Dual Targeting Myc G-Quadruplex And Topoisomerase I: Lead Discovery Indenoisoquinolines For Cancer Therapy

G-quadruplexes are noncanonical four-stranded DNA or RNA secondary structures that have emerged as novel therapeutic targets. MYC is one of the most important oncogenes and is overexpressed in a number of human cancers. The G-quadruplex formed in the MYC oncogene promoter (MycG4) is a transcriptional silencer and amenable to small molecule targeting. Indenoisoquinolines are human topoisomerase I inhibitors with improved physicochemical and biological properties as compared to the clinically used camptothecin anticancer drugs. Three indenoisoquinolines, indotecan (LMP400), indimitecan (LMP776), and LMP744, have entered phase I clinical trials in adults with relapsed solid tumors and lymphomas. However, some indenoisoquinolines showed potent anticancer activity without being strong topoisomerase I inhibition activity, suggesting additional mechanisms of action. Herein, we demonstrate that a large number of anticancer indenoisoquinolines strongly bind and stabilize MycG4 in vitro, using fluorescence, nuclear magnetic resonance (NMR), and circular dichroism (CD) spectroscopy, as well as gel electromobility shift assay (EMSA). In addition, MycG4-interactive indenoisoquinolines lower MYC mRNA and protein levels in vivo, as shown by western blotting and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) experiments. These results indicate that targeting the MYC promoter G4 to downregulate MYC is a likely mechanism of action for the anticancer activities of these particular indenoisoquinolines. In addition, structure-activity-relationships of MycG4 recognition by indenoisoquinolines were investigated and elucidated the essential structural requirements for strong binding. The analysis of indenoisoquinoline derivatives for their MYC inhibitory, topoisomerase I inhibitory, and anticancer activity led to the discovery of a synergistic effect of MYC inhibition and topoisomerase I inhibition on anticancer activity. In conclusion, our result reveals MYC downregulation by MycG4 binding as a new mechanism of action for anticancer indenoisoquinolines. Moreover, we suggest dual targeting of MycG4 and topoisomerase I as a novel strategy for cancer therapy. Citation Format: Kaibo Wang. Dual targeting MYC G-quadruplex and topoisomerase I: Lead discovery indenoisoquinolines for cancer therapy [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 6433.