Release date:2023/6/16 18:43:10

The design concept of antibody-drug conjugate (ADC) has a long history. As early as 1913, Professor Paul Ehrlich, the father of chemotherapy and Nobel laureate, first proposed the concept of "magic bullet". That is, cytotoxic drugs are installed on specific monoclonal antibodies to achieve targeted killing of tumor cells. Currently available ADC drugs are based on this theory. By connecting cytotoxic drugs to monoclonal antibodies, monoclonal antibodies can be used as carriers to efficiently transport small molecule cytotoxic drugs to the target tumor cells in a targeted manner.

Antibody-drug conjugates (ADCs) have developed rapidly in recent years, anti-tumor ADC drugs are a class of targeted drugs with high attention. By the end of 2022, a total of 15 ADC drugs have been approved for marketing worldwide, and more than 140 are in clinical trials. It is estimated that by 2030, the ADC market will exceed $15 billion.

ADC drugs are used as drug delivery vehicles due to the high target specificity and long half-life of antibodies. Theoretically, by targeting monoclonal antibodies, ADCs can precisely find the lesion and achieve a real " prescribe the right medicine ". Therefore, the selection of the antibody target is very important. Target antigens should be highly expressed in tumor tissues, but not expressed in normal tissues. Currently, popular targets include CD family (CD33, CD30, CD22, CD79B, CD19), BCMA, HER2, TROP2, Tissue factor (TF), Nectin-4, FRα, EGFR, etc. Here, we briefly introduce some ADC targets in solid tumors and hematological tumors.

ADC Targets For Solid Tumors

To date, FDA-approved ADC targets for the treatment of solid tumors include HER2, TROP2, Tissue factor (TF), Nectin-4, FRα, and EGFR.

ADC target for solid tumors
Figure 1. ADC targets for solid tumors
(Source: References [1])


HER2 is a 185kda transmembrane glycoprotein belonging to the EGFR family. Amplification of the HER2/neu gene is a known driver of human malignancy and metastasis. HER2 has been a therapeutic target for decades because of its role in cancer. HER2 has also been a target for ADCs, and both T-DM1 and T-DXT are approved for use in patients with HER2-positive metastatic breast cancer.

There are several mechanisms of HER2 endocytosis. The first is CME. Co-immunoprecipitation clearly shows that HER2 binds directly to AP-2. In addition, dynasore can completely block HER2 endocytosis in SKBR3 cells. In addition, HER2 has been shown to utilize the fossa mediated endocytosis pathway and the CLIC/GEEC endocytosis pathway.

Currently, there are three ADC drugs targeting HER2 on the market, namely Kadcyla, Enhertu, and RC48.


TROP-2 is a 46kDa monomeric glycoprotein with properties such as selective overexpression, structural endocytosis, and directed lysosomal, making it a very attractive target for ADCs. The internalization mechanism of Trop2 is related to CME.

In addition, Trop2 binds to a variety of ligands, such as claudin-1, claudin-7, cyclin D1, and IGF1. However, none of these ligands have been shown to be internalized when binding or interacting with Trop2. Therefore, Trop2 endocytosis is more intense in tumor cells than in normal cells, suggesting that Trop2 is a good target for ADC.

TF (Tissue factor)

TF (Tissue factor), also known as thromboplastin factor III or CD142, is a transmembrane glycoprotein with procoagulant activity that has the ability to induce intracellular signaling in combination with proteolytic enzyme factor VIIa (FVIIa). TF is thought to promote cancer progression through FVIA-dependent intracellular signaling pathways that regulate cell survival, proliferation, metastasis, and angiogenesis. It is upregulated in various solid tumors and tumor vascular systems due to hypoxia-induced signaling.

The internalized properties of this antigen are ideal for developing of TF-targeted ADCs. In addition, a TF-FVIIa-mediated mechanism of induction of surface TF expression has been reported, in which the formation of the TF-FVIIa complex leads to the release of TF from the Golgi apparatus and then transport to the membrane, resulting in enhanced cell surface TF expression. If this effect can be induced by anti-TF ADCs, then this could allow repeated targeting of malignant cells expressing TF.


Nectin-4 is a 66 kDa type I transmembrane protein whose primary role is to facilitate intercellular contacts. Nectin-4 is attractive as an ADC target because it has been shown to be overexpressed in several tumor types but barely present in normal adult tissues.

At present, there is no information has been found on endocytosis of natural ligands or complexes of mAb/ADC and nectin-4, but the research on endocytosis of nectin-4 combined with pathogens can be used for reference. Nectin-4 is also a receptor for measles virus, and studies have shown that measles virus enters MCF7, HTB-20 breast cancer and DLD-1 colorectal cancer cells through macropinocytosis. Viral entry requires PAK1, whereas the dynamin inhibitor Dynasore had no effect on viral entry. In addition, cells expressing the dominant negative fossa protein did not eliminate the endocytosis of the virus. Based on these indirect studies, nectin-4 exhibits the robust endocytosis required by the viral receptor.


FRα (folate receptor alpha) is a membrane-bound metabolic folate receptor involved in the intracellular transport of folate. Once bound to folate, the receptor-ligand complex is internalized through a non-classical lipid raft endocytosis mechanism. FRα is highly expressed in ovarian, breast, endometrial, mesothelioma, and lung cancers, but barely expressed in normal cells, making this receptor well suited for ADC targeting.

Furthermore, FRα is thought to assist pro-tumor signaling by binding to folate, inducing downstream effects such as activation of STAT3, intracellular transport of FRα as a transcription factor as a growth pathway, and intracellular transport of folate for DNA biosynthesis.


Epidermal growth factor receptor (EGFR) belongs to the human epidermal growth factor receptor family, which is highly expressed in a variety of solid tumors, and the signaling pathway triggered by EGFR can cause tumor proliferation. Antibody-targeted drugs against EGFR, ranging from monoclonal antibodies such as cetuximab and panitumumab, to today's bispecific antibodies and ADC drugs, have significantly improved efficacy and specificity.

The application of chimeric antibody cetuzumab in ADC drugs has been reported. In addition, MAb806 is an IgG1 κtype monoclonal antibody targeting EGFRvIII (a tumor-specific target that does not occur under normal physiological conditions), with good specificity and internalization properties, and can be used as a small molecule drug delivery vector.

ADC Targets For Hematological Tumors

For hematologic tumors, immune lineage-specific biomarkers such as CD19, CD20, CD22, CD33, CD79B and BCMA are widely and uniformly expressed at high levels on malignant blood cells, and thus have been extensively explored as candidate targets for ADC development.  In addition, the target antigens of approved ADCs are easily internalized after binding, which is an important feature that contributes to the efficacy of ADCs.

ADC targets for hematological tumors
Figure 2. ADC targets for hematological tumors
(Source: References [1])


CD19 is considered to be a pan-B cell marker and a major signaling component of multimolecular complexes on the surface of mature B cells. The expression of CD19 is highly conserved in most B-cell malignancies, and in addition, CD19 has rapid internalization kinetics and does not shed into circulation, making it an ideal ADC target antigen.


CD22 is a 140 kDa transmembrane glycoprotein that, like CD33, is a member of the Siglec family and shares several structural features with the family. The key difference is that CD22 is much larger than CD33 because it has multiple Ig domains and ITIM/ ITIM-like motifs. Expression of CD22 is limited to B cells, and CD22 is expressed at elevated levels in most mother cells of a variety of B-cell malignancies, including ALL.

CD22 is endocytized by the CME. Native-like ligands accumulate intracellularly through constitutive rapid endocytosis of CD22. These ligands are sorted for degradation in lysosomes, while CD22 is recycled back to the cell surface. In addition, CD22 ligand-induced endocytosis activates intracellular pools that replenish or increase the expression levels of CD22 on the cell surface. Therefore, CD22 has favorable endocytic properties for ADCs.


CD30 is a 120kda transmembrane glycoprotein belonging to the tumor necrosis factor receptor (TNFR) superfamily. Its extracellular portion consists of six cysteine-rich domains (CRDs) in an extended conformation. CD30 is expressed on activated T cells and B cells, as well as various lymphoid neoplasms, including Hodgkin lymphoma and ALCL.

CD30 is not endocytotic, instead, it is shed by proteolytic cleavage, and the shedding of CD30 is mediated by matrix metalloproteinases (MMPs). Shedding is a feature of CD30 biology, and high concentrations of circulating soluble CD30 can be used as a serum marker to monitor tumor progression. For the efficacy of ADCs, elevated CD30 circulating levels appear to isolate injected ADCs, thereby reducing the number of ADCs able to target CD30-positive tumor sites. Therefore, the lack of endocytosis suggests that CD30 is not an ideal ADC target.


CD33, a 67kda transmembrane glycoprotein receptor, is a member of the sialic-binding immunoglobulin-like lectin (SIGLEC) family, which is normally expressed on normal myeloid cells and is the target of Gemtuzumab ozogamicin due to its preferential overexpression on AML cells. The immunoreceptor tyrosine-based inhibitory motif (ITIM) of CD33 regulates CD33 endocytosis, which can be activated by clathrin-mediated endocytosis (CME). Regarding endocytosis efficiency, there was no correlation between the expression level of CD33 in AML cells and its endocytosis rate. CD33 is a slowly internalized antigen, and CD33 cross-linking does not improve endocytosis. AML patients who do not respond to GO may be related to poor function of CD33 receptor ingestion.


CD79b is expressed only in immature and mature B cells and is overexpressed in ≥80% of B cells in malignant tumors. CD79a and CD79b are two non-covalently bound transmembrane proteins that mediate signaling and endocytosis. For the latter, the CD79a-CD79b heterodimer is a scaffold that controls BCR endocytosis. The endocytosis of BCR is mainly accomplished by CME and mediated by AP-2. Interestingly, CD79a directly interacts with the μ-subunit of AP-2, which in turn activates CD79b and leads to endocytosis of the entire BCR complex.

In addition, for ADCs, CD79a can be internalized as a monomer, but CD79b cannot. If the proximal membrane tyrosine (Y195) of CD79b is mutated, the binding of AP-2 to CD79a is blocked, and endocytosis is also blocked. In 18% of activated B-cell-like DLBCL specimens, Y195 was mutated. In summary, there is evidence that CD79b's endocytosis depends on the internalization of the entire BCR complex, rather than as a monomer.


BCMA or CD269, also known as TNFR superfamily member 17, transduces signals that induce B cell survival and proliferation. BCMA has a molecular weight of only 20.2 kDa, and its ligand-bound extracellular region has an "arm-chair" conformation, consisting of six CRDS. In addition to multiple myeloma, BCMA is also expressed in many hematological malignancies such as Hodgkin's lymphoma and non-Hodgkin's lymphoma.

However, little is known about the precise endocytic pathway utilized by BCMA. Related to endocytosis, sialylation is a regulatory function that may induce BCMA to endocytosis using CME.


At present, 15 ADC drugs have been approved for marketing in the world, and more than 450 ADC drugs are in different stages of research and development. From the perspective of the target distribution of ADC products, HER2 is the most popular target for ADC research and development, followed by TROP2. The competition for ADC drugs is increasing. If pharmaceutical companies want to stand out, the selection of targets is the key.

In recent years, the development of ADCs has also exploded as improvements have been made through better selection of cytotoxic drugs, bioconjugation methods, better targeting of antigens, and optimized antibody engineering. In 2022 alone, 249 clinical trials evaluating ADCs have been initiated, a 35% increase compared to 2021, and there is significant overlap in the tumor targets being studied. In the future, ADC drugs will have a huge potential in the anti-tumor market. On this track, if a company wants to stand out, a differentiated layout that meets clinical needs is the key, especially in the selection of targets, which determines the company's position in the future commercial competition.

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Target Antigen Attributes and Their Contributions to Clinically Approved Antibody-Drug Conjugates (ADCs) in Haematopoietic and Solid Cancers. Cancers (Basel).2023 Mar; 15(6): 1845.
Impact of EndocytosisMechanisms for the Receptors Targeted by the Currently Approved Antibody-DrugConjugates (ADCs)—A Necessity for Future ADC Research and Development. Pharmaceuticals(Basel). 2021 Jul; 14(7): 674.

Related articles:
Summary of Approved HER2 ADCs on The Market & in Clinical Trials
[2]. Overview of HER2-targeted Drugs
[3]. Clinical Development of ADC Drugs Targeting TROP-2
[4]. The Rise of the TROP2-Directed ADCs for Solid Tumors
[5]. FDA Approves Tivdak - First Tissue Factor (TF)-Targeted Antibody Conjugate Drug (ADC)
[6]. Nectin-4-Directed Drugs for Solid Tumors
[7]. Novel FRα-targeting Antibody-drug Conjugates (ADCs)
[8]. EGFR-Directed ADCs for Cancer Treatment

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