Antibody–drug conjugates (ADCs) have matured into a mainstream oncology platform over the past two decades, propelled by engineering innovations and a series of pivotal regulatory approvals. ADCs comprise a monoclonal antibody for selective antigen recognition, a cytotoxic payload, and a linker that governs stability and intracellular release. Collectively, improvements in each component have translated to expanded indications, better efficacy, and more manageable safety profiles across hematologic malignancies and solid tumors 13412.
A major inflection point occurred in 2019, when the FDA approved three new ADCs and expanded indications for an existing agent—nearly doubling the number of approved ADCs in a single year. This moment validated decades of design iteration and signaled the platform’s readiness for broader clinical utility 14. Subsequent advances, including HER2-low targeting and novel payload classes, have further expanded ADC applications from niche settings to heterogeneous and lower antigen–expressing disease 711.
Defining ADC Generations and Lessons Learned
While “generations” are not rigidly codified, the literature consistently frames ADC evolution in three waves, each defined by key design improvements that address limitations revealed in early clinical experience.
First era (pre-2010)
Early programs suffered from unconjugated antibodies, unstable linkers, and inadequate payload potency. These issues led to premature payload release and off-target toxicity, undermining efficacy and safety. The lessons catalyzed focused improvements in linker chemistry, conjugation stability, and payload selection 412.
Second era (2010–2018)
Clinically validated ADCs (e.g., brentuximab vedotin, trastuzumab emtansine, inotuzumab ozogamicin) established design principles: robust linker stability, potent microtubule inhibitors (auristatins, maytansinoids) and DNA-damaging agents (e.g., calicheamicin), and strategic antigen selection with high tumor expression. These successes positioned ADCs for broader adoption in hematologic malignancies and solid tumors 4912.
Third generation (2019–2026)
Composition advances—cleavable linkers enabling bystander killing, hydrophilic linker modifications to reduce aggregation, site-specific conjugation for homogeneous drug-to-antibody ratio (DAR), and new payload classes (topoisomerase I inhibitors, targeted inhibitors)—have driven efficacy improvements and expanded indications (e.g., HER2-low). Clinical programs and reviews highlight 13 ADCs commercially available and >300 in exploration, with combination strategies and novel formats (bispecifics, probodies) poised to overcome resistance 38111221.
Technical Advances Shaping Therapeutic Index
Site-specific conjugation and DAR control
Site-specific conjugation (e.g., cysteine mutants like trastuzumab-A114C) produces more homogeneous products with predictable pharmacology and stability, exemplified by neolymphostin-based ADC precursors 6. Accurate DAR determination via standard-free LC–HRMS enables robust CMC control linking structure to efficacy and pharmacokinetics, a prerequisite for scale-up and regulatory approval 13.
Linker innovations
Cleavable linkers (e.g., protease-sensitive valine-citrulline or pH/reduction-sensitive chemistries) have become dominant in next-generation designs to enable intracellular release and bystander effect, while non-cleavable linkers remain valuable for certain payloads and indications 8111220. Hydrophilic linker modifications and early-stage payload hydrophobicity assessment via UPLC-based assays help mitigate aggregation, improve conjugation yields, and optimize DAR 18.
Payload diversification
Beyond traditional microtubule inhibitors (MMAE/MMAF, DM1/DM4), payload classes now include DNA-damaging agents (e.g., duocarmazine, calicheamicin), topoisomerase I inhibitors (deruxtecan, SN-38), and targeted small-molecule inhibitors (e.g., neolymphostin targeting PIKK pathways). Repurposed conventional agents (vincristine) and emerging immunomodulatory payloads (e.g., STING agonists) highlight the expanding arsenal 611121521.
Bystander effect
Cleavable linkers coupled with membrane-permeable payloads enable cytotoxic diffusion into neighboring antigen-low or heterogeneous tumor cells—an approach explicitly designed to overcome resistance to earlier non-cleavable constructs (e.g., trastuzumab deruxtecan vs. trastuzumab emtansine) 117.
Novel formats and retargeting
Bispecific strategies and probody concepts aim to refine tumor selectivity and minimize off-tumor effects; dataset summaries flag probodies as an emerging format, though no clinical evidence was found in the retrieved materials 21. Adaptabodies—bispecific nanobodies that retarget existing monoclonal antibodies to new antigens—demonstrate feasibility in infectious disease models and could accelerate therapeutic development 5.
###Clinical Impact and Regulatory Milestones
The ADC field’s regulatory maturation is marked by approvals across hematologic malignancies and solid tumors. Key milestones and their design attributes are summarized below.
Approved ADCs and Pivotal Indications (Selected)
| ADC | Target | Linker/Payload Highlights | Approved Indications (selected) | Notes |
|---|---|---|---|---|
| Brentuximab vedotin | CD30 | Cleavable vc-MMAE | Lymphomas | Cornerstone in HL/ALCL; combination strategies in older patients (BV-nivolumab; BV-CHOP) 11019 |
| Polatuzumab vedotin | CD79b | Cleavable vc-MMAE | DLBCL | 2019 approval as part of a regulatory inflection year 114 |
| Loncastuximab tesirine | CD19 | PBD dimer payload | DLBCL | Expands payload diversity in hematologic malignancies 1 |
| Trastuzumab emtansine (T-DM1) | HER2 | Non-cleavable DM1 | Breast cancer | Solid-tumor proof of concept; indication expansions 414 |
| Trastuzumab deruxtecan (T-DXd) | HER2 | Cleavable linker; TOP1 inhibitor | Breast cancer (later HER2-low), gastric, colorectal | Designed to overcome T-DM1 resistance and heterogeneity; broader therapeutic reach 7111421 |
| Enfortumab vedotin | Nectin-4 | Cleavable vc-MMAE | Urothelial carcinoma | First ADC for urothelial; strategic target selection 1420 |
| Inotuzumab ozogamicin | CD22 | Calicheamicin | Hematologic malignancies | Design evolution from earlier DNA-damaging payloads 412 |
| Sacituzumab govitecan | TROP2 | SN-38 | Breast cancer; gastric (China) | Expands TOP1 payload utility; TROP2 is a broadly expressed solid-tumor target 21 |
| Datopotamab deruxtecan | TROP2 | TOP1 inhibitor | Breast cancer (USA) | Next-gen TOP1 class extension 21 |
A 2019 review documented approvals of enfortumab vedotin, polatuzumab vedotin, and fam-trastuzumab deruxtecan, alongside expanded indications for ado-trastuzumab emtansine—cementing a “positive inflection point” for ADCs 14. Comprehensive reviews estimate 13 ADCs available as of 2024, with continued growth in approvals and indications thereafter 821.
Applications by Therapeutic Area
Hematologic malignancies
-
Lymphomas: ADCs are established therapies. Brentuximab vedotin, polatuzumab vedotin, and loncastuximab tesirine are approved for CD30+, CD79b+, and CD19+ lymphoid malignancies, respectively 1. Combination strategies (BV–nivolumab, BV–CHOP) yielded effective, chemotherapy-sparing regimens in older or unfit HL patients, with 5-year follow-up showing durable benefit and a subset cured; ongoing work focuses on dose/schedule optimization 1019. Design principles emphasize high antigen density, efficient internalization, and payload selection matched to disease biology 9.
-
Acute lymphoblastic leukemia (ALL) and related B-cell malignancies: Inotuzumab ozogamicin underscores the utility of DNA-damaging payloads (calicheamicin) in hematologic contexts 412.
Solid tumors
-
Breast cancer: HER2-targeted ADCs remain foundational. Trastuzumab emtansine (non-cleavable DM1) validated solid-tumor ADCs, while trastuzumab deruxtecan’s cleavable linker and potent TOP1 payload expanded efficacy into HER2-low disease, addressing antigen heterogeneity through bystander killing 4711. TROP2-directed agents (sacituzumab govitecan, datopotamab deruxtecan) further broaden applicability in heterogeneous tumors 21.
-
Gastric cancer: HER2-directed agents (including T-DXd) have approvals and late-stage activity. The pipeline emphasizes CLDN18, TROP2, c-MET, and HER3/EGFR bispecific approaches, reflecting a shift toward tissue-selective and heterogeneity-aware targets 21.
-
Urothelial carcinoma: Enfortumab vedotin (nectin-4) exemplifies successful target selection and linker/payload choices adapted to solid tumor biology and prior therapy lines (post PD-1/PD-L1 and platinum) 2014.
-
Colorectal cancer: While earlier-stage overall, HER2, HER3 (e.g., patritumab deruxtecan), c-MET, TROP2, CEACAM5, and B7-H3 are active targets, with T-DXd approved in the USA and multiple Phase II/III programs underway 21.
-
Lung cancer (adenosquamous and broader NSCLC subsets): Early-phase exploration includes HER2, TROP2, FOLR1, mesothelin, and B7-H3, with increasing focus on novel tissue-specific antigens and bispecific formats 21.
-
Hepatic cancer: Emerging programs target c-MET, TROP2, nectin-4, and DLK1, reflecting the field’s expansion into historically challenging indications 21.
Exploratory non-oncology
While the dataset is oncology-focused, adaptabodies demonstrate proof-of-concept in infectious disease by retargeting a monoclonal antibody to neutralize tetanus toxin in a mouse model—survival extended under lethal challenge when combining adaptabodies and increased antibody dosing 5. While non-oncology applications have been proposed, the retrieved materials did not identify clinical-stage ADC programs in autoimmunity or infectious disease. 21.
Key Challenges, Resistance, and Future Directions
Resistance mechanisms
Payload extrusion via ABC transporters (e.g., P-gp/MDR1) and lysosomal sequestration can blunt ADC efficacy post-internalization. In vitro, MMAE’s high efflux ratio (44.5) and increased EC50 in MDR1-high cells were reversed with P-gp inhibitors (elacridar) and lysosomotropic agents (chloroquine), and boosted brentuximab vedotin cytotoxicity in CD30+ cells when combined—highlighting the need to assess transporter interactions early in development 2. Broader resistance drivers include antigen downregulation, internalization/processing bottlenecks, and payload choice 3.
Safety considerations
Design instability (premature release) and off-target payload diffusion are historical concerns that motivated linker and conjugation improvements 4. Reviews also outline safety/effect trade-offs across linkers and payloads, and provide systematic assessments for combinations involving bispecific antibodies and chemotherapy—informing risk management in complex regimens 1217.
CMC/manufacturing
DAR heterogeneity impacts pharmacokinetics and efficacy; robust DAR measurement (LC–HRMS) and early payload hydrophobicity profiling (UPLC assay) mitigate aggregation and improve conjugation yields, supporting scale-up and regulatory quality standards 1318.
Combination strategies
Checkpoint inhibitors and chemotherapy combinations can enhance outcomes. BV–nivolumab achieved durable remissions with long-term survival in older HL patients; BV–CHOP is an active approach in advanced-stage HL 1019. Reviews advocate combination therapy to enhance efficacy and overcome resistance 3.
Emerging payloads and formats
Novel payloads (e.g., neolymphostin-based inhibitors, repurposed vincristine with cleavable linkers) deliver potent anti-tumor activity, including complete regression in vivo after single administration in HER2-positive models—illustrating continued payload innovation 615. Platform innovations include bispecific ADCs, probodies (conditionally activated antibodies), adaptabodies for rapid retargeting, and immune-modulating payloads (e.g., STING agonists), positioning the field for next-generation therapeutics 521.
Market and geographic trends
The number of commercially available ADCs reached 13 as of a 2024 review, with over 300 in clinical exploration. China’s rapid growth features strong activity in HER2/TROP2/CLDN18 targets, while the USA/EU maintain leadership in novel payloads and formats 3821. The 2019 regulatory surge validated platform confidence and catalyzed investment across therapeutic areas 14.
Selected Challenges and Mitigation Strategies
| Challenge | Evidence | Implication for Design |
|---|---|---|
| ABC transporter-mediated payload efflux | MMAE is P-gp/MDR1 substrate; inhibition or lysosomal modulation restores cytotoxicity 2 | Screen payloads for transporter interactions; consider linker/payload properties to avoid lysosomal sequestration; explore combination with transporter inhibitors |
| Linker instability and off-target toxicity | Early failures linked to unstable linkers and unconjugated antibodies 4 | Prioritize linker stability and controlled cleavage; leverage site-specific conjugation to reduce heterogeneity |
| Antigen heterogeneity | T-DXd designed for bystander killing, enabling efficacy in HER2-low and heterogeneous tumors 711 | Use cleavable linkers and membrane-permeable payloads; consider bispecific targeting strategies |
| DAR heterogeneity and aggregation | LC–HRMS DAR measurement; UPLC hydrophobicity assay for payloads 1318 | Implement robust analytical controls; optimize hydrophilic–hydrophobic balance early |
Opportunities for Innovation in ADC Development
The ADC platform’s trajectory from constrained early efficacy to broad, durable impact reflects a disciplined integration of chemical, biological, and clinical insights. With HER2-low targeting now a reality, nectin-4 validated in urothelial carcinoma, and lymphoid malignancies anchored by multiple ADCs, the field stands at a point of converging innovation—site-specific conjugation, smarter linkers, diversified payloads, and synergy with immunotherapy 137142021.
Yet key challenges remain: resistance through intracellular trafficking and efflux, safety trade-offs inherent to potent warheads, and manufacturing rigor to guarantee consistent DAR and product quality. The literature’s emphasis on combination strategies, transporter-aware payload selection, and next-generation formats—including bispecific ADCs, probodies, and rapid antibody retargeting via adaptabodies—signals clear pathways to further expand the therapeutic window and address unmet needs 2351721.
In sum, ADCs have entered a phase of sustained growth and diversification, supported by validated design principles and a rich pipeline spanning hematologic malignancies and solid tumors. Continued progress will hinge on aligning molecular engineering with disease biology, optimizing CMC robustly, and thoughtfully integrating ADCs into combination regimens—advances already visible across the 2019–2026 horizon 381421.