Abstract C58: p53/p16-independent cytoreduction of chemoresistant pancreatic adenocarcinoma by metabolically optimized epigenetic therapy
Authors: Tohme Rita , Gu Xiaorong , Desoky Asmaa El , Schuerger Caroline , Lindner Daniel , Sohal Davendra , Saunthararajah Yogen 
Published: March 2020

Abstract
Although several reasons for chemorefractoriness of pancreatic ductal adenocarcinoma (PDAC) response have been proposed, a well-established basis is genetic inactivation of the p53/p16 apoptosis axis. Therefore, there is a need for p53/p16-independent PDAC treatments. The key epigenetic regulator DNA methyltransferase 1 (DNMT1) has been scientifically validated as a molecular target for p53-independent cytoreduction of pancreatic cancer, e.g., by activating terminal epithelial programs. Moreover, the pyrimidine nucleoside analog prodrugs decitabine (Dec) or 5-azacytidine (5Aza) are metabolically processed into a nucleotide analog that depletes DNMT1. Motivated by the scientific data, several clinical trials evaluating Dec, 5Aza, or analogs to treat PDAC have been completed. However, results from these trials have been discouraging and contradict encouraging preclinical results. To model the clinical situation, we evaluated Dec and 5Aza efficacy in gemcitabine-resistant PDAC in vitro and in mice. Gemcitabine-resistant PDAC cells were cross-resistant to Dec but not 5Aza. Measurement of the pyrimidine metabolism enzymes needed for prodrug conversion demonstrated the reason: DCK that activates gemcitabine and Dec was suppressed, but UCK2 that activates 5Aza was preserved and upregulated. Also, the catabolic enzyme cytidine deaminase (CDA), which rapidly catabolizes Dec and 5Aza into inactive uridine counterparts, was upregulated. Importantly, these shifts in pyrimidine metabolism that were observed in established gemcitabine-resistant cells were rapidly recapitulated (within hours/days) upon exposure of naïve PDAC cells to Dec or 5Aza, although 5Aza exposure immediately upregulated DCK, whereas Dec exposure immediately upregulated UCK2. Measurements of nucleotide levels by LC-MS/MS indicated that these contrasting shifts were caused by opposite effects of Dec and 5Aza on dCTP levels. These observations suggested potential solutions to overcome resistance: We evaluated alternating Dec with 5Aza, to exploit their mutual priming effects on DCK and UCK2 expression, and the addition of tetrahydrouridine (THU) to inhibit the catabolism of Dec and 5Aza by CDA. This optimized regimen was very efficacious in a murine xenograft model of gemcitabine-resistant PDAC (median vehicle control tumor measurements 972 mm3(range 726-1267.5); median THU-Dec/THU-5Aza 16 mm3 (range 0-87.5); P&lt;0.00001). Terminal epithelial-differentiation of gemcitabine-resistant PDAC cells was confirmed by a significant increase in pancreatic epithelial markers PTF1A, MIST1, and N-cadherin, while apoptosis markers were not induced. In sum, the pyrimidine metabolism network, which senses and regulates nucleotide amounts, automatically dampens Dec (and gemcitabine) and 5Aza activity. Nevertheless, these reactions can be anticipated and exploited instead to extend and deepen DNMT1 depletion and noncytotoxic tumor cytoreduction.
Citation Format: Rita Tohme, Xiaorong Gu, Asmaa El Desoky, Caroline Schuerger, Daniel Lindner, Davendra Sohal, Yogen Saunthararajah. p53/p16-independent cytoreduction of chemoresistant pancreatic adenocarcinoma by metabolically optimized epigenetic therapy [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr C58.