Protein Binding Of Quinidine Research Articles

Vitamin D receptor activation induces P-glycoprotein and increases brain efflux of quinidine: an intracerebral microdialysis study in conscious rats.

Since the vitamin D receptor (VDR) was found to up-regulate cerebral P-glycoprotein expression in vitro and in mice, we extend our findings to rats by assessing the effect of rat Vdr activation on brain efflux of quinidine, a P-gp substrate that is eliminated primarily by cytochrome P450 3a. We treated rats with vehicle or the active VDR ligand, 1α,25-dihydroxyvitamin D3 [1,25(OH)2D3] (4.8 or 6.4 nmol/kg i.p. every 2nd day × 4) and examined P-gp expression and cerebral quinidine disposition via microdialysis in control and treatment studies conducted longitudinally in the same rat. The 6.4 nmol/kg 1,25(OH)2D3 dose increased cerebral P-gp expression 1.75-fold whereas hepatic Cyp3a remained unchanged. Although there was no change in systemic clearance elicited by 1,25(OH)2D3, brain extracellular fluid quinidine concentrations were lower in treated rats. We noted that insertion of indwelling catheters increased plasma protein binding of quinidine and serial sampling decreased the blood:plasma concentration ratio, factors that alter distribution ratios in microdialysis studies. After appropriate correction, KECF/P,uu and KECF/B,uu, or ratios of quinidine unbound concentrations in brain extracellular fluid to plasma or blood at steady-state, were more than halved. We demonstrate that VDR activation increases cerebral P-gp expression and delimits brain penetration of P-gp substrates.

Influence of the ORM1 phenotypes on serum unbound concentration and protein binding of quinidine

Background: Only unbound or free drug in plasma can be transported to its site of action. The fraction of unbound drug in plasma varies widely for highly bound drugs among individuals. The genetic polymorphism of orosomucoid (ORM) could be related to the interindividual variability in plasma binding of basic drugs, as ORM is the transport protein for these drugs in plasma. The ORM is a major binding protein in plasma for various basic drugs and is coded by two loci, ORM1 and ORM2, which are closely linked on chromosome 9q31→34.1. ORM1 locus is highly polymorphic and the ORM2 locus is monomorphic in most population. Methods: Twenty-eight healthy volunteers were selected with three ORM1 phenotypes, containing homozygotes ORM1 F1 ( n=10) and ORM1 S ( n=8), and heterozygote ORM1 F1S ( n=10), identified by isoelectric focusing on polyacrylamide gels followed immunoblotting after desialylation of sera. After a single oral dose of quinidine 200 mg, serum total (HPLC) and unbound concentrations in ultrafiltrate (ultrafiltration/HPLC) were determined, and the pharmacokinetic parameters and protein binding rate were calculated. Results: Serum concentrations of ORM (553.8–573.2 mg/l) and albumin proteins (57.5–58.4 mg/l) were similar in the three groups ( P>0.05). Unbound quinidine concentration in ORM1 F1 phenotype subjects was higher than that in ORM1 S and ORM1 F1S phenotype; the free drug percentage for the subjects with ORM1 F1 phenotype (19.79%) was twice as high as that with ORM1 S phenotype (10.96%) ( P<0.01) at 24 h after administration of oral quinidine when the state of disposition equilibrium occurred. The elimination t 1/2 values and the other pharmacokinetic parameters of quinidine were not affected by the different ORM1 phenotypes. Conclusions: Different ORM1 phenotypes may affect the disposition of quinidine, a basic drug, rather than its hepatic metabolism and elimination. The functional heterogeneity of ORM1 could be responsible for the differences in plasma binding of quinidine. Therefore, monitoring of the unbound quinidine concentration would be important for the patients with different ORM1 phenotypes who are treated with quinidine.

Lack of effect of treatment with human recombinant‐tumour necrosis factor (HrTNF) on the binding of quinidine to alpha 1‐acid glycoprotein (AGP).

Tumour necrosis factor (TNF) is known to be a key mediator in the acute phase response and its administration has been shown to cause a five fold increase in serum alpha 1-acid glycoprotein (AGP) concentration in the rat. Since, in man, plasma AGP level determines the protein binding of many important drugs (e.g. narcotic analgesics, phenothiazines, antiarrhythmics, calcium channel blockers) likely to be given to patients who will be treated with TNF, it is important to determine if TNF treatment of humans causes a similar increase in AGP concentration and drug binding. Therefore, the plasma protein binding of quinidine and the serum level of AGP were studied over a 4 day period in each of five cancer patients who were treated with human recombinant-tumour necrosis factor (HrTNF) using a dosage schedule of 6-8 x 10(+5) units/m2 daily for 5 days. It was observed that the quinidine binding ratio (the quotient of bound and free concentration in plasma) was highly correlated with the plasma concentration of AGP (r = 0.818) and that the mean pretreatment AGP concentration in the patients was about three times that found in normal subjects. However, no effects of the TNF treatment regime used in the present study could be demonstrated on either plasma AGP concentration or quinidine free fraction. These observations allow the tentative conclusion that HrTNF does not cause a significant increase in serum AGP level in cancer patients whose baseline AGP concentration is high. However, further study of the relationship between TNF treatment and serum AGP level is needed.

Analysis of nonlinear tissue distribution of quinidine in rats by physiologically based pharmacokinetics.

The nonlinear tissue distribution of quinidine in rats was investigated by a physiologically based pharmacokinetic model. Serum protein binding of quinidine showed a nonlinearity over the in vivo plasma concentration range. The blood-to-plasma concentration ratio (Cb/Cp) of quinidine also showed a concentration dependence. The steady-state volume of distribution (Vss) determined over the plasma concentration range from 0.5 to 10 micrograms/ml was 6.0 +/- 0.45 L/kg. The tissue-to-plasma partition coefficient (Kp) of muscle, skin, liver, lung, and gastrointestinal tract (GI) showed a nonlinearity over the in vivo plasma concentration range of quinidine, suggesting saturable tissue binding. The concentration of quinidine in several tissues and plasma was predicted by a physiologically based pharmacokinetic model using in vitro plasma protein binding and the Cb/Cp of quinidine. The tissue binding parameters were estimated from in vivo Kp values. The predicted concentration curves of quinidine in each tissue and in plasma showed good agreement with the observed values.

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.

Factors Affecting Quinidine Protein Binding in Humans

The free (unbound) concentration of drug in plasma is often an important determinant of pharmacological and toxicological effects. Unfortunately, studies examining the factors influencing the free fraction of quinidine in plasma have yielded inconsistent results. It is probable that differences in the type of blood collection tubes utilized and the analytical procedure employed biased some of these estimates of quinidine binding. The present study was executed in a manner free of factors now known to introduce artifacts into estimates of the free fraction of quinidine. In healthy volunteers, the free fraction of quinidine (1.0 μg/mL) was 0.129 ± 0.019 (mean ± SD) and was constant throughout the therapeutic range. A high-affinity, low-capacity binding site (K = 1.17 × 105 M−1; nP = 3.49 × 10−5 M) and a low-affinity, high-capacity binding site (K = 1.33 × 103 M−1; nP = 3.14 × 10−3 M) were identified. The characteristics of quinidine binding in a 4.5-g/dL solution of human serum albumin (K = 3.05 × 103 M−1; nP = 1.36 × 10−3 M) suggested that the low-affinity, high-capacity binding site was on this protein. In the presence of tris(butoxyethyl) phosphate (75 μg/mL), the quinidine free fraction increased from 0.114 to 0.231. A lidocaine concentration of 250 μg/mL caused a similar increase. Patients suffering traumatic injury had a significant increase in α1-acid glycoprotein concentration (197 mg/dL) and a decreased quinidine free fraction (0.075 ± 0.019). Patients with hyperlipidemia had free fractions similar to those observed in healthy individuals (0.118 ± 0.019). These data suggest that the high-affinity, low-capacity binding site is on α1-acid glycoprotein and that lipoproteins are of little importance in the protein binding of quinidine.

Factors Affecting Quinidine Protein Binding in Man

Factors Affecting Quinidine Protein Binding in Man

Factors Influencing the Apparent Protein Binding of Quinidine

Various factors influencing the apparent protein binding of quinidine were examined. Different binding values in rabbit plasma were obtained by equilibrium dialysis techniques employing three commonly used buffers. Binding values comparable to those found by ultrafiltration were achieved after dialysis against isotonic phosphate buffer for ~ 4 hr. Dialysis beyond 8 hr gave an increased free fraction with time. The reported effect of in vitro added heparin on plasma protein binding could be prevented by reducing the final concentration in blood from 20 to 5 U/ml, a concentration still sufficient to prevent clotting of the blood sample. Daily freezing and thawing of plasma samples over 1 week did not alter the binding of quinidine. The samples were stable for at least 2 months at —20°.

The effect of age and cyanosis on the protein binding of quinidine in the pediatric patient

The effect of age and cyanosis on the protein binding of quinidine in the pediatric patient

Age-Related Differences in the Protein Binding of Quinidine

The in vitro serum protein binding of quinidine was determined in 26 pediatric patients. Group I consisted of 6 cord blood samples obtained at the time of delivery, group II of 8 infants aged 8-18 months, and group III of 12 children over the age of 2 years. The percentage of free quinidine in group I was 39.2 +/- 10.8, group II 24.4 +/- 10.6 and group III 16.6 +/- 6.5, demonstrating a larger proportion of unbound quinidine in the newborn group and an increase in protein binding with age (p less than 0.0001). Therefore, the protein binding of quinidine is diminished in the neonate and young infant and may result in enhanced quinidine activity.

Plasma protein carbamylation and decreased acidic drug protein binding in uremia.

The effects of in vitro carbamylation of plasma with potassium cyanate on drug-protein binding have been investigated. Potassium cyanate added to samples of normal plasma and incubated for 30 to 150 min induced time-related plasma protein carbamylation. Carbamylation of plasma did not influence quinidine protein binding, but resulted in decreased salicylate binding. The increased free fraction of salicylate in plasma correlated with the degree of carbamylation of plasma proteins (r = 0.99; p less than 0.001). Plasma from patients with chronic renal disease showed varying degrees of plasma protein carbamylation, correlating with the values of free plasma salicylate (r = 0.80; p less than 0.05). Scatchard plots for sulfadiazine binding in plasma from patients with uremia and in normal plasma carbamylated in vitro with potassium cyanate showed changes in the 2 groups when compared with those in normal individuals. If cyanate is produced in vivo from urea in patients with uremia, plasma protein carbamylation may play a role in the decreased plasma protein binding of some acidic drugs.

Pharmacokinetic and Clinical Implications of Quinidine Protein Binding

Equilibrium dialysis was used to assess factors influencing quinidine binding to serum proteins. Binding was not influenced by prolonged storage at −20° or by variation in total serum quinidine concentrations over a clinically relevant range. Protein binding decreased with increasing temperature between 24 and 37° (p<0.001) and when total serum protein concentrations were decreased from 6.0 to 2.6g/100ml by dilution with phosphate buffer. The addition of therapeatic concentrations of salicylic acid, phenylbutazone, and tolbutamide significantly reduced quinidine binding (p<0.025), but 10 other commonly coadministered drugs did not. Displacement of quinidine by salicylic acid was nonlinear; the extent of displacement increased with increasing total quinidine concentrations. The unbound quinidine fraction among 12 healthy subjects varied almost twofold and was partly explained by differences in serum albumin concentrations within the usual range of normal (3.9–4.8g/100ml). Pharmacokinetic analysis was performed on simultaneous total and unbound serum quinidine concentrations in 12 volunteers who received single intravenous doses of quinidine lactate. The mean elimination half-lives of total and unbound quinidine were not significantly different. However, the mean volume of distribution and total metabolic clearance of unbound quinidine were considerably greater than those determined using total drug concentrations. Renal clearance of free quinidine exceeded creatinine clearance, consistent with tubular secretion of the unbound fraction. Between-subject variability in elimination half-life and metabolic clearance of quinidine was less when free rather than total serum concentrations were analyzed. Acute ECG changes due to intravenous quinidine correlated better with unbound than with total drug concentrations. Thus, measurement of unbound as well as total serum quinidine concentrations may provide additional understanding of variations between individuals in pharmacokinetics and clinical effects of quinidine.

Blood collection techniques, heparin and quinidine protein binding.

With the use of glass syringes without heparin and all glass equipment, the percent of unbound quinidine was measured by ultrafiltration and a double-extraction assay method after addition of 2 microgram/ml of quinidine sulfate. Compared to the all-glass method, collection of blood using Vacutainers resulted in an erroneous and variable decrease in quinidine binding related to blood to rubber-stopper contact. With glass, the unbound quinidine fraction was (mean +/- standard error) 10 +/- 1% in 10 normal volunteers, 8.5 +/- 1.5% in 10 patients with congestive heart failure, and 11 +/- 2% in 11 patients with chronic renal failure (although in 8 of the latter 11 patients the percent of unbound quinidine was 4 or more standard errors from the mean of the normal group). During cardiac catheterization, patients had markedly elevated unbound quinidine fractions: 24 +/- 2% (p less than 0.001). This abnormality coincided with the addition of heparin in vivo and was less apparent after the addition of up to 10 U/ml of heparin in vitro (120% and 29% increase in unbound quinidine fractions, respectively). Quinidine binding should be measured with all glass or equivalent equipment.

The Effect of Renal Transplantation on Plasma Protein Binding

The in vitro plasma protein binding was determined in nine maintenance hemodialysis patients who later underwent renal transplantation. The organic acid fluorescein (10 micrograms/ml) or the organic base quinidine (5 micrograms/ml) was added to the pre and post transplant serum of these patients. Drug concentrations were measured spectrophotofluorometrically after equilibrium dialysis. The results were compared with the plasma protein binding of eight normal volunteers. The patients on maintenance hemodialysis had lower plasma protein binding of fluorescein than normals (78 +/- 5% vs 89 +/- 4, p less than 0.001). Plasma protein binding improved significantly after renal transplantation (85 +/- 3, p less than 0.01) but was still lower than in normals (p 0.05). Plasma protein binding of quinidine was not significantly different than in normal volunteers (77 +/- 8%) either prior to (72 +/- 10%) or after (73 +/- 12%) kidney transplantation. Plasma protein binding of quinidine remains unaffected by renal transplantation. However, the abnormal plasma protein binding or organic acids in chronic renal failure may be significantly improved by renal transplantation.

Binding of quinidine to plasma proteins in normal subjects and in patients with hyperlipoproteinemias.

Quinidine in the plasma binds to various lipoproteins as well as to albumin. With the use of computer simulations the effect of varying the serum level of each of the lipoproteins as well as albumin was examined. The results suggested that the overall binding of quinidine is relatively insensitive to changes in any one serum protein provided the others are present in normal concentrations. Evaluation of the degree of protein binding of quinidine in plasma from normal subjects and patients with types IV and IIa hyperlipoproteinemias supported the computer predictions. The degree of protein binding in plasma from normal subjects and patients with types IV and IIA hyperlipoproteinemias were 71.2% +/- 11.4%, 75.9% +/- 8.7%, and 69.9% +/- 15.0%, respectively. The quinidine binding in plasma of patients with hyperlipoproteinemias did not differ statistically from that in the normal subjects.

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.

455 The nature of quinidine-protein binding with reference to myocardial effects of quinidine

455 The nature of quinidine-protein binding with reference to myocardial effects of quinidine