Albumin binding and brain uptake of 6-fluoro-DL-tryptophan: competition with L-tryptophan

We investigated potential competition between L-tryptophan (TRP) and 6-fluoro-DL-tryptophan (6-F-TRP) for binding to albumin and for passage through the blood-brain barrier (BBB). In experiments based on equilibrium dialysis, albumin (600 microM) bound about 80% of TRP and 50% of 6-F-TRP with affinity constants (Ka) of 3.7 +/- 0.04 x 10(4) and 0.62 +/- 0.01 x 10(4) M-1, respectively. Competitive inhibition was assessed as the decrease in the apparent Ka (K' a) of TRP in the presence of 6-F-TRP, with no modification of the N value. Competition between TRP, 6-F-TRP and L-valine (VAL) for passage across the BBB was demonstrated using two approaches. When administered concomitantly with TRP or 6-F-TRP to rats, VAL decreased brain uptake of TRP and 6-F-TRP and reversed their action on serotonin. In Oldendorf's model, 6-F-TRP and VAL decreased the brain uptake of TRP.

Measurement of free drug in phase I: is it useful?

Early investigation of protein binding of a new drug is mandatory. The following questions have to be answered: is unbound fraction constant over tested concentrations? Which proteins are involved? What are the binding parameters? Can the drug compete with other therapeutic agents for the binding sites or in other words can drug displacements be predicted? What is the interindividual variability in protein binding? Is the binding stereoselective? All this information is necessary in predicting the pharmacokinetic behaviour of the drug and in assisting in the design of future pharmacokinetic protocols in phases II and III. The use of free drug concentration should also be considered when comparing the bioavailability of regular vs sustained release dosage forms of drugs exhibiting concentration-dependent binding and when studying concentration-effect relationships.

A re-evaluation of the HSA-piroxicam interaction

Piroxicam binding to HSA was studied using equilibrium dialysis and fluorescence methods. It was shown that this drug, like its analogs isoxicam and tenoxicam, binds to the apazone locus (site I area) and to a lesser extent to the diazepam site (site II). The piroxicam binding to HSA can be modulated by various specific ligands--apazone, warfarin, diazepam, ibuprofen--and these drug interactions have to be considered not only as potential displacement from the HSA binding sites but also in terms of induced allosteric effects.

Disease-induced variations in plasma protein levels. Implications for drug dosage regimens (Part II)

Part I of this article, which appeared in the previous issue of the Journal, discussed the implications of variations in plasma protein levels in a number of diseases: hepatic and renal disease, acute myocardial infarction, burns, cancer, diabetes mellitus, hyperlipidaemia and inflammatory diseases. In Part II the authors continue their review with a further range of disease states, and consider their import for drug dosages.

Disease-induced variations in plasma protein levels. Implications for drug dosage regimens (Part I)

Many diseases appear to lead to a decrease of drug plasma binding due either to hypoalbuminaemia or to a modification of albumin structure. In other diseases, the binding of a drug may increase due to elevated concentrations of alpha 1-acid glycoprotein or lipoproteins. However that may be, the free fraction of a drug may vary in different pathologies. But an increase or decrease of the drug free fraction does not automatically mean an increase or decrease of the free drug concentration. Whatever the drug, a variation in the volume of distribution more or less proportional to the variation in the plasma free fraction can be expected. With respect to the clearance, the problem is much more complex and depends on the hepatic extraction ratio of drug. If the extraction is related to the free fraction (fu) of drug, a variation in fu will lead to a variation in the total drug concentration but no variation in the free drug concentration and no change in the pharmacological effect. If the extraction of a drug is dependent on hepatic flow, a variation in fu will lead to a change in the free drug concentration (with no change in the total drug concentration) and hence changes in the pharmacological effect. The aim of this article is to review the literature concerning disease-induced variations in plasma protein levels during the past 10 years. Finally, possible implications for drug dosage regimens are discussed generally from examples studies in the literature.

How chronically administered valproate increases chlordiazepoxide transfer through the blood-brain barrier

Sodium valproate (VPA) is a drug widely used in the treatment of epileptics often in association with benzodiazepines. Recent animal studies have shown that the addition of valproate increases diazepam levels in the cortex and the cerebellum (Hariton et al, 1985). The aim of our study was to determine the effect of VPA on the transfer of benzodiazepines through the blood-brain barrier. They were investigated using the intracarotid injection technique in rats as described by Oldendorf (1971). Our results show that the 14C-chlordiazepoxide brain extraction is significantly higher in rats on prolonged valproate treatment than in controls. With regard to plasma protein binding effects on chlordiazepoxide transport, our data indicate that a fraction of the protein-bound chlordiazepoxide could transfer from the intracapillary space to the brain tissue space because of enhanced drug dissociation from albumin in the brain microcirculation (Kd in vitro = 74.1 microM; Kd in vivo = 793.7 microM). Two distinct mechanisms can be deduced from this study: 1) chlordiazepoxide is displaced from HSA by valproate, 2) in addition, this fatty acid could increase drug permeation through the blood brain barrier (PS/F (chlordiazepoxide) = 0.60 in controls, PS/F (chlordiazepoxide) = 0.97 in treated rats). On the contrary, the washout of the benzodiazepine from the rat brain does not seem to be modified by the addition of valproate.

Blood thiamine and thiamine phosphate concentrations in excessive drinkers with or without peripheral neuropathy

The aim of our study was to answer the following questions: (1) is thiamine deficient in chronic excessive drinkers; and (2) is peripheral neuropathy associated with thiamine deficiency or with alcohol intake itself? We performed direct assays of blood concentrations of free thiamine and thiamine phosphate in excessive drinkers with or without peripheral neuropathy and in control subjects. We found no difference in free thiamine concentrations between excessive drinkers with and without neuropathy, and no difference in free thiamine concentrations between the two groups of excessive drinkers and the control group. By contrast, a deficiency in thiamine phosphate was observed in each group of excessive drinkers compared to the control group. This was reflected in blood concentrations of total thiamine which were also lower in excessive drinkers than in controls. Finally, the thiamine phosphate: free thiamine ratio was slightly but significantly lower in the two groups of excessive drinkers than in the control group. Both groups of excessive drinkers showed typical moderate liver disease of alcoholic origin. In conclusion, the free thiamine fraction was not diminished in this group of alcoholic hospital inpatients. Thiamine deficiency would not therefore appear to play a determining role in the onset of peripheral neuropathy. In contrast, the phosphorylated fraction was slightly reduced, probably owing to the liver disease in these subjects. Contrary to studies using indirect assay techniques, our results suggest that thiamine deficiency is either slight or absent in chronic drinkers.

Onset and development of cyclosporin A effects on lymphocytes of cardiac patients before heart transplantation

The effects of cyclosporin A (CSA) in low dosage (4 mg/kg/24 h i.v.) were studied in 17 patients awaiting heart transplantation. The lymphocyte subsets were typed, enumerated, and their proliferation measured before CSA perfusion, after infusion (over 24 h), and then again after two further 24 h intervals (T0 h, T24 h, T48 h, and T72 h). No significant change was found in T or B enumerations although lymphocyte function was markedly modified. For all 17 patients, there was a significant decrease in lymphocyte proliferation that was, however, re-established after 72 h for 14 of the patients, but not for the remaining 3. Inhibition of the proliferative response was found to occur rapidly, to be potent although rapidly reversible in most cases, while yet subject to wide interindividual variability. These four features suggest that (a) CSA in fixed doses may later the balance between helper and suppressor cells in varying degrees according to the patient, and that (b) CSA given immediately or shortly after heart transplantation could be beneficial.

Blood distribution of tenoxicam in humans: a particular HSA drug interaction

Blood binding of tenoxicam was studied in vitro by equilibrium dialysis. Isolated human plasma proteins and blood cells were checked, and the distribution of the bound form was then calculated. The results showed that tenoxicam is mainly bound to HSA and that binding percentages are not different when measured in plasma (98.4%) and in an HSA solution at physiological concentration (704 microM, 98.15%). In these conditions, within the range of 1-150 microM, the tenoxicam binding percentage remained constant, evidence of a nonsaturable process. When a lower HSA concentration (10 microM) was used, the binding parameters of the tenoxicam interaction were calculated by using the same equilibrium dialysis data, by 3 methods of analysis- a stoichiometric method and site-oriented methods, fixing or not the number of HSA binding sites (n) as integer values. The best fit was observed with the first method, suggesting that two main interactions occurred. The site-oriented method gave lesser fits, the better being observed when n was not fixed. Its value, 1.77, suggest the possibility of two binding sites, one of them not preformed. The effects of known markers of site I, warfarin and apazone, of site II, diazepam and ibuprofen and of palmitic acid showed that tenoxicam is bound simultaneously to both sites I and II. The binding capacity of site I for tenoxicam is enhanced by diazepam: as this compound alone is bound to site II, this result suggests that the two HSA binding sites are not independent.

Evidence for isoxicam binding to site I as a primary site and to site II as a secondary site of human serum albumin

Isoxicam binding to HSA was studied using equilibrium dialysis and fluorescence methods. It was shown that this drug binds to or near site I (warfarin or azapropazone site) and to site II (the diazepam site) as a secondary site, although it is generally considered that their respective drug structural requirements are often exclusive. The binding parameters were calculated with different mathematical models; a site oriented model with or without fixing the number of binding sites as integer values and a stoichiometric model. The relevant results are in good agreement under the selected experimental conditions. The stoichiometric method indicates that no positive cooperativity occurred during the binding process but other interactions between the two sites cannot be excluded.

In vitro studies on the blood distribution of almitrine

Blood binding of almitrine, a highly lipophilic drug, was investigated in vitro. [3H]-Almitrine was incubated in a serum pool and isolated protein and lipoprotein fractions. The investigations were performed by using ultracentrifugation and another method which measures the uptake by proteins from glass beads coated with almitrine. Our results with ultracentrifugation show that the distribution of almitrine in serum takes place predominantly in the lipoprotein fraction (78%) and to a minor extent (22%) in the fraction of d greater than 1.20 (albumin-rich fraction). Experiments using glass beads coated with almitrine were then conducted to measure the binding of almitrine to isolated plasma proteins. The maximal uptake values (mol almitrine/mol lipoprotein) of almitrine by isolated lipoproteins decrease from VLDL (260) to LDL (20) to HDL (3) and seem to be related to the lipid content of the particles. The uptake by albumin and alpha 1-acid glycoprotein was low. The molar ratios of [almitrine]/[lipoprotein] are roughly proportional to almitrine concentrations within the therapeutic range. When almitrine was incubated in erythrocytes suspended in several dilutions of serum, almitrine partitioned less in erythrocytes as the serum protein concentration increased in the suspension.

[Prolonged anticoagulation following chlorophacinone poisoning]

In 1985 and 1986 the Swiss Toxicologic Information Center registered 152 cases of rodenticide poisoning. Among those substances chlorophacinone, an indanedione derivative, has a prolonged antivitamin K effect. We report here the case of an eighteen-year-old female hospitalized 3 days after deliberately ingesting some 100 mg chlorophacinone. Her Quick time at admission was less than 10% (Prothrombin time 79 sec., normal control 12 sec.). Under high dose vitamin K therapy the Quick was rapidly corrected but fell again on each vitamin K withdrawal. In a search for a relation between the variations of prothrombin time and chlorophacinone plasma levels, these were assessed by HPLC. Prothrombin time (and vitamin K dependent factors VII and X) finally normalized only 7 weeks after chlorophacinone ingestion. Clinical condition remained satisfactory throughout and other biological parameters unaffected. This case emphasizes the need for prolonged clinical and laboratory follow-up for rodenticide intoxications and for vitamin K administration for several weeks.

Effects of the binding of imipramine to erythrocytes and plasma proteins on its transport through the rat blood-brain barrier

Brain extraction of a tricyclic antidepressant, imipramine, was investigated using the carotid injection technique in the rat. The extent to which drug binding to plasma proteins and erythrocytes could inhibit the brain extraction was measured. Equilibrium dialysis showed that imipramine is highly bound to human serum albumin (HSA), alpha 1-acid glycoprotein (AAG), lipoproteins, and erythrocytes. The free dialyzable drug fraction was inversely related to the protein concentration. Despite this degree of binding, no significant reduction in the brain extraction of the drug was observed in the presence of HSA, lipoprotein, or erythrocytes. Only AAG reduced the brain transport of this drug in a ratio related to the protein concentration. However, the rat brain extraction was higher than expected from the in vitro measurement of the dialyzable fraction. These data indicate that the amount of circulating imipramine available for penetration in brain exceeds widely the dialyzable fraction of the drug as measured in vitro.

Binding of indapamide to serum proteins and erythrocytes

The binding of indapamide to isolated serum proteins and erythrocytes was studied in order to understand its blood distribution. In serum, indapamide was mainly bound to alpha 1-acid glycoprotein with a high affinity (K = 73.4/mM), and to albumin and lipoproteins. Indapamide was bound to erythrocytes via a saturable process with a high affinity (K = 385/mM and N = 57 microM for an hematocrit value of 0.48), and erythrocytes were the main binding component in blood (more than 80% of indapamide was associated to erythrocytes in blood). The binding to serum proteins affected indapamide distribution in blood, and alpha 1-acid glycoprotein was shown to be the more effective protein in decreasing the amount of indapamide associated to erythrocytes.

Blood binding and tissue uptake of drugs. Recent advances and perspectives

The free drug hypothesis, which states that only the unbound moiety of drug in blood is available for tissue diffusion, is discussed according to recent investigations. In some experimental conditions, it must be assumed that part of the protein-bound drug in plasma is extracted during a single passage through the organ studied. The mechanisms underlying these observations are not unequivocal and remain hypothetical. In the liver, high-affinity binding sites for serum albumin have been demonstrated, and they would explain the high extraction by liver of endogenous and exogenous compounds. However, these experiments measure the unidirectional transfer of a drug from the vascular to the extravascular space in non-steady-state conditions. Hence, in steady-state conditions, the free drug hypothesis cannot be ruled out because it is supported by numerous pharmacokinetic studies.

Pharmacological criteria for risk-benefit evaluation of NSAIDs

Evaluation of new non-steroidal anti-inflammatory drugs (NSAIDs) must compare efficacy and toxicity with existing compounds. Real progress involves maintaining effectiveness while decreasing toxicity. It is relatively easy to assess the effects of NSAIDs in animal models, and to determine gastrointestinal toxicity. However, although the ratio of active and toxic doses in animals can be extrapolated to man, the approach is limited and the NSAID needs to be assessed in a clinical setting as early as possible. In France, a national survey system has reported a wide range of adverse effects related to NSAIDs and shown important differences between compounds. Overdosage may be one of the factors responsible for toxicity, therefore pharmacokinetic evaluation is useful. In some disease states e.g. rheumatoid arthritis, there is a higher possibility of saturation pharmacokinetics with some drugs. Other pharmacokinetic parameters of interest are half-life, functions limiting activity, and hepatotoxicity. Furthermore, different pharmacokinetic parameters are required for different forms of disease. In acute states, the NSAID should have a short half-life and low protein binding and vice versa in chronic states. An important goal is to develop more selective NSAIDs regarding mechanisms of action or distribution into diseased tissues.

Critical risk/benefit analysis of pefloxacine use in children under 15 years--the problem of arthralgias

Pefloxacine belongs to a group of new quinolone antibiotics with more general indications than the urinary quinolones marketed about twenty years ago. The contraindication of the quinolones in children under 15 years of age limits their usage exclusively to adults. In this paper, the adverse arthralgic effects of these quinolones, which have largely motivated the contraindication, have been analyzed from an experimental, clinical and pathophysiological point of view. It is concluded that the pediatric benefits associated with the marked antibacterial activity of pefloxacine, particularly in pseudomonas and enterobacteriae infections, should be balanced against the risks associated with arthralgia whenever the condition of the patient is grave and decisions vital to a favorable prognosis for the sick child are necessary.

Binding of two anthranilic acid derivatives to human albumin, erythrocytes, and lipoproteins: evidence for glafenic acid high affinity binding

The binding of two anthranilic acid derivatives, glafenic and floctafenic acids, to human erythrocytes and plasma proteins has been investigated in vitro by equilibrium dialysis. Despite their close chemical structures it was shown that the binding of the two compounds to serum albumin, lipoproteins, and erythrocytes was dramatically different both in quality and quantity. Using various techniques including fluorometry and circular dichroism, it was shown that glafenic acid binds to the human serum albumin (HSA) warfarin/azapropazone site and that floctafenic acid binds to both warfarin/azapropazone and benzodiazepine sites. Glafenic acid is strongly bound to HSA with n = 1, k = 2.4 X 10(6) liters/mol and to erythrocytes with N = 12.4 mumol/liter, K = 1.7 X 10(6) liters/mol. Floctafenic acid is bound with a weaker affinity to HSA, n = 2, k = 0.3 X 10(6) liters/mol and to erythrocytes, N = 2900 mumol/liter and K = 0.007 X 10(6) liters/mol.

In vitro studies on the distribution of probucol among human plasma lipoproteins

The role of human plasma lipoproteins as carriers in the blood transport of the cholesterol-lowering and water-insoluble drug, probucol, was investigated in in vitro studies. [14C]Probucol was incubated in whole human blood, a serum pool, individual diluted sera, and isolated protein and lipoprotein fractions. In whole blood, about 90% partitioned in plasma. Following ultracentrifugal fractionation of the serum, it was found that less than 5% distributed in the d greater than 1.20 protein fraction (albumin-rich fraction) and more than 95% in the lipoprotein fractions. The distribution of probucol in the lipoprotein fractions correlated with the lipoprotein total lipid volume under saturation conditions (incubation of isolated lipoprotein fractions) as well as nonsaturation conditions (fractionation of serum exposed to [14C]probucol). Incubation of the albumin-rich fraction and of apolipoproteins originating from the isolated lipoprotein fractions showed that they account for a negligible part in the interaction of probucol with blood components. The probucol uptake of individual sera was shown to be correlated to the lipid content of the serum. When probucol was incubated in erythrocyte suspensions containing variable amounts of lipoproteins, probucol partitioned less in erythrocytes as the lipoprotein concentration increased in the suspension.