Trastuzumab Deruxtecan With Nivolumab in HER2-Expressing Metastatic Breast or Urothelial Cancer: Analysis of the Phase Ib DS8201-A-U105 Study

Purpose: This multicenter phase Ib study investigated trastuzumab deruxtecan (T-DXd) plus nivolumab in patients with HER2-expressing metastatic breast cancer (mBC) and metastatic urothelial cancer (mUC). Patients and Methods: Part 1 determined the recommended dose for expansion (RDE) of T-DXd plus nivolumab. Part 2 evaluated efficacy and safety; the primary endpoint was confirmed objective response rate (cORR) by independent central review. Results: Seven patients with mBC were enrolled in part 1 and received T-DXd 3.2 mg/kg (4 patients) or 5.4 mg/kg (3 patients) plus nivolumab. The RDE for T-DXd was 5.4 mg/kg plus nivolumab 360 mg intravenously/3 weeks. In part 2, 32 patients with HER2-positive mBC (cohort 1; inclusive of 3 administered 5.4 mg/kg in part 1), 16 with HER2-low mBC (cohort 2), 30 with HER2-high mUC (cohort 3), and 4 with HER2-low mUC (cohort 4) were enrolled. At data cutoff (July 22, 2021), the cORR (95% CI) for cohorts 1-4 was 65.6% (46.8%-81.4%), 50.0% (24.7%-75.3%), 36.7% (19.9%-56.1%), and not assessed due to small sample size, respectively. Median treatment duration (range) with T-DXd in cohorts 1-4 was 8.9 (1-23), 6.9 (1-21), 3.9 (1-21) months, and not assessed; most common treatment-emergent adverse event was nausea (55.2%, 62.5%, 73.3%, 75.0%). Adjudicated drug-related ILD/pneumonitis rates (cohorts 1-3) were 20.7%, 0%, and 20.0% (1 grade 5 each, cohorts 1 and 3). Conclusion: T-DXd plus nivolumab demonstrated promising antitumor activity in HER2-expressing mBC or mUC and safety consistent with the known profile of T‑DXd. ILD/pneumonitis is an important risk and requires careful monitoring and prompt intervention.