Abstract 3713: Inhibition of NQO2 by imidozoacridinones, including C1311 (Symadex, NSC645809): Implications for their mechanism of action
Authors: Humphries Matthew P. , Barnes John , Caraher Mary C. , Bryce Richard , Stratford Ian J. , Nolan Karen A. 
Published: February 2011

Abstract
INTRODUCTION: Imidazoacridinones (IAs) have well documented anti-tumour activity in vitro and in vivo. They are thought to act as intercalating agents and inhibitors of topoisomerase II. More recently C1311 and other IAs have been shown to act as inhibitors of receptor tyrosine kinases such as FLT3 (J Clin Oncol 2008. Vol 26(15S) 2576). We have recently demonstrated that a structurally related series of triazoloacridinones can act as potent inhibitors of the oxidoreductases NQO1 and NQO2 (Bioorg Med Chem, 2009. In Press). Consequently, we conducted a computational substructure screen of the NCI chemical database to identify compounds with an IA scaffold. Here we describe these compounds, as novel inhibitors of NQO2, with low nanomolar potency. Moreover, some of these agents are up to 50-fold more efficient than the standard inhibitor, resveratrol.
EXPERIMENTAL: Compounds were evaluated spectrophotometrically for their ability to inhibit human NQO2 using NRH and DCPIP as co-substrates. Inhibitory potency (IC50) was related to computational binding affinity in order to derive potential structure/activity relationships. Toxicity was assessed in HCT116 cells (measured as the concentration required to reduce proliferation by 50%) using the MTT assay. The capacity of the compounds to interact with DNA was measured by changes in DNA melting temperature. Intercalative properties were confirmed by ethidium bromide displacement in SDS-PAGE gel electrophoresis
RESULTS: Seventeen compounds were evaluated for inhibitory potency which ranged from 6 to 1000nM. IC50 values were correlated with computationally derived binding affinities with a correlation coefficient of 0.93. However, the 200 fold difference in NQO2 inhibitory potency was not associated with any major change in toxicity which ranged from 460 to 3000nM. All compounds (at 10μM) were able to increase the melting temperature of calf thymus DNA in low salt solution. Changes in Tm ranged from 4.5 to 9oC and correlated well with ethidium bromide displacement. Though, as previously reported for IAs (Biochem Pharma, 2002. Vol 63, 1653), no obvious relationship could be determined between toxicity and Tm or the ability to displace ethidium bromide from DNA.
CONCLUSION: We have identified IAs as novel inhibitors of NQO2, with some in the series being the most potent inhibitors yet reported. Therefore, these agents could have applications in processes, such as inflammation, where NQO2 may be important. Furthermore, one of the IAs in the series is C1311 (IC50 = 148nM), an agent currently in clinical trial. It has been reported that tyrosine kinase inhibitors, such as imatinib, can also interact with NQO2 (BMC Struct Biol, 2009. Vol 9(7) 1472) and this may compromise drug action. Hence, when reviewing the activity of C1311, the cellular content of NQO2 should be considered since drug/enzyme interactions could influence overall activity.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3713.