Physiologically based pharmacokinetic model for the renal clearance of phenolsulfonphthalein and the interaction with probenecid and salicyluric acid in the dog.

Plasma kinetics and renal excretion of intravenous phenolsulfonphthalein (PSP, 1.0 g), with and without concomitant administration of probenecid or salicyluric acid (SUA), were studied in the Beagle dog. Pharmacokinetic analysis revealed that tubular secretion is the predominant route of excretion, and that secretion is inhibited by probenecid and SUA. A physiologically based kidney model was developed that incorporates the functional characteristics of the kidney that determine the excretion of PSP, i.e., renal plasma flow, urine flow, nonlinear protein binding, glomerular filtration, tubular secretion, and tubular accumulation. The model enabled an accurate description and analysis of the measured plasma levels and renal excretion rates. The interaction with probenecid and SUA could be adequately described with the model by inhibition of the carrier-mediated uptake of PSP into the proximal tubular cells. However, both compounds clearly differed in their inhibitory action. Whereas probenecid showed simple competitive inhibition, for SUA a considerably more complex interaction (two-site competitive system) had to be taken into consideration. Especially in the interaction experiments, only satisfactory fits to the model were obtained when secretion was assumed to be dependent on unbound PSP concentrations. Model calculations showed that in the control experiments tubular secretion was accompanied by a pronounced accumulation of PSP within the proximal tubular cells, which was clearly diminished in presence of probenecid or SUA. The predicted accumulation ratios were in good agreement with previous studies.