P01.18.b Modeling Glioblastoma Treatment Responses: Insights Into Tumor Plasticity and the Role of the Tumor Microenvironment

BACKGROUND Glioblastoma relapse remains inevitable, placing these tumors among the most difficult to treat. Treatment challenges include inherent drug resistance, tumour’s plastic nature, and suppressive tumour microenvironment (TME). In other cancers, treatment-induced plasticity has been associated with the emergence of drug-tolerant persister cells, which survive therapy by undergoing reversible phenotypic changes towards resistant states. The role of persisters in glioblastoma remains to be unravelled. It is unclear to what extent the responses are patient- and treatment-specific, and how much they depend on the tumor microenvironment. MATERIAL AND METHODS We applied a cohort of molecularly characterized patient-derived models to investigate patient-specific responses to chemotherapeutics (Temozolomide, VAL-083). Treatment efficacy and its correlation with molecular features were assessed ex vivo using organoids and stem-like cultures as well as in vivo through orthotopic xenografts. To mimic clinical treatment schedules, we assessed functional responses in time, incorporating prolonged exposure to treatment and drug holiday period over several weeks. Analysis included tumor cell viability, cell cycle, DNA damage and repair, senescence and quiescence features. Phenotypic adaptation of glioblastoma and microenvironmental cells in vivo were analysed with MRI, single-cell RNA-seq, spatial transcriptomics and IHC. RESULTS Assessment of drug efficacy across a cohort of patient-derived models revealed resistant versus adaptive responder groups, which were treatment-specific. While resistant cells did not show functional changes in cell viability and cell cycle upon prolonged drug exposure, sensitive cells show a biphasic growth curve characteristic of persister cells. The adaptation towards the least proliferative phenotype was reversible during drug holiday period, further confirming a role of a transient drug-tolerant state. Chemotherapeutic treatment in orthotopic xenografts partially reduced tumor growth in vivo and activated apoptosis and DNA repair pathways in glioblastoma cells. These changes coincided with an increased infiltration of tumor-associated macrophages. CONCLUSION Our findings highlight the critical roles of both tolerance and intrinsic resistance mechanisms. The responder groups appear to be specific to the type of treatment and the unique characteristics of each patient’s tumor, emphasizing the need to evaluate treatment-induced plasticity in a patient-specific context. Glioblastoma cells are further supported by the adaptation of the microenvironment towards a tumor-supportive crosstalk.