Purpose Using real-world data, interstitial lung disease (ILD) prevalence before and after HER2-directed therapy was estimated. Potential ILD risk factors in patients receiving HER2-directed therapy for metastatic breast cancer (mBC) were evaluated. Methods Adults with HER2-directed therapy for mBC initiated between September 25, 1998, and February 22, 2020 were, included. ILD was defined broadly as one or more of 64 lung conditions. Patients were followed until incident ILD, death, last contact, or study end. Results In total, 533 patients were identified with median age at mBC of 57, 51% had de novo mBC, 43% were ever smokers, 30% had lung metastases, 9% had thoracic radiation, 6% had chronic obstructive pulmonary disease, and 16% had prevalent ILD. ILD cumulative incidence at one year was 9% (95% CI 6%, 12%), with a median follow-up of 23 months. Smoking (HR 2.2, 95% CI 1.1, 4.8) and Black/African-American race (HR 3.4, 95% CI 1.6, 7.5) were significantly associated with ILD; HRs for preexisting lung conditions (HR 1.8, 95% CI 0.9, 3.8) and thoracic radiation (HR 2.3, 95% CI 0.8, 7.1) were not statistically significant. Prevalent ILD was associated with 13-fold greater occurrence of incident ILD. 85% of patients with prevalent or incident ILD were symptomatic. Conclusions This real-world population of patients with mBC had a high prevalence of ILD prior to HER2-directed therapy, reflecting the multifactorial causation of interstitial lung changes. The cumulative incidence of ILD in patients receiving HER2-directed therapy for mBC augments prior reports. Symptomatic presentation suggests an opportunity for early intervention.
