Stereoselective binding of doxazosin enantiomers to plasma proteins from rats,dogs and humans in vitro

Aim:

(±)Doxazosin is a long-lasting inhibitor of α1-adrenoceptors that is widely used to treat benign prostatic hyperplasia and lower urinary tract symptoms. In this study we investigated the stereoselective binding of doxazosin enantiomers to the plasma proteins of rats, dogs and humans in vitro.

Methods:

Human, dog and rat plasma were prepared. Equilibrium dialysis was used to determine the plasma protein binding of each enantiomer in vitro. Chiral HPLC with fluorescence detection was used to measure the drug concentrations on each side of the dialysis membrane bag.

Results:

Both the enantiomers were highly bound to the plasma proteins of rats, dogs and humans [(−)doxazosin: 89.4%–94.3%; (+)doxazosin: 90.9%–95.4%]. (+)Doxazosin exhibited significantly higher protein binding capacities than (−)doxazosin in all the three species, and the difference in the bound concentration (C_b) between the two enantiomers was enhanced as their concentrations were increased. Although the percentage of the plasma protein binding in the dog plasma was significantly lower than that in the human plasma at 400 and 800 ng/mL, the corrected percentage of plasma protein binding was dog>human>rat.

Conclusion:

(−)Doxazosin and (+)doxazosin show stereoselective plasma protein binding with a significant species difference among rats, dogs and humans.     

Stereoselective,competitive, and nonlinear plasma protein binding of ibuprofen enantiomers as determinedin vivo in healthy subjects

The plasma protein binding and competitive inhibition parameters of R(–)- and S(+)-ibuprofen were determined in vivo in 12 healthy subjects. Subjects participated in a 4×4 Latin square design in which oral solutions of drug were administered as 300 mg R (–)-ibuprofen, 300 mg S (+)-ibuprofen, 300 mg R (–)-+300 mg S (+)-ibuprofen, and 300 mg R(–)-+600 mg S (+)-ibuprofen. Unlabeled ibuprofen enantiomers were quantitated using a stereospecific reversed-phase HPLC assay, and plasma protein binding experiments were performed using radiolabeled¹⁴C-enantiomers and an ultrafiltration method at 37C. At therapeutic drug concentrations, the protein binding of each enantiomer was greater than 99%. Furthermore, the binding of ibuprofen enantiomers was Stereoselective and mutually competitive, as well as nonlinear. The bound-free data were fitted to a model in which the non-linearity of plasma protein binding and competition between enantiomers for binding sites could be accommodated. There were substantial differences in the affinity of ibuprofen enantiomers for protein binding sites (RP2=0.358±0.185 vs. SP2=0.979 ±0.501 g/ml; X±SD) but no differences in their binding capacity (RP1=160±86 vs. SP1=161 ±63 g/ml). Although statistically significant, the differences in competitive inhibition parameters were more modest (SKI=0.661 ±0.363 vs. RKI=0.436 ±0.210 g/ml). As a result, the intrinsic binding (i.e.), P1/P2J of R(–)-ibuprofen was greater than S(±)-ibuprofen, and the unbound fraction was significantly greater for S-enantiomer vs. R-enantiomer after a given dose of R-ibuprofen or racemate.This work was supported in part by a grant from The Upjohn Company, and by grants R01 GM 35498 and MO1 RR00042 from the National Institutes of Health. During the course of this work, J. K. Paliwal was supported by a CONRAD Fellowship.     

Stereoselective binding of chiral drugs to plasma proteins

Chiral drugs show distinct biochemical and pharmacological behaviors in the human body. The binding of chiral drugs to plasma proteins usually exhibits stereoselectivity, which has a far-reaching influence on their pharmacological activities and pharmacokinetic profiles. In this review, the stereoselective binding of chiral drugs to human serum albumin (HSA), α1-acid glycoprotein (AGP) and lipoprotein, three most important proteins in human plasma, are detailed. Furthermore, the application of AGP variants and recombinant fragments of HSA for studying enantiomer binding properties is also discussed. Apart from the stereoselectivity of enantiomer-protein binding, enantiomer-enantiomer interactions that may induce allosteric effects are also described. Additionally, the techniques and methods used to determine drug-protein binding parameters are briefly reviewed.     

Melphalan concentration dependent plasma protein binding in healthy humans and rats

Summary The binding of melphalan to plasma proteins from four healthy humans and from rats was measured by centrifugal ultrafiltration. Melphalan concentrations were determined by HPLC and by measuring ¹⁴C-melphalan activity. In whole blood, melphalan was distributed preferentially in plasma. However, a constant fraction, 37%, which was independent of the total melphalan concentration in whole blood, was present within the red blood cells. The binding of melphalan to plasma proteins from humans was less than that from rats. In both, however, the fraction bound was constant throughout the concentration range (0.1 to 9.0 µM) that is achieved during standard-dose melphalan therapy. Albumin was the primary binding protein. At concentrations equal to or in excess of 33 µM, which have been achieved during high-dose melphalan therapy, free plasma melphalan concentrations were no longer linearly related to total drug concentrations, and the plasma protein binding of melphalan in the human became concentration dependent. This occurred at concentrations of 70 µM in the rat. Scatchard analysis of the data indicated the presence of 2 groups of binding sites. Class I sites had 0.03 and 0.4 binding sites per albumin molecule in humans and rats, with respective association constants of 4.43 × 10⁴M^–1 and 1.92 × 10⁴M^–1. Class II sites had 5.18 and 2.60 binding sites per molecule, with repective association constants of 3.82 × 10²M^–1 and 2.01 × 10²M^–1.     

高效液相色谱法测定布洛芬与人血浆蛋白的结合率

目的:建立人血浆中布洛芬浓度的测定方法,测定布洛芬在人血浆中的蛋白结合率,并计算相关参数。方法:用平衡透析法以高效液相色谱法为检测手段测定血浆蛋白结合率。结果:布洛芬与人标准血浆的蛋白结合率为(95.4±0.3)%。结论:布洛芬与人血浆蛋白为高强度结合。     

银杏内酯B在血浆中的蛋白结合率研究

目的测定银杏内酯B(ginkgolide B,GB)与大鼠和人的血浆蛋白结合率。方法将血浆样品加入内标23-羟基白桦酸,乙醚液液萃取后,高效液相色谱-质谱联用仪分析,采用平衡透析法测定GB在大鼠及人血浆中的血浆蛋白结合率。结果人和大鼠血浆中GB药物浓度在0.05～5.00 mg.L-1范围内血浆蛋白结合呈线性关系;GB与大鼠血浆蛋白结合率约为25%,与人血浆蛋白结合率约为20%。结论 GB的血浆蛋白结合率较低,在人血浆的蛋白结合率略低于大鼠,但不存在显著性差异;约80%的药物以游离态形式存在。     

平衡透析法测定具栖冬青苷和毛冬青酸的大鼠血浆蛋白结合率

目的建立测定具栖冬青苷、毛冬青酸血药质量浓度的方法,并测定其大鼠体外血浆蛋白结合率。方法采用高效液相色谱法和平衡透析法测定具栖冬青苷、毛冬青酸在大鼠体外血浆中的血浆蛋白结合率。结果低、中、高3种质量浓度,具栖冬青苷在大鼠体外血浆中的蛋白结合率分别为39.72%±2.68%,52.05%±3.35%和52.32%±0.76%;毛冬青酸在大鼠体外血浆中的蛋白结合率分别为55.18%±3.66%,60.79%±2.20%和47.00%±1.59%。结论建立HPLC法对具栖冬青苷和毛冬青酸进行分离,方法简便。体外实验,具栖冬青苷和毛冬青酸与大鼠血浆属中等结合型药物,且蛋白结合率与药物质量浓度无关。     

高效液相色谱法测定间尼索地平不同对映体在血浆中的蛋白结合率

目的:建立间尼索地平不同对映体在大鼠血浆、人血浆和牛血清白蛋白中蛋白结合率的测定方法,并计算不同种属血浆蛋白的相关参数。方法:采用平衡透析法测定蛋白结合率,用高效液相色谱法测定血浆中药物总浓度及游离药物浓度。结果:R-与S-间尼索地平的血浆蛋白结合率分别为:大鼠血浆为(97.4±8.2)%、(93.0±13.4)%;人血浆为(90.8±10.5)%、(89.2±9.9)%;牛血清白蛋白为(95.3±8.7)%、(91.0±5.7)%。最低定量下限为0.01ng。结论:在体外间尼索地平不同对映体与大鼠血浆、人血浆和牛血清白蛋白结合率很高,R-间尼索地平的结合率及蛋白结合药物的表观最大能力βp均较S-间尼索地平高。随着药物血浆浓度升高,R-间尼索地平结合率下降,S-间尼索地平结合率升高,均有一定的浓度依赖性。     

TJ0711盐酸盐的血浆蛋白结合率及其在大鼠体内的组织分布

目的:建立生物样品中TJ0711盐酸盐的浓度的测定方法,测定TJ0711盐酸盐在人标准血浆和大鼠血浆中的蛋白结合率,考察其在大鼠组织中的分布特性。方法:用平衡透析法测定血浆蛋白结合率,用高效液相-荧光检测法测定血浆中药物总浓度及游离药物浓度。大鼠灌胃给予TJ0711盐酸盐30 mg.kg-1,测定10,25,60,180 min TJ0711盐酸盐在各组织中的含量。结果:当TJ0711盐酸盐的血药浓度为0.5,2.0,4.0,8.0μg.mL-1时,其在大鼠血浆中的蛋白结合率分别为(88.7±2.3)%,(87.8±1.0)%,(85.8±2.0)%和(86.8±0.9)%,在人血浆中的蛋白结合率分别为(92.1±0.8)%,(89.8±1.4)%,(90.4±1.0)%和(90.4±1.7)%。药物在大鼠体内以肾、肝、肺中分布较多,脑和脂肪组织也能检测到药物,绝大多数组织的药物含量在给药后10 min或25 min最高1,80 min后药物含量显著下降。结论:TJ0711盐酸盐在人和大鼠血浆中的蛋白结合率均属于高度结合(>80%),且不随药物浓度的增加发生变化,但在人和大鼠血浆中的蛋白结合率差异具有显著性(...     

美沙拉嗪PEG化前药在不同种属血浆中蛋白结合率的测定

目的:测定美沙拉嗪PEG化前药在不同种属血浆中的蛋白结合率,考察PEG化对药物蛋白结合率的影响。方法:采用血浆平衡透析法,以紫外分光光度法对透析内液与外液中的药物含量进行测定,计算药物与小鼠血浆、犬血浆、兔血浆及牛血浆的蛋白结合率。结果:美沙拉嗪PEG化物的低、中、高(1.08,1.35,1.62 mg·mL^-1)3种质量浓度的小鼠血浆蛋白结合率分别为(22.31±5.28)%,(17.15±3.21)%,(16.11±4.55)%;犬血浆为(25.16±5.87)%,(22.47±4.14)%,(23.14±3.46)%;兔血浆为(17.13±8.26)%,(13.87±6.32)%,(15.93±2.31)%;牛血浆为(14.41±6.21)%,(18.84±7.80)%,(14.18±5.23)%。结论:美沙拉嗪PEG化物血浆蛋白结合程度较低,种属间存在一定差异。与原型药物相比,药物经PEG化后,血浆蛋白结合呈下降趋势。     

The use of α-adrenoceptor antagonists in lower urinary tract disease

α-adrenoceptor antagonists have traditionally been used in the treatment of hypertension but in recent years they have become increasingly common in the treatment of benign prostatic enlargement (BPE), where they reduce the ‘dynamic’ component of bladder outlet obstruction and appear to have additional actions to reduce irritative symptoms of the disease. Prazosin (Hypovase^®, Alza), doxazosin (Cardura^®, Pfizer), indoramin (Doralese^®, Wyeth-Ayerst Pharmaceuticals Inc.) and terazosin (Hytrin^®, Abbott Laboratories) are currently available in the UK for BPE but these agents have cardiovascular actions in a significant number of patients, inducing effects which must be considered adverse unless the patient also requires treatment for mild-to-moderate hypertension. The uroselective α-adrenoceptor antagonists tamsulosin (Flomax^®, Yamanouchi Pharmaceutical Co. Ltd.) and alfuzosin (Xatral^®, Sanofi-Synthelabo) have recently been introduced. These agents exert their selectivity via different mechanisms; selective tissue distribution for alfuzosin and α-adrenoceptor subtype selectivity for tamsulosin. The incidence of cardiovascular side effects for both drugs is similar to placebo. Several lines of evidence suggest that the α-adrenoceptor antagonists may relieve lower urinary tract (LUT) symptoms by other mechanisms additional to those which account for the reduction in bladder outlet obstruction. If correct, these agents may be of use in the treatment of other bladder conditions.     

Concentration‐dependent plasma protein binding of the novel dipeptidyl peptidase 4 inhibitor BI 1356 due to saturable binding to its target in plasma of mice,rats and humans

Objectives The purpose of this study was to characterise the plasma protein binding of BI 1356. Methods BI 1356 (proposed trade name ONDERO) is a novel dipeptidyl peptidase 4 (DPP‐4) inhibitor, which is under clinical development for the treatment of type 2 diabetes. DPP‐4 is expressed in various tissues but soluble DPP‐4 is also present in plasma. Therefore, binding to soluble DPP‐4 may influence the pharmacokinetics of BI 1356. Plasma protein binding of BI 1356 was determined in vitro for wild type mice and rats and the results compared with those for DPP‐4 knockout mice and DPP‐4 deficient Fischer rats. In addition, protein binding of BI 1356 was examined in plasma from healthy human volunteers and renal excretion of the compound in the DPP‐4 knockout mice was compared with that occurring in wild type mice. Key findings The results showed that BI 1356 exhibited a prominent concentration‐dependent plasma protein binding due to a saturable high affinity binding to the DPP‐4 target in plasma. Differences in renal excretion of BI 1356 between DPP‐4 knockout mice and wild type mice suggested that saturable binding of BI 1356 to DPP‐4 in the body also influenced elimination. Conclusions High affinity, but readily saturable binding of BI 1356 to its target DPP‐4 accounted primarily for the concentration‐dependent plasma protein binding at therapeutic plasma concentrations of BI 1356.     

Protein binding of warfarin enantiomers in serum of humans and rats

The protein binding of racemic ¹⁴ C-warfarin and ³ H-S(-)-warfarin was determined in individual undiluted serum samples from 31 human subjects and 11 rats. The free fraction value of the R(+) enantiomer was determined indirectly from the free fraction values of the S(–) enantiomer and racemic warfarin, a method which was verified by direct determination of the free fraction of R(+)-warfarin with the unlabeled compound at higher concentrations. The ratio (mean± sd)of free fraction values, R(+):S(–), was 1.32±0.256in man and 1.56±0.351 in the rat;the difference between species is statistically significant (p<0.025).This ratio was independent of the free fraction value of either enantiomer even though there were pronounced intersubject differences in serum protein binding of the anticoagulant. S(–)-Warfarin is eliminated more rapidly than the R(+) enantiomer in man, while the opposite occurs in rats. The results of this study rule out a species difference in serum protein binding as a factor contributing to the difference in elimination kinetics but help to explain the strong correlation in the elimination kinetics of the two enantiomers in individual rats.Supported by Grant GM 20852 from the National Institute of General Medical Sciences of the National Institutes of Health.This is paper XXVI in the series Comparative Pharmacokinetics of Coumarin Anticoagulants. Previous paper: J. T. Slattery, A. Yacobi, and G. Levy,J. Pharm. Sci. (in press).     

Pharmacokinetics of quinidine related to plasma protein binding in man

Summary The disposition and plasma protein binding of quinidine after intravenous administration were studied in 13 healthy subjects. Plasma protein binding, expressed as the fraction of quinidine unbound ranged from 0.134–0.303 (mean 0.221). Elimination rate constant () varied from 0.071 to 0.146 h^–1 (mean 0.113), and apparent volume of distribution (V) varied from 1.39–3.20 l · kg^–1 (mean 2.27). Total body clearance was 2.32–6.49 ml min^–1 · kg^–1. There was a positive linear correlation between the plasma fraction of unbound quinidine and both V (r=0.885, p<0.01) and total body clearance (r=0.668, p<0.05). No significant correlation existed between the fraction of unbound quinidine in plasma and the elimination rate constant. The results show that both the apparent volume of distribution and total body clearance of quinidine are proportional to the unbound fraction in plasma. This implies that the total plasma concentration of quinidine at steady state will change with alterations in plasma binding, whilst the concentration of unbound compund and its elimination rate will remain unaffected.     

Pharmacokinetics of reboxetine in healthy volunteers. Single oral doses,linearity and plasma protein binding

The pharmacokinetics of reboxetine, a new antidepressant agent, were found to be close to linear in a crossover study comparing administration of single 2, 3, 4 and 5 mg capsule doses in 15 healthy male volunteers, and in the same study the capsules were bioequivalent to the proposed therapeutic tablet formulation (4mg). Kinetic analysis was based on HPLC assay of reboxetine in plasma and urine collected up to 72 h after each administration. Plasma levels indicated a rapid absorption (t_max?2h) and an elimination half-life of about 13 h. Clearance and volume of distribution were modest (ratios to bioavailability: CL/F?29 mL min^?1; V_z/F?32L); urinary excretion was ～9% of dose, corresponding to a renal clearance of only 3 mL min^?1 (a value consistent with the rate of glomerular filtration of unbound drug). In vitro, binding to plasma proteins, estimated from radioactivity levels following dialysis of ¹⁴C-labelled reboxetine, appeared to be dominated by α₁-acid glycoprotein without marked saturation up to plasma concentrations of over 500 ng mL^?1 (2.8–3.1% unbound with human plasma from three additional volunteers; 1.8–2.0% for 2gL^?1 orosomucoid α₁-acid glycoprotein, and 46.4–47.4% for 40 gL^?1 albumin), whilst the mean C_max in the current study was much lower (164 ng mL^?1 after a 5 mg dose).     

The influence of free fatty acids on valproic acid plasma protein binding during fasting in normal humans

Summary The effect of physiologic variations of free fatty acid levels on in vivo valproic acid plasma protein binding was studied in 6 healthy adult subjects. 14 blood samples were taken during a 12-h dosing interval at steady state while in a fed condition and also during a 27 h fast. Free fraction and total valproate concentration were determined by equilibrium dialysis and GLC, respectively. Free fatty acid levels were determined from both fresh samples and samples incubated at 37°C for 12 h, the latter in order to simulate equilibrium dialysis conditions. Fasting resulted in increased serum free fatty acid levels in all subjects, ranging from 34–182% (p<0.01). Incubation also caused free fatty acid levels to rise, more so in fed samples (50–87%,p<0.01) than in fasting samples (10–50%,p<0.01). Fasting resulted in a 9% increase in the mean free fraction for all subjects combined (p<0.01). Regression analysis of 180 sets of values for free fraction, total valproate concentration and free fatty acid level suggested that valproate concentration accounts for 17% and free fatty acid level for 37% of the variation in free fraction. Mean clearance was unchanged by fasting despite an increased free fraction suggesting decreased intrinsic clearance (i.e. decreased metabolism) of valproate under these conditions.     

齐墩果酸在人血浆蛋白和血清白蛋白中结合率的测定

齐墩果酸(oleanolic acid,OA)又名庆四素(图1),系五环三萜类化合物,以游离或结合成苷的形式广泛存在于白花蛇舌草、山楂、丁香、大枣、女贞子、枇杷叶、橡木和夏枯草等植物中。临床上主要用于急     

二-(2, 4-二氯苯甲酰异羟肟酸) 二苯基合锡大鼠血浆蛋白结合率的测定

目的 建立二-(2, 4-二氯苯甲酰异羟肟酸) 二苯基合锡（DPDCT）在大鼠血浆中蛋白结合率的测定方法。方法 采用HPLC法测定大鼠血浆中药物总浓度及游离药物浓度,结合平衡透析法测定血浆蛋白结合率。结果 DPDCT在质量浓度为0.2、1.0、5.0 μg/mL时与大鼠的血浆蛋白结合率分别为（58.9±2.0）%、（57.4±1.2）%和（58.6±0.8）%。不同浓度组血浆蛋白结合率差异无统计学意义。结论 该方法灵敏度高,重复性好,操作简单,能够满足生物样品分析的要求。DPDCT具有中等强度的血浆蛋白结合率。     

Assessment of in vitro metabolic stability,plasma protein binding,and pharmacokinetics of E‐ and Z‐guggulsterone in rat

Guggulsterone is a racemic mixture of two stereoisomers (E‐ and Z‐), obtained from the gum resin of Commiphora mukul and it is marketed as an antihyperlipidemic drug. The aim of our study was to assess the in vitro and in vivo absorption, distribution, metabolism, and excretion (ADME) properties namely solubility, in vitro metabolism, plasma protein binding and oral pharmacokinetic studies of E‐ and Z‐guggulsterone. In vitro metabolism experiments were performed by using rat liver and intestinal microsomes. In vitro intrinsic clearance (CL_int) was found to be 33.34 ± 0.51 and 39.23 ± 8.12 μL/min/mg protein in rat liver microsomes for E‐ and Z‐isomers, respectively. Plasma protein binding was determined by equilibrium dialysis method and in vivo pharmacokinetic studies were performed in male Sprague Dawley (SD) rats. Both isomers were highly bound to rat plasma proteins (>95% bound). Plasma concentration of E‐ and Z‐isomers decreased rapidly following oral administration and were eliminated from systemic circulation with a terminal half‐life of 0.63 ± 0.25 and 0.74 ± 0.35 h, respectively. The clearance (CL) for E‐isomer was 2.79 ± 0.73 compared to 3.01 ± 0.61 L/h/kg for Z‐isomer, indicating no significant difference (student t test; p <0.05) in their elimination.The pharmacokinetics of both isomers was characterized by extensive hepatic metabolism as seen with rat liver microsomes with high clearance and low systemic availability in rats. In brief, first‐pass metabolism seems to be responsible factor for low bioavailability of guggulsterone. Copyright © 2015 John Wiley & Sons, Ltd.     

Apparent volumes of distribution and drug binding to plasma proteins and tissues

Summary A pharmacokinetic model that incorporates linear binding of drug to plasma proteins and tissue indicates the same relationship between apparent volume of distribution and drug binding as that proposed by Gillette (1971) based on a simple distribution model. Apparent volume of distribution (V) is directly proportional to free fraction of drug in plasma (f_p) and indirectly proportional to free fraction of drug in tissue (f_T). In the case of a constant f_T, a plot of V versus f_p will be linear with an intercept equal to plasma volume (V_p). If f_T changes with f_p, an apparently linear plot may result but the intercept will exceed V_p. An approach to the calculation of f_T, a composite binding parameter, is presented and illustrated by comparing the tissue binding of tolbutamide in patients during acute viral hepatitis and upon recovery.Supported in part by Grant GM-20852 from the National Institutes of Health.