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

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.

Construction and analysis of competing endogenous RNA network of MCF‑7 breast cancer cells based on the inhibitory effect of 6‑thioguanine on cell proliferation

“…Initially, 6-TG was only employed to treat acute and chronic myeloid leukemia (9). Following further research, 6-TG has proven to have therapeutic effects on inflammatory bowel disease, children's gliomas and pancreatic cancer (9)(10)(11). In a previous study, 6-TG was shown to inhibit MCF-7 breast cancer cell proliferation by increasing FAS expression, thereby activating the apoptosis pathway (12).…”

6-Thioguanine and Its Analogs Promote Apoptosis of Castration-Resistant Prostate Cancer Cells in a BRCA2-Dependent Manner

“…The thiopurine 6-thioguanine (6-TG) is a chemotherapeutic drug that induces DNA damage and is successfully used in the treatment of childhood acute lymphoblastic leukemia and other forms of leukemias [24]. Previous studies on its efficacy in various solid tumors have shown infrequent but positive results and have suggested that combinations of 6-TG with other agents may enhance the antitumor effects [25,26]. Recently, 6-TG was found to be a potent inhibitor of ubiquitin-specific protease 2, which plays a critical role in prostate tumor cell survival [27].…”

<p>Transcriptomics Analysis of the Tumor-Inhibitory Pathways of 6-Thioguanine in MCF-7 Cells via Silencing DNMT1 Activity</p>

“…22 In addition, 6-TG, as an immunosuppressive drug, was used in autoimmune and chronic inflammatory diseases, such as Crohn's disease, by regulating GTPase activity, and in pancreatic ductal adenocarcinoma (PDAC) cells, 6-TG inhibited the BRAF-MEK-ERK pathway to induce cell apoptosis. 23,24 More notably, 6-TG induced an inhibitory effect on the growth of BRCA-defective tumors, similar to a PARP inhibitor, which was induced because of the inability to repair DNA damage. 25,26 Nevertheless, the effect of 6-TG on the most common breast cancer MCF-7 cell line has not been investigated clearly.…”