ici-198583 and lometrexol

A series of 7 human squamous carcinoma cell lines of the head and neck (HNSCC), grown in standard medium containing high folate concentrations and in "folate-conditioned" medium containing nanomolar concentrations of folates, were all found to be sensitive (IC50: less than or equal to 50 nM) in growth-inhibition studies to methotrexate (MTX) following drug exposure for 7 days. However, when MTX exposure was limited to 24 hr, only 2 out of 7 HNSCC cell lines were sensitive to MTX (IC50: less than 500 nM), 2 were moderately sensitive (IC50: 1-2 microM), and 3 exhibited inherent resistance to MTX (IC50: greater than 250 microM). In these last 3 cell lines, the mechanism of resistance was not correlated with altered membrane transport of MTX or changes in dihydrofolate reductase activity, but rather was associated with a 3-fold lower activity of intracellular folylpolyglutamate synthase (FPGS) activity compared to MTX-sensitive HNSCC cells. The 3 cell lines exhibiting inherent resistance to a short exposure to MTX, however, did not show inherent cross-resistance after exposure for 24 hr to one or more of 3 novel antifolate compounds. These compounds, which appear to be more efficiently transported and polyglutamylated than MTX, include: 10-ethyl-10-deazaaminopterin (10-EdAM), 2-desamino-2-methyl-N10-propargyl-5,8-dideazafolic acid (ICI-198,583), and 5,10-dideazatetrahydrofolic acid (DDATHF). These results indicate that antifolate membrane transport and intracellular FPGS activity are important factors in determining sensitivity or resistance of HNSCC cells to short-term antifolate compound exposures.

Topics: Aminopterin; Biological Transport; Carcinoma, Squamous Cell; Cell Division; Cell Line; Cell Membrane; Drug Resistance; Drug Screening Assays, Antitumor; Folic Acid; Folic Acid Antagonists; Head and Neck Neoplasms; Humans; Methotrexate; Tetrahydrofolate Dehydrogenase; Tetrahydrofolates; Tumor Cells, Cultured

5,10-Dideazatetrahydrofolate (DDATHF) is an antifolate drug, the cytotoxic effects of which can be fully reversed by hypoxanthine, suggesting that DDATHF exerts its effects by inhibiting de novo purine biosynthesis. ICI198583 is a quinazoline based inhibitor of thymidylate synthase. In this study we examine the interaction between treatment of mouse leukaemic L1210 cells with these drugs. The addition of DDATHF with ICI198583 was correlated with a decrease in ICI198583 cytotoxicity in a dose dependent manner. This protection was associated with a decrease in DNA fragmentation, and a drop in intracellular dATP pools. These results support the hypothesis that inhibitory effects on de novo purine biosynthesis by inhibitors of dihydrofolate reductase may limit cytotoxicity, and indicate that a rise in dATP pools may be an important cytotoxic signal.

Topics: Animals; Antineoplastic Agents; Cell Cycle; Cell Survival; Deoxyadenine Nucleotides; DNA, Neoplasm; Drug Antagonism; Folic Acid; Folic Acid Antagonists; Hypoxanthine; Hypoxanthines; Leukemia L1210; Mice; Tetrahydrofolates; Thymine Nucleotides; Tumor Cells, Cultured