Trastuzumab deruxtecan (T-DXd) is approved for HER2-positive and HER2-low metastatic breast cancer (BC), yet robust predictive biomarkers remain lacking. Both primary and acquired resistance limit durable clinical benefit. We aimed to identify determinants of response and resistance to T-DXd using PDX models of ER-positive BC.

Twenty-nine PDX models established from primary or metastatic ER positive BC were treated in vivo with T-DXd. Acquired resistance models were generated through prolonged exposure of initially T-DXd sensitive PDXs. Molecular characterization included genomic profiling, RNA sequencing, and IHC analysis of HER2 and ER expression at baseline and after treatment.

T-DXd induced tumor regression or complete response in 45% of models. HER2 expression alone did not discriminate responders from non-responders. Resistance was associated with EMT, hypoxia, KRAS, and TNFα/NF-κB gene signatures. In contrast, response to T-DXd was significantly associated with estrogen response, G2M, and E2F targets gene signatures and higher ER protein score. Moreover, pharmacodynamic analyses revealed ER gene and protein downregulation in T-DXd treated tumors of multiple responding models, suggesting a potential role of ER downregulation in T-DXd response. T-DXd treatment was also associated with downregulation of G2M checkpoint and E2F target gene signatures accompanied by reduced cell cycle–related protein expression level in two responding models. A decreased expression of many genes involved in cholesterol homeostasis was also observed in different PDX models following T-DXd treatment. Acquired resistance was characterized by decreased HER2 and ER expression and marked transcriptional reprogramming involving estrogen response, cholesterol homeostasis, and lipid metabolism pathways. Lipidomic analyses showed accumulation of fatty acids, glycerophospholipids, and Gb3—a glycosphingolipid component of cell membranes—in the resistant models as compared to the responding parental model, suggesting a metabolic reprogramming that may contribute to resistance. Current investigations focus on elucidating the mechanisms driving ER downregulation in T-DXd–treated tumors, characterizing T-DXd internalization dynamics, and developing combination strategies that exploit metabolic vulnerabilities in resistant models exhibiting alterations in structural and signaling lipid pathways.

T-DXd sensitivity in ER-positive BC is associated with ER expression at baseline and with inhibition of ER signalling, E2F targets and G2M checkpoints on-treatment. Acquired resistance is accompanied by loss of HER2 and ER expression and metabolic rewiring toward cholesterol and lipid pathways, suggesting potential therapeutic vulnerabilities.

Héloïse Derrien, Ahmed Dahmani, Laura Sourd, Léa Huguet, Gabriel Champenois, André Nicolas.