June 25, 2024

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How to choose ADC Targets for Hematological and Solid Tumors?

How to choose ADC Targets for Hematological and Solid Tumors?


How to choose ADC Targets for Hematological and Solid Tumors?

From the perspective of targets, HER2 experienced T-DM1’s Me too, and DS-8201’s Me too is the focus of fast follow plus differentiated research and development, including HER2 double-antibody ADCs, with a maximum of 24 models .

Trop2 and Claudin18.2 followed closely, reaching 13 and 12 models respectively. Other popular ADC targets include FRα, Nectin-4, B7-H3, EGFR, CD20, cMET, CD79b, etc.


In the development of ADCs, selection of target antigens that are highly expressed on malignant cells but low on normal tissues and immune cells is considered critical to achieve selectivity and potency while minimizing off-target toxicity.

Beyond that, the selection of antigens for use in ADCs requires consideration of several factors related to the expression pattern and biological characteristics of the target antigen. This article introduces some ADC targets of hematological tumors & solid tumors.


ADC targets for hematological malignancies

For hematological malignancies, immune lineage-specific biomarkers such as CD19, CD20, CD22, CD33, BCMA, and CD79 are widely and uniformly expressed at high levels on malignant hematological cells and thus have been extensively explored as candidate targets for ADC development.

In addition, the target antigens of approved ADCs are all readily internalized after binding, an important feature that contributes to the efficacy of ADCs.


How to choose ADC Targets for Hematological and Solid Tumors?



CD33 is a 67kda transmembrane glycoprotein receptor, a member of the sialic acid-binding immunoglobulin-like lectin ( SIGLEC ) family, normally expressed on normal myeloid cells, due to its preferential overexpression on AML cells, Gemtuzumab ozogamicin target.

The intracellular immunoreceptor tyrosine-based inhibitory motif ( ITIM ) of CD33 regulates the endocytosis of CD33 and can activate endocytosis through clathrin-mediated endocytosis ( CME ).

Regarding endocytic efficiency, there was no correlation between the expression level of CD33 in AML cells and its endocytic rate.

CD33 is a slowly internalized antigen, and in addition, CD33 cross-linking did not improve endocytosis. AML patients who do not respond to GO may be related to the hypofunction of CD33 receptor endocytosis.


CD22 is a 140 kDa transmembrane glycoprotein that, like CD33, is a member of the Siglec family and shares several structural features with this family. The key difference is that CD22 is much larger than CD33 because of its multiple Ig domains and ITIM/ITIM-like motif.

CD22 expression is restricted to B cells, and CD22 is expressed at elevated levels in most blasts of various 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.


CD19 is considered a pan-B cell marker and a major signaling component of a multimolecular complex on the surface of mature B cells.

CD19 expression is highly conserved in most B-cell malignancies, and in addition CD19 has rapid internalization kinetics and is not shed into the circulation, making it an ideal ADC target antigen.


CD79b is expressed only in immature and mature B cells and is overexpressed in ≥80% of B cells in malignancies.

CD79a and CD79b are two non-covalently associated transmembrane proteins that mediate signaling and endocytosis. For the latter, the CD79a-CD79b heterodimer is the scaffold that controls BCR endocytosis.

BCR endocytosis 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.

Furthermore, 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.

Y195 is mutated in 18% of activated B-cell-like DLBCL specimens. Taken together, there is evidence that CD79b’s endocytic activity depends on 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.

The molecular weight of BCMA is only 20.2 kDa, and its ligand-binding extracellular domain has an “armchair” 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 utilize CME for endocytosis.


CD30 is a 120kda transmembrane glycoprotein that belongs 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 hallmark of CD30 biology, and high concentrations of circulating soluble CD30 can serve as serum markers for monitoring tumor progression.

Regarding the efficacy of ADCs, elevated circulating levels of CD30 appear to sequester injected ADCs, thereby reducing the number of ADCs able to localize to CD30-positive tumor sites.

Therefore, the lack of endocytosis results suggest that CD30 is not an ideal ADC target.




ADC targets for solid tumors


ADCs designed to treat solid tumors target a range of antigens that often include tumor-associated membrane proteins or receptors that may be involved in pro-tumor pathways.

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

How to choose ADC Targets for Hematological and Solid Tumors?




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.

Because of its role in cancer, HER2 has been a therapeutic target for decades. HER2 has also been a target of ADCs, and both T-DM1 and T-DXT are approved for patients with HER2-positive metastatic breast cancer.

There are multiple mechanisms for the endocytosis of HER2. The first is CME. Co-immunoprecipitation clearly shows that HER2 directly binds to AP-2.

In addition, dynasore can completely block the endocytosis of HER2 in SKBR3 cells.

In addition, studies have demonstrated that HER2 can utilize caveolae-mediated endocytosis pathways and CLIC/GEEC endocytosis pathways.


Trop2 is a 46kDa monomeric glycoprotein with properties such as selective overexpression, constitutive endocytosis, and lysosome targeting, making it a very attractive target for ADCs.

The internalization mechanism of Trop2 is related to CME.

Furthermore, 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 upon binding or interacting with Trop2.

Therefore, Trop2 is more strongly endocytized in tumor cells than in normal cells, suggesting that Trop2 is a good target for ADCs.


Nectin-4 is a 66 kDa type I transmembrane protein whose main function is to facilitate cell-to-cell contact. 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 on endocytosis of natural ligands or complexes of mAb/ADC and nectin-4, but the research on nectin-4 binding to pathogen endocytosis 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.

Furthermore, cells expressing a dominant-negative caveolin did not eliminate viral endocytosis. Based on these indirect studies, nectin-4 exhibits robust endocytic activity required for viral receptors.


TF ( tissue factor ), also known as thromboplastin factor III or CD142, is a transmembrane glycoprotein with procoagulant activity that has the ability to complex with the proteolytic enzyme factor VIIa ( FVIIa ) to induce intracellular signaling.

TF is thought to contribute to cancer progression through FVIIa-dependent intracellular signaling pathways that regulate cell survival, proliferation, metastasis, and angiogenesis. It is upregulated in various solid tumors and tumor vasculature due to hypoxia-induced signaling.

The internalization properties of this antigen are ideal for the development of TF-targeting ADCs.

In addition, the reported mechanism of TF-FVIIa-mediated induction of surface TF expression, in which the formation of the TF-FVIIa complex leads to the release of TF from the Golgi apparatus, followed by trafficking to the membrane, leads to enhanced expression of TF on the cell surface.

If this effect could be induced by anti-TF ADCs, this could allow reproducible targeting of TF-expressing malignant cells.


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 by a non-classical mechanism of lipid raft endocytosis.

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 trafficking of FRα as a transcription factor for growth-promoting pathways, and intracellular trafficking of folate for DNA biosynthesis .




Factors in Target Antigen Selection

The use of highly efficient cytotoxic payloads in ADCs requires rational selection of target antigens with the aim of maximizing tumor selectivity and antitumor efficacy while minimizing off-target dose-limiting toxicity.

Therefore, delivering as much payload as possible to cancer cells is critical for ADC design while trying to balance factors such as safety and efficacy.

According to currently accepted principles, the ideal target antigen for an effective ADC should be expressed with sufficient density and uniformity on the surface of tumor cells and minimally expressed on normal cells to limit off-target toxicity and optimize the therapeutic index.

In addition to specificity and overexpression, the optimal target antigen needs to be extracellular in order for the antibody to bind the epitope.

In addition, ADC efficacy usually depends on effective target-mediated internalization, and the internalization rate and internalized lysosomal trafficking kinetics after tumor antigen binding to ADC may directly affect payload release and cancer cell killing.

It is also crucial to understand that antigenic targets are primarily directed towards the circulation or lysosomal targeting pathways.

Recycling of antigen-ADC complexes to the plasma membrane is thought to affect the efficient delivery of ADCs to lysosomes and may impede the release of payloads to the cytosol, compromising ADC potency.

Finally, another factor affecting the effectiveness of ADCs is the rate at which antigens are removed from the cell surface, often mediated by proteases produced by tumor cells, a process known as antigen shedding.


Target expression level

The threshold level of antigen expression to achieve ADC activity varies significantly according to several parameters, many of which have not been fully elucidated. Expression levels are known to depend on the specific target, epitope recognized, and cancer indication, which is especially evident for ADCs targeting solid tumors.

For example, clinical experience evaluating the efficacy of Kadcyla in HER2-positive metastatic breast cancer has shown that the high-expression subgroup has better survival compared with the low-HER2 expression subgroup.

However, the outstanding efficacy of Enhertu in breast cancer patients with low HER2 expression indicates that there is no broadly applicable target antigen expression threshold to ensure the efficacy of ADC .

Some HER2-negative cell lines may still maintain active HER2 signaling and be sensitive to anti-HER2 therapies in vitro and in vivo.

This suggests that lower levels of tumor-promoting signaling due to lower levels of surface-expressed HER2 may still support tumor growth.

Other targets also varied, for example selecting patient groups with only high expression of FRα target antigens appeared to be associated with therapeutic benefit.

In contrast to the ADC target CD70 in RCC clinical studies, only a limited correlation between antigen expression levels and the sensitivity of CD70-targeted ADCs was observed.

Collectively, preclinical studies and clinical evaluations of ADCs for several cancer types have shown that there is no overarching paradigm that correlates antigen expression levels with ADC activity. Therefore, the ideal cut-off value for antigen expression for each tumor type and ADC needs to be determined empirically.

Off-target toxicity

Clinically observed toxicity of ADC therapy is most commonly due to off-target effects, whereas on-target-off-target tumor toxicity of ADCs may be affected by the choice of target antigen.

To mitigate toxicity, the physiological role of the target antigen and the mechanism by which it exerts that role must also be considered.

Therefore, preclinical toxicity studies of new ADC targets require not only the study of the differential expression of the target between tumor and normal tissues, but also the study of the physiological function of the target to determine the potential toxicity .

For example, a phase I trial of the CD44 antibody bivatuzumab in combination with an ADC of DM-1 in squamous cell carcinoma reported fatal skin toxicity, which may be attributed to expression of CD44 by healthy keratinocytes.

However, although normal cell expression of the target antigen is an important factor to consider, it does not necessarily hinder the development and eventual success of ADCs.

For example, although TROP-2 is expressed at high levels in some normal tissues, Trodelvy has been successfully developed and received FDA approval for the treatment of metastatic triple-negative breast cancer.

This suggests that, for anti-TROP-2 ADCs, differential expression of the antigen in normal versus malignant tissues may be sufficient to avoid severe toxicity.

In addition, it has also been suggested that expression of TROP-2 intracellularly rather than on the cell surface on normal cells, or on the luminal side of the duct or glandular epithelium inaccessible to antibodies or ADCs, may play an important role.

Antigen shedding

When selecting an ADC target, the shedding rate of the antigen may be an important consideration. In antibody-based therapeutics, antigen shedding refers to the removal of target antigens expressed on the cell surface, a process usually mediated by proteases, as a means of functional modulation.

Earlier studies with immunotoxins suggested that increased antigen shedding may reduce the amount of ADC available for targeting and tumor binding, compromising ADC effectiveness.

However, other studies using mathematical and experimental models have shown that whether high antigen shedding rates may increase or decrease ADC effectiveness depends on several factors, including ADC endocytosis rates, ADC recycling rates, and extravasation rates through the tumor microenvironment.

For example, the same model for anti-mesothelin and anti-CD25 immunotoxins showed that increased shedding increases the potency of the former but decreases the potency of the latter.






For ADCs to select target antigens for solid tumors and hematological tumors, several factors may need to be considered, including:

(1) the degree of expression of the antigen in tumor and healthy tissue; (

2) the physiological function of the antigen in normal cells and tumor cells;

(3) Endocytic properties and endocytic mechanism of antigen;

(4) Whether, where and how antigen is shed, and the potential impact of shedding on the effectiveness of ADC;

(5) Antigen circulation and its impact on the mechanism of action of ADC.


Gaining insight into these properties of the target antigen, along with careful consideration of how the choice of antigen may affect other aspects of ADC performance, will help us design optimal ADCs that are clinically safe and effective for the treatment of solid and hematological malignancies.











1. 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.

2. Impact of Endocytosis Mechanisms for the Receptors Targeted by the Currently Approved Antibody-Drug Conjugates (ADCs)—A Necessity for Future ADC Research and Development. Pharmaceuticals(Basel). 2021 Jul; 14(7): 674.

How to choose ADC Targets for Hematological and Solid Tumors?

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