P18.10.B INTER- AND INTRATUMORAL DRUG RESPONSE HETEROGENEITY IN GLIOBLASTOMA: A COMPARATIVE ORGANOID-BASED ANALYSIS OF TEMOZOLOMIDE AND VORTIOXETINE

Abstract BACKGROUND Inter- and intratumoral heterogeneity represents a major obstacle to the effective treatment of glioblastoma (GBM), contributing to therapy resistance and tumor recurrence. Vortioxetine, a multimodal antidepressant, has recently been suggested to exert antitumor effects in GBM. This study investigates the heterogeneous intra- and intertumoral response to vortioxetine in patient-derived organoids (PDOs) in comparison to the standard chemotherapeutic drug temozolomide (TMZ). MATERIAL AND METHODS Initially, IC₅₀ concentrations for TMZ and vortioxetine were determined for established GBM cell lines (U138, GaMG, U87), and further validated in PDOs. These concentrations were then used to treat PDOs from three different patients as well as multiple spatially distinct regions from a single tumor to determine the temporal dynamics of vortioxetine response. Over a 10-day period, cell viability was monitored every other day using the Alamar Blue assay. Subsequently, PDOs from eight GBM patients and from multiple tumor regions within each patient were treated with TMZ and vortioxetine at the identified optimal time point to assess inter- and intratumoral differences in drug response by the same assay. RESULTS IC₅₀ values were determined in 2D cell culture as 1.0 mM for TMZ and 9.79 µM for vortioxetine, and refined for PDOs to 3.3 mM and 22.15 µM, respectively. Day 8 was identified as the optimal time point for assessing vortioxetine response and therefore used for the subsequent experiment. Vortioxetine showed significantly greater efficacy than TMZ in PDOs from all 8 patients (p < 0.0001). Significant intertumoral differences in sensitivity were observed (p = 0.015). Intratumoral heterogeneity was only evident in one single patient with distinct regional responses to vortioxetine (p = 0.0128). CONCLUSION Vortioxetine exhibits superior efficacy compared to TMZ in GBM PDOs, with significant intertumoral variability in treatment response. These findings highlight the importance of accounting for tumor heterogeneity when evaluating novel therapeutic strategies and support the potential of vortioxetine as a candidate for further investigation in GBM therapy.