HER2 Expression-Independent Antitumor Effect of Trastuzumab Deruxtecan (T-Dxd) on Pediatric Solid Tumors.

e15016 Background: Treatment of pediatric solid tumors shows limited improvement even with intensification of conventional chemotherapy, suggesting novel approaches such as targeted therapies are needed. Trastuzumab Deruxtecan (T-DXd), a HER2-targeting antibody–drug conjugate, exhibits significant anti-cancer activity in breast cancers with a broad range of HER2 expression. T-DXd might be a promising agent for refractory or relapsed pediatric solid tumors, but its efficacy in pediatric tumors needs to be studied. Methods: Anti-cancer efficacy of T-DXd and its payload DXd were assessed in vitro by utilizing our in-house cancer cell line models. Cytotoxicity assays were performed for 60 pediatric cancer cell lines, including Neuroblastoma (NB) = 25, Ewing sarcoma / Ewing sarcoma family of tumors (EWS / EWSFT) = 17, Rhabdomyosarcoma (RMS) = 10, Others (Osteosarcoma, Brain tumor, Wilms tumor, Hepatoblastoma, and Malignant rhabdoid tumor (MRT) = 8, and 2 breast cancer cell lines expressing HER2 high or low (IHC score 2+ or 0) as positive or low control, respectively. We analyzed HER2 expression on the cell surface by flow cytometry and evaluated relative mean fluorescence intensity (MFI) values. Results: In the flow cytometric assessment, pediatric cancer cell lines showed smaller median MFI of 1.40 (0.67 - 2.71) compared with breast cancer control with HER2 high expression (33.77). The median MFI of each cell lines were 0.94 in NB, 1.76 in EWS / EWSFT, 1.42 in RMS, 1.52 in other pediatric cancers. Although low or no expression of HER2, certain inhibitory activities to the cell growth were observed for T-DXd against a part of NB, EWS, RMS and MRT cell lines, with the minimum IC50 values of 14.5 nM, 6.5 nM, 10.2 nM, 8.1 nM, respectively. In NB, EWS and RMS, a wide range of sensitivity to T-DXd were seen among different cell lines in the same disease. Most of cell lines showed high sensitivity to DXd (Median IC50 value: 0.90 nM [0.08 - 6.46 nM]), but, among them, cell lines with lower sensitivity to DXd tend to show less sensitivity to T-DXd. There was no clear correlation between sensitivity to T-DXd and HER2 expression among pediatric cancer cell lines, indicating that other mechanism such as intrinsic sensitivity to payload might affect antitumor activity of T-DXd. Conclusions: T-DXd exhibited antitumor efficacy against pediatric solid tumors in vitro irrespective of HER2 expression. Our results suggest that T-DXd might be an alternative therapy for pediatric solid tumor cases for whom conventional chemotherapies and other targeting therapies are ineffective. This study supports further investigation for T-DXd in this population.