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

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 36 h. 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 (alpha), these are the only parameters which can be reported without qualification as to the concentration. The mean +/-DS 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 alpha, while the renal clearance of total disopyramide is dependent upon alpha.

Synthesis and anticholinergic properties of the enantiomers of 4-(isopropylamino)-2-(2-pyridyl)-2-phenylbutyramide, the mono-N-dealkylated metabolite of disopyramide

The 2R and 2S enantiomers of 4-isopropyl-2-(2-pyridyl)-2-phenylbutyramide [(2R)-2 and (2S)-2] were prepared from the respective 2R and 2S enantiomers of disopyramide [(2R)-1 and (2S)-1] by oxidation with peracid, Cope elimination, and subsequent zinc/HCl reduction of the resulting hydroxylamines (2R)-3 and (2S)-3. The enantiomers were tested as antagonists to the contraction of guinea pig ileum longitudinal muscle produced in response to electrically stimulated release of acetylcholine. The enantiomers showed IC50 values of 5.0 X 10(-6) and 14 X 10(-6) M for (2S)-2 and (2R)-2 respectively, about a 3-fold difference between enantiomers. Data are presented showing direct antagonism of acetylcholine in the guinea pig ileum assay. In a comparison of the anticholinergic effects of 2 and 1, the metabolite (2) was slightly less potent than disopyramide (1).

Evaluation of methods for producing renal dysfunction in rats

The following methods for producing renal dysfunction in rats were compared: single-step 5/6th nephrectomy, two-step 5/6th nephrectomy, bilateral ureteral ligation, and uranyl nitrate injection. Control groups consisted of single- and two-step sham-operated animals and animals that received an injection of normal saline solution. The methods were evaluated on the basis of the following criteria, which were assessed daily for 6 days: survival, body weight, hematocrit, serum creatinine concentration, serum urea nitrogen concentration, serum glutamic pyruvic transaminase activity, serum albumin concentration, and serum protein binding of salicylate (determined every other day). Animals with bilateral ureteral ligation survived only 2 days, single-step 5/6th nephrectomy caused a high incidence of fatalities. Some of the methods were associated with the development of hypoalbuminemia, but no significant elevation of transaminase activity occurred. Serum protein binding of salicylate was reduced in rats with renal dysfunction. A strong positive correlation between the creatinine and urea nitrogen concentrations in the serum of animals with renal dysfunction (r = 0.91, p less than 0.001) and a negative correlation between the serum albumin concentration and salicylate free fraction (r = -0.71, p less than 0.001) were found. Uranyl nitrate injection has the advantages of technical simplicity, a high survival rate (no deaths in this study), and relatively consistent and sustained diminution of renal function (as reflected by serum creatinine and urea nitrogen concentrations).

The stereoselective disposition of disopyramide in the dog

The disposition of d-, 1-, and d,1-disopyramide was studied in 5 conscious dogs after intravenous administration (15 mg/kg) of each compound using a balanced crossover design. The clearance of d-disopyramide (15.4 +/- 5.10 ml/min/kg) was significantly greater than that of the l-isomer (9.45 +/- 2.52 ml/min/kg) (p < 0.001). The clearance of the d,1-mixture was intermediate between that obtained for the d- and l-isomers. The steady-state volume of distribution of the three compounds was similar (approximately 1.4 liters/kg). The elimination half-life reflected differences in clearance, being 76.4 +/- 7.30 min for d-disopyramide, 112 +/- 23.4 min for 1-disopyramide, and 97.2 +/- 15.1 min for d,1-disopyramide. The effect of general anesthesia with urethane and chloralose on the disposition of the compounds was also examined. General anesthesia decreased the clearance and increased the half-life of all three compounds. No consistent differences in the volume of distribution were observed with anesthesia as compared to control. Thus, there is stereoselective elimination of the optical isomers of disopyramide in the dog, and general anesthesia decreases the clearance of d-, 1-, and d,1-disopyramide.

Propoxyphene and norpropoxyphene plasma concentrations after oral propoxyphene in cirrhotic patients with and without surgically constructed portacaval shunt

Plasma concentrations of propoxyphene and its major metabolite, norpropoxyphene, were determined over at least 12 hr after oral administration of 130 mg dextropropoxyphene hydrochloride to eight men with hepatic cirrhosis, of whom four had a surgically constructed portacaval shunt, and to seven healthy men. Propoxphene concentrations were appreciably higher and norpropoxyphene concentrations were much lower in the patients than in the normal subjects. The ratio of areas under the plasma concentration-time curve from 0 to 12 hr, norpropoxyphene: propoxyphene, was 0.70 +/- 0.46 (x +/SD) in patients and 3.94 +/ 0.83 in normal subjects. A similar decrease in this ratio was observed previously in otherwise healthy dogs after surgical construction of portacaval shunt when propoxyphene was given orally, but not after intravenous injection of the drug. A woman with portacaval shunt and essentially complete renal failure was also studied; she exhibited the highest propoxyphene peak concentration in this investigation and had no detectable norpropoxyphene in plasma. Most of the patients, unlike the normal subjects, experienced considerable sedation after propoxyphene. These results are probably due to increase systemic availability of orally administered propoxyphene in patients with hepatic cirrhosis and possibly to increased receptor response to the drug by these patients. It is concluded that propoxyphene should be administered cautiously and in reduced doses to patients with hepatic dysfunction.

Propoxyphene and norpropoxyphene plasma concentrations in the anephric patient

The widely used analgesic propoxyphene is subject to extensive presystemic (first-pass) biotransformation after oral administration. There have been indications that presystemic biotransformation of a drug may be less in anephric patients than in healthy subjects. Plasma concentrations of propoxyphene (a drug with dangerous adverse effects at high concentrations) and its major and pharmacologically active metabolite norpropoxyphene have been compared in 7 anephric patients and 7 healthy subjects after oral administration of a 130-mg dose. Maximum propoxyphene concentrations were much higher (177 +/- 16 vs 81 +/- 35 ng/ml, mean +/- SD, p less than 0.001), and areas under the concentration-time curve over 12 hr were much larger (4,310 +/- 1,520 vs 2,250 +/- 1,050 ng hr/ml, p less than 0.02) in the anephric patients than in the normal subjects. These differences were statistically significant even after normalization for dose per body weight. Norpropoxyphene concentrations were also higher and more persistent in the anephric patients. These differences, which appear to result from decreased presystemic biotransformation of propoxyphene and decreased elimintation of norpropoxyphene, indicate that propoxyphene should be used cautiously and at reduced doses in patients with renal failure.

Effect of hemodialysis on propoxyphene and norpropoxyphene concentrations in blood of anephric patients

Our purpose was to determine whole-blood hemodialysis clearances and the effect of hemodialysis on blood propoxyphene concentrations and of its major metabolite, norpropoxyphene, in anephric patients under apparent steady-state conditions with respect to propoxyphene. Propoxyphene hydrochloride 130 mg was given orally every 8 hr for 7 doses to 4 patients. Blood propoxyphene and norpropoxyphene levels were determined repeatedly during the sixth dosing interval (before hemodialysis) and during the seventh dosing interval (during hemodialysis). There were no statistically significant differences in the areas under the blood level/time curves of propoxyphene and norporopoxyphene during the sixth and seventh dosing intervals, indicating that hemodialysis contributes negligible to their total clearance from the body. The low hemodialysis clearances of propoxyphene and norpropoxyphene were confirmed by direct in vivo determination of their hemodialyzer extraction ratios. Propoxyphene produces much higher propoxyphene plasma levels and higher as well as more persistent norpropoxyphene plasma levels in anephric patients than in normal subjects. In view of their substantive cumulation during repeated propoxyphene administration, their central nervous system and cardiac toxicity at high concentrations, their low hemodialysis clearance, and the apparent sensitivity of patients with renal failure to narcotics, propoxyphene should be used cautiously in anephric patients.

Plasma protein binding of d-propoxyphene in normal subjects and anephric patients

The free fraction of d-propoxyphene in the plasma of normal subjects is 0.24 +/- 0.02 (mean +/- S.D.) and is not concentration dependent in the usual therapeutic and toxic conentration range. Renal failure and relatively high concentrations of d-norpropoxyphene, the major metabolite of d-propoxyphene, have no significant effect on the plasma protein binding of d-propoxyphene.