Results from a Phase 0/1 clinical trial of the breast cancer drug ribociclib (Kisqali®; Novartis), a kinase inhibitor, plus everolimus (Afinitor®; Novartis), a derivative of sirolimus and an inhibitor of tor serine-threonine kinases, in patients with high-grade glioma (HGG) demonstrate that ribociclib achieved pharmacologically-relevant concentrations in gadolinium (Gd)-non-enhancing tumor, consistent with the observed tumor pharmacodynamics (PD) effects. In contrast, everolimus demonstrated minimal penetration in the (Gd)-non-enhancing compartment of the tumor.
In high-grade glioma (HGG), the RB-CDK4/6 and mTOR signaling pathways are deregulated and mTOR activation is a potential mechanism of resistance to CDK4/6 inhibition.
The purpose of the study, conducted by the Ivy Brain Tumor Center at Barrow Neurological Institute, the largest early-phase drug development program for brain cancer in the world, was to determine the combined effect of RB-CDK4/6 and mTOR inhibitors on recurrent HGGs by evaluating the tumor pharmacokinetics (PK) and tumor pharmacodynamics (PD). The study was funded by the Ben and Catherine Ivy Foundation and the individual donors of the Barrow Neurological Foundation. 
The results were published in the Journal of Neuro-Oncology and will also be presented at the European Association for Neuro-Oncology (EANO) Meeting on September 26, 2021, by Nader Sanai, MD, director of the Ivy Brain Tumor Center and director of neurosurgical oncology at Barrow Neurological Institute.
“Glioblastoma presents singular, complex challenges as compared to other types of cancer,” Sanai explained.
“You are not dealing with a single entity, but rather a collection of genetic variants that differ from patient to patient,” he added.
The Ivy Brain Tumor Center has been investigating ribociclib, an agent approved by the U.S. Food and Drug Administration (FDA) for advanced breast cancer and part of a newly-discovered class of targeted therapy that undermines cancer cell division and could form the backbone of a new drug cocktail for patients with malignant brain tumors, in a monotherapy clinical trial for glioblastoma.
While ribociclib exhibited good brain penetration, this single-drug therapy had limited clinical efficacy. Longitudinal analysis of samples from the monotherapy trial provided the rationale for this first combination trial with ribociclib and everolimus to undermine resistance mechanisms.
“Our primary focus is to discover new therapeutic drug combinations that can penetrate the blood-brain barrier, modulate their targets, and ultimately prevent the regrowth of cancer cells,” noted Shwetal Mehta, Ph.D., chief operating officer and deputy director of the Ivy Brain Tumor Center.
A total of 21 patients, eligible to participate in the trial, had recurrent high-grade WHO Grade III (n=2) and WHO Grade IV (n=19) glioma (HGG) with intact RB expression, CDKN2A/B deletion or CDK4/6 amplification, and PTEN loss or PIK3CA mutations. In the study, six of the participating patients received five days of presurgical ribociclib (400mg QD) plus everolimus (2.5mg QD) and then underwent tumor resection at 2, 8 or 24 hours following the last dose.
As part of subsequent dose-escalations, the investigators enrolled three additional patients in each in 5 seperate cohorts, reaching a maximum dose-level of ribociclib (600mg QD) plus everolimus (60mg QW). The researchers collected tumor tissue (gadolinium [Gd]-enhancing and non-enhancing regions), CSF, and plasma and determined total and unbound drug concentrations using validated LC-MS/MS methods. Tumor PD effects, including RB and S6 phosphorylation, were compared to matched archival tissue. A PK ‘trigger’ (i.e., unbound concentration > 5-fold biochemical IC50) and a PD ‘trigger’ (>30% decrease in both pRB and pS6) were set for each drug. Gd-nonenhancing tissue exhibiting both PK and PD effects in excess of these thresholds qualified patients for postoperative combination therapy.
The investigators did not observe any dose-limiting toxicities. All patients demonstrated a marked decrease in Gd-enhancement on preoperative MRI following treatment with the presurgical drug.
In Gd-nonenhancing tumor regions, the median unbound concentration of ribociclib was 719 nM (i.e., > 5-fold biochemical IC50 for CDK4/6 inhibition), whereas the unbound everolimus tumor concentrations in all patients were below the lower limit of quantitation (i.e., < 0.2 nM). 
The investigators observed that in adult HGG, ribociclib achieved pharmacologically relevant concentrations in Gd-nonenhancing tumor, which was consistent with the observed tumor PD effects. However, everolimus only exhibited very limited penetration into human glioma tissue.
“[The results of] this study demonstrated that everolimus does not sufficiently penetrate the brain tumor and its further development for this patient population is not recommended. Whereas, we continue to see positive results from ribociclib and believe there is promise in CDK4/6 inhibitors combined with other targeted therapies for the treatment of brain cancer,” Mehta said.
In the last 30 years, there has only been one drug approved by the FDA with a survival benefit for glioblastoma, the most common brain cancer. The standard-of-care is not curative and most patients experience tumor progression after treatment.
Unlike conventional Phase 1, 2 or 3 trials, the Ivy Center’s Phase 0 trials bridge the gap between initial drug testing and definitive efficacy studies. The goal is to quickly identify how a drug works in patients and whether it should be fast-tracked for further development.
“An integral component of our Phase 0 clinical trials is genetic testing. We match new targeted drug combinations to our patients and can measure their effect in about 10 days,” Sanai said
“The speed in which our clinical trials operate are paramount to our patients who desperately need effective treatment options for their case without losing valuable time,” he concluded.
A Phase 0 /II Study of Ribociclib (LEE011) in Combination With Everolimus in Preoperative Recurrent High-Grade Glioma Patients Scheduled for Resection – NCT03834740
 Sanai N, Tien A, Jiang J, Chang Y, Pennington-Krygier C, DeSantis A, Fujita Y, Kim S, Li J, Mehta S, OS05.8.A A Phase 0/1 ‘Trigger’ Trial of Ribociclib Plus Everolimus in Recurrent High-Grade Glioma, Neuro-Oncology, Volume 23, Issue Supplement_2, September 2021, Page ii8, https://doi.org/10.1093/neuonc/noab180.024
Featured Image: Nader Sanai, MD, director of the Ivy Brain Tumor Center and director of neurosurgical oncology at Barrow Neurological Institute. Photo courtesy: © 2018 – 2012 Ivy Brain Tumor Center. Used with permission.