Interstitial pneumonitis related to trastuzumab deruxtecan, a human epidermal growth factor receptor 2‐targeting Ab–drug conjugate, in monkeys

Trastuzumab deruxtecan (T‐DXd: DS‐8201a) is an anti‐human epidermal growth factor receptor 2 (HER2) Ab–drug conjugated with deruxtecan (DXd), a derivative of exatecan. The objective of this study was to characterize T‐DXd‐induced lung toxicity in cynomolgus monkeys. Trastuzumab deruxtecan was injected i.v. into monkeys once every 3 weeks for 6 weeks (10, 30, and 78.8 mg/kg) or for 3 months (3, 10, and 30 mg/kg). To evaluate the involvement of DXd alone in T‐DXd‐induced toxicity, DXd monohydrate was given i.v. to monkeys once a week for 4 weeks (1, 3, and 12 mg/kg). Interstitial pneumonitis was observed in monkeys given T‐DXd at 30 mg/kg or more. The histopathological features of diffuse lymphocytic infiltrates and slight fibrosis were similar to interstitial lung diseases (ILD)/pneumonitis related to anticancer drugs in patients, with an incidence that was dose‐dependent and dose‐frequency‐dependent. Monkeys receiving DXd monohydrate did not suffer lung toxicity, although the DXd exposure level was higher than that of DXd in the monkeys given T‐DXd. The HER2 expression in monkey lungs was limited to the bronchial level, although the lesions were found at the alveolar level. Immunohistochemical analysis confirmed that T‐DXd localization was mainly in alveolar macrophages, but not pulmonary epithelial cells. These findings indicate that monkeys are an appropriate model for investigating T‐DXd‐related ILD/pneumonitis. The results are also valuable for hypothesis generation regarding the possible mechanism of T‐DXd‐induced ILD/pneumonitis in which target‐independent uptake of T‐DXd into alveolar macrophages could be involved. Further evaluation is necessary to clarify the mechanism of ILD/pneumonitis in patients with T‐DXd therapy. Trastuzumab deruxtecan (T‐DXd; DS‐8201a), an anti‐human epidermal growth factor receptor 2 Ab–drug conjugate with a derivative of exatecan (DXd), has been associated with interstitial lung diseases (ILD)/pneumonitis in clinical trials. This work indicates that the histopathological features of T‐DXd‐induced lung toxicity in monkeys are similar to ILD/pneumonitis associated with anticancer drugs in patients.