The binding of sulphonylureas to serum albumin

Characterization of binding by sulfonylureas with normal or modified human serum albumin using affinity microcolumns prepared by entrapment

Modification of proteins can occur during diabetes due to the formation of advanced glycation end-products (AGEs) with reactive dicarbonyls such as glyoxal (Go) and methylglyoxal (MGo). Human serum albumin (HSA) is a serum protein that binds to many drugs in blood and that is known to be modified by Go and MGo. This study examined the binding of various sulfonylurea drugs with these modified forms of HSA by using high-performance affinity microcolumns prepared by non-covalent protein entrapment. Zonal elution experiments were employed to compare the retention and overall binding constants for the drugs with Go- or MGo-modified HSA vs normal HSA. The results were compared to values from the literature, such as measured or estimated using affinity columns containing covalently immobilized HSA or biospecifically-adsorbed HSA. The entrapment-based approach provided estimates of global affinity constants within 3-5 min for most of the tested drugs and with typical precisions of ±10-23%. Each entrapped protein microcolumn was stable for over at least 60-70 injections and one month of use. The results obtained with normal HSA agreed at the 95% confidence level with global affinity constants that have been reported for the given drugs in the literature. It was found for HSA that had been modified with clinically-relevant levels of either Go or MGo that an increase in the global affinity constant of up to 2.1-fold occurred for some of the tested drugs. The information acquired in this study can be used in the future to adapt this entrapment-based approach to study and evaluate interactions between other types of drugs and normal or modified binding agents for clinical testing and biomedical research.

Glibenclamide transfer across the perfused human placenta is determined by albumin binding not transporter activity

The placenta mediates the transfer of maternal nutrients into the fetal circulation while removing fetal waste products, drugs and environmental toxins that may otherwise be detrimental to fetal development. This study investigated the role of drug transporters and protein binding in the transfer of the antidiabetic drug glibenclamide across the human placental syncytiotrophoblast using placental perfusion experiments and computational modeling. In the absence of albumin, placental glibenclamide uptake from the fetal circulation was not affected by competitive inhibition with bromosulphothalein (BSP), indicating that OATP2B1 does not mediate placental glibenclamide uptake from the fetus. In the presence of maternal and fetal albumin, BSP increased placental glibenclamide uptake from the fetal circulation by displacing glibenclamide from BSA, increasing the free fraction of glibenclamide driving diffusive transport. The P-gp and BCRP inhibitor GF120918 did not affect placental glibenclamide uptake from the maternal circulation and as such this study did not find any evidence for the apical efflux transporters in placental glibenclamide transfer. Computational modeling confirmed that albumin binding and not transporter activity, is the dominant factor in the transfer of glibenclamide across the human placenta. The effect of BSP binding to albumin on promoting the diffusive transfer of glibenclamide highlights the importance of drug-protein binding interactions and their interpretation using computational modeling.

Synthesis and evaluation of NLRP3-inhibitory sulfonylurea [11C]MCC950 in healthy animals

The diaryl sulfonylurea MCC950/CRID3 is a potent NLRP3 inhibitor (IC₅₀ = 8 nM) and, in animal models, MCC950 protects against numerous NLRP3-related neurodegenerative disorders. To evaluate the brain uptake and investigate target engagement of MCC950, we synthesised [¹¹C-urea]MCC950 via carrier added [¹¹C]CO₂ fixation chemistry (activity yield = 237 MBq; radiochemical purity >99%; molar activity = 7 GBq/µmol; radiochemical yield (decay-corrected from [¹¹C]CO₂) = 1.1%; synthesis time from end-of-bombardment = 31 min; radiochemically stable for >1 h). Despite preclinical efficacy in neurodegeneration studies, preclinical positron emission tomography (PET) imaging studies in mouse, rat and rhesus monkey revealed poor brain uptake of low molar activity [¹¹C]MCC950 and rapid washout. In silico prediction tools suggest efflux transporter liabilities for MCC950 at microdoses, and this information should be taken into account when developing next generation NLRP3 inhibitors and/or PET radiotracers.

Characterization of tolazamide binding with glycated and normal human serum albumin by using high-performance affinity chromatography

Sulfonylurea drugs are antidiabetic drugs that are utilized in the treatment of type II diabetes and often have significant binding with human serum albumin (HSA). Immobilized samples of normal or glycated HSA in affinity microcolumns were used to investigate interactions of these proteins with the sulfonylurea drug tolazamide. HPLC and frontal analysis were used to first examine the overall binding of this drug with these samples of HSA. It was found that tolazamide had two general classes of binding sites (i.e., high and low affinity) for normal and glycated HSA. The higher affinity sites had binding constants of around 4.3-6.0 × 10⁴ M^-1 for these interactions at pH 7.4 and 37 °C, while the lower affinity sites had binding strengths of 4.9-9.1 × 10³ M^-1. Zonal competition studies between tolazamide and probes for Sudlow sites I and II on HSA were also performed and used to provide site-specific affinities for tolazamide at these sites. A decrease of 22% in affinity was observed for tolazamide at Sudlow site I and an increase up to 58% was seen at Sudlow site II when comparing glycated HSA with normal HSA. These observed changes were compared to those of other first-generation sulfonylurea drugs, providing information on how glycation can alter the total and local binding strength of tolazamide and related compounds with HSA under levels of glycation seen in patients with diabetes.

Biological evaluations of newly-designed Pt(II) and Pd(II) complexes using spectroscopic and molecular docking approaches

To perform biological evaluations of newly-designed Pt(II) and Pd(II) complexes, the present study was conducted with targeted protein human serum albumin (HSA) and HCT116 cell line as model of human colorectal carcinoma. The binding of Pt(II) and Pd(II) complexes to HSA was analyzed using fluorescence spectroscopy and molecular docking. The thermal stability and alterations in the secondary structure of HSA in the presence of Pt(II) and Pd(II) complexes were investigated using the thermal denaturation method and circular dichroism (CD) spectroscopy. The cytotoxicity of the Pt(II) and Pd(II) complexes was studied against the HCT116 cell line using MTT assay. The binding analysis revealed that the fluorescence findings were well in agreement with docking results such that there is only one binding site for each complex on HSA. Binding constants of 8.7 × 10³ M^-1, 2.65 × 10³ M^-1, 0.3 × 10³ M^-1, and 4.4 × 10³ M^-1 were determined for Pd(II) and Pt(II) complexes (I-IV) at temperature of 25 °C, respectively. Also, binding constants of 1.9 × 10³ M^-1, 15.17 × 10³ M^-1, 1.9 × 10³ M^-1, and 13.1 × 10³ M^-1 were determined for Pd(II) and Pt(II) complexes (I-IV) at temperature of 37 °C, respectively. The results of CD and thermal denaturation showed that the molecular structure of HSA affected by interaction with Pt(II) and Pd(II) complexes is stable. Cytotoxicity studies represented the growth suppression effect of the Pt(II) and Pd(II) complexes toward the human colorectal carcinoma cell line. Therefore, the results suggest that the new designed Pt(II) and Pd(II) complexes are well promising candidates for use in cancer treatment, particularly for human colorectal cancer. Communicated by Ramaswamy H. Sarma.

Chromatographic studies of chlorpropamide interactions with normal and glycated human serum albumin based on affinity microcolumns

Sulfonylurea drugs have significant binding to proteins in blood, with most of this binding believed to occur with human serum albumin (HSA). High performance affinity chromatography and affinity microcolumns containing immobilized HSA were used to investigate binding by the sulfonylurea drug chlorpropamide to normal HSA and glycated HSA, which is a modified form of HSA that has an increased serum concentration in diabetes. Experiments employing frontal analysis indicated that the binding by chlorpropamide gave a good fit to a two-site model for both normal HSA and glycated HSA samples that were representative of controlled or advanced diabetes. These interactions involved a set of moderate-to-high affinity sites and a set of lower affinity sites, with binding constants in the range of 6.2-9.9 × 10⁴ M^-1 and 0.18-0.57 × 10⁴ M^-1, respectively, at pH 7.4 and 37 °C. Competition studies utilizing a zonal elution format demonstrated that chlorpropamide could interact at both Sudlow sites I and II of HSA, with affinities in the range expected for the moderate-to-high affinity sites of this drug. The affinity of chlorpropamide at Sudlow site I had a small increase of up to 1.2-fold when comparing the normal HSA and glycated HSA samples. Chlorpropamide gave a larger 1.4- to over 1.5-fold increase at Sudlow site II when the affinity of this drug was compared between normal HSA and the same samples of glycated HSA. These results were compared to those obtained previously with other sulfonylurea drugs to help determine how glycation can change the overall and site-selective binding strength of these drugs with HSA at levels of protein modification that are seen in patients with diabetes.

VLDL/LDL acts as a drug carrier and regulates the transport and metabolism of drugs in the body

Only free drugs have been believed to be carried into tissues through active or passive transport. However, considering that lipoproteins function as carriers of serum lipids such as cholesterol and triglycerides, we hypothesized that lipoproteins can associate with certain drugs and mediate their transport into tissues in lipid-associated form. Here, in vitro and in vivo studies with low density lipoprotein receptor (LDLR)-overexpressing or -knockdown cells and wild-type or LDLR-mutant mice were used to show the association of various drugs with lipoproteins and the uptake of lipoprotein-associated drugs through a lipoprotein receptor-mediated process. In clinical studies, investigation of the effect of lipoprotein apheresis on serum drug concentrations in patients with familial hypercholesterolemia demonstrated that lipoprotein-mediated drug transport occurs in humans as well as in mice. These findings represent a new concept regarding the transport and metabolism of drugs in the body and suggest that the role of lipoprotein-mediated drug transport should be considered when developing effective and safe pharmacotherapies.

Analysis of glipizide binding to normal and glycated human serum albumin by high-performance affinity chromatography

In diabetes, the elevated levels of glucose in the bloodstream can result in the nonenzymatic glycation of proteins such as human serum albumin (HSA). This type of modification has been shown to affect the interactions of some drugs with HSA, including several sulfonylurea drugs that are used to treat type II diabetes. This study used high-performance affinity chromatography (HPAC) to examine the interactions of glipizide (i.e., a second-generation sulfonylurea drug) with normal HSA or HSA that contained various levels of in vitro glycation. Frontal analysis indicated that glipizide was interacting with both normal and glycated HSA through two general groups of sites: a set of relatively strong interactions and a set of weaker interactions with average association equilibrium constants at pH 7.4 and 37 °C in the range of 2.4-6.0 × 10(5) and 1.7-3.7 × 10(4) M(-1), respectively. Zonal elution competition studies revealed that glipizide was interacting at both Sudlow sites I and II, which were estimated to have affinities of 3.2-3.9 × 10(5) and 1.1-1.4 × 10(4) M(-1). Allosteric effects were also noted to occur for this drug between the tamoxifen site and the binding of R-warfarin at Sudlow site I. Up to an 18% decrease in the affinity for glipizide was observed at Sudlow site I ongoing from normal HSA to glycated HSA, while up to a 27% increase was noted at Sudlow site II. This information should be useful in indicating how HPAC can be used to investigate other drugs that have complex interactions with proteins. These results should also be valuable in providing a better understanding of how glycation may affect drug-protein interactions and the serum transport of drugs such as glipizide during diabetes.

Analysis of free drug fractions by ultrafast affinity extraction: interactions of sulfonylurea drugs with normal or glycated human serum albumin

Ultrafast affinity extraction and a multi-dimensional affinity system were developed for measuring free drug fractions at therapeutic levels. This approach was used to compare the free fractions and global affinity constants of several sulfonylurea drugs in the presence of normal human serum albumin (HSA) or glycated forms of this protein, as are produced during diabetes. Affinity microcolumns containing immobilized HSA were first used to extract the free drug fractions in injected drug/protein mixtures. As the retained drug eluted from the HSA microcolumn, it was passed through a second HSA column for further separation and measurement. Items that were considered during the optimization of this approach included the column sizes and flow rates that were used, and the time at which the second column was placed on-line with the HSA microcolumn. This method required only 1.0 μL of a sample per injection and was able to measure free drug fractions as small as 0.09-2.58% with an absolute precision of ±0.02-0.5%. The results that were obtained indicated that glycation can affect the free fractions of sulfonylurea drugs at typical therapeutic levels and that the size of this effect varies with the level of HSA glycation. Global affinity constants that were estimated from these free drug fractions gave good agreement with those predicted from previous binding studies or determined through a reference method. The same approach could be utilized with other drugs and proteins or modified binding agents of clinical or pharmaceutical interest.

Studies of drug interactions with glycated human serum albumin by high-performance affinity chromatography

Diabetes is a health condition associated with elevated levels of glucose in the bloodstream and affects 366 million people worldwide. Type II diabetes is often treated with sulfonylurea drugs, which are known to bind tightly in blood to the transport protein human serum albumin (HSA). One consequence of the elevated levels of glucose in diabetes is the non-enzymatic glycation of proteins such as HSA. Several areas of HSA are now known to be affected by glycation-related modifications, which may in turn affect the binding of sulfonylurea drugs and other solutes to this protein. This review discusses some recent studies that have examined these changes in drug-protein binding by employing high-performance affinity chromatography (HPAC). A description of the theoretical and experimental techniques that were used in these studies is given. The information on drug interactions with glycated HSA, as obtained through this method, is also summarized. In addition, the potential advantages of this approach in the areas of biointeraction analysis and personalized medicine are considered.

Effects of Fatty Acids and Glycation on Drug Interactions with Human Serum Albumin

The presence of elevated glucose concentrations in diabetes is a metabolic change that leads to an increase in the amount of non-enzymatic glycation that occurs for serum proteins. One protein that is affected by this process is the main serum protein, human serum albumin (HSA), which is also an important carrier agent for many drugs and fatty acids in the circulatory system. Sulfonylureas drugs, used to treat type 2 diabetes, are known to have significant binding to HSA. This study employed ultrafiltration and high-performance affinity chromatography to examine the effects of HSA glycation on the interactions of several sulfonylurea drugs (i.e., acetohexamide, tolbutamide and gliclazide) with fatty acids, whose concentrations in serum are also affected by diabetes. Similar overall changes in binding were noted for these drugs with normal HSA or glycated HSA and in the presence of the fatty acids. For most of the tested drugs, the addition of physiological levels of the fatty acids to normal HSA and glycated HSA produced weaker binding. At low fatty acid concentrations, many of these systems followed a direct competition model while others involved a mixed-mode interaction. In some cases, there was a change in the interaction mechanism between normal HSA and glycated HSA, as seen with linoleic acid. Systems with only direct competition also gave notable changes in the affinities of fatty acids at their sites of drug competition when comparing normal HSA and glycated HSA. This research demonstrated the importance of considering how changes in the concentrations and types of metabolites (e.g., in this case, glucose and fatty acids) can alter the function of a protein such as HSA and its ability to interact with drugs or other agents.

Analysis of drug interactions with modified proteins by high-performance affinity chromatography: binding of glibenclamide to normal and glycated human serum albumin

High-performance affinity chromatography (HPAC) was used to examine the changes in binding that occur for the sulfonylurea drug glibenclamide with human serum albumin (HSA) at various stages of glycation for HSA. Frontal analysis on columns containing normal HSA or glycated HSA indicated glibenclamide was interacting through both high affinity sites (association equilibrium constant, K(a), 1.4-1.9 × 10(6)M(-1) at pH 7.4 and 37 °C) and lower affinity sites (K(a), 4.4-7.2 × 10(4)M(-1)). Competition studies were used to examine the effect of glycation at specific binding sites of HSA. An increase in affinity of 1.7- to 1.9-fold was seen at Sudlow site I with moderate to high levels of glycation. An even larger increase of 4.3- to 6.0-fold in affinity was noted at Sudlow site II for all of the tested samples of glycated HSA. A slight decrease in affinity may have occurred at the digitoxin site, but this change was not significant for any individual glycated HSA sample. These results illustrate how HPAC can be used as tool for examining the interactions of relatively non-polar drugs like glibenclamide with modified proteins and should lead to a more complete understanding of how glycation can alter the binding of drugs in blood.

Characterization of the binding of sulfonylurea drugs to HSA by high-performance affinity chromatography

Sulfonylurea drugs are often prescribed as a treatment for type II diabetes to help lower blood sugar levels by stimulating insulin secretion. These drugs are believed to primarily bind in blood to human serum albumin (HSA). This study used high-performance affinity chromatography (HPAC) to examine the binding of sulfonylureas to HSA. Frontal analysis with an immobilized HSA column was used to determine the association equilibrium constants (Ka) and number of binding sites on HSA for the sulfonylurea drugs acetohexamide and tolbutamide. The results from frontal analysis indicated HSA had a group of relatively high-affinity binding regions and weaker binding sites for each drug, with average Ka values of 1.3 (+/-0.2) x 10(5) and 3.5 (+/-3.0) x 10(2) M(-1) for acetohexamide and values of 8.7 (+/-0.6) x 10(4) and 8.1 (+/-1.7) x 10(3) M(-1) for tolbutamide. Zonal elution and competition studies with site-specific probes were used to further examine the relatively high-affinity interactions of these drugs by looking directly at the interactions that were occurring at Sudlow sites I and II of HSA (i.e., the major drug-binding sites on this protein). It was found that acetohexamide was able to bind at both Sudlow sites I and II, with Ka values of 1.3 (+/-0.1) x 10(5) and 4.3 (+/-0.3) x 10(4) M(-1), respectively, at 37 degrees C. Tolbutamide also appeared to interact with both Sudlow sites I and II, with Ka values of 5.5 (+/-0.2) x 10(4) and 5.3 (+/-0.2) x 10(4) M(-1), respectively. The results provide a more quantitative picture of how these drugs bind with HSA and illustrate how HPAC and related tools can be used to examine relatively complex drug-protein interactions.

Predicting plasma protein binding of drugs: a new approach

In spite of the large amount of plasma protein binding data for drugs, it is not obvious and there is no clear consensus among different disciplines how to deal with this parameter in multidimensional lead optimization strategies. In this work, we have made a comprehensive study on the importance of plasma protein binding and the influencing factors in order to get new insights for this molecular property. Our analysis of the distribution of percentage plasma protein binding among therapeutic drugs showed that no general rules for protein binding can be derived, except for the class of chemotherapeutics, where a clear trend towards lower binding could be observed. For the majority of indication areas, however, empirical rules are missing. We present here an extensive list of multiply determined primary association constants for binding to human serum albumin (HSA) for 138 compounds from the literature. Correlating these binding constants with the percentage fraction of protein bound showed that the percentage data above 90%, corresponding to a binding constant below 6 microM, are of insufficient accuracy. Furthermore, it could be demonstrated that the lipophilicity of drugs, traditionally felt to dominate binding to HSA, is not the only relevant descriptor. Here, we report a generic model for the prediction of drug association constants to HSA, which uses a pharmacophoric similarity concept and partial least square analysis (PLS) to construct a quantitative structure-activity relationship. It is able to single out the submicromolar to nanomolar binders, i.e. to differentiate between 99.0 and 99.99% plasma protein binding. Depending on the system, this can be important in medicinal chemistry programs and may together with other computed physicochemical and ADME properties assist in the prioritization of synthetic strategies.

Association of upregulated activity of K(ATP) channels with impaired insulin secretion in UCP1-expressing insulinoma cells

Insulin-secreting MIN6 cells overexpressing uncoupling protein-1 (UCP1) were studied regarding insulin secretion in response to various secretagogues. Overexpression of UCP1 prevented an increase of cytosolic ATP levels induced by glucose. In contrast, glucose utilization was not affected, nor was glycerol phosphate flux. The UCP1-expressing cells showed an inability to increase cytosolic Ca(2+) concentration ([Ca(2+)](i)) in response to glucose or alpha ketoisocaproate and this resulted in less insulin secretion, whereas initial reduction in [Ca(2+)](i) occurring upon either nutrient addition was not affected. Moreover, the effectiveness of tolbutamide on [Ca(2+)](i) increase was reduced and the dose-response relations for insulin secretion induced by the agent was shifted toward the right in the UCP1-expressing cells. The resting membrane potential of the UCP1-expressing cells was significantly hyperpolarized by 6.2 mV compared with control cells. In the perforated and conventional whole-cell patch-clamp configurations, the conductance density of ATP-sensitive K(+) (K(ATP)) channels of the UCP1-expressing cells was 6-fold and 1.7-fold greater than that of the control cells, respectively. The sensitivity of K(ATP) channels for tolbutamide was not different between two groups, indicating that in intact cells more than 6-fold higher concentrations of tolbutamide were required to reduce the K(ATP) channel currents of UCP1-expressing cells to the same levels as of the control cells. The current density of the voltage-dependent Ca(2+) channels was not influenced. In conclusion, UCP1-expressing cells showed a refractoriness to respond to tolbutamide as well as nutrients. An upregulated activity of K(ATP) channels was associated with unresponsiveness to the agent in the cells with impaired mitochondrial function.

KATP channels regulate mitogenically induced proliferation in primary rat hepatocytes and human liver cell lines. Implications for liver growth control and potential therapeutic targeting

To determine whether K(ATP) channels control liver growth, we used primary rat hepatocytes and several human cancer cell lines for assays. K(ATP) channel openers (minoxidil, cromakalim, and pinacidil) increased cellular DNA synthesis, whereas K(ATP) channel blockers (quinidine and glibenclamide) attenuated DNA synthesis. The channel inhibitor glibenclamide decreased the clonogenicity of HepG2 cells without inducing cytotoxicity or apoptosis. To demonstrate the specificity of drugs for K(+) channels, whole-cell patch-clamp recordings were made. Hepatocytes revealed K(+) currents with K(ATP) channel properties. These K(+) currents were augmented by minoxidil and pinacidil and attenuated by glibenclamide as well as tetraethylammonium, in agreement with established responses of K(ATP) channels. Reverse transcription of total cellular RNA followed by polymerase chain reaction showed expression of K(ATP) channel-specific subunits in rat hepatocytes and human liver cell lines. Calcium fluxes were unperturbed in glibenclamide-treated HepG2 cells and primary rat hepatocytes following induction with ATP and hepatocyte growth factor, respectively, suggesting that the effect of K(ATP) channel activity upon hepatocyte proliferation was not simply due to indirect modulation of intracellular calcium. The regulation of mitogen-related hepatocyte proliferation by K(ATP) channels advances our insights into liver growth control. The findings have implications in mechanisms concerning liver development, regeneration, and oncogenesis.

Electrical remodeling due to atrial fibrillation in chronically instrumented conscious goats: roles of neurohumoral changes, ischemia, atrial stretch, and high rate of electrical activation

BACKGROUND

Recently, we developed a goat model of chronic atrial fibrillation (AF). Due to AF, the atrial effective refractory period (AERP) shortened and its physiological rate adaptation inversed, whereas the rate and stability of AF increased. The goal of the present study was to evaluate the role of (1) the autonomic nervous system, (2) ischemia, (3) stretch, (4) atrial natriuretic factor (ANF), and (5) rapid atrial pacing in this process of electrical remodeling.

METHODS AND RESULTS

Twenty-five goats were chronically instrumented with multiple epicardial atrial electrodes. Infusion of atropine (1.0 mg/kg; n=6) or propranolol (0.6 mg/kg; n=6) did not abolish the AF-induced shortening of AERP or interval (AFI). Blockade of K+(ATP) channels by glibenclamide (10 micromol/kg; n=6) slightly increased the AFI from 95+/-4 to 101+/-5 ms, but AFI remained considerably shorter than during acute AF (145 ms). Glibenclamide had no significant effect on AERP after electrical cardioversion of AF (69+/-14 versus 75+/-15 ms). Volume loading by 0.5 to 1.0 L of Hemaccel (n=12) did not shorten AERP. The median plasma level of ANF increased from 42 to 99 pg/mL after 1 to 4 weeks of AF (n=6), but ANF infusion (0.1 to 3.1 microg/min, n=4) did not shorten AERP. Rapid atrial pacing (24 to 48 hours; n=10) progressively shortened AERP from 134+/-10 to 105+/-6 ms and inversed its physiological rate adaptation.

CONCLUSIONS

Electrical remodeling by AF is not mediated by changes in autonomic tone, ischemia, stretch, or ANF. The high rate of electrical activation itself provides the stimulus for the AF-induced changes in AERP.

Oral antidiabetic drug use in the elderly

Non-insulin-dependent diabetes mellitus (NIDDM) is a metabolic disease that is common in the elderly, and is characterised by insulin insufficiency and resistance. Measures such as bodyweight reduction and exercise improve the metabolic defects, but pharmacological therapy is the most frequently used and successful therapy. The sulphonylureas stimulate insulin secretion. Metformin and troglitazone increase glucose disposal and decrease hepatic glucose output without causing hypoglycaemia. Acarbose is a dietary aid that spreads the dietary carbohydrate challenge to endogenous insulin over time. These pharmacological agents, either alone or in combination, should improve blood glucose regulation in patients with NIDDM.

The effect of co-administered drugs on oxaprozin binding to human serum albumin

The binding of the non-steroidal anti-inflammatory drug oxaprozin to human serum albumin was studied by bioaffinity high-performance liquid chromatography using a column based on immobilized human serum albumin. Displacement studies using marker compounds for the major drug binding sites showed that oxaprozin has a high affinity for the benzodiazepine/indole site and binds to the warfarin site but with a significantly lower affinity. Biochromatography and ultrafiltration techniques were used to screen for possible competition and allosteric interactions between oxaprozin and potential co-administered drugs including non-steroidal anti-inflammatory drugs, antipyretics, hypoglycaemics, inhibitors of angiotensin-converting enzyme, anaesthetics, metal ions and anticancer agents. Competition occurred mainly with drugs bound at the benzodiazepine site (benzodiazepines, various non-steroidal anti-inflammatories).

Pharmacokinetics of the anticancer agent sulofenur in mice, rats, monkeys, and dogs

The absorption and pharmacokinetics of sulofenur [N-(indan-5-sulfonyl)-N'-(4-chlorophenyl)urea, LY186641] and its major metabolites were examined in mice, rats, monkeys, and dogs. The compound is a diarylsulfonylurea currently being evaluated as an oncolytic agent in phase I and II trials. In all species, sulofenur was well absorbed after an oral dose, but over a prolonged period, and sulofenur exhibited a fairly long half-life of elimination from plasma. These values ranged from 6 h in rats up to 30, 110, and 200 h in mice, monkeys, and dogs, respectively, at doses (240-1000 mg/m2) within the range of those used in clinical trials. Experiments describing the high degree of binding of sulofenur to plasma proteins (consistently > 99%) help to explain these relatively long half-lives. There is, however, a large difference between these plasma half-lives in the species studied. Sulofenur was previously found to be extensively metabolized to products that are excreted primarily into the urine. In this study, its major metabolites, which are found mainly in the urine, were also minor components of the drug-related material (< 10% of the sulofenur concentrations) in the plasma of rats treated with sulofenur. The absorption, binding characteristics, and elimination of these major metabolites after their administration to rats were also compared with sulofenur.(ABSTRACT TRUNCATED AT 250 WORDS)

Blockade of the ATP-sensitive potassium channel modulates reactive hyperemia in the canine coronary circulation

The mechanism of reactive hyperemia remains unknown. We hypothesized that reactive hyperemia was related to the opening of ATP-sensitive potassium channels during coronary occlusion. The resulting hyperpolarization of the smooth muscle cell plasma membrane might reduce calcium influx through voltage-dependent calcium channels and result in relaxation of smooth muscle tone and vasodilation. In eight open-chest, anesthetized dogs, 30-second coronary occlusions resulted in an average flow debt repayment of 200 +/- 41%. After low-dose (0.8 mumol/min) and high-dose (3.7 mumol/min) infusion of intracoronary glibenclamide, flow debt repayment fell to 76 +/- 14% and 50 +/- 8%, respectively (p less than 0.05 compared with control for both). The decline in flow debt repayment was due to a significant reduction both in maximum coronary conductance during reactive hyperemia and in its duration. In addition, there was a significant decline in the sensitivity of the coronary circulation to adenosine-induced vasodilation after glibenclamide. While more variable, there was no overall change in the sensitivity of the coronary vasculature to acetylcholine-induced vasodilation after glibenclamide. We conclude that reactive hyperemia is determined in a large part by the ATP-sensitive potassium channel, probably through its effect on membrane potential and voltage-sensitive calcium channels. Because reactive hyperemia was never fully abolished at the highest doses of glibenclamide tested, it is possible that additional mechanisms are involved in the genesis of this complex phenomenon.

The management of diabetes mellitus in older individuals

Nearly 50% of individuals with type II diabetes mellitus are over the age of 65 years. There are numerous reasons to maintain blood glucose levels below 11.1 nmol/L (200 mg/dl) in older persons, and there are a number of changes often seen with advancing age that persons, and there are a number of changes often seen with advancing age that may interfere with the management of diabetes mellitus, e.g. hypodipsia, anorexia, visual disturbance, altered renal and hepatic function, depression, impaired basoreceptor response and multiple medications. Hyperglycaemia appears to produce cognitive impairment which may lead to poor compliance. It is often difficult to manipulate diet in older people, and in fact dietary changes can lead to severe protein energy malnutrition. High maximum voluntary oxygen intake has been correlated with increased glucose disposal, but there is little evidence that physical exercise can improve diabetic control in the elderly. Oral sulphonylurea hypoglycaemic agents are extremely useful in the treatment of diabetes in these patients, but it should be remembered that they are more liable to develop hypoglycaemia than are younger diabetics. The role of metformin in the management of older diabetic patients is poorly studied. Many older persons can cope well with insulin therapy, but those with visual disturbances often make errors when drawing up insulin and require special attention. Combination therapy of insulin with oral hypoglycaemic agents is not recommended in this group of patients, and serum fructosamine is preferred to glycated haemoglobin to monitor control. Successful management of elderly diabetic patients thus requires an interdisciplinary team approach.

Evidence for more than one binding site for sulfonylureas in insulin-secreting cells

Specific binding of both [3H]glibenclamide and [3H]gliquidone has been observed in a particulate fraction of insulin-secreting rat tumour (RIN m5F) cells. The binding of both the labels was time-dependent, of high affinity (including a low affinity binding site), saturable and reversible. The rank order of inhibition of [3H]glibenclamide binding was glibenclamide greater than gliquidone greater than AG-EE 388 = AG-EE 86 = AG-EE 319 greater than AG-EE 436 (AG coded drugs are benzoic acid derivatives which lack the sulfonylurea moiety of sulfonylureas). The Kds of high affinity binding for glibenclamide and gliquidone were 0.08 and 1.3 nM, respectively. When [3H]gliquidone was used as the labelled compound this rank order of binding and the affinities of drugs were different, e.g. glibenclamide was less potent than gliquidone. The Kd values of high affinity binding to the [3H]gliquidone binding site were 810 and 79 nM with respect to glibenclamide and gliquidone. The binding site labelled by [3H]gliquidone, in contrast to that labelled by [3H]glibenclamide, was not able to discriminate between the two enantiomers AG-EE 319 and AG-EE 436. The data indicate that there are different binding sites for glibenclamide and gliquidone in RIN m5F cells. In extension to data of other groups it is speculated that there exists more than one specific binding site for sulfonylureas and other related compounds, e.g. benzoic acid derivatives and that sulfonylureas behave differently not only in quantitative but in qualitative terms as well.

Possible protein binding displacement interaction between glibenclamide and metolazone

The effects of metolazone on the protein binding of glibenclamide were studied. It was found that increasing metolazone concentrations up to 100 ng/ml had no significant effect on the protein binding of glibenclamide studied at 10 micrograms/ml. Metolazone is unlikely to cause a clinically significant increase in the free fraction of glibenclamide in patients receiving both drugs.

Control of insulin secretion by sulfonylureas, meglitinide and diazoxide in relation to their binding to the sulfonylurea receptor in pancreatic islets

Sulfonylureas inhibit an ATP-dependent K+ channel in the B-cell plasma membrane and thereby initiate insulin release. Diazoxide opens this channel and inhibits insulin release. In mouse pancreatic islets, we have explored whether other targets for these drugs must be postulated to explain their hypo- or hyperglycaemic properties. At non-saturating drug concentrations the rates of increase in insulin secretion declined in the order tolbutamide = meglitinide greater than glipizide greater than glibenclamide. The same rank order was observed when comparing the rates of disappearance of insulin-releasing and K+ channel-blocking effects. The different kinetics of response depend on the lipid solubility of the drugs, which controls their penetration into the intracellular space. Allowing for the different kinetics, the same maximum secretory rates were caused by saturating concentrations of tolbutamide, meglitinide, glipizide and glibenclamide. A close correlation between insulin-releasing and K+ channel-blocking potencies of these drugs was observed. The relative potencies of tolbutamide, meglitinide, glipizide and glibenclamide corresponded well to their relative affinities for binding to islet-cell membranes, suggesting that the binding site represents the sulfonylurea receptor. The biphasic time-course of dissociation of glibenclamide binding indicates a complex receptor-drug interaction. For diazoxide there was no correlation between affinity of binding to the sulfonylurea receptor and potency of inhibition of insulin secretion. Thus, opening or closing of the ATP-dependent K+ channel by diazoxide or sulfonylureas, respectively, appears to be due to interaction with different binding sites in the B-cell plasma membrane. The free concentrations of tolbutamide, glipizide, glibenclamide and diazoxide which are effective on B-cells are in the range of therapeutic plasma concentrations of the free drugs. It is concluded that the hypo- and hyperglycaemic effects of these drugs result from changing the permeability of the ATP-dependent K+ channel in the B-cell plasma membrane.

Pharmacokinetic-pharmacodynamic relationships of oral hypoglycaemic agents. An update

Oral hypoglycaemic drugs, sulphonylureas and biguanides, occupy an important place in the treatment of Type II (non-insulin-dependent) diabetic patients who fail to respond satisfactorily to diet therapy and physical exercise. Although the precise mechanisms of action of these compounds are still poorly understood, there is sufficient agreement that sulphonylureas have both pancreatic and extrapancreatic effects, whereas biguanides have predominantly extrapancreatic actions. By using labelled compounds or measuring the circulating concentrations, the main pharmacokinetic properties of oral hypoglycaemic agents have been assessed and, in some cases, their pharmacokinetic-pharmacodynamic relationships have been evaluated. A correlation between diabetes control and plasma sulphonylurea or biguanide concentrations is generally lacking at the steady-state, with the possible exception of long-acting agents; after either oral or intravenous dosing, the reduction of plasma glucose is usually related to the increased circulating drug concentrations. The toxic effects of oral hypoglycaemic drugs are more frequent in the elderly and in the presence of conditions that may lead to drug accumulation or potentiation (increased dosage, use of long-acting compounds, hepatic and renal disease, interaction with other drugs); however, a relationship between toxic effects and drug plasma levels has been reported only for biguanides.

Highly potent and stereoselective effects of the benzoic acid derivative AZ-DF 265 on pancreatic beta-cells

1. Mouse islets were used to define the characteristics and study the mechanisms of the stimulation of insulin release by compound AZ-DF 265, 4-[[N-(alpha-phenyl-2-piperidino-benzyl) carbamoyl]methyl] benzoic acid, a substituted benzoic acid with an asymmetric carbon atom. 2. At a non-stimulatory concentration of glucose (3 mM), (-)-AZ-DF 265 reversibly inhibited 86Rb efflux from islet cells, depolarized the beta-cell membrane, induced electrical activity, stimulated 45Ca efflux, and triggered insulin release. Maximum inhibition of 86Rb efflux occurred at 0.03 microM (-)-AZ-DF 265, whereas the threshold concentration for stimulation of release was 0.1 microM. Omission of extracellular Ca2+ abolished all effects of the drug but the inhibition of 86Rb efflux. 3. At a stimulatory concentration of glucose (10 mM), (-)-AZ-DF 265 reversibly increased 86Rb efflux, potentiated electrical activity, augmented 45Ca efflux, and increased insulin release. Maximum stimulation of 86Rb efflux and insulin release was obtained with 0.03 microM (-)-AZ-DF 265. Omission of extracellular Ca2+ abolished all effects of the drug. 4. The potency of (-)-AZ-DF 265 was similar to that of glibenclamide, whereas the (+)-enantiomer was about 10 times less potent on 86Rb efflux and insulin release. 5. It is concluded that, like sulphonylureas, compound AZ-DF 265 decreases K+ permeability of the beta-cell membrane and thereby causes depolarization. This activates voltage-dependent Ca channels, permits Ca2+ influx and eventually stimulates insulin release. Its stereoselectivity may help to elucidate the mechanisms of K channel blockade and, hence, lead to the design of more potent and specific insulinotropic drugs.

The second generation sulfonylureas: glipizide and glyburide

Glyburide and glipizide are approved by the FDA for treatment of symptomatic and asymp tomatic NIDDM patients in whom hyperglycemia cannot be satisfactorily controlled by diet and exercise. Pharmacology studies suggest that glyburide and glipizide sensitize the beta cells of the pancreas to release insulin only in the presence of elevated serum glucose levels. Long-term efficacy appears to depend on extrapancreatic effects, suppressing hepatic glucose production, and improved postreceptor insulin activity.

Nonenzymatic glucosylation of human serum albumin and its influence on binding capacity of sulfonylureas

To estimate the functional change occurring in human serum albumin by nonenzymatic glucosylation, glucosylated human serum albumin was prepared by in vitro incubation with glucose. The rate of glucosylation proceeded as a first-order reaction. The binding of sulfonylureas to serum albumin was determined by equilibrium gel filtration. Through this method, it was possible to estimate the binding capacity of a low water solubility drug with a high affinity to protein. The amounts of the sulfonylureas bound to glucosylated HSA decreased by 44% with tolazamide and acetohexamide, 50% with glibenclamide, and 52% with tolbutamide, compared to human serum albumin (HSA). This suggests that a high concentration of glucosylated HSA in diabetic patients may possibly cause an increase in free drug concentration exceeding normal levels. This study shows that the decrease in the binding capacity of sulfonylureas with protein is due to the modification of albumin molecules by the covalent binding of glucose.

Binding of tolmetin and salicylic acid to human serum albumin as a function of temperature

When drug-protein binding data are evaluated thermodynamically standard free energy (delta G0), standard enthalpy (delta H0) and standard entropy (delta S0) are usually estimated from association constants (Ka) derived from binding data obtained at only two temperatures. Estimation of delta H0 involves the assumption of its constancy in the temperature range studied and linearity of a van't Hoff plot of ln Ka versus 1/T. Sometimes the assumption of such linearity is invalid for theoretical reasons and data obtained at only two temperatures contain no information concerning linearity of this plot. We present data for the binding of both tolmetin and salicylic acid to human serum albumin as a function of temperature which make doubtful the validity of using association constants of these drugs to derive thermodynamic constants other than delta G0 values.

Tissue distribution and metabolism of drugs VI: Effect of second drugs on pancreatic distribution and insulin-releasing activity of sulfonylureas in perfused rat pancreas

The distribution of sulfonylureas and their insulin-releasing potency as a pharmacological response were studied in isolated perfused rat pancreas. Sulfonylurea concentrations in the perfused pancreas in the presence or absence of a second drug were determined after perfusion for 15 min. Sulfonylureas could be distributed throughout the pancreas readily, and the tissue sulfonylurea concentration was reduced by the addition of sulfaphenazole, sulfadimethoxine, and salicylic acid. The insulin secretion rate stimulated by tolbutamide also was reduced by these three drugs; sulfanilamide, which could not displace the tolbutamide distribution, did not affect the tolbutamide-mediated secretion of insulin. These results document the importance of drug concentration in the tissue or receptor site with regard to insulin secretion and show that the sulfonylurea-mediated secretion of insulin can be modified easily by concomitant perfusion of a second drug that displaces sulfonylurea in the pancreas. These findings suggest that the drug interaction at the target organ or receptor site should be understood to provide adequate drug therapy.

Method of obtaining drug-macromolecule binding parameters directly from dynamic dialysis data

A new method of treating dynamic dialysis data to obtain binding parameters for drug-macromolecule interactions is presented. This method allows the determination of binding parameters directly from dialysis data according to a theoretical model. It is not necessary to determine the dialysis rate constant accurately in a separate experiment, and bias is not introduced due to differentiation. The proposed method should be applicable where the drug is substantially bound to the dialysis membrane.

Plasma protein binding of diazepam and tolbutamide in chronic alcoholics

The increased incidence of drowsiness in hypoalbuminemic patients administered diazepam and more rapid clearance of tolbutamide in cirrhotics may be due to changes in plasma protein binding. The binding of diazepam and tolbutamide was studied by equilibrium dialysis at 37degreesC over a total drug concentration range of 1 to 10 mug/ml and 50 to 300 mug/ml, respectively, in plasma from 21 normal and 14 alcoholic subjects. At 1 mug/ml, diazepam plasma protein binding (+/- S.D.) was 98.5+/-0.4 per cent in normals and 97.8+/-1.2 per cent in alcoholics; at 100 mug/ml, tolbutamide binding was 97.8+/-0.3 per cent in normals and 95.1+/-4.2 per cent in alcoholics. For both agents at all concentrations, the binding to plasma from alcoholics was significantly decreased (P less than 0.01-less than 0.02). The extent of binding of both drugs was dependent on the albumin concentration. These findings suggest that important changes in pharmacologic effect, distribution, and clearance of diazepam and tolbutamide can be anticipated in alcoholics with hypoalbuminemia.