Determination of unbound fraction of disopyramide in plasma: a comparison of equilibrium dialysis, ultrafiltration through dialysis membranes and ultrafree anticonvulsant drug filters

Determination of drug protein-binding by high-performance liquid chromatography using a chemically bonded bovine albumin stationary phase

A liquid chromatographic method for the determination of the degree of protein-binding of drugs has been established, using a stationary phase to which bovine serum albumin has been bonded chemically. In a structurally heterogeneous group of compounds, results of the method correlate well with protein-binding data obtained by equilibrium dialysis (r = 0.89, n = 23, p less than 0.001). Within a series of analogous piperazines a good correlation is found (r = 0.981, n = 11, p less than 0.001). The chromatographic method allows automation of the measurement of protein-binding of large series of compounds with protein-binding ranging between 10 and 99%. The method is not expensive and is less time consuming than equilibrium dialysis. Only 1 mg of technical-grade material is required to determine the protein-binding, and radioactive labelling of the material is not necessary.

The pharmacokinetics and protein binding of disopyramide in pigs

The pharmacokinetics of disopyramide were investigated in 5 pigs. Disopyramide was administered as intravenous bolus injections at different doses and as intravenous infusions at different rates. By measuring the free and total concentrations in plasma of parent compound and desisopropyldisopyramide (the main metabolite in humans) and the amounts excreted in urine it was found that disopyramide had a concentration dependent protein binding and that free and total concentrations could be described by a two-compartment model with a t1/2, beta of approximately 2 hours. The free concentrations were found to be more valuable for estimating the kinetic parameters. The total clearance of disopyramide in the pig was found to approximately 3 ml/min./kg which is about 3 times greater than in man. In contrast to humans the free clearance in the pig increased with declining concentration. Apart from this the kinetics of disopyramide in the pig were very similar to those in man. In conclusion the pig could be a relevant model for studying the pharmacokinetics in humans.

The influence of age and smoking on the elimination of disopyramide

The influences of smoking and age on the elimination kinetics of disopyramide were studied in 27 subjects. Total elimination clearance of disopyramide was measured after an infusion to steady state. The total elimination clearance was significantly (P less than 0.05) decreased in elderly non-smoking patients compared with young non-smoking subjects (1.54 +/- 0.33 vs 2.12 +/- 0.67 ml kg-1 min-1) (mean +/- s.d.). Smoking more than 20 cigarettes per day significantly (P less than 0.05) increased total elimination clearance in elderly (2.02 +/- 0.35 vs 1.54 +/- 0.33 ml kg-1 min-1), while no significant induction by tobacco was observed in young healthy persons. Serum concentrations of alpha 1-acid glycoprotein, the major binding protein of disopyramide, were significantly higher (P less than 0.001) in the elderly patients. However, the volume of distribution (V) was significantly (P less than 0.001) greater in the elderly patients (2.44 +/- 0.64 vs 1.16 +/- 0.15 1 kg-1). Steady-state serum concentrations of the free drug were significantly (P less than 0.01) lower in the young volunteers (0.75 +/- 0.13 micrograms ml-1) than in the elderly (0.90 +/- 0.10 micrograms ml-1). The half-life of disopyramide was significantly shorter (P less than 0.01) in the young volunteers than in the elderly patients. No difference was observed in the relationship between the serum concentration of disopyramide and its main dealkylated metabolite in the groups studied. The results indicate that it might be advisable to reduce the dosage of disopyramide by approximately 30% in elderly non-smokers compared with young subjects.

Drug level monitoring: cardiovascular drugs

Methods for the determination of cardiovascular drugs in blood and plasma are critically reviewed with emphasis on gas and liquid chromatographic techniques. The importance of the various procedures is discussed, in particular sample work-up where the conditions for isolation and derivatization of the compounds are decisive for the accuracy and precision of the methods. Compared with other assay techniques chromatographic methods are generally to be preferred owing to their better selectivity. In the review the following groups are discussed: digitalis glycosides, antiarrhythmic agents, beta-adrenoceptor antagonists, vasodilating agents, antihypertensive compounds, and diuretics.

Protein binding of propisomide

This paper describes the protein binding of propisomide to human serum and isolated proteins using equilibrium dialysis. The drug is exclusively bound to alpha1-acid glycoprotein with high affinity. The binding is saturable even at low concentrations of the drug. Thus, the fraction unbound varied from 0.05 to 0.60 with decreasing serum concentration. The major metabolite of the drug or other drugs with affinity for alpha1-acid glycoprotein can displace propisomide from its binding site only when present in serum at high levels. Two ultrafiltration techniques are compared to equilibrium dialysis for the determination of serum protein binding of propisomide. Ultrafiltration does not give reliable results. Equilibrium dialysis is retained as an accurate method for the determination of the fraction unbound of propisomide.

Pharmacokinetic-pharmacodynamic analysis of unbound disopyramide directly measured in serial plasma samples in man

The pharmacokinetics and pharmacodynamics of the antiarrhythmic drug, disopyramide, were investigated in 12 volunteers who took 300 mg doses of 3 different capsule preparations and an aqueous oral solution of the drug at 1-week intervals. Concentrations of drug unbound to plasma proteins were measured by a sensitive immunoenzyme assay after ultrafiltration of plasma samples taken serially after dosing. QT interval was measured on serial ECG recordings with correction for changes in heart rate. Unbound concentrations of disopyramide were modelled by an open one-compartment pharmacokinetic model with a zero-order absorption rate and a lag time. There was no significant difference in parameter estimates between the four preparations, except for the lag time, which was significantly shorter for the solution preparation. The saturable protein binding of disopyramide was described by a hyperbolic model including a specific binding site and additional nonspecific binding. The pharmacodynamic relationship between unbound drug concentration and QT prolongation was fit by a simple linear model. This fit was better using unbound concentration of the drug than using total concentrations.

Binding of disopyramide to alpha 1-acid glycoprotein in plasma measured by competitive equilibrium dialysis

A modification of equilibrium dialysis in which alpha 1-acid glycoprotein and plasma compete directly for disopyramide has been used in conjunction with binding curves to measure the extent of the alpha 1-acid glycoprotein-disopyramide interaction. At concentrations in the therapeutic range, 80-90% of disopyramide was bound to alpha 1-acid glycoprotein for plasma from each of six healthy adults. Also, equilibrium dialysis data are presented, indicating that pH does not influence the binding of disopyramide within the therapeutic range.