CSIG-06. CROSSTALK BETWEEN ONCOGENIC SIGNALING PATHWAYS AS DRIVERS OF THERAPY RESISTANCE AGAINST SMALL MOLECULE INHIBITORS IN GLIOBLASTOMA

Abstract

Abstract Patients with glioblastoma multiforme (GBM) have a median survival of 15 months. Despite an intensive treatment schedule with resection, radio- and chemotherapy, recurrence is inevitable. An important challenge to overcome GBM treatment resistance involves intratumoral heterogeneity, characterized by molecularly, phenotypically, and clinically distinctive GBM subtypes. Different small molecule inhibitors have been designed to inhibit signaling proteins in oncogenic driver pathways, including the EGFR, PDGFR, MET, CDK4, and PI3K/Akt pathway, in GBM. However, these small molecule inhibitors have been unsuccessful in improving patient outcome. Here, we investigate if crosstalk and redundancy in signaling pathways is responsible for single-agent resistance using patient-derived glioblastoma 2D stem cell cultures (pdGBM). Eight brain-penetrable and clinically approved small molecule inhibitors (SMI) were used to target key GBM driver genes. We measured cell viability in triplicate with a CellTiter-Glo assay. The half-maximal inhibitory concentration (IC50) for each inhibitor was defined for six molecularly distinct pdGBM 2D cell cultures. Although inhibition of downstream effector proteins reduced cell viability more effectively (IC50 70nM-1µM), compared to inhibiting upstream membrane-bound tyrosine kinase receptors (IC50 1-15µM), remaining cell proliferation was seen in all pdGBM using the different SMIs as single agent. To identify resistance mechanisms, we investigated changes in phosphokinase activity after SMI monotherapy in the different pdGBM 2D cell cultures. Based upon these results, we could identify combinations of SMIs targeting compensatory pathways that drive treatment resistance. In conclusion, this study presents a mechanistically driven selection of tumor tailored combination treatment to overcome resistance to SMI monotherapy in pdGBM. The efficacy of these novel SMI combinations will be challenged next, moving from pdGBM 2D stem cell cultures towards pdGBM organoids.