THE RELATIONSHIP BETWEEN INTRINSIC THYMIDYLATE SYNTHASE EXPRESSION AND SENSITIVITY TO THYMITAQ(TM) IN HUMAN LEUKEMIA AND COLORECTAL-CARCINOMA CELL-LINES

Thymidylate synthase (TS) expression has been characterized for a pane l of eight human colorectal carcinoma and five human leukaemia cell li nes, to relate differences in intrinsic TS activity, protein and mRNA levels to growth inhibition caused by continuous exposure to THYMITAQ( TM), a specific non-classical antifolate TS inhibitor. Although a 20-f old variation in sensitivity to THYMITAQ(TM) was found within the colo rectal cell line panel (IC50 0.12-2.7 mu M), sensitivity was not relat ed to TS activity, TS protein or TS mRNA levels. For the leukaemic cel l lines, only a twofold range in sensitivity to THYMITAQ(TM) was obser ved (IC50 0.87-2.3 mu M), and this did not correlate with TS activity, TS protein or TS mRNA levels. Across all of the cell lines, TS activi ty was linearly related to TS protein levels (r(2) = 0.87, P < 0.0001) . However, for both the colorectal and leukaemia cell line panels, no relationship was found between TS mRNA/18S rRNA ratios and either TS a ctivity or TS protein, consistent with the importance of post-transcri ptional mechanisms in regulating TS activity. Two of the colorectal ce ll lines (BE and HCT116) and one of the human leukaemic cell lines (HL 60), were intrinsically resistant to THYMITAQ(TM) (IC50 > 2 mu M) in t he absence of TS overexpression, suggesting that, subsequent to TS inh ibition, events such as DNA repair and tolerance to apoptotic stimuli are also important determinants of sensitivity to THYMITAQ(TM).