Rbn-2397: A Potent And Selective Small Molecule Inhibitor Of Parp7 That Induces Tumor-Derived Antitumor Immunity Dependent On Cd8 T Cells.

Abstract Targeting cytosolic nucleic acid sensing pathways and the Type I interferon (IFN) response is an emerging therapeutic strategy being explored in oncology. The PARP family consists of seventeen enzymes that regulate fundamental biological processes including response to cellular stress. In contrast to PARP1, PARP7 (TIPARP) is a monoPARP that catalyzes the transfer of single units of ADP-ribose onto substrates (MARylation) to change their function and plays a role in suppressing the Type I IFN response. RBN-2397 selectively inhibits PARP7 compared to the approved PARP1 inhibitors and demonstrates > 50-fold selectivity for inhibition of PARP7 over all PARP family members as measured by biochemical assays. The inhibition of PARP1-mediated ADP-ribosylation has been well-characterized for several PARP1 inhibitors using a cellular hydrogen peroxide-induced PARylation assay. Here, we show that RBN-2397 inhibits PARP7-dependent MARylation with an IC50 of 2 nM exhibiting a 300-fold window over PARP1-driven PARylation. Using the mouse CT26 cell line, we showed that RBN-2397, but not the PARP1 inhibitor olaparib, induced Type I IFN signaling demonstrated by STAT1 phosphorylation. The effect on pSTAT1 was phenocopied by PARP7 knockout (KO). To further demonstrate specificity, we show that simultaneous KO of PARP7 prevented any additional increase of STAT1 phosphorylation by RBN-2397; however, KO of PARP1 had no effect on the induction of Type I IFN signaling by RBN-2397. We had previously reported that RBN-2397 dosing of CT26 tumor bearing immune competent BALB/c mice led to complete and durable tumor regressions which could be reversed by interfering with tumor-derived IFN signaling (1). Here we show that in contrast, RBN-2397 showed modest activity with no tumor regressions in CT26-tumor bearing immunodeficient NOG mice. To assess which immune cell populations are involved in the antitumor effects of RBN-2397, CT26 tumor-bearing BALB/c mice were depleted of CD4 T, CD8 T or NK cells. Depletion of CD4 T or NK cells had no effect on RBN-2397 antitumor activity; however, depletion of CD8 T cells significantly reversed the effects of RBN-2397, suggesting that CD8 T cells are responsible for much of the antitumor immunity induced by RBN-2397. We have discovered and developed RBN-2397, a first-in-class, potent and selective inhibitor of PARP7. We show RBN-2397 restores Type I IFN signaling in cancer cells and that this is an on-target effect of inhibiting the catalytic activity of PARP7 and not PARP1. We further show that the adaptive immune response was required for the antitumor effects of RBN-2397. RBN-2397 is the first agent to enter clinical trials that targets this tumor-intrinsic vulnerability, and a Phase I clinical trial is underway (NCT04053673). (1) AACR Jun 22-24, 2020: Cancer Res 2020;80 (16 Suppl): Abstract nr 3405. Citation Format: Joseph M. Gozgit, Melissa M. Vasbinder, Ryan P. Abo, Kaiko Kunii, Kristy G. Kuplast-Barr, Bin Gui, Alvin Z. Lu, Jennifer R. Molina, Elena Minissale, Kerren K. Swinger, Tim J. Wigle, Danielle J. Blackwell, Christina R. Majer, Yue Ren, Mario Niepel, Ellen Bamberg, Jan-Rung Mo, David Church, Jeff Song, Luke Utley, Patricia E. Rao, Timothy J. Mitchison, Kevin W. Kuntz, Victoria M. Richon, Heike Keilhack. RBN-2397: A potent and selective small molecule inhibitor of PARP7 that induces tumor-derived antitumor immunity dependent on CD8 T cells [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 48.