Pharmacokinetics, Safety, and Efficacy of Trastuzumab Deruxtecan with Concomitant Ritonavir or Itraconazole in Patients with HER2-Expressing Advanced Solid Tumors.

Purpose: To evaluate drug–drug interactions between the human epidermal growth factor receptor 2 (HER2)–targeted antibody-drug conjugate trastuzumab deruxtecan (T-DXd; DS-8201a) and the OATP1B/CYP3A inhibitor ritonavir or the strong CYP3A inhibitor itraconazole. Patients and Methods: Patients with HER2-expressing advanced solid tumors were enrolled in this phase I, open-label, single-sequence crossover study (NCT03383692) and received i.v. T-DXd 5.4 mg/kg every 3 weeks. Patients received ritonavir (cohort 1) or itraconazole (cohort 2) from day 17 of cycle 2 through the end of cycle 3. Primary endpoints were maximum serum concentration (Cmax) and partial area under the concentration-time curve from beginning of cycle through day 17 (AUC17d) for T-DXd and deruxtecan (DXd) with (cycle 3) and without (cycle 2) ritonavir or itraconazole treatment. Results: Forty patients were enrolled (cohort 1, n = 17; cohort 2, n = 23). T-DXd Cmax was similar whether combined with ritonavir [cohort 1, cycle 3/cycle 2; 90% confidence interval (CI): 1.05 (0.98–1.13)] or itraconazole [cohort 2, 1.03 (0.96–1.09)]. T-DXd AUC17d increased from cycle 2 to 3; however, the cycle 3/cycle 2 ratio upper CI bound remained at ≤1.25 for both cohorts. For DXd (cycle 3/cycle 2), Cmax ratio was 0.99 (90% CI, 0.85–1.14) for cohort 1 and 1.04 (0.92–1.18) for cohort 2; AUC17d ratio was 1.22 (1.08–1.37) and 1.18 (1.11–1.25), respectively. The safety profile of T-DXd plus ritonavir or itraconazole was consistent with previous studies of T-DXd monotherapy. T-DXd demonstrated promising antitumor activity across HER2-expressing solid-tumor types. Conclusions: T-DXd was safely combined with ritonavir or itraconazole without clinically meaningful impact on T-DXd or DXd pharmacokinetics.