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BACKGROUND Despite decades of research, nearly all patients with glioblastoma (GBM) face recurrent disease. Temozolomide (TMZ), the drug of choice for GBM, methylates guanine bases at the O6 position (O6meG). These lesions are normally repaired by O6meG-methyltransferase (MGMT). When MGMT is deficient (e.g. in the case of promoter hypermethylation), O6meG lesions generate mispairs with thymine and activate the mismatch repair system (MMR), which is thought to be critical for the cytotoxicity of TMZ. Numerous studies have established that acquired MMR deficiency is a common mechanism of TMZ resistance that emerges in recurrent GBM. However, reversion of MGMT-methylation status is not common in recurrent GBM. Recent work from our group demonstrated that TMZ synergizes with ATR inhibitors (ATRi’s) in MGMT-methylated GBM cells. The goal of this project was to identify the specific types of alkylating agents that synergize with ATRi’s in an MGMT-methylated, MMR-deficient molecular context. RESULTS We used GBM cell lines U251 and LN229, which are both MGMT-methylated and exquisitely sensitive to TMZ in combination with ATRi at baseline, and created MMR-deficient versions through stable knockdown of MSH2. We determined that MSH2 knockdown conferred remarkable resistance in U251 and LN229 to TMZ treatment in comparison to their respective MMR-proficient counterparts. In addition, we were unable to detect synergy between ATRi’s and TMZ in the setting of MSH2 knockdown. In contrast to TMZ, we found that treatment with chloroethyl nitrosourea (CNU) alkylating agents--including lomustine--with ATRi’s was extremely effective in killing MGMT-methylated, MMR-deficient GBM cells. CONCLUSION Here, we report an exquisite synergistic interaction between ATRi’s and CNU agents in MMR-deficient, MGMT-methylated GBM cells. Future work in the lab will examine the interaction of CNU and ATRi’s with radiation therapy (RT). Because ATRi’s are known radiosensitizers, we believe that novel combinations of ATRi’s, lomustine, and re-RT may have promise in recurrent GBM.

EXTH-55. TEMOZOLOMIDE-RESISTANT GLIOMA CELLS ARE SENSITIVE TO CHLOROETHYLATING NITROSOUREA COMPOUNDS IN COMBINATION WITH ATR INHIBITORS