Abstract
N-[5-[ N-(3,4-Dihydro-2-methyl-4-oxoquinazolin-6-ylmethyl)- N-methylamino]-2-thenoyl]-L-glutamic acid (ZD1694) is a folate-based thymidylate synthase (TS; EC 2.1.1.45) inhibitor. Metabolism to higher chain length polyglutamates is essential for its optimal cytotoxic effect. A ZD1694-resistant (300-fold) human ileocecal carcinoma cell line (HCT-8/DW2) was developed, and its mechanism of resistance was evaluated. TS activities in situ and TS protein levels in the HCT-8 parental line and HCT-8/DW2 were similar (168 ± 47 vs 137 ± 25 pmol/hr/10 6 cells and 2.05 ±0.28 vs 2.07 ± 0.19 pmol/mg protein, respectively). The ic 50 values of ZD1694 for TS inhibition in cell-free extracts were similar in both lines, but the ic 50 of ZD1694 for TS inhibition in situ in HCT-8/DW2 cells was 27- and 268-fold higher than that in HCT-8 cells at 0 and 24 hr, respectively, after a 2-hr drug exposure. Folylpolyglutamate synthetase (FPGS; EC 6.3.2.17) activity was significantly lower in resistant HCT-8/DW2 cells as compared with parental HCT-8 cells (88 ± 40 vs 1065 ± 438 pmol/hr/mg protein when ZD1694 was used as substrate). The combined endogenous pool of methylenetetrahydrofolate and tetrahydrofolate in HCT-8/DW2 cells was also decreased. In addition, HCT-8/DW2 cells accumulated lower levels of methotrexate (MTX) in a 2-hr period, although the initial velocity of MTX transport was similar to that in parental HCT-8 cells. The lower level of FPGS activity and the lower level of (anti)folate accumulation in HCT-8/DW2 correlated with drug resistance and with the higher ic 50 of ZD1694 for in situ TS inhibition. In addition, drug resistance was also correlated with the rapid recovery of in situ TS activity after drug treatment. In brief, in this highly ZD1694-resistant HCT-8 cell line, resistance is associated with decreased FPGS activity, which, in turn, affects the metabolism of ZD1694 and consequently the extent and duration of in situ TS inhibition by the drug.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
