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To investigate the hypothesis that the pharmacokinetics of imirestat, an aldose reductase inhibitor, are influenced by saturable binding to tissues, three experiments were done. (1) The nature of the dose dependence was characterized in rats. Two groups of nine adult male Sprague–Dawley rats received iv 14C-imirestat at doses of 2 or 8 mg/kg. Serial blood samples were obtained over 15 days. Volume of distribution at steady-state was significantly different between the high- and the low-dose groups (0.744 ± 0.103 1 and 1.10 ± 0.228 L, respectively). Clearance was independent of dose over this fourfold range (∼15 ml/hr). (2) The effect of either statil or AL3152, both aldose reductase inhibitors and potential competitors for aldose reductase binding, on the pharmacokinetics of a single 0.2-mg/kg iv dose of imirestat was assessed. A 2.4-mg/kg loading dose of statil was administered and a constant-rate infusion (56 µg/hr/kg) was begun 16 hr before imirestat. A 2-mg/kg loading dose of AL3152 and a constant-rate infusion (115 µg/kg/hr) were also administered 16 hr before imirestat. The infusions were maintained throughout the study. AL3152 administration decreased the imirestat steady-state volume of distribution by a mean of 63%. Statil administration decreased it by a mean of 39%. (3) The dosing regimen of the second study was repeated and, at two sampling times, nine tissues and plasma were obtained from four rats per sampling time for determination of imirestat tissue-to-plasma concentration ratio. The tissue/ plasma imirestat concentration ratio in the adrenals 24 hr after imirestat administration was 56.9 ± 20.0 in the imirestat group, 17.7 ± 1.27 in the statil-coadministered group, and 12.3 ± 2.59 in the AL3152-coadministered group. A similar trend of decrease in the ratios was observed in all tissues at both 24 and 168 hr. The results suggest that a saturable tissue binding phenomenon at least partially accounts for the nonlinear pharmacokinetics of imirestat.