Studies concerned with overcoming resistance to methotrexate: a comparison of the effects of methotrexate and 2,4-diamino-5-(3',4'-dichlorophenyl)-6-methylpyrimidine (BW50197) on the colony forming ability of L5178Y cells.

The effects of methotrexate (MTX) and 2,4-diamino-5-(3',4'-dichlorophenyl)-6-methylpyrimidine (BW50197) on the colony forming ability of L5178Y cells were compared. Two sub-lines of cells were used, one sensitive to methotrexate and the other resistant. In addition, the inhibitory effects of BW50197 against dihydrofolate reductase (DHFR) extracted from the MTX resistant cells were compared with those of MTX and pyrimethamine. It was found that although BW50197 was a less effective inhibitor of DHFR than MTX, it was superior to MTX at high concentrations in killing MTX resistant cells, and this superiority increased with the time of exposure to the drugs. These findings suggest that (a) when antifolate compounds are screened for antitumour activity it is insufficient simply to assess them on the basis of their ability to inhibit DHFR and (b) BW50197 should be given clinically so as to achieve the highest possible tissue concentration for the longest possible time consistent with the safety of the patient.