Effect of sulfadimethoxine on thiopental distribution and elimination in rats

The effect of sulfadimethoxine on the distribution and elimination of thiopental was examined by comparing the change in the steady-state volume of distribution (Vss) determined from both in vivo plasma elimination and in vitro serum and tissue binding studies in rats. The plasma disappearance of thiopental after a 12-mg/kg iv dose followed a biexponential decline in both the control and sulfadimethoxine-treated rats. The plasma thiopental concentrations under the steady-state plasma sulfadimethoxine concentration (500 micrograms/ml) were significantly lower than those of the control rats. In the sulfadimethoxine-treated rats, the pharmacokinetic parameter beta significantly decreased while Vss significantly increased to 3.6-fold that of the control rats. With sulfadimethoxine, a significant increase was observed in the apparent dissociation constant (Kd) of thiopental to serum protein by equilibrium dialysis, but the total number of binding sites was not altered. The in vitro serum free fraction of thiopental was increased to about 2.6-fold in the presence of sulfadimethoxine. The free fraction of thiopental in the main distribution tissues (liver, muscle, and adipose) was determined by equilibrium dialysis with and without sulfadimethoxine. No significant changes were observed in the presence of sulfadimethoxine. The calculated Vss, determined by the free fractions from in vitro binding experiments, also showed a significant increase. The ratio of Vss with sulfadimethoxine to that of the control rats was 2.8. The total clearance did not change, but the intrinsic clearance decreased to one-half of that of the control rats due to the increase of the serum free fraction by sulfadimethoxine. It was concluded that sulfadimethoxine caused a displacement of thiopental in plasma protein binding, which significantly increased the free fraction of thiopental, and this result may explain the significant increase of Vss and the decrease of both beta and intrinsic clearance. Tissue binding of thiopental, however, was unaffected by sulfadimethoxine.