Abstract LB480: Cryogenic volume electron microscopy reveals restoration of organelle integrity by abemaciclib in hormone receptor-positive, HER2-negative metastatic breast cancer organoids

Abstract Resistance to CDK4/6 inhibitors (CDK4/6i) remains a major clinical challenge in hormone receptor-positive, HER2-negative metastatic breast cancer (HR+/HER2- MBC). Despite advances in multi-omics and histopathology, these techniques often introduce artifacts and fail to capture the ultrastructural dynamics of tumor response. This study pioneers the application of cryogenic volume electron microscopy (cryo-vEM) with a focused ion beam/scanning electron microscopy (cryo-FIB/SEM) to patient-derived organoids (PDOs) as a means of investigating the subcellular impact of abemaciclib, a potent CDK4/6i.Using PDOs generated from a liver metastasis of a CDK4/6i-naïve patient who demonstrated durable clinical benefit from CDK4/6i therapy, we revealed significant abemaciclib-induced alterations in cellular architecture and organelle morphology. Initial room-temperature FIB/SEM imaging identified hallmark metastatic features, including hollow organoid structures, misshapen nuclei, enlarged mitochondria, and pseudopodia with invasive morphology. Following abemaciclib treatment, PDOs exhibited normalized nuclear shapes, prominent nucleoli, reduced pseudopodia, and a loss of hollow organoid architecture, indicating diminished metastatic potential.Cryo-FIB/SEM of vitrified organoid enabled artifact-free imaging of these structural changes in their native state. Mitochondria in treated PDOs showed fewer aberrant cristae folds, while untreated cells displayed nuclear-wrapped mitochondria, a feature absent post-treatment. This suggests a treatment-induced disruption of tumor-specific energy supply routes. Additionally, cytoskeletal remodeling was evident, reflecting a functional shift associated with reduced invasive capacity.These findings underscore the power of cryogenic imaging techniques in bridging the gap between cellular ultrastructure and therapeutic response. By integrating future molecular and multi-omics analyses, this approach has the potential to revolutionize drug evaluation, offering a pathway to personalized, artifact-free assessment of treatment efficacy in metastatic breast cancer. This work not only highlights abemaciclib’s mechanistic impact but also sets the stage for cryo-vEM as a transformative tool in cancer research. Citation Format: Gong-Her Wu, Mei-Kuang Chen, Kelly Hunt, Debu Tripathy, Khandan Keyomarsi, Wah Chiu. Cryogenic volume electron microscopy reveals restoration of organelle integrity by abemaciclib in hormone receptor-positive, HER2-negative metastatic breast cancer organoids [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB480.