Inhibition of clonogenic potential by the glycinamideribonucleosyl
transformylase inhibitor
5,10-dideazatetrahydrofolic acid (
DDATHF,
Lometrexol) was evaluated in vitro in a human ovarian
carcinoma cell line, SW626.
Drug-induced inhibition of clonogenic potential is a function of the dose and time of exposure and is independent of the formation of
DNA single-strand breaks or de novo synthesis of
protein. Simultaneous treatment with 100 microM
hypoxanthine completely prevented the inhibition of clonogenic potential caused by 0.5 microM
DDATHF.
DDATHF blocked cells in the early-middle S-phases of the cell cycle, and there was a corresponding marked reduction in the rate of
DNA synthesis after
drug withdrawal. The cytotoxic potential of
DDATHF was modulated by the
folic acid concentration present in the medium. In a medium containing 0.22 microM
folic acid,
DDATHF cytotoxicity was at least 100 times that in a regular medium containing 2.22 microM
folic acid, levels which, however, are about 100 times those found in human plasma.
DDATHF cytotoxicity differed moderately when
folic acid concentrations varied between 0.22 and 0 microM, suggesting that
folic acid does not necessarily antagonise
DDATHF anti-tumour activity.
Folinic acid at a concentration as low as 0.1 microM can completely rescue cells when given simultaneously with 0.5 microM
DDATHF. When
folinic acid was given 24 h after
DDATHF, a reversal of cytotoxicity was observed at 0.5 and 1 microM, but to a much lesser extent than simultaneous treatment. When
folinic acid was added after 48 or 72 h of
DDATHF washout, even at a high concentration and for a long time, no reduction in
DDATHF cytotoxicity was found. In conclusion, the study highlights the modulation of
DDATHF cytotoxicity by
folic acid or by
folinic acid and provides further rationale for in vivo clinical investigation with these combinations.