Abstract GS3-01: Trastuzumab deruxtecan (T-DXd; DS-8201a) vs. trastuzumab emtansine (T-DM1) in patients (pts) with HER2+ metastatic breast cancer (mBC): subgroup analyses from the randomized phase 3 study DESTINY-Breast03
Authors: Hurvitz Sara , Kim Sung-Bae , Chung Wei-Pang , Im Seock-Ah , Park Yeon Hee , Hegg Roberto , Kim Min-Hwan , Tseng Ling-Ming , Petry Vanessa , Chung Chi-Feng , Iwata Hiroji , Hamilton Erika , Curigliano Giuseppe , Xu Binghe , Lee Caleb , Liu Yali , Cathcart Jillian , Bako Emarjola , Verma Sunil , Cortés Javier 
Published: March 2022

Background: T-DXd is a HER2-targeting antibody-drug conjugate approved for the treatment of pts with advanced HER2+ mBC based on the DESTINY-Breast01 study (NCT03248492). DESTINY-Breast03 (NCT03529110) isa randomized, multicenter, open-label, phase 3 study assessing the efficacy and safety of T-DXd vs. T-DM1 in pts with HER2+ mBC previously treated with trastuzumab and taxane. In the primary analysis, T-DXd demonstrated a clinically meaningful and statistically significant improvement in PFS vs. T-DM1 (Corteset al, ESMO 2021). In this exploratory analysis, we provide additional efficacy and safety data in subgroups, including in pts with brain metastases (BMs). Methods: Pts were randomly assigned 1:1 to receive 5.4 mg/kg T-DXd or 3.6 mg/kg T-DM1 Q3W. Pts with clinically stable BMs were eligible. Lesions were measured per modified Response Evaluation Criteria in Solid Tumors version 1.1. The primary endpoint was progression-free survival (PFS) determined by blinded independent central review (BICR). PFS and overall response rate (ORR) were analyzed for subgroups.Sites of progression and post-end-of-study therapies were also investigated. Results: At data cutoff (May 21, 2021), 524 pts were randomly assigned to T-DXd (n=261) orT-DM1 (n=263). T-DXd demonstrated superior PFS by BICR vs. T-DM1 (HR, 0.28 [95%CI, 0.22-0.37]; P=7.8 x 10-22); median (m) PFS by BICR was not reached (95% CI, 18.5-NE) for T-DXd compared with 6.8 mo (95% CI, 5.6-8.2) forT-DM1. For pts with stable BMs at baseline (n=82), mPFS was 15.0 mo (95% CI,12.5-22.2) for T-DXd vs. 3.0 mo (95% CI, 2.8-5.8) for T-DM1 (HR, 0.25 [95% CI,0.31-0.45)]. Overall, confirmed ORR for T-DXd was 79.7% vs. 34.2% for T-DM1.For patients with stable BMs at baseline, ORR was 67.4% for T-DXd vs. 20.5% forT-DM1. Consistent PFS and ORR benefit was also observed across other subgroups(Table 1). At data cutoff, 84 (32.2%) pts treated with T-DXd had progressive disease (PD) versus 155 (58.9%) with T-DM1. In pts with stable BMs in the T-DXd arm, 48.8% of pts (21/43) had PD. In pts with stable BMs in the T-DM1 arm, 69.2%of pts (27/39) had PD. Data on sites of progression will be presented. Further analyses are underway and will be presented. Overall, the safety profile of T-DXd was manageable and comparable with its known safety profile. Adjudicated drug-related interstitial lung disease/pneumonitis was reported in 27 (10.5%) pts treated with T-DXd and 5 (1.9%) pts treated with T-DM1 overall, with no grade 4 or 5 events. Additional new safety data will be presented. Conclusion: DESTINY-Breast03,the first-reported randomized phase 3 trial comparing T-DXd to standard of care, met the primary endpoint with T-DXd demonstrating superior PFS vs. T-DM1and T-DXd had a manageable safety profile. In this exploratory analysis, consistent PFS and ORR benefit with T-DXd vs. T-DM1 was observed across subgroups in pts with HER2+ mBC previously treated with trastuzumab and taxane, including in pts with BMs.

Table 1.Subgroup Analyses forPFS and ORR of T-DXd versus T-DM1PFS by BICR HR (95% CI)Absolute ORR Difference (T-DXd-T-DM1) (95% CI)All patients (N=524)0.28 (0.22-0.37)45.5 (37.6-53.4)Hormone receptorPositive (n=272)0.32 (0.22-0.46)47.3 (36.1-58.4)Negative (n=248)0.30 (0.20-0.44)43.2 (31.5-55.0)Prior pertuzumabYes (n=320)0.31 (0.22-0.43)46.7 (36.5-56.9)No (n=204)0.30 (0.19-0.47)43.6 (30.5-56.7)Prior lines of therapya0-1 (n=258)0.33 (0.23-0.48)39.3 (27.3-51.2)≥2 (n=266)0.28 (0.19-0.41)51.6 (40.9-62.4)Visceral disease Yes (n=384)0.28 (0.21-0.38)48.3 (39.1-57.6)No (n=140)0.32 (0.17-0.58)39.1 (23.6-54.6)Brain metastases at baseline Yes (n=82)0.25 (0.13-0.45)46.9 (25.6-68.3)No (n=442)0.30 (0.22-0.40)45.5 (36.9-54.1)

Citation Format: Sara Hurvitz, Sung-Bae Kim, Wei-Pang Chung, Seock-Ah Im, Yeon Hee Park, Roberto Hegg, Min-Hwan Kim, Ling-Ming Tseng, Vanessa Petry, Chi-Feng Chung, Hiroji Iwata, Erika Hamilton, Giuseppe Curigliano, Binghe Xu, Caleb Lee, Yali Liu, Jillian Cathcart, Emarjola Bako, Sunil Verma, Javier Cortés. Trastuzumab deruxtecan (T-DXd; DS-8201a) vs. trastuzumab emtansine (T-DM1) in patients (pts) with HER2+ metastatic breast cancer (mBC): subgroup analyses from the randomized phase 3 study DESTINY-Breast03 [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr GS3-01.