A drug‐repositioning screen for primary pancreatic ductal adenocarcinoma cells identifies 6‐thioguanine as an effective therapeutic agent for TPMT‐low cancer cells

Inki Kim,Sojung Park,Je‐Keun Rhee,Yeon‐Sook Choi,Jae Hwi Song,Suhwan Chang,Song Cheol Kim,Eun Ji Lee,Eun A Choi

doi:10.1002/1878-0261.12364

Abstract

Pancreatic cancer is one of the most difficult cancers to cure due to the lack of early diagnostic tools and effective therapeutic agents. In this study, we aimed to isolate new bioactive compounds that effectively kill pancreatic ductal adenocarcinoma (PDAC) cells, but not untransformed, human pancreatic ductal epithelial (HPDE) cells. To this end, we established four primary PDAC cell lines and screened 4141 compounds from four bioactive‐compound libraries. Initial screening yielded 113 primary hit compounds that caused over a 50% viability reduction in all tested PDAC cells. Subsequent triplicate, dose‐dependent analysis revealed three compounds with a tumor cell‐specific cytotoxic effect. We found that these three compounds fall into a single category of thiopurine biogenesis. Among them, 6‐thioguanine (6‐TG) showed an IC50 of 0.39–1.13 μm toward PDAC cells but had no effect on HPDE cells. We propose that this cancer selectivity is due to differences in thiopurine methyltransferase (TPMT) expression between normal and cancer cells. This enzyme is responsible for methylation of thiopurine, which reduces its cytotoxicity. We found that TPMT levels were lower in all four PDAC cell lines than in HPDE or Panc1 cells, and that knockdown of TPMT in HPDE or Panc1 cells sensitized them to 6‐TG. Lastly, we used a patient‐derived xenograft model to confirm that 6‐TG has a significant antitumor effect in combination with gemcitabine. Overall, our study presents 6‐TG as a strong candidate for use as a therapeutic agent against PDAC with low levels of TPMT.

Highlights

Despite the recent advances in surgical techniques and other treatment strategies, pancreatic cancer remains a cancer type with the lowest 5-year survival rate (He and Yuan, 2014; Kim, 2008)
We suggest thiopurine analogs as such candidates. 6-TG seems to be a PDACspecific antitumor agent, and we confirmed its efficacy in a patient-derived xenograft model
Western blot analysis of SMAD4 and P53, as well as vimentin and SMA, revealed that the 34 629-cell line has a stellate cell-like molecular phenotype but is not exactly the same

Summary

Introduction

Despite the recent advances in surgical techniques and other treatment strategies, pancreatic cancer remains a cancer type with the lowest 5-year survival rate (He and Yuan, 2014; Kim, 2008). There is a desmoplastic reaction, which means accumulation of the extracellular matrix along with activated (fibrotic) stellate cells (Bahrami et al, 2017), resulting in a barrier to drug delivery (Lunardi et al, 2014) Despite these challenges, there are newly developed therapeutic options, including FOLFIRINOX and Paclitaxel-Nab, that have shown progress in chemotherapy (Sohal et al, 2016). Bazedoxifene, which is approved as an estrogen modulator, was revealed as an antiGP130 signaling agent that can inhibit pancreatic cancer cell growth (Wu et al, 2016) These findings reveal novel therapeutic agents based on the cytotoxicity to PDAC cells, often these drugs exert toxicity toward normal cells. Considering the heterogeneity of PDAC (Iovanna and Dusetti, 2017), there is a need to apply drug repositioning to other PDAC cell lines including primary cells

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