Abstract 381: Elevated PARP7 expression in select cancers identifies a target population for RBN-2397 therapy
Authors: Wong Jodie , Kuplast-Barr Kristy , Abo Ryan P. , Kulkarni Anupriya S. , Nieman Linda T. , Neyaz Azfar , Xu Katherine H. , Sharaf Radwa , Albacker Lee A. , Ting David T. , Keilhack Heike , McEachern Kristen A. 
Published: July 2021

PARP7 (TIPARP) is a monoPARP which catalyzes the transfer of single units of ADP-ribose onto substrates to change their function. Its expression is upregulated during cellular stress, including viral infection or through the activation of the aryl hydrocarbon receptor after exposure to cigarette smoke. We and others have shown that PARP7 activity suppresses the Type I interferon (IFN) response following activation by cytosolic nucleic acid sensing pathways. RBN-2397 is a first-in class PARP7 inhibitor, which induces cancer cell autonomous and immune stimulatory effects in preclinical models through enhanced Type I IFN signaling in cancer cells. Here we describe the presence of PARP7 genomic amplification with corresponding increased mRNA expression across select cancers. Elevated PARP7 expression or amplification may identify cancer patients that could derive benefit from treatment with RBN-2397.

In characterizing PARP7 copy number and mRNA expression from The Cancer Genome Atlas (TCGA) database, we found the presence of PARP7 copy number amplification in a subset of tumor types, particularly those of squamous histology, as well as ovarian cancer that corresponded to higher mRNA expression levels. High PARP7 expression correlated with poor survival in squamous cancers, while it had no effect on survival in ovarian cancer. Interestingly, tumor types with high PARP7 expression also expressed higher levels of baseline interferon stimulated genes (ISGs). This parallels our findings that cancer cell lines with higher ISGs at baseline are more responsive to PARP7 inhibition.

To explore PARP7 copy number variations (CNVs) in advanced cancer patients, we queried the FoundationCore® (Foundation Medicine, Inc) database. Similar to TCGA, squamous cancers as well as ovarian, breast, and pancreatic ductal adenocarcinoma (PDAC) were among the tumor types with PARP7 amplifications. Moreover, PARP7 was found to be amplified both on the background of chromosome 3q arm-level amplifications as well as focally.

Congruent to our analysis of PARP7 amplifications, we evaluated PARP7 mRNA expression in both squamous and non-squamous non-small cell lung carcinoma (NSCLC), breast cancer, and PDAC primary tumor samples. Using a validated RNAscope ISH probe set, we analyzed over 1,000 patient samples and found that PARP7 was more highly expressed in tumor cells relative to corresponding normal tissues. Overall, there were varying levels of PARP7 expression across samples with higher expression levels of PARP7 in tumor cells, compared to stroma, across all cancers examined.

Our analyses describing the presence of PARP7 amplifications as well as high expression levels in several tumor types including NSCLC, breast, and PDAC provides evidence for the therapeutic relevance of PARP7 inhibition and highlights potential patient selection strategies to identify those patients more likely to benefit from RBN-2397 treatment.

Citation Format: Jodie Wong, Kristy Kuplast-Barr, Ryan P. Abo, Anupriya S. Kulkarni, Linda T. Nieman, Azfar Neyaz, Katherine H. Xu, Radwa Sharaf, Lee A. Albacker, David T. Ting, Heike Keilhack, Kristen A. McEachern. Elevated PARP7 expression in select cancers identifies a target population for RBN-2397 therapy [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 381.