The protein binding of phenytoin, propranolol, diazepam, and AL01576 (an aldose reductase inhibitor) in human and rat diabetic serum

Structural and functional integrity of human serum albumin: Analytical approaches and clinical relevance in patients with liver cirrhosis

Human serum albumin (HSA) is the most abundant circulating plasma protein. Besides a significant contribution to the osmotic pressure, it is also involved in the fine regulation of many other physiological processes, including the balance of the redox state, the inflammatory and/or immunological responses, and the pharmacokinetic and pharmacodynamics of many drugs. Growing evidence suggests that HSA undergoes structural and functional damage in diseases characterized by an enhanced systemic inflammatory response and oxidative stress, as it occurs in chronic liver disease. Based on their clinical relevance, this review provides a summary of the most common post-translational modifications affecting HSA structural integrity and functions and their clinical relevance in the field of liver disease. The review also provides a critical description of the analytical approaches employed for the investigation of conformational alterations and the identification/quantitation of specific post-translational modifications affecting HSA. Finally, the analytical methods available for the assessment of two of the most clinically relevant non-oncotic properties of HSA, namely the binding capacity and the antioxidant activity, are critically reviewed. Among the available techniques particular attention is given to those proposed for the in vitro and in vivo investigation of structurally modified albumin.

Glycated Serum Albumin and AGE Receptors

In vivo modification of proteins by molecules with reactive carbonyl groups leads to intermediate and advanced glycation end products (AGE). Glucose is a significant glycation reagent due to its high physiological concentration and poorly controlled diabetics show increased albumin glycation. Increased levels of glycated and AGE-modified albumin have been linked to diabetic complications, neurodegeneration, and vascular disease. This review discusses glycated albumin formation, structural consequences of albumin glycation on drug binding, removal of circulating AGE by several scavenger receptors, as well as AGE-induced proinflammatory signaling through activation of the receptor for AGE. Analytical methods for quantitative detection of protein glycation and AGE formation are compared. Finally, the use of glycated albumin as a novel clinical marker to monitor glycemic control is discussed and compared to glycated hemoglobin (HbA1c) as long-term indicator of glycemic status.

Analysis of drug-protein interactions by high-performance affinity chromatography: interactions of sulfonylurea drugs with normal and glycated human serum albumin

High-performance affinity chromatography (HPAC) is a type of liquid chromatography that has seen growing use as a tool for the study of drug-protein interactions. This report describes how HPAC can be used to provide information on the number of binding sites, equilibrium constants, and changes in binding that can occur during drug-protein interactions. This approach will be illustrated through recent data that have been obtained by HPAC for the binding of sulfonylurea drugs and other solutes to the protein human serum albumin (HSA), and especially to forms of this protein that have been modified by non-enzymatic glycation. The theory and use of both frontal analysis and zonal elution competition studies in such work will be discussed. Various practical aspects of these experiments will be presented, as well as factors to consider in the extension of these methods to other drugs and proteins or additional types of biological interactions.

Changes in the pharmacokinetic of sildenafil citrate in rats with Streptozotocin-induced diabetic nephropathy

Aim

The present investigates deals with the change in the pharmacokinetic of Sildenafil citrate (SIL) in disease condition like diabetic nephropathy (DN).

Method

Diabetes was induced in rats by administering Streptozotocin i.e. STZ (60 mg/kg, IP) saline solution. Assessment of diabetes was done by GOD-POD method and conformation of DN was done by assessing the level of Creatinine, Blood Urea Nitrogen (BUN) and Albuminurea. After the conformation of DN single dose of drug SIL (2.5 mg/kg, p.o.) were given orally and Pharmacokinetic Parameters like [AUC o-t (ug.h/ml), AUC 0-∞, C_max, T_max, Kel, Clast] were estimated in the plasma by the help of HPLC-UV.

Result

There was significant increase (p < 0.01) in the Pharmacokinetic parameters of SIL in DN rat (AUC_0-t, AUC_0-∞, C_max, T_max and T_1/2) compare to normal control rat and significant increase Kel in the DN rat compare to control rat.

Conclusion

The study concluded that there was significant (p < 0.01) increase in the bioavailability of SIL in DN.

Review: Glycation of human serum albumin

Glycation involves the non-enzymatic addition of reducing sugars and/or their reactive degradation products to amine groups on proteins. This process is promoted by the presence of elevated blood glucose concentrations in diabetes and occurs with various proteins that include human serum albumin (HSA). This review examines work that has been conducted in the study and analysis of glycated HSA. The general structure and properties of HSA are discussed, along with the reactions that can lead to modification of this protein during glycation. The use of glycated HSA as a short-to-intermediate term marker for glycemic control in diabetes is examined, and approaches that have been utilized for measuring glycated HSA are summarized. Structural studies of glycated HSA are reviewed, as acquired for both in vivo and in vitro glycated HSA, along with data that have been obtained on the rate and thermodynamics of HSA glycation. In addition, this review considers various studies that have investigated the effects of glycation on the binding of HSA with drugs, fatty acids and other solutes and the potential clinical significance of these effects.

Effect of diabetes mellitus on pharmacokinetic and pharmacodynamic properties of drugs

The effects of diabetes mellitus on the pharmacokinetics and pharmacodynamics of drugs have been well described in experimental animal models; however, only minimal data exist for humans and the current knowledge regarding the effects of diabetes on these properties remains unclear. Nevertheless, it has been observed that the pharmacokinetics and pharmacodynamics of drugs are changed in subjects with diabetes. It has been reported that diabetes may affect the pharmacokinetics of various drugs by affecting (i) absorption, due to changes in subcutaneous adipose blood flow, muscle blood flow and gastric emptying; (ii) distribution, due to non-enzymatic glycation of albumin; (iii) biotransformation, due to regulation of enzymes/transporters involved in drug biotransformation; and (iv) excretion, due to nephropathy. Previously published data also suggest that diabetes-mediated changes in the pharmacokinetics of a particular drug cannot be translated to others. Although clinical studies exploring the effect of diabetes on pharmacodynamics are still very limited, there is evidence that disease-mediated effects are not limited only to pharmacokinetics but also alter pharmacodynamics. However, for many drugs it remains unclear whether these influences reflect diabetes-mediated changes in pharmacokinetics rather than pharmacodynamics. In addition, even though diabetes-mediated pharmacokinetics and pharmacodynamics might be anticipated, it is important to study the effect on each drug and not generalize from observed data. The available data indicate that there is a significant variability in drug response in diabetic subjects. The discrepancies between individual clinical studies as well as between ex vivo and clinical studies are probably due to (i) the restricted and focused population of subjects in clinical studies; (ii) failure to consider type, severity and duration of the disease; (iii) histopathological characteristics generally being missing; and (iv) other factors such as varying medication use, dietary protein intake, age, sex and obesity. The obesity epidemic in the developed world has also inadvertently influenced the directions of pharmacological research. This review attempts to map new information gained since Gwilt published his paper in Clinical Pharmacokinetics in 1991. Although a large body of research has been conducted and significant progress has been made, we still have to conclude that the available information regarding the effect of diabetes on pharmacokinetics and pharmacodynamics remains unclear and further clinical studies are required before we can understand the clinical significance of the effect. An understanding of diabetes-mediated changes as well as of the source of the variability should lead to the improvement of the medical management and clinical outcomes in patients with this widespread disease.

Strategies for proteomic analysis of non-enzymatically glycated proteins

Among post-translational modifications of proteins, non-enzymatic glycation is one of the less frequently studied by experts in proteomics. However, the relevance of protein glycation has been widely shown up in several pathological conditions. In fact, non-enzymatic glycation has been strongly related to hyperglycemic conditions and, thus, to chronic complications associated to diabetes mellitus and renal failure as well as degenerative changes occurring in the course of aging. Two different glycation levels are distinguished whether the structure of the protein is seriously damaged or not. The biochemical and clinical significance of both glycations have been already described. Several reasons have contributed to the lack of highly sensitive and selective methods for identification and quantitation of glycated proteins. These are mainly (1) the low concentration of glycated proteins in humans due to the low efficiency of the glycation process, (2) the modification of enzymatic digestion patterns, (3) the low ionization efficiency of glycated peptides, and (4) the lack of software including tools to identify this post-translational modification. The aim of this review is to provide the analytical guidelines required to succeed in the analysis of glycated proteins. For this purpose, different analytical approaches are considered to solve the main drawbacks derived from this gap in the proteomics field. Some challenges are finally proposed to be taken into account in future research.

Characterization of glycation adducts on human serum albumin by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

BACKGROUND

Non-enzymatic glycation of human serum albumin (HSA) is associated with the long-term complications of diabetes. We examined the structure and location of modifications on minimally-glycated HSA and considered their possible impact on the binding of drugs to this protein.

METHODS

Minimally-glycated and normal HSA (used as a control) were digested with trypsin, Glu-C or Lys-C, followed by fractionation of the resulting peptides and their analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to determine the structures and locations of glycation adducts.

RESULTS

Several specific lysine and arginine residues were identified as modification sites in minimally-glycated HSA. Residues K12, K51, K199, K205, K439 and K538 were found to be modified through the formation of fructosyl-lysine, while the modification of K159 and K286 involved the formation of pyrraline or N(epsilon)-carboxymethyl-lysine, respectively. Lysine K378 was found to give N(epsilon)-carboxyethyl-lysine in some forms of glycated HSA but fructosyl-lysine in other forms. Residues R160 and R472 produced a modification based on N(epsilon)-(5-hydro-4-imidazolon-2-yl)ornithine. Lysine R222 was modified to produce argpyrimidine, N(epsilon)-[5-(2,3,4-trihydroxybutyl)-5-hydro-4-imidazolon-2-yl]ornithine or tetrahydropyrimidine.

CONCLUSIONS

With the exception of K12, K199, K378, K439 and K525, all of the observed sites of modification for minimally-glycated HSA were new to this current study. The fact that many of these glycation-related modifications are located at or near known drug binding sites on HSA explains why some differences have been previously noted in the binding of certain drugs to normal vs glycated HSA.

Stereospecific recognition of a spirosuccinimide type aldose reductase inhibitor (AS-3201) by plasma proteins: A significant role of specific binding by serum albumin in the improved potency and stability

AS-3201 [(3R)-2'-(4-bromo-2-fluorobenzyl)spiro[pyrrolidine-3,4'(1'H)-pyrrolo[1,2-a]pyrazine]-1',2,3',5(2'H)-tetrone] is a structurally novel and stereospecifically potent aldose reductase (AKR1B; EC 1.1.1.21) inhibitor, which contains a succinimide ring that undergoes ring-opening at physiological pH levels. To delineate intermolecular interactions governing its favorable pharmacokinetic profile, the interaction of AS-3201 (R-isomer) with plasma proteins, especially human serum albumin (HSA), was examined in comparison with that of the optical antipode (S-isomer). Fluorescence, kinetic, and high-performance frontal analyses showed that the R-isomer is more strongly bound than the S-isomer to sites I and II on HSA, and the R-isomer is particularly protected from hydrolysis, suggesting that the stable HSA-R-isomer complex contributes to its prolonged activity. The thermodynamic parameters for the specific binding indicated that in addition to hydrophobic interactions, hydrogen bonds contribute significantly to the R-isomer complex formation. (13)C NMR observations of the succinimide ring (5-(13)C enriched), which are sensitive to its ionization state, suggested the presence of a hydrogen bond between the R-isomer and HSA, and (19)F NMR of the pendent benzyl ring (2-(19)F) evaluated the equilibrium exchange dynamics between the specific sites. Furthermore, fatty acid binding or glycation (both are site II-oriented perturbations) inhibited the binding to one of the specific sites and reduced the stereospecificity of HSA toward the isomers, although the clinical influence of these perturbations on the R-isomer binding ratio seemed to be minor. Thus, the difference in the interaction mode at site II might be a major cause of the stereospecificity; this is discussed on the basis of putative binding modes. The present results, together with preliminary absorption and distribution profiles, provide valuable information on the stereospecific pharmacokinetic and pharmacodynamic properties of the R-isomer relevant for the therapeutic treatment of diabetic complications.

A novel model for prediction of human drug clearance by allometric scaling

Sixty-one sets of clearance (CL) values in animal species were allometrically scaled for predicting human clearance. Unbound fractions (f(u)) of drug in plasma in rats and humans were obtained from the literature. A model was developed to predict human CL: CL=33.35 ml/min x (a/Rf(u))(0.770), where Rf(u) is the f(u) ratio between rats and humans and a is the coefficient obtained from allometric scaling. The new model was compared with simple allometric scaling and the "rule of exponents" (ROE). Results indicated that the new model provided better predictability for human values of CL than did ROE. It is especially significant that for the first time the proposed model improves the prediction of CL for drugs illustrating large vertical allometry.

Age-related alteration of carbamazepine-serum protein binding in man

To determine whether biological maturation influences the kinetics of carbamazepine-serum protein binding, the carbamazepine free fraction (%) was investigated in the serum of 66 patients, ranging from 4 to 83 years, with epilepsy or trigeminal neuralgia, treated with carbamazepine alone or carbamazepine in combination with phenytoin, phenobarbital, and/or valproic acid, over a relatively long period. Biochemical parameters such as levels of albumin and non-glycated albumin showed a significant relationship with carbamazepine free fraction (r = -0.521, P < 0.001 for albumin; r = -0.700, P < 0.001 for non-glycated albumin). Non-glycated albumin was more strongly correlated with carbamazepine free fraction. The biochemical parameters showed a significant relationship with age (r =-0.243, P < 0.1 for albumin; r =0.666, P < 0.001 for glycated albumin; r = -0.459, P < 0.001 for non-glycated albumin; r = 0.640, P < 0.001 for carbamazepine free fraction). Glycated albumin (%), non-glycated albumin and carbamazepine free fraction (%) were strongly correlated with age, whereas albumin showed only a weak correlation with age. To evaluate the effects of ageing on carbamazepine-serum protein binding, the patients were divided into three groups according to age: children, 4-15 years; adults, 16-64 years; elderly, 65-83 years. Albumin and non-glycated albumin were much lower, and glycated albumin (%) and carbamazepine free fraction (%) much higher in the elderly group than in the other two groups. The results of this study showed that the major ligand of carbamazepine in the serum was non-glycated albumin, which decreased with age. These observations suggested that in elderly patients, the elevation of free carbamazepine concentrations in the serum caused by reduced non-glycated albumin levels, induces increases in the sensitivity of the pharmacological effects of carbamazepine and the risk of drug interactions.

Effect of glycosylation on carbamazepine-serum protein binding in humans

Effect of glycosylation on carbamazepine-serum protein binding was investigated in vitro using the serum from 24 diabetics and 10 healthy subjects, and in vivo using the serum from 49 patients receiving carbamazepine. In both binding studies, nonglycosylated albumin levels were strongly correlated with the carbamazepine free fraction (%). To evaluate the effect of glycosylation in vivo, the patients were divided into two groups according to glycosylated albumin levels (%): a healthy group (10-15) and a high group (15 and over). The high group had decreased nonglycosylated albumin levels and an increased carbamazepine free fraction. Our results suggest that one should not use total concentrations for the monitoring of serum carbamazepine concentrations, but free concentrations, especially in poorly controlled diabetics.

Theophylline metabolism in healthy nonsmokers and in patients with insulin-dependent diabetes mellitus

This study examined the pharmacokinetics of theophylline and formation of its metabolites in patients with insulin-dependent diabetes mellitus (IDDM) and in sex-, age-, and weight-matched healthy nonsmokers (n = 8 per group). Subjects received a single dose of 5 mg/kg theophylline intravenously. The pharmacokinetic parameter values of theophylline (plasma clearance, elimination half-life, and volume of distribution) and the formation clearance of theophylline metabolites (3-methylxanthine, 1-methyluric acid, and 1,3-dimethyluric acid) were similar between the two groups. Plasma free fraction of theophylline was higher in the subjects with diabetes than in the healthy subjects (0.61 +/- 0.04 versus 0.56 +/- 0.02; p < 0.001). In the group of subjects with diabetes, there was a positive correlation between hemoglobin A1c values and plasma theophylline clearance (r = 0.76; p < 0.05), formation clearance of 1,3-dimethyluric acid (r = 0.78; p < 0.05), and formation clearance of 1-methyluric acid (r = 0.71; p < 0.05). These results suggest that patients with IDDM and poor glycemic control are more likely to have an increased rate of theophylline metabolism.

Protein binding of digitoxin, valproate and phenytoin in sera from diabetics

Chronic hyperglycaemia results in glycation of serum albumin and might affect the binding of drugs. The aim of the present study was to compare, using an equilibrium dialysis method, the protein binding of therapeutic concentrations digitoxin, valproate and phenytoin in sera from 70 insulin-dependent diabetics and 25 controls. Drug concentrations were measured by fluorescence immunopolarisation. Glycated albumin was measured by laser nephelometry after affinity chromatography. In sera from diabetics, protein binding of digitoxin (88.8 versus 89.9%) was unchanged; the protein binding of valproate (75.2 versus 80.7%) and phenytoin (67.9 versus 75.3%) was significantly decreased, but with no correlation with the concentration of glycated albumin. We conclude that the difference in protein binding between diabetic and control sera is due to glucose-independent modification of albumin in diabetics.

The effects of diabetes mellitus on pharmacokinetics and pharmacodynamics in humans

The article reviews the effect of diabetes on the pharmacokinetics and pharmacodynamics of drugs in humans. For most drugs which cross the gastrointestinal wall by passive diffusion, oral absorption is unlikely to be affected by diabetes, although a delay in the absorption of tolazamide and a decrease in the extent of absorption of ampicillin have been reported. Subcutaneous absorption of insulin is more rapid in diabetic patients, whereas the intramuscular absorption of several drugs is slower. The binding of a number of drugs in the blood is reduced in diabetes, which may be due to glycosylation of plasma proteins or displacement by plasma free fatty acids, the level of which is increased in diabetic patients. Plasma concentrations of albumin and alpha 1-acid glycoprotein do not appear to be changed by the disease. The distribution of drugs with little or no binding in the blood is generally not altered, although the volume of distribution of phenazone (antipyrine) is reduced by 20% in insulin-dependent diabetes mellitus (IDDM). In contrast to animal studies, the metabolic clearance of most drugs in humans appears to be unaffected or slightly reduced by the disease. The presence of fatty liver in non-insulin-dependent diabetes mellitus (NIDDM) may contribute to a reduced hepatic clearance, whereas decreased binding in the blood may cause an increase in clearance. The effect of diabetes on hepatic blood flow in humans appears to be unknown. Diabetes affects kidney function in a significant number of diabetic patients. During the first 10 years after the onset of the disease, glomerular filtration is elevated in these patients. Thus, the renal clearance of a number of antibiotics has been shown to be increased in diabetic children. As the disease progresses, renal function is impaired and glomerular function declines from the initial elevated state. In diabetic adults the renal clearance of drugs either is comparable with that found in nondiabetic individuals or is reduced. A limited number of studies have been conducted comparing the dose-response of cardiovascular drugs in diabetic patients with that in nondiabetic controls. Decreased, increased and unchanged responses have been reported. It is apparent that in some cases an altered response may be observed for a drug when administered to a diabetic patient compared with a similar nondiabetic individual. At the present time, it is not possible to ascertain whether these studies reflect true pharmacodynamic changes or merely alterations in pharmacokinetics.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclosporine pharmacokinetics and effect in the type I diabetic rat model

Recent clinical studies have demonstrated the potential benefit of the T-cell-specific immunosuppressant, cyclosporine, in the treatment of Type I insulin-dependent diabetes. In the present study, steady-state cyclosporine pharmacokinetics, fasting glucose and insulin levels and renal function were examined in stable insulin-dependent diabetic rats and compared to non-diabetic rats. Mean creatinine clearance 30 days following diabetes induction was not significantly different from saline controls. Cyclosporine treatment (5 mg/kg/day i.v. for 13 days) did not significantly alter creatinine clearance in either group; however, renal function of vehicle-treated diabetic rats was markedly reduced compared to other groups. Serum insulin concentrations were significantly greater in diabetic rats treated with cyclosporine compared to the control group (35.1 +/- 22.7 vs. 16.0 +/- 8.1 microU/ml; P less than 0.05). Glucose levels were proportionately reduced in diabetic rats treated with cyclosporine. Area under the concentration-time curve, half-life and volume of distribution of cyclosporine were significantly reduced in diabetic rats compared to non-diabetic controls. In summary, the pharmacokinetics and pharmacodynamics of cyclosporine were significantly different in the insulin-dependent diabetic rat model compared to normal controls. Furthermore, short-term cyclosporine therapy reduced the extent of experimental diabetic nephropathy observed in this model.