Birth, Development and Prospect of ADC Drugs

Antibody-drug conjugates (ADCs) are heating up home and abroad, but ADCs are not new. Let's take a brief look at the magical "magic bullet"-ADC.

Figure 1 ADC structure Image source: Biological latitude and longitude

01  The birth of ADC drugs and its tortuous development history

Figure 2 ADC development history

ADC (antibody-conjugated drug) is not a new concept. As the early 20th century, the Nobel Prize winner in medicine, German scientist Paul Ehrlich, had proposed the concept of ADC and called ADC drugs " Magic Bullet". But it wasn't until the 1950s that research on ADC drugs took off.

In 1958, Mathe firstly combined mouse antibody with guanidinium for the treatment of leukemia. Due to difficulties in immunogenicity and antibody preparation, ADC drugs were stalled for decades until the emergence of monoclonal antibodies in 1975, and later the emergence of humanized antibodies.

In 2000, the first antibody-drug conjugate was approved by the FDA for the treatment of acute myeloid leukemia, but limited by the problems of conjugation technology, targeting, and effectiveness, the antibody-drug conjugate was unstable in the blood, and the toxins were small molecules. Early release, with severe toxicity, led to the withdrawal of the first drug from the market in 2010.

After more than 10 years of precipitation, the new antibody-conjugated drug Adcetris was approved by the FDA for the treatment of Hodgkinson's lymphoma and systemic anaplastic large cell lymphoma in 2011, and the Kadcyla drug was approved by the FDA for HER2 positive in 2013. In breast cancer, the emergence of these two drugs has rekindled enthusiasm for ADC research.

02  Structure of ADC

Figure 3 ADC structure

ADC generally consists of three parts, monoclonal antibody drug targeting specific antigen, drug with cytotoxicity, linker linking antibody and toxin small molecule, this structure makes ADC drug more advantageous, making it compatible with traditional small molecule chemotherapy The powerful killing effect and tumor targeting of antibody drugs.

Among them, the antibody is responsible for finding tumor cells, and the linker allows the antibody to be entrained, bringing toxic small molecules into the tumor cells, and the small molecules are the sharp swords to kill the tumor cells. In addition, there are also antibodies that have anti-tumor effects at the same time.

Due to the large differences between different ADC drug designs, even if different drugs of the same target have different recognition sites, linking sites, linkers and small molecules to which they are attached, there will be great differences in drug toxicity. The factors that need to be considered in the design of ADC drugs are the factors that affect drug toxicity, including:

The selection of antibodies, a good antibody should have a clear target, high expression in tumor cells, low expression in normal tissues, support drug loading, and stable loading, good internalization by cells, good PK characteristics, and less non-specific binding;

Selection of connection sites: whether directional coupling can be performed, the number of connections that can be made, etc.;

Linker: It is stable in the internal circulation of the body to ensure that the toxin will not be released in the internal environment, but can be well released in the cell, and can be released by lysosomal enzyme cleavage, or released after the antibody is degraded;

Cytotoxic drugs: highly effective pharmacodynamics, non-immunogenic, and can be combined with linkers through modification.

03  ADC mechanism of action

Figure 4 The mechanism of action of ADC 

Image source: Baidu Encyclopedia

Internal circulation: Due to the poor oral bioavailability of ADC drugs, intravenous injection is used. Since the relative molecular mass of small molecule drugs is very small compared to that of antibodies, the circulation of ADC drugs in the body is similar to that of common antibodies. resemblance.

Binding to antigens: Antibodies recognize specific antigenic sites for targeted binding.

Internalization: After the antibody binds to the antigen receptor, the tumor cell membrane begins to undergo endocytosis, and the ADC drug is ingested into the cell. Drug release: After the conjugate enters the cell, the linker is cleaved by lysosome in the cell, the linker is cleaved, and the small molecule cytotoxic drug is released.

Play the role of drugs: Small molecule cytotoxics work to kill tumor cells, target cells undergo apoptosis, and when target cells die, an active cytotoxic load may also kill surrounding tumor cells, also known as the bystander effect.

04  The third generation of ADC

The first generation of ADC drugs: In the early 1990s, ADCs based on humanized and chimeric monoclonal antibodies were reported, and then until 2001, the world's first ADC drug-Pfizer's Mylotarg was approved by the FDA for the treatment of CD33 Targeted acute myeloid leukemia, however, the Phase III study of Mylotarg was launched in 2004 and found that it had fatal dry damage. Pfizer voluntarily applied for its delisting in 2010. This is the first generation ADC drug, which is different from the first generation ADC. Different from the use of mouse-derived monoclonal antibodies, the second-generation ADC drugs use humanized monoclonal antibodies represented by trastuzumab, which has improved targeting to tumor cells. At the same time, the second generation ADC uses more toxic small molecules, the coupling method is still similar to the first generation, and the stability of the linker still needs to be improved. The representative drugs are Adcetris produced by Seattle Genetics, launched in 2011, and Kadcyla, produced by Roche, launched in 2013. Kadcyla is also the first ADC drug approved for the treatment of solid tumors.

The evolution of third-generation ADC drugs is mainly due to the development of site-specific conjugation technology. More precise conjugation technology can control the position and number of highly active drug molecules conjugated on the antibody, with higher homogeneity and improved drug Purity, quality control, etc. It ensures that the DAR is stable and controllable, and reduces the toxicity of the drug.

05  Several coupling techniques of ADC

1. Conjugation of Lysine Residues

This method attaches the payload to the lysine residue of the antibody through a linker containing an active carboxylate site.

2. Modified disulfide bond reduction cysteine coupling and

Taking IgG1 as an example, there are 4 pairs of interchain disulfide bonds that can be reduced, and 8 cysteine sulfhydryl groups are obtained after reduction. Groups such as maleimide on Linker can react with sulfhydryl groups to form stable conjugates. The DAR value of the final product can be optimized by the degree of disulfide reduction

3. Unnatural amino acid conjugation

This method introduces the unnatural amino acid into the antibody by constructing an unnatural amino acid expression system (for example, using tRNA carrying the unnatural amino acid, etc.), and connects with the linker at the unnatural amino acid site.

4. Enzyme-catalyzed coupling

In this method, the specific amino acid sequence on the antibody is recognized by the enzyme, and the corresponding site is modified to generate a conjugation site.

5. Coupling via transpeptidase-mediated transpeptidation

This method relies on the catalytic properties of the transpeptidase. Using this feature of Sortase A, various types of molecules can be linked to oligo-G to achieve conjugation to specific sites of antibodies.

6. Coupling via MTGase (microbial transglutaminase)-mediated transpeptidation

The primary amine-containing linker is covalently attached to the primary amine of the specific glutamine (Q295) of the deglycosylated antibody using an MTG enzyme-catalyzed transpeptidation reaction, since each heavy chain of the antibody has one binding site, Therefore, the DAR value of ADC molecules obtained by this method is fixed at 2.

7. Conjugation by engineering the N-glycan on the aspartic acid residue of the antibody

06  ADC Drug Outlook

Due to the advantages of clear targets, mature technology, and good selectivity, antibody-drug conjugate research will still be a hot spot in the future. ADC is like a 5G track in communication. It is a challenge and an opportunity for domestic companies to overtake in this curve. Although the ADC has been developed for decades, there is still huge room for improvement.

Existing studies have shown that the proportion of ADC compounds delivering effector molecules to target cells is far less than 1%, but the targeting of drug delivery is still much higher than the traditional systemic drug delivery mode, and the incidence of adverse reactions is significantly lower than that of traditional drugs. , it can be seen that ADC still has many shortcomings in some places, but it is more of an advantage over traditional drugs, which means that whoever can overcome the existing shortcomings can seize the first opportunity in the ADC market.