Bispecific ADCs: Solving Antigen Heterogeneity or Masking Payload Resistance?

Bispecific antibody-drug conjugates (ADCs) have become one of the most visible areas of oncology drug development. The rationale is compelling: adding a second binding specificity may broaden tumor-cell recognition, reduce dependence on a single antigen, and, in selected settings, improve receptor clustering or internalization. These properties are particularly attractive in solid tumors, where antigen expression can vary between lesions and among subclones within the same tumor ^[1-2].

The issue is not whether bispecific ADCs have biological value; clearly, they can. The more interesting question is whether the industry has begun to treat bispecificity itself as evidence of a next-generation ADC. That assumption becomes harder to sustain as resistance increasingly shifts from the cell surface to the intracellular pharmacology of the payload.

The Value of Dual Targeting

Antigen heterogeneity provides the clearest rationale for a bispecific ADC. A conventional ADC depends on sufficient expression of a single surface antigen, making loss or heterogeneous distribution of that antigen a potential route of escape. Engaging a second target can preserve recognition across a broader tumor-cell population and may provide additional opportunities for internalization ^[3].

Figure 1. In comparison to traditional ADCs, bispecific antibody drug conjugates present distinct design considerations and properties as the next-generation ADCs ^[4].

The biology, however, is more demanding than simply pairing two established targets. Co-expression patterns, receptor density, epitope accessibility, internalization kinetics, normal-tissue distribution, and antibody geometry all influence whether dual targeting creates a meaningful advantage. A pair that appears attractive from expression data alone may behave very differently once both binding arms are incorporated into the same molecule. The important question is therefore not how many targets an ADC recognizes, but whether the second target solves a defined limitation of the first ^[1-2].

When Resistance Moves Beyond the Target

After an ADC enters a tumor cell, activity depends on intracellular trafficking, linker processing, payload release, and cellular sensitivity to the payload. Changes in drug transport, DNA-damage response, apoptotic signaling, or metabolism can therefore reduce efficacy even when antigen recognition remains intact ^[5].

This is increasingly relevant for topoisomerase I (Topo I) inhibitor payloads, whose potency and bystander activity have driven widespread use across ADC pipelines. As exposure to related Topo I mechanisms increases, resistance may persist even when the antibody target changes ^[6].

Figure 2. The role of ABC transporter in topo-active cancer drug resistance ^[7].

Increased drug efflux, altered DNA-damage responses, or reduced sensitivity to Topo I-mediated injury may therefore limit the benefit of simply delivering another Topo I payload through a different targeting scaffold ^[3,5]. For next-generation ADCs, a key challenge is maintaining activity and therapeutic window as tumors become less naïve to dominant payload classes.

Topo I Resistance Is Pushing Payload Innovation Forward

Topo I resistance is driving interest in payloads with distinct mechanisms, improved control of intracellular release, and dual-payload ADCs that deliver two mechanistically different agents ^[8]. The rationale for dual-payload designs is mechanistic complementarity: resistance to one payload may leave activity through the second intact, while distinct forms of cellular stress may make escape through a single resistance pathway more difficult ^[9].

The challenge is molecular control. Payloads differ in potency, hydrophobicity, stability, permeability, and release requirements. Combining them requires control over loading, ratio, distribution, and product heterogeneity, making antibody, linker, payload, and conjugation increasingly interdependent ^[8–10].

Figure 3. The evolution of antibody–drug conjugate (ADC) payloads ^[11].

Overcoming Topo I resistance is therefore unlikely to depend on payload substitution alone. It is more likely to require broader optimization of ADC architecture.

Complexity Raises the Bar for Molecular Control

As payload and antibody formats become more complex, linker and conjugation design become increasingly important. Linker stability, drug-to-antibody ratio, and conjugation site directly affect release, hydrophobicity, clearance, aggregation, and pharmacokinetics ^[12]. ADC development is therefore shifting from optimizing individual components to integrating them as a single molecular system. A potent payload is of limited value in an unstable conjugate, while a sophisticated antibody format may still fail if internalization, trafficking, or physicochemical properties are unfavorable. Conversely, a conventional antibody can remain highly effective when the payload and delivery architecture are well matched to tumor biology ^[13].

Figure 4. Next-generation antibody-drug conjugates ^[14].

Antibody Discovery Should Anticipate the Final ADC

Epitope, internalization efficiency, receptor trafficking, specificity, stability, and developability all influence whether an antibody can support effective payload delivery. In bispecific ADCs, these requirements become more demanding because two binding interfaces must be balanced while maintaining suitable molecular behavior.

Figure 5. Design considerations on types of BsAb in BsADCs ^[4].

For this reason, antibody discovery and preclinical ADC evaluation increasingly benefit from being considered as a connected workflow. Cyagen's HUGO-Ab™ platform supports fully human antibody discovery across monoclonal, bispecific, and other advanced antibody formats (e.g. VHH). Once ADC candidates are available, Cyagen's cell-based platforms can evaluate target binding, cellular internalization, payload activity, cytotoxicity, and functional effects, while target-humanized and multiplex humanized mouse models enable more translationally relevant assessment of complex targeting strategies. Its preclinical CRO capabilities further support efficacy, biodistribution, PK/PD, and safety studies.

Figure 6. Cyagen’s end-to-end services in the ADC/AOC development process.

The connection between these stages is scientifically useful. Internalization data can influence antibody prioritization; humanized target models can reveal whether activity depends on a biologically realistic antigen context; and in vivo pharmacology can show whether advantages observed at the binding or cellular level remain meaningful at the whole-organism level. For bispecific ADCs in particular, multiplex humanized models can help evaluate dual-target biology within the same in vivo system.

What Is Actually Behind the Bispecific ADC Bubble?

The next generation of ADCs may be defined less by structural novelty than by how precisely each design addresses the biology that limits activity. As resistance mechanisms become more diverse, the field may also need to rethink how ADCs are selected, sequenced, and evaluated across treatment lines. An important question is whether future ADC development will require resistance-informed biomarkers that can distinguish when a tumor needs a new target, a new payload mechanism, or an entirely different delivery strategy.

Ultimately, the meaningful measure of ADC innovation is not structural novelty, but whether the architecture addresses where activity is actually being lost. As ADC strategies continue to evolve, Cyagen supports researchers with the humanized antibody platform and end-to-end preclinical services to evaluate the biology behind these increasingly complex therapeutic designs.

👉 [Explore our ADC and AOC Development Platform]

👉 [Download the HUGO-Ab™ Fully Human Antibody Discovery Brochure]

👉 [Discuss Your Next ADC Project with Our Experts]

References

[1] Ji X, Yang Y, Ma C, Huang W, Guo S, Wang L, Zheng H, Wu X. Recent advances in bispecific antibody-drug conjugates for breast cancer therapy. Cancer Chemother Pharmacol. 2026 Jan 24;96(1):15. doi: 10.1007/s00280-025-04863-9. PMID: 41579215; PMCID: PMC12831798.

[2] Esapa B, Jiang J, Cheung A, Chenoweth A, Thurston DE, Karagiannis SN. Target Antigen Attributes and Their Contributions to Clinically Approved Antibody-Drug Conjugates (ADCs) in Haematopoietic and Solid Cancers. Cancers (Basel). 2023 Mar 19;15(6):1845. doi: 10.3390/cancers15061845. PMID: 36980732; PMCID: PMC10046624.

[3] Li S, Zhao X, Fu K, Zhu S, Pan C, Yang C, Wang F, To KKW, Fu L. Resistance to antibody-drug conjugates: A review. Acta Pharm Sin B. 2025 Feb;15(2):737-756. doi: 10.1016/j.apsb.2024.12.036. Epub 2024 Dec 31. PMID: 40177568; PMCID: PMC11959940.

[4] Gu Y, Wang Z, Wang Y. Bispecific antibody drug conjugates: Making 1+1>2. Acta Pharm Sin B. 2024 May;14(5):1965-1986. doi: 10.1016/j.apsb.2024.01.009. Epub 2024 Jan 20. PMID: 38799638; PMCID: PMC11119582.

[5] Pommier Y. Molecular Determinants of Response and Rational Drug Combinations for Antibody-Drug Conjugates (ADCs) with Topoisomerase I (TOP1) Inhibitor Payloads. J Mol Biol. 2026 Aug 15;438(16):169841. doi: 10.1016/j.jmb.2026.169841. Epub 2026 May 4. PMID: 42092629; PMCID: PMC13231283.

[6] Conilh L, Sadilkova L, Viricel W, Dumontet C. Payload diversification: a key step in the development of antibody-drug conjugates. J Hematol Oncol. 2023 Jan 17;16(1):3. doi: 10.1186/s13045-022-01397-y. PMID: 36650546; PMCID: PMC9847035.

[7] Tao J, Gu Y, Zhou W, Wang Y. Dual-payload antibody-drug conjugates: Taking a dual shot. Eur J Med Chem. 2025 Jan 5;281:116995. doi: 10.1016/j.ejmech.2024.116995. Epub 2024 Oct 23. PMID: 39481229.

[8] Sharma NK, Bahot A, Sekar G, Bansode M, Khunteta K, Sonar PV, Hebale A, Salokhe V, Sinha BK. Understanding Cancer's Defense against Topoisomerase-Active Drugs: A Comprehensive Review. Cancers (Basel). 2024 Feb 6;16(4):680. doi: 10.3390/cancers16040680. PMID: 38398072; PMCID: PMC10886629.

[9] Wen M, Yu A, Park Y, Calarese D, Gerber HP, Yin G. Homogeneous antibody-drug conjugates with dual payloads: potential, methods and considerations. MAbs. 2025 Dec;17(1):2498162. doi: 10.1080/19420862.2025.2498162. Epub 2025 May 5. PMID: 40322862; PMCID: PMC12054377.

[10] Géraud A, Gougis P, de Nonneville A, Beaufils M, Bertucci F, Billon E, Brisou G, Gravis G, Greillier L, Guerin M, Mezni E, Mitry E, Noel R, Pignon J, Sabatier R, Seguin L, Spano JP, Vicier C, Viret F, Goncalves A, Ciccolini J. Pharmacology and pharmacokinetics of antibody-drug conjugates, where do we stand? Cancer Treat Rev. 2025 Apr;135:102922. doi: 10.1016/j.ctrv.2025.102922. Epub 2025 Mar 25. PMID: 40157134.

[11] Wang Z, Li H, Gou L, Li W, Wang Y. Antibody-drug conjugates: Recent advances in payloads. Acta Pharm Sin B. 2023 Oct;13(10):4025-4059. doi: 10.1016/j.apsb.2023.06.015. Epub 2023 Jun 30. PMID: 37799390; PMCID: PMC10547921.

[12] Hasan MM, Laws M, Jin P, Rahman KM. Factors influencing the choice of monoclonal antibodies for antibody-drug conjugates. Drug Discov Today. 2022 Jan;27(1):354-361. doi: 10.1016/j.drudis.2021.09.015. Epub 2021 Sep 28. PMID: 34597756.

[13] Aoyama M, Tada M, Yokoo H, Ito T, Misawa T, Demizu Y, Ishii-Watabe A. Linker and Conjugation Site Synergy in Antibody-Drug Conjugates: Impacts on Biological Activity. Bioconjug Chem. 2024 Oct 16;35(10):1568-1576. doi: 10.1021/acs.bioconjchem.4c00348. Epub 2024 Oct 3. Erratum in: Bioconjug Chem. 2026 May 20;37(5):1070-1071. doi: 10.1021/acs.bioconjchem.6c00229. PMID: 39363433; PMCID: PMC11488503.

[14] Etessami JD, Valenza C, Tolcher AW, LoRusso P, Curigliano G. Advancing Antibody-Drug Conjugates: Current Perspectives and Future Directions. Am Soc Clin Oncol Educ Book. 2026 Jun;46(3):e517110. doi: 10.1200/EDBK-26-517110. Epub 2026 May 14. PMID: 42133892.