Trastuzumab deruxtecan (T-DXd) vs treatment of physician’s choice (TPC) in patients (pts) with HER2-low, hormone receptor-positive (HR+) unresectable and/or metastatic breast cancer (mBC): Exploratory biomarker analysis of DESTINY-Breast04.
Shanu Modi, Naoki Niikura, Toshinari Yamashita, William Jacot, Joohyuk Sohn, Eriko Tokunaga, María Vidal, Yeon Hee Park, Keun Seok Lee, Y.S. Chae, Naoto T. Ueno, Aleix Prat, Fumitaka Suto, Yusuke Kuwahara, Robert McEwen, Wenqin Feng, Hiroki Goto, Cecilia Orbegoso, David Cameron, Junji Tsurutani
Abstract
1020 Background: DESTINY-Breast04 (NCT03734029) showed improved progression-free survival (PFS) and overall survival for T-DXd vs TPC in pts with HER2-low (IHC 1+ or 2+/ISH-negative) mBC. We present exploratory biomarker analysis in pts with HER2-low, HR+ mBC. Methods: Biopsy specimens collected from 326 pts after prior treatment were analyzed using RNA-sequencing and intrinsic subtypes estimated by PAM50 gene expression. ESR1 and PIK3CA mutations and known gene alterations associated with resistance to CDK4/6 inhibitors (CDK4/6i) were assessed in baseline (BL) circulating tumor DNA (ctDNA) samples from 414 pts by Guardant OMNI. Association with objective response rate (ORR) and PFS was evaluated. Results: Frequencies of BL intrinsic subtypes in the T-DXd and TPC arms were 41.3% and 46.6% for Luminal A, 48.0% and 37.9% for Luminal B, and 9.0% and 11.7% for HER2 enriched, respectively. According to ctDNA results in the T-DXd and TPC arms, respectively, ESR1 mutations were observed in 51.3% and 54.0% of pts, PIK3CA mutations in 36.1% and 41.6% of pts, and at least one CDK4/6i resistance marker (pts with prior CDK4/6i) was detected in 71.5% and 70.2% of pts. Improved T-DXd efficacy was seen regardless of intrinsic subtype (Luminal A, Luminal B, HER2-enriched), ESR1 mutation, PIK3CA mutation, or CDK4/6i resistance markers (Table). Conclusions: Greater clinical benefit was consistently observed with T-DXd vs TPC independent of intrinsic subtype, ESR1 mutation, PIK3CA mutation, or known CDK4/6i resistance marker status. Clinical trial information: NCT03734029 . [Table: see text]