Concentration and pH dependent steady-state volume of distribution of methotrexate estimated by a simple physiologically based method.

The effects of plasma concentration and pH on the steady-state volume of distribution, Vss, of methotrexate (MTX) were studied in five conditioned male beagle-mongrel dogs. Steady-state plasma MTX concentrations of approximately 1, 20, and 100 micrograms/ml were targeted for by i.v. bolus doses followed by i.v. infusions. An isotonic solution of sodium bicarbonate or ammonium chloride was simultaneously infused for the purpose of inducing plasma pH change, while the infusion of an isotonic solution of sodium chloride served as a control. Plasma and urine concentrations of MTX were quantitated by a sensitive high-performance liquid chromatographic method, and the Vss of MTX was estimated by a recently reported physiologically based method of Chiou and Lam. Statistically significant (p less than 0.05) concentration and plasma pH dependent Vss of MTX were observed. Concentration dependence of Vss was noted in sodium chloride and ammonium chloride infused dogs, but not in bicarbonate treated dogs. There was an average 50.0 and 44.8% increase in Vss at 1 microgram/ml relative to the two higher concentrations (20 and 100 micrograms/ml) for dogs treated with ammonium and sodium chloride, respectively. However, Vss of MTX at the targeted concentrations of 20 and 100 micrograms/ml was relatively constant. Plasma pH dependence of Vss was observed only at the plasma concentration of 1 microgram/ml, and on the average, ammonium chloride and sodium chloride treatments resulted in 50.0 and 31.3% higher Vss, respectively, when compared with the bicarbonate treatment. These phenomena appear to be adequately explained by the reported tissue uptake kinetics of MTX.