Abstract 5461: Triapine-mediated ABCB1 induction via PKC induces widespread therapy unresponsiveness but is not underlying acquired triapine resistance
Authors: Miklos Walter , Pelivan Karla , Kowol Christian , Dornetshuber-Fleiss Rita , Spitzwieser Melanie , Cichna-Markl Margit , Köllensperger Gunda , Keppler Bernhard , Berger Walter , Heffeter Petra 
Published: August 2015

Due to their enhanced proliferation rate, tumor cells are highly susceptible for ribonucleotide pool disruption. Therefore, several thiosembicarbazone-based ribonucleotide reductase inhibitors have been developed, out of which Triapine is the most promising candidate. Triapine is currently tested in clinical phase I and II studies and shows promising effects in haematological diseases. Unfortunately, triapine is rather ineffective in solid cancer types. However, the mechanism underlying this failure is yet not fully understood. One possible theory could be development of rapid acquired resistance against the chemotherapeutic drug. To investigate this issue we generated a triapine-resistant cell line (SW480/tria) by stepwise selection of human colon carcinoma SW480 cells. SW480/tria cells displayed a broad cross-resistance especially against several well-known ABCB1 substrates (e.g. vincristine). In accordance, strong ABCB1 expression was detected in SW480 Tria cells. The induction of ABCB1 was not based on gene amplification but on hypomethylation of the ABCB1 promoter. As a next step, rhodamine-123 and ATPase assays were performed to investigate if triapine does interact with the ABCB1 transport function. However, no ABCB1 inhibitory potential of triapine could be found. Further on, combined treatment in SW480/tria cells with triapine and the known ABCB1 inhibitors cyclosporine A (CSA) and verapamil did not restore triapine sensitivity. In addition, the intracellular triapine levels were comparable between parental and triapine-selected SW480 cell lines. Moreover, increased ABCB1 expression was found to be a consequence of triapine stress-induced PKC activation. This suggests that the strong ABCB1 expression of SW480/tria cells might rather be a consequence of triapine-induced cell stress than a major driver for specific triapine resistance.

Taken together our data reveal that, although triapine is only a weak substrate for ABCB1, strong ABCB1 expression is induced by short- and long-term triapine exposure resulting in distinct cross resistance against other anticancer drugs. This definitely has to be considered in the selection of combination schemes and in second line therapy following triapine failure.

Citation Format: Walter Miklos, Karla Pelivan, Christian Kowol, Rita Dornetshuber-Fleiss, Melanie Spitzwieser, Margit Cichna-Markl, Gunda Köllensperger, Bernhard Keppler, Walter Berger, Petra Heffeter. Triapine-mediated ABCB1 induction via PKC induces widespread therapy unresponsiveness but is not underlying acquired triapine resistance. [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 5461. doi:10.1158/1538-7445.AM2015-5461