Abstract B24: Ataxia-telangiectasia and Rad3-related (ATR) phosphorylation as a pharmacodynamic biomarker of ATR activation in solid tumor tissue models

Abstract

Abstract Inhibitors of checkpoint kinases, such as ATR, Chk1 or Wee1, in combination with cytotoxic agents could enhance therapeutic efficacy compared to monotherapy, and these combination approaches are currently being extensively explored. The presence of replicative stress or deregulated S-phase in cancer has been recognized as a rationale for the use of ATR and Chk1 inhibitors with chemotherapy and efforts are underway to define genetic determinants that sensitize cancer cells to ATR inhibition. Pharmacodynamic (PD) biomarkers of drug activity are valuable tools in clinical trials using targeted agents to determine whether each investigational agent is acting upon its intended target, the expected intracellular signaling pathways are modulated, and the dosage regimen and sequencing are optimal. In addition, investigations of the potential effects of cell cycle distribution within the tumor (in animal models) on drug efficacy may point to alternative dosing regimens. The first clinical trials evaluating inhibitors of the ataxia telangiectasia and Rad3-related kinase (ATR) in combination with cytotoxic agents are enrolling patients. The current method for quantifying ATR inhibition used in preclinical in vitro and in vivo models is via indirect measurement of downstream effector proteins, such as phosphorylated Chk1, rather than measurement of direct target engagement. We propose that an immunofluorescence microscopy assay of ATR autophosphorylation status as a biomarker of DNA damage activation combined with measurement of the downstream effector proteins will reveal more specific information about target engagement and intended PD effect than would be obtained from a single-biomarker assay. Here we propose that the measurement of ATR pT1989, Chk1 pS345 and γH2AX by validated quantitative immunofluorescence assays will be useful in PD evaluation of tumor responses in clinical trials as a biomarker for ATR pathway activation or inhibition. In addition, evaluation of the variability of expression of activated ATR in tumor models of different histologies may provide valuable insight into tumor types likely to be sensitive to ATR inhibitors. Citation Format: Deborah Wilsker, Allison M. Marrero, Angie Dull, Thomas D. Pfister, Scott M. Lawrence, John Carter, Michelle Gottholm-Ahalt, Melinda Hollingshead, James Doroshow, Ralph E. Parchment, Robert J. Kinders. Ataxia-telangiectasia and Rad3-related (ATR) phosphorylation as a pharmacodynamic biomarker of ATR activation in solid tumor tissue models. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Cancer Cell Cycle - Tumor Progression and Therapeutic Response; Feb 28-Mar 2, 2016; Orlando, FL. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(11_Suppl):Abstract nr B24.