Abstract B40: Selective suppression of Myc transcription with a G-quadruplex stabilizing small molecule

The transcription factor Myc is disregulated in some 70% of human cancers, where it triggers aberrant growth and signaling, and contributes to tumorigenesis. Thus, Myc is a highly sought after drug target. However, targeting the Myc protein directly with small molecules has proven to be a significant challenge due to a lack of small molecule binding sites and rapid protein turnover in the cell. To date, most successes in inhibiting Myc function have come through alternative indirect approaches such as BET bromodomain inhibition. Here, we present a novel class of small molecules that silences Myc expression by stabilization of the G-quadruplex structure present in the NHEIII region of the Myc promoter. We used a small molecule microarray screen to identify molecules that selectively bind to the Myc G-quadruplex. Biophysical studies, including thermal melt and SPR analysis, demonstrated that the compound binds reversibly to the quadruplex structure with low micromolar affinity. Next, we demonstrated that the compound inhibited polymerase progression in an in vitro PCR-stop assay in a dose dependent fashion, while the compound had no effect on amplification of an oligo incapable of forming a quadruplex. Target specificity was demonstrated using a cell-based assay where one copy of the Myc gene has a translocation and is no longer under the control of a quadruplex. In this assay, Myc is only silenced upon treatment when there is a quadruplex in the promoter. Further studies demonstrate dose- and time-dependent effects on cell viability and Myc protein levels in a panel of myeloma cells. The compound also had minimal effects on viability of PBMCs from a healthy donor. Finally, gene expression analysis demonstrated that in contrast to other quadruplex-stabilizing small molecules, the compound reported here does not silence several other genes that are also under the control of quadruplexes. Citation Format: John K. Simmons, Lindsey B. Saunders, Kenneth Felsenstein, Peter Gareiss, Shuling Zhang, Beverly Mock, John S. Schneekloth, Jr.. Selective suppression of Myc transcription with a G-quadruplex stabilizing small molecule. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B40.