Quinidine and dihydroquinidine interactions in human plasma

Entrapment of alpha1-acid glycoprotein in high-performance affinity columns for drug-protein binding studies

A slurry-based method was developed for the entrapment of alpha1-acid glycoprotein (AGP) for use in high-performance affinity chromatography to study drug interactions with this serum protein. Entrapment was achieved based on the physical containment of AGP in hydrazide-activated porous silica supports and by using mildly oxidized glycogen as a capping agent. The conditions needed for this process were examined and optimized. When this type of AGP column was used in binding studies, the association equilibrium constant (Ka) measured by frontal analysis at pH 7.4 and 37°C for carbamazepine with AGP was found to be 1.0 (±0.5)×10(5)M(-1), which agreed with a previously reported value of 1.0 (±0.1)×10(5)M(-1). Binding studies based on zonal elution were conducted for several other drugs with such columns, giving equilibrium constants that were consistent with literature values. An entrapped AGP column was also used in combination with a column containing entrapped HSA in a screening assay format to compare the binding of various drugs to AGP and HSA. These results also agreed with previous data that have been reported in literature for both of these proteins. The same entrapment method could be extended to other proteins and to the investigation of additional types of drug-protein interactions. Potential applications include the rapid quantitative analysis of biological interactions and the high-throughput screening of drug candidates for their binding to a given protein.

Analysis of free drug fractions in human serum by ultrafast affinity extraction and two-dimensional affinity chromatography

Ultrafast affinity extraction and a two-dimensional high performance affinity chromatographic system were used to measure the free fractions for various drugs in serum and at typical therapeutic concentrations. Pooled samples of normal serum or serum from diabetic patients were utilized in this work. Several drug models (i.e., quinidine, diazepam, gliclazide, tolbutamide, and acetohexamide) were examined that represented a relatively wide range of therapeutic concentrations and affinities for human serum albumin (HSA). The two-dimensional system consisted of an HSA microcolumn for the extraction of a free drug fraction, followed by a larger HSA analytical column for the further separation and measurement of this fraction. Factors that were optimized in this method included the flow rates, column sizes, and column switching times that were employed. The final extraction times used for isolating the free drug fractions were 333-665 ms or less. The dissociation rate constants for several of the drugs with soluble HSA were measured during system optimization, giving results that agreed with reference values. In the final system, free drug fractions in the range of 0.7-9.5% were measured and gave good agreement with values that were determined by ultrafiltration. Association equilibrium constants or global affinities were also estimated by this approach for the drugs with soluble HSA. The results for the two-dimensional system were obtained in 5-10 min or less and required only 1-5 μL of serum per injection. The same approach could be adapted for work with other drugs and proteins in clinical samples or for biomedical research.

High-throughput analysis of drug dissociation from serum proteins using affinity silica monoliths

A noncompetitive peak decay method was used with 1 mm×4.6 mm id silica monoliths to measure the dissociation rate constants (kd) for various drugs with human serum albumin (HSA) and α1-acid glycoprotein (AGP). Flow rates up to 9 mL/min were used in these experiments, resulting in analysis times of only 20-30 s. Using a silica monolith containing immobilized HSA, dissociation rate constants were measured for amitriptyline, carboplatin, cisplatin, chloramphenicol, nortriptyline, quinidine, and verapamil, giving values that ranged from 0.37 to 0.78 s(-1). Similar work with an immobilized AGP silica monolith gave kd values for amitriptyline, nortriptyline, and lidocaine of 0.39-0.73 s(-1). These kd values showed good agreement with values determined for drugs with similar structures and/or affinities for HSA or AGP. It was found that a kd of up to roughly 0.80 s(-1) could be measured by this approach. This information made it possible to obtain a better understanding of the advantages and possible limitations of the noncompetitive peak decay method and in the use of affinity silica monoliths for the high-throughput analysis of drug-protein dissociation.

Dihydroquinidine versus disopyramide: efficacy in patients with chronic stable ventricular ectopy

Dihydroquinidine (DQ) is contained in substantial amounts in quinidine salts, but its direct antiarrhythmic action has not been studied. The efficacy of oral DQ (300 mg t. i. d.) compared to disopyramide (D) (200 mg t.i.d.) was thus investigated using a double-blind crossover placebo-controlled protocol in 12 patients, aged 13 to 67 years, with chronic stable high frequency premature ventricular beats (PVB), defined as greater than 100 PVB/h during 48-72-h control Holter monitoring. The protocol included three 72-h treatment periods: DQ, D, and placebo at random. On days 2 and 3 of each period a 24-h Holter recording was carried out; drug blood levels were determined at peak (days 2 and 3) and trough time (day 3). No significant difference in the mean PVB/h was found between control (735 +/- 400) and placebo periods (564 +/- 388), or between the two Holter recordings of each period. Compared to placebo both DQ (106 +/- 113, p less than 0.005) and D (240 +/- 263, p less than 0.05) reduced the mean PVB/h, but the decrease was significantly higher with DQ (78 versus 53%, p less than 0.02). Nine patients (75%) on DQ and 5 (42%) on D had a greater than 70% decrease in mean PVB/h; complex PVBs were abolished in 3 of 6 patients on both treatments. On day 3, DQ plasma levels were 1.31 +/- 0.44 (peak) and 0.92 +/- 0.45 (trough) mg/l; D plasma levels were 2.88 +/- 0.64 (peak) and 2.02 +/- 0.31 (trough) mg/l; no significant difference was found between day 2 and day 3 samples.(ABSTRACT TRUNCATED AT 250 WORDS)

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 microgram/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 X 10(5) M-1; nP = 3.49 X 10(-5) M) and a low-affinity, high-capacity binding site (K = 1.33 X 10(3) M-1; nP = 3.14 X 10(-3) M) were identified. The characteristics of quinidine binding in a 4.5-g/dL solution of human serum albumin (K = 3.05 X 10(3) M-1; nP = 1.36 X 10(-3) M) suggested that the low-affinity, high-capacity binding site was on this quinidine free fraction increased from 0.114 to 0.231. A lidocaine concentration of 250 micrograms/mL caused a similar increase. Patients suffering traumatic injury had a significant increase in alpha 1-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).(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption of quinidine and dihydroquinidine in humans

Quinidine and dihydroquinidine were administered as the sulphates in an oral solution to seven healthy volunteers. In all subjects, dihydroquinidine was absorbed to a lesser extent than quinidine. On the basis of comparative AUC pu to 6 h after administration, dihydroquinidine was 73% as available as quinidine. Rates of elimination of the compounds were similar. No correlation could be found between saliva and plasma levels for either compound. Limits for content of dihydroquinidine in commercial quinidine preparations seem essential to ensure adequate bioavailability.

Clinical pharmacokinetics of quinidine

The elimination of quinidine is accomplished by a combination of renal excretion of the intact drug (15 to 40% of total clearance) and hepatic biotransformation to a variety of metabolites (60 to 85% of total clearance). Many of the metabolites appear to be pharmacologically active. Typical ranges for kinetic properites of quinidine in healthy persons are: apparent volume of distribution 2.0 to 3.5 litres/kg; elimination half-life 5 to 12 hours; clearance, 2.5 to 5.0 ml/min/kg. Quinidine clearance is reduced in the elderly, in patients with cirrhosis, and in those with congestive heart failure. Oral quinidine is available either as relatively rapidly absorbed conventional tablets (usually quinidine sulphate) or as a variety of slowly absorbed sustained release preparations. Absolute systemic availability generally is 70% or greater. Quinidine is 70 to 95% bound to plasma protein, primarily to albumin but also to a number of other plasma constituents. Binding is reduced in patients with cirrhosis, partly because of hypoalbuminaemia, but is not influenced by renal insufficiency. Clinical interpretation of total serum or plasma quinidine concentrations must be altered in patients with reduced or increased binding, since it is the unbound fraction which is pharmacologically active.

Pharmacokinetics of dihydroquinidine in congestive heart failure patients after intravenous quinidine administration

The pharmacokinetics of dihydroquinidine were studied in 8 patients with congestive heart failure following a 22 min intravenous infusion of a quinidine preparation that contained 5.9% dihydroquinidine as an impurity. Using a thin layer chromatography-fluorometric assay procedure for dihydroquinidine, the post-infusion plasma dihydroquinidine concentrations declined biexponentially. The half-life of the fast and slow dispositional processes was 4.42 +/- 1.81 min and 6.52 +/- 2.40 h, respectively. The central compartment volume for dihydroquinidine in these patients was 0.44 +/- 0.11 l/kg with an overall apparent volume of distribution of 1.14 +/- 0.38 l/kg. The computed values of total body plasma clearance of dihydroquinidine ranged from 1.29 to 2.69 ml/min/kg with a mean value of 1.94 +/- 0.60 ml/min/kg. In these patients, approximately 16% of the administered dihydroquinidine dose was excreted intact into the urine in 48 h. The estimated value of renal clearance was 0.314 +/- 0.129 ml/min/kg. When compared to control cardiac patients, the data showed that the apparent volume of distribution for dihydroquinidine is smaller in patients with congestive heart failure and as a result of this diminished volume, the clearance rate of dihydroquinidine was slower. The net effect of these differences was the production of higher plasma concentrations of dihydroquinidine in the heart failure group.