The Influence of Protein Binding on the Pharmacokinetics of Sulphadimethoxine in Rabbits

Abstract The pharmacokinetics of sulphadimethoxine was investigated in rabbits at five dose levels. A significant nonlinearity of dose dependent pharmacokinetics was demonstrated. A large dose dependent change in per cent of sulphadimethoxine bound to plasma proteins was shown. It is concluded that the nonlinearity of the pharmacokinetics of sulphadimethoxine is mainly caused by the dose dependent changes in per cent of drug bound to plasma proteins.     

Protein Binding of Cocaine in Human Serum

The protein binding characteristics of cocaine have not been extensively studied. Since cocaine is related to other local anesthetic compounds which are highly protein bound, we examined the binding of cocaine in human serum using an ultrafiltration method. The free fraction averaged 0.083 ± 0.018 in the serum of 12 healthy volunteers. Binding was studied at concentrations ranging from 0.1 to 500 µg/ml and was concentration dependent, with increases being most pronounced at concentrations above 5 µg/ml. Two classes of binding sites were identified with affinity and capacity constants consistent with binding to alpha-1-acid glycoprotein (AAG) and albumin. The addition of AAG to serum resulted in a decrease in the free fraction from 0.079 to 0.041, while tris(butoxyethyl)phosphate increased the free fraction to 0.233. The binding ratio was found to be highly correlated with the AAG concentration (r = 0.89). In addition, the predicted free fraction in the absence of AAG (0.67) was in good agreement with the observed value of 0.647 in a solution of human serum albumin (4.5 g/dl). Of the metabolites of cocaine, only norcocaine displaced the parent drug from serum binding sites. These results indicate that cocaine is highly bound to serum proteins, primarily albumin and AAG. The significance of concentration-dependent binding to cocaine toxicity remains to be established.     

间尼索地平在大鼠体内的组织分布、排泄及血浆蛋白结合率的测定

建立了HPLC法测定生物样品中间尼索地平的含量，并研究了其在大鼠体内的组织分布、排泄及血浆蛋白结合率等特征。以尼莫地平为内标，样品采用液-液萃取。采用C18柱，流动相为乙腈-20mmol／LKH2PO4溶液（60：40），检测波长237nm。间尼索地平在2～1000ng／ml范围内，线性关系良好。间尼索地平在大鼠体内分布广泛，经粪便及胆汁的排泄量均较少，而尿中则未检出药物原型，间尼索地平的血浆蛋白结合率达97％以上。     

Pharmacokinetics of Zopolrestat,a Carboxylic Acid Aldose Reductase Inhibitor,in Normal and Diabetic Rats

The pharmacokinetics of zopolrestat, a carboxylic acid aldose reductase inhibitor, were examined in normal male rats dosed intravenously at 2 mg/kg and in normal and streptozotocin-diabetic male rats after oral administration at 50 mg/kg. After oral dosing, C _max was 127 µg/ml for normal rats and 144 µg/ml for diabetic rats. AUC(0–), however, was lower for diabetic rats than for normal rats and plasma half-life was longer in normal rats (8.0 vs 6.6 hr). Half-lives of zopolrestat in nerve, kidney, and lens were longer than plasma half-life and were similar for both diabetic and normal rats. Less than 2% of the dose was excreted in the urine as unchanged zopolrestat during the 48-hr period following dosing by diabetic or normal rats. Protein binding of zopolrestat was less extensive in plasma from diabetic rats than in plasma from normal rats. Similar kinetics were observed in diabetic animals receiving-five daily doses of zopolrestat at 50 mg/kg/day. There was no plasma or liver accumulation of zopolrestat at steady state, consistent with the observed half-lives. However, zopolrestat did accumulate in nerve, kidney, and lens to varying degrees during multiple dosing, reflecting the longer half-lives of zopolrestat in these tissues.     

Pharmacokinetics of Dietary Phenethyl Isothiocyanate in Rats

Purpose Phenethyl isothiocyanate (PEITC) is a dietary component present in cruciferous vegetables and reported to have chemopreventive properties. Previous reports of PEITC pharmacokinetics have measured total ITC (PEITC and its metabolites) in plasma. Our objective was to examine the dose-dependent pharmacokinetics and oral bioavailability of unchanged PEITC, as well as its pH- and temperature-dependent stability and its serum protein binding. Methods Stability was studied at different pH values at room temperature and 4°C. Protein binding was determined by equilibrium dialysis. For the pharmacokinetics study, male Sprague–Dawley rats were administered with PEITC at doses of 2, 10, 100, or 400 μmol/kg intravenously or 10 or 100 μmol/kg orally. Plasma samples were analyzed by liquid chromatography–tandem mass spectrometry. Pharmacokinetic analysis was conducted by WinNonlin and ADAPT II. Results Phenethyl isothiocyanate was stable in aqueous buffers at pH 7.4 with half-lives of 56.1 and 108 h at room temperature and 4°C, respectively. The free fraction of PEITC in rat serum was 0.019. The clearance (Cl) at a low dose of PEITC (2 μmol/kg) was 0.70 ± 0.17 L h⁻¹ kg⁻¹ with an apparent volume of distribution (V_ss) of 1.94 ± 0.42 L/kg. At higher doses, Cl tended to decrease, whereas V_ss increased. Oral bioavailability of PEITC was 115 and 93% at doses of 10 and 100 μmol/kg, respectively. A three-compartment model with Michaelis–Menten elimination and distribution was found to best characterize the plasma concentration profiles. Conclusions Phenethyl isothiocyanate is stable in biological samples, with increased stability under refrigerated conditions. It has high oral bioavailability, low clearance, and high protein binding in rats; nonlinear elimination and distribution occur following the administration of high doses. This investigation represents the first report of the pharmacokinetics of dietary PEITC.     

Saturable Tissue Binding and Imirestat Pharmacokinetics in Rats

To investigate the hypothesis that the pharmacokinetics of imirestat, an aldose reductase inhibitor, are influenced by saturable binding to tissues, three experiments were done. (1) The nature of the dose dependence was characterized in rats. Two groups of nine adult male Sprague–Dawley rats received iv ¹⁴C-imirestat at doses of 2 or 8 mg/kg. Serial blood samples were obtained over 15 days. Volume of distribution at steady-state was significantly different between the high- and the low-dose groups (0.744 ± 0.103 1 and 1.10 ± 0.228 L, respectively). Clearance was independent of dose over this fourfold range (15 ml/hr). (2) The effect of either statil or AL3152, both aldose reductase inhibitors and potential competitors for aldose reductase binding, on the pharmacokinetics of a single 0.2-mg/kg iv dose of imirestat was assessed. A 2.4-mg/kg loading dose of statil was administered and a constant-rate infusion (56 µg/hr/kg) was begun 16 hr before imirestat. A 2-mg/kg loading dose of AL3152 and a constant-rate infusion (115 µg/kg/hr) were also administered 16 hr before imirestat. The infusions were maintained throughout the study. AL3152 administration decreased the imirestat steady-state volume of distribution by a mean of 63%. Statil administration decreased it by a mean of 39%. (3) The dosing regimen of the second study was repeated and, at two sampling times, nine tissues and plasma were obtained from four rats per sampling time for determination of imirestat tissue-to-plasma concentration ratio. The tissue/ plasma imirestat concentration ratio in the adrenals 24 hr after imirestat administration was 56.9 ± 20.0 in the imirestat group, 17.7 ± 1.27 in the statil-coadministered group, and 12.3 ± 2.59 in the AL3152-coadministered group. A similar trend of decrease in the ratios was observed in all tissues at both 24 and 168 hr. The results suggest that a saturable tissue binding phenomenon at least partially accounts for the nonlinear pharmacokinetics of imirestat.     

Potential Use of Microdialysis in Pharmacokinetics: A Protein Binding Study

Pharmaceutical Research -     

Influence of Lipophilicity on the Protein Binding Affinity of Cephalosporins

Pharmaceutical Research -     

Binding of Digitoxin to Human Serum Proteins: Influence of pH on the Binding of Digitoxin to Human Albumin

Abstract: The binding of digitoxin to albumin was studied under various conditions with regard to electrolytes and pH. Considerable variations of Na, K, Ca and Mg within the range of clinical relevance did not influence the binding of digitoxin to albumin. Under the influence of excessive concentrations of digitoxin this binding followed the simple law of mass action with an average number of binding sites of about 0.5 independent of pH. The intrinsic association constants were about twice the apparent association constants, calculated in another way and with the assumption of one binding site per molecule of albumin. The binding of digitoxin to albumin was shown to depend on the pH of the medium with a maximum association constant at pH = 4.8 (intrinsic association constant, K = 5 x 10⁵ l/mol). The binding of digitoxin to human serum proteins was shown to be equal to that of the binding to albumin. Under conditions relevant for clinical interpretations the binding of digitoxin to other proteins was insignificant.     

Serum Protein Binding of AL01576, a New Aldose Reductase Inhibitor

Pharmaceutical Research -     

奎尼丁对心律失常家兔脂质过氧化物含量的影响

肾上腺素(adrenaline Adr)诱发家兔室性心律失常(VA)后,血浆及组织匀浆中脂质过氧化物(LPO)代谢产物丙二醛(MDA)的含量明显增加。而奎尼丁(quinidine,Qui)6mg/kg iv 能有效地防止 Adr诱发的室性心律失常,并能显著抑制 MDA 的生成,结果提示 Qui 抗 Adr 性心律失常可能与抑制脂质过氧化作用有关。     

Serum Protein Binding of Nonsteroidal Antiinflammatory Drugs: A Comparative Study

The unbound fraction in serum f _u , is a critical parameter in describing and understanding the pharmacokinetics of NSAIDs. We compared f _u for 6 different NSAIDs using ultrafiltration of pooled serum at pH 7.4 and 24C. Measurements covered a wide concentration range in order to define binding affinity and number of binding sites. HPLC was used to measure drug concentrations in serum and ultrafiltrate. Direct injection of ultrafiltrate and serum (diluted 250X) permitted quantitation down to approximately 70 nM for most of the NSAIDs, i.e., approximately 15–20 ng/ml. Assuming binding only to albumin, the data were fitted to a model of two classes of binding sites with dissociation constants K1 and K2. The lowest K1 (highest affinity) was found with flurbiprofen, 0.0658 M, the highest with ketoprofen, 5.23 M, an 80-fold difference. At low drug concentrations, f _u becomes virtually constant and approaches a lower limit, . The following values were calculated: diclofenac 0.21% fenoprofen 0.25%, flurbiprofen 0.022%, ketoprofen 0.52%, naproxen 0.039%, and tolmetin 0.37%. Thus the least bound NSAID, ketoprofen, had a value 24-fold that of the most highly bound, flurbiprofen. The NSAIDs also differed widely with regard to the extent of variation in f _u within the range of therapeutic concentrations, and hence with regard to their potential as displacers of other drugs.     

Unchanged Protein Binding and the Increase of Serum Diazepam Levels after Food Intake

Abstract Seven subjects received diazepam 0.3 mg/kg intravenously twice with a 2-week interval between the doses. The subjects ingested a fatty or carbohydrate meal in a cross-over fashion 4 hours after the injection on both experimental days. Venous blood samples were drawn 2, 3, 4, 5 and 6 hours after the injection of diazepam for measurement of the serum levels of total and free (unbound) diazepam, N-desmethyldiazepam, and free fatty acids. Serum levels of diazepam decreased progressively with time until the food intake, after which a significant (P < 0.01) postprandial increase (average 23 %) occurred with both diets as compared to the preprandial levels at 4 hours (average 240 ng/ml). Serum levels of free fatty acids decreased significantly both after a fatty (P < 0.01) and a carbohydrate (P < 0.05) meal. Diazepam was extensively (96 to 98 %) bound to proteins and no changes in its protein binding was found. It is concluded that the late impairment of psychomotor skills that occurs with an increase in the diazepam serum level after its intravenous administration is due rather to its re-mobilization from a storage site than to variations in its protein binding.     

Dextran Induced Hypoalbuminaemia as a Model for the Study of Influences of Protein Binding on the Pharmacokinetics of Drugs

Abstract A model for examination of the effect of hypoalbuminaemia on drug pharmacokinetics in laboratory animal is described. The model has been used to investigate the effect of protein binding on the kinetics of sulphadimethoxine in rabbits. A greater volume of distribution of sulphadimethoxine is shown in the dextran induced hypoalbuminaemic rabbits in comparison with control animals. It is concluded that the suggested model is convenient for investigation of the influences of protein binding on drug pharmacokinetics.     

Kinetics of Phenytoin in Pregnant and Non-pregnant Rats

Abstract The plasma kinetics of phenytoin were studied in pregnant and nonpregnant rats after a single intravenous dose. The apparent volume of distribution was greater in the pregnant rats resulting in lower plasma concentrations in these rats within the first hour of injection. The beta half-life in pregnant rats (5.0 hours) was prolonged compared to the non-pregnant rats (2.2 hours). The plasma clearance values which take the altered volume of distribution into account, showed a smaller but still significant difference between the two groups of rats. This indicates a lower capacity of the pregnant rats to metabolize the drug.     

Effect of Corticosteroid Binding Globulin on the Pharmacokinetics of Prednisolone in Rats

Purpose. The effect of exogenous corticosteroid binding globulin (CBG) on the pharmacokinetics of intravenous prednisolone was determined in rats to test the free hormone hypothesis. Methods. A dose of CBG to yield 95% binding with 1000 ng/ml of prednisolone in vitro in rat plasma or saline was administered before dosing 2 mg/kg of prednisolone hemisuccinate or methylprednisolone intravenously. Drug concentrations in plasma samples were assayed by HPLC. Results. Single administration of CBG decreased apparent prednisolone clearance by 56% (155 to 66 ml/min/kg) and reduced apparent V_ss by 35% (4.1 to 2.7 L/kg) (p<0.001). Methylprednisolone pharmacokinetics, studied as a negative control because the drug does not bind to CBG, did not change. Conclusions. The corticosteroid bound to CBG does not appear to be available for removal by clearance organs.     

Binding of Digitoxin to Human Serum Albumin: Influence of Ionic Strength and Ionic Medium on the Digitoxin-Albumin Complex Formation

Abstract: The binding of digitoxin to human serum albumin was studied under varying conditions with regard to the ionic strength and the ionic medium. At neutral pH and low ionic strength a substitution of Cl^- by I^- or SCN^- was followed by an increased digitoxin-albumin complex formation. The maximum binding affinity was obtained for Cl^- > 0.4 mol/l, I^- > 0.1 mol/l and SCN^- > 0.05 mol/l. CCl₃COO^- was shown to be a powerful inhibitor of the digitoxin-albumin interaction. At pH = 5 the binding affinity was uninfluenced by a substitution of Cl^- by I^- or SCN^-. Under similar conditions substantial variations of the ionic strength too, did not influence the binding affinity. The interpretation of this binding pattern was discussed with regard to a molecular mechanism for the digitoxin-albumin interaction.     

Effects of Acute and Chronic Inflammation on the Pharmacokinetics of Prednisolone in Rats

The effects of acute and chronic stages of carrageenan-induced air-pouch inflammation on the pharmacokinetics of prednisolone were studied in male Wistar rats. Chronic inflammation produced a significant increase in the area under the curve (AUC) of prednisolone compared to control animals (6594 ± 2144 vs 3530 ± 2164 µg · hr/ L). The effect of acute inflammation was not significant (AUC = 4996 ± 3813). Both acute and chronic inflammation also reduced thein vitro plasma protein binding of prednisolone, the reduction being much greater after chronic inflammation. The AUC of free prednisolone after chronic inflammation was 3141 µg · hr/L, compared to 1121 µg · hr/L in the control group and 1823 µg · hr/L after acute inflammation. The mean values of half-life and apparent volume of distribution at steady-state in each group were similar. These results indicate that prednisolone must be used with caution in the treatment of inflammatory diseases because of higher free concentrations of the steroid.     

羟基红花黄色素A在血瘀和正常大鼠体内药动学研究

目的：研究羟基红花黄色素A（HSYA）在血瘀和正常大鼠体内的药物动力学特征。方法：采用HPLC．法测定血瘀与正常大鼠血浆中HSYA的血药浓度,应用DAS2．0求得药动学参数。结果：血瘀大鼠和正常大鼠Cmax分别为（8．36±1．09）和（4．61±0．19）mg·L^-1;tma分别为（1．19±0．55）和（0．75±0．00）h;AUC06分别为（23．16±2．88）和（8．68±0．93）mg·L^-1·h;t1/2β分别为（1．00±0．30）和（0．98±O．15）h。结论：HSYA在急性血瘀大鼠体内吸收和代谢均明显慢于正常大鼠体内,说明血瘀证可以改变药物的代谢过程。