Abstract 695: Glucuronidation in human colon cancer cells acts as an intrinsic mechanism of resistance against tubulin depolymerizers that are UGT substrates
Authors: Cosenza Stephen C. , Cho Sool Yeon , Tantravahi Ramana , Panda Gayatri , Pallela Venket , Reddy M.V. Ramana , Roboz John , Reddy E. Premkumar 
Published: June 2012

Abstract
Tumor cell resistance towards chemotherapeutic agents is thought to arise through a number of different mechanisms, induced by prior exposure, activation of intrinsic resistance mechanisms or a combination of both and is a major problem in oncology. ON013105 is a novel compound with a potent anti-tumor activity against both solid and leukemic tumor cell lines. This compound was found to induce mitotic arrest of tumor cells leading to activation of caspase activity and apoptotic death. This compound is currently in phase I clinical trials for mantle cell lymphoma therapy. While studying the kinetics of cell death induced by ON013105 (phosphate prodrug) and ON013100 (free acid), we identified a colorectal cancer cell line that was 1000 fold resistant to both forms of the compound. Studies were undertaken to identify the mechanism of resistance. We first sought to determine the frequency and degree of resistance by testing a large panel of cell lines from a variety of tumor types. We found that 4 out of 9 colon tumor lines were resistant while all other tumor cell types were sensitive. This led us to explore the possibility of metabolism and/or transport mechanisms that could result in drug resistance. The compounds are not substrates for the MDR-1 pumps, but transcriptome and protein expression studies identified a variety of UDP-glucuronosyltransferases (UGTs) expressed in the resistant cell types. Cross resistance studies with compounds that are known UGT substrates showed that only combretastatin A4 cell killing activity correlated with sensitivity towards our compounds. Uptake studies showed that sensitive cells retained ON013100 while ON013100 was rapidly removed in resistant cells. ON013100-glucuronide was found to be a metabolite (571 Da) in both the cell pellet and the medium and its formation correlated with cell sensitivity. In cell pellet, (a) from 0 to 2 h, metabolite quantities (in terms of peak intensities) were 50 to 250 times higher in HT-29 (resistant) than in HCT-15 (sensitive); (b) from 2 to 24 h, rapidly decreased to baseline; (c) metabolite quantities in HCT-15 were essentially zero with very small amount (peak intensities) detected from 0 to 4 h. We could also show specific competition by known acyl glycones whose treatment resulted in reversal of resistance. The recent identification of glucuronidation of combretastatin A4 by UGT1A9 also points to UGT activity as the mechanism of resistance. We find it interesting that both of these compounds are agents that interact with tubulins. Identification of the UGT isoforms responsible for ON013100 glucuronidation demonstrate the possibility of intrinsic resistance due to UGT activity of colon tumors and should be considered as a clinically important factor when determining possible therapies that include tubulin poisons which are also UGT substrates.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 695. doi:10.1158/1538-7445.AM2011-695