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Release date:2019/10/30 17:21:08
Antibody-drug conjugates (ADCs) are a new class of targeted drugs consisting of "mAbs, cytotoxic drugs, and linkers that link the two." The ADC was originally designed to increase the effectiveness of chemotherapy and reduce its toxicity. Since the antibody is targeted (can recognize cancer cell surface antigens), the cytotoxic molecules can be selectively "transported" directly into the tumor cells to exert an anti-cancer effect while avoiding effects on healthy cells. 
 
So, how is the ADC drug designed? What is the principle of action? What is the current status of clinical application and development? Which of the research drugs may be approved for marketing in the future?

1. The Design Element of ADCs

As described above, the components that make up the ADC include tumor antigen-specific monoclonal antibodies, stable chemical linkers, and potent cytotoxic molecules (also called payloads). There are many important factors to consider when designing an ADC.

Image source:  The Lancet

a) Antibody and target antigen

The desirable properties of the ADC antibody portion include: 1) minimal immunogenicity; 2) high affinity and avidity for tumor antigen, and efficient internalization (ADC-target antigen complexes need to be internalized by receptor-mediated endocytosis, allowing them to release potent cytotoxic loads in cells); 3) has a longer circulating half-life.
In terms of specificity, an ideal target antigen needs to have two characteristics at the same time: 1) high expression on the surface of target cells; 2) low expression in healthy tissues. In addition, the ideal shedding of the ADC should be as small as possible to prevent the free antigen from binding to the antibody in the circulation.
 
b) Cytotoxic Payload

The cytotoxic payload (molecule/drug) is the final effector component of the ADC. The toxic payload of the ADC can target either DNA or tubulin.

Viral molecules that target DNA include duocarmycins, calicheamicins, pyrrolobenzodiazepines (PBDs), and SN-38 (active metabolites of irinotecan). Among them, the action mechanism of calicheamicins is to induce double bond cleavage, and the action mechanism of duocarmycins and PBDs is to cause DNA alkylation.

The effect of tubulin inhibitors MMAE (auristatins monomethyl auristatin E) and MMAF (monomethyl auristatin F) is to inhibit microtubule polymerization, resulting in G2/M phase cell cycle arrest.

The basic parameters for selecting an effective toxic payload for the ADC include conjugating, solubility, and stability. The structure of the selected toxic molecule should be such that it can be coupled to a linker. In addition, the water solubility of the toxic molecule and the long-term stability in the blood are important because the ADCs are prepared in an aqueous solution and administered intravenously.

c) Linker

The linker is responsible for linking the cytotoxic payload to the mAb and maintaining ADC stability during the systemic circulation. The chemical nature of the linker and the conjugating site play a crucial role in the stability, pharmacokinetic and pharmacodynamic properties of the ADC, as well as the therapeutic window. 
Biochempeg has developed a series of peptides with discrete PEG chain which mitigates aggregation & immunogenicity in ADC. 

An ideal linker must have sufficient stability to ensure that the ADC molecules do not break apart early, safely circulate through the bloodstream, and reach the target site; they must also be able to break quickly during internalization to release toxic payloads. According to the release mechanism of the load, currently available linkers are classified into two types: cleavable and noncleavable. The former relies on physiological environment to release payloads. A noncleavable linker is a non-reducible bond with an amino acid residue in a mAb and is, therefore, more stable in the blood; such a linker (such as a thioether linker) is dependent on the lysosomal degradation of the mAb to release its payload.

The conjugating characteristics of the connector are critical to controlling the therapeutic window of the ADC. The drug's drug to antibody ratio (DAR) or the amount of toxic drug attached to the mAb determines the potency and toxicity of the ADC. Although high drug loading can increase the potency of the ADC, it also increases off-target effects. In order to overcome the ADC drugs that produce various DARs in the production process, some studies have adopted innovative methods of site-specific conjugating to reduce variability, improve conjugating stability and pharmacokinetic properties, and ultimately produce more ADC drugs. 

2. Action Mechanism of ADCs
Image source:  The Lancet

Briefly, the action mechanism of ADC is divided into five steps: 1) the ADC binds to the antigen on the target cell; 2) the ADC-antigen complex is internalized by endocytosis; 3) the ADC degrades in the lysosome; 4) the cytotoxic payload (drug) release and function; 5) target cell apoptosis.

Because of the low oral bioavailability of ADCs, such drugs are administered by intravenous injection. ADCs circulating in the blood first find and bind their target cells. After binding, the ADC-antigen complex is internalized by clathrin-mediated endocytosis to form an early endosome containing an ADC-antigen complex (Fig. 2A). The early endosome eventually develops into a secondary endosome prior to fusion with the lysosome. For ADCs with cleavable linkers, the cleavage mechanism (eg hydrolysis, protease cleavage, disulfide bond cleavage) may occur either in the early endosome or in the secondary endosome, but don’t occur in Lysosomal transport phase. However, for ADCs with noncleavable linkers, the release of toxic payloads (drugs) is achieved by complete protein degradation in lysosomes: proton pumps in lysosomes create an acidic environment that promotes proteases (eg cathepsin-B, plasmin) mediated proteolytic cleavage.

3. Ten FDA Approved Antibody Drug Conjugates

To date, a total of ten ADCs have been approved by the FDA, including: ado-trastuzumab emtansine (Kadcyla™), brentuximab vedotin (Adcetris™), inotuzumab ozogamicin (Besponsa™), gemtuzumab ozogamicin (Mylotarg™) , Moxetumomab pasudotox (Lumoxiti
), polatuzumab vedotin-piiq (Polivy), Enfortumab vedotin (Padcev), Trastuzumab deruxtecan (Enhertu), and belantamab mafodotin-blmf (Blenrep). Table 1 summarizes the design, approved indications, doses, and adverse events of these drugs.
 
Drug Maker Condition Trade name Approval Year
Gemtuzumab ozogamicin Pfizer/Wyeth relapsed acute myelogenous leukemia (AML) Mylotarg 2017;2000
Brentuximab vedotin Seattle Genetics, Millennium/Takeda relapsed HL and relapsed sALCL Adcetris 2011
Trastuzumab emtansine Genentech, Roche HER2-positive metastatic breast cancer (mBC) following treatment with trastuzumab and a maytansinoid Kadcyla 2013
Inotuzumab ozogamicin Pfizer/Wyeth relapsed or refractory CD22-positive B-cell precursor acute lymphoblastic leukemia Besponsa 2017
Moxetumomab pasudotox Astrazeneca  adults with relapsed or refractory hairy cell leukemia (HCL)  Lumoxiti 2018
Polatuzumab vedotin-piiq Genentech, Roche relapsed or refractory (R/R) diffuse large B-cell lymphoma (DLBCL) Polivy 2019
Enfortumab vedotin Astellas/Seattle Genetics adult patients with locally advanced or metastatic urothelial cancer who have received a PD-1 or PD-L1 inhibitor, and a Pt-containing therapy Padcev 2019
Trastuzumab deruxtecan AstraZeneca/Daiichi Sankyo adult patients with unresectable or metastatic HER2-positive breast cancer who have received two or more prior anti-HER2 based regimens Enhertu 2019
Sacituzumab govitecan Immunomedics adult patients with metastatic triple-negative breast cancer (mTNBC) who have received at least two prior therapies for patients with relapsed or refractory metastatic disease Trodelvy 2020
Belantamab mafodotin-blmf GlaxoSmithKline (GSK) adult patients with relapsed or refractory multiple myeloma Blenrep 2020
FDA Approved ADCs, Table Source:  Wikipedia

4. Antibody-Drug Conjugate Clinical Trials

In addition to the ten FDA approved ADC drugs, a large number of ADCs are currently
under clinical development, and the indications include various hematological malignancies as well as solid tumors. Table 2 lists some promising antibody drug conjugates in clinical trials. (Find the full list of ADCs in clinical trials here.)


ADC clinical Trial
Promising antibody drug conjugates in clinical development. Image source:  The Lancet

5. Conclusion

As one of the research and development hotspots in the field of medicine, more than 100 ADCs are currently in different stages of clinical development, and there are hundreds of ongoing clinical trials. According to the authors of the review, with reference to ClinicalTrials.gov and PubMed, the number of ADC-related clinical trials and publications has increased this year compared to 2018. 
Biochempeg provides the most comprehensive media for conjugation research.

In summary, scientists believe that as technology advances, ADCs continue to iterate, and the choice of targets, linkers, and Cytotoxic Payloads are gradually improving. At the same time, with the development of immunotherapy, the choice of developing ADC Conjugation therapy is becoming more and more extensive. Therefore, in the future, ADC-based treatment options are expected to be used earlier for the treatment of certain types of cancer patients. 

Related Articles:
[1] History and Development of Antibody Drug Conjugates (ADCs)

[2] ADCs for Clinical Research in the Global Market
[3] ADCs Against Cancer: Clinical Landscape and Challenges
[4] 
Innovative Linker Technology for Antibody Drug Conjugates (ADCs)
[5] 
Cleavable vs. Non-Cleavable Linkers in Antibody-Drug Conjugates
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