Binding of Sulfonamides to Carbonic Anhydrase: Influence on Distribution Within Blood and on Pharmacokinetics

Four new aromatic sulfonamides were synthesized and purified by standard techniques. Two were unsubstituted, primary sulfonamides and two possessed substituents on the sulfonamide nitrogen. The affinity of the inhibitors for the enzyme carbonic anhydrase was determined in terms of the inhibitory potency, which was found to be dependent on the presence of an unsubstituted sulfonamide group. Binding studies were performed in erythrocyte suspensions using a range of concentrations and the unbound, extracellular concentrations were determined by high-performance liquid chromatographic (HPLC) assay. The dissociation constant of binding and the total binding capacity of the erythrocytes were estimated by nonlinear regression using a two-site binding model. The affinity of the compounds for erythrocytes reflected their inhibitory potency against the enzyme. Binding to plasma proteins was more dependent on lipophilicity and pK _a and was stronger for the substituted sulfonamides. Pharmacokinetic studies in rats showed that the unsubstituted sulfonamides with a high affinity for carbonic anhydrase in erythrocytes have longer half-lives and lower clearance values than the substituted sulfonamides which were more strongly bound to plasma proteins. However, comparison of unbound clearance values showed that the variations in molecular structure, which produced differences in carbonic anhydrase binding and in distribution, also produced variations in susceptibility to elimination processes.     

Binding of amobarbital,pentobarbital and diphenylhydantoin to blood cells and plasma proteins in healthy volunteers and uraemic patients

Summary By equilibrium dialysis of human plasma it has been shown that the binding of pentobarbital and diphenylhydantoin to plasma proteins is decreased in uraemic patients (46 and 74 per cent bound, respectively) compared to healthy volunteers (61 and 88 per cent bound). The degree of binding of pentobarbital was significantly correlated with that of diphenylhydantoin and amobarbital, which suggests similarity of their binding sites. Appreciable proportions of the drugs were found in blood cells both in healthy and uraemic subjects. As expected, the distribution of drugs in whole blood was different in the uraemics from healthy subjects, because of the decreased plasma protein binding and the lowered red cell count in uraemia. Analysis of the data showed that the ratio between the concentrations in blood cells and plasma water in uraemic patients was not significantly different from that in healthy subjects.     

An automated flow injection-serial dynamic dialysis technique for drug-protein binding studies

The interface of an automated flow injection analyzer with a dialysis unit to study drug-protein interactions using flow injection serial dynamic dialysis (FISDD) procedure is described. The method is based on the study of the kinetics of dialysis of the ligand in the absence and presence of protein. A study of the binding of sulfamethoxazole. sulfamethizole and sulfisoxazole to bovine serum albumin by means of such an automated system was undertaken to investigate the utility of FISDD technique for protein binding studies. The determination of dialysable sulfonamides was performed automatically by the FIA analyser. The influence of ionic strength and viscosity on the rate of dialysis was investigated. It was found that both variables did not affect the kinetic profile. Binding by the cellophane membrane was not encountered as a problem with the compounds studied. Binding parameters estimated for sulfamethoxazole were found to agree well with those reported in the literature. The Scatchard plots for the binding of sulfamethizole and sulfisoxazole with bovine serum albumin, revealed two classes of binding sites for each sulfonamide. The system was also used for the calculation of the dialytic rate constants. Experimental variables can be readily controlled to yield favoured conditions to study the protein binding phenomenon.     

Displacement of albumin-bound antimicrobial agents by bilirubin.

The ability of bilirubin to displace antimicrobial agents from their binding sites on albumin was studied in vitro by equilibrium dialysis. Sulfonamides, tetracyclines, penicillins and cephalosporins of different degrees of protein binding were tested. It was found that bilirubin reduced the protein binding of all four classes of antimicrobial agents, although by varying degrees. This effect was most pronounced with the compounds which had the highest degree of protein binding, such as cloxacillin, cephazolin, methacycline and sulfisoxazole, all of which are bound greater than 80% by albumin. On the other hand, the drugs with less than 25% binding, such as ampicillin, cephalexine and tetracycline were not ostensibly displaced by bilirubin. Scatchard plots of the binding of sulfamethoxazole to albumin in the presence of 400 mumol/1 of bilirubin, showed that bilirubin almost completely displaced the sulfonamide molecule from the high affinity site on the albumin molecule. Reduced protein binding of drugs in hyperbilirubinemic infants may have pharmacokinetic significance.     

Characterization of salicylate binding to synovial fluid and plasma protein in patients with rheumatoid arthritis

Summary Protein binding of salicylate in synovial fluid and plasma from patients with rheumatoid arthritis was studied by equilibrium dialysis. Protein binding in the synovial fluid was considerably lower at all salicylate concentrations studied (0.07 – 2.2 mM). Scatchard plots of the data were analyzed assuming binding to two classes of binding sites, each plasma sample being diluted to an albumin concentration equal to that in synovial fluid from the same patient. Binding to the primary binding sites was considerably decreased in synovial fluid in comparison with plasma. The affinity of the secondary binding sites was slightly lower. Thus, at a low therapeutic drug concentration, the decreased binding of salicylate to synovial fluid protein in patients with rheumatoid arthritis could mainly be accounted for by decreasing affinity of binding to the primary binding sites.     

The Binding of Aripiprazole to Plasma Proteins in Chronic Renal Failure Patients

The binding of drugs to plasma protein is frequently altered in certain types of renal diseases. We recently reported on the effects of oxidation and uremic toxins on the binding of aripiprazole (ARP) to human serum albumin. In our continuing investigations, we examined the binding of ARP to plasma pooled from patients with chronic renal dysfunction. We examined the issue of the molecular basis for which factors affect the changes in drug binding that accompany renal failure. The study was based on the statistical relationships between ARP albumin binding and biochemical parameters such as the concentrations of oxidized albumin and uremic toxins. The binding of ARP to plasma from chronic renal patients was significantly lower than healthy volunteers. A rational relationship between the ARP binding rate and the concentration of toxins, including indoxyl sulphate (IS) and p-cresyl sulphate (PCS), was found, particularly for IS. Moreover, multiple regression analyses that involved taking other parameters such as PCS or oxidized albumin ratio to IS into account supports the above hypothesis. In conclusion, the limited data reported in this present study indicates that monitoring IS in the blood is a very important determinant in the dosage plan for the administration of site II drugs such as ARP, if the efficacy of the drug in renal disease is to be considered.     

The Distribution of Chlorpromazine between Plasma and Erythrocytes

Abstract: Chlorpromazine is found to be taken up by the erythrocytes in a dissociable binding similar to that of the plasma protein binding so that an equilibrium is attained between these binding sites. The plasma/cell ratio is on an average 2 but is subject to large interindividual variations. This ratio is only moderately influenced by the presence of other drugs and anticoagulants in the blood. The consequences of the cell binding of drugs for the analytical result when using plasma or whole blood and the evaluation of blood concentrations of their relation to the therapeutic effect are discussed.     

Protein Binding of Doxepin and Desmethyldoxepin

Abstract: The protein binding of doxepin (DOX) and desmethyldoxepin (DDOX) were studied in serum and plasma samples from healthy volunteers and psychiatric patients. Binding was measured by equilibrium dialysis (16 hrs at 37°) and drug concentrations by radioimmunoassay. In addition, albumin and α₁-acid glycoprotein concentrations of the samples were measured by radial immunodiffusion. The mean ± SEM percentages of unbound DOX were: 20.4 ± 1.2 and 15.9 ± 1.2 in healthy subjects (n = 16) and patients (n= 15) respectively. and those of DDOX: 21.4 ± 0.9 and 19.0 ± 1.4 for healthy subjects and patients, respectively. There was a significant negative correlation between serum α₁-acid glycoprotein concentration and free fraction of DOX in both groups. In healthy subjects a significant negative correlation was also found between albumin concentrations and free fraction of both DOX and DDOX. Binding experiments with isolated protein fractions revealed that all of the total binding in plasma could be explained by binding to albumin and α₁-acid glycoprotein. The observed 2–4-fold interindividual variability in the free fractions of these drugs is probably less important than the much larger variability in the total serum concentrations.     

Effects of glycosylation of hypoglycaemic drug binding to serum albumin

The binding properties of hypoglycaemic drugs to glycosylated human serum albumin (G-HSA) were investigated using a fluorescence quenching method. Displacement patterns between tolbutamide and Sudlow's-site-specific drugs to G-HSA were also investigated. The order of the binding affinities of these drugs to HSA was glibenclamide>acetohexamide>tolbutamide≥glicrazide>metfolmin. The order of the binding affinities were the same for G-HSA as for HSA. The ability of G-HSA to bind hypoglycaemic drugs, however, was much lower than that of HSA. Scatchard plots for the binding of tolbutamide to both albumins were biphasic. The glycosylation affected saturable binding sites (I and II), whereas it did not influence non-saturable binding sites. The displacement patterns of tolbutamide binding between both albumins were not affected in the presence of site-I- or III-specific drugs, whereas the relative binding of tolbutamide to site-II-specific drugs between the two albumins was remarkably changed. The glycosylation of HSA not only increases the unbound drug concentration but also changes the displacement pattern at site II. Our results suggest that the extensive glycosylation of plasma proteins in diabetic patients complicates drug–drug interactions beyond those seen in normal people. © 1997 John Wiley & Sons, Ltd.     

Removal of sulfonamides by hemofiltration

Hemofiltration is a relatively new technique for removing toxic substances from the body. Unlike hemodialysis or hemoperfusion, the driving force behind hemofiltration is ultrafiltration. There have been several studies examining the clearance of drugs by hemofiltration but to date no study has investigated in a systematic way the effects of protein binding, perfusate flow, transmembrane pressure, and the duration of treatment on drug clearance by hemofiltration. The influence of these factors on the hemofiltration clearance of three sulfonamides with differing degrees of protein binding was investigated. It was found that hemofiltration drug clearance decreased with the duration of hemofiltration and protein binding but increased with perfusate flow and transmembrane pressure.     

Distribution of pentobarbital and diphenylhydantoin between plasma and cells in blood: Effect of salicylic acid,temperature and total drug concentration

Summary The binding of pentobarbital, diphenylhydantoin and salicylic acid to cells in blood was found to be independent of total drug concentration within therapeutic levels. Salicylic acid displaced pentobarbital and diphenylhydantoin from plasma protein binding sites, but high levels of salicylic acid had no effect on the distribution of the other two drugs to washed blood cells. Thus, in whole blood the presence of salicylic acid decreased the fraction of pentobarbital or diphenylhydantoin bound to plasma protiens and increased the fraction of the drug in plasma water and in blood cells. Diphenylhydantoin was shown not to be bound irreversibly to blood cells and equilibration in between the inside and outside of the cells was found to be rapid (within 5 min), even at high concentrations. Binding to washed blood cells was the same at 37°C and 25°C, in contrast to plasma protein binding. It is pointed out that these effects may cause certain analytical errors, resulting in changes in plasma concentration if plasma is separated at a low temperature.     

Kinetics of diphenylhydantoin in uraemic patients: Consequences of decreased plasma protein binding

Summary Diphenylhydantoin (2 mg/kg) was infused intravenously in four uraemic patients and four healthy volunteers and its plasma concentration measured during and after the infusion. The plasma concentrations were considerably lower in the uraemic subjects and the apparent volume of distribution was higher. These observations could be explained by the lower plasma protein binding of diphenylhydantoin in the uraemics. The overall elimination rate constant was greater (shorter half-life) in the uraemic patients. This difference could not be explained by reduced plasma protein binding, but it might be due to induction of diphenylhydantoin metabolism in the uraemic state. it is concluded that monitoring of the plasma levels of drugs in uraemic patients should be combined with determination of the extent to which the compounds are bound to plasma proteins.     

Accumulation kinetics of drugs with nonlinear plasma protein and tissue binding characteristics

The purpose of this investigation was to study, by digital computer simulation, the accumulation kinetics of drugs which exhibit concentration-dependent binding to tissues and either linear (constant free fraction) or concentration-dependent (increasing free fraction with increasing drug concentration) binding to plasma proteins. It was assumed that elimination rate is proportional to free drug concentration in plasma and that there occurs instantaneous equilibration of drug between vascular and nonvascular spaces. Nonlinear binding can yield, under certain conditions, apparently biexponential plasma concentrationtime curves which may be misinterpreted as being representative of a linear and biexponential-system. Such misinterpretation would cause the following errors in the prediction of drug accumulation and elimination kinetics during and after constantrate infusion: (a) the time required to reach steady state may be overestimated, and (b) the prominence of the apparent distribution phase after cessation of infusion may be underestimated. Drugs with linear and nonlinear plasma protein binding characteristics differ with respect to the relationship between infusion rate and steadystate concentration. This relationship is linear when plasma protein binding is linear. Steadystate concentration increases less than proportionally with increasing infusion rate if plasma protein binding is drug concentration dependent.     

Protein binding of non-steroidal anti-inflammatory drugs in plasma and synovial fluid of arthritic patients

1 The protein binding of seven non-steroidal anti-inflammatory drugs (indomethacin, tolmetin, salicylic acid, ibuprofen, flurbiprofen, naproxen and GP53,633) and warfarin was investigated by equilibrium dialysis in simultaneous samples of synovial fluid and plasma from 12 arthritic patients. 2 The protein binding of all drugs studied except warfarin and flurbiprofen was significantly lower in synovial fluid than in plasma. 3 The decreased protein binding of these drugs is likely to explain the lower total drug concentrations found in synovial fluid in comparison to plasma. 4 The lower albumin concentration plays an important role in determination of reduced drug binding in synovial fluid compared to plasma and the fatty acid concentration in synovial fluid may also influence the protein binding of some of these drugs.     

Differences in the binding of drugs to plasma proteins from newborn and adult man. I

Summary The binding of various drugs to plasma proteins from adult and cord plasma was determined by equilibrium dialysis. For almost all the drugs binding to cord plasma was lower than to plasma from adults. No evidence was found that the difference in binding was due to the different protein concentration in cord and adult plasma or to an influence of substances in ultrafiltrates of the plasmas.     

Managing drug reactions to sulfonamides and other drug in hiv infection: Desensitization rather than rechallenge

Drug reactions in patients with hiv infection, e.g. fever or rash, are a frequently occurring clinical problem. These side effects particularly are observed with sulfonamides; however, many other drugs have also shown to induce allergic reactions when given to patients with hiv infection. The production of hydroxylamines has been put forward as one of the explanations for these high incidence of reactions on drugs. Since sulfonamides are the first choice of therapy for the treatment and prophylaxis of Pneumocystis carinii pneumonia, several strategies have been developed to circumvent drug reactions. In general rechallenge or desensitization are recommended in literature. This article discusses the results and risks of rechallenge and desensitization with sulfonamides or other drugs, as mentioned in the literature. Furthermore preliminary results of rechallenge with a sulfonamide, which is not metabolized into hydroxylamines, are presented. From the data in the literature it is concluded that desensitization should be preferred to rechallenge.     

High incidence of multisystemic reactions to zimeldine

Summary Four metabolites of dipyrone, 4-methyl-aminoantipyrine (MAA), 4-aminoantipyrine (AA), 4-formylaminoantipyrine (FAA) and 4-acetylamino-antipyrine (AAA) can be identified in human plasma after its oral administration. The plasma protein binding of the metabolites in samples from 20 healthy volunteers was determined by ultrafiltration. None of the metabolites were found to be extensively bound to plasma proteins. The binding of MAA and AA was relatively higher than of FAA and AAA, as expected from their chemical structure. The mean percentage plasma protein binding was 57.6% for MAA, 47.9 for AA, 17.8 for FAA and 14.2% for AAA. The correlation between the unbound concentration in plasma and the total concentrations of MAA, AA, FAA and AAA was linear. No association was evident between the total protein plasma concentration and the extent of binding. The possible therapeutic implications related to protein binding of several analgesic and non-steroidal anti-inflammatory drugs are discussed.     

The effect of prednisone and hydrocortisone on the plasma protein binding of prednisolone in man

Summary The protein binding of prednisolone was studied in plasma obtained from healthy volunteers in the presence of added amounts of prednisone and hydrocortisone. The plasma protein binding was determined using in vitro equilibrium dialysis for 16 h at 37°C against isotonic Krebs-Ringer buffer using radioactive prednisolone. Prednisone appeared to have no effect on prednisolone binding. This surprising result was observed even when prednisone concentrations were more than 35 fold greater than prednisolone concentrations. In contrast, a marked competition between hydrocortisone and prednisolone was observed. These binding data were fit using a nonlinear least squares regression computer program and the capacity and affinity constants for the binding of prednisolone to transcortin and albumin were estimated including the competition for binding sites between prednisolone and hydrocortisone. The results from these studies compare favorably with recent parameter calculations, and our previous work where differences in binding were noted between cushingoid and noncushingoid patients.     

Plasma protein binding of tamsulosin hydrochloride in renal disease: role of α1-acid glycoprotein and possibility of binding interactions

  Objective: To investigate the factors that affect the plasma protein binding of tamsulosin in patients with lowered renal function compared with that in healthy subjects and to evaluate the possibility of binding interactions. Methods: Blood was donated from patients with renal dysfunction and from healthy subjects. The binding of ¹⁴C-tamsulosin to plasma proteins was determined by the ultrafiltration method. In addition, plasma protein binding interactions were investigated between tamsulosin and other drugs used concomitantly. Results: The mean percentage of unbound ¹⁴C-tamsulosin was 0.90% in the healthy subjects (control) and was 0.71% in the patients. The unbound fraction in the patients with α₁-acid glycoprotein (α₁-AGP) levels over 0.9 mg/ml was significantly lower than that in the healthy subjects. In contrast, the unbound fraction in the patients with α₁-AGP levels less than 0.7 mg/ml, which is the mean normal level, was almost equal to the control levels. A significant correlation existed between the C_b/C_u of tamsulosin and plasma α₁-AGP level (r ² = 0.580, P < 0.001), while no correlation existed between the C_b/C_u and plasma albumin level (r ² = 0.021, P = 0.381) in both groups. No apparent binding interactions were observed between tamsulosin and the other drugs examined. Conclusions: Tamsulosin is highly bound to α₁-AGP. The extent of plasma protein binding of tamsulosin correlated with the α₁-AGP level but not with the albumin level. Furthermore, there appears to be no or little possibility of binding interactions between tamsulosin and other drugs in clinically concomitant use, despite its strong binding to α₁-AGP. Received: 27 October 1998 / Accepted in revised form: 29 March 1999     

Likelihood and mechanisms of cross-allergenicity between sulfonamide antibiotics and other drugs containing a sulfonamide functional group

Concerns about cross-allergenicity between sulfonamide antibiotics and nonantibiotic, sulfonamide-containing drugs persist and can complicate patients' drug therapy unnecessarily. No interaction between the human immune system and the sulfonamide functional group has been demonstrated. The immunologic determinant of type I, immediate hypersensitivity responses to sulfonamide antibiotics is the N1 heterocyclic ring. Nonantibiotic sulfonamides do not contain this structural feature. Non-type I hypersensitivity responses to sulfonamide antibiotics are largely attributable to reactive metabolites that may cause either direct cytotoxicity or immunologic response. Formation of these metabolites is a stereospecific process that occurs at the N4 amino nitrogen of the sulfonamide antibiotics, a structure also not found on any nonantibiotic sulfonamide drugs. The stereospecificity of these reactions implies that cross-reactivity with nonantibiotic sulfonamide-containing drugs is highly unlikely; this assertion is supported by recent literature. However, T-cell recognition of unmetabolized, nonhaptenated parent sulfonamide antibiotic appears to occur in a small subset of hypersensitive patients. Several of the severe cutaneous reactions associated with sulfonamide antibiotics are mediated by T cells. It is not known whether T-cell recognition of antibiotic is related to the sulfonamide functional group. Until the mechanism of this recognition is elucidated, cross-reactivity with nonantibiotic sulfonamides appears to remain at least theoretically possible.