Lometrexol (5,10-dideazatetrahydrofolic
acid;
DDATHF), is a specific inhibitor of glycinamideribonucleosyl (
GAR) transformylase with anti-tumour activity in murine and human
carcinomas. The cytotoxicity activity of
DDATHF was evaluated in vitro in NIH/3T3 cells transfected with human alpha-
folate-
binding protein (FBP)
complementary DNA to examine the role of the receptor. In FBP-transfected NIH/3T3 (FBP-tNIH/3T3) cells, which internalised about three times more
5-methyltetrahydrofolic acid than the mock-transfected cells, the cytotoxtic potential of
DDATHF showed a clear increase. Subsequently, we analysed four ovarian
carcinoma cell lines (OVCAR3, IGROV1, SKOV3, and SW626) expressing different amounts of FBP. Cells were conditioned to grow in medium depleted of
folic acid then tested by MOv18 and
folic acid binding. Only SKOV3 and SW626 cells grown in
folic acid-depleted medium showed increased FBP expression, about 3- and 8-fold respectively. The cytotoxic potential of
DDATHF was evaluated by a standard clonogenic assay. In a medium containing 2.27 microM
folic acid the
DDATHF IC50 values were 50 nm on OVCAR3, 500 nM on SW626 and 1000 nM on IGROV1. In
folic acid-free medium IC50 values were 2 nM on OVCAR3 and Sw626 and 40 nM on IGROV1. Only on SKOV3 cells was
DDATHF cytotoxicity the same regardless of the amount of
folic acid in the medium (IC50 8 nM). Thus,
DDATHF did not inhibit the growth of IGROV1 cells depleted of
folic acid after stripping FBP with
phosphatidylinositol-
phospholipase C, even at a dose toxic for cells constitutively expressing FBP. Although FBP expression is certainly one of the parameters affecting
drug toxicity, taken alone it is not a sufficiently reliable predictor of
cancer cell sensitivity to
DDATHF.