Antibody-drug Conjugates: A Comprehensive Guide

Antibody-drug conjugate (ADC) is an effective combination of chemotherapy and immunotherapy. The concept was first proposed more than 100 years ago by German scientist Paul Ehrlich. He likens ADCs to "wonder drugs" because they can clearly identify targets (cancer cells) without harming the body, much like "snipers". In recent years, with the continuous approval and marketing of ADC drugs, this kind of drug has become one of the hottest technology fields in the global biotech drug research and development. So why are ADCs so amazing? How do they achieve "precise guidance"? This article has collected several common ADC problems, including:

1. What are Antibody-drug Conjugates?

2. Antibody-drug Conjugates Structure

3. How do Antibody-drug Conjugates work?

4. Antibody-drug Conjugates Components

5. Advantages of Antibody-drug Conjugates

6. Do Antibody-drug Conjugates have any weaknesses?

7. FDA-approved Antibody-drug Conjugates

1. What are Antibody-drug Conjugates?
Antibody-drug conjugate (adc) is a new type of biologic drug, which combines 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 side effects.

2. Antibody-drug Conjugates Structure

An ADC consists of three main components: an antibody that selectively recognizes antigens on the surface of a cancer cell, a payload that kills the cancer cell, and a linker that connects the antibody to the payload.

Fig.1 the structure of ADCs1

3. How do Antibody-drug Conjugates work?

First, the injected ADC binds to the target cell antigen after a number of obstacles, and the ADC-antigen complex is introduced into the cell through reticulin-mediated endocytosis. After the complex enters the cell, the lysosome fuses with the endosome, causing the intracellular linker to cleave and release and activate small cytotoxic molecules. When released into the cytoplasm, drugs can insert DNA or inhibit microtubule polymerization, killing tumor cells and causing apoptosis of target cells. When target cells die, the active payload may also kill surrounding tumor cells, also known as the bystander effect.

Fig.2 the working mechanism of ADCs 1

4. Antibody-drug Conjugates Components

4.1 Selection of Targets

The successful development of an ADC depends on the specific binding of the antibody to the target. The ideal target for ADC is high expression on the surface of tumor cells and low or no expression in normal cells, with minimal toxicity to both targeted and untargeted tumors. In addition to specificity and full expression, the optimal target should be able to produce effective internalization effect.

Fig.3 Target for ADCs in development and in the clinic and developed (A). Target antigens overexpressed in cancer cells; (B). Target antigens regulated from

driver oncogenes; (C). Target antigens in the tumor vasculature and stroma

4.2 Selection of Antibodies

Antibodies should have a high antigenic affinity and a long circulating half-life in order to specifically enrich the cytotoxins at the tumor site. Considering the half-life, structural stability, immune function of Fc fragment and the convenience of coupling of different IgG types of antibodies, IgG1 is preferred for ADC, and IgG4 is rarely used. According to the immunogenicity of antibodies, they can be divided into fully humanized antibodies, humanized antibodies and chimeric antibodies. To reduce the immune response, more fully humanized or humanized antibodies are selected.

4.3 Selection of Payloads

The toxin molecule (payload) is a key factor in the success of ADC development. Since adc needs to go through multiple steps from entering the human body to the final release of cytotoxic agents, considering the efficiency of each step, the payload should have high anti-tumor activity. Therefore, toxic molecules at the nanomole level (IC50 value is 0.01-0.1 nM) should be the appropriate payload. In addition, the payload must have suitable functional groups that can be coupled, strong cytotoxicity, appropriate hydrophilic and hydrophobic balance, and high stability.

4.4 Selection of Linker

The linker acts as a bridge to the ADC and is connected to the antibody via either a cut-out connector or a non-cut-out connector. The connector requires a sophisticated design that is stable, prevents the chain from breaking during physiological conditions, and features high release efficiency in specific locations.

5. Advantages of Antibody-drug Conjugates

Compared with traditional fully or partially humanized antibodies or antibody fragments, ADCs can release highly active cytotoxins in tumor tissues and theoretically have higher efficacy. ADC, on the other hand, has higher tolerance and lower side effects than fusion proteins. They can accurately identify the target without affecting normal cells, which greatly improves the efficacy and reduces the toxic side effects. Therefore, they have attracted the attention of the pharmaceutical research and development field.

6. Do Antibody-drug Conjugates have any weaknesses?

The complex structure and diverse design of ADC bring difficulties to the production and quality control related to CMC research. At the same time, the superposition of ADC in vivo and the complex biological process also make non-clinical research and clinical research face multiple challenges. As the production of ADC mostly involves highly active cytotoxic drugs, it requires high requirements for hardware equipment, process design and personnel training, and requires a large amount of capital input and technical reserves, so the production threshold is relatively high.

Currently, ADC development faces three major challenges and opportunities:

6.1 Instability of Linker

This instability can lead to premature release of the payload into the bloodstream and lead to non-specific uptake and targeted toxicity of the adc.

6.2 Nonspecific Endocytosis

Hydrophobicity promotes aggregation and non-specific endocytosis of ADCs, especially ADCs with a high DAR(drug-antibody ratio), resulting in targeted off-target effects. Adcs with a high DAR are also cleared by other non-specific, endocytotic cells. Therefore, optimizing DAR is also an important strategy to improve TI(therapeutic index).

6.3 Receptor-mediated Uptake Mechanism

The off-target toxicity of FC-γRS mediated ADC was mainly hematotoxicity. Hematotoxicity is the most common off-target dose-limiting toxicity (dlt) of ADCs containing Auristatin (MAME, MMAF), Calicheamicin, and Maytansinoid (DM-1).

7. FDA-approved Antibody-drug Conjugates

Of the 10 ADCs approved for marketing, six are for hematologic tumors and the rest for solid tumors (Table 1). Currently, more than 80 ADCs are in active clinical trials, most of which are in phase I and I/II. More than 80% of clinical trials study the safety and efficacy of ADCs in various solid tumors, with the rest involving hematologic malignancies. This suggests that with the early success of T-DM1 and the recent approval of sacituzumab govitecan and Loncastuximab tesirine, ADC research has gradually shifted toward solid tumors in recent years.

Table1 FDA-approved ADCs

Drug Name	Trade Name	Company	Target	Payload	Indications	Approval year
Gemtuzumab ozogamicin	Mylotarg	Pfizer/Wyeth	CD33	Calicheamicin	AML	2000 2017
Brentuximab vedotin	Adcetris	Seattle Genetics, Millennium/Takeda	CD30	MMAE	cHL、sALCL、PTCL	2011
Trastuzumab emtansine	Kadcyla	Genentech/Roche	HER2	DM1	mBC	2013
Inotuzumab ozogamicin	Besponsa	Pfizer/Wyeth	CD22	Calicheamicin	ALL	2017
Polatuzumab vedotin	Polivy	Genentech/Roche	CD79b	MMAE	r/r DLBCL	2019
Enfortumab vedotin	Padcev	Astellas/Seattle Genetics	Nectin-4	MMAE	mUC	2019
Trastuzumab deruxtecan	Enhertu	AstraZeneca/ Daiichi Sankyo	HER2	Dxd	mBC、mGC	2019
Sacituzumab govitecan	Trodelvy	Immunomedics	TROP2	SN-38	mTNBC	2020
Belantamab mafodotin	Blenrep	GlaxoSmithKline	BCMA	MMAF	MM	2020
Loncastuximab tesirine	Zynlonta	ADC Therapeutics	CD19	SG3199	r/r DLBCL	2021

Since the concept of "biological missile" was proposed, ADC has been continuously optimized and innovated, and has been greatly improved, becoming one of the important means in the field of cancer treatment. As more ADCs enter the clinical stage, the industry is gradually moving away from traditional technologies to more innovative techniques to develop this complex product, including exploring new tumor antigens, new antibody structures, new payloads, new linkers and advanced coupling methods. With the deepening of ADC research, the molecular structure design of ADC will be more reasonable, the uniformity will be greatly improved, and the in vivo stability will be continuously improved, so as to reduce the toxic side effects, improve the efficacy and activity, expand the treatment window, and bring new hope for tumor patients.

References

1. Abuhelwa Z, Alloghbi A, Nagasaka M. A comprehensive review on antibody-drug conjugates (ADCs) in the treatment landscape of non-small cell lung cancer (NSCLC)[J]. Cancer treatment reviews, 2022, 106.