Modl-12. Repurposing Ssri/Snri Targeting Neurotransmitter Signaling to Sensitize DMG to Cdk9 Inhibitors

Abstract BACKGROUND Diffuse Midline Glioma (DMG) comprise a subset of aggressive pediatric brain tumors with few effective therapies. Cyclin-dependent kinase 9 inhibitors (CDK9i) like zotiraciclib (ZTR) are in clinical trials for adult and pediatric glioma, but resistance frequently develops. In this study, we find that co-treatment with CDK9i and brain penetrant, FDA-approved drugs targeting neurotransmitter signaling alters DMG transcription and reduces DMG growth and tumorigenesis. METHODS We conducted drug screens to assess DMG response to a sub-threshold dose of ZTR combined with a library of ~220 bioactive and FDA-approved compounds. Combinatorial effects of CDK9i+SSRI/SNRI treatment were confirmed through in vitro dose curve studies and in orthotopic DMG xenograft models. We identified downstream targets of CDK9i+SSRI/SNRI treatment through phospho-proteomic profiling, RNAseq analysis, immunofluorescence staining, and Western blotting. RESULTS Our targeted drug screens identified SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin and norepinephrine reuptake inhibitors) that increase DMG sensitivity to a low dose of zotiraciclib (ZTR). Follow-up studies confirm that combinations of structurally distinct CDK9i (ZTR, AZD4573) and SSRI/SNRI (sertraline, duloxetine) synergistically reduce DMG growth in neurospheres, induce the apoptotic marker cleaved caspase 3, and reduce the expression of proliferation markers. Additional proteogenomic analyses define downstream targets of CDK9i+SSRI co-treatment, including differential phosphorylation of chromatin regulators and RNA Polymerase 2 (Pol2)-associated elongation factors and splicing machinery. Further RNAseq studies reveal robust transcriptional changes induced by CDK9i+SSRI treatment characterized by altered expression of developmental and synaptic signaling genes. Finally, pilot pre-clinical studies find that CDK9i+SSRI treatment reduces tumor growth and increases progression free survival in a DMG xenograft model. CONCLUSIONS We have identified synergistic CDK9i+SSRI drug combinations that can be applied to alter malignant gene regulation and reduce DMG growth in vitro and in xenografts. These studies provide a basis for a drug-repurposing strategy to combine SSRI/SNRI and CDK9 inhibitors for DMG treatment.