Abstract 552: Fenretinide enhances trastuzumab efficacy in trastuzumab-resistant HER2+ breast cancer cell lines

Abstract Introduction: HER2-positive (HER2+) breast cancer (BC) treatments have significantly improved patient outcomes, but treatment resistance remains a critical challenge, often driven by breast cancer stem cells (BCSCs). These resilient cells evade targeted therapies, leading to disease relapse and progression. Fenretinide (Fen), a synthetic retinoic acid (RA) derivative, has shown promise in acute myeloid leukemia by promoting cellular differentiation. Given BCSCs' similarities to embryonic cells, RA-based approaches may induce BCSC differentiation, reducing their resistance to HER2-targeted therapies. This project aims to identify BCSC markers in resistant HER2+ BC cell lines and evaluate the effects of Fen induced differentiation. Methods: RNA-Seq gene expression data was downloaded from CCLE and analysed using RStudio (R version 4.1.2). HER2+ BC cell lines with innate (HCC1569, HCC1954) or acquired (SKBR3-T, BT474-T) trastuzumab (Tras) resistance were studied. Acquired resistance was induced by continuous exposure to 10 ng/mL Tras for 6 months. Cells were treated with 1 µM Fen for 24 hours. qPCR analysis employed the comparative Ct method (ΔΔCt), expressed as fold change to control. Statistical significance was determined using the Student’s t-test or one-way ANOVA with Dunnett’s multiple comparison. Results are presented as the mean of biological triplicate experiments ± standard deviation. Results: DepMap analysis showed enhanced CD44 expression in innate-resistant HCC1569 and HCC1954 compared to parental SKBR3 and BT474. qPCR confirmed significantly higher CD44 levels in HCC1569 than SKBR3 (p = 0.0318) and significantly lower CD24 levels in HCC1569 and HCC1954 compared to SKBR3 (p = 0.008 for both). Acquired-resistance models SKBR3-T and BT474-T exhibited reduced BCSC markers compared to their parental lines. Fen had no significant effect on the expression of BCSC markers in innate or acquired resistant cell lines. Fen significantly decreased colony formation (CFA) across all cell lines and enhanced the growth-inhibitory effect of Tras in the SKBR3-T cell line in a dose-dependent manner. Conclusions: Preliminary findings indicate that Fen influences CFA in both innate and acquired Tras-resistant HER2+ BC cell lines and enhances Tras efficacy in acquired resistant SKBR3-T. However, its impact on BCSC markers is limited. Further investigation is needed to elucidate the role of Fen in overcoming Tras resistance mechanisms in HER2+ BC. Citation Format: Debbie O'Reilly, John Crown, Denis M. Collins. Fenretinide enhances trastuzumab efficacy in trastuzumab-resistant HER2+ breast cancer cell lines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 552.