Twenty-one gliomas in patients aged 0-21 years were evaluated for drug sensitivity by ex vivo expanded circulating tumor cells (CTC). The results were correlated with clinical outcomes. Venous blood samples were obtained prior to drug treatment. Peripheral blood mononuclear cells were processed in a 3D cell culture system (EVA Select™, Cancer Free Biotech Ltd., Taipei, Taiwan) and cultured for 3 weeks. Expanded CTCs were successfully cultured into organoids from 18 out of 21 patients and were analyzed for ATP abundance. Staining with CD45, a marker for blood cells, and pancytokeratin, a marker for keratinocytes, was performed on the cultured cells. Staining of GFAP, a marker of glioma cells, was performed in a subset of samples. These cells were then tested in cytotoxicity assays in triplicate with a panel of chemotherapeutic and targeted agents at clinically relevant concentrations. The surviving fraction was normalized to a buffer-only control. Based on the percentage of cell viability, the agent was chosen for clinical treatment. Comparing the results among low-grade glioma (LGG; n = 6), diffuse midline glioma (DMG; n = 4), and high-grade glioma (HGG, n = 8; including glioblastoma multiforme [GBM; n = 5]), the mean surviving fraction to temozolomide was similarly high across the three tumor types (LGG vs. DMG vs. HGG = 57.5% vs. 50.6% vs. 49.5%, respectively). 6 of 6 patients in the LGG group showed CTC sensitivity to at least one chemotherapeutic agent tested. The clinical response of patients treated with selected agents was evaluated with the RANO criteria at 6 months after initiation of treatment. Among the 24 agents tested with clinical correlation, the CTC surviving fraction after exposure to the agent was significantly higher in patients who had progressive disease within 6 months (n = 11; 68%) vs. in patients with no progression at 6 months (n = 13; 39%; P = 0.039). Treating CTCs with histone deacetylase inhibitors in vitro resulted in a consistently lower surviving fraction (15.1% ± 12.0%) for DMG and HGG/GBM; however, clinical correlation was not available. The 1 patient with clinical correlation with HGG had a 34.9% surviving fraction to a Tyrosine kinase inhibitor (TKI) in vitro and showed a 42.9% shrinkage at 6 months after treatment with the TKI. The expansion of CTCs in patients with relapsed/refractory pediatric gliomas provides the ability to test drug sensitivity of patient-derived organoids. Our data suggest a correlation between the ex vivo drug sensitivity of CTCs and clinical response.Citation Format: Yen-Lin Liu, Yin-Ju Chen, Shu-Huey Chen, Yu-Mei Liao, Wu Shih-Pei, Yi-Hsuan Chen, Wan-Ling Ho, Liang-Yi Juo, Chia-Yau Chang, Jinn-Li Wang, Min-Yu Su, Pei-Chin Lin, Shih-Chung Wang, James S. Miser, Tai-Tong Wong, Yuan-Hung Wu, Peng Yuan Wang, Thierry Burnouf, Jeng-Fong Chiou, Long-Sheng Lu. Application of in vitro drug screening of circulating tumor cells in pediatric glioma therapy. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6723.