Genome-wide CRISPR-Cas 9 screens identify BCL family members as modulators of response to regorafenib in experimental glioma

Registered systemic treatment options for glioblastoma patients are limited. The phase II REGOMA trial suggested an improvement of median overall survival in progressive glioblastoma by the multi-tyrosine kinase inhibitor regorafenib. This has not been confirmed by GBM AGILE. So far, regorafenib has been administered as monotherapy or as an addition to standard of care in newly diagnosed glioblastoma. Rational combination therapies involving regorafenib might be a reasonable strategy. Here, we aimed at identifying functionally-instructed combination therapies involving regorafenib. We applied a genome-wide CRISPR-Cas9-based functional genomics target discovery approach using activation and knockout screens followed by genetic, pharmacological, functional validations. Regorafenib-induced molecular alterations were assessed by RNAsequencing and DigiWest. We investigated selected functionally-instructed combination therapies in three orthotopic glioma mouse models in vivo (syngeneic SMA560/VM/Dk model and two xenograft models) and performed immunohistochemistry of post-treatment brains. We identified potential modifiers of regorafenib response including BCL2, BCL2L1, ITGB3, FOXC1, SERAC1, ARAF, and PLCE1. The combination of regorafenib with Bcl-2/Bcl-xL inhibition was superior to both monotherapies alone in vitro, ex vivo and in vivo. We identified regorafenib-induced regulations of the Bcl-2 downstream target chemokine receptor 1 (CCR1) as one potential underlying molecular mediator. Furthermore, regorafenib led to changes in the myeloid compartment of the glioma-associated microenvironment. This preclinical study uses a functional genomics-based target discovery approach with subsequent validations involving regorafenib. It serves as a biological rationale for clinical translation. Particularly, an investigation of the combination of regorafenib plus navitoclax within a clinical trial is warranted.