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Targeting MCL-1 Dependence in GBM Using Transcriptional CDK Inhibitors CYC065 and THZ1

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posted on 21.03.2022, 16:01 by Viktorija JuricViktorija Juric

Glioblastoma multiforme (GBM) is the most aggressive form of primary malignant brain tumours in adults, accounting for nearly 50% of all central nervous system (CNS) tumours and currently treated with surgery and aggressive chemo- and radiotherapy. Median survival for GBM patients is 12- 14 months with 25% of patients living longer than one year and only 5% of patients surviving more than five years. Current standard of care (SOC) enables only limited prolonged survival for patients and novel strategies to combat this disease are urgently needed.

Dysregulation of cyclin-dependent kinase (CDK) expression leads to uncontrolled tumour proliferation and growth. Several small molecules have been developed to target these abnormalities and are now applied clinically. Equally, overexpression of anti-apoptotic proteins often leads to evasion of programmed cell death in many tumours and drugs targeting these proteins could restore apoptotic function.

In this study, we investigated the benefits of transcriptional CDK inhibitors (CKIs) as anti-tumour agents in TMZ resistant primary and recurrent GBM patient-derived cell lines (PDCLs) and semi-in vivo models of GBM. We demonstrate that CYC065 (CDK2/9i) and THZ1 (CDK7i) cause loss of cell viability and disrupt sphere formation in primary and recurrent patient-derived gliomaspheres while sparing primary cortical neurons. Moreover, CYC065 and THZ1 suppressed colony formation and invasion in the majority of PDCLs, irrespective of their mutational background. Viability loss arose from G2/M cell cycle arrest following treatment, and subsequent induction of apoptotic cell death. Treatment efficacies and treatment durations required to induce cell death were associated with proliferation rates, and apoptosis induction correlated with abolishment of expression of MCL-1, which is a cell cycle- regulated antiapoptotic BCL-2 family member. GBM models were generally dependent on MCL-1 expression for cell survival, as demonstrated by pharmacological MCL-1 inhibition or genetic depletion of MCL-1. Further analyses identified CKI-induced MCL-1 loss as a prerequisite to establish conditions at which the BH3-only protein BIM can efficiently induce apoptosis, with cellular BIM concentrations strongly correlating with treatment efficacy. Moreover, we showed that BH3 profiling can be used to predict treatment response to CKIs in patient-derived gliomaspheres.

Additionally, high levels of chemo-resistant glioma stem cells (GSCs) were found in recurrent gliomasphere cultures. CYC065 and THZ1 induced apoptosis in isolated populations of CD133 and CD44 biomarker-positive cells while TMZ was ineffective, highlighting CKI’s anti-tumour activity in these chemotherapy resistant cell subpopulations which are believed to contribute to GBM tumour relapse and aggressiveness. Finally, CKIs reduced proliferation and promoted apoptosis in chick embryo xenograft models of primary and recurrent GBM.

Collectively, these data demonstrate that CYC065 and THZ1 display high anti- tumour activity in primary and recurrent GBM and provide scientific rationale for the further development of CKIs to potentiate their clinical utilisation in the future.




First Supervisor

Prof. Brona Murphy

Second Supervisor

Prof. Markus Rehm


A thesis submitted for the degree of Doctor of Philosophy from the Royal College of Surgeons in Ireland in 2021

Published Citation

Juric V,. Targeting MCL-1 Dependence in GBM Using Transcriptional CDK Inhibitors CYC065 and THZ1 [PhD Thesis] Dublin: Royal College of Surgeons in Ireland; 2021

Degree Name

Doctor of Philosophy (PhD)

Date of award



  • Doctor of Philosophy (PhD)

Research Area

  • Cancer