Rapid emergence of methotrexate resistance in cultured mouse cells.

We have examined the time required for mouse 3T6 cells to become resistant to 200 nM methotrexate by three selection protocols: (a) a single-step 0 to 200 nM dose; (b) a two-step 0 to 80 to 200 nM dose; and (c) a multistep 0 to 40 to 80 to 120 to 160 to 200 nM dose. An initial inoculum of 5 X 10(5) cells was grown to 10(6) cells at each increment of methotrexate, reduced to 5 X 10(5) cells, and again grown to 10(6) cells at the next increment. The total elapsed time required for an initial inoculum of 5 X 10(5) cells to grow to 1 X 10(6) cells resistant to 200 nM methotrexate was 45, 21, and 6.5 days, respectively, for the three drug dosage schedules. The single-step resistant variants did not contain amplified dihydrofolate reductase genes, whereas cells resistant to 200 nM methotrexate by the two stepwise selections were resistant as a result of a 6-fold amplification of the dihydrofolate reductase gene. We conclude that the resistance to 200 nM methotrexate resulting from gene amplification did not preexist in the initial population but was generated during the selection process. These results are discussed in terms of the emergence of drug resistance during the course of chemotherapy of tumors.