Mechanisms of acquired resistance to methotrexate in a human squamous carcinoma cell line of the head and neck, exposed to different treatment schedules

Mechanisms of acquired resistance to methotrexate (MTX) were evaluated in HNSCC-11B cells which were made resistant to methotrexate either by continuous (11B-MTX-C) or by pulse exposure (11B-MTX-P) to the drug. 11B-MTX-C cells were 91-fold resistant to methotrexate and 30-fold or 49-fold crossresistant to trimetrexate and 10-EdAM, respectively. Dihydrofolate reductase (DHFR) activity was increased 63-fold in 11B-MTX-C cells together with a decrease in [3H]-methotrexate transport and folylpolyglutamate synthase (FPGS) activity (2.5-fold and 3.8-fold, respectively). Against two novel antifolates targetting enzymes other than DHFR, minor crossresistance was observed for ICI-198, 583, but full sensitivity was retained for DDATHF. 11B-MTX-P cells were 46-fold resistant to methotrexate and 47-fold crossresistant to ICI-198,583 in short-term drug exposure, but showed only minor changes in methotrexate sensitivity following prolonged drug exposure. The resistant phenotype in 11B-MTX-P cells were characterised by a 5.6-fold decrease in FPGS activity. These results suggest that different mechanisms of methotrexate resistance in HNSCC cells in vitro can be obtained dependent on the schedule of exposure to the drug.