The effect of saturable binding to plasma proteins on the pharmacokinetic properties of disopyramide

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Abstract
Disopyramide exhibits saturable binding to plasma proteins in the therapeutic plasma concentration range. Because of this property, controversy exists in the literature regarding the pharmacokinetic properties of the drug. The purposes of this study were to reassess the pharmacokinetic properties of disopyramide in humans, taking into consideration both total and unbound concentrations and to use disopyramide as a model compound to study the effect of drug binding on the renal clearance of both total and unbound drug. A single intravenous dose of disopyramide (1.5 mg/kg) was administered to eight normal volunteers. Blood and urine samples were collected for 36h. Total concentrations of disopyramide in plasma and urine were determined by high pressure liquid chromatography. Binding of disopyramide to plasma proteins was determined by equilibrium dialysis. In all subjects, the binding of disopyramide to plasma proteins was saturable, but there were considerable differences in binding between subjects. The volume of distribution, total body clearance, and renal clearances of both total and unbound drug were calculated. Because only the total body clearance and renal clearance of unbound compound are not dependent upon unbound fraction (α), these are the only parameters which can be reported without qualification as to the concentration. The mean ± SD total body clearance of unbound drug in the eight subjects was 5.40± 2.80 ml/min/kg. About 50% of this was due to renal elimination. A statistically significant negative correlation of the renal clearance of total disopyramide with time was observed in seven of eight subjects, whereas a significant correlation between the renal clearance of unbound disopyramide and time was observed in only one subject. This suggests that the renal clearance of unbound disopyramide is independent of α, while the renal clearance of total disopyramide is dependent upon α.