Abstract

The effect of folinic acid (CF) on the tissue residence and renal excretion of methotrexate (MTX) in the rat model is described in an experiment designed to resemble a clinical regimen of high-dose MTX therapy with CF rescue. Concentrations of MTX in the kidney, liver, and plasma were determined following the sequential killing of the animals up to 24 hr after an iv bolus injection of 100 mumol/kg MTX. The treated group received repeated iv doses of CF (5.86 mumol/kg) for 9.3 hr. The tissue retention property of MTX was characterized by noncompartmental parameters, the mean residence time (MRT), and the single-pass mean residence time (MRTSP) in organs. The values of MRT in the kidney and liver were greater than that in plasma and were shortened by the CF treatment, indicating that CF is effective in reducing the duration of exposure to MTX in those organs. For drugs whose duration of exposure is a key factor in their toxicity, MRTSP was shown to be a useful parameter in toxicokinetic study. The renal clearance of MTX was virtually constant over a wide range of plasma MTX levels (0.1-165 microM) and was inhibited by p-aminohippurate. CF accelerated the renal excretion of MTX, which effect depended on the plasma concentration of MTX. Since reabsorption and filtration were not affected by CF, the increased excretion of MTX was attributed to a stimulation of tubular secretion. These observations show that CF exerts a beneficial effect in accelerating the excretion of MTX residing in the organs, in addition to supplying active folate to aid the resumption of de novo DNA synthesis.