Enhertu Side Effects
Enhertu (trastuzumab deruxtecan) causes common gastrointestinal and hematologic toxicities, with interstitial lung disease (ILD) being the most serious and potentially fatal adverse event requiring vigilant monitoring.
Most Common Side Effects (≥20% of patients)
Based on the FDA label and clinical trial data, the most frequent adverse events include 1:
Gastrointestinal Effects
- Nausea (70-76% any grade, 0.5-4.6% grade ≥3)
- Vomiting (34-40% any grade, 1.4-1.6% grade ≥3)
- Constipation (32-34% any grade, 0.7-0.8% grade ≥3)
- Diarrhea (27-34% any grade, 1.3-2.3% grade ≥3)
- Decreased appetite (26-32%)
Hematologic Toxicities
- Neutropenia (75% any grade, 27% grade ≥3)
- Anemia (69% any grade, 8-9% grade ≥3)
- Thrombocytopenia (48% any grade, 6% grade ≥3)
- Leukopenia (86% decreased WBC count)
General Symptoms
- Fatigue (53-54% any grade, 4.4-4.8% grade ≥3)
- Alopecia (40-48% - note this is cosmetic but very common)
Hepatic
- Elevated liver enzymes (ALT 30-44%, AST 27-41%)
Critical Safety Concern: Interstitial Lung Disease
ILD/pneumonitis is the most serious toxicity and can be fatal 2, 1:
- Occurs in 8-12.1% of patients across studies
- Grade 5 (fatal) events occur in approximately 1-2% of cases 3, 4
- In DESTINY-Breast09, ILD occurred in 12.1% receiving trastuzumab deruxtecan plus pertuzumab, with 2 grade 5 deaths 3
- Most cases are grade 1-2, but vigilance is essential
ILD Risk Factors to Monitor
Real-world data identifies specific risk factors 5:
- Prior immunotherapy (HR 3.22, P=0.052)
- Smoking history (HR 2.71, P=0.062)
- Pre-existing lung disease
- Lung metastases
ILD Management Algorithm
- Baseline pulmonary assessment before starting therapy
- Monitor for new or worsening respiratory symptoms (dyspnea, cough) at each visit
- Immediately hold drug if ILD suspected
- Obtain chest imaging and pulmonary consultation
- Permanently discontinue for grade 3-4 ILD or recurrent grade 2 ILD
- May resume at reduced dose for grade 1 or first occurrence of grade 2 after resolution
Dose Modifications and Treatment Discontinuation
From clinical trials 2:
- Dose reductions: 20-23% of patients
- Treatment discontinuation: 8-16% of patients
- Dose interruptions: 39% of patients
Most common reasons for dose modification:
- Neutropenia
- Fatigue
- Nausea
- Thrombocytopenia
- Elevated transaminases
Cardiovascular Toxicity
While less prominent than with traditional trastuzumab, cardiac monitoring remains important 6, 7:
- LVEF decline occurs in approximately 10-13% of patients
- Most cases are asymptomatic
- Appears more reversible than anthracycline-induced cardiotoxicity
- Monitor LVEF at baseline and periodically during treatment
Infusion-Related Reactions
- Occur in approximately 1.2% of patients 1
- Premedication with antiemetics is standard
- May require infusion rate reduction or temporary interruption
Dose-Dependent Toxicity Profile
The 5.4 mg/kg dose is preferred over 6.4 mg/kg based on safety data 2:
- Lower rates of grade ≥3 neutropenia (0% vs 24%)
- Lower rates of pneumonitis (2% vs 8%)
- Similar efficacy with improved tolerability
- FDA-approved dose is 5.4 mg/kg every 3 weeks
Special Population Considerations
Elderly Patients
- Similar adverse event profile to younger patients 8
- May require dose adjustments starting at 4.4 mg/kg in frail patients 9
- More vigilant monitoring for myelosuppression and fatigue
Neutropenia Risk Stratification
Low neutrophil-to-lymphocyte ratio (NLR) at cycle 3 day 1 predicts higher risk of grade ≥3 neutropenia (HR 0.10, P<0.001) 5
Common Pitfalls to Avoid
- Underestimating ILD risk: Even mild respiratory symptoms warrant immediate evaluation
- Inadequate premedication: Always give antiemetics before infusion
- Delayed dose modifications: Don't wait for severe toxicity to adjust dosing
- Ignoring smoking history: Smokers need enhanced pulmonary monitoring
- Overlooking drug interactions: Review all concurrent medications, especially immunotherapy history