The ex vivo plasma protein binding of theophylline in renal disease.

The plasma protein binding of theophylline was measured in 21 patients with renal disease and 21 healthy age and sex matched controls. The percentage of theophylline unbound to plasma was greater in patients with nephrotic syndrome and in chronic renal failure than in controls. In nephrotic syndrome the impairment of drug binding mirrored the marked degree of hypoalbuminaemia seen in this condition but in chronic renal failure the impairment of protein binding was greater than would be expected from the plasma albumin concentration changes. The percentage of theophylline free in plasma in renal disease may be increased (by as much as 50%). Such changes should be taken into account in interpreting the relationship between total plasma theophylline concentration and drug effect in renal disease.     

The plasma protein binding of metoclopramide in health and renal disease.

The plasma protein binding of metoclopramide was measured after addition of the drug (60 ng ml-1) to plasma from 18 patients with renal disease and 18 age and sex matched healthy individuals. The mean free fraction in renal disease (0.59 range 0.41-0.71) was not significantly different from controls (mean 0.6 range 0.56-0.69). In both groups the binding ratio of metoclopramide was significantly related to plasma alpha 1-acid glycoprotein (AAG) concentration but not to albumin or plasma non-esterified fatty acids concentration. Metoclopramide bound to human serum albumin (HSA) to a limited extent and to human AAG to a greater extent indicating that AAG is the major binding protein for the drug in plasma.     

Determinants of free theophylline clearance in asthma.

1. The free and total plasma and saliva concentrations of theophylline were measured during a dosing interval at steady state in nineteen asthmatic subjects receiving a once-daily theophylline preparation (Riker TCR-1). 2. Saliva theophylline clearance (mean +/- s.d. 5.8 +/- 2.1l h-1) was closely related to total plasma theophylline clearance (mean 3.6 +/- 1.2l h-1) (r = 0.958, n = 19, P less than 0.001). 3. Saliva theophylline clearance was closely related (r = 0.967, n = 19, P less than 0.001) and numerically very similar to the free plasma theophylline clearance (mean 5.8 +/- 1.9l h-1) (mean difference = 0.06 +/- 0.12 s.e. mean). 4. Free plasma theophylline clearance was significantly, although weakly, related to body weight and to the plasma free thyroxine concentration which together accounted for over 40 per cent of the variability in free clearance. 5. Theophylline administered for 2 weeks did not affect plasma free thyroxine (FT4) free tri-iodothyronine (FT3) or reverse tri-iodothyronine (rT3) concentrations compared with placebo.     

Characterization of chloroquine plasma protein binding in man.

Chloroquine protein binding was determined by equilibrium dialysis of purified plasma proteins and plasma samples from 20 healthy subjects and 14 patients with rheumatoid arthritis. The mean binding was 61 +/- 9% in plasma from healthy subjects (range 46-74%) and 64 +/- 7% in plasma from rheumatoid arthritis patients (range 55-79%). Albumin and alpha 1-acid glycoprotein at physiological concentrations bound chloroquine to an approximately equal extent. Protein binding is unlikely to be an important determinant of chloroquine pharmacokinetics or response.     

Comparison of ultrafiltration devices for assessing theophylline protein binding

The ability of two commercially available ultrafiltration devices to assess the protein binding of theophylline was compared using human serum albumin in pH 7.4 buffer at 37 degrees C. The devices compared were the Millipore-MC filter units (Catalog number UFC3LGC00) and the Amicon Centrifree Micropartition System; both rely on centrifugal force to separate the unbound and bound molecules. The results from each device were essentially identical provided that the Millipore units were centrifuged using conditions that would not allow heat buildup to occur and thereby cause a decrease in the binding. This problem could be minimized if the Millipore units were available in a low-binding 30,000 dalton cutoff membrane instead of the 10,000 dalton membrane currently available. The use of the Millipore 30,000 dalton cutoff polysulfone membrane (UFC3TTK00) reduced the centrifugation time necessary to obtain a sufficient volume of ultrafiltrate for assay, thereby minimizing the possibility of heat generation. Furthermore, the binding of theophylline to this device was low and comparable to the binding to the Centrifree devices. Because this low degree of membrane binding may not hold for other drugs, we would suggest that a Millipore unit supplied with a low-binding, 30,000 dalton cutoff membrane could provide binding data with an efficacy comparable to that achieved with the Centrifree devices.     

Variability of the serum protein binding of theophylline in patients with asthma and cystic fibrosis.

1. The serum protein binding of theophylline was studied in 28 asthmatics and 11 patients with cystic fibrosis (CF) who were receiving the drug regularly. Peak theophylline samples were collected at 2 week intervals on four occasions in each asthmatic and on three occasions in each CF patient. The binding was measured using ultrafiltration at 37 degrees C and pH 7.4. The total and free (unbound) theophylline concentrations were measured using high-performance liquid chromatography. 2. The mean free-fractions (+/- s.d.) in asthmatics (0.50 +/- 0.03) and in CF patients (0.51 +/- 0.04) were not significantly different. The intra- and inter-subject variability in the free-fraction (fu) was relatively small in both patient groups. The binding was found to be concentration-independent at serum theophylline concentrations up to 30.9 micrograms ml-1. The effects of age, gender, serum albumin and total serum protein on the free fraction were evaluated. 3. The results indicate that the binding of theophylline is similar in the two disease states. The low degree of variability in serum theophylline binding indicates that measurements of total serum theophylline concentrations will reflect unbound serum concentrations with acceptable accuracy in both patient groups studied.     

The plasma protein binding of basic drugs.

The plasma protein binding of basic drugs appears to vary more than was at first assumed and is related to the marked intra-and interindividual differences in one of the chief binding proteins, AAG. Changes in AAG concentrations will result in alterations in the distribution and metabolism of basic drugs which will complicate the interpretation of the relationship between total drug concentration and drug efficacy or toxicity. For some drugs, e.g. lignocaine, direct measurement of free concentrations may improve their clinical use but rapid and reliable techniques are as yet not readily available.     

Cefpiramide kinetics and plasma protein binding in cholestasis.

Cefpiramide is a new parenteral cephalosporin mainly excreted in the bile. Eight patients with cholestasis and 11 healthy subjects received a single 1 g i.v. dose. Cefpiramide concentrations in plasma and urine were measured by h.p.l.c. and plasma binding was determined by ultrafiltration. Total clearance of cefpiramide (mean +/- s.d.) was 15.5 +/- 7.1 ml min-1 in patients and 25.6 +/- 4.6 ml min-1 in healthy subjects. As a result, the terminal elimination half-life was longer in patients (12.0 +/- 2.9 h vs 5.3 +/- 0.9 h). Owing to impaired biliary elimination of cefpiramide in cholestasis, the urinary recovery of unchanged drug in patients was about five times greater than in healthy subjects (85.1 +/- 10.3% vs 16.2 +/- 3.9%). Plasma binding was significantly lower in cholestasis (fu = 0.23 +/- 0.13 vs 0.02 +/- 0.004 in healthy subjects). Accordingly, the dosage regimen of cefpiramide should be modified in patients with cholestasis.     

Sex-related differences in the plasma protein binding of lignocaine and diazepam.

1 The percentage of lignocaine free in the plasma of ten females receiving oral contraceptive medication was significantly greater than in 17 males of similar age (18--42 years). 2 In the same subjects the percentage of diazepam free in plasma was significantly greater in the contraceptive treated group than in 11 contraceptive-free females and significantly greater in contraceptive-free females than in males. 3 The differences in lignocaine binding were almost completely attributable to changes in alpha 1-acid glycoprotein concentration, which is reduced by oestrogens. The binding of diazepam was significantly related to albumin, alpha 1-acid glycoprotein and non-esterified fatty acid concentrations which together were related to 55% of the variation in the binding of this basic compound.     

Determinants of plasma alpha 1-acid glycoprotein (AAG) concentrations in health.

The concentration of alpha 1-acid glycoprotein (AAG) was measured in plasma from 200 healthy subjects belonging to 78 family units. The AAG concentration varied markedly between individuals (mean 0.77, range 0.36-1.46 g 1(-1]. When the genetic contribution to the variability was assessed, the only significant correlation observed was that between husband and wife and this was weak. We conclude that in addition to the known effects of age and gender, environmental (rather than genetic) factors largely determine the variance of AAG concentrations.     

Drug-protein interaction: plasma protein binding of furocoumarins.

The binding of six furocoumarins (angelicin, psoralen, 8-methoxypsoralen, 5-methoxypsoralen, 8-methylpsoralen, 4,5'8-trimethylpsoralen) to human serum and human serum albumin was studied by equilibrium dialysis using tritium labelled compounds. The results indicated that in serum all furocoumarins are bound mostly by albumin, the extent of binding being related to the structure of the furocoumarins; at any rate, high values of the bound drug, ranging from 84 to 97% were observed. The percentage of binding is strictly related to the water solubility of the compounds. A limited number of binding sites, n = 1-2.4, were detected in the albumin molecule, indicating a high specificity in the binding process. The association constants of the furocoumarins to albumin. Ka, ranged from 1.2 X 10(4) M-1 (8-methoxypsoralen) to 1.9 X 10(5) M-1 (4,5'8-trimethylpsoralen).     

Predicting plasma protein binding of drugs--revisited

Plasma protein binding of drugs has been studied for almost 100 years, but despite the accumulation of large amounts of data, the accurate prediction of this ADME parameter continues to be problematic. This review outlines recent efforts on the development of prediction tools for plasma protein binding of drugs, specifically human serum albumin, in the context of its relevance and its influencing factors. The issue of why it is difficult to achieve prediction of sufficient quality for a diverse dataset will also be considered.