Maximum tolerated doses of methotrexate and 7-hydroxy-methotrexate in a model of acute toxicity in rats

Purpose: After more than 50 years of methotrexate (MTX) treatment of acute lymphoblastic leukaemia (ALL), it is currently believed that as long as dos e escalations are followed by adequate leucovorin rescue guided by monitori ng MTX serum concentrations, hydration and urinary alkalinization, high-dos e MTX (HD-MTX) can be tolerated without life-threatening toxicity. However, our recent experimental animal studies of the major metabolite of MTX, 7-O H-MTX, indicate that this concept may have some limitations. Animals with l evels of 7-OH-MTX of 1 mM, which is below the levels routinely found in pat ients on HD-MTX, demonstrate intolerable toxicity and some animals die with in 8 h. Electron microscopy indicates that endothelial cell and platelet fu nctions are perturbed. Since animal data are lacking, and interspecies diff erences not known, we wanted to investigate the maximum tolerated doses of MTX and 7-OH-MTX in a rat model of short-term effects. The maximum tolerate d dose was chosen instead of LD50 for reasons of animal welfare. Methods: W e infused MTX and 7-OH-MTX into anaesthetized male Wistar rats and monitore d the animals for 8 h. The drugs were given as a bolus plus continuous infu sion. The dose-finding ranges were 1.8-11.3 g/kg MTX and 0.1-1.2 g/kg 7-OH- MTX. Results: The maximum tolerated dose was between 3 and 5 g/kg for MTX a nd lower than 0.1 g/kg for 7-OH-MTX. The mean serum concentrations of MTX a nd 7-OH-MTX in animals that did not survive the 8-h period were 21.9 and 1. 6 mM, respectively. The animals that received the highest MTX or 7-OH-MTX d oses and concentrations died after sudden reductions in heart rate and bloo d pressure. Conclusions: We demonstrated a lower maximum tolerated dose of 7-OH-MTX than of MTX in rats after 8 h. The 7-OH-MTX concentrations were in the therapeutic range after HD-MTX. If the rat/human interspecies differen ces are not large, our data may indicate that HD-MTX regimens should not be further dose intensified, due not so much to the effects of MTX as to thos e of 7-OH-MTX.