Nonlinear kinetics of the thiamine cation in humans: saturation of nonrenal clearance and tubular reabsorption.

The pharmacokinetics of thiamine in plasma and urine was investigated in 13 healthy and 3 renal-insufficient volunteers. Doses ranging from 5 to 200 mg thiamine hydrochloride were administered either as an iv bolus or a 50-min infusion. A sum of 3 exponentials was used as the unit impulse response function to characterize plasma kinetics. Drug input was mathematically described as a rectangular pulse of length 2 or 50 min. Total clearance, defined as the reciprocal of the area under the unit impulse response function, was found to depend on dose and creatinine clearance, as shown by a multiple nonlinear regression analysis. The nonrenal component of the total clearance was demonstrated to be dose-dependent, whereas its mean renal component was only dependent on creatinine clearance. At high plasma concentrations renal clearance approached renal plasma flow, and remained constant during the decline to near physiological plasma levels. With further decline under a characteristic threshold concentration, renal clearance decreased far below the glomerular filtration rate, indicating tubular reabsorption. Binding to plasma proteins was excluded by ultrafiltration experiments. The process of renal excretion can be described by a combination of glomerular filtration, flow-dependent tubular secretion, and saturable tubular reabsorption. The concentration dependency of renal clearance was reflected in its mean value, which was only 76% of its maximum value measured in the higher concentration range. In the dose range studied, most of the given dose had already been linearly excreted before tubular reabsorption became evident, and consequently the measured mean renal clearances did not differ enough from one another to exhibit the expected dose dependency. With increasing dose a shift of the cleared dose fraction from the nonrenal to the renal side was observed. Saturated nonrenal clearance alone could explain this effect.