TMET-35. CDK12 Inhibition Synergizes with Temozolomide to Effectively Eradicate Glioblastoma

Glioblastoma (GBM) IDH-wildtype, the most common and aggressive primary brain tumor in adults, has a dismal prognosis with a median survival of just 1-2 years despite current treatments including surgery, temozolomide, and radiation. In this study, we investigated the therapeutic potential of targeting CDK12 in GBM using patient-derived xenograft (PDX) models. CDK12 inhibition was achieved through genetic manipulation, shRNA knockdown, and the novel inhibitor SR-4835. Metabolic changes were assessed using extracellular flux analysis and LC/MS-based carbon tracing. Our screening revealed that CDK12 is essential for GBM growth, with inhibition leading to reduced tumor growth in GBM PDX lines in vitro, but no effect on human astrocytes. Notably, SR-4835 demonstrated significant inhibition of GBM growth at low nanomolar concentrations and showed a synergistic effect when combined with temozolomide. In vivo, SR-4835 activated intrinsic apoptotic pathways, resulting in tumor shrinkage. Analysis of extracellular flux and metabolite profiling, involving the tracking of carbon, U-13C-Glucose, and U-13C-Glutamine, demonstrated that disrupting the function of CDK12 significantly impaired cellular respiration, which was coupled with an activation of GSK3β and downregulation of PPARD. This led to cellular energy depletion and apoptotic cell death, in part through upregulation of the pro-apoptotic protein Noxa. In vivo, SR-4835 induced apoptosis and tumor regression in orthotopic GBM models, significantly extending overall survival without evident toxicity. Notably, combination therapy with TMZ resulted in complete tumor eradication in all treated animals. These findings provide strong preclinical evidence supporting CDK12 inhibition as a potential novel therapeutic strategy for glioblastoma, laying the foundation for future clinical trials.