High-performance frontal analysis for the study of protein binding of troglitazone (CS-045) in albumin solution and in human plasma

Assessing OATP1B1- and OATP1B3-Mediated Drug-Drug Interaction Potential of Vemurafenib Using R-Value and Physiologically-Based Pharmacokinetic Models

Organic anion transporting polypeptides (OATP) 1B1 and OATP1B3 are important determinants of transporter-mediated drug-drug interactions (DDIs). Current studies assessed the OATP1B1 and OATP1B3-mediated DDI potential of vemurafenib, a kinase inhibitor drug with high protein binding and low aqueous solubility, using R-value and physiologically-based pharmacokinetic (PBPK) models. The total half-maximal inhibitory concentration (IC50,total) values of vemurafenib against OATP1B1 and OATP1B3 were determined in 100% human plasma in transporter-overexpressing human embryonic kidney 293 stable cell lines. The unbound fraction of vemurafenib in human plasma before (fu,plasma) and after addition into the uptake assay plate (fu,plasma,inc) were determined by rapid equilibrium dialysis. There was no statistically significant difference between fu,plasma and fu,plasma,inc. Vemurafenib IC50,total values against OATP1B1 and OATP1B3 are 175 ± 82 and 231 ± 26 μM, respectively. The R-values [R = 1 + fu,plasma × Iin,max/(fu,plasma,inc × IC50,total)] were then simplified as R = 1+Iin,max/IC50,total, and were 1.76 and 1.57 for OATP1B1 and OATP1B3, respectively. The simulated pravastatin AUC ratio was 1.28 when a single dose of pravastatin (40 mg) was co-administered with vemurafenib (960 mg, twice daily) at steady-state, compared to pravastatin alone. Both R-value and PBPK models predict that vemurafenib has the potential to cause OATP1B1- and OATP1B3-mediated DDIs.

Glitazones exert multiple effects on β-cell stimulus-secretion coupling

Earlier studies suggest that glitazones exert beneficial effects in patients with type 2 diabetes by directly affecting insulin secretion of β-cells, besides improving the effectiveness of insulin in peripheral tissues. The effects of glitazones on stimulus-secretion coupling (SSC) are poorly understood. We tested the influence of troglitazone and pioglitazone on different parameters of SSC, including insulin secretion (radioimmunoassay), cell membrane potential, various ion currents (patch-clamp), mitochondrial membrane potential (ΔΨ), and cytosolic Ca(2+) concentration (fluorescence). Troglitazone exerted stimulatory, inhibitory, or no effects on insulin secretion depending on the drug and glucose concentration. It depolarized the ΔΨ, thus lowering ATP production, which resulted in opening of ATP-dependent K(+) channels (K(ATP) channels) and reduced insulin secretion. However, it also exerted direct inhibitory effects on K(ATP) channels that can explain enhanced insulin secretion. Troglitazone also inhibited the currents through voltage-dependent Ca(2+) and K(+) channels. Pioglitazone was less effective than troglitazone on all parameters tested. The effects of both glitazones were markedly reduced in the presence of bovine serum albumin. Glitazones exert multiple actions on β-cell SSC that have to be considered as undesired side effects because the influence of these compounds on β-cells is not controllable. The final effect on insulin secretion depends on many parameters, including the actual glucose and drug concentration, protein binding of the drug, and the drug by itself. Troglitazone and pioglitazone differ in their influence on SSC. It can be assumed that the effects of pioglitazone on β-cells are negligible under in vivo conditions.

Endogenous bile acid disposition in rat and human sandwich-cultured hepatocytes

Sandwich-cultured hepatocytes (SCH) are used commonly to investigate hepatic transport protein-mediated uptake and biliary excretion of substrates. However, little is known about the disposition of endogenous bile acids (BAs) in SCH. In this study, four endogenous conjugated BAs common to rats and humans [taurocholic acid (TCA), glycocholic acid (GCA), taurochenodeoxycholic acid (TCDCA), and glycochenodeoxycholic acid (GCDCA)], as well as two BA species specific to rodents (α- and β-tauromuricholic acid; α/β TMCA), were profiled in primary rat and human SCH. Using B-CLEAR® technology, BAs were measured in cells+bile canaliculi, cells, and medium of SCH by LC-MS/MS. Results indicated that, just as in vivo, taurine-conjugated BA species were predominant in rat SCH, while glycine-conjugated BAs were predominant in human SCH. Total intracellular BAs remained relatively constant over days in culture in rat SCH. Total BAs in control (CTL) cells+bile, cells, and medium were approximately 3.4, 2.9, and 8.3-fold greater in human than in rat. The estimated intracellular concentrations of the measured total BAs were 64.3±5.9 μM in CTL rat and 183±56 μM in CTL human SCH, while medium concentrations of the total BAs measured were 1.16±0.21 μM in CTL rat SCH and 9.61±6.36 μM in CTL human SCH. Treatment of cells for 24h with 10 μM troglitazone (TRO), an inhibitor of the bile salt export pump (BSEP) and the Na⁺-taurocholate cotransporting polypeptide (NTCP), had no significant effect on endogenous BAs measured at the end of the 24-h culture period, potentially due to compensatory mechanisms that maintain BA homeostasis. These data demonstrate that BAs in SCH are similar to in vivo, and that SCH may be a useful in vitro model to study alterations in BA disposition if species differences are taken into account.

CHROMATOGRAPHIC ANALYSIS OF DRUG INTERACTIONS IN THE SERUM PROTEOME

The binding of drugs with serum proteins and binding agents such as human serum albumin, α1-acid glycoprotein, and lipoproteins is an important process in determining the activity and fate of many pharmaceuticals in the body. A variety of techniques have been used to study drug interactions with serum proteins, but there is still a need for faster or better methods for such work. High-performance liquid chromatography (HPLC) is one tool that has been utilized in many formats for these types of measurements. Advantages of using HPLC for this application include its speed and precision, its ability to be automated, its good limits of detection, and its compatibility with a wide range of assay formats and detectors. This review will discuss various approaches in which HPLC can be employed for the study of drug-protein interactions. These techniques include the use of soluble proteins in zonal elution and frontal analysis methods or vacancy techniques such as the Hummel-Dreyer method. Zonal elution and frontal analysis methods that make use of immobilized proteins and high-performance affinity chromatography will also be presented. A variety of applications will be examined, ranging from the determination of free drug fractions to the measurement of the strength or rate of a drug-protein interaction. Newer developments that will be discussed include recent work in the creation of novel mathematical approaches for HPLC studies of drug-protein binding, the use of HPLC methods for the high-throughput screening of drug-protein binding, and the creation and use of affinity monoliths or affinity microcolumns for examining drug-protein systems.

Inhibition of monoacylglycerol lipase by troglitazone, N-arachidonoyl dopamine and the irreversible inhibitor JZL184: comparison of two different assays

BACKGROUND AND PURPOSE

Drugs used clinically usually have a primary mechanism of action, but additional effects on other biological targets can contribute to their effects. A potentially useful additional target is the endocannabinoid metabolizing enzyme monoacylglycerol lipase (MGL). We have screened a range of drugs for inhibition of MGL and compared the observed potencies using different MGL enzyme assays.

EXPERIMENTAL APPROACH

MGL activity was screened using recombinant human MGL (cell lysates and purified enzyme) with 4-nitrophenyl acetate (NPA) as substrate. 2-Oleolyglycerol metabolism by rat cerebellar cytosolic MGL and by recombinant MGL was also investigated.

KEY RESULTS

Among the 96 compounds screened in the NPA assay, troglitazone, CP55,940, N-arachidonoyl dopamine and AM404 inhibited NPA hydrolysis by the lysates with IC(50) values of 1.1, 4.9, 0.78 and 3.1µM, respectively. The potency for troglitazone is in the same range as its primary pharmacological activity, activation of peroxisome proliferator-activated receptor (PPAR) γ. Among PPARγ ligands, the potency order towards human MGL was troglitazone > ciglitazone > rosiglitazone > 15-deoxy-Δ(12,14) -prostaglandin J(2) ≈ CAY 10415 > CAY 10514. In contrast to the time-dependent inhibitor JZL184, the potency of troglitazone was dependent upon the enzyme assay system used. Thus, troglitazone inhibited rat cytosolic 2-oleoylglycerol hydrolysis less potently (IC(50) 41µM) than hydrolysis of NPA by the human MGL lysates.

CONCLUSIONS AND IMPLICATIONS

'Hits' in screening programmes for MGL inhibitors should be assessed in different MGL assays. Troglitazone may be a useful lead for the design of novel, dual action MGL inhibitors/PPARγ activators.

Palmitate-induced down-regulation of sortilin and impaired GLUT4 trafficking in C2C12 myotubes

Elevated saturated FFAs including palmitate (C16:0) are a primary trigger for peripheral insulin resistance characterized by impaired glucose uptake/disposal in skeletal muscle, resulting from impaired GLUT4 translocation in response to insulin. We herein demonstrate that palmitate induces down-regulation of sortilin, a sorting receptor implicated in the formation of insulin-responsive GLUT4 vesicles, via mechanisms involving PKC and TNF-α-converting enzyme, but not p38, JNK, or mitochondrial reactive oxygen species generation, leading to impaired GLUT4 trafficking in C2C12 myotubes. Intriguingly, unsaturated FFAs such as palmitoleate (C16:1) and oleate (C18:1) had no such detrimental effects, appearing instead to effectively reverse palmitate-induced impairment of insulin-responsive GLUT4 recycling along with restoration of sortilin abundance by preventing aberrant PKC activation. On the other hand, shRNA-mediated reduction of sortilin in intact C2C12 myotubes inhibited insulin-induced GLUT4 recycling without dampening Akt phosphorylation. We found that the peroxisome proliferator-activated receptor γ agonist troglitazone prevented the palmitate-induced sortilin reduction and also ameliorated insulin-responsive GLUT4 recycling without altering the palmitate-evoked insults on signaling cascades; neither highly phosphorylated PKC states nor impaired insulin-responsive Akt phosphorylation was affected. Taken together, our data provide novel insights into the pathogenesis of PKC-dependent insulin resistance with respect to insulin-responsive GLUT4 translocation, which could occur not only through defects of insulin signaling but also via a reduction of sortilin, which directly controls trafficking/sorting of GLUT4 in skeletal muscle cells. In addition, our data suggest the insulin-sensitizing action of peroxisome proliferator-activated receptor γ agonists to be at least partially mediated through the restoration of proper GLUT4 trafficking/sorting events governed by sortilin.

Determination of troglitazone stereoisomers in rat plasma using semi-micro HPLC with electrochemical detection

A highly sensitive determination method for troglitazone stereoisomers was developed by high-performance liquid chromatography with electrochemical detection (HPLC-ECD). The oxidation behavior of troglitazone was investigated for the application of ECD by measuring the cyclic voltammogram. The separation was performed on a semi-micro chiral column (Chiralcel OJ-RH) using a mobile phase consisting of methanol-acetic acid (1000:1, v/v) containing 50mM LiClO4 at a flow rate of 20 microl/min. The peak areas of the stereoisomers separated from 0.1 to 50 ng/ml of troglitazone had good linearity with correlation coefficients of >0.999, and had similar response. The limit of detection was 1.3 fmol (signal-to-noise ratio of 3). This method was applied to the determination of troglitazone stereoisomers in rat plasma. The levels of troglitazone stereoisomers in rat plasma could be monitored until 24h after the oral administration.

Measurements of drug–protein binding by using immobilized human serum albumin liquid chromatography–mass spectrometry

An HPLC/MS based method was used for fast and convenient determination of drug plasma-protein interactions in early drug discovery screening by employing a human serum albumin affinity column. Results from this methodology were compared with data from ultrafiltration or dialysis methods, and good agreement was observed. A compound not suitable for ultrafiltration due to the very high non-specific binding to artificial membrane of ultrafiltration device was also successfully analyzed by this method, and the protein binding determined by this chromatography method was very similar to data obtained by dialysis technique employing biological membranes. The immobilized HSA column LC/MS method also proved to be more reproducible and precise compared to ultrafiltration method in drug protein binding measurements.

Synergistic effects of troglitazone and oleate on the translatability of preproinsulin mRNA from INS-1 cells

Glitazones are known to modulate fatty acid-induced effects on insulin secretion in the pancreatic beta-cell. The present study focused on combined effects of troglitazone and oleate on preproinsulin (PPI) biosynthesis. Insulin-producing INS-1 cells were incubated for 4 hr at 11.2mM glucose in the presence (O(+)) or absence (O(-)) of 200 microM oleate with (T(+)) or without (T(-)) 10 microM troglitazone. After cell lysis, cytoplasmic RNA was extracted and employed for Northern blotting and corresponding in vitro translation. Compared with untreated controls (CTRL=O(-)/T(-)), the cellular content of PPI-mRNA from cells which had been simultaneously treated by troglitazone and oleate (O(+)/T(+)) was significantly diminished (O(+)/T(+)=75+/-10% x CTRL; P=0.015). The PPI-mRNA content from those cells which had been exclusively exposed either to oleate (O(+)/T(-)) or troglitazone (O(-)/T(+)) did not significantly differ from that of the untreated controls. In spite of that decreased PPI-mRNA content, in vitro translation revealed the highest yield of newly synthesized PPI in RNA samples from those cells which had been simultaneously exposed to oleate and troglitazone before (O(+)/T(+)=1.6+/-0.3 x CTRL; P=0.01). It is concluded that troglitazone and oleate synergistically affect the translational rate at the level of the PPI-mRNA molecule.

Troglitazone: the discovery and development of a novel therapy for the treatment of Type 2 diabetes mellitus

Prior to the introduction of troglitazone, it had been more than 30 years since the last significant improvement in antidiabetic therapy. In view of the pressing need for more effective oral agents for the treatment of Type 2 diabetes mellitus, troglitazone was granted priority review by the FDA and was launched in the USA in 1997. The first of the thiazolidinedione insulin sensitizing agents, troglitazone was quickly followed by rosiglitazone and pioglitazone. The glitazones proved to be effective not only in lowering blood glucose, but also to have beneficial effects on cardiovascular risk. Troglitazone was subsequently withdrawn because of concerns about hepatotoxicity, which appears to be less of a problem with rosiglitazone and pioglitazone. Recent insights into the molecular mechanism of action of the glitazones, which are ligands for the peroxisome proliferator-activated receptors, open the prospect of designing more effective, selective and safer antidiabetic agents. This document will review the history of troglitazone from discovery through clinical development.

Direct chiral separation of troglitazone stereoisomers using reversed-phase high-performance liquid chromatography

A simple HPLC method for the direct chiral separation of troglitazone stereoisomers was developed. The separation was performed on a reversed-phase cellulose-derivertized chiral column (Chiralcel OJ-R) using a mobile phase consisting of methanol-acetic acid (1000:1, v/v) at a flow rate of 0.5 ml/min. The peak areas of stereoisomers separated from 0.13 to 0.75 mg/ml of troglitazone had good linearity, with correlation coefficients > 0.999 in the reversed-phase mode. The repeatability of the ratios of stereoisomers isolated from 0.5 mg/ml of troglitazone had a relative standard deviation of 0.1-0.2%. The relative sensitivities of the four isomers at UV 285 nm were similar, as each response factor was within the range of 0.99-1.01. Troglitazone racemized at the chiral center of the thiazolidine ring in methanol solution, but was found to be stable for 24 h in methanol-acetic acid (1000:1, v/v). This method was applied to the stereoisomeric analysis of troglitazone in pharmaceutical formulations and used to evaluate the constancy of the stereoisomer ratio in the manufacturing process and stability testing.

Troglitazone does not protect rat pancreatic beta cells against free fatty acid-induced cytotoxicity

Thiazolidinediones are a novel class of antidiabetic drugs that reduce insulin resistance through interaction with nuclear peroxisome proliferator-activated receptor (PPAR)gamma. One of these agents, troglitazone, was also proposed to protect beta cells against FFA-induced toxicity, but this effect has not yet been directly demonstrated. We recently reported in vitro conditions under which free fatty acids (FFA) cause beta cell death by necrosis or apoptosis. The present study investigates whether troglitazone (10 microM) interferes with this FFA-induced toxicity. Addition of this compound did not protect against oleate- or palmitate-induced toxicity. On the contrary, it increased palmitate-induced necrosis during the first two days of culture, and elevated (increase by 10-20%, P<0.05) both oleate- and palmitate-induced apoptosis after 8 days. These results do not support the view that troglitazone exerts a direct protective effect on beta cells that are exposed to cytotoxic FFA concentrations. They instead indicate that the agent may sensitize pancreatic beta cells to FFA-induced damage, raising the possibility that its use facilitates the deleterious effect of increased FFA levels on the pancreatic beta cell mass.

Plasma protein binding study of oxybutynin by high-performance frontal analysis

Plasma protein binding of oxybutynin (OXY) was investigated quantitatively and enantioselectively using high-performance frontal analysis (HPFA). An on-line HPLC system which consists of HPFA column, extraction column and analytical column was developed to determine the unbound concentrations of OXY enantiomers in human plasma, in human serum albumin (HSA) solutions, and in human alpha1-acid glycoprotein (AGP) solutions. OXY is bound in human plasma strongly and enantioselectively. The bound drug fraction in human plasma containing 2-10 microM (R)- or (S)-OXY was higher than 99%, and the unbound fraction of (R)-OXY was 1.56 times higher than that of (S)-isomer. AGP plays the dominant role in this strong and enantioselective plasma protein binding. The total binding affinities (nK) of (R)- and (S)-OXY to AGP were 6.86 x 10(6) and 1.53 x 10(7) M(-1), respectively, while the nK values of (R)- and (S)-OXY to HSA were 2.64 x 10(4) and 2.19 x 10(-4) M(-1), respectively. The binding affinity of OXY to AGP is much higher than that to HSA, and shows high enantioselectivity (SIR ratio of nK values is 2.2). It was found that both enantiomers are bound competitively at the same binding site on an AGP molecule. The binding property between OXY and low density lipoprotein (LDL) was investigated by using the frontal analysis method incorporated in high-performance capillary electrophoresis (HPCE/FA). It was found the binding is non-saturable and non-enantioselective.

Chromatographic and electrophoretic studies of protein binding to chiral solutes

Protein interactions are important in determining the transport, metabolism and/or activity of many chiral compounds within the body. This review examines data that have been obtained on these interactions by various chromatographic and electrophoretic methods, especially those based on either high-performance liquid chromatography or capillary electrophoresis. Zonal elution, frontal analysis and vacancy methods are each considered, as are approaches that employ either soluble or immobilized proteins. There are a variety of different items that can be learned about a solute-protein system through these techniques. This includes information on the binding constants and number of binding sites for a solute-protein system, as well as the thermodynamic parameters, rate constants, interaction forces and binding site structure for the protein and solute. Numerous examples are provided throughout this review, as taken from the literature and from work performed within the author's laboratory.

Method development and validation for the HPLC potency assay of troglitazone tablets

This paper describes the development and validation of an isocratic, reversed-phase, high performance liquid chromatographic (HPLC) method for the assay of 200-mg troglitazone tablets. The chromatographic conditions of the method employ a YMC ODS-A, 120 A (4.6 x 150 mm, 5 microm) column, isocratic elution with (50 mM aqueous NaH2PO4, pH 4.0):acetonitrile:methanol, (35:50:15, v/v/v) as the mobile phase at a flow rate of 1.0 ml/min, a 10 microl injection volume, and ulltraviolet (UV) detection at 225 nm. The active was analyzed at ambient column temperature, using peak area responses.

Role of glucose and insulin in thiazolidinedione-induced alterations in hepatic gluconeogenesis

Previous studies from our laboratory as well as from others have suggested that the thiazolidinediones have the capacity to act as insulinomimetic agents, especially in the liver. In order to further characterize this insulinomimetic action, we evaluated the effect of troglitazone, a representative thiazolidinedione, on lactate- and glucagon-stimulated gluconeogenesis, in the presence or absence of insulin (10 nM) in isolated rat hepatocytes. The antigluconeogenic effect of troglitazone under basal (lactate-stimulated) conditions was found to be due to an elevation in the fructose 2,6-bisphosphate content, which was, however, not mediated by an activation of 6-phosphofructo 2-kinase. Troglitazone (125 and 250 microM) in the absence of insulin, produced a dose-dependent reduction in glucagon-stimulated gluconeogenesis, thereby suggesting an insulinomimetic effect. In addition, troglitazone (125 and 250 microM), in combination with insulin, produced an additive inhibition of gluconeogenesis during glucagon-stimulated conditions. However, unlike insulin, the metabolic mechanism responsible for these effects (in the presence or absence of insulin) does not involve fructose 2,6-bisphosphate.

Troglitazone protects human erythrocytes from oxidant damage

The antidiabetic drug troglitazone contains the active chromanol ring of alpha-tocopherol, which should give it antioxidant properties within cells. In these studies, the antioxidant effects of troglitazone were tested in human erythrocytes and in their ghosts. Troglitazone bound to erythrocyte ghosts in a linear manner and was retained even after centrifugation washes. In response to an oxidant stress generated by a water-soluble free radical initiator, troglitazone that was bound to erythrocyte ghosts was oxidized, but induced a lag-phase in the disappearance of endogenous alpha-tocopherol and in the appearance of lipid hydroperoxides. Troglitazone also delayed loss of endogenous alpha-tocopherol and hemolysis in washed intact erythrocytes in response to free radical-induced extracellular oxidant stress. To mimic exposure of erythrocytes to lipid hydroperoxides in vivo, erythrocytes were incubated with phospholipid liposomes that contained small amounts of preformed lipid hydroperoxides. This induced an oxidant stress in both the liposomes and cells. Troglitazone in concentrations above 4 microM almost completely prevented further appearance of lipid hydroperoxides in the liposomes, and also completely preserved alpha-tocopherol in the erythrocytes. The present results suggest that troglitazone will help to prevent peroxidative damage to erythrocytes in areas of excessive oxidant stress in the vascular bed.

Chronic and acute effects of thiazolidinediones BM13.1258 and BM15.2054 on rat skeletal muscle glucose metabolism

1 New thiazolidinediones BM13.1258 and BM15.2054 were studied with regard to their PPARgamma-agonistic activities and to their acute and chronic effects on glucose metabolism in soleus muscle strips from lean and genetically obese rats. 2 Both BM13.1258 and BM15.2054 revealed to be potent PPARgamma-activators in transient transfection assays in vitro. 3 In insulin-resistant obese rats, but not in lean rats, 10 days of oral treatment with either compound increased the stimulatory effect of insulin on muscle glycogen synthesis to a similar extent (insulin-induced increment in micromol glucose incorporated into glycogen g-1 h-1: control, +1.19+/-0.28; BM13.1258, +2.50+/-0.20; BM15.2054, +2.55+/-0.46; P<0.05 vs control each). 4 In parallel to insulin sensitization, mean glucose oxidation increased insulin-independently in response to BM13.1258 (to 191 and 183% of control in the absence and presence of insulin, respectively; P<0.01 each), which was hardly seen in response to BM15.2054 (to 137 and 124% of control, respectively; ns). 5 Comparable effects on PPARgamma activation and on amelioration of insulin resistance by BM13.1258 and BM15.2054 were therefore opposed by different effects on glucose oxidation. 6 In contrast to chronic oral treatment, acute exposure of muscles to BM13.1258 or BM15.2054 in vitro elicited a distinct catabolic response of glucose metabolism in specimens from both lean and obese rats. 7 The results provide evidence that BM13.1258 and BM15.2054 can affect muscle glucose metabolism via more than one mechanism of action. 8 Further efforts are required to clarify, to what extent other mechanisms besides insulin sensitization via the activation of PPARgamma are involved in the antidiabetic actions of thiazolidinediones.

Binding study of semotiadil and levosemotiadil with alpha(1)-acid glycoprotein using high-performance frontal analysis

High-performance frontal analysis (HPFA) was used to investigate the binding properties of human alpha(1)-acid glycoprotein (AGP) with semotiadil ((R)-isomer, Ca-channel blocker) and its antipode levosemotiadil ((S)-isomer, Ca- and Na-channel blockers). An on-line HPLC system consisting of a HPFA column, an extraction column, and an analytical HPLC column was used to determine the unbound concentrations of these enantiomers, and the experimental data were subsequently subjected to the Scatchard analyses to estimate their binding parameters. The binding affinity of the (R)-isomer (K = 3.17 x 10(7) M, n = 0.74) is approximately 1.2 times stronger than that of (S)-isomer (K = 2.59 x 10(7) M, n = 0.74). An enantioselective competitive binding study indicated that both enantiomers are bound at the same site on AGP molecules.

Thiazolidinediones--the new insulin enhancers

Insulin resistance, characterized by reduced responsiveness to normal circulating levels of insulin, leads to hyperglycemia and hyperinsulinemia resulting in a deadly quartet of non-insulin dependent diabetes mellitus, obesity, hypertension and dyslipidemia These complications, also referred to as 'Syndrome X' have been associated with an increased risk of coronary heart disease. A number of non-pharmacological and pharmacological interventions are available for prevention and treatment of insulin resistance. However, introduction of thiazolidinediones, the new orally active class of drug, has proved to be a major breakthrough in this field. These agents have been shown to reduce insulin resistance by a novel mechanism of action. By interacting with a family of nuclear receptors known as peroxisome proliferator activated receptors thiazolidinediones are thought to enhance the actions of insulin, thereby increasing insulin dependent glucose disposal and reducing hepatic glucose output. A series of animal and clinical studies in patients with impaired Glucose Tolerance and NIDDM have demonstrated the safety and effect of various thiazolidinediones including ciglitazone, pioglitazone and troglitazone. Thus, thiazolidinediones by unlocking insulin resistance act as a key to glycemic control and hence are likely to prove a useful and rational therapy in NIDDM and possibly other disorders resulting from insulin resistance.

Acute troglitazone action in isolated perfused rat liver

1. The thiazolidinedione compound, troglitazone, enhances insulin action and reduces plasma glucose concentrations when administered chronically to type 2 diabetic patients. 2. To analyse to what extent thiazolidinediones interfere with liver function, we examined the acute actions of troglitazone (0.61 and 3.15 microM) on hepatic glucose and lactate fluxes, bile secretion, and portal pressure under basal, insulin- and/or glucagon-stimulated conditions in isolated perfused rat livers. 3. During BSA-free perfusion, high dose troglitazone increased basal (P < 0.01), but inhibited glucagon-stimulated incremental glucose production by approximately 75% (10.0 +/- 2.5 vs control: 40.0 +/- 7.2 micromol g liver(-1), P < 0.01). In parallel, incremental lactate release rose approximately 6 fold (13.1 +/- 5.9 vs control: 2.2 +/- 0.8 mmol g liver(-1), P < 0.05), while bile secretion declined by approximately 67% [0.23 +/- 0.02 vs control: 0.70 +/- 0.05 mg g liver(-1) min(-1)), P < 0.001]. Low dose troglitazone infusion did not enhance the inhibitory effect of insulin on glucagon-stimulated glucose production, but rapidly increased lactate release (P < 0.0005) and portal venous pressure (+0.17 +/- 0.07 vs +0.54 +/- 0.07 cm buffer height, P < 0.0001). 4. These results indicate that troglitazone exerts both insulin-like and non-insulin-like hepatic effects, which are blunted by addition of albumin, possibly due to troglitazone binding.

High-performance frontal analysis for drug-protein binding study

High-performance frontal analysis (HPFA) is a novel analytical method which enables simultaneous determination of total and unbound drug concentrations under drug-plasma protein binding condition. HPFA can be achieved using separation systems such as HPLC and CE. This paper deals with the principle and feature of HPFA method and its application to the stereoselective protein binding study. HPFA allows a simple analysis following direct sample injection, and does not suffer from undesirable drug adsorption on membrane nor leakage of bound drug through the membrane which are often encountered in conventional ultrafiltration and dialysis methods. HPFA can be easily incorporated into on-line HPLC system. By coupling HPFA with a chiral HPLC column, the unbound concentration of a racemic drug can be determined enantioselectively. The detection limit can be improved by coupling of HPFA with a preconcentration column. High-performance capillary electrophoresis/frontal analysis (HPCE/FA) enables to determine unbound concentrations enantioselectively with ultramicro injection volume, and is hence useful especially for the binding study of proteins which are scarce and difficult to obtain.

Column switching in capillary liquid chromatography-tandem mass spectrometry for the quantitation of pg/ml concentrations of the free basic drug tolterodine and its active 5-hydroxymethyl metabolite in microliter volumes of plasma

A capillary column switching system was developed for the determination of low, unbound concentrations of the basic drug tolterodine and its active 5-hydroxymethyl (5-HM) metabolite in human plasma. Free concentrations of tolterodine and 5-HM at pM and nM (pg/ml and ng/ml) levels were obtained by ultrafiltration of 40-400 microliters plasma at 37 degrees C. The free fraction (%) was independent of the plasma concentrations of the analytes. Detection of the analytes was performed by sheathless electrospray tandem mass spectrometry in the multiple-reaction monitoring mode. The selectivity of the mass spectrometric detection and the additional clean-up on the pre-column allowed direct injection of the ultrafiltrated plasma samples. Tolterodine and 5-HM were pre-concentrated on a reversed-phase capillary pre-column (1 cm x 200 microns) and subsequently backflushed onto the separation column (25 cm x 200 microns). The stability of the chromatographic system was good; a large number of ultrafiltrated plasma samples could be injected and the relative standard deviation of the retention times was typically < or = 1% (within-day). The accuracy was between 86 and 105% and the precision was between 1 and 7% without the use of an internal standard. Linear calibration curves were obtained between 100 pM and 100 nM.

Influence of glycosylation on the drug binding of human serum albumin

The influence of glycosylation on the drug binding of human serum albumin (HSA) was studied using HSA containing different amounts and degrees of glycosylated HSA. The drugs used were furosemide, naproxen, procaine, phenylbutazone, salicylic acid, sulphamethoxazole, tolbutamide and warfarin. The drug-HSA parameters (lognK) were measured by the ultrafiltration method, frontal analysis and a modified Hummel-Dreyer method. The modified Hummel-Dreyer method was the simplest method with high precision and required the smallest amounts of proteins. The lognK values were well correlated with the octanol-water partition coefficients; the correlation coefficients were over 0.95. The results suggested that hydrophobic interaction is the predominant force for the drug binding. The early stage of glycosylation of HSA did not significantly affect the drug-binding capacity. Generally, the binding affinity of HSA decreased, perhaps due to a conformational change or steric hindrance (except naproxen) when further glycosylation occurred.

Acute non-insulin-like stimulation of rat muscle glucose metabolism by troglitazone in vitro

1. The direct short-term effects of troglitazone on parameters of glucose metabolism were investigated in rat soleus muscle strips. 2. In muscle strips from Sprague-Dawley rats, troglitazone (3.25 micromol l(-1)) increased basal and insulin-stimulated glucose transport by 24% and 41%, respectively (P<0.01 each). 3. In the presence of 5 nmol l(-1) insulin, stimulation of glucose transport by 3.25 micromol l(-1) troglitazone was accompanied by a 36% decrease in glycogen synthesis, while glycolysis was increased (112% increase in lactate production) suggesting a catabolic response of intracellular glucose handling. 4. Whereas insulin retained its stimulant effect on [3H]-2-deoxy-glucose transport in hypoxia-stimulated muscle (by 44%; c.p.m. mg(-1) h(-1): 852+/-77 vs 1229+/-75, P<0.01), 3.25 micromol l(-1) troglitazone failed to increase glucose transport under hypoxic conditions (789+/-40 vs 815+/-28, NS) suggesting that hypoxia and troglitazone address a similar, non-insulin-like mechanism. 5. No differences between troglitazone and hypoxia were identified in respective interactions with insulin. 6. Troglitazone acutely stimulated muscle glucose metabolism in a hypoxia/contraction-like manner, but it remains to be elucidated whether this contributes to the long-term antidiabetic and insulin enhancing potential in vivo or is to be regarded as an independent pharmacological effect.

Recent advances in chromatographic and electrophoretic methods for the study of drug-protein interactions

Drug-protein binding is an important process in determining the activity and fate of a pharmaceutical agent once it has entered the body. This review examines various chromatographic and electrophoretic methods that have been developed to study such interactions. An overview of each technique is presented along with a discussion of its strengths, weaknesses and potential applications. Formats that are discussed include the use of both soluble and immobilized drugs or proteins, and approaches based on zonal elution, frontal analysis or vacancy peak measurements. Furthermore, examples are provided that illustrate the use of these methods in determining the overall extent of drug-protein binding, in examining the displacement of a drug by other agents and in measuring the equilibrium or rate constants for drug-protein interactions. Examples are also given demonstrating how the same methods, particularly when used in high-performance liquid chromatography or capillary electrophoresis systems, can be employed as rapid screening tools for investigating the binding of different forms of a chiral drug to a protein or the binding of different proteins and peptides to a given pharmaceutical agent.

Thiazolidinediones

The thiazolidinediones are a unique class of compounds that exert direct effects on the mechanisms of insulin resistance and result in improved insulin action and reduced hyperinsulinemia. Troglitazone is the first of these compounds to be approved for use in humans and has the potential not only to reduce glycemia and insulin requirements in type II diabetes but to improve other components of the insulin resistance syndrome including dyslipidemia, hypertension, and accelerated cardiovascular disease. Such compounds also hold promise for the prevention of type II diabetes and for the treatment of other insulin-resistant states including polycystic ovary disease. In addition to the novel mechanism of action through binding and activation of PPARs, troglitazone has other unique advantages, including once-a-day administration, a low incidence of minor side effects, no known drug interactions, hepatic metabolism and secretion, and potent antioxidant properties. Thiazolidinedione compounds such as troglitazone provide an important additional resource for the health care provider in the management of type II diabetes and other components of the insulin resistance syndrome.

Enantioselective protein binding of semotiadil and levosemotiadil determined by high-performance frontal analysis

An on-line frontal analysis HPLC system was developed for the determination of the unbound concentrations of semotiadil, a new calcium antagonist with non-dihydropyridine structure, and its antipode (Levosemotiadil), and was applied to the enantioselective investigation of their plasma protein binding properties. This system consists of a high-performance frontal analysis (HPFA) column, an extraction column, and an analytical column, which are connected via two switching valves. After the direct injection of the sample solution into the HPFA column, the drug was eluted as a zonal peak with a plateau region. The unbound drug concentration was determined as the drug concentration in the plateau. As low as 1.04 nM of the unbound drug was determined with good reproducibility. Semotiadil (R-isomer) and levosemotiadil (S-isomer) are bound strongly and enantioselectively to human serum albumin (HSA) and human alpha 1-acid glycoprotein (AGP), and the enantioselectivity was reversed between these plasma proteins. While HSA binds S-isomer more strongly than the antipode, human AGP binds R-isomer more strongly. In human plasma, the unbound drug fraction was less than 1%, and the enantioselectivity was similar to that observed in AGP solution.

Protein-binding high-performance frontal analysis of (R)- and (S)-warfarin on HSA with and without phenylbutazone

Applicability of high-performance frontal analysis (HPFA) to the stereoselective study of drug-drug interaction upon plasma protein binding has been investigated. Racemic warfarin and phenylbutazone were used as model drugs. An on-line HPFA/HPLC system consisting of a HPFA column (diol-silica column), an extraction column, and a chiral separation column was developed, and human serum albumin solution containing racemic warfarin and/or phenylbutazone was injected directly to the HPFA column. When the injection volume was large enough, the binding equilibrium in the sample solution was reproduced in the column, and consequently a plateau region appeared on the chromatogram. This plateau region contains unbound drug(s). A given volume of eluent in the plateau part was transferred into the extraction column by column-switching. The concentrated drug(s) was then transferred to the chiral separation column to determine the unbound concentrations of the enantiomers and/or the competitor. The results agreed with those obtained by a conventional ultrafiltration-HPLC method. The influence of phenylbutazone upon the protein binding of warfarin is enantioselective. In warfarin and human serum albumin mixed solution, the unbound concentration of (R)-warfarin was 1.22 times higher than that of the S-isomer. By addition of phenylbutazone, the unbound concentration of (S)-warfarin increased more than that of (R)-warfarin, resulting in the reversed enantioselectivity, i.e., the unbound concentration of (S)-warfarin became 1.19 times larger than that of (R)-warfarin. The present method was also applicable to human plasma samples.