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The lack of knowledge about the relationship between tumor genotypes and therapeutic responses remains one of the most important gaps in enabling the development of effective targeted cancer therapies. Increasing evidence supports the notion that DNA damage response (DDR) signaling plays an important role in inducing radiation and temozolomide (TMZ) resistance in glioblastoma (GBM) and hence, it has emerged as a molecular target for therapeutic development. The cellular response to TMZ therapy is critically mediated by MGMT and DDR signaling networks that are regulated by Ataxia Telangiectasia Mutated (ATM) and Ataxia Telangiectasia and Rad3-related protein (ATR). ATR is a primary sensor of DNA damage initiating subsequent repair leading to cell survival. Here, we show that three clinical grade ATR inhibitors (Bay1895344, AZD6738 and VE-822) had similar selective sensitivity pattern across eight glioma-like stem cells (GSC) lines tested with three (GSC28, GSC11, GSC274) being the most sensitive, and three (GSC300, GSC272, and GSC20) being the most resistant cell lines. Interestingly, all three ATR inhibitors showed a significant synergism with TMZ (Combination index and Bliss model) in GSC300, GSC272, and GSC20, and antagonism in GSC28, GSC11, and GSC274. Further analysis showed that the synergistic group was resistant to single agent ATR inhibitor, whereas antagonistic group was inherently sensitive to ATR inhibitor. The synergistic response was not associated with any major genetic alterations such as MGMT status, EGFR amplification, PTEN deletion/mutation, or p53 deletion/mutation. However, GSEA analysis of baseline RNA-seq data of the 6 cell lines showed an enhanced expression of six DNA damage/repair/response pathways in the antagonistic group. It suggests that a low level of DNA damage/repair/response pathways could be a marker of synergy for TMZ with ATR inhibitors in GBM. Further studies are underway to validate this observation in more GSC lines in vitro and in vivo.

DDRE-05. DECREASED EXPRESSION OF DNA DAMAGE RESPONSE PATHWAYS CORRELATES WITH SYNERGISTIC ACTIVITY OF TEMOZOLOMIDE WITH ATR INHIBITORS IN GLIOBLASTOMA