Effect of probenecid on the renal and nonrenal clearances of zidovudine and its distribution into cerebrospinal fluid in the rabbit

The effect of probenecid on the renal excretion of zidovudine (3′-azido-3′-deoxythymidine; AZT) and its distribution into CSF was studied in the rabbit. Although probenecid is chemically unrelated to AZT, it has been shown that probenecid inhibits AZT elimination in Acquired Immunodeficiency Syndrome (AIDS) patients. The effect of probenecid on the renal clearance of AZT after an iv bolus dose was studied in crossover experiments in the absence (control) and the presence of a continuous iv infusion of probenecid. Probenecid coadministration increased the AZT plasma AUC by 70%, by proportionally decreasing the total body clearance. The renal clearance decreased by 50%. The effect of probenecid on the renal clearance of AZT at steady state was studied by measuring the renal clearance of AZT at different steady-state plasma probenecid concentrations. The renal clearance of AZT decreased with increasing probenecid concentration, suggesting competitive inhibition of the secretion of AZT in the renal tubule. The relationship between AZT renal clearance and probenecid plasma concentrations, during and after probenecid iv infusion in conscious and in anesthetized uretercannulated rabbits, showed hysteresis, indicating that probenecid plasma concentration is different from the concentration at the site of interaction. This suggests the presence of an effect compartment for the inhibition of AZT renal excretion by probenecid. The effect of probenecid on the CSF distribution of AZT was also studied in the rabbit. Probenecid coadministration caused a sevenfold increase in the AZT AUCCSF in probenecid-treated rabbits when compared with controls. The AUCCSF:AUCplasma ratio increased from 0.152 ± 0.023 in the control rabbits to 0.568 ± 0.222 in the probenecid-treated rabbits. The CSF-to-plasma AZT concentration ratio during the postdistributive phase increased from 0.24 ± 0.05 in controls to 0.65 ± 0.12 in the probenecid-treated rabbits.