Antibody-drug conjugates (ADCs), which use antibodies to selectively deliver cytotoxic drugs to the tumor site, are currently in rapid development. To date, a total of twelve ADCs have been approved by the FDA, they are Kadcyla, Adcetris, Besponsa, Mylotarg, Lumoxiti, Polivy, Padcev, Trodelvy, Enhertu, Blenrep, ZYNLONTA and Tivdak. In addition, there are currently more than 80 ADCs under investigation being evaluated in approximately 150 active clinical trials. Despite the increasing popularity of ADCs, scaling up their therapeutic index (better efficacy and less toxicity) remains a challenge.
Drug research and development must achieve the unity of clinical effectiveness and safety, both of which are indispensable. Mylotarg's history from listing to delisting to re-listing fully demonstrates the importance of safety in drug research and development. Major pharmaceutical companies start from the various components of ADC in order to obtain ADC products with high clinical value. However, ADC drugs, as a very complex entity, cannot simply combine several elements to bring clinical benefits to patients. The current fixed-point coupling technology used in the latest generation of ADC drugs cannot guarantee the development of products with good curative effect. Representative examples such as vadastuximab talirine developed by Genentech and MEDI4276 developed by AstraZeneca/MedImmune have all failed due to safety issues. It can be seen that the development of ADC still faces many challenges.
Complex Pharmacokinetic Characteristics
After ADCs are administered (mainly intravenously), they mainly exist in the systemic circulation in the form of intact ADC, naked antibody and free form of cytotoxic payload, and are dynamically changing as ADC recognizes target antigen, internalizes and decouples these three forms of existence. Typical ADC pharmacokinetics are characterized by a continuous decrease in the concentration of structurally intact ADC and naked antibody as the ADC is internalized and the antibody is cleared. Factors that influence antibody clearance include the mononuclear phagocyte system and Fc receptor (FcRn) -mediated recycling. By binding to ADCs in endocytic vacuoles, FcRn exports ADCs to the extracellular compartment for reuse. As a result, antibodies, including structurally intact ADCs and naked antibodies, typically have longer half-lives than traditional small molecule drugs. As for the free form of cytotoxic payload, it is mainly metabolized in the liver and excreted in the urine or feces, and this process may be affected by drug interactions and liver and kidney function impairment. All of these factors, together with individual patient differences, make it difficult to establish PK and PD models to describe the clinical characteristics of ADCs and assist in the design of novel ADCs.
ADC Can't Demonstrate Its Benefit
Despite the increasing number of ADC approvals, there are still challenges for ADCs that have demonstrated superior safety and efficacy in the clinic. An unexpected challenge faced by many developers during clinical evaluations is the inability of ADCs to demonstrate benefit compared to controls, such as MM-302. MM-302 is an anti-HER2 monoclonal antibody conjugated to doxorubicin.
The Phase II HERMIONE trial (NCT02213744) was discontinued due to a lack of benefit from a placebo treatment. Another ADC that reported a similar situation was Rovalpitumab tesirine (RovA-T), encouraging results from the Phase I trial reported 18% ORR in assessable patients, 38% ORR in patients with high DLL3 expression (NCT01901653). However, safety and efficacy issues were raised due to the results of the Phase II trial TRINITY (NCT02674568), which did not meet the primary endpoint and reported high toxicity rates. The most common event in patients was pleural effusion, which was thought to be toxic in association with PBDE dimers. Ultimately, results from the Phase III trials TAHOE (NCT03061812) and MERU (NCT03033511) resulted in AbbVie completely discontinuing the development of RovA-T, with a lack of survival benefit compared to the control group.
The design of ADC drug
Unavoidable Toxic Side Effects of ADCs
One of the major challenges of ADCs is unavoidable toxic side effects, which is caused by the premature release of cytotoxic small molecules into the bloodstream. "Off-target, off-tumor"-related adverse events appear to dominate the toxicity profile of most existing ADCs. A meta-analysis of available data revealed that different drug-loaded small molecules have distinct toxicity profiles independent of the target antigen. Antibodies are an awkward vehicle for transport, difficult to navigate through the tumor's alleys, and it is estimated that only 0.1% of the drug reaches the tumor tissue. In order to ensure that the other 99% of highly toxic warheads do not bring systemic toxicity, the chemical link between the warhead and the antibody must be sufficiently stable. However, in order to release the warhead in the cell without being too stable, this obviously brings trouble to drug design.
The associated increased risk depends on the toxicity profile associated with cytotoxic small molecules. Off-target toxicities of maytansine (DM1) were hepatotoxicity and thrombocytopenia. MMAE was associated with the likelihood of peripheral neuropathy, neutropenia, and anemia. MMAF was associated with ocular toxicity. In terms of the metabolic characteristics of ADC, the hydrophobicity of ADC increases the clearance rate due to the high drug load of hydrophobic small molecules. An in vivo study involving a xenograft model compared the effect of a high dose of MMAE with different DARS (2, 4, and 8) conjugation against CD30 mab on ADC clearance. The results showed that the higher the drug loading, the higher the clearance rate. In this study, ADCs with a DAR of 8 were cleared fastest.
Of the 12 ADCs currently approved for marketing, the most common grade 3 or greater severe side effect is hematotoxicity, including neutropenia, thrombocytopenia, leukopenia, and anemia. Off-target toxicities such as hematotoxicity, hepatotoxicity, and gastrointestinal reactions may be related to the premature release of loaded drugs into the blood circulation, non-tumor tissues, or tumor microenvironment, and the subsequent effects of loaded drugs on healthy tissues. Such side effects are consistent with the effects of conventional chemotherapy drugs on rapidly proliferating healthy cells.
Tumor Targeting And Payload Release
Compared with small molecules of traditional chemical poisons, ADCs obviously have a much larger molecular weight because of the conjugated antibodies, which will limit the efficiency of ADCs to penetrate the tumor microenvironment and reach tumor cells. According to reports, only 2% of intravenously injected ADC drugs can finally reach tumor tissue, which is why the toxicity of drug loading needs to be considered when designing ADCs.
On the one hand, in order to reduce the possible systemic toxicity caused by most of the highly toxic drugs that do not reach the tumor tissue, the linker between the antibody and the drug should be stable enough. On the other hand, in order to ensure that the drug can be released smoothly after entering the cell , the linker cannot be too stable, so this needs to be weighed when designing the ADC.
After arriving at the tumor tissue, the ADC is internalized into the cell by forming an ADC-antigen complex. The ADC with an acidic cleavable linker is transported to the early endosome, and the ADC that requires protease cleavage is transported to the late endosome and lysosome. The high heterogeneity of target antigen expression has important implications for the antitumor activity of ADCs internalized into cells. When designing an ADC, the choice of a cleavable non-polar drug carrier can effectively cross the cell membrane, and better exert the "side-killer effect" of the ADC to kill the surrounding tumor cells that do not express antigens.
Aggregation of ADCs
In addition, ADCs are easy to aggregate. ADC aggregation leads to modifications that reduce its ability to bind antigens.
Protein aggregation is a major obstacle to ADC development. It can occur at every stage as well as during transportation and long-term storage. The aggregation is immunogenic. In addition, protein aggregation can lead to product loss. Overall, any chemical or physical degradation can lead to structural changes in ADCs and lead to excessive protein aggregation. There are various other factors that can cause aggregation, such as frequent freezing/thawing, high protein and salt concentrations, elevated temperature, or low pH. In addition, most payloads are hydrophobic, and binding the payload at a high DAR on the protein surface can lead to excessive protein aggregation, hindering the successful development of ADCs. In some ADCs, the payload binds to Cys residues after breaking existing disulfide bonds, and hydrophobic-hydrophobic interactions increase.
Drug Resistance of ADCs
Another challenge factor is drug resistance. The internalization of the payload and its efficacy are mainly affected by acquired resistance mechanisms, such as down-regulation of tumor cell antigen expression. Drug resistance occurs when a treatment fails or becomes less effective, or when tumor cells escape. Drug resistance can arise at the start of treatment or after drug treatment.
There are many mechanisms by which drug resistance arises, some of which include: down-regulation of antigen level expression, drug efflux pumps, endocytosis and migration, defective lysosomal function, altered signaling pathways, and dysregulated apoptosis.
Immunogenicity of ADCs
ADC is a synthetic biomacromolecular drug, which is also a foreign substance to the human body, and has the potential risk of inducing immunogenicity. With the increase in the types and wide application of macromolecular protein drugs, the related immunogenicity problems have gradually surfaced. Protein drugs have potential factors to induce immunogenicity, the consequences of which may affect efficacy and even be life-threatening. Antibody-drug conjugates (ADCs) have the same immunogenicity as antibody drugs, and their immunogenicity risks affect the safety and effectiveness of the drugs for patients, and may even bring fatal new diseases to patients due to ADA (anti-drug antibody) and endogenous protein cross. This must be assessed in clinical studies.
Along with the success and excitement of the ADC development process, there also have its complexities and limitations. This is an era of innovative drugs, with newer iterations of different technologies, and ADCs need more extensive clinical coverage and confirmation as more questions are studied in depth.
In the future, ADC drugs will have a huge potential in the anti-tumor market. Biopharma PEG is a worldwide leader of PEG linker supplier that offers a wide array of different ADC linkers to empower our customer's advanced research.
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 FDA Approved Antibody-Drug Conjugates (ADCs) Up To 2022
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 History and Development of Antibody Drug Conjugates (ADCs)
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 ADCs, A Highly Targeted Drug Therapy For Cancer