Abstract BACKGROUND Diffuse midline gliomas (DMGs) are malignant pediatric brain tumors characterized by H3K27M mutation and global genome hypomethylation. This results in enhanced transcription of MGMT, a DNA-repair enzyme, rendering alkylating chemotherapeutics ineffective. Therefore, we investigated the efficacy of a functionally different DNA damaging agent, Val-083, that induces double strand breaks and triggers S/G2 phase cell cycle arrest for homologous DNA repair. We further evaluated its combined efficacy with Wee1 (G2/M checkpoint) inhibitor, AZD1775, to override cell cycle arrest and propel cells with DNA damage into premature mitosis and apoptosis. METHODS DMG cells were treated with increasing concentration of Val-083 and/or AZD1775 and their viability was evaluated using CellTiter-Glo luminescent assay. Next, the drugs’ mechanisms of action were investigated by immunoblotting for biomarkers involved in DNA damage response and cell cycle arrest. Flow cytometry was used to investigate cell cycle profile and apoptosis. Additionally, in vivo tumor growth inhibition and survival benefit were evaluated using zebrafish and mouse xenograft DMG models. RESULTS The IC50 values of Val-083 range between 0.69 and 4.38 µM while the IC50 values of AZD1775 range between 0.22 to 0.98 µM. Combination of both drugs showed high areas of synergy on multiple DMG lines, DNA double strand breaks and significant increase in cell population at the S (p<0.01) and G2/M (p<0.05) phases. Importantly, on zebrafish xenograft tumor model, combination treatment significantly (p<0.001) inhibited tumor growth in comparison to control and single treatment groups. Furthermore, the combined treatment demonstrated significant (p<0.01) survival benefit (median survival (MS) = 62 days) on tumor-bearing nude mice models, in comparison to vehicle (MS= 44 days), Val-083 (MS= 54.5 days) and AZD1775 (MS= 47 days) treatment groups. CONCLUSION Val-083 in combination with AZD1775 demonstrate promising efficacy in DMG preclinical models, providing a strong rationale for positioning these arms for clinical testing.