The effect of trimethoprim on cellular transport of methotrexate and its cytotoxicity to human lymphoblastoid cells in vitro.

A study has been made of the effects of the antifolate drug trimethoprim (2,4-diamino-5-(3'4',5'-trimethoxybenzyl)pyrimidine) on the cellular transport of natural folates and of methotrexate in a human lymphoblastoid cell line grown in culture. The results indicated that cellular uptake of 5-methyltetrahydrofolate, folic acid and methotrexate could be inhibited by trimethoprim; the inhibition, for methotrexate at least, was competitive, but it was in every case rather weak. Calculations showed that under conditions used during therapy with methotrexate, trimethoprim could be employed as an antibacterial agent without the cellular uptake of methotrexate being impaired by more than 1%. The cytotoxic effect of trimethoprim against the cell line used was 2000 times less, on a molar basis, than that of methotrexate. However, the degree of growth inhibition caused by the two agents in combination was greater than the sum of the individual toxicities. Again, this effect was not large at clinically useful concentrations of trimethoprim.