RESUMO
Although 5-fluorouracil (5-FU) is one of the most effective single agents in treating solid tumors, its low effectiveness as a single agent has led to development of a number of modulators intended to enhance its therapeutic effectiveness. Of these, methotrexate (MTX) and trimetrexate (TMTX) have been shown to have synergistic anticancer activity with 5-FU. The effect of these two drugs on the uptake and the intratumoral metabolism of 5-FU was studied in two rat tumor models using 19F-nuclear magnetic resonance spectroscopy: on excised samples of Walker 256 carcinosarcoma and noninvasively (in vivo) in Novikoff hepatoma. In the rats bearing the Walker 256 tumor, a 4-hr pretreatment with MTX showed the maximal increase in the rate of conversion from 5-FU to its fluorinated nucleotides/nucleosides. In the rats bearing the Novikoff hepatoma, both modulators increased the amounts of cyctotoxic anabolites of 5-FU, but at the doses administered, the cumulative amounts of 5-FU anabolites formed after MTX were significantly higher than those formed after TMTX or after saline control. On the other hand, the increase in the levels of the fluorinated nucleotides/nucleosides after TMTX peaked at a later time. The possible significance of these findings is that timing of administration of a modulator is important because it affects both transport and metabolism of 5-FU. The two modulators studied, both antifolates, act differently on transport and on metabolism: MTX affects both, whereas TMTX, at the level studied, appears to affect predominantly the metabolic process. In addition, significant differences exist between tumor models. These data suggest possible mechanisms and processes that should be studied further in humans, using these noninvasive pharmacokinetic imaging methods for monitoring 5-FU targeting and metabolism.
