DESTINY-CRC01 (NCT03384940) was a multicenter, open-label, phase 2 trial assessing the efficacy and safety of trastuzumab deruxtecan (T-DXd) in patients with HER2-expressing metastatic colorectal cancer (mCRC) that progressed after ≥2 prior regimens; results of the primary analysis are published. Patients received T-DXd 6.4 mg/kg every 3 weeks and were assigned to either: cohort A (HER2-positive, immunohistochemistry [IHC] 3+ or IHC 2+/in situ hybridization [ISH]+), cohort B (IHC 2+/ISH−), or cohort C (IHC 1+). Primary endpoint was objective response rate (ORR) by independent central review in cohort A. Secondary endpoints included ORR (cohorts B and C), duration of response, disease control rate, progression-free survival, overall survival, pharmacokinetics, and safety of T-DXd. 86 patients were enrolled (53 in cohort A, 15 in cohort B, and 18 in cohort C). Results of the primary analysis are published, reporting an ORR of 45.3% in cohort A. Here, we report the final results. No responses occurred in cohorts B or C. Median progression-free survival, overall survival, and duration of response were 6.9, 15.5, and 7.0 months, respectively. Overall serum exposure (cycle 1) of T-DXd, total anti-HER2 antibody, and DXd were similar regardless of HER2 status. Most common grade ≥3 treatment-emergent adverse events were decreased neutrophil count and anemia. Adjudicated drug-related interstitial lung disease/pneumonitis occurred in 8 patients (9.3%). These findings support the continued exploration of T-DXd in HER2-positive mCRC.

PURPOSE: To investigate efficacy and safety of trastuzumab deruxtecan (T-DXd) in human epidermal growth factor receptor 2 (HER2)–low gastric or gastroesophageal junction (GEJ) adenocarcinoma. METHODS: Patients with locally advanced or metastatic HER2-low (cohort 1, immunohistochemistry 2+/in situ hybridization–negative; cohort 2, immunohistochemistry 1+) gastric/GEJ adenocarcinoma treated with at least two prior regimens, including fluoropyrimidine and platinum, but anti-HER2 therapy naive, received T-DXd 6.4 mg/kg intravenously once every 3 weeks. The primary end point was confirmed objective response rate by independent central review. RESULTS: Among 21 patients enrolled in cohort 1 and 24 enrolled in cohort 2, 19 and 21 patients, respectively, had central HER2 confirmation, received T-DXd, and had measurable tumors at baseline. The confirmed objective response rate was 26.3% (95% CI, 9.1 to 51.2) from five partial responses in cohort 1 and 9.5% (95% CI, 1.2 to 30.4) from two partial responses in cohort 2. Thirteen patients (68.4%) in cohort 1 and 12 (60.0%) in cohort 2 experienced reduced tumor size. The median overall survival was 7.8 months (95% CI, 4.7 to nonevaluable) in cohort 1 and 8.5 months (95% CI, 4.3 to 10.9) in cohort 2; the median progression-free survival was 4.4 months (95% CI, 2.7 to 7.1) and 2.8 months (95% CI, 1.5 to 4.3), respectively. The most common grade ≥ 3 treatment-emergent adverse events in cohorts 1 and 2 were anemia (30.0% and 29.2%), decreased neutrophil count (25.0% and 29.2%), and decreased appetite (20.0% and 20.8%). Drug-related interstitial lung disease/pneumonitis occurred in one patient in each cohort (grade 1 or 2). No drug-related deaths occurred. CONCLUSION: This study provides preliminary evidence that T-DXd has clinical activity in patients with heavily pretreated HER2-low gastric/GEJ adenocarcinoma.

Trastuzumab deruxtecan (T-DXd) showed statistically significant clinical improvement in patients with human epidermal growth factor receptor 2-positive (HER2⁺) gastric cancer in the DESTINY-Gastric01 trial. Exploratory results from DESTINY-Gastric01 suggested a potential benefit in patients with HER2-low gastric cancer. Spatial and temporal heterogeneity in HER2 expression or gene alteration, an inherent characteristic of gastric cancer tumors, presents a challenge in identifying patients who may respond to T-DXd. Specific biomarkers related to therapeutic response have not been explored extensively. Exploratory analyses were conducted to assess baseline HER2-associated biomarkers in circulating tumor DNA and tissue samples, and to investigate mechanisms of resistance to T-DXd. Baseline HER2-associated biomarkers were correlated with objective response rate (ORR) in the primary cohort of patients with HER2⁺ gastric cancer. The primary cohort had 64% concordance between HER2 positivity and HER2 (ERBB2) plasma gene amplification. Other key driver gene amplifications, specifically MET, EGFR and FGFR2, in circulating tumor DNA were associated with numerically lower ORR. Among 12 patients with HER2 gain-of-function mutations, ORR was 58.3% (7 of 12). ORR was consistent regardless of timing of immunohistochemistry sample collection. Further investigations are required in larger studies.