Abstract 1228: Novel small molecule inhibitors that target the exportin binding pocket of TOP2A for the treatment of multiple myeloma

Abstract Introduction Certain cancers have been shown to express a specific type of drug-resistance involving the export of drug target proteins from the nucleus of the cell to the cytoplasm. This mechanism of drug-resistance has been shown to be important in multiple myeloma (MM) and recently a nuclear export inhibitor, selinexor, has received approval by the FDA in MM. However, general nuclear export signal (NES) inhibitors like selinexor exhibit off-target toxicities when used in patients. We have begun to develop novel drugs that specifically prevent the export of the drug-target topoisomerase IIα (TOP2A). Methods In this study, we 1) generated an atomic homology model of human TOP2A to identify lead compounds targeting the NES of TOP2A; NES inhibitors (NESi), 2) examined binding specifically to TOP2A and inhibit nuclear export by immunofluorescence microscopy (IF) , nuclear-cytoplasmic fractionation, immunoprecipitation (IP), proximity ligation assay (PLA), and Biacore kinetic/affinity assay, and 3) tested the anticancer activity of these compounds in MM in vitro, ex vivo and in vivo MM. Results The atomic homology model of human TOP2A provided the basis for in silico molecular docking screenings of 139,735 small molecules (NCI) (MW < 500) to identify lead compounds targeting the NES of TOP2A. NESi compounds with the highest docking scores were assayed for their ability to induce apoptosis in human MM cells when used with the TOP2 inhibitor doxorubicin (DOX). We isolated four lead compounds that induced apoptosis when used with DOX and that inhibited nuclear export of TOP2A, as shown by IF and nuclear-cytoplasmic fractionation, IP, a PLA and Biacore kinetic/affinity assays. Inhibition by the lead compound, NCI-9138, was specific to TOP2A because p53 trafficking was unaffected and the inhibitor did not affect TOP2A protein expression or enzymatic function (decatenation). This NESi was found to sensitize human MM cells in vitro, in vivo mouse models, and patient MM cells exposed ex vivo to DOX but did not affect normal cell lines. These TOP2A-specific nuclear export inhibitors may potentially lead to a new approach in circumventing drug resistance. Conclusions Our previous studies have shown that TOP2A is exported from the nucleus of MM cells and that this is a significant cause of resistance to DOX therapy. In this study, we have shown that blocking export of TOP2A can sensitize human MM cells to DOX both in vitro, ex vivo in MM cells from patients, and in in vivo mouse models of MM. A drug that specifically inhibits the export of a single cancer-related protein has not been published. We intend to show that this unique approach to cancer-treatment may significantly advance the treatment of MM. This paper also demonstrates a proof of concept for the development of highly specific small molecule inhibitors that could be used as cancer therapeutics. Citation Format: Joel G. Turner, David Ostrov, Nicholas Lawrence, Jana Dawson, Juan Gomez, Alexis Bauer, Mark Meads, Alex Achille, Kenneth Shain. Novel small molecule inhibitors that target the exportin binding pocket of TOP2A for the treatment of multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1228.