Probenecid inhibition of methotrexate cytotoxicity in mouse L1210 leukemia cells

The effect of probenecid on methotrexate cytotoxicity has been measured using mouse L1210 leukemia cells in vitro. Cytotoxic effects were measured using a soft agar cloning technique. Cell exposure to varying concentrations of methotrexate at fixed concentrations of probenecid resulted in increased cell survival when compared with exposure to methotrexate alone. Cell exposure to a fixed concentration of methotrexate and varying concentrations of probenecid showed this effect to be dependent on the concentration of probenecid. Intracellular methotrexate concentrations were unchanged by the presence of probenecid. However, there was an effect of probenecid on progression of cells through the cell cycle which would tend to decrease the number of cells susceptible to the cytotoxic effects of methotrexate.