The “1+1>2” effect of dual-target ADCs is not a simple addition of the killing effects of two targets. Its core lies in achieving enhanced antitumor activity, reduced resistance, and widened therapeutic windows through optimized dual-target selection, payload combinations, and conjugation technologies. This synergistic effect is particularly valuable clinically—traditional single-target ADCs face the challenge of “intra-class cross-resistance.” For example, the TROPION-PanTumor01 study showed that patients resistant to Topo1i ADCs had a confirmed ORR of only 15% to the new Topo1i ADC datopotamab deruxtecan, whereas dual-target ADCs can overcome this through complementary mechanisms. The synergy is reflected in superior tumor clearance at the same dose without significant toxicity enhancement, or more efficient outcomes than combining single-target ADCs while avoiding issues like PK mismatches and toxicity overlap. For instance, HER2/EGFR dual-target ADCs can simultaneously block two oncogenic pathways, preventing signal compensation triggered by single-target inhibition—this is the fundamental difference between “synergy” and “additivity.”
Currently, dual-target ADCs have moved from preclinical to early clinical stages, with Chinese companies leading the development. Beijing Betta Biotech’s EGFR/HER3 dual-antibody ADC BL-B01D1 achieved a clinical ORR >60%, significantly outperforming similar single-target drugs; Innovent’s IBI3020 (CEACAM5-targeting, Topo1i+MMAE) showed efficacy in single-payload resistant models, with Phase I enrolling mainly third-line colorectal cancer and other solid tumors with CEA >10 ng/mL. Internationally, Sutro’s STRO-227 (PTK7-targeting, Exatecan×8+MMAE×2) remains effective in tumor models resistant to sequential Dxd and MMAE ADCs, with better tumor inhibition in breast cancer models than single-payload ADCs; Callio’s CLIO-8221 (HER2-targeting, Topo1i+ATRi, DAR 4+4) achieves effective tumor regression with a single dose in T-DXd resistant models, with no unexpected toxicity at 30 mpk in NHPs, offering a therapeutic window over 13-fold. Kyinhong Biotech’s TROP2-targeting dual-payload ADC KH815 has entered clinical trials (NCT068856450), further confirming the potential of synergistic killing.
Despite promising prospects, multiple obstacles remain in achieving synergistic killing. First is blind target combination—some dual-targets fail to consider pathway relevance, resulting in “ineffective synergy.” For example, MedImmune’s MMAE+SG3457 (a highly potent PBD dimer) ADC with DAR 2+2, despite dual mechanisms, did not show notable in vitro potency improvement compared with the respective single-drug ADCs. Second, payload activity imbalance—Topo1i (such as Dxd and Exatecan, IC50 in sub-nanomolar to nanomolar range) and MMAE (IC50 typically in picomolar range) differ by 10–100 fold; improper DAR ratios can render the weaker payload ineffective. Third, heterogeneous conjugation—the heterogeneity of multi-payload ADCs can narrow the therapeutic window. Fourth, overlapping toxicity—dual cytotoxic payloads may exacerbate hematologic toxicity. Fifth, PK/PD mismatch—differences in release and metabolism of the two payloads can desynchronize effects.
Four Core Dimensions for Achieving Synergistic Killing
The “1+1>2” synergy of dual-target ADCs is not simple target addition; it requires precise synergy across target selection, payload pairing, conjugation technology, and DAR-linker optimization. Each technological breakthrough corresponds to a core subproblem in achieving synergy, and only systematic resolution enables dual-target ADCs to overcome the efficacy bottlenecks of single-target drugs.
1. Target Combination: The Premise of Synergy
Target combination is the primary prerequisite for dual-target ADC synergy. Ineffective combinations may result in “1+1<2” with additive toxicity or neutralized efficacy. Scientific target selection follows the principle of “functional complementarity, mechanistic synergy,” either targeting core tumor survival pathways or forming a loop addressing tumor heterogeneity or resistance, laying the foundation for subsequent payload efficacy.
Current mainstream strategies fall into three categories: first, “core pathway dual inhibition,” targeting two key signaling pathways essential for tumor cell proliferation, such as HER2 and EGFR, which are often co-expressed and functionally complementary in solid tumors like pancreatic cancer. Dual targeting avoids pathway compensation after single-target inhibition; second, “synthetic lethal pairs,” combining Topo1i targets with DNA damage repair (DDR) pathway targets. Topo1i induces DNA damage, while DDR inhibition prevents repair, creating a synthetic lethal effect; third, “tumor microenvironment and cell killing synergy,” combining tumor surface antigens with immune-related targets for direct killing and immune activation.
Preclinical and clinical examples validate the value of rational target selection. Dr. Cai Xianghai’s team focuses on dual-target ADCs selecting core tumor antigens as the targeting basis. In pancreatic cancer models, the HER2/EGFR dual-target drug AAA-PE24 specifically cross-links the two receptors, enhancing tumor binding efficiency and receptor-mediated endocytosis, extending tumor retention 360%–460% compared to monovalent drugs—directly reflecting target synergy. Innovent’s IBI3020 uses CEACAM5 as a single target with dual payloads, demonstrating the importance of target selection—high CEACAM5 expression in colorectal cancer enables precise dual-payload delivery, showing potential efficacy in third-line patients.
Industry consensus emphasizes the “synergy” of targets rather than quantity. Not all dual-target combinations yield synergy; functional relevance must be verified by bioinformatics and in vitro experiments to avoid meaningless overlap. In the future, personalized target combinations based on individual tumor genomic profiles will further enhance precise synergistic killing.
2. Payload Combination: Releasing Synergistic Potency
If target combination is the “navigation system,” payload combination is the “killing weapon,” directly determining the efficacy ceiling of “1+1>2.” The key is selecting complementary payloads that enhance direct killing or activate immune response to overcome resistance while maintaining balanced potency to avoid additive toxicity or insufficient efficacy.
Current mature strategies fall into three main categories, each with clear synergistic mechanisms and practical examples. The first is “cytotoxic + cytotoxic,” the most established strategy, attacking different cellular structures. Topo1i + MMAE is the typical example: MMAE disrupts microtubule dynamics to block cell division; Topo1i induces lethal DNA damage. Sutro’s STRO-227 (PTK7-targeting, Exatecan DAR8 + MMAE DAR2) remains effective in tumors resistant to sequential Dxd and MMAE ADCs and shows superior inhibition in breast cancer models, with HNSTD 25 mpk in NHPs comparable to Dxd ADC, demonstrating synergistic balance of efficacy and safety. Another cytotoxic combination is “dual DNA damage mechanisms,” e.g., PBD dimer + camptothecin analogs, enhancing damage through DNA crosslinking and topoisomerase trapping, but potency matching is crucial—MedImmune’s MMAE+SG3457 did not outperform single-drug ADCs.
The second is “cytotoxic + immune agonist,” a high-risk, high-reward approach. Cytotoxic agents induce immunogenic cell death (ICD), releasing tumor antigens; immune agonists activate APCs and promote T cell infiltration, converting “cold” tumors to “hot” and generating long-term immune memory. Sutro and Astellas’ Her2-iADC (Her2-targeting, Exatecan DAR4 + STING agonist DAR2) shows preclinical tumor inhibition and NK/monocyte proliferation, outperforming Exatecan ADC or STING ADC alone, with NHP MTD 25 mpk and minimal STING agonist release (~0.01%), thanks to stable linker design mitigating immune toxicity.
The third is “synthetic lethal” combinations, exploiting tumor genetic defects. Topo1i + DDR inhibitor is a typical example. Tumor resistance to Topo1i often upregulates DDR repair, while ATRi specifically blocks this, sensitizing tumor cells to Topo1i. Callio’s CLIO-8221 (HER2-targeting, Topo1i+ATRi DAR4+4) achieves effective tumor regression with a single dose in T-DXd resistant models, with no unexpected toxicity at 30 mpk in NHPs, offering >13-fold therapeutic window, demonstrating synthetic lethal synergy.
The key principle in payload combination is “potency balance.” Materials indicate Dxd/Exatecan activity is sub-nanomolar to nanomolar, while MMAE is picomolar—10–100 fold difference—necessitating DAR ratio design for balance, e.g., Sutro’s 8+2 Exatecan+MMAE. Conversely, Scripps’ MMAF+PNU-159682 lacked potency balance, showing no in vitro advantage over single-drug ADCs. Industry trends show payload combinations evolving from traditional cytotoxic addition to “cytotoxic+immune” and “cytotoxic+targeted inhibitors,” especially with immune agonists offering solutions for resistant solid tumors.
3. Precision Conjugation: Ensuring Synergistic Delivery
Even with optimal targets and payloads, without precise conjugation, dual-target ADCs cannot achieve synergy. Heterogeneous conjugates cause efficacy fluctuations, increased toxicity, or failure to co-deliver dual payloads. Precision conjugation aims for “site-specificity, controllable ratio, and uniform product,” providing a stable delivery vector for synergy.
Mature conjugation techniques include site-specific and multi-site methods. Site-specific attaches dual payloads at the same antibody site using methods like maleimide chemistry, microbial transglutaminase (mTG) + click chemistry, and disulfide bridging. Seattle Genetics used maleimide and inter-chain disulfide conjugation with PEG24 modification, achieving high DAR (MMAE+MMAF DAR8+8) without aggregation, significantly enhancing antitumor activity. mTG + click chemistry installs orthogonal groups; University of Texas connected MMAE+MMAF via azide-DBCO and MTz-TCO, ensuring high uniformity and superior efficacy in HER2-low heterogeneous HCC1954-TDR models. Site-specific conjugation, however, requires multiple purification steps and has short inter-payload distance, optimized with PEG linkers.
Multi-site conjugation attaches dual payloads at different antibody sites, addressing single-site limitations and is the mainstream approach. Methods include noncanonical amino acid (ncAA) incorporation, genetic code expansion (GCE), and orthogonal enzyme conjugation. ncAA allows site-specific modification via amber suppression tRNA/aaRS systems, yielding uniform products but requiring special expression systems. GCE can integrate selenocysteine into antibodies for dual modification, e.g., HER2 PNU-159682/MMAF construct with S396U-HC and A114C-HC, DAR_PNU-159682 1.9, DAR_MMAF 1.5. Orthogonal enzyme conjugation (e.g., mTG + LpIA) enables specific conjugation for payloads with different release kinetics.
Shanghai Institute of Materia Medica combined glycoengineering and enzymatic conjugation to develop a “one-pot” multi-site method: N297 glycosylation and K248 lysine sites were conjugated sequentially with endo-S2 and FcBP-TE-payload complexes, simplifying purification and improving yield. The resulting dual-payload ADC demonstrated comparable tumor inhibition to high-DAR single-payload ADCs in NCI-N87 models.
4. DAR and Linker Optimization: Balancing Synergy
Dual-target ADC synergy requires precise control of “dose” and “release timing,” corresponding to DAR ratio optimization and linker design. DAR ratio determines intracellular concentration balance of dual payloads, while linker controls release kinetics, jointly defining synergy and therapeutic window.
DAR optimization centers on “potency matching,” adjusting ratios based on payload activity differences. MMAE IC50 is picomolar, Dxd/Exatecan sub-nanomolar to nanomolar—a 10–100-fold difference—so Topo1i+MMAE uses “high-potency payload low DAR, low-potency payload high DAR.” Sutro’s STRO-227 (Exatecan DAR8 + MMAE DAR2) showed superior inhibition and safe NHP HNSTD 25 mpk, confirming efficacy-safety balance. Innovent’s IBI3020 used Topo1i+MMAE 2+2, demonstrating high antitumor activity in colorectal mouse models and adherence to individualized DAR principle in Phase I.
DAR also accounts for payload hydrophobicity. Seattle Genetics showed PEG24-modified maleimide linkers allowed MMAE+MMAF DAR8+8 without aggregation, enhancing antitumor activity. Without hydrophilic modification, high DAR induces ADC aggregation in circulation, increasing off-target toxicity. Dual-payload ADC DAR ratios are diversifying: 2+2, 4+4, 2+4, offering more flexibility than fixed single-payload DARs (2, 4, 8).
Linker design focuses on “orthogonality” and “stability.” Orthogonality ensures independent payload release; stability maintains payloads in circulation until tumor-specific release. Seagen’s dual-protected cysteine linker allows sequential conjugation via orthogonal protection, ensuring controlled release. Sutro’s Her2-iADC uses stable linker to minimize STING agonist detachment (~0.01%), preventing systemic toxicity, with NHP MTD 25 mpk.
University of Texas’ tri-arm linker uses two orthogonal click reactions to connect MMAE and MMAF, avoiding cross-reaction and enabling synchronized release in HER2-low HCC1954-TDR models, outperforming single-drug combination, confirming orthogonal linker design value. Future linkers will evolve to “smart responsive” types (pH/enzymatic), controlling release order based on tumor microenvironment for enhanced synergy precision.
Future Prospects and Innovation Focus
Dual-target ADC achieving “1+1>2” centers on four-dimensional synergy: “scientific target combination + complementary payload mechanisms + controllable conjugation + precise toxicity management.” Targets anchor core tumor pathways (e.g., EGFR×HER3 blocking HER family compensation, Betta BL-B01D1); payloads match potency and logic (e.g., Sutro STRO-227 Exatecan+MMAE, Callio CLIO-8221 Topo1i+ATRi synthetic lethal); conjugation relies on ncAA or enzyme-based site-specific methods for uniformity; PK/PD optimization prevents toxicity overlap, overcoming single-target ADC resistance and tumor heterogeneity bottlenecks.
Future innovation focuses on three directions:
Technologically, ncAA conjugation and responsive linkers (pH/enzymatic) enable precise payload release. Strategically, “cytotoxic + immune agonist” combinations (e.g., Sutro Her2-iADC Exatecan+STING agonist) convert “cold” tumors to “hot.” Application-wise, biomarker-guided patient stratification improves response rates. Chinese companies, with Innovent IBI3020 (CEACAM5-targeting) and Kyinhong KH815 (TROP2-targeting), hold clinical first-mover advantage. With subsequent data readouts, dual-target ADCs are poised to become core treatment options for resistant solid tumors, advancing precision oncology.
