Abstract 5702: BTX-9341, a bifunctional degrader of CDK4 and CDK6 for glioblastoma multiforme

Abstract Glioblastoma multiforme (GBM) is an aggressive form of brain cancer with limited treatment options. GBM has a high frequency of dysregulation of the CDKN2A-cyclin D-CDK4/CDK6 signaling node and should subsequently be sensitive to CDK4/6 inhibition. Despite this, the CDK4/6 inhibitors that are approved for HR+/HER2- breast cancer have shown limited efficacy in GBM due to poor blood brain barrier (BBB) penetration. BTX-9341 is a Cereblon (CRBN) mediated CDK4/6 bifunctional degrader that we have developed for HR+/HER2- breast cancer. This degrader shows good exposure in brain tissues with a high brain to plasma ratio. Given the exposure in the brain, we explored the in vitro and in vivo efficacy of BTX-9341 in GBM cell line and xenograft models. GBM cell lines treated with BTX-9341 showed up to 86% degradation of CDK4 and CDK6 with DC50s <1nM. CDK4/6 phosphorylates the protein RB which releases the transcription factor E2F, inducing the expression of genes which promote cell cycle progression. We examined RB phosphorylation by immunoblot and cell cycle progression by propidium iodide staining followed by flow cytometry. BTX-9341 was potent in all downstream assays, with phospho-RB IC50s <10nM, and G0/G1 cell cycle arrest at concentrations as low as 10nM. We used a 2D colony formation assay (CFA) to assess inhibition of proliferation by cell cycle arrest. BTX-9341 potently inhibited cell proliferation with CFA IC50s of 13-100nM in GBM cell lines. BTX-9341 displays excellent pharmacokinetic properties with good oral bioavailability and high brain to plasma ratios which allowed for oral dosing in both subcutaneous and orthotopic xenograft studies. We examined tumor growth inhibition efficacy in two GBM xenograft models: an intracranial U87 model and a subcutaneous U118 model and saw robust tumor growth inhibition in both models with BTX-9341 treatment. The U118 cells have partially unmethylated MGMT, which lead to a loss of efficacy of temozolomide over time, while BTX-9341 maintained inhibition of tumor growth. This indicates that BTX-9341 may be effective in patient populations for which temozolomide is ineffective. The U87 intracranial xenograft model also showed dose-dependent tumor growth inhibition upon treatment with BTX-9341, indicating that BTX-9341 can cross the BBB and reach concentrations high enough to execute cell proliferation inhibition effects. BTX-9341 treated mice also had much better survival than abemaciclib treated mice, the only CDK4/6i which has BBB penetration. These results show that BTX-9341 displays excellent single agent activity in vitro and in vivo in GBM models. The xenograft data indicates that BTX-9341 can inhibit growth of MGMT methylated and unmethylated GBM cell lines, and that BTX-9341 can inhibit tumor growth in the brain more effectively than BBB penetrant CDK4/6i. Together this data shows that BTX-9341 may be a promising candidate for treating GBM. Citation Format: Hannah Majeski, Kirti Chahal, Akinori Okano, Angela Pasis, Casey Carlson, Arvind Shakya, Qiao Liu, Shenlin Huang, Aparajita Hoskote Chourasia, Leah Fung. BTX-9341, a bifunctional degrader of CDK4 and CDK6 for glioblastoma multiforme [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 5702.