What is Enhertu and Does it Cause Neutropenia?
Enhertu (trastuzumab deruxtecan or T-DXd) is an antibody-drug conjugate that combines a HER2-targeting antibody with a topoisomerase I inhibitor, and yes, it definitively causes neutropenia—with 51-72% of patients experiencing decreased neutrophil counts depending on the dose used. 1
What is Enhertu?
Enhertu is a HER2-targeted antibody-drug conjugate (ADC) composed of three components 2:
- Trastuzumab: The HER2-targeting monoclonal antibody
- A cleavable tetrapeptide-based linker: Allows targeted drug release
- DXd (deruxtecan): A cytotoxic topoisomerase I inhibitor payload that causes DNA damage and apoptotic cell death
The drug works by binding to HER2 receptors on cancer cells, getting internalized, and releasing the cytotoxic payload inside the cell to cause DNA damage and cell death 3, 4.
FDA-Approved Indications
Enhertu is approved for:
- HER2-positive metastatic breast cancer (after prior anti-HER2 therapy) 5, 6, 7
- HER2-low metastatic breast cancer (IHC 1+ or 2+/ISH negative) 8
- HER2-positive metastatic gastric/gastroesophageal junction adenocarcinoma (second-line or later) 2
- HER2-mutant non-small cell lung cancer 9
Does Enhertu Cause Neutropenia? Absolutely Yes.
Neutropenia Incidence by Dose:
At 5.4 mg/kg dose (breast cancer, NSCLC, solid tumors):
- 65% of patients experienced decreased neutrophil count (any grade)
- 19% had Grade 3 or 4 neutropenia
- 1.2% developed febrile neutropenia
- Median time to onset: 22 days (range: 2-939 days) 1
At 6.4 mg/kg dose (gastric cancer):
- 72% of patients experienced decreased neutrophil count (any grade)
- 51% had Grade 3 or 4 neutropenia (significantly higher than the 5.4 mg/kg dose)
- 4.8% developed febrile neutropenia
- Median time to onset: 16 days (range: 4-187 days) 1
Clinical Management of Neutropenia
Grade 3 neutropenia (ANC <1.0 to 0.5 × 10⁹/L):
- Interrupt Enhertu until resolved to Grade 2 or less
- Resume at the same dose 1
Grade 4 neutropenia (ANC <0.5 × 10⁹/L):
- Interrupt Enhertu until resolved to Grade 2 or less
- Resume at a reduced dose (one level down) 1
Febrile neutropenia (ANC <1.0 × 10⁹/L + fever >38.3°C):
- Interrupt Enhertu until resolved
- Resume at a reduced dose (one level down) 1
Monitoring Requirements
The FDA label explicitly mandates 1:
- Complete blood counts prior to initiation of Enhertu
- CBC prior to each dose
- CBC as clinically indicated between doses
Real-World Data
Recent real-world studies confirm the clinical trial findings. In a 2025 multicenter Portuguese study, Grade ≥3 neutropenia occurred in 9% of patients, with low neutrophil-to-lymphocyte ratio at cycle 3 day 1 being a significant predictor (HR 0.10,95% CI 0.02-0.53, P <0.001) 10.
Other Critical Safety Concerns
While neutropenia is common, interstitial lung disease (ILD)/pneumonitis is the most serious toxicity:
- Occurs in 10-12% of patients
- Fatal in 0.9-2.2% of cases
- Requires permanent discontinuation if symptomatic (Grade ≥2) 1, 11
Common adverse events (>30% incidence) include nausea, anemia, vomiting, fatigue, constipation, and diarrhea 12, 4.
Clinical Efficacy Context
Despite these toxicities, Enhertu demonstrates remarkable efficacy:
- 60.9% objective response rate in heavily pretreated HER2-positive breast cancer 3
- Median progression-free survival of 16.4 months in third-line setting 3
- Superior to trastuzumab emtansine (T-DM1) in head-to-head comparison 5, 6
The benefit-risk profile strongly favors the 5.4 mg/kg dose over 6.4 mg/kg due to lower neutropenia rates (19% vs 51% Grade 3-4) while maintaining efficacy 13.
Common Pitfalls to Avoid
- Do not confuse Enhertu with trastuzumab or ado-trastuzumab emtansine (T-DM1)—check vial labels carefully 1
- Do not delay CBC monitoring—neutropenia onset can be as early as 2 days 1
- Do not ignore pulmonary symptoms—ILD can be fatal and requires immediate evaluation 1
- Do not use in patients with baseline LVEF <50%—cardiac toxicity is a concern 1