Abstract 348: Amino acid-based prodrugs of gemcitabine - a therapeutic option to overcome chemoresistance in pancreatic cancer
Authors: Bhutia Yangzom D. , Babu Ellappan , Singh Pankaj K. , Ganapathy Vadivel 
Published: August 2015

Abstract
Pancreatic cancer is the deadliest of all cancers. Currently, gemcitabine is the only drug of choice to treat this cancer but the results are modest as the disease itself is diagnosed at an advanced stage and often times develops resistance to gemcitabine via silencing of the influx transporters for the drug. We hypothesized that the delivery of gemcitabine into pancreatic cancer cells in a different form via some other transporters would provide a logical alternative approach. It would even be better if we can exploit for this purpose the transporters that are selectively upregulated in pancreatic cancer. Several nucleoside drugs are transportable substrates for the amino acid transporter SLC6A14 if used in the form of amino acid-based prodrugs. Such strategy would work for gemcitabine if SLC6A14 is upregulated in pancreatic cancer. With this in mind, we analyzed publically available microarray datasets for the amino acid transporters that are upregulated in pancreatic cancer. We found four transporters (SLC1A5, SLC7A5, SLC7A11, and SLC6A14) overexpressed in tumors compared to normal tissue. The fold-increase in expression is the highest for SLC6A14 in all datasets (range, 1.3 - 163.3; p, 0.05 - 2×10−14). The fold-increase for the other three transporters is 2 at the maximum. We confirmed these findings using primary pancreatic cancer tissues and cancer cell lines. First, we used pancreatic cancer tissues and compared with the normal tissues. SLC6A14 mRNA is increased at least by 7-fold in tumors versus normal tissue. We then evaluated the expression of SLC6A14 mRNA and protein in normal and pancreatic cancer cell lines. The expression is higher in all cancer cell lines than in a normal cell line. This was further confirmed with a tissue microarray containing pancreatic tumor tissues from 52 patients. The delivery of gemcitabine in the form of amino acid-based prodrugs via SLC6A14, an exceptionally energy-coupled transporter, as a means to overcome gemcitabine resistance is a novel idea that has never been tested. As a proof of principle, we have shown that SLC6A14 can transport several nucleoside drugs (acyclovir, ganciclovir, and cytarabine) in the form of amino acid-based prodrugs. The parent nucleosides are not substrates for the transporter. To specifically assess the interaction of amino acid-based prodrugs of gemcitabine with SLC6A14, we synthesized the valyl ester of gemcitabine and investigated its transport via SLC6A14 using two different heterologous expression systems: a mammalian cell expression system and the Xenopus oocyte expression system. In both experimental approaches, we found that the valyl ester of gemcitabine, but not the parent drug, is a transportable substrate for SLC6A14. The transport process is Na/Cl-coupled and electrogenic. We conclude that amino acid-based prodrugs of gemcitabine represent a viable alternative to overcome resistance to gemcitabine in the treatment of pancreatic cancer.
Citation Format: Yangzom D. Bhutia, Ellappan Babu, Pankaj K. Singh, Vadivel Ganapathy. Amino acid-based prodrugs of gemcitabine - a therapeutic option to overcome chemoresistance in pancreatic cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 348. doi:10.1158/1538-7445.AM2015-348