Executive Summary
The human epidermal growth factor receptor 2 (HER2)-directed antibody–drug conjugate (ADC) landscape has undergone transformative change from 2024 to 2026. Trastuzumab deruxtecan (T-DXd; Enhertu; Daiichi Sankyo/AstraZeneca) has consolidated its position as the benchmark HER2 ADC, now holding regulatory approvals across HER2-positive and HER2-low breast cancer, HER2-positive gastric/gastroesophageal junction (GEJ) cancer, HER2-mutant non-small cell lung cancer (NSCLC), and HER2-positive colorectal cancer (CRC) in major global markets. A competitive ecosystem of next-generation agents — including disitamab vedotin (RC48), SHR-A1811, trastuzumab pamirtecan (DB-1303/BNT323), trastuzumab duocarmazine (SYD985), ARX788, and BL-M07D1 — is advancing through late-stage development with differentiated designs. This review provides a structured overview of the clinical evidence, ADC design principles, competitive positioning, and future directions for medical professionals.
1. Why T-DXd Has Become the HER2 ADC Benchmark
T-DXd comprises a humanized anti-HER2 IgG1 antibody (trastuzumab), a cathepsin B-sensitive cleavable glycine–glycine–phenylalanine–glycine (GGFG) peptide linker, and deruxtecan (DXd), a potent topoisomerase I inhibitor. Its drug-to-antibody ratio (DAR) of 8 — substantially higher than first-generation T-DM1 (DAR 3.5) — maximizes intratumoral drug delivery. DXd's superior membrane permeability generates a pronounced bystander effect, enabling cytotoxic activity in neighboring HER2-negative or HER2-low tumor cells following lysosomal payload release. This mechanism underpins T-DXd's unprecedented clinical activity across a wide HER2-expression spectrum, including HER2-low (immunohistochemistry [IHC] 1+ or IHC 2+/ISH-negative) and HER2-ultralow (IHC 0 with any membrane staining) disease — a population comprising approximately 50% of breast cancers 34.
T-DXd is the only HER2 ADC in the current dataset with approvals spanning breast, gastric, NSCLC, and CRC across the United States, European Union, China, and Japan, making it the clear global standard-setter 1.
2. Enhertu: Clinical Evidence by Tumor Type and HER2-Expression Level
Table 1. Pivotal T-DXd Trial Evidence by Indication (Selected)
| Trial | Setting | Population | Key Efficacy | Key Safety Signal |
|---|---|---|---|---|
| DESTINY-Breast03 | 2L HER2+ metastatic breast cancer (MBC) | Prior trastuzumab + taxane | mPFS 28.8 vs 6.8 months vs T-DM1; OS 52.6 vs 42.7 months (updated analysis; HR 0.67); ORR 79.7% vs 34.2% | ILD/pneumonitis 16.7%; neutropenia, nausea |
| DESTINY-Breast02 | Post-T-DM1 HER2+ MBC | Prior T-DM1 required | mPFS 17.8 vs 6.9 months; mOS 39.2 vs 26.5 months | ILD/pneumonitis 10.4%; grade 5 ILD 0.5% |
| DESTINY-Breast09 | 1L HER2+ MBC | Untreated metastatic HER2+ | mPFS 40.7 vs 26.9 months vs THP; ORR 85.1% vs 78.6%; OS immature | Grade ≥3 TEAEs 63.5%; ILD 12.1% incl. 2 fatal |
| DESTINY-Breast04 | HER2-low MBC | IHC 1+ or IHC 2+/ISH-; HR+ | mPFS 10.1 vs 5.4 months (HR+); mOS 23.9 vs 17.5 months (HR+ cohort) | ILD critical; myelosuppression |
| DESTINY-Breast06 | HR+, HER2-low/ultralow MBC | After ≥1 endocrine therapy; no prior MBC chemotherapy | mPFS 13.2 vs 8.1 months; ORR 65.7% vs 30.8% (HER2-low); HER2-ultralow mPFS 15.1 vs 8.3 months | ILD 11.3% (3 grade 5 events) |
| DESTINY-Breast05 | HER2+ early breast cancer adjuvant | Residual disease post-neoadjuvant | 3-year IDFS 92.4% vs 83.7% vs T-DM1; HR 0.47 | ILD; nausea; myelosuppression |
| DESTINY-Breast11 | HER2+ early neoadjuvant | Untreated early HER2+ | pCR 67.3% vs 56.3% vs ddAC-THP | ILD monitored |
| DESTINY-Breast12 | Brain metastases cohort (HER2+ MBC) | Active/stable brain metastases | 12-month PFS 61.6%; CNS ORR 62.3% active, 79.2% stable lesions | ILD 16.0% in BM cohort; fatal ILD reported |
| DESTINY-Gastric01 | 2L HER2+ gastric/GEJ | Prior trastuzumab-based therapy | ORR 40.5% vs 11.3%; mOS 12.5 vs 8.4 months | ILD 13.9% |
| DESTINY-Gastric02 | HER2+ gastric/GEJ (US/EU) | Prior trastuzumab-based therapy | Confirmed ORR 42%; 4 CR, 29 PR | ILD observed |
| DESTINY-CRC01/02 | HER2+ metastatic CRC | IHC 3+ or IHC 2+/ISH+ (cohort A) | ORR 45.3% (cohort A); mPFS 6.9 months; 5.4 mg/kg ORR 37.8% (CRC02) | ILD 9.3%; grade 5 ILD 3.5% in CRC01 |
| DESTINY-Lung02 | HER2-mutant metastatic NSCLC | Randomized 5.4 vs 6.4 mg/kg | 5.4 mg/kg: ORR 49.0%, mPFS 9.9 months, mOS 19.5 months | ILD 14.9% at 5.4 mg/kg vs 32.0% at 6.4 mg/kg |
| DESTINY-PanTumor02 | Pan-tumor HER2-expressing solid tumors | IHC 3+/2+ (7 non-breast/gastric/lung tumor types) | Overall ORR 37.1%; IHC 3+ ORR 61.3%; mPFS 6.9 months; mOS 13.4 months | ILD monitored |
mPFS = median progression-free survival; mOS = median overall survival; ORR = objective response rate; CR = complete response; pCR = pathological complete response; IDFS = invasive disease-free survival; ILD = interstitial lung disease; THP = taxane + trastuzumab + pertuzumab; BM = brain metastases 237
Safety Profile
Across tumor types, ILD/pneumonitis is the principal serious safety concern associated with T-DXd, occurring in approximately 9–27% of patients depending on indication and dose. Fatal ILD events have been documented across multiple trials, and a black box warning is in place. Importantly, DESTINY-Lung02 demonstrated that the 5.4 mg/kg dose substantially reduces ILD risk (14.9%) compared to 6.4 mg/kg (32.0%) without meaningful efficacy compromise, establishing 5.4 mg/kg as the preferred NSCLC dose 2. Nausea (up to 74.6%), vomiting (41.6%), neutropenia, anemia, fatigue, and thrombocytopenia are frequent; dose interruptions and reductions are needed in 40–62% of patients. Clinicians should implement proactive pulmonary monitoring and follow institutional ILD management algorithms 27.
A real-world study of 38 patients with active brain metastases confirmed T-DXd's intracranial activity: intracranial objective response rate (iORR) was 65.5% in HER2-positive and 66.7% in HER2-low patients, with higher iORR observed in untreated brain metastases (78.6% HER2-positive, 83.3% HER2-low) versus previously treated lesions 5.
3. Emerging HER2 ADC Competitors: Design and Clinical Positioning
Table 2. Approved and Late-Stage HER2 ADCs: Design Feature Comparison
| Agent | Antibody | Payload Class | Linker | DAR | Bystander Effect | Approval/Stage (2026) | Key Indication(s) |
|---|---|---|---|---|---|---|---|
| T-DXd (Enhertu) | Trastuzumab (IgG1) | DXd (Topo I inhibitor) | Cleavable GGFG peptide | 8 | Strong (membrane-permeable DXd) | Approved (US, EU, China, Japan) | HER2+ breast, gastric, NSCLC, CRC; HER2-low breast |
| Disitamab vedotin (RC48) | Hertuzumab | MMAE (tubulin inhibitor) | Cleavable valine-citrulline | 4 | Yes | Approved (China: gastric); BLA/NDA (China: breast) | Gastric, breast, NSCLC (China); urothelial |
| SHR-A1811 | Trastuzumab | SHR169265 (Topo I inhibitor) | Cleavable GGFG | 6 | Yes | Approved (China: NSCLC); Phase III (breast, gastric, CRC) | HER2+ and HER2-low breast, NSCLC, gastric, CRC |
| Trastuzumab duocarmazine (SYD985) | Trastuzumab | DUBA (duocarmycin; DNA alkylator) | Cleavable cathepsin B | ~2–3 | Yes (DUBA membrane-permeable) | BLA/NDA (US breast); Phase III (EU breast) | HER2+ breast |
| DB-1303/BNT323 (trastuzumab pamirtecan) | Trastuzumab | Topo I inhibitor (DITAC platform) | Cleavable | ~6–8 | Yes | Phase III (US, China, Germany); BLA accepted NMPA Apr 2026 | HER2+ breast; HER2-low breast (DYNASTY-Breast02) |
| Zanidatamab zovodotin (ZW49) | Zanidatamab (bispecific, dual-epitope) | Zovodotin | Cleavable vc | 2 | Yes | Phase I (discontinued) | Multiple HER2-expressing solid tumors |
| ARX788 | Trastuzumab | MMAF (auristatin; tubulin inhibitor) | Site-specific conjugation | Controlled | Yes | Phase II/III (China, US) | HER2+ breast |
| BL-M07D1 | Trastuzumab | Ed-04 | Cathepsin B-cleavable | 8 | Yes | Phase III (China breast); Phase II (gastric, NSCLC) | Breast, gastric, NSCLC |
| T-DM1 (Kadcyla) | Trastuzumab (IgG1) | DM1 (maytansine; tubulin inhibitor) | Non-cleavable SMCC | 3.5 | No | Approved (US, EU, China, Japan) | HER2+ breast (adjuvant residual disease; 2L MBC) |
Disitamab Vedotin (RC48)
RC48 employs hertuzumab, a novel anti-HER2 antibody with reportedly higher HER2 affinity than trastuzumab, conjugated to monomethyl auristatin E (MMAE) — a tubulin polymerization inhibitor — via a cleavable valine-citrulline linker (DAR 4). In a pooled phase I/Ib analysis (n=136), confirmed ORR was 42.9% in HER2-overexpression and 33.3% in HER2-low advanced breast cancer at the recommended 2.0 mg/kg Q2W dose. Critically, ILD incidence was 0.7% — markedly lower than T-DXd. Peripheral neuropathy (mitigated by dexamethasone pretreatment) and neutropenia are the primary safety signals 4. A retrospective real-world Chinese study (n=81) demonstrated median PFS of 5.9 months, ORR 29.6%, and importantly no observed cross-resistance with HER2 tyrosine kinase inhibitors (TKIs) 13. Preclinical data suggest RC48 may retain activity in T-DXd–resistant tumor models 11.
SHR-A1811 (Trastuzumab Rezetecan)
SHR-A1811 uses a trastuzumab backbone, a GGFG cleavable linker, and a novel Topo I inhibitor payload (SHR169265) at an optimized DAR of 6. The DAR 6 design aims to balance efficacy with reduced systemic toxicity compared to T-DXd's DAR 8. A global phase I trial (n=307) reported overall ORR of 59.9%, with ORR 76.3% in HER2-positive breast cancer and 60.4% in HER2-low breast cancer. ILD occurred in only 2.6% of patients — substantially lower than T-DXd across comparable trials. SHR-A1811 is notably already approved in China for stage IV NSCLC and has broad phase III programs underway in breast, gastric, and CRC 110.
Trastuzumab Pamirtecan (DB-1303/BNT323)
This BioNTech/DualityBio agent uses the proprietary DITAC platform with a Topo I inhibitor payload. Its phase III trial in China met its primary PFS endpoint in HER2-positive metastatic breast cancer in September 2025, and a BLA was accepted by China's NMPA in April 2026. Global phase III development (DYNASTY-Breast02) is ongoing in HER2-low breast cancer. The FDA granted Breakthrough Therapy designation for endometrial cancer 1.
Zanidatamab Zovodotin (ZW49)
ZW49 employs a biparatopic design — zanidatamab binds two distinct HER2 epitopes simultaneously, potentially preventing antigen shedding and overcoming a key resistance mechanism. Phase I results (n=77) across multiple HER2-expressing tumor types demonstrated confirmed ORR 31% and disease control rate 72% at 2.5 mg/kg Q3W. Zymeworks discontinued further clinical development of ZW49 following the Phase I trial 1.
4. Resistance Mechanisms, Sequencing, and Combination Approaches
HER2 ADC resistance develops through multiple convergent pathways: HER2 downregulation or antigen shedding following prior trastuzumab exposure; impaired ADC internalization or lysosomal trafficking; upregulation of drug efflux pumps (e.g., P-glycoprotein); activation of bypass signaling (PI3K/AKT, MAPK, JAK/STAT); loss of tumor suppressors (PTEN, TP53); and upregulation of DNA repair genes (e.g., SLX4) 8. The post-T-DXd therapeutic space remains a major unmet need: no retrieved data establish a standard-of-care approach after T-DXd progression.
Rational sequencing strategies supported by available data include:
- Alternative HER2 ADCs with different payloads (e.g., MMAE-based RC48 after Topo I–based T-DXd) — preclinical models support this approach 11
- HER2 TKIs (tucatinib, neratinib) to restore HER2 pathway suppression
- mTOR inhibitor rechallenge: two case reports demonstrated that everolimus followed by RC48 rechallenge produced partial responses (PFS 13.5 and 9.0 months) in PIK3CA-mutant HER2-low breast cancer 9
- Immunotherapy combinations: immune checkpoint inhibitors to address immune evasion
- CDK4/6 inhibitors or endocrine therapy: in HR-positive HER2-low disease
- Bispecific antibodies: dual-epitope targeting to overcome antigen shedding 1
5. Future Directions Through 2030
Table 3. Key Unmet Needs and Forward-Looking Catalysts
| Priority Area | Current Gap | Anticipated Development |
|---|---|---|
| Post-T-DXd sequencing | No established standard after T-DXd progression | Alternative-payload ADCs (RC48, ARX788), bispecific designs, TKI combinations |
| Earlier-line use | T-DXd moving to 1L MBC (DESTINY-Breast09) and neoadjuvant/adjuvant (DESTINY-Breast05/11) | Further early-stage approvals expected 2026–2028 |
| HER2-low/ultralow | DB-1303, SHR-A1811 in active HER2-low trials | Expansion of Topo I ADC activity across expression spectrum |
| CNS metastases | T-DXd active; dedicated CNS trials needed | Intracranial activity trials; BBB-penetrant ADC engineering |
| ILD risk reduction | 11–27% ILD across T-DXd studies | DAR-optimized (e.g., DAR 6 SHR-A1811), novel payload ADCs with lower ILD risk |
| Pan-tumor HER2 | DESTINY-PanTumor02 across 7 tumor types | Ongoing pan-tumor basket trials; endometrial, biliary tract, ovarian, pancreatic |
| Biomarker refinement | HER2-ultralow threshold; PIK3CA/PTEN predictive role | Quantitative IHC standardization; genomic companion diagnostics |
| Combination strategies | Early-phase data only | Immunotherapy, CDK4/6 inhibitor, and TKI combination trials |
From a regulatory standpoint, T-DXd received FDA approval for HER2-positive early breast cancer (neoadjuvant/adjuvant) in May 2026, based on DESTINY-Breast11 (pCR 67.3% vs 56.3%) and DESTINY-Breast05 (3-year IDFS 92.4% vs 83.7% vs T-DM1; HR 0.47; p<0.0001). China's NMPA BLA acceptance for DB-1303/BNT323 in April 2026 represents the first meaningful regulatory challenge to T-DXd dominance in China's HER2+ breast cancer market. The companion diagnostic landscape is also evolving: the PATHWAY anti-HER-2 (4B5) antibody assay was analytically and clinically validated (OPA ≥97.9%) for HER2-ultralow patient identification, supporting expanded T-DXd eligibility 6.
Next-generation ADC engineering priorities include: improved linker stability to reduce premature payload release; site-specific conjugation for homogeneous DAR; dual-payload ADCs combining two cytotoxic mechanisms; and masked/probody designs for tumor-selective activation. Emerging agents with lower ILD burdens (SHR-A1811 at 2.6%, RC48 at 0.7%) compared to T-DXd (9–27% depending on indication) represent a meaningful tolerability differentiator, particularly in patients with pre-existing pulmonary comorbidities, though cross-trial safety comparisons must be interpreted cautiously given population and protocol heterogeneity 410.
Clinical Practice Implications
T-DXd has established the new standard across multiple HER2-stratified cancer populations, and clinicians should anticipate continued label expansion into earlier disease stages and additional solid tumor types through 2027–2030. Vigilant ILD monitoring — including baseline pulmonary assessment, regular symptom review, and low threshold for CT imaging — is non-negotiable regardless of indication. Dose optimization at 5.4 mg/kg Q3W across breast, NSCLC, and CRC settings reflects the favorable efficacy-to-toxicity balance. Emerging competitors, particularly RC48 and SHR-A1811, are poised to carve niches in markets where ILD risk, tolerability, or post-T-DXd activity are prioritized. China represents the most competitive regional market, with multiple domestically approved or BLA-stage HER2 ADCs. Cross-trial efficacy comparisons between agents remain hypothesis-generating only; randomized head-to-head data between T-DXd and competitors are currently absent from the retrieved literature 12.
The 2026 HER2 ADC landscape is transitioning from a single-agent paradigm to a multi-agent ecosystem, with defined sequencing, rational combination strategies, and biomarker-driven patient selection expected to characterize clinical practice through 2030. Clinicians, trialists, and biopharma professionals should anticipate that T-DXd's benchmark status will be challenged incrementally — particularly in the post-T-DXd and tolerability-sensitive spaces — rather than displaced comprehensively in the near term.