With the rapid development of antibody-drug conjugates (ADCs), more and more companies have entered the field of ADC research and development, and various types of ADC technologies and branches have emerged. Bispecific antibody conjugates (BsAb ADCs) are one of the emerging new technologies. The high specificity of bispecific antibodies enables more precise targeting of tumor cells. On the other hand, promoting the coordinated endocytosis of the two targets through cross-linking can improve the efficiency of toxins entering tumor cells. Currently, bispecific ADCs are still in the early stages of development globally, with only a few products entering the clinical development stage, and all of them are in early clinical stages.
Bispecific antibodies (BsAb), also known as bifunctional antibodies, can simultaneously recognize and bind two different antigens and epitopes, and block two different signaling pathways to exert their effects. According to different structures, bispecific antibody structures can be divided into two main categories: IgG-like bispecific antibodies and non-IgG-like bispecific antibodies.
Fig. 1. Various bispecific antibodies (bsAbs) are currently in clinical development (Drug Discovery Today. 2015, 20(7): 838-847).
IgG-like bispecific antibodies: IgG-like BsAb has an Fc part and has Fc-mediated effector functions, such as antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC) and antibody dependent cell-mediated phagocytosis (ADCP). The molecular weight of IgG-like BsAb is relatively large, and its Fc part helps in the later purification of the antibody and improves its solubility and stability. And the Fc part can bind to the receptor FcRn, increasing the serum half-life of the antibody. Such antibody structures mainly include Triomabs/quadroma, DVD-Ig (dual variable domain Ig), CrossMAb, Two-in-one IgG, and scFv2-Fc.
Non-IgG-like bispecific antibodies: non-IgG-like BsAb lacks the Fc fragment and exerts therapeutic effects only through antigen binding. It has the characteristics of low immunogenicity, easy production, and small molecular weight. Due to its smaller relative molecular weight, non-IgG-like BsAb has higher penetration into tumor tissue and therefore has a stronger therapeutic effect. These BsAbs come in many forms, mainly including TandAb (tandem diabody), scFv-HSA-scFv, BiTE (bispecific T-cell engager), DART (dual affinity retargeting) and Nanobody.
Compared with ordinary antibodies, BsAb adds a specific antigen-binding site and shows the following advantages in treatment: Mediating immune cell killing of tumors: An important mechanism of action of bispecific antibodies is to mediate immune cell killing. Bispecific antibodies have two antigen-binding arms, one of which binds to the target antigen and the other to the labeled antigen on the effector cells. The latter can activate the effector cells to target and kill tumor cells.
Dual target signal blocking exerts unique or overlapping functions to effectively prevent drug resistance: Blocking dual signaling pathways is another important mechanism of action of bispecific antibodies because it can bind dual targets simultaneously. Receptor tyrosine kinases (RTKs) are the largest class of enzyme-linked receptors and play an important regulatory role in the cell proliferation process, such as the Her family. RTKs are abnormally highly expressed on the surface of tumor cells, leading to malignant proliferation of tumor cells, and are therefore also important targets for tumor treatment. Single-target monoclonal antibodies against RTKs have been widely used in tumor treatment. However, tumor cells can activate intracellular HER family members by switching signaling pathways or by homo- or heterodimers between HER family members themselves or between different members. Signals for immune evasion. Therefore, the use of bispecific antibody drugs to simultaneously block two or more RTKs or their ligands can reduce the escape of tumor cells and improve the therapeutic effect.
Fig. 2. Mode of action of bispecific antibodies (Drug Discovery Today. 2015, 20(7): 838-847).
With stronger specificity, targeting and low off-target toxicity: the two antigen-binding arms of bispecific antibodies can combine the characteristics of different antigens. The two antigen-binding arms respectively bind to two antigens on the surface of cancer cells, which can effectively enhance the binding specificity and targeting of the antibody to cancer cells and reduce side effects such as off-target.
Effectively reduce treatment costs: Compared with traditional antibodies, BsAb is highly competitive in terms of tissue penetration rate, tumor cell killing efficiency, off-target rate and clinical indications, and has significant clinical advantages. Especially in terms of dosage, since its therapeutic effect can reach 100-1000 times that of ordinary antibodies, the dosage can be as low as 1/2000 of the original, significantly reducing the cost of drug treatment. Relative to combination therapy, the cost of bispecific antibodies is also much lower than that of two single-agent combination therapies.
Currently, bispecific antibody ADCs entering clinical practice are divided into two categories: dual-epitope ADCs and dual-target ADCs, which mainly include dual-epitope ADCs with three targets (HER2 dual-epitope, Met dual-epitope, and FRα dual-epitope), while dual-target ADCs mainly combine EGFR and other targets. Compared with monoclonal antibodies, bispecific antibody ADCs are more effective and safer. Their performance is as follows:
(1) Two antigen-binding sites can bind tumor cells and immune cells, redirect immune cells, recruit immune cells to the surroundings of tumor cells, and enhance the killing power of tumors;
(2) It can block two different signaling pathways at the same time to enhance cell killing toxicity;
(3) Dual antibodies can target tumor cells more specifically, overcome drug resistance, increase drug safety, and relatively reduce side effects caused by off-target effects.
Fig. 3. Bispecifics and antibody-drug conjugates (Drug Discovery Today: Technologies. 2018, 30: 55-61).
ZW-49 has been recognized as the leader in bispecific antibody ADCs. It is a bispecific antibody ADC drug developed by Zymeworks that can specifically bind to two non-overlapping epitopes of the HER2 receptor at the same time. This product uses a protease-cleavable linker to couple HER2 dual antibodies and ZymeLink Auristatin toxin. The parent monoclonal antibodies of ZW-49 are two already on the market, namely pertuzumab and trastuzumab. ZW-49 targets both the ECD2 (pertuzumab binding site) and ECD4 (trastuzumab binding site) epitopes of HER2.
MEDI4276 is a bi-epitope quadrivalent HER2 ADC drug. It uses 39S (epitope similar to Pertuzumab, fully human antibody, recognizes domain 2) as the backbone, connected to the scFv structure of trastuzumab (recognizes domain 4). The recognition epitopes of 39S and trastuzumab are located at opposite ends of the HER2 ECD and are >90 Å apart from each other. The DAR is 4, the linker is maleimidocaproyl linker, and the toxin is tubulysinwarhead. However, development of MEDI4276 has been suspended due to toxicity issues.
Regenron has a bispecific ADC product REGN5093-M114 that targets two different epitopes of MET. It consists of an asymmetric bispecific antibody connected to the toxin M24 (maytansine derivative) through a linker. The DAR value is about 3.2. It is worth noting that the REGN5093-M114 naked antibody itself also has a certain effect in inhibiting tumors and has demonstrated good efficacy in various preclinical models.
The targets of most bispecific ADCs are two different epitopes of the same target, such as the three products above. However, Merck's bispecific ADC pipeline M1231 also targets EGFR/MUC1, which is co-expressed in a variety of tumor cells, including non-small cell lung cancer (NSCLC), esophageal squamous cell carcinoma (ESCC), head and neck squamous cell carcinoma cell carcinoma, triple-negative breast cancer, and ovarian cancer, among others. M1231 is developed based on the SEED (Strand Exchange Engineering Domain) antibody scaffold. The antibody part targeting MUCI is scFv, while the antibody part targeting EGFR is in Fab form. The DAR value is about 4. M1231 is linked to the hemiasterlin-related microtubule inhibitor via a cleavable Val-Cit linker. In preclinical studies, M1231 demonstrated payload delivery and positive anti-tumor activity against tumor cells. Currently, M1231 is undergoing clinical research on solid tumors, and no data has been released yet.
ADCs and bsAbs are greatly improved forms of antibody drugs with strong tumor specificity. More than 100 ADCs and bsAbs are currently under clinical evaluation. However, in order to break the competition between dual-antibody and ADC, dual-antibody ADC came into being. Although many bsAb ADC drug candidates are still in the early stages of clinical trials. By conjugating bsAbs and ADCs (bsADCs), it is expected to achieve synergistic effects in the future, leading to the development of more effective drugs.