Abstract B127: Evaluating PRKACA as a therapeutic target for Fibrolamellar Carcinoma
Authors: Schalm Stefanie S , O’Hearn Erin , Wilson Kevin , LaBranche Timothy , Silva Grace , DiPietro Lucian , Bifulco Neil , Woessner Richard , Stransky Nicolas , Sappal Darshan , Shutes Adam , Campbell Robert , Lobbardi Riadh , Palmer Michael , Kim Joseph , Miller Stephen , Dorsch Marion , Lengauer Christoph , Guzi Timothy , Kadambi Vivek , Garner Andrew , Hoeflich Klaus P 
Published: March 2020

Abstract
Introduction: Fibrolamellar Carcinoma (FLC) is a rare primary liver malignancy, affecting children and young adults without chronic liver disease. FLC tumors are largely resistant to chemotherapy, making the identification of effective treatment options urgently needed. Recent genomic data strongly suggest that DNAJB1-PRKACA kinase fusions are the drivers of the vast majority of FLC cases. However, it has not been assessed whether FLC tumors remain dependent on DNAJB1-PRKACA expression and whether PRKACA inhibition could be a therapeutic approach for FLC. Here we summarize the preclinical evaluation of PRKACA as a potential therapeutic target for FLC. Methods: We established a xenograft model from a FLC- patient and then developed inducible PRKACA shRNA cell lines from this model. We also designed potent tool compounds that selectively inhibit the PRKACA protein to assess PRKACA as a potential therapeutic target for FLC. Results: We characterized a patient-derived xenograft (PDX) model of FLC (LI5132) and confirmed DNAJB1-PRKACA fusion expression and constitutive PRKACA pathway activation measured by phospho-VASP. The model also shows fibrolamellar type histology by H&E staining and expression of typical FLC markers like cytokeratin 7 and CD68 by IHC. Using inducible PRKACA-specific shRNA cell lines from this PDX model we demonstrated that the FLC transcriptional gene signature correlates strongly with expression of the DNAJB1-PRKACAfusion protein. Importantly, we demonstrated for three inducible PRKACA shRNA cell-line-derived xenograft models that the in vivo tumor growth remained dependent on DNAJB1-PRKACA fusion expression (TGI-72%-78%, day 22). PRKACA knockdown tumors displayed reduced Ki67 index (6.4 %) when compared to non-induced controls (37.1 %) further confirming that proliferation of the tumors depends on the fusion expression. To investigate the PRKACA catalytic dependency of the FLC model, we designed potent and selective PRKACA inhibitors based on starting points from our proprietary kinase inhibitor library. These investigational compounds are the first selective and potent PRKACA inhibitors and provide excellent tools to assess in vitro and in vivo PRKACA dependency. These compounds achieved potent PRKACA pathway inhibition and dose-dependent inhibition of FLC-specific gene expression, including genes such as carbamoyl phosphate synthetase (CPS1) and forkhead box C1 (FoxC1). We established a pharmacokinetic/ pharmacodynamic relationship and demonstrated in vivo PRKACA pathway inhibition in PDX tumors, as measured by phospho-VASP. Importantly, oral delivery of a potent and selective PRKACA inhibitor achieved up to 80% PRKACA kinase inhibition and led to statistically significant FLC tumor growth inhibition (54%, day 34) on a tolerated schedule. These data demonstrate that FLC depends on PRKACA kinase activity. Conclusion: This study is the first evaluation of PRKACA kinase inhibition as a therapeutic approach for FLC. The results from these preclinical experiments provide strong evidence that FLC depends on PRKACA catalytic activity and that novel PRKACA inhibitors may significantly decrease tumor growth in vivo.
Citation Format: Stefanie S Schalm, Erin O’Hearn, Kevin Wilson, Timothy LaBranche, Grace Silva, Lucian DiPietro, Neil Bifulco, Richard Woessner, Nicolas Stransky, Darshan Sappal, Adam Shutes, Robert Campbell, Riadh Lobbardi, Michael Palmer, Joseph Kim, Stephen Miller, Marion Dorsch, Christoph Lengauer, Timothy Guzi, Vivek Kadambi, Andrew Garner, Klaus P Hoeflich. Evaluating PRKACA as a therapeutic target for Fibrolamellar Carcinoma [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B127. doi:10.1158/1535-7163.TARG-19-B127