Abstract PR004: Second generation RNA Polymerase I inhibitor PMR-116 targets ribsomal RNA synthesis to potently treat a broad spectrum of malignancies

Abstract

Abstract Purpose and rational for the study: The purpose of the study was to develop novel 2nd generation ribosomal RNA synthesis inhibitors for cancer treatment. Ribosome biogenesis (RiBi) is essential for cell growth and proliferation. Transcription of ribosomal RNA genes by Pol I is a critical step in RiBi and is tightly regulated by tumor suppressors and oncoproteins. Indeed, the ability of the potent oncoprotein, MYC, to drive RiBi is necessary for its oncogenic activity (Bywater et al., Cancer Cell, 2011). These data suggested that targeting RiBi, might be a promising strategy to treat malignancies, especially those driven by MYC. To test this, we developed the 1st small molecule inhibitor of Pol I transcription, CX-5461 (Drgyin et al., Cancer Research 2011). While CX-5461 has shown promising activity in pre-clinical studies (Bywater et al., Cancer Cell, 2011; Rebello et al., Clin Cancer Res., 2016; Hein et al., Blood, 2017) and in early Phase trials (Khot et al., Cancer Discovery, 2019; Hilton et al., Nat Commun. 2022) its mechanism of action remains controversial with evidence of off-target activity inducing DNA damage through inhibition of TOPO2a (Cameron et al., Biomedicines, 2024; Koh et al., Nature Genetics, 2024). Thus, to determine if selective inhibition of Pol I transcription in the absence of DNA damage can be used to therapeutical treat cancer in vivo, we developed a series of more selective 2nd generation Pol I inhibitors with our lead compound being PMR-116. Methods and summary of data: We demonstrate that PMR-116 is selective for inhibition of Pol I over Poll II transcription and functions by stalling the Pol I complex at the rDNA promoter. PMR-116, exhibits improved drug-like properties and pharmacokinetics compared to 1st generation CX-5461. As a single agent PMR-116 efficiently treats a range of pre-clinical models of hematologic and solid tumors including lymphoma, AML, hepatocellular carcinoma and prostate cancer. MYC expression positively correlates with sensitivity to PMR-116 in human cancer cell lines and high MYC tumors in vivo are exceptionally sensitive to PMR-116. In contrast to CX-5461, the therapeutic response to PMR-116 occurs in the absence of DNA damage suggesting that DNA damage is not an obligatory response to Pol I transcription inhibition but rather an off-target effect of the 1st generation drug. Small molecule drug combination screens in AML cells demonstrate that PMR-116 works synergistically to reduce cell viability with a range of drugs including carfilzomib and quisinostat used for treatment of hematologic malignancies. Conclusions: Our data establishes inhibition of RNA polymerase I as a bone fide therapeutic target for cancer therapy and demonstrated that PMR-116 is a promising new broad- spectrum anti-cancer drug particularly for those tumors driven by MYC, which are highly refractory to standard therapies. Based on these data we have launched a Phase I clinical trial of PMR-116 in patients with advanced solid tumors (CTRN12620001146987). Citation Format: Rita Ferreira, Katherine M. Hannan, Konstantin Panov, Thejanni Udumanne, Amee J George, Zaka Yuen, Perlita Poh, Eric Kusnadi, Alisee Huglo, Mitchell Lawrence, Mustapha Haddach, Denis Drygin, Luc Furic, Ross D Hannan, Nadine Hein. Second generation RNA Polymerase I inhibitor PMR-116 targets ribsomal RNA synthesis to potently treat a broad spectrum of malignancies [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: RNAs as Drivers, Targets, and Therapeutics in Cancer; 2024 Nov 14-17; Bellevue, Washington. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(11_Suppl):Abstract nr PR004.