Abstract 1385: In vitro evaluation of platinum chemotherapy combinations with the ATR inhibitor, ceralasertib (AZD6738)
Authors: Hall Sally E. , Lau Alan , Dean Emma , Martin Elizabeth , Ottley Christopher , Drew Yvette , Veal Gareth 
Published: September 2020

Abstract
Introduction: The combination of platinum-based chemotherapy with inhibitors of the DNA damage response (DDR) is therapeutically challenging due to overlapping toxicities, such as myelosuppression. The optimal sequence and schedule of drug delivery to mitigate toxicity and maximise efficacy has not been determined for many combinations. Ceralasertib is an oral inhibitor of ATR, a key constituent of the DDR. Ceralasertib has entered early phase clinical trials in combination with carboplatin (NCT02264678). The aim of the current study was to investigate the mechanism of interaction of ceralasertib with platinum (Pt) chemotherapy, characterising the formation and removal of Pt-induced DNA adducts, in order to provide insight into the repair and recovery of DNA following combination treatment.
Methods: The growth inhibitory effects of ceralasertib alone and in combination with cisplatin were tested using sulphorhodamine B (SRB) colorimetric assays in a panel of non-small cell lung (H460 and H23) and breast (MCF7, T47D, HCC1806, MDA-MB-436 and MDA-MB-468) cancer cell lines. Median effect analysis using CalcuSyn software (Biosoft, Cambridge, UK) was used to evaluate synergistic combinations. Time- and schedule-dependent analyses of Pt-DNA adduct formation in cells treated with the combination were carried out using inductively-coupled mass spectrometry (ICP-MS). Cell cycle analysis and DNA replication combing assays were performed to explore alternative mechanisms of synergy between ceralasertib and cisplatin.
Results: Ceralasertib was synergistic with cisplatin, inhibiting growth across the cell line panel. However, neither pre-exposure with either agent or co-exposure of cisplatin treated cells to ceralasertib affected the total number, nor longevity, of Pt-DNA adducts compared with cisplatin treatment alone. Continued exposure to ceralasertib for up to 48 hours following wash out of cisplatin (after 4, 8 or 24 hours treatment) did not maintain Pt-induced adducts within the DNA. Peak Pt-DNA adduct formation did not consistently correlate with cell line sensitivity to either drug, combination synergy score, or the effects of ceralasertib or cisplatin on cell cycle progression. Combination treatment did result in a reduction in replication fork speed, but this effect was no greater than that seen with ceralasertib monotherapy.
Conclusion: Synergy was observed between cisplatin and ceralasertib in the cell lines tested. However, our in vitro data suggests that this is not associated with an interaction at the level of Pt-DNA adduct formation.
Citation Format: Sally E. Hall, Alan Lau, Emma Dean, Elizabeth Martin, Christopher Ottley, Yvette Drew, Gareth Veal. In vitro evaluation of platinum chemotherapy combinations with the ATR inhibitor, ceralasertib (AZD6738) [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1385.