Background: Glioblastoma multiforme (GBM) is characterized by a high frequency of cyclin-dependent kinase (CDK)4 and CDK6 pathway dysregulation. CDK4/6 inhibitors palbociclib and abemaciclib are approved for the treatment of breast cancer, but poor blood–brain barrier (BBB) penetration limits their efficacy in GBM. GLR2007 is a novel CDK4/6 inhibitor with potential for improved penetration across the BBB. Here, we report on the activity of GLR2007 in GBM cell lines and its anti-tumor efficacy in mouse xenograft models. Methods: Three in vitro assays were used to assess the activity of GLR2007. Inhibition of CDK4/6 enzymatic activity by GLR2007 or palbociclib was calculated, and cell cycle stages were analyzed in U87-MG cells treated with vehicle control or GLR2007 for 24 h. Cell viability was evaluated in U87-MG and U118-MG cell lines after culture for 72 h with GLR2007 or abemaciclib. In vivo evaluation of the anti-tumor efficacy of GLR2007 versus vehicle, abemaciclib, and/or palbociclib was performed in BALB/c nude mouse GBM xenograft models. Quantitative whole-body autoradiography was used to determine the distribution of [¹⁴C]GLR2007 in the tissues of Sprague Dawley rats. Results: GLR2007 potency toward CDK4 and CDK6 was 33.1 and 3.8 times that of palbociclib, respectively. At concentrations >13.72 nM, GLR2007 caused G1 arrest of U87-MG cells. GLR2007 inhibited proliferation in U87-MG cells (IC₅₀ 15.6±2.4 nM) and U118-MG cells (IC₅₀ 23.2±5.2 nM). Anti-tumor efficacy of GLR2007 versus vehicle control was observed in two mouse GBM xenograft models (Table). Studies performed in rats demonstrated the distribution of [¹⁴C]GLR2007 in whole brain tissue following a single oral dose, with total radioactivity levels in the brain exceeding those in plasma by 2.3–4.5-fold from 2–6 h after dosing. Conclusions: These preclinical studies demonstrate the potential of GLR2007 as a novel CDK4/6 inhibitor for treatment of GBM. GLR2007 showed numerically greater anti-tumor efficacy than approved CDK4/6 inhibitors in xenograft models, and evidence of substantial central nervous system penetration.

^aCalculated as change in mean tumor volume from baseline for treatment group, as a percentage of change in vehicle group, measured at treatment day 21. ^bCalculated as percentage increase in median survival time of treatment group versus vehicle group. ^cPalbociclib and abemaciclib were tested only at the doses stated.