The kinetics of methotrexate polyglutamation in human breast cancer cells

The polyglutamation kinetics of methotrexate (MTX) in MCF-7 human breast cancer cells have been formulated mathematically. The model takes account of glutamation and hydrolysis kinetics up through the pentaglutamate level, increased synthesis of dihydrofolate reductase following exposure to drug, reversible tight-binding to reductase, and membrane transport of polyglutamates. The glutamation, hydrolysis, and efflux parameters have been determined from fits to experimental MTX polyglutamate uptake and efflux data. The preferred substrate for folypolyglutamyl synthase in the intact cell appears to be MTX diglutamate, on average being two to three times as reactive as either the parent drug or the triglutamate. Hydrolysis rate constants range from 0.03 to 0.19 h-1, but no clear trend with chain length is observable given the large uncertainty of each parameter estimate. However, the efflux of MTX polyglutamates from MCF-7 cells does show a trend with chain length decreasing with increasing length as expected. The best characteristic time of MTX diglutamate efflux is 4.1 h, about one-third that of the higher polyglutamate species, in agreement with observations on the MDA.MB.436 breast cancer cell line. The model shows quantitative agreement with the fraction of MTX polyglutamates found still to be bound to reductase in MCF-7 cells following 24 h of efflux, and qualitative agreement with the time dependence of bound MTX-polyglutamate concentration profiles obtained on the ZR-75 breast cancer line.