Abstract 2082: NUC 3373 induces a cytoplasmic translocation of thymidylate synthase in colorectal carcinoma cell lines
Authors: McKissock Fiona G. , Um In Hwa , Mullen Peter , Harrison David J. 
Published: August 2019

BACKGROUND: Although 5-fluorouracil-based (5-FU) chemotherapies remain the cornerstone of combination therapies for colorectal cancer (CRC), their clinical utility is limited by key cancer resistance mechanisms associated with breakdown, transport, and activation. This includes the intracellular conversion of 5-FU into its active metabolite, fluorodeoxyuridine-monophosphate (FUDR-MP), before core anti-cancer activity can be exerted via inhibition of the enzyme thymidylate synthase (TS). Although widely studied, there are mixed reports as to whether high or low basal tumor TS levels are prognostic of response to 5-FU therapy. NUC-3373, a phosphoramidate transformation of FUDR-MP, is designed to bypass the key resistance mechanisms associated with 5-FU, resulting in a potentially more effective and safer treatment option. This study aimed to determine the prognostic value of cancer cell TS protein expression in predicting therapeutic response to NUC-3373 and to further explore the effects of NUC-3373 on TS biology.

METHODS: IC50 values for NUC-3373 were established in nine CRC cell lines by sulforhodamine B assay, after which one sensitive (HCT116) and one resistant (SW480) cell line were selected for further characterization. In these two cell lines, endogenous, induced, and ternary complex TS expression was measured by Western blot analysis and the cellular localization of TS was confirmed by immunocytochemistry and assessed by stereological methods.

RESULTS: TS protein expression was measured in nine CRC cell lines treated with NUC-3373 and no correlation was found between TS protein levels and the IC50 for NUC-3373. This indicates that the sensitivity of these cell lines to NUC-3373 is not dependent on basal TS protein expression levels. When the two cell lines selected for their sensitivity/resistance to NUC-3373 were treated with NUC-3373, TS expression was induced after 12 hours and remained elevated at 48 hours. In addition, treatment with NUC-3373 was associated with a translocation of TS from the nucleus to the cell cytoplasm, which was more pronounced in the NUC-3373 sensitive cells. Translocated TS did not localize to vesicles or the Golgi apparatus; the cytoplasmic localization remains to be determined.

CONCLUSIONS: These results suggest that basal TS levels are not prognostic of response to NUC-3373 in CRC cells, instead NUC-3373 induces TS expression and causes nuclear to cytoplasmic translocation. This novel finding is indicative of an alternative mechanism of action for FUDR-MP in promoting anti-cancer activity that is independent of the DNA damage pathway.

Citation Format: Fiona G. McKissock, In Hwa Um, Peter Mullen, David J. Harrison. NUC 3373 induces a cytoplasmic translocation of thymidylate synthase in colorectal carcinoma cell lines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2082.