Abstract B21: In a patient-derived xenograft model of occult hepatic metastasis from pancreatic cancer the MEK inhibitor trametinib delays tumor outgrowth and prolongs survival
Authors: Newhook Timothy , Lindberg James , Adair Sara , Parsons J. Thomas , Bauer Todd W. 
Published: May 2015

Abstract
Background: Survival for patients with pancreatic ductal adenocarcinoma (PDAC) remains dismal and the majority of patients succumb to metastatic disease. Even for those with localized PDAC, most will die from metastatic disease despite margin-negative resection and adjuvant therapy. Therefore, these patients must harbor occult metastatic PDAC at presentation. There is a compelling need for the development of preclinical models that efficiently recapitulate occult metastatic liver PDAC to identify molecular and cellular pathways that drive metastatic cellular survival and growth. Towards this aim, we have developed a PDAC model of occult liver metastases using patient-derived xenografts (PDXs) to study the growth of PDAC within the metastatic environment and evaluated the effect of MEK inhibitor therapy on tumor progression.
Methods and Results: Extensively characterized patient-derived KRAS-mutant (Tumors 608, 366, and 654) and wild-type (Tumors 738 and 215) PDAC cells were transduced with luciferase and injected into the spleens of athymic, nude mice, allowed to circulate for 10 minutes, after which a splenectomy was performed. To evaluate metastatic cell growth kinetics in the liver, tumor burden was monitored by sequential bioluminescent imaging. All mice exhibited defined phases of survival/dormancy and the proliferative outgrowth; however, differential kinetics were observed for each tumor cell line. To evaluate the effect of MEK inhibition of occult metastatic PDAC cells in the liver, Tumor 608 cells were injected and mice either received the MEK inhibitor trametinib (0.3 mg/kg, daily) or vehicle control beginning 48 hour post-injection. Trametinib significantly reduced metastatic tumor burden, delayed time to proliferative outgrowth, and greatly prolonged survival as compared to control (med. survival: 114 vs. 43 days, p&lt;0.001). In contrast, in an orthotopic model with 250-500 mm3 tumors trametinib led to no inhibition in tumor growth for Tumor 608. To characterize metastatic PDAC cells within the liver after similar treatment conditions, we isolated cells from the liver 48 and 72 hours, 10 and 28 days after splenic injection. Tumor 608 cells were injected and metastatic PDAC cells were retrieved from the liver at the aforementioned timepoints post injection using magnetic column separation with human EpCAM (CD326)-targeted magnetically labeled microbeads. Flow cytometric analyses of retrieved cells revealed that decreased cellular markers of proliferation and increased caspase-3 cleavage correlated with decreased tumor burden observed at these timepoints in mice treated with trametinib.
Conclusions: In a model of occult liver metastatic PDAC, patient-derived tumors exhibit different growth kinetics in the liver environment. Furthermore, MEK inhibition with trametinib decreased metastatic cellular proliferation, increased apoptosis, prolonged metastatic tumor outgrowth, and significantly increased survival of mice harboring occult hepatic metastases from PDAC. This efficacy of single agent trametinib is unique to occult metastatic disease and is not seen in orthotopic models of advanced, established pancreatic tumors. This finding illustrates the importance of using specific preclinical models that best recapitulate the clinical aspects of patients who will be evaluated in future clinical trials. Further investigation into the cellular and molecular factors promoting PDAC cell survival within the hepatic microenvironment utilizing this model will lead to development of rational therapeutic strategies for patients with occult metastatic disease.
Citation Format: Timothy Newhook, James Lindberg, Sara Adair, J. Thomas Parsons, Todd W. Bauer. In a patient-derived xenograft model of occult hepatic metastasis from pancreatic cancer the MEK inhibitor trametinib delays tumor outgrowth and prolongs survival. [abstract]. In: Proceedings of the AACR Special Conference: The Translational Impact of Model Organisms in Cancer; Nov 5-8, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(11 Suppl):Abstract nr B21.