Relative biochemical aspects of low and high doses of methotrexate in mice.

During low infusion rates of methotrexate (1.0 microng/hr/mouse; plateau plasma concentration, 2 X 10(-8) M), [3H]deoxyuridine incorporation into DNA was inhibited to a significant degree in small intestine and femur marrows. However, incorporation of [3H]thymidine into intestinal DNA was stimulated at this low infusion rate. During high infusion rates of methotrexate (10 microng/hr/mouse, plateau plasma concentration, 4 X 10(-7) M), inhibition of the incorporation of [3H]deoxyuridine at the steady state levels of plasma methotrexate in both the small intestine and femur marrow was significant. In contrast to stimulation at the low infusion rate, incorporation of [3H]thymidine into intestinal DNA at this high infusion rate was inhibited to a significant degree. Inhibition was not statistically significant in femur marrow DNA. The inhibition of [3H]thymidine into intestinal DNA could be reversed by the simultaneous infusion of inosine. Thus, in the in vivo system, an antipurine effect on DNA Synthesis at high methotrexate plasma concentration in the small intestine was observed. This antipurine effect was not apparent at the lower concentrations. The lower concentration, however, could still inhibit [3H]deoxyuridine incorporation into intestinal and femur marrow DNA to a significant enough degree that, if prolonged, it would resultin lethality to the mice. The thymineless state can be maintained for at most 60 hr in mice without lethal toxicity, whereas the antipurine state can be maintained for no longer than 18 hr in mice without some lethal toxicity. These data have important implications in rescue studies using thymidine or leucovorin.