PARP7 negatively regulates the type I interferon response in cancer cells and its inhibition triggers antitumor immunity.

PARP7 is a monoPARP that catalyzes the transfer of single units of ADP-ribose onto substrates to change their function. Here, we identify PARP7 as a negative regulator of nucleic acid sensing in tumor cells. Inhibition of PARP7 restores type I interferon (IFN) signaling responses to nucleic acids in tumor models. Restored signaling can directly inhibit cell proliferation and activate the immune system, both of which contribute to tumor regression. Oral dosing of the PARP7 small-molecule inhibitor, RBN-2397, results in complete tumor regression in a lung cancer xenograft and induces tumor-specific adaptive immune memory in an immunocompetent mouse cancer model, dependent on inducing type I IFN signaling in tumor cells. PARP7 is a therapeutic target whose inhibition induces both cancer cell-autonomous and immune stimulatory effects via enhanced IFN signaling. These data support the targeting of a monoPARP in cancer and introduce a potent and selective PARP7 inhibitor to enter clinical development.

Targeted Degradation of PARP14 Using a Heterobifunctional Small Molecule.

PARP14 is an interferon-stimulated gene that is overexpressed in multiple tumor types, influencing pro-tumor macrophage polarization as well as suppressing the antitumor inflammation response by modulating IFN-γ and IL-4 signaling. PARP14 is a 203 kDa protein that possesses a catalytic domain responsible for the transfer of mono-ADP-ribose to its substrates. PARP14 also contains three macrodomains and a WWE domain which are binding modules for mono-ADP-ribose and poly-ADP-ribose, respectively, in addition to two RNA recognition motifs. Catalytic inhibitors of PARP14 have been shown to reverse IL-4 driven pro-tumor gene expression in macrophages, however it is not clear what roles the non-enzymatic biomolecular recognition motifs play in PARP14-driven immunology and inflammation. To further understand this, we have discovered a heterobifunctional small molecule designed based on a catalytic inhibitor of PARP14 that binds in the enzyme's NAD+ -binding site and recruits cereblon to ubiquitinate it and selectively target it for degradation.

A potent and selective PARP14 inhibitor decreases protumor macrophage gene expression and elicits inflammatory responses in tumor explants.

PARP14 has been implicated by genetic knockout studies to promote protumor macrophage polarization and suppress the antitumor inflammatory response due to its role in modulating interleukin-4 (IL-4) and interferon-γ signaling pathways. Here, we describe structure-based design efforts leading to the discovery of a potent and highly selective PARP14 chemical probe. RBN012759 inhibits PARP14 with a biochemical half-maximal inhibitory concentration of 0.003 µM, exhibits >300-fold selectivity over all PARP family members, and its profile enables further study of PARP14 biology and disease association both in vitro and in vivo. Inhibition of PARP14 with RBN012759 reverses IL-4-driven protumor gene expression in macrophages and induces an inflammatory mRNA signature similar to that induced by immune checkpoint inhibitor therapy in primary human tumor explants. These data support an immune suppressive role of PARP14 in tumors and suggest potential utility of PARP14 inhibitors in the treatment of cancer.

In Vitro and Cellular Probes to Study PARP Enzyme Target Engagement.

Poly(ADP-ribose) polymerase (PARP) enzymes use nicotinamide adenine dinucleotide (NAD+) to modify up to seven different amino acids with a single mono(ADP-ribose) unit (MARylation deposited by PARP monoenzymes) or branched poly(ADP-ribose) polymers (PARylation deposited by PARP polyenzymes). To enable the development of tool compounds for PARP monoenzymes and polyenzymes, we have developed active site probes for use in in vitro and cellular biophysical assays to characterize active site-directed inhibitors that compete for NAD+ binding. These assays are agnostic of the protein substrate for each PARP, overcoming a general lack of knowledge around the substrates for these enzymes. The in vitro assays use less enzyme than previously described activity assays, enabling discrimination of inhibitor potencies in the single-digit nanomolar range, and the cell-based assays can differentiate compounds with sub-nanomolar potencies and measure inhibitor residence time in live cells.

Enabling drug discovery for the PARP protein family through the detection of mono-ADP-ribosylation.

Poly-ADP-ribose polymerases (PARPs) are a family of enzymes responsible for transferring individual or chains of ADP-ribose subunits to substrate targets as a type of post-translational modification. PARPs regulate a wide variety of important cellular processes, ranging from DNA damage repair to antiviral response. However, most research to date has focused primarily on the polyPARPs, which catalyze the formation of ADP-ribose polymer chains, while the monoPARPs, which transfer individual ADP-ribose monomers, have not been studied as thoroughly. This is partially due to the lack of robust assays to measure mono-ADP-ribosylation in the cell. In this study, the recently developed MAR/PAR antibody has been shown to detect mono-ADP-ribosylation in cells, enabling the field to investigate the function and therapeutic potential of monoPARPs. In this study, the antibody was used in conjunction with engineered cell lines that overexpress various PARPs to establish a panel of assays to evaluate the potencies of literature-reported PARP inhibitors. These assays should be generally applicable to other PARP family members for future compound screening efforts. A convenient and generalizable workflow to identify and validate PARP substrates has been established. As an initial demonstration, aryl hydrocarbon receptor was verified as a direct PARP7 substrate and other novel substrates for this enzyme were also identified and validated. This workflow takes advantage of commercially available detection reagents and conventional mass spectrometry instrumentation and methods. Ultimately, these assays and methods will help drive research in the PARP field and benefit future therapeutics development.