DDDR-45. INVESTIGATING TEMOZOLOMIDE RESISTANCE IN GLIOBLASTOMA USING A CLINICALLY RELEVANT DOSING REGIMEN

Abstract The standard of care of management for glioblastoma, the most common primary malignant brain tumor in adults, involves maximal safe surgical resection followed by adjuvant chemoradiotherapy and six cycles of adjuvant temozolomide (TMZ). Approximately half of all treated patients develop chemoresistance to TMZ. The evolution of chemoresistance to temozolomide in vitro has historically involved continuous exposure to drug, as opposed to clinical dosing schedules, which involve five days of treatment followed by 23 days of recovery. Thus, in this study, we examined the emergence of chemoresistance to TMZ in vitro, using a clinical dosing regimen. Temozolomide dose response to RN1, an MGMT promoter unmethylated, IDH-wildtype, patient-derived glioblastoma cell line, was determined using cell titer glo. Cells were treated with TMZ for five days, followed by 23 days of recovery, in cycles. In each subsequent cycle, the dose of TMZ used was adjusted to previous cycle’s IC50, the concentration at which 50% of cell growth is inhibited. This protocol was repeated over three rounds. Cells treated with TMZ, as compared to the DMSO-treated control population, gradually acquired chemoresistance with a continuous increase in IC50. Our protocol therefore highlights how a clinically relevant dosing regimen may help capture the trajectory by which chemoresistance emerges, also allowing for the exploration of the molecular mechanisms underlying this phenomenon.