Abstract 270: Temozolomide resistant MGMT negative/MMR proficient cancer cells rely on ATR signaling and homologous recombination for DNA repair and survival

Abstract

MGMT loss is used to predict response to temozolomide (TMZ), however, some patients do not benefit, suggesting that additional criteria for selection remain to be identified. Using patient derived and well-established cancer lines cancer lines we observed a 100% association between mismatched repair (MMR) defects and TMZ resistance; however, only 16% of MGMT-/MMR+ lines were sensitive to TMZ at clinically relevant concentrations. Activation of DNA damage and repair signaling was noted in both sensitive and resistant bladder cancer cell lines in response to TMZ but protracted DNA damage, accompanied by persistent G2/M arrest, was observed only in the sensitive models. ATR inhibition sensitized MGMT-/MMR proficient resistant models to TMZ resulting in measurable DNA damage, prolonged G2/M arrest and persistent growth inhibition: an effect that was mediated by Chk1. Homologous recombination and repair has been shown to play an important role in repairing TMZ-induced DNA damage. We demonstrate that ATR inhibition abrogated homologous recombination (HR) activity in a MGMT-/MMR proficient TMZ resistant bladder model and that a TMZ sensitive bladder model had minimal HR (assessed by pDR-GFP/SceI assay). Moreover, HR -deficiency (mediated by BRCA1 defects or RAD51 inhibition) conferred sensitivity to TMZ in combination with an ATR inhibitor. Our in vitro data suggest that a MGMT-/MMR+/HR-Deficiency profile identifies models that respond to TMZ/ATR inhibitor combination. Indeed, one Patient-derived Xenograft (PDX) model (available in the Patient Derived Models Repository), selected by this criterion (MGMT-/MMR+ and LOH>40%) showed delayed tumor growth and increased survival in the TMZ-VX970 combination arm, compared to single agents. Overall, our data suggest that MGMT-/MMR+ cells rely on ATR-dependent signaling for repair of TMZ-induced DSBs and that HR defects should be evaluated as criteria for selecting patients that will benefit from an ATR inhibitor/TMZ combination regimen. Funded by NCI Contract No. HHSN261200800001E. This research was supported, in part, by the Developmental Therapeutics Program in the Division of Cancer Treatment and Diagnosis of the National Cancer Institute. Citation Format: Lara El Touny, Erik Harris, Curtis Hose, John Connelly, Diana Vengsarkar, Anne Monks, Ralph E. Parchment, James H. Doroshow, Beverly A. Teicher, Annamaria Rapisarda. Temozolomide resistant MGMT negative/MMR proficient cancer cells rely on ATR signaling and homologous recombination for DNA repair and survival [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 270.