Antibody-drug Conjugates: A Comprehensive Guide

1. What are Antibody-drug Conjugates?

Antibody-drug conjugates (ADCs) are new biological drugs, which combine the high specificity of monoclonal antibody drugs with the high activity of small molecule cytotoxic drugs to improve the targeting of tumor drugs and reduce the toxic and side effects.

2. Antibody-drug Conjugates Structure

ADCs consist of three main parts: antibodies responsible for selectively recognizing cancer cell surface antigens, payloads responsible for killing cancer cells, and linker used to connect antibodies with Payloads.

Fig.1 the structure of ADCs ^[1]

3. How do Antibody-drug Conjugates work?

First, the ADCs injected into the body bind to the target cell antigen after many obstacles, and the ADCs-antigen complex formed by it enters the cell through reticulin-mediated endocytosis. After the complex enters the cell, lysosome fusion with endosome causes linker cleavage in the cell, releasing and activating small molecule cytotoxic drugs. After the drug is released into cytoplasm, it can be inserted into DNA or inhibit microtubule polymerization, killing tumor cells and causing apoptosis of target cells. When the target cell dies, the active payloads may also kill the surrounding tumor cells, also known as Bystander effect.

Fig.2 the working mechanism of ADCs ^[1]

4. Antibody-drug Conjugates Components

4.1 Selection of Targets

The successful development of ADCs depends on the specific binding of antibodies to targets. The ideal targets of ADCs are high expression on the surface of tumor cells, low expression or no expression in normal cells, and minimizing toxicity of on-target and off-tumor. In addition to specificity and sufficient expression, the best target should also cause an efficient internalization effect.

Fig.3 Target for ADCs in development and in the clinic and developed

4.2 Selection of Antibodies

Antibodies should have high antigen affinity and long circulating half-life, so that they can specifically enrich cytotoxins in tumor sites. Considering the half-life, structural stability, Fc fragment immune function and coupling convenience of different IgG types antibodies, ADCs mostly choose IgG1 but seldom use IgG4. According to the immunogenicity of antibodies, they can be divided into fully humanized antibodies, humanized antibodies and chimeric antibodies. In order to reduce immune response, completely humanized or humanized antibodies are more selected.

4.3 Selection of Payloads

Toxin molecule (Payload) is the key factor for the success of ADCs research and development. Because ADCs have to go through multiple steps from entering human body to finally releasing cytotoxic agents, considering the efficiency of each step, payloads should have high anti-tumor activity, so toxic molecules with nanomolar level (IC50 value is 0.01-0.1 nM) are the appropriate payloads. In addition, payloads must have suitable functional groups that can be coupled, strong cytotoxicity, proper hydrophilic-hydrophobic balance, and high stability.

4.4 Selection of Linker

Linker, as the bridge of ADCs, connects with antibodies through cleavable connectors or non-cleavable connectors. Linker needs exquisite design, which not only needs stability to prevent chain breaking in the physiological state but also has the characteristics of high release efficiency in specific parts.

5. Advantages of Antibody-drug Conjugates

Compared with traditional completely or partially humanized antibodies or antibody fragments, ADCs have a higher theoretical curative effect because they can release highly active cytotoxins in tumor tissues. On the other hand, ADCs have a higher tolerance and lower side effects than fusion protein. They can accurately identify targets and not affect normal cells, which greatly improves the efficacy and reduces the toxic and side effects, so they have attracted the attention of people in the field of pharmaceutical research and development.

6. Do Antibody-drug Conjugates have any weaknesses?

The structure of ADCs is complex and the design is diversified, which brings difficulties to the production and quality control related to CMC research. At the same time, the superposition of ADCs and complex biological process in vivo also makes non-clinical research and clinical research face multiple challenges. Because the production of ADCs mostly involves highly active cytotoxic drugs, it requires high hardware equipment, process design and personnel training, and requires a large amount of capital investment and technical reserves, so the production threshold is high.

At present, there are three main challenges and opportunities in the development of ADCs:

6.1 Instability of Linker

This instability can lead to premature release of payload into blood, and lead to nonspecific uptake and miss-target toxicity of ADCs.

6.2 Nonspecific Endocytosis

Hydrophobicity will promote the aggregation and nonspecific endocytosis of ADCs, especially ADCs with high DAR (drug-antibody ratio), resulting in miss-target effect. ADCs with high DAR will also be cleared by other cells with nonspecific and strong endocytosis ability. So optimizing DAR is also an important strategy to improve TI (therapeutic index).

6.3 Receptor-mediated Uptake Mechanism

The off-target toxicity of ADC mediated by Fc γ Rs is mainly manifested in blood toxicity. Blood toxicity is the most common off-target dose-limiting toxicity (DLTs) of ADCs containing Auristatin (MAME, MMAF), Calicheamicin and Maytansinoid (DM-1).

7. FDA-approved Antibody-drug Conjugates

Among the ten ADCs approved for marketing, six of them are for the treatment of hematological tumors, and the others are for solid tumors (Table 1). At present, more than eighty ADCs are undergoing active clinical trials, most of which are in Phase I and Phase I/II. More than 80% of clinical trials are studying the safety and effectiveness of ADCs in various solid tumors, while the rest involve hematological malignant tumors. This indicates that following the success of T-DM1 early and the recent approval of sacituzumab govitecan and Loncastuximab tesirine, research on ADCs have gradually turned into the solid tumor in recent years.

Table1 FDA-approved ADCs

ADCs have been continuously innovated and optimized since the concept of "biological missile" was put forward and have been greatly improved, making it one of the important means in the field of cancer treatment. As more and more ADCs enter the clinical stages, the industry is gradually shifting from traditional technologies to more innovative technologies to develop this complex product, which includes exploring new tumor antigens, new antibody structures, new payloads, new linkers and advanced coupling methods. With the in-depth study of ADCs, the molecular structure design of ADCs will be more reasonable, the uniformity will be greatly improved, and the stability in vivo will be continuously improved, thus reducing the toxic and side effects, improving the efficacy and activity, expanding the treatment window, and bringing new hope to tumor patients.

8. Hot Products

CUSABIO has developed a series of products with different species and tags for a variety of hot targets, which are suitable for immune, antibody screening, SPR, cell activity detection and other experiments. CUSABIO is committed to assisting you in drug development, and all activity test protocols are available for free. In addition, CUSABIO also provides related research antibodies and ELISA kits. If you don't find the targets you desired, you can leave message or chat to us by livechat.