Association of early tumor shrinkage with survival in patients with HER2-positive advanced gastric cancer treated with trastuzumab deruxtecan.

360 Background: Trastuzumab deruxtecan (T-DXd), an antibody-drug conjugate composed of an anti-human epidermal growth factor receptor 2 (HER2) antibody and a cytotoxic topoisomerase I inhibitor, has been approved as third or later-line treatment for unresectable HER2-positive advanced gastric cancer (AGC) based on the results of phase II DESTINY-Gastric 01 trial. However, there are few studies on real-world clinical outcomes and clinicopathological factors associated with efficacy of T-DXd in patients with HER2-positive AGC. Methods: This single-center retrospective study enrolled patients with AGC treated with T-DXd as third- or later-line chemotherapy between March 2018 and December 2023. Early tumor shrinkage (ETS) was defined as the relative change in the sum of longest diameters of RECIST target lesions at 8 ± 2 weeks compared with baseline. Prognostic outcomes were assessed using the log-rank test and Cox proportional hazards regression model. Results: A total of 57 patients were enrolled. The median age was 66 years (range, 31–82); 68% were male; 79% had HER2 3+; 23% had pulmonary metastasis; 67% were treated with T-DXd in the third-line setting. At a median follow-up of 27.6 months, the median progression-free survival (PFS) was 4.5 months (95% confidence interval [CI], 3.8–5.5), and the median overall survival (OS) was 7.3 months (95% CI, 4.9–9.4). Among 40 patients with target lesions, the overall response rate was 35.0% and the disease control rate was 90.0%. Patients with ETS had notably longer PFS (median PFS (months) 8.6 vs 4.1, P < 0.01) and OS (median OS (months) 9.1 vs 6.1, P < 0.01) than those without ETS. In multivariate analysis, ETS was associated with significantly longer PFS (hazard ratio [HR], 0.23; 95% CI, 0.08–0.63; P = 0.004) and OS (HR, 0.27; 95% CI, 0.12–0.65; P = 0.004), whereas pulmonary metastasis was associated with significantly shorter OS (HR 2.46, 95% CI, 1.07–5.65; P = 0.03). Conclusions: This study showed that achieving an ETS was significantly associated with a favorable prognosis in patients with HER2-positive AGC treated with T-DXd.