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ATR inhibition reverses the resistance of homologous recombination deficient MGMTlow/MMRproficient cancer cells to temozolomide

Lara H. El Touny, Curtis Hose, John Connelly, Erik Harris, Anne Monks, Angie B. Dull, Deborah F. Wilsker, Melinda G. Hollingshead, Michelle Gottholm-Ahalt, Sergio Y. Alcoser, Michael E. Mullendore, Ralph E. Parchment, James H. Doroshow, Beverly A. Teicher and Annamaria Rapisarda _

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Oncotarget. 2021; 12:2114-2130. https://doi.org/10.18632/oncotarget.28090

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Lara H. El Touny1,8, Curtis Hose1, John Connelly1, Erik Harris1, Anne Monks1, Angie B. Dull2, Deborah F. Wilsker2, Melinda G. Hollingshead3, Michelle Gottholm-Ahalt3, Sergio Y. Alcoser3, Michael E. Mullendore4, Ralph E. Parchment2, James H. Doroshow5,6, Beverly A. Teicher6,7 and Annamaria Rapisarda1

1 Molecular Pharmacology Laboratory, Leidos Biomedical Research Inc., FNLCR, Frederick, MD, USA

2 Clinical Pharmacodynamic Biomarkers Program, Applied/Developmental Research Directorate, Leidos Biomedical Research Inc., FNLCR, Frederick, MD, USA

3 Biological Testing Branch, NCI, Frederick, MD, USA

4 In Vivo Evaluation Program, Leidos Biomedical Research Inc., FNLCR, Frederick, MD, USA

5 Division of Cancer Treatment and Diagnosis, NCI, Bethesda, MD, USA

6 Developmental Therapeutics Branch, Center for Cancer Research, NCI, National Institutes of Health, Bethesda, MD, USA

7 Molecular Pharmacology Branch, Developmental Therapeutics Program, NCI, Rockville, MD, USA

8 Current address: Division of Preclinical Innovation, National Center for Advancing Translational Sciences, National Institutes of Health, NIH, Bethesda, MD, USA

Correspondence to:

Annamaria Rapisarda, email: [email protected]

Keywords: TMZ; MMR; ATR; HR; REV3L

Received: June 24, 2021     Accepted: September 24, 2021     Published: October 12, 2021

Copyright: © 2021 El Touny et al. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


The therapeutic efficacy of temozolomide (TMZ) is hindered by inherent and acquired resistance. Biomarkers such as MGMT expression and MMR proficiency are used as predictors of response. However, not all MGMTlow/−ve/MMRproficient patients benefit from TMZ treatment, indicating a need for additional patient selection criteria. We explored the role of ATR in mediating TMZ resistance and whether ATR inhibitors (ATRi) could reverse this resistance in multiple cancer lines. We observed that only 31% of MGMTlow/−ve/MMRproficient patient-derived and established cancer lines are sensitive to TMZ at clinically relevant concentrations. TMZ treatment resulted in DNA damage signaling in both sensitive and resistant lines, but prolonged G2/M arrest and cell death were exclusive to sensitive models. Inhibition of ATR but not ATM, sensitized the majority of resistant models to TMZ and resulted in measurable DNA damage and persistent growth inhibition. Also, compromised homologous recombination (HR) via RAD51 or BRCA1 loss only conferred sensitivity to TMZ when combined with an ATRi. Furthermore, low REV3L mRNA expression correlated with sensitivity to the TMZ and ATRi combination in vitro and in vivo. This suggests that HR defects and low REV3L levels could be useful selection criteria for enhanced clinical efficacy of an ATRi plus TMZ combination.

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