The Effect of Urinary pH and Flow Rate on the Renal Elimination of Zopolrestat and Zopolrestat Glucuronide in Humans.

Zopolrestat is an aldose reductase inhibitor that may be useful in the treatment of diabetic complications by reducing flux through the polyol pathway. The plasma half-life of zopolrestat in man is approximately 30 h, and approximately 45% of an orally administered 1000-mg dose is eliminated in the urine as unchanged drug. Because active secretion accounts for much of the renal clearance for zopolrestat, a carboxylic acid with a pK(a) of 5.46, the effect of urinary pH and flow rate on renal clearance of drug was investigated in a series of studies. Renal clearance of zopolrestat following oral administration of 200 mg was determined in normal male volunteers under basal conditions and after treatment with NH(4)Cl and NaHCO(3) to alter urinary pH. Plasma concentrations of zopolrestat were similar under basal and NaHCO(3) treatment but were approximately twofold higher under NH(4)Cl treatment. However, the half-life of zopolrestat under NH(4)Cl treatment (29.5 h) was similar to the half-life of zopolrestat in untreated subjects. Renal clearance decreased by a factor of 2.54 for each unit decrease in urinary pH. In a second study, there was no effect of urine flow rate on renal clearance following an oral dose of 400 mg. Renal elimination of zopolrestat and zopolrestat glucuronide was also examined in volunteers with normal urine flow dosed at either 600 or 1000 mg/day. Whereas renal clearance of zopolrestat decreased with decreasing urinary pH, renal elimination of zopolrestat glucuronide was not affected by pH.