Toxicity of folic acid analogs in cultured human cells: a microtiter assay for the analysis of drug competition.

We have used a microtiter assay to study the toxicity of various folate analogs in a series of cultured human cell lines that exhibit different degrees of resistance to methotrexate, an inhibitor of dihydrofolate reductase. These cells retain their sensitivity to the lipophilic antifolate BW301U despite the amplification of dihydrofolate reductase genes. Because the cell lines under investigation grow very slowly and have poor plating efficiencies in unconditioned medium, an assay was developed that relies on cell proliferation rather than colony formation as a measure of toxicity. This approach is easily generalized to provide a rapid and inexpensive assay of drug competition. Two-dimensional studies indicate that methotrexate and BW301U show differences in patterns of toxicity, competition, and rescue by folinic acid, suggesting that the two drugs act on different targets. Further applications of the microtiter assay to the analysis of multidrug interactions are discussed.