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

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

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

Summary The binding of certain drugs to isolated fractions of plasma proteins obtained from newborn and adult man has been studied by equilibrium dialysis. For thiopental, desipramine, nitrofurantoin, sulfamethoxydiazine, meticillin and salicylic acid no difference was found between binding to the albumin fraction from newborns and adults. However, for thiopental, desipramine and promethazine binding to the globulin fraction was smaller in the newborns than in adults. Addition of bilirubin to the albumin fraction decreased the binding of nitrofurantoin, sulfamethoxydiazine and meticillin. No difference in the binding of meticillin to the albumin or globulin fractions from newborns and adults was found. The binding decreased, however, if both fractions were combined. Four mechanisms to explain the difference in binding between newborns and adults are discussed: (1) Displacement of drugs by bilirubin, (2) different binding properties of cord and adult albumin, (3) different properties of the globulins and (4) interaction of albumin with globulins in the newborn.This study was partially supported by grant Ku 156/4 of the Deutsche Forschungsgemeinschaft     

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.     

The effect of fever on quinine and quinidine disposition in the rat.

The pharmacokinetics of quinine and its diastereoisomer quinidine has been investigated in normal and febrile rats. Endotoxin-induced fever in rats resulted in an increased quinine clearance (CL) (4.49 +/- 1.45 vs 1.38 +/- 0.65 L h-1 kg-1, P less than 0.001) and volume of distribution (Vd) (42.6 +/- 8.8 vs 28.9 +/- 10.3 L kg-1, P less than 0.05) with a concomitant shortening of the elimination half-life (t1/2) (7.1 +/- 2.5 vs 15.9 +/- 5.9 h, P less than 0.01). With quinidine, however, fever resulted in an increased CL (3.95 +/- 1.05 vs 1.89 +/- 0.60 L h-1 kg-1, P less than 0.002) with no change in Vd and a significant decrease in t1/2 (5.1 +/- 0.7 vs 10.1 +/- 2.8 h, P less than 0.001). In both studies there was no significant difference in hepatic microsomal protein or cytochrome P450 content. Neither drug accumulated in the liver but low concentrations of quinidine were present in the heart 24 h after administration. In-vitro studies suggest that temperature does not alter the binding of either drug. These data suggest that fever enhances the clearance of quinine and quinidine. These findings may offer some additional explanation of the lack of serious quinine and quinidine toxicity during the treatment of malaria infection, even after large dosages of the drug administered during the initial period of treatment when fever is most intense.     

Disposition of the diastereoisomers quinine and quinidine in the ovine fetus

The disposition of the diastereoisomers quinine and quinidine was investigated in the near-term pregnant ewe. Five sheep were administered quinine and quinidine separately in random order by a combination of bolus and 30-h iv infusion. On a subsequent occasion, four of the five sheep were also administered the two drugs simultaneously. After separate dosage, systemic clearance of quinine tended to be greater than that of quinidine (714 +/- 299 versus 422 +/- 146 mL/min, p = 0.08). Maternal renal clearance exhibited no stereoselectivity and represented less than 2% of total clearance. Simultaneous administration did not alter the disposition of either drug in the mother. After separate dosage, fetal total concentrations (Cf) of quinine and quinidine were substantially lower than maternal total concentrations, as reflected in Cf:Cm ratios of 0.15 +/- 0.06 versus 0.10 +/- 0.08, respectively. Similarly, fetal unbound concentrations (Cfu) were substantially lower than maternal unbound concentrations (Cmu; Cfu/Cmu = 0.46 +/- 0.09 for quinine and 0.23 +/- 0.09 for quinidine). This indicates the presence of fetal elimination of both isomers. Fetal renal clearances of quinine and quinidine were similar (0.34 +/- 0.24 mL/min versus 0.38 +/- 0.24 mL/min) and less than that of endogenous creatinine, indicating the absence of net renal tubular secretion. After simultaneous dosage of quinine and quinidine, Cf:Cm (0.48 +/- 0.24 and 0.31 +/- 0.19, respectively) and Cfu:Cmu (0.73 +/- 0.14 and 0.52 +/- 0.20, respectively) were greater than for separate dosages. Fetal renal clearance of both drugs was unchanged, suggesting that the higher Cfu:Cmu ratios after simultaneous dosage were due to mutual inhibition of the fetal metabolism of these drugs.     

The binding of aurothiomalate to plasma proteins in vitro.

The physicochemical factors responsible for the aurothiomalate-albumin interaction were studied by a thermodynamic analysis of the binding of the anion to bovine albumin. The binding process was entropically driven and it was concluded that electrostatic bonding formed the basis of the aurothiomalate-albumin interaction. The binding of aurothiomalate to human plasma proteins at 37 degrees C and pH 7.45 and the modification of its binding by indomethacin and phenylbutazone was studied by ultrafiltration. Aurothiomalate was bound to human plasma albumin at a single site with an affinity constant of 6.1 X 10(3) M-1 and also at several sites of lower affinity. The plasma protein binding of the anion was increased in the presence of indomethacin and phenylbutazone. At therapeutic concentrations in vitro phenylbutazone significantly increased the plasma protein binding of aurothiomalate.     

Methotrexate binding to human plasma proteins

The percentage of methotrexate (MTX) binding to plasma protein was 50.4 +/- 1.9 in healthy subject, and 32.3 +/- 3.6 in patients with cancer (breast carcinoma in most cases). Drugs used in combination with MTX in therapy (vincristine, vinblastine, cyclophosphamide, 5-fluorouracil, prednisone) depress the MTX binding to albumins. MTX binds to two kinds of albumins binding sites, with different binding constants: K1 = 8.88 mM-1 and K2 = 1.76 mM-1.     

Comparative effects of quinine and quinidine on glucose metabolism in healthy volunteers.

1. To investigate the relative effects of quinine and quinidine on glucose metabolism, 11 healthy males aged 17-32 years were given three separate 1 h intravenous infusions; normal saline alone, quinine dihydrochloride 10 mg base kg-1 body weight (BW) in normal saline, and quinidine dihydrochloride 10 mg base kg-1 BW in normal saline. A constant infusion of 5 mg glucose kg-1 ideal BW min-1 was given for 1 h before and during each study. 2. Assessment of pancreatic beta cell function and tissue insulin sensitivity from plasma glucose and insulin concentrations at the end of the first hour using the Continuous Infusion of Glucose with Model Assessment (CIGMA) technique confirmed normal glucose tolerance for each subject on each test day. 3. Plasma glucose concentrations at 1 h were similar to those at 2 h. There was no significant difference between the plasma glucose profiles during the three infusion regimes (P greater than 0.05). Plasma insulin rose significantly during the second hour (P less than 0.0001); increments after quinine (geometric mean [-1 s.d- +1 s.d.]; 47.0 [27.8-79.4] mu l-1) were significantly greater than those after quinidine (19.8 [6.1-65.2] mu l-1) and saline (7.5 [0-21.5] mu l-1; P less than 0.05). Plasma quinine concentrations at the end of the infusion (6.5 +/- 4.4 mg l-1) correlated with insulin increments during the second hour (r = 0.662, P = 0.028) and were significantly greater than those of quinidine (3.0 +/- 0.8 mg l-1; P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Nonaqueous titration of sulfates of quinine and quinidine using barium acetate.

A nonaqueous titrimetric method is proposed for determining the diastereomeric sulfates of quinine and quinidine. The sulfuric acid content of the alkaloid salts is precipitated, in the form of barium sulfate, with acetous barium acetate solution before the liberated alkaloid is titrated; the necessary calculations are provided. A favorable characteristic of the proposed procedure is the accuracy, speed, and ease of performance. The mean percent recoveries (p = 0.05) obtained with the proposed method for the sulfates of quinine and quinidine were 98.84 +/- 1.00 and 99.74 +/- 1.27, respectively, compared with 100.73 +/- 1.44 and 100.82 +/- 1.16, respectively, when the BP 1968 procedure was applied.     

The binding of flurbiprofen to plasma proteins

The binding of flurbiprofen to human serum albumin and normal plasma was measured using ultracentrifugation. Flurbiprofen was bound in excess of 99% in all cases. Over the concentration range normally associated with therapy the data could be adequately described by a one-site binding model. The association constant decreased with increasing protein concentration, being 3.04 X 10(6) M-1 at an albumin concentration, of 2 g/100 ml and 1.19 X 10(6) M-1 at a concentration of 4 g/100 ml. Flurbiprofen binding showed no dependence on pH over the range 7.0 to 8.0.     

Comparative binding of etretinate and acitretin to plasma proteins and erythrocytes.

The interactions of etretinate and its main metabolite acitretin with human plasma proteins have been investigated in vitro by an erythrocyte partitioning technique that allows a quantitative estimation of the plasma and erythrocyte binding. Etretinate was extensively lipoprotein-bound (75% of plasma etretinate), with a binding constant for its main low density lipoprotein carrier of 40 x 10(6) M-1, accounting for 48% of the total plasma-bound drug. Acitretin was mainly albumin-bound (91% of plasma acitretin), with a binding constant of 0.7 x 10(6) M-1. The total plasma binding of both drugs was > 99% and, in blood, the fractions associated with erythrocytes were 14.5 and 8.1% of the total amount for etretinate and acitretin, respectively.     

Induction of quinidine metabolism and plasma protein binding by phenobarbital in dogs

Two porta-caval transposed mongrel dogs were studied for phenobarbital (PB) induction of quinidine disposition after separate quinidine infusions via normal intravenous route and via portal vein. The plasma concentrations of quinidine and of three metabolites measured (3-OH quinidine, quinidine N-oxide, quinidine 10,11-dihydrodiol) were quite similar between i.v. and portal vein infusions, suggesting that the liver extraction ratio for quinidine in dogs is very low. After PB pretreatment plasma quinidine concentrations at the end of a 10 hr infusion increased about twofold while the half-life decreased from a control value of about 16 hr to 6 hr. Plasma concentrations of the three major metabolites measured were also increased following PB treatment. Plasma protein binding for quinidine and two of its three measured metabolites (3-hydroxy quinidine and quinidine N-oxide) were increased after PB treatment. Pharmacokinetic analysis of the data showed a decrease in steady-state volume of distribution (Vdss) of quinidine from an average value of 153 L to 54 L after PB treatment, while the total clearance did not change (6.6 vs. 5.6 L/hr). This decrease in Vdss could be explained by an increase in plasma protein binding of quinidine after PB treatment. The unbound nonrenal clearance of quinidine was induced by PB treatment. The decrease in fraction free in plasma and increase in unbound nonrenal (hence total) clearance resulted in little or no change in total plasma clearance for quinidine. The formation rate constants calculated for two quinidine metabolites, 3-hydroxy quinidine and quinidine N-oxide, were increased after PB treatment, suggesting an induction in these two metabolic pathways. Only quinidine 10,11-dihydrodiol was found in the bile after quinidine infusion, and the biliary clearance of this metabolite was also induced after PB treatment.     

The binding of the antimalarial arteether to human plasma proteins in-vitro.

The binding of the novel antimalarial drug, arteether, to human plasma, pure albumin and alpha 1-acid glycoprotein has been investigated by ultrafiltration, using [14C]arteether. The protein binding in plasma obtained from 11 healthy male subjects ranged from 73.4 to 81.8% bound, with a mean of 78.7 +/- 2.1%. The binding of drug in plasma was mainly accounted for by binding to albumin and alpha 1-acid glycoprotein. Scatchard analysis of the binding data revealed that the binding affinity of arteether to alpha 1-acid glycoprotein is much greater (20-fold) than that to albumin. This suggests that alpha 1-acid glycoprotein is the more important binding protein in plasma. This may have clinical importance due to alterations in plasma protein binding in patients with malaria, as the concentration of alpha 1-acid glycoprotein is markedly increased during malarial infection.     

Increased plasma binding and decreased blood cell binding of quinidine in blood from anuric rats

The blood cell/plasma concentration ratio of quinidine, as influenced by the plasma protein binding, was studied in normal and anuric rats by applying incubation and equilibrium dialysis techniques on blood and plasma, respectively, from normal and anuric rats. The plasma protein binding of quinidine in anuria was increased at concentrations of unbound drug of less than 1.75 X 10(-4) M and decreased above this concentration. At an assumed "therapeutic" quinidine concentration (1 X 10(-5) M), the mean concentration ratio (total quinidine in blood cells)/(total quinidine in plasma) was 1.84 in normals and 0.46 in anuria, and the mean ratio (total quinidine in blood cells)/(unbound in plasma) was 4.45 and 1.81, respectively. As the latter ratios were concentration dependent and greater than could be accounted for by pH-dependent distribution, quinidine is presumably bound in/on the blood cells. Reduced distribution ratio in anuria, even when related to unbound quinidine in plasma, also indicates changed binding in blood cells, a finding confirmed by applying the data to modified Scatchard plot. this may have implication for the use of blood cell/plasma concentration ratio as screening procedure for the altered plasma binding of quinidine in patients.