Purpose: We show that expression of GRP78, the master regulator of the endoplasmic reticulum (ER) stress response, leads to chemotherapy resistance in pancreatic ductal adenocarcinoma (PDAC) and utilize a small molecule inhibitor to block the activity of GRP78, increasing the efficacy of currently available therapies in vitro and in vivo.

Background: Gemcitabine (GEM) is the standard chemotherapeutic agent for adjuvant therapy of PDAC and a commonly used agent in other treatment settings, including neoadjuvant treatment of borderline resectable PDAC and palliative treatment of metastatic PDAC. However, recurrent disease is observed in an alarming number of patients treated with GEM or GEM-based treatment regimens. A molecular-level understanding of the mechanisms that contribute to chemoresistance is essential for the development of more effective treatment strategies. ER chaperone proteins protect cells from apoptosis in response to cellular insults, including inflammation and pancreatitis. Elevated expression of chaperone proteins also causes the activation of many “pro-survival” pathways that promote cell survival and resistance to routinely utilized therapeutic regimens. We hypothesize that expression of ER chaperone protein GRP78, is responsible for innate and acquired resistant to GEM and that therapies targeting GRP78 by blocking its activity with the small molecule inhibitor, IT-139, could potentially improve patient response to GEM-based treatment regimens.

Methods: We examined if GRP78 overexpression correlated with chemoresistance in PDAC through establishing a patient-derived orthotopic xenograft (PDX) model. Treating tumor-bearing mice with GEM continuously or releasing mice from GEM treatment after 3-4 weeks of exposure identified PDX lines that were either sensitive or resistant to GEM. Tumors from these cohorts were analyzed for GRP78 expression. To examine the role of GRP78 in chemoresistance in vitro, GRP78 expression was induced in GEM-sensitive cell lines prior to GEM treatment. Further, to investigate the effect of GRP78 loss on chemoresistance, siRNA knockdown of GRP78, followed by GEM treatment, was used in GEM-resistant cell lines. Finally, the efficacy of combination treatment with GEM and IT-139, a ruthenium-based anticancer drug and GRP78 inhibitor, was determined in vitro and in vivo.

Results: Here, we report that PDX lines that result in GEM resistant PDAC have higher GRP78 expression than that of GEM sensitive PDX lines. In vitro, increased GRP78 induces chemoresistance in GEM-sensitive PDAC cell lines while knockdown of GRP78 reduces chemoresistance in GEM-resistant PDAC cell lines. Finally, we found that IT-139 can overcome GRP78 mediated chemoresistance. In vitro, IT-139 restores sensitivity to cytotoxic drugs in drug resistant PDAC cells and induces twice as much cell death in combination treatment compared with GEM alone. In vivo, a single weekly IT-139 treatment in combination with GEM caused a 35% increase in median survival and a 25% increase in overall survival compared to GEM alone.

Conclusions: Collectively, our data show that GRP78 expression promotes chemoresistance in PDAC and therapeutic strategies blocking the activity of GRP78 increase the efficacy of currently available therapies.

Citation Format: Jenifer B. Gifford, Wei Huang, Ann E. Zeleniak, Antreas Hindoyan, Hong Wu, Timothy R. Donahue, Reginald Hill.{Authors}. ER chaperone GRP78 increases chemoresistance in pancreatic ductal adenocarcinoma. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr B08.