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Bystander Effects of Antibody-Drug Conjugates

Release time:2023/4/25 16:44:14

The bystander effect of ADC is expected to solve heterogeneous expression of target antigens (Ag) in metastatic tumor tis…

Previously, conventional chemotherapy had a narrow therapeutic window due to the inability to precisely deliver chemotherapeutic drugs to tumor cells. Antibody-drug conjugates (ADCs) effectively expand the therapeutic window through the combination of targeting plus chemotherapy, breaking the limitations of conventional cytotoxic drugs.

ADCs consist of three main components, including a monoclonal antibody, a cytotoxin, and a linker that connects the two. The antibody is used to specifically target tumor cells, the cytotoxin plays a cell-killing effect, and the linker plays the role of connecting and releasing the toxin.

Figure 1. Schematic diagram of antibody drug conjugate. Source: reference [2]

In 2013, the approval of T-DM1 marked the entry of ADC drugs into the field of solid tumor treatment. The most difficult aspect of ADC treatment for solid tumors is the heterogeneous expression of target antigens (Ag) in metastatic tumor tissues, which can lead to the ineffectiveness of drugs acting on primary tumors against metastatic tumor tissues. The bystander effect of ADC is expected to solve this problem, let's learn more about it.

Tumour Heterogeneity

Tumor heterogeneity is one of the characteristics of malignant tumors. It refers to the fact that tumors undergo multiple divisions and proliferation during the growth process, and their daughter cells show distinct morphological and phenotypic profiles, including cellular morphology, gene expression, metabolism, motility, proliferation, and metastatic potential. The heterogeneity of cancer cells introduces significant challenges in designing effective treatment strategies. It is difficult for a single target to cover all cancer cells for solid tumors. After targeted drugs kill target-positive cells, the originally non-dominant negative cancer cell lines may develop further, leading to cancer progression, as well as the emergence of drug resistance.

Bystander Effect of ADCs

Bystander killing refers to the process in which the drug from an ADC is released either from the target cell after internalization and degradation of the ADC or from the extracellular space. This drug then enters and destroys the neighboring or bystander cells, regardless of whether or not they express the ADC target antigen. The degree of bystander killing induced by an ADC is largely influenced by factors such as the extent of ADC internalization upon binding to the target antigen, the presence of a cleavable or non-cleavable linker, and the hydrophobicity of the attached cytotoxic agent.

1. Targeted antibodies

The chemical stability of the antibody-linker is critical. A balance between ADC specificity and damage to normal tissues is required if a cytotoxic payload is to be released prior to ADC internalization, thereby producing a killing effect on cells surrounding the target antigen-positive cells.

2. Linkers

When designing ADCs that target antigens with heterogeneous expression in tumors, cleavable linkers are often preferred as they tend to enhance the bystander effect. These linkers are designed to be cleaved by a specific protease or in response to a particular pH after internalization, thereby releasing the free drug. This free drug is intended to directly kill the target cell that is positive for the antigen. However, in certain cases, it can also diffuse out of the target cell and kill the neighboring antigen-negative cells, thereby causing a bystander effect.

3. Payloads

The payload with bystander-killing effect generally requires good membrane permeability, and typically such molecules are required to be neutral or uncharged molecules with high hydrophobicity. However, hydrophobicity also needs to be balanced as excessive hydrophobicity can result in aggregation or tissue absorption of the ADC drug, causing high toxicity, which is not suitable for ADC drug development. According to publicly available information, Payloads with bystander killing effects mainly include MMAE, DXD, Eribulin, while Payloads without bystander killing effects mainly include Amanitin and MMAF (with negative charge).

Figure 2. Possible mechanism(s) of action are shown for an ADC. Source: Reference [2]

According to the study, the bystander killing effect mainly includes the following possible mechanisms of action:

1. Upon binding to the target antigen expressed on cancer cells, ADCs are internalized through endocytosis and subsequently degraded in the lysosome, resulting in the release of the cytotoxic payload. The membrane permeable cytotoxic payload may pass out of the cell from which it was released and exert a bystander killing effect on surrounding tumor cells.

2. Once the ADC binds to its surface tumor antigen, it is internalized into endosomes that subsequently mature and fuse with lysosomes. In the lysosomes, the drug is released via cleavage of the linker by specific proteases such as cathepsin B or by the degradation of the ADC. Thus, the toxin moieties are directly released extracellularly and penetrate the membrane into the neighboring cells to exert the killing effect.

3. ADC-bound target tumor cells may be internalized through Fc-mediated phagocytosis, and toxin molecules are released through the cell membrane and penetrate the membrane into the neighboring cells to exert the killing effect.

ADCs with  Bystander Killing Effect

Trastuzumab deruxtecan (DS-8201)

As a new generation ADC drug, Trastuzumab deruxtecan (Enhertu®: Daiichi Sankyo/AstraZeneca) has unique structural and mechanistic properties that confer potent antitumor activity. It is comprised of a topoisomerase I inhibitor payload, deruxtecan (DXd) via a cleavable tetrapeptide based linker. The cleavable linker is structurally stable in circulation with a low drug-shedding rate, thus reducing toxic side effects, and DS-8201 has an efficient bystander effect. DS-8201 has been studied in breast, gastric, lung and colorectal cancers and has shown good anti-tumor activity.

Figure 3. Structure of DS8201, Source: Astrazeneca official website

At SABCS in December 2020, DS-8201 presented an update on data from DENSTINY-Breast01, a clinical study in patients with advanced HER2-positive breast tumors: ORR of 61.4%, DOR of 20.8 months, DCR of 97.4%, and mPFS of 19.4 months.

According to data from a Phase 3 clinical trial (DESTINY-Breast03) of DS-8201 in December 2022, DS-8201 showed a significant improvement in overall survival versus T-DM1 in patients with HER2-positive metastatic breast cancer, as well as the longest reported median progression-free survival, reaffirming trastuzumab deruxtecan as the standard of care in the second-line setting. A manageable safety profile of trastuzumab deruxtecan was confirmed with longer treatment duration.

Figure 4. DS8201 & T-DM1, Source: Astrazeneca official website

DS-8201 significantly prolonged the median progression-free survival of patients by up to 22 months compared to T-DM1, with a statistically significant and clinically meaningful benefit in terms of progression-free survival (PFS). The confirmed objective response rate (ORR) in the DS-8201 group was 78.5%, with 21.1% of patients in complete response(CR).The median duration of response (DoR) for DS-8201 was 36.6 months.

In February 2022, DESTINY-Breast04 showed a significant improvement in progression-free survival compared to chemotherapy in patients with MBC with low HER2 expression, a positive result that may redefine disease classification and hopefully change the landscape of breast cancer treatment.


As the competitive ADC drug landscape further evolves, the development of new payloads will play an increasingly important role. Given the importance of payloads with bystander-killing effects on heterogeneous tumor cells, such payload development is a promising direction. However, it is undeniable that the bystander killing effect may also lead to the non-specific killing of normal cells. Therefore, the rational selection and design of payload and linker according to the actual situation of the target and specific indications are needed to reduce or avoid the adverse effect of bystander killing effect and give full play to the bystander killing effect.

Huateng Pharma is dedicated to being your most reliable partner to provide chemical synthesis and high-quality PEG linkers for ADC drugs. We are committed to promoting the progress of your ADC discovery and development projects.


[1] Ogitani Y, Hagihara K, Oitate M, Naito H, Agatsuma T. Bystander killing effect of DS-8201a, a novel anti-human epidermal growth factor receptor 2 antibody-drug conjugate, in tumors with human epidermal growth factor receptor 2 heterogeneity. Cancer Sci. 2016 Jul;107(7):1039-46.
[2] Staudacher AH, Brown MP. Antibody drug conjugates and bystander killing: is antigen-dependent internalisation required? Br J Cancer. 2017 Dec 5;117(12):1736-1742.
[3] Hurvitz SA, Hegg R, Chung WP, et al. Trastuzumab deruxtecan versus trastuzumab emtansine in patients with HER2-positive metastatic breast cancer: updated results from DESTINY-Breast03, a randomised, open-label, phase 3 trial [published correction appears in Lancet. 2023 Feb 18;401(10376):556]. Lancet. 2023;401(10371):105-117. doi:10.1016/S0140-6736(22)02420-5

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