Clinical pharmacokinetics of quinidine

Quinidine for ventricular arrhythmias: A comprehensive review

Quinidine, the first antiarrhythmic drug, was widely used during the 20th century. Multiple studies have been conducted to provide insights into the pharmacokinetics and pleiotropic effects of Class Ia antiarrhythmic drugs. However, safety concerns and the emergence of new drugs led to a decline in their use during the 1990s. Despite this, recent studies have reignited the interest in quinidine, particularly for ventricular arrhythmias, where other antiarrhythmics have failed. In conditions such as Brugada syndrome, idiopathic ventricular fibrillation, early repolarization syndrome, short QT syndrome, and electrical storms, quinidine remains a valuable asset. Starting from the European and American recommendations, this comprehensive review aimed to explore the various indications for quinidine and the studies that support its use. We also discuss the potential future of quinidine, including the necessary research to optimize its use and patient selection. Additionally, it addresses the imperative task of mitigating the iatrogenic burden associated with quinidine usage and confronts the challenge of ensuring drug accessibility.

Quantitatively Predicting Effects of Exercise on Pharmacokinetics of Drugs Using a Physiologically Based Pharmacokinetic Model

Exercise significantly alters human physiological functions, such as increasing cardiac output and muscle blood flow and decreasing glomerular filtration rate (GFR) and liver blood flow, thereby altering the absorption, distribution, metabolism, and excretion of drugs. In this study, we aimed to establish a database of human physiological parameters during exercise and to construct equations for the relationship between changes in each physiological parameter and exercise intensity, including cardiac output, organ blood flow (e.g., muscle blood flow and kidney blood flow), oxygen uptake, plasma pH and GFR, etc. The polynomial equation P = ΣaiHRi was used for illustrating the relationship between the physiological parameters (P) and heart rate (HR), which served as an index of exercise intensity. The pharmacokinetics of midazolam, quinidine, digoxin, and lidocaine during exercise were predicted by a whole-body physiologically based pharmacokinetic (WB-PBPK) model and the developed database of physiological parameters following administration to 100 virtual subjects. The WB-PBPK model simulation results showed that most of the observed plasma drug concentrations fell within the 5th-95th percentiles of the simulations, and the estimated peak concentrations (Cmax) and area under the curve (AUC) of drugs were also within 0.5-2.0 folds of observations. Sensitivity analysis showed that exercise intensity, exercise duration, medication time, and alterations in physiological parameters significantly affected drug pharmacokinetics and the net effect depending on drug characteristics and exercise conditions. In conclusion, the pharmacokinetics of drugs during exercise could be quantitatively predicted using the developed WB-PBPK model and database of physiological parameters. SIGNIFICANCE STATEMENT: This study simulated real-time changes of human physiological parameters during exercise in the WB-PBPK model and comprehensively investigated pharmacokinetic changes during exercise following oral and intravenous administration. Furthermore, the factors affecting pharmacokinetics during exercise were also revealed.

Pharmacokinetics and Safety of Atogepant Co-administered with Quinidine Gluconate in Healthy Participants: A Phase 1, Open-Label, Drug-Drug Interaction Study

Atogepant, an oral calcitonin gene-related peptide receptor antagonist, is approved for the preventive treatment of migraine. Atogepant is a substrate of P-glycoprotein (P-gp), breast cancer resistance protein, organic anion transporting polypeptide transporters, and cytochrome P450 (CYP)3A4 and 2D6. Quinidine is a strong P-gp and CYP2D6 inhibitor. A phase 1 open-label study evaluated the effect of P-gp and CYP2D6 inhibition by quinidine on the pharmacokinetics of atogepant, and the safety and tolerability of atogepant and quinidine gluconate (QG) when co-administered and when given alone in 33 healthy adults. There was no significant change in the atogepant maximum plasma concentration with QG co-administration. The overall systemic exposure, the area under the plasma concentration-time curve (from time 0 to time t or to infinity), of atogepant increased by 25% when co-administered with QG. However, such an increase was not considered clinically relevant. Atogepant did not alter the mean plasma concentration of quinidine at steady state. The incidence of treatment-emergent adverse events (TEAEs) was highest when QG was administered alone (42.4%), which was primarily due to QT prolongation. Most TEAEs reported were mild in severity and resolved within 1-2 days. Co-administration of atogepant with QG did not result in any unexpected tolerability findings in this phase 1 study in healthy participants. The increase in atogepant exposure during QG co-administration could be due to inhibition of CYP2D6 (a minor contributor to atogepant clearance) as well as inhibition of P-gp.

Hypoalbuminemia and Pharmacokinetics: When the Misunderstanding of a Fundamental Concept Leads to Repeated Errors over Decades

Surprisingly, misinterpretation of the influence of hypoalbuminemia on pharmacokinetics and the clinical effects of drugs seems to be a current problem, even though hypoalbuminemia has no impact on the pharmacologically active exposure. Exceptions to this fact are highly protein-bound anaesthetics with high elimination capacity (i.e., <5 drugs on the market). To assess the frequency of misinterpretation of the influence of hypoalbuminemia on pharmacokinetics and the clinical effects of drugs between 1975 and 2021, a PubMed literature review was conducted. Each paragraph on albumin binding was classified as correct, ambiguous or incorrect, creating two acceptable categories: (1) content without any errors, and (2) content containing some incorrect and/or ambiguous statements. The analyses of these articles showed that fewer than 11% of articles contained no interpretation errors. In order to contain this misinterpretation, several measures are proposed: (1) Make the message accessible to a wide audience by offering a simplified and didactic video representation of the lack of impact of albumin binding to drugs. (2) Precise terminology (unbound/free form/concentration) should be used for highly bound drugs. (3) Unbound/free forms should be systematically quantified for highly plasma protein bound drugs for clinical trials as well as for therapeutic drug monitoring.

Prediction of drug pro-arrhythmic cardiotoxicity using a semi-physiologically based pharmacokinetic model linked to cardiac ionic currents inhibition

Drug-induced torsades de pointes (TdP) risks are responsible for the withdrawal of many drugs from the market. Nowadays, assessments of drug-induced TdP risks are mainly based on maximum effective free therapeutic plasma concentration (EFTPCmax) and cardiac ionic current inhibitions using the human ventricular myocytes model (Tor-ORd model). Myocytes are targets of drug-induced TdP. The TdP risks may be directly linked to myocyte drug concentrations. We aimed to develop a semi-physiologically based pharmacokinetic (Semi-PBPK) model linked to cardiac ionic current inhibition (pharmacodynamics, PD) (Semi-PBPK-PD) to simultaneously predict myocyte drug concentrations and their TdP risks in humans. Alterations in action potential duration (ΔAPD90) were simulated using the Tor-ORd model and ionic current inhibition parameters based on myocyte or plasma drug concentrations. The predicted ΔAPD90 values were translated into in vivo alterations in QT interval(ΔQTc) induced by moxifloxacin, dofetilide, or sotalol. Myocyte drug concentrations of moxifloxacin, dofetilide, and sotalol gave better predictions of ΔQTc than plasma. Following validating the developed semi-PBPK-PD model, TdP risks of 37 drugs were assessed using ΔAPD90 and early afterdepolarization occurrence, which were estimated based on 10 × EFTPCmax and 10 × EFTMCmax (maximum effective free therapeutic myocyte concentration). 10 × EFTMCmax gave more sensitive and accurate predictions of pro-arrhythmic cardiotoxicity and the predicted TdP risks were also closer to clinic practice than 10 × EFTPCmax. In conclusion, pharmacokinetics and TdP risks of 37 drugs were successfully predicted using the semi-PBPK-PD model. Myocyte drug concentrations gave better predictions of ΔQTc and TdP risks than plasma.

Therapeutic Drug Monitoring of Quinidine in Pediatric Patients with KCNT1 Genetic Variants

Quinidine (QND) is an old antimalarial drug that was used in the early 20th century as an antiarrhythmic agent. Currently, QND is receiving attention for its use in epilepsy of infancy with migrating focal seizures (EIMFS) due to potassium sodium-activated channel subfamily T member 1 (KCNT1) genetic variants. Here, we report the application of Therapeutic Drug Monitoring (TDM) in pediatric patients carrying KCNT1 genetic variants and orally treated with QND for developmental and epileptic encephalopathies (DEE). We measured plasma levels of QND and its metabolite hydroquinidine (H-QND) by using a validated method based on liquid chromatography coupled with mass spectrometry (LC-MS/MS). Three pediatric patients (median age 4.125 years, IQR 2.375–4.125) received increasing doses of QND. Cardiac toxicity was monitored at every dose change. Reduction in seizure frequency ranged from 50 to 90%. Our results show that QND is a promising drug for pediatric patients with DEE due to KCNT1 genetic variants. Although QND blood levels were significantly lower than the therapeutic range as an anti-arrhythmic drug, patients showed a significant improvement in seizure burden. These data underlie the utility of TDM for QND not only to monitor its toxic effects but also to evaluate possible drug–drug interactions.

Physiologically-based Pharmacokinetic Modeling to Assess the Impact of CYP2D6-Mediated Drug-Drug Interactions on Tramadol and O-Desmethyltramadol Exposures via Allosteric and Competitive Inhibition

Tramadol is an opioid medication used to treat moderately severe pain. Cytochrome P450 (CYP) 2D6 inhibition could be important for tramadol, as it decreases the formation of its pharmacologically active metabolite, O‐desmethyltramadol, potentially resulting in increased opioid use and misuse. The objective of this study was to evaluate the impact of allosteric and competitive CYP2D6 inhibition on tramadol and O‐desmethyltramadol pharmacokinetics using quinidine and metoprolol as prototypical perpetrator drugs. A physiologically based pharmacokinetic model for tramadol and O‐desmethyltramadol was developed and verified in PK‐Sim version 8 and linked to respective models of quinidine and metoprolol to evaluate the impact of allosteric and competitive CYP2D6 inhibition on tramadol and O‐desmethyltramadol exposure. Our results show that there is a differentiated impact of CYP2D6 inhibitors on tramadol and O‐desmethyltramadol based on their mechanisms of inhibition. Following allosteric inhibition by a single dose of quinidine, the exposure of both tramadol (51% increase) and O‐desmethyltramadol (52% decrease) was predicted to be significantly altered after concomitant administration of a single dose of tramadol. Following multiple‐dose administration of tramadol and a single‐dose or multiple‐dose administration of quinidine, the inhibitory effect of quinidine was predicted to be long (≈42 hours) and to alter exposure of tramadol and O‐desmethyltramadol by up to 60%, suggesting that coadministration of quinidine and tramadol should be avoided clinically. In comparison, there is no predicted significant impact of metoprolol on tramadol and O‐desmethyltramadol exposure. In fact, tramadol is predicted to act as a CYP2D6 perpetrator and increase metoprolol exposure, which may necessitate the need for dose separation.

Simulations of Cytochrome P450 3A4-Mediated Drug-Drug Interactions by Simple Two-Compartment Model-Assisted Static Method

In order to predict cytochrome P450 3A4 (CYP3A4)-mediated drug-drug interactions (DDIs), a simple 2-compartment model-assisted, overall inhibition activity (A_i,overall) method was derived based on 2 concepts. One concept was that the increase in blood victim level and fold increase in the area under the blood victim level curve produced by DDI are determined entirely by A_i,overall, the hepatic availability of the victim and fraction of urinary excreted unchanged victim, where A_i,overall is determined by the perpetrator-specific CYP isoform inhibition activities (A_i,CYPs, DDI predictor-1) and victim-specific fractional CYP isoform contributions (f_m,CYPs, predictor-2). The other concept was that a DDI can be bridged to other DDIs, so that any possible DDI produced by a given victim or a given perpetrator can be predicted by using these predictors. The A_i,CYP3A4s of 12 common CYP3A4 inhibitors were able to be determined and shown to be useful for the prediction of CYP3A4-mediated DDIs wherein victims were metabolized by multiple CYP isoforms. Additionally, it was demonstrated that f_m,CYP values with high confidence can be estimated by bridging DDIs produced by the same victim and different perpetrators. This bridging approach will accelerate prediction of DDIs produced by new chemical entities from the existing DDI database.

High risk of QT interval prolongation and torsades de pointes associated with intravenous quinidine used for treatment of resistant malaria or babesiosis

Cardiac toxicity may be associated with drugs used for malaria. Torsades de pointes (TdP) is a well-known adverse effect of quinidine when used for atrial fibrillation. Intravenous quinidine doses for resistant malaria are 2 to 3 times higher than those used for arrhythmias. Among 6 patients receiving quinidine for malaria or babesiosis, 4 developed QT interval prolongation and 2 experienced TdP. Clinicians should be aware that recommended doses of quinidine for malaria carry a high TdP risk.

Interaction between topically and systemically coadministered P-glycoprotein substrates/inhibitors: effect on vitreal kinetics

The objective of the present study was to investigate the effect of topically coadministered P-glycoprotein (P-gp) substrates/inhibitors on the vitreal kinetics of a systemically administered P-gp substrate. Anesthetized male rabbits were used in these studies. The concentration-time profile of quinidine in the vitreous humor, after intravenous administration, was determined alone and in the presence of topically coadministered verapamil, prednisolone sodium phosphate (PP), and erythromycin. The vitreal pharmacokinetic parameters of quinidine in the presence of verapamil [apparent elimination rate constant (λ(z)), 0.0027 ± 0.0002 min(-1); clearance (CL_F), 131 ± 21 ml/min; area under the curve (AUC(0-∞)), 39 ± 7.0 μg · min/ml; and mean residence time, 435 ± 20 min] were significantly different from those of the control (0.0058 ± 0.0006 min(-1), 296 ± 46 ml/min, 17 ± 3 μg · min/ml, and 232 ± 20 min, respectively). A 1.7-fold decrease in the vitreal λ(z) and a 1.5-fold increase in the vitreal AUC of quinidine were observed in the presence of topical PP. Statistically significant differences between the vitreal profiles of the control and erythromycin-treated group were also observed. Plasma concentration-time profiles of quinidine, alone or in the presence of the topically instilled compounds, remained unchanged, indicating uniform systemic quinidine exposure across groups. This study demonstrates an interaction between topically and systemically coadministered P-gp substrates, probably through the modulation of P-gp on the basolateral membrane of the retinal pigmented epithelium, leading to changes in the vitreal kinetics of the systemically administered agent.

Pharmacokinetics/Pharmacodynamics of Antiarrhythmic Drugs

This article describes the pharmacology of antiarrhythmic medications. Although these medications are broadly considered in terms of their blockade of either sodium or potassium channels, they act by a variety of pharmacodynamic mechanisms. Elimination may be via hepatic metabolism or renal mechanisms, or a combination. In particular, interactions between antiarrhythmic medications and other drugs that interfere with hepatic metabolism by P450 enzymes is a source for toxicity.

Pharmacokinetic analysis of transcellular transport of quinidine across monolayers of human intestinal epithelial Caco-2 cells

To investigate the mechanism responsible for the intestinal absorption of a lipophilic organic cation, quinidine, we performed a pharmacokinetic analysis of transcellular transport across Caco-2 cell monolayers grown on a porous membrane. Basolateral-to-apical transport of the drug was almost constant in the concentration range of 100 nM-100 microM. Transcellular transport was greater in the apical-to-basolateral direction than in the opposite direction. Apical-to-basolateral transport was greater at a concentration of 100 microM than 100 nM. The calculated influx clearance value of the apical membrane was much greater than the other influx/efflux clearance values of cell membranes, and was 5.6-fold the influx clearance value of the basolateral membrane at the drug concentration of 100 microM. We also investigated the uptake of quinidine at the apical membrane of Caco-2 cells grown on plastic dishes. The uptake was markedly increased by alkalization of the apical medium at 37 degrees C, and was decreased at low temperature (4 degrees C). In addition, it was inhibited by diphenhydramine and levofloxacin, but not by carvedilol, rifamycin SV, or L-carnitine. These findings indicated that the influx at the apical membrane was the direction-determining step in the transcellular transport of quinidine across Caco-2 cell monolayers, and that some specific transport system was involved in this influx.

Prediction of in vivo drug-drug interactions from in vitro data: impact of incorporating parallel pathways of drug elimination and inhibitor absorption rate constant

AIMS

Success of the quantitative prediction of drug-drug interactions via inhibition of CYP-mediated metabolism from the inhibitor concentration at the enzyme active site ([I]) and the in vitro inhibition constant (K(i)) is variable. The aim of this study was to examine the impact of the fraction of victim drug metabolized by a particular CYP (f(mCYP)) and the inhibitor absorption rate constant (k(a)) on prediction accuracy.

METHODS

Drug-drug interaction studies involving inhibition of CYP2C9, CYP2D6 and CYP3A4 (n = 115) were investigated. Data on f(mCYP) for the probe substrates of each enzyme and k(a) values for the inhibitors were incorporated into in vivo predictions, alone or in combination, using either the maximum hepatic input or the average systemic plasma concentration as a surrogate for [I]. The success of prediction (AUC ratio predicted within twofold of in vivo value) was compared using nominal values of f(mCYP) = 1 and k(a) = 0.1 min(-1).

RESULTS

The incorporation of f(mCYP) values into in vivo predictions using the hepatic input plasma concentration resulted in 84% of studies within twofold of in vivo value. The effect of k(a) values alone significantly reduced the number of over-predictions for CYP2D6 and CYP3A4; however, less precision was observed compared with the f(mCYP). The incorporation of both f(mCYP) and k(a) values resulted in 81% of studies within twofold of in vivo value.

CONCLUSIONS

The incorporation of substrate and inhibitor-related information, namely f(mCYP) and k(a), markedly improved prediction of 115 interaction studies with CYP2C9, CYP2D6 and CYP3A4 in comparison with [I]/K(i) ratio alone.

Inhibition of CYP2D6 by quinidine and its effects on the metabolism of cilostazol

OBJECTIVE

In vitro results are inconclusive as to whether cilostazol is metabolised by cytochrome P450 isoenzyme 2D6 (CYP2D6). The goals of this study were (1) to assure the dose of quinidine and timing relative to cilostazol used in this study were adequate to cause inhibition of CYP2D6, (2) to evaluate carryover effects of quinidine administration, and (3) to evaluate the effect of CYP2D6 deficiency and administration of quinidine (a CYP2D6 inhibitor) on the pharmacokinetics of a single 100 mg oral dose of cilostazol.

DESIGN

This study was conducted as a single-centre, open-label, randomised sequence, 2-period, crossover pharmacokinetic trial. Water alone (treatment without quinidine) or two 200 mg oral doses of quinidine sulfate with water were administered 25 hours and 1 hour prior to a single 100 mg dose of cilostazol in period 1. Study participants were crossed over to opposite treatment in period 2. Metoprolol 25 mg, used as a positive control, was administered 1 hour after quinidine sulfate with water or using water alone to assess the magnitude of CYP2D6 inhibition by quinidine.

STUDY PARTICIPANTS

22 healthy nonsmoking Caucasian (14 male and 8 female) volunteers participated in the study.

MAIN OUTCOME MEASURES

Serial blood and urine samples were collected at predose and after cilostazol administration to characterise cilostazol and its metabolite pharmacokinetics. Additional plasma samples were taken to assess the pharmacokinetics of quinidine. Urine samples were collected to measure metoprolol and hydroxymetoprolol.

RESULTS

Administration of metoprolol with quinidine caused a significant (p < 0.001) decrease in the urinary 4-hydroxymetoprolol/metoprolol ratio compared with administration of metoprolol alone (42-fold decrease, 0.065 vs 2.707). Hence, quinidine effectively converted extensive metabolisers of CYP2D6 to poor metabolisers of CYP2D6. The 21-day washout period was adequate to have complete recovery from quinidine inhibition of CYP2D6. The analysis of variance demonstrated that the mean maximum plasma concentration (Cmax) for cilostazol, both adjusted and unadjusted for the free fraction, was higher in the control group than in the quinidine group (p = 0.023). However, the time to Cmax (p = 0.669), the area under the plasma concentration-time curve from time zero to infinity (AUC infinity; p = 0.133), and the apparent oral clearance (p = 0.135) were unchanged. The geometric mean ratios (90% confidence interval) comparing with quinidine (test) and without quinidine (reference) coadministration for Cmax and AUC infinity are 0.86 (0.77, 0.95) and 0.92 (0.84, 1.00), respectively. Similar patterns were observed for OPC-13015 and OPC-13213 with regard to Cmax, area under the plasma concentration-time curve from time zero to the last measurable concentration at time t, and AUC infinity (where determinable). The slight decrease in the systemic availability of cilostazol and its metabolites was thought to be a result of the increased gastrointestinal motility secondary to quinidine.

CONCLUSIONS

Administration of quinidine sulfate 200 mg profoundly inhibited CYP2D6-mediated metabolism. The effects of quinidine inhibition of CYP2D6 metabolism were completely reversible during the 21-day washout period. Coadministration of quinidine with cilostazol had no substantial effect on cilostazol or its metabolites (OPC-13015 and OPC-13213). Hence, CYP2D6 does not have a significant contribution in the metabolic elimination of cilostazol.

Inhibition of debrisoquine hydroxylation with quinidine in subjects with three or more functional CYP2D6 genes

AIMS

To study whether the CYP2D6 capacity in ultrarapid metabolizers of debrisoquine due to duplication/multiduplication of a functional CYP2D6 gene, can be 'normalised' by low doses of the CYP2D6 inhibitor quinidine and whether this is dose-dependent.

METHODS

Five ultrarapid metabolizers of debrisoquine with 3, 4 or 13 functional CYP2D6 genes were given single oral doses of 5, 10, 20, 40, 80 and 160 mg quinidine. Four hours after quinidine intake, 10 mg debrisoquine was given. Urine was collected for 6 h after debrisoquine administration. Debrisoquine and its 4-hydroxymetabolite were analysed by h.p.l.c. and the debrisoquine metabolic ratio (MR) was calculated.

RESULTS

Without quinidine the MR in the ultrarapid metabolizers ranged between 0.01 and 0.07. A dose-effect relationship could be established for quinidine with regard to the inhibitory effect on CYP2D6 activity. To reach an MR of 1-2, subjects with 3 or 4 functional genes required a quinidine dose of about 40 mg, while the sister and brother with 13 functional genes required about 80 mg quinidine. After 160 mg quinidine, the MRs, in the subjects with 3, 3, 4, 13 and 13 functional genes, were 12.6, 10.1, 9.2, 2.4 and 2.2, respectively.

CONCLUSIONS

A dose-effect relationship could be established for quinidine inhibition of CYP2D6 in ultrarapid metabolizers. The clinical use of low doses of quinidine as an inhibitor of CYP2D6 might be considered in ultrarapid metabolizers taking CYP2D6 metabolized drugs rather than giving increased doses of the drug. Normalizing the metabolic capacity of CYP2D6, by giving a low dose of quinidine, may solve the problem of 'treatment resistance' caused by ultrarapid metabolism.

Effect of diclofenac, disulfiram, itraconazole, grapefruit juice and erythromycin on the pharmacokinetics of quinidine

AIMS

In vitro studies suggest that the oxidation of quinidine to 3-hydroxyquinidine is a specific marker reaction for CYP3A4 activity. To assess the possible use of this reaction as an in vivo marker of CYP3A4 activity, we studied the involvement of cytochromes CYP2C9, CYP2E1 and CYP3A4 in the in vivo oxidative metabolism of quinidine.

METHODS

An open study of 30 healthy young male volunteers was performed. The pharmacokinetics of a 200 mg single oral dose of quinidine was studied before and during daily administration of 100 mg diclofenac, a CYP2C9 substrate (n=6); 200 mg disulfiram, an inhibitor of CYP2E1 (n=6); 100 mg itraconazole, an inhibitor of CYP3A4 (n=6); 250 ml single strength grapefruit juice twice daily, an inhibitor of CYP3A4 (n=6); 250 mg of erythromycin 4 times daily, an inhibitor of CYP3A4 (n=6). Probes of other enzyme activities, caffeine (CYP1A2), sparteine (CYP2D6), mephenytoin (CYP2C19), tolbutamide (CYP2C9) and cortisol (CYP3A4) were also studied.

RESULTS

Concomitant administration of diclofenac reduced the partial clearance of quinidine by N-oxidation by 27%, while no effect was found for other pharmacokinetic parameters of quinidine. Concomitant administration of disulfiram did not alter any of the pharmacokinetic parameters of quinidine. Concomitant administration of itraconazole reduced quinidine total clearance, partial clearance by 3-hydroxylation and partial clearance by N-oxidation by 61, 84 and 73%, respectively. The renal clearance was reduced by 60% and the elimination half-life increased by 35%. Concomitant administration of grapefruit juice reduced the total clearance of quinidine and its partial clearance by 3-hydroxylation and N-oxidation by 15, 19 and 27%, respectively. The elimination half-life of quinidine was increased by 19%. The caffeine metabolic index was reduced by 25%. Concomitant administration of erythromycin reduced the total clearance of quinidine and its partial clearance by 3-hydroxylation and N-oxidation by 34, 50 and 33%, respectively. Cmax was increased by 39%.

CONCLUSIONS

The results confirm an important role for CYP3A4 in the oxidation of quinidine in vivo, and this applies particularly to the formation of 3-hydroxyquinidine. While a minor contribution of CYP2C9 to the N-oxidation of quinidine is possible, a major involvement of the CYP2C9 or CYP2E1 enzymes in the oxidation of quinidine in vivo is unlikely.

Itraconazole increases plasma concentrations of quinidine

BACKGROUND

Quinidine is eliminated mainly by CYP3A4-mediated metabolism. Itraconazole interacts with some but not all of the substrates of CYP3A4; it is therefore important to study the possible interaction of itraconazole with quinidine.

METHODS

A double-blind, randomized, two-phase crossover study design was used with nine healthy volunteers. Itraconazole (200 mg) or placebo was ingested once a day for 4 days. A single 100 mg oral dose of quinidine sulfate was ingested on day 4. Plasma concentrations of quinidine, itraconazole, and hydroxyitraconazole, as well as cumulative excretion of quinidine into urine, were determined up to 24 hours. The ECG, heart rate, and blood pressure were also recorded up to 24 hours.

RESULTS

On average the peak plasma concentration of quinidine increased to 1.6-fold (p < 0.05), and the area under the concentration-time curve of quinidine increased to 2.4-fold (p < 0.01) by itraconazole. The elimination half-life of quinidine was prolonged 1.6-fold (p < 0.001), and the area under the 3-hydroxyquinidine/quinidine ratio-time curve decreased to one-fifth (p < 0.001) by itraconazole. The renal clearance of quinidine decreased 50% (p < 0.001) by itraconazole, whereas the creatinine clearance was unaffected. The QTc interval correlated with the concentrations of quinidine during both itraconazole and placebo phases (r2 = 0.71 and r2 = 0.79, respectively; p < 0.01), although only minor changes between the phases were observed in other pharmacodynamic variables.

CONCLUSIONS

Itraconazole increases plasma concentrations of oral quinidine, probably by inhibiting the CYP3A4 isozyme during the first-pass and elimination phases of quinidine. The decreased renal clearance of quinidine might be the result of the inhibition of P-glycoprotein-mediated tubular secretion of quinidine by itraconazole. The concentrations of quinidine should be closely monitored if itraconazole or some other potent CYP3A inhibitors are used with quinidine.

Pharmacokinetic and pharmacodynamic interactions between diltiazem and quinidine

OBJECTIVES

To examine the pharmacokinetic and pharmacodynamic interactions between quinidine and diltiazem because both drugs can inhibit drug metabolism.

METHODS

Twelve fasting, healthy male volunteers (age, 24 +/- 5 years; weight, 75 +/- 10 kg) received a single oral dose of diltiazem (60 mg) or quinidine (200 mg), alone and on a background of the other drug, in a crossover study. Background treatment consisted of 100 mg quinidine twice a day or 90 mg sustained-release diltiazem twice a day for 2 day before the study day.

RESULTS

Pretreatment with diltiazem significantly (p < 0.05) increased the area under the curve of quinidine from 7414 +/- 1965 to 11,213 +/- 2610 ng.hr/ml and increased its terminal elimination half-life (t1/2) from 6.8 +/- 1.1 to 9.3 +/- 1.5 hours. Its oral clearance was decreased from 0.39 +/- 0.1 to 0.25 +/- 0.1 L/hr/kg, whereas the maximal concentration was not significantly affected. Diltiazem disposition was not significantly affected by pretreatment with quinidine. Diltiazem pretreatment increased QTc and PR intervals and decreased heart rate and diastolic blood pressure. No significant pharmacodynamic differences were shown for diltiazem alone versus quinidine pretreatment.

CONCLUSION

Diltiazem significantly decreased the clearance and increased the t1/2 of quinidine, but quinidine did not alter the kinetics of diltiazem with the dose used. No significant pharmacodynamic interaction was shown for the combination that would not be predicted from individual drug administration.

Effect of grapefruit juice on the pharmacokinetics and pharmacodynamics of quinidine in healthy volunteers

A study was conducted to examine the effect of grapefruit juice on the disposition of quinidine sulfate and changes of QT intervals after oral administration to twelve healthy male volunteers. Participants received two oral doses of quinidine sulfate tablets (400 mg) with 240 mL of water or grapefruit juice, each separated by a 1-week washout period. Plasma samples for analysis of quinidine and its major metabolite, 3-hydroxyquinidine, were collected for a 24-hour period and analyzed by a high-performance liquid chromatography method. For pharmacodynamic data, the electrocardiograms (ECGs) were performed for 12 hours, and the recordings were marked for ECG interval at all blood collection time periods. There was no significant difference in pharmacokinetic parameters of quinidine when administered with grapefruit juice or water, except for time to maximum concentration (tmax), which was 1.6 hours after administration with water and 3.3 hours after administration with grapefruit juice. Administration with grapefruit juice also resulted in a 33% decrease in the area under the concentration-time curve (AUC) of 3-hydroxyquinidine compared with water, but did not increase the AUC of quinidine or change the ratio of AUC of 3-hydroxyquinidine to the AUC of quinidine. Pharmacodynamic parameters, including changes in the rate-corrected QT (QTc) interval, closely paralleled the pharmacokinetic data, in that administration with grapefruit juice led to delayed maximal effect on QTc and reduction in maximal effect. Administration with grapefruit juice therefore delays the absorption of quinidine and inhibits the metabolism of quinidine to 3-hydroxyquinidine.

Interactions of ethanol and quinidine on contractility and myocyte action potential in the rat ventricle

The combined effects of ethanol and quinidine on cardiac electromechanical coupling are unknown, but both drugs affect cardiac conduction and can cause myocardial depression. Isolated left ventricular papillary and ventricular myocytes were used to assess the combined effects of quinidine and ethanol on the electrophysiologic and mechanical properties of rat myocardium. The combination of quinidine (1-300 microM) and ethanol (120-240 mg/dL) depressed active papillary muscle tension within the clinically useful concentration range. In electrophysiologic studies of isolated ventricular myocytes, quinidine prolonged the action potential duration at 50% (APD50) and 90% (APD90) repolarization, the absolute refractory period, and the relative refractory period, but decreased the maximum rate of change of depolarization in phase 0 (Vmax). When cells were exposed to ethanol (240 mg/dL) and quinidine (1.5 microM) together, a significant decrease in the quinidine-induced prolongation of the absolute refractory and relative refractory periods was seen. Additional changes in action potential parameters from the quinidine values included slight reductions in Vmax and in APD50 and APD90, but these reductions were not consistently displayed, nor were they statistically significant.

Smooth nonparametric maximum likelihood estimation for population pharmacokinetics, with application to quinidine

The seminonparametric (SNP) method, popular in the econometrics literature, is proposed for use in population pharmacokinetic analysis. For data that can be described by the nonlinear mixed effects model, the method produces smooth nonparametric estimates of the entire random effects density and simultaneous estimates of fixed effects by maximum likelihood. A graphical model-building strategy based on the SNP method is described. The methods are illustrated by a population analysis of plasma levels in 136 patients undergoing oral quinidine therapy.

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.

Practical optimisation of antiarrhythmic drug therapy using pharmacokinetic principles

The optimisation of antiarrhythmic drug therapy is dependent on the definitions and methods of short term efficacy testing and the characteristics of those drugs used for rhythm disturbances. The choice of an initial antiarrhythmic drug dosage is highly empirical, and will remain so until the measurement of free concentrations, enantiomeric fractions and genetic phenotyping becomes routine. However, the clinician can devise an efficient initial dosage for efficacy testing procedures based on pharmacokinetic principles and disposition variables in the literature. In this regard, a nomogram for commonly used agents and dosages was constructed and is offered as a guide to accomplish this goal. Verification of the accuracy and usefulness of this nomogram in a prospective manner in patients with symptomatic tachyarrhythmias is still required. On a long term basis, dosage regimens can be modified by the use of pharmacokinetic principles and patient-specific target concentrations, in accordance with the methods used to monitor arrhythmia recurrence and drug-related side effects.

Poisoning due to class IA antiarrhythmic drugs. Quinidine, procainamide and disopyramide

Quinidine, procainamide and disopyramide are antiarrhythmic drugs in the class 1A category. These drugs have a low toxic to therapeutic ratio, and their use is associated with a number of serious adverse effects during long term therapy and life-threatening sequelae following acute overdose. Class 1A agents inhibit the fast inward sodium current and decrease the maximum rate of rise and amplitude of the cardiac action potential. Prolonged Q-T interval and, to a lesser extent, QRS duration may be observed at therapeutic concentrations of quinidine. With increasing plasma concentrations, progressive depression of automaticity and conduction velocity occur. 'Quinidine syncope' (a transient loss of consciousness due to paroxysmal ventricular tachycardia, frequently of the torsade de pointes type) occurs with therapeutic dosing, often in the first few days of therapy. Extracardiac adverse effects of quinidine include potentially intolerable gastrointestinal effects and hypersensitivity reactions such as fever, rash, blood dyscrasias and hepatitis. Procainamide produces electrophysiological changes that are similar to those of quinidine, although Q-T interval prolongation with the former is less pronounced at therapeutic concentrations. Hypersensitivity reactions including fever, rash and (more seriously) agranulocytosis are associated with procainamide, and a frequent adverse effect requiring cessation of therapy is the development of systemic lupus erythematosus. Of the 3 drugs, disopyramide has the most pronounced negative inotropic effects, which are especially significant in patients with pre-existing left ventricular dysfunction. As with quinidine, unexpected 'disopyramide syncope' at therapeutic concentrations has been described. Anticholinergic side effects are common with this drug and may require cessation of therapy. Disopyramide therapy may unpredictably induce severe hypoglycaemia. Severe intoxication with the class 1A agents may result from acute accidental or intentional overdose, or from accumulation of the drugs during long term therapy. Acute overdose can result in severe disturbances of cardiac conduction and hypotension, frequently accompanied by central nervous system toxicity. Decreased renal function can cause significant accumulation of procainamide and its active metabolite acecainide (N-acetyl-procainamide), resulting in severe intoxication. Mild to moderate renal dysfunction is less likely to lead to quinidine or disopyramide intoxication, unless renal failure is severe or concurrent hepatic dysfunction is present. Management of acute intoxication with class 1A drugs includes gut decontamination with provision of respiratory support and treatment of seizures as needed. Hypertonic sodium bicarbonate, by antagonising the inhibitory effect of quinidine on sodium conductance, may reverse many or all manifestations of cardiovascular toxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Potential usefulness of quinine to circumvent the anthracycline resistance in clinical practice

Quinine, the widely used antimalaria agent, was found to increase the cytotoxicity of epideoxorubicin (epiDXR) in resistant DHD/K12 rat colon cancer cells in vitro. Quinine appeared as slightly less effective than quinidine or verapamil for anthracycline potentiation but its weaker cardiotoxicity could counterbalance this disadvantage in vivo. Serum from six patients treated by conventional doses of quinine (25-30 mg kg-1 day-1) was demonstrated to enhance the accumulation of epiDXR in DHD/K12 cells as judged by fluorescence microscopy and HPLC assay (1.6 to 6-fold compared with control serum). In this patients quinine concentrations in serum ranged from 4.4 to 10.1 micrograms ml-1. Our results suggest that quinine could be safely used as anthracycline resistance modifier in clinical practice.

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)

Comparative bioavailability characteristics of commercial quinidine polygalacturonate and sulfate tablets

This study compared the relative bioavailability characteristics of quinidine polygalacturonate (QP) and quinidine sulfate (QS) after oral administration of commercial tablets and a liquid form prepared from crushed tablets in 13 healthy adult male volunteers. Each subject received the following four single-dose treatments in a randomized, crossover manner with a one-week washout period between treatments: 400 mg QS liquid, two 200-mg QS tablets, 550 mg QP liquid, and two 275-mg QP tablets. All four treatments were equivalent in terms of the dose of quinidine base. Multiple serum samples and two 24-hour urine specimens were collected over 24 and 48 hours, respectively, and assayed for quinidine with a specific HPLC assay method. For the absorption and disposition parameters measured (maximum serum concentration, time to reach maximum concentration, area under the concentration-time curve [0-48 hours], absorption and elimination rate constants, absorption and elimination half-lives, apparent total body clearance, apparent volume of distribution, and dose fraction excreted in the urine) no significant differences were observed for any of the parameters among the four treatments (p greater than 0.05). The results of the present investigation demonstrated that QP and QS produced identical serum quinidine concentration-time curves when given in the form of a tablet or liquid. The clinical implications of these observations with respect to the dosing of QP are discussed.

Quinidine oxidative metabolism. Identification and biosynthesis of quinidine 10,11-dihydrodiol stereoisomers

The isocratic reversed phase high performance liquid chromatographic method proposed for quinidine metabolic studies facilitates particularly the separation of 10(R) and (S) isomers of quinidine 10,11-dihydrodiols. The finding of each of these forms following a new synthetic pathway allows us to identify and quantify them in biological fluids. These two isomers have especially been observed in rat bile and hepatocyte secretions. The metabolic inducing effect of phenobarbital on the oxidative metabolism of quinidine is verified in rat isolated hepatocytes. Simultaneous secretion of the two dihydrodiols is also verified in human urine by a gas chromatography/mass spectrometry procedure.

A dose-effect study of the in vivo inhibitory effect of quinidine on sparteine oxidation in man

1. Twelve healthy extensive metabolisers of sparteine were sparteine tested daily for 6 days (19.00 h to 07.00 h). A small but statistically significant rise in sparteine metabolic ratio (MR) was observed. 2. Following 100 mg quinidine sulphate given to four of the subjects at 16.00 h, sparteine tests were carried out 19.00 h to 07.00 h on the same day and then daily for 6 days. Quinidine caused an immediate twenty-fold increase in sparteine-MR which then gradually returned to normal over the following 4-6 days. Quinidine concentrations in plasma were measurable only up to 20 h after the quinidine test dose. 3. At weekly intervals, all 12 subjects received single doses of quinidine sulphate of 5, 10, 20, 40 and 80 mg at 16.00 h, each time followed by a sparteine test 19.00 h to 07.00 h on the same day. A clear dose-effect relationship was found with a significant rise in the sparteine-MR even after 5 mg quinidine. After 80 mg quinidine, 8 of 12 subjects became phenotypically poor metabolisers (MR greater than 20).

Quinidine kinetics after a single oral dose in relation to the sparteine oxidation polymorphism in man

The kinetics at a single oral dose (400 mg) of quinidine were studied in four extensive metabolizers (EM) and four poor metabolizers (PM) of sparteine. The clearance of quinidine by 3-hydroxylation was significantly lower in PM than in EM, but the difference was small (25-30%). This finding suggests that 3-hydroxylation, in part, is catalyzed by the same isoenzyme of cytochrome P450, P450db1 which oxidizes sparteine. Otherwise, no significant phenotypic differences in total or metabolic clearance were found and it is concluded that the metabolism of quinidine is largely carried out by P450 isoenzymes different from P450db1. A biexponential decline in the log plasma quinidine concentration vs time curves was observed in all subjects, and the mean elimination half-life was 11-12 h. This is about twice as long as generally reported in the literature.

Effect of cimetidine on quinidine bioavailability

Elevations in quinidine steady-state serum concentrations have been reported in patients who received cimetidine concurrently. Studies in normal volunteers have shown that areas under the serum concentration-time curve of orally administered quinidine are higher when quinidine is given during chronic cimetidine therapy as compared to under control conditions. The mechanism for this interaction is generally ascribed to decreased hepatic clearance as a consequence of enzyme inhibition. In this study, we show that cimetidine also decreases the bioavailable fraction of quinidine.

Differences in the binding of quinine and quinidine to plasma proteins

1. Little is known about the comparative plasma protein binding of the antimalarial agents quinine (QN) and its isomer quinidine (QD). We have examined the in vitro binding of QN and QD to albumin, alpha 1-acid glycoprotein, normal human plasma, and maternal and foetal umbilical cord plasma. 2. QN was more avidly bound than QD, and binding of both drugs was substantially higher to alpha 1-acid glycoprotein than to albumin, indicating that alpha 1-acid glycoprotein is the more important binding protein. 3. Protein and drug concentration dependent binding was evident for both QN and QD. The unbound fraction of both drugs fell with increasing albumin (10 to 60 g l-1) and alpha 1-acid glycoprotein (0.5 to 2.0 g l-1) concentration, and there was a marked increase in unbound fraction of QN (6 to 19%) and QD (13 to 36%) in human plasma when drug concentrations were increased over the antimalarial therapeutic range (0.5 to 10 mg l-1). 4. In human volunteer plasma, the unbound fractions of QN and QD were 7.5 +/- 2.2% and 12.3 +/- 2.3% respectively, whilst the unbound fractions for both drugs were significantly higher in maternal plasma (QN = 13.0 +/- 5.4%, QD = 18.3 +/- 2.5%) and significantly higher still in foetal umbilical cord plasma (QN = 25.7 +/- 10%, QD = 35 +/- 5.3%).

Lack of effect of smoking on the metabolism and pharmacokinetics of quinidine in patients

The urinary metabolite profile of quinidine and the oral clearance of this drug were studied under steady state conditions in five smoking and nine non-smoking patients. No significant differences were observed in the urinary recovery of unchanged quinidine, 3S-3-hydroxyquinidine, 2'-oxoquinidinone or quinidine-N-oxide between smokers and non-smokers. In addition, the plasma clearance of quinidine was not affected by the smoking status of subjects. These results suggest that cigarette smoke does not induce any of the main pathways for quinidine metabolism in a typical patient population and that the consideration of smoking status is of little utility in aiding in the selection of initial dosage regimens for this drug.

Effects of cimetidine and ranitidine on the pharmacokinetics of quinine

The pharmacokinetics of orally administered quinine were determined in six normal volunteers before and after a 7-day course of cimetidine (1 g day-1) or ranitidine (300 mg day-1). Peak plasma quinine concentration and the time of peak concentration were not altered after cimetidine or ranitidine pretreatment. After cimetidine pretreatment there was a significant reduction in the apparent oral clearance of quinine, from 0.182 +/- 0.063 (mean +/- s.d.) to 0.133 +/- 0.055 1 h-1 kg-1 (P less than 0.05). This was reflected in a 49% (range 17 to 90%) increase in the mean elimination half-life from 7.6 +/- 1.3 to 11.3 +/- 3.7 h (P less than 0.05). In contrast to cimetidine, ranitidine had no significant effect on the clearance or half-life of quinine. The apparent interaction between quinine and cimetidine may have therapeutic implications. Special care should be taken in patients taking these two common drugs concomitantly. Additionally, to avoid unnecessary risks due to drug interaction, the use of ranitidine may be preferable in the patients in whom it is desirable to administer an H2-receptor antagonist together with quinine.

Sparteine oxidation is practically abolished in quinidine-treated patients

In eight patients a sparteine-test was carried out immediately before and after 1 week of treatment with quinidine 600-800 mg day-1. Before treatment one patient was classified as a poor metaboliser (metabolic ratio: greater than or equal to 20), and seven patients as extensive metabolisers. During quinidine treatment, the formation of sparteine metabolites (2- and 5-dehydrosparteine) was practically abolished. Patients initially classified as extensive metabolisers thus exhibited the phenotype of poor metabolisers during quinidine treatment.

Pharmacokinetics of hydroxy-3(S)-dihydroquinidine in healthy volunteers after intravenous and oral administration

The pharmacokinetics of hydroxy-3(S)-dihydroquinidine (HDHQ) were studied in 6 healthy volunteers following a 15 min intravenous infusion of a 300 or 400 mg dose, a 300 mg oral dose in solution and a 300 mg tablet administration on three separate occasions (random order) with at least one week intervals. Using a specific HPLC assay for HDHQ, the post-infusion and post-absorption plasma HDHQ concentrations declined bi-exponentially. Both oral forms of HDHQ were absorbed rapidly (tmax 1 h-1.2 h) with an absolute bioavailability of the oral solution (F = 0.54 to 0.93) which was not significantly different from that of the tablet (F = 0.66 to 0.90). HDHQ was rapidly and extensively distributed to the tissues with a high steady-state volume of distribution (6.82 +/- 1.85 l X kg-1). Mean elimination half-life was 6.7 +/- 1.4 h after IV infusion, 8.4 +/- 1.7 h after the oral solution and 11.3 +/- 4.4 h after the tablet administration. HDHQ was partially eliminated from the body in the unchanged non-conjugated form by the urine and renal clearance represented approximately 50% of the total body clearance. These results show that HDHQ is rapidly and almost completely absorbed and has potential for a twice daily administration regimen for the treatment of cardiac arrhythmias.

Quinidine-nifedipine interaction

Quinidine pharmacokinetics are known to be altered by a number of drugs. We present a case where dose-related increases in quinidine serum concentrations were significantly suppressed by concurrent nifedipine therapy. Clinicians should be alert to the possibility of an alteration in quinidine serum concentrations when instituting or discontinuing nifedipine in patients receiving quinidine.

Quinidine induced lupus syndrome

Five patients with a lupus-like syndrome secondary to quinidine are described. Eleven other cases have previously been reported. The quinidine induced lupus syndrome is similar to that seen with procainamide and hydralazine treatment but occurs less frequently. The lower incidence may reflect a difference in the metabolism of quinidine. Quinidine should be considered as potentially responsible when multisystem disease appears in patients receiving this drug.