Introduction of Bispecific ADC
Antibody-drug conjugates (ADCs) have become a cornerstone of targeted cancer therapy, coupling the antigen specificity of monoclonal antibodies with the potent cytotoxicity of small-molecule drugs. By selectively delivering their payload to tumor cells, ADCs can achieve high efficacy while reducing the systemic toxicity of conventional chemotherapy.
However, most approved ADCs rely on a single antigen for targeting. In heterogeneous tumors, where antigen expression varies across cells, this single target approach can leave some cells untreated. Antigen downregulation or mutation can further drive resistance, while expression of the target in healthy tissues increases the risk of “on-target, off-tumor” toxicity.
Bispecific antibody-drug conjugates (BsADCs) are an emerging strategy to overcome the limitations of traditional ADCs. By binding to two distinct antigens or epitopes, they can broaden tumor coverage, limit escape caused by antigen loss, and increase binding specificity. Certain designs further enhance internalization or redirect intracellular trafficking to optimize payload delivery, improving tumor cell kill while minimizing off-target toxicity. By integrating the targeting flexibility of bispecific antibodies with the potent cytotoxicity of ADCs, BsADCs offer a promising next-generation platform to expand the therapeutic window and better tackle resistant or heterogeneous cancers.
Figure 1. Comparison of conventional ADC and Bispecific ADC.
Design Consideration for Bispecific ADCs
- The development of BsADCs requires coordinated optimization of all three ADC components, antibody, linker, and payload, with the added complexity of dual-target binding. Simply replacing a monospecific antibody with a bispecific format is not enough. The unique pharmacology of BsADCs demands a holistic approach to maximize efficacy and safety.
· Targeting Strategy
- Both targets should have high tumor selectivity and minimal expression in healthy tissues to widen the therapeutic window. Binding mode considerations include biparatopic versus dual-antigen engagement. Additional considerations encompass antigen internalization rate, recycling versus lysosomal trafficking, and expression heterogeneity across tumor subtypes.
· Antibody Format
- Fc-containing designs (IgG-like) provide longer half-life and immune effector functions but risk nonspecific uptake. FC-lacking formats (Fab or VH-VL) improve tumor penetration but have shorter half-life and stability issues. Fc engineering helps fine-tune this balance between efficacy and safety.
· Linker-Payload Optimization
- The linker must balance stability in circulation with efficient intracellular release. Cleavable linkers exploit tumor-specific conditions, while non-cleavable linkers may improve safety when off-target risk is high. Payloads should match the targeting strategy and tumor biology, with consideration for potency, bystander effect, and payload permeability.
· Integrated Design
- Changes in any component can alter BsADC pharmacology. Antibody engineering, linker chemistry, and payload selection must be considered together, with preclinical evaluation of potency, trafficking, biodistribution, and toxicity.
Figure 2. Design considerations in bispecific ADC.
Main mechanisms of BsADCs
BsADCs can be designed to suit different types of tumors, but most clinical designs follow three main strategies: aggregation, receptor inhibition, and hijacking intracellular transport. These approaches respectively enhance internalization through receptor clustering, block multiple signaling pathways to suppress tumor growth, or exploit alternative trafficking routes to maximize payload release inside cancer cells. Less common mechanisms include cell bridging, piggybacking the barrier, and co-factor minetic effects.
· Aggregation
One of the most effective ways to boost ADC internalization is by inducing receptor clustering on the tumor cell surface. Bispecific ADCs can achieve this via biparatopic targeting, where both binding arms engage different sites of the same receptor. This forces receptor clustering, triggering internalization and lysosomal trafficking for payload release.
An example is ZW49, a biparatopic HER2-targeting ADC designed to improve efficacy in tumors with low to moderate HER2 expression and to overcome resistance seen with conventional HER2 ADCs. Built on a modified asymmetric IgG-scFv design, one arm is a full Fab recognizing the ECD2 domain of HER2, while the other is an scFv binding to the ECD4 domain. This non-overlapping epitope recognition enables the antibody to “grip” HER2 at two distinct sites, inducing extensive receptor clustering on the tumor cell surface. The clustering triggers rapid receptor internalization and efficient trafficking to lysosomes, where the auristatin payload is released. This dual epitope strategy enhances payload delivery compared with monospecific HER2 ADCs and increases tumor cell killing, even in cancers with low to moderate HER2 expression. Early-phase clinical studies have shown that ZW49 has a manageable safety profile and promising anti-tumor activity in heavily pretreated HER2-positive patients.
Figure 3. Aggregation mechanism of Bispecific ADCs. A. Illustrative structure of ZW-49. B. Schematic illustration of aggregation mechanism of ZW-49 in tumor cells.
· Receptor Inhibition
Another powerful strategy for bispecific ADCs is to block multiple signaling pathways that drive tumor growth and survival by dual-antigen targeting. This simultaneous inhibition reduces the likelihood of resistance, can enhance internalization for better payload delivery, and may also lower on-target, off-tumor toxicity by requiring co-expression of both targets for binding.
An example is AZD9592, an EGFR-cMet bispecific ADC developed to address resistance and limited durability of single target EGFR therapies. Constructed on the DuetMab bispecific IgG platform, it binds both EGFR and cMet but with more than 15-fold higher affinity for cMet, reducing EGFR-mediated binding in healthy tissues and lowering on-target, off-tumor toxicity. By blocking two key oncogenic pathways often co-expressed in tumors, AZD9592 shuts down compensatory signaling that drives resistance, while dual engagement also enhances internalization and trafficking for efficient payload release. It employs a cleavable linker to deliver a proprietary topoisomerase I inhibitor, causing DNA double-strand breaks. Following promising preclinical results, AZD9592 is now in a first-in-human phase I trial with advanced solid tumors.
Figure 4. Receptor inhibition mechanism of Bispecific ADCs. A. Illustrative structure of AZD9592. B. Schematic illustration of receptor inhibition mechanism of AZD9592 in tumor cells.
· Hijacking intracellular transport
Some tumor-associated receptors, such as HER2, are inefficiently trafficked to lysosomes after internalization, instead recycling back to the cell surface. This limits payload release. Bispecific ADCs can overcome this by pairing a primary tumor target with a “helper” receptor that naturally undergoes rapid lysosomal trafficking, redirecting the ADC complex for more efficient payload release.
An example is HER2-PRLR bispecific ADCs. These BsADCs were designed to overcome the inefficient lysosomal trafficking of HER2, which often recycles back to the cell surface instead of delivering its payload. This construct pairs one binding arm against HER2 with another against the prolactin receptor (PRLR), a membrane protein known for rapid lysosomal routing after internalization. By co-engaging HRE2 and PRLR on the same tumor cell, the ADC “hijacks” PRLR’s trafficking pathway, directing the HER2-ADC complex more efficiently to lysosomes where the cytotoxic payload can be released. Preclinical studies have shown improved lysosomal delivery and in vitro potency compared with monospecific HER2 ADCs, supporting this strategy as a way to enhance payload release without altering the HER2-binding arm or the payload chemistry.
Figure 5. Hijacking intracellular transport mechanism of Bispecific ADCs. A. Illustrative structure of HER2xPRLR bispecific ADC. B. Schematic illustration of hijacking intracellular transport mechanism of HER2xPRLR bispecific ADC in tumor cells.
Conclusion
BsADCs build on the proven concept of ADCs while overcoming key limits of single-antigen targeting. Through aggregation, receptor inhibition, and hijacking intracellular transport, they can enhance internalization, block multiple tumor-promoting pathways, improve payload delivery, and in some designs, reduce on-target, off-tumor toxicity by increasing tumor selectivity. As preclinical and clinical development advances, BsADCs hold strong potential to deliver more durable responses, expand patient eligibility, and improve outcomes in heterogeneous and treatment-resistant cancers.
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