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Profound, durable and MGMT‐independent sensitivity of glioblastoma cells to cyclin‐dependent kinase inhibition

TG02 is a novel cyclin‐dependent kinase (CDK) inhibitor and thought to act mainly via CDK‐9 inhibition‐dependent depletion of short‐lived oncoproteins such as MCL‐1 or c‐MYC. We studied the activity of TG02 in 9 human long‐term glioma cell lines (LTC) and 5 glioma‐initiating cell lines (GIC) using various cell death assays in vitro and in the LN‐229 LTC and ZH‐161 GIC models in vivo. TG02 exhibits strong anti‐tumor cell activity with EC50 concentrations in the nanomolar range. Median survival in the LN‐229 and ZH‐161 models was moderately prolonged by TG02. Neither constitutive CDK levels nor those of MCL‐1 or c‐MYC correlated with sensitivity to TG02. Cdk‐9 or cdk‐5 gene silencing alone did not fully reproduce the effects of TG02. C‐myc gene silencing inhibited cell growth, but did not modulate TG02 activity. Electron microscopy revealed cell death to be essentially apoptotic. High concentrations of TG02 induced annexin V binding and minor caspase 3 cleavage, but the pan‐caspase inhibitor, zVAD‐fmk, or BCL‐2 or MCL‐1 gene transfer only moderately attenuated TG02‐induced cell death, and caspase inhibition did not prevent loss of MCL‐1 or c‐MYC. TG02 activity was independent of O6‐methylguanine DNA methyltransferase expression. Repetitive exposure to TG02 did not generate an acquired TG02 resistance phenotype, but accumulation of MCL‐1, loss of c‐MYC, or senescence. TG02 is a highly potent apoptosis‐inducing agent in glioma cells in vitro. Caspase inhibition does not rescue TG02‐treated cells and repetitive exposure fails to confer acquired resistance, supporting the clinical evaluation of TG02 in glioblastoma.