E-002 - FURTHER EVIDENCE ON ABCB1 DRUG TRANSPORTER IN THE RESPONSE TO TEMOZOLOMIDE, CARMUSTINE AND LOMUSTINE IN GLIOBLASTOMA MULTIFORME.

L. Radtke¹, A. Majchrzak-CeliÅ„ska², C. Awortwe¹, I. Vater¹, I. Nagel¹, I. Cascorbi¹, M. Kaehler¹; ¹University Hospital Schleswig-Holstein, Kiel, Germany, ²Medical University of Poznan, Poznan, Poland.

Background: With a five-year survival rate of 6.8%, glioblastoma multiforme (GBM) is one of the most malignant brain tumors. Alkylating agents, e.g. temozolomide and the nitrosoureas lomustine and carmustine are the main pillars of therapy. However, therapy success still is limited. One restraining factor is drug resistance caused by drug transporters of the ATP-binding cassette family, e.g. ABCB1 and ABCG2. Nevertheless, their role in drug resistance of GBM cells remains controversial. Here, we investigated the role of ABCB1 and ABCG2 in the response to temozolomide, carmustine and lomustine in GBM cells.

Methods: An ABCB1 knockout T98G glioblastoma cell model was established using CRISPR/Cas9-genome editing. Indirect drug transport assays were performed using rhodamine 123, Hoechst 33342 and the inhibitors ciclosporin A and Ko143. Drug susceptibility was analyzed by cell viability, colony formation, cell cycle distribution and apoptosis.

Results: ABCB1-, but not ABCG2-mediated transport, was reduced by temozolomide (1.6-fold, p< 0.001). Cotreatment of temozolomide and cyclosporin A led to a 2-fold reduction in cell viability (p < 0.001). Using CRISPR/Cas9, homozygous ABCB1 knockout cell lines were established. Cell fitness of ABCB1 knockout cells was decreased in the presence of temozolomide and carmustine, but not lomustine compared to wild-type cells.

Conclusion: Our data indicates that ABCB1 is highly relevant for the outward-directed transport, as well as the treatment response of temozolomide and carmustine, but not lomustine, in GBM cells. ABCB1 seems to be not only important at the blood-brain barrier, but also at the blood-tumor barrier. Novel ways of inhibiting, downregulating or silencing ABCB1 should be evaluated to increase the efficacy of currently available anti-GBM drugs.

Radtke, L., Majchrzak-Celińska, A., Awortwe, C., Vater, I., Nagel, I., Sebens, S., Cascorbi, I., Kaehler, M.. CRISPR/Cas9-induced knockout reveals the role of ABCB1 in the response to temozolomide, carmustine and lomustine in glioblastoma multiforme. Pharmacol Res 2022 (in press).