Preclinical Evaluation of the Efficacy of the Cyclin-Dependent Kinase Inhibitor Ribociclib in Combination with Letrozole against Patient-Derived Glioblastoma Cells

Ongoing studies suggest that letrozole (LTZ), a drug used in the treatment of breast cancer, can potentially be repurposed as a novel therapeutic for glioblastoma (GBM). In a phase 0/I trial in patients with recurrent GBM, we observed that LTZ permeates into the GBM tissue and triggers dose-dependent changes in the expression of genes regulating the cell cycle [e.g., cyclin-dependent kinase (CDK) inhibitor 2A/2B, CDK4]. Based on these observations, we hypothesized that a combination of CDK4/6 inhibitors and LTZ may result in synergistic anti-GBM activity. Therefore, we assessed the antitumor effects of LTZ in combination with ribociclib, a third-generation CDK4/6inhibitor, and the brain pharmacokinetics of ribociclib. Using cell viability and neurosphere growth assays against a panel of patient-derived GBM lines, both compounds were found to be cytotoxic when used as single agents and were strongly synergistic when used in combination. We then assessed the DNA-damaging effects (γH2AX induction), cell-cycle arrest, and the induction of apoptosis (Annexin V-FITC/propidium iodide) of both compounds as single agents and when used in combination. LTZ potentiated ribociclib-induced DNA damage and cell-cycle arrest, leading to apoptosis. Systemic and brain pharmacokinetic analysis of ribociclib in Sprague-Dawley rats by serial blood and brain extracellular fluid sampling showed that ribociclib penetrates the blood-brain barrier with a partitioning coefficient (Kpu,u,brain) of about 10%. Overall, our studies suggest that a combination of ribociclib and LTZ is likely to be strongly synergistic against GBM at concentrations of the drugs that can be achieved in the brain.