC) is to link the biologically active small molecule drug to the monoclonal antibody through a chemical link, and the monoclonal antibody acts as a carrier to transport the small molecule drug in a targeted manner.
into target cells
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1 The budding stage (1910-1980) Paul Ehrlich began with the idea of ​​a cure for syphilis, and in 1913 first proposed the concept of a "magic bullet"
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Based on this, the first antibody-cytotoxic conjugated drug came out in 1958, combining antibodies with methotrexate for the treatment of leukemia, but it was unsuccessful
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The advent of hybridoma technology in the 1970s put ADC research and development on the right track, and ADC drugs entered animal experiments for the first time
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2 Exploring stage (1980-2000) Among the first-generation ADC drugs, antitumor drugs such as mitomycin C and vinca alkaloids are mainly coupled with mouse monoclonal antibodies through non-cleavable linkers (amide or succinimide).
associated with severe immunogenicity problems
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Following the creation of humanized monoclonal antibody technology by Greg Winter in the late 1980s, Mylotarg became the first approved ADC drug in 2000, but a phase III study found that it had no more significant clinical advantages and had severe liver toxicity.
Withdrawn from the market; it was re-approved by the FDA in 2017 after Pfizer adjusted its dosing regimen
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3 Maturity stage (2000-present) Representative drugs of the second generation ADC are Adcetris (Seattle Genetics) launched in 2011 and Kadcyla (Roche) launched in 2013, using humanized monoclonal antibodies represented by trastuzumab As well as more efficient small molecule drugs, they jointly drive the upgrading and iteration of ADC technology
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However, the coupling method is still similar to the first generation, and the stability of the linker still needs to be improved
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In addition, due to off-target toxicity, the presence of unbound antibodies, and the large drug-to-antibody ratio (DAR) causing ADC aggregation or rapid clearance, most current second-generation ADCs show a narrow therapeutic window, which is difficult to meet the needs of patients
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The research is never ending, and soon the third generation ADC drugs enter people's field of vision
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It utilizes the site-specific binding of small-molecule drugs to monoclonal antibodies to ensure antibody-drug conjugates with clear DARs; in addition, the optimization of antibodies, linkers, and small-molecule drugs can improve drug activity, stability and pharmacokinetics.
The pharmacology is greatly improved, the drug toxicity and coupling shedding speed are lower, and the cell viability is high at low antigen levels
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Since 2019 alone, 8 drugs have been launched, expanding the therapeutic indications to bladder cancer and gastric cancer
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There are currently 13 ADC drugs on the market in the world.
The treatment areas are mainly focused on hematological tumors and solid tumors, and they are mainly used for the late-line treatment of patients, including advanced, relapsed/refractory and metastatic tumor indications.
It is expected to continue to be a research hotspot in the field of anticancer in the next few years
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Simply put, ADC is a combination therapy of "monoclonal antibody + chemotherapy", and ADC drug is a combination of three elements of "antibody + linker + effector molecule", thus taking into account the selective characteristics of monoclonal antibodies and the killing ability of chemotherapy
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1 ADC drug action mechanism ADC first enters the blood circulation through intravenous injection, and the antibody recognizes and binds to the specific antigen on the cell surface
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Then, under the mediation of cell surface receptors that specifically bind to cell surface antibodies, the ADC drug is endocytosed and entered into lysosomes for degradation, and the effector is released in a sufficient amount in the cell in a highly active form to play a role.
Directly kills or induces apoptosis
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2 Five core technologies Therefore, from the perspective of mechanism of action, target selection, antibody modification, linker design, selection of toxic small molecules, and coupling methods have become the five core factors affecting ADC drug design
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1) Selection of targets Targets are one of the important factors in determining ADC indications and are the basis for ADC design
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To become a target, the following conditions must be met, that is, high expression in tumor cells and low expression in ordinary cells, localization on the cell surface, and internalization function
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The three are indispensable, which is the guarantee for the precise role of ADC, full contact, and full function of entering the cell.

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2) Antibody transformation There are five main types of antibodies: IgM, IgD, IgG, IgE and IgA
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Among them, IgG1, a subtype of IgG, is the most commonly used antibody for cancer immunotherapy
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A typical IgG1 antibody consists of two heavy chains and two light chains, and is divided into two regions
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The Fc region is responsible for antibody recognition by immune effectors; the Fab region is responsible for providing antigen specificity
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Why does the IgG1 scaffold stand out? From the table data comparison, it can be seen that IgG1 has relatively strong antibody-dependent cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) and is easy to produce
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And long half-life, it is an ideal choice for ADC drugs
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3) Design of linker Usually, the requirements of linker design need to be stable and release the effect factor when it reaches the appropriate position
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The linker is divided into cleavable and non-cleavable types: Cleavable linker: The linker mainly utilizes the environmental differences between the blood system and tumor cells.
When the environment changes, the linker breaks and releases the drug; the cleavable linker can be divided into three types major subclasses: acid-dependent bond breakage, redox-dependent bond breakage, and enzymatically cleavable bond breakage
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Non-cleavable linker: The linker is more stable in plasma, and there is evidence that the release of the linker payload occurs mainly in lysosomes after ADC internalization, with low systemic toxicity
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4) Selection of toxic small molecules As the key to the lethality of ADC, there are two main mechanisms: one is to cause DNA damage; the other is to inhibit tubulin polymerization
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It has the characteristics of strong toxicity, good water solubility and stability, and a group that can be coupled with linker.

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5) Conjugation method There are many drawbacks in the early ADC random coupling method, and site-specific conjugation has gradually developed into a mainstream technology, mainly in the following four categories: a.
The introduction of reactive cysteine ​​through genetic engineering into specific points of the antibody to insert half cystine residue, and then the hydroxyl group on cysteine ​​is coupled to toxin to form a site-specific antibody-drug conjugate
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b.
Introduction of unnatural amino acids Unnatural amino acids are introduced into specific sites of any target protein in the organism.
The residues on these unnatural amino acids can be combined with linkers to form site-specific ADCs
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c.
Enzymatic method By inserting specific amino acid markers into the antibody sequence that can be recognized by some specialized enzymes, the toxin can be selectively linked to the antibody
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d.
Disulfide bond reduction method By reducing the disulfide bond of the monoclonal antibody itself, a dibromo reagent is used to react with the reduced interchain disulfide to provide a re-bridged mAb
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1 Technical advantages As a relatively advanced and mature targeted drug technology in current anti-cancer therapy, ADC drugs show advantages in many aspects: 1) Strong therapeutic efficacy: the bystander effect exerts good lethality, and multiple targets show the potential of pan-cancer therapy
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2) The specific recognition rate is low: the drug is directly delivered to the target site, with high targeting
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3) Weak immunogenicity: Humanized carriers are often used, which is not easy to produce drug resistance
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4) Long action cycle time: prolonged action time in serum
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5) Low toxicity and side effects: low toxicity to non-target cells, high safety treatment window
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2 Challenges ADC drugs are still in a stage of vigorous development as a whole, and they are facing huge challenges both in the research and development process and in clinical applications
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Off-target toxicity: Due to the difficulty of avoiding off-target during delivery, toxic small molecules will be released into the blood circulation in advance, causing the risk of poisoning
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Protein aggregation: a major hurdle in ADC development
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Repeated freezing and thawing and changes in temperature, concentration and pH may cause structural changes in ADCs to induce protein aggregation, thereby affecting immunogenicity and product loss
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Drug resistance: The drug resistance of ADC drugs is related to the down-regulation of target antigens, cell cycle, signaling pathways, and gene mutations
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Clinical adverse reactions: In the clinical trials of ADC drugs, various adverse reactions have occurred, which are related to various factors such as their target properties, connector selection, and number of toxic carriers.
1 Market 1.
Today, the development of ADC drugs is in a state of blowout , the number of related patents, R&D projects and clinical drug experiments are all showing an upward trend
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2.
Its future market expectations also show a high development trend
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According to data model estimates, the global ADC drug market is huge, with total sales exceeding $16.
4 billion by 2026
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2 R&D Prospects 1.
Target Diversification At present, there is a high repetition of target selection in ADC research and development.
Most drugs are concentrated on a few popular targets, especially the HER2 target drugs under development far outnumber others
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But in fact, there are more than 200 targets under research and development in the world, the choices are very diverse, and the potential of the targets needs to be further explored
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2.
Re-selection of indications As far as indications are concerned, ADC drugs are highly concentrated in the field of disease research and are basically oriented towards solid tumors
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However, relevant studies have shown that ADC drugs have great potential for non-cancer indications, and they have found medicinal power in autoimmune diseases, bacterial infections, liver fibrosis, viral infections, etc.
, which opens up new directions for their research and development
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About the author Drug-seeking truth group They are students of Nanjing University's new drug research and development strategy course.
This article is jointly completed by four students, Lv Jingwen, Ma Shanchi, Tan Xi, and Wang Yan, in the 11 groups of the course
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They are full of vigor, their fighting spirit is high, they are down-to-earth to learn the knowledge of new drug research and development, and they firmly believe that I love my teacher and I love the truth more
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They are the successors of the new generation of pharmaceutical industry, and they are the future light in the field of new drug research and development! References: [1] ADCs inspire large pharma investments.
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