Venetoclax Cooperates with Ionizing Radiation to Attenuate Diffuse Midline Glioma Tumor Growth

Purpose: Tumor relapse after radiotherapy is a major hurdle in treating pediatric H3K27M-mutant diffuse midline gliomas (DMG). Radiotherapy-induced stress increases association of BCL2 family of proteins with BH3 pro-apoptotic activators preventing Apoptosis. We hypothesized that inhibition of radiotherapy-induced BCL2 with a clinically relevant inhibitor, venetoclax, will block BCL2 activity leading to increased Apoptosis. BCL2 has never been implicated in DMG as a radiotherapy-induced resistant mechanism.

Experimental design: We performed an integrated genomic analysis to determine genes responsible for radioresistance and a targeted drug screen to identify drugs that synergize with radiation in DMG. Effect of venetoclax on radiation-naïve and 6 Gy radiation on cells was evaluated by studying cell death, changes in BCL2 phosphorylation, Reactive Oxygen Species (ROS), and Apoptosis, as well as BCL2 association with BH3 Apoptosis initiators. The efficacy of combining venetoclax with radiation was evaluated in vivo using orthotopic xenograft models.

Results: BCL2 was identified as a key regulator of tumor growth after radiation in DMGs. Radiation sensitizes DMGs to venetoclax treatment independent of p53 status. Venetoclax as a monotherapy was not cytotoxic to DMG cells. Postradiation venetoclax treatment significantly increased cell death, reduced BCL2-BIM association, and augmented mitochondrial ROS leading to increased Apoptosis. Combining venetoclax with radiotherapy significantly enhanced the survival of mice with DMG tumors.

Conclusions: This study shows that venetoclax impedes the antiapoptotic function of radiation-induced BCL2 in DMG, leading to increased Apoptosis. Results from these preclinical studies demonstrate the potential use of the BCL2 inhibitor venetoclax combined with radiotherapy for pediatric DMG.