[Methotrexate and its drug resistance].

Methotrexate (MTX), a folic acid antagonist, is among the most active known drugs for clinical cancer therapy. The mechanisms of resistance to MTX have been extensively studied, and several mechanisms have been reported so far. Those are decreased cell permeability, increased synthesis of dihydrofolate reductase (DHFR), which is the target enzyme of MTX, decreased polyglutamation of MTX, and decreased binding capacity of MTX to DHFR. Recent basic studies revealed that an increase of DHFR synthesis is the result of a proportional amplification of DHFR gene, and that polyglutamation of MTX plays an important role in its action. High-dose methotrexate (HDMTX) therapy has a good rationale for overcoming MTX resistance. However, many clinical trials of HDMTX during past decade have not demonstrated any apparent clinical efficacy except childhood ALL and ostosarcoma. Further research would be necessary for the improvement of the therapy using MTX.