Mechanism of warfarin potentiation by amiodarone: dose--and concentration--dependent inhibition of warfarin elimination

Chinese Patients With Heart Valve Replacement Do Not Benefit From Warfarin Pharmacogenetic Testing on Anticoagulation Outcomes

BACKGROUND

Genotype-guided warfarin dosing has been shown in some randomized trials to improve anticoagulation outcomes in individuals of European ancestry; yet, its utility in Chinese patients with heart valve replacement remains unresolved.

METHODS

A total of 2264 patients who underwent heart valve replacement at Wuhan Asia Heart Hospital were enrolled in this study. Patients were randomly divided into 2 groups, namely, a genotype-guided and a traditional clinically guided warfarin dosing group. In the genotype-guided group (n = 1134), genotyping for CYP2C9 and VKORC1 (-1639 G→A) was performed using TaqMan genotyping assay. Warfarin doses were predicted with the International Warfarin Pharmacogenetics Consortium algorithm. Patients in the control group (n = 1130) were clinically guided. The primary outcome was to compare the incidence of adverse events (major bleeding and thrombotic) during a 90-day follow-up period between 2 groups. Secondary objectives were to describe effects of the pharmacogenetic intervention on the first therapeutic-target-achieving time, the stable maintenance dose, and the hospitalization days.

RESULTS

A total of 2245 patients were included in the analysis. Forty-nine events occurred during follow-up. Genotype-guided dosing strategy did not result in a reduction in major bleeding (0.26% versus 0.63%; hazard ratio, 0.44; 95% confidence interval, 0.13-1.53; P = 0.20) and thrombotic events (0.89% versus 1.61%; hazard ratio, 0.56; 95% confidence interval, 0.27-1.17; P = 0.12) compared with clinical dosing group. Compared with traditional dosing, patients in the genotype-guided group reached their therapeutic international normalized ratio in a shorter time (3.8 ± 2.0 versus 4.4 ± 2.0 days, P < 0.001). There was no difference in hospitalization days (P = 0.28).

CONCLUSIONS

Warfarin pharmacogenetic testing according to the International Warfarin Pharmacogenetics Consortium algorithm cannot improve anticoagulation outcomes in Chinese patients with heart valve replacement.

Clinical outcomes in patients with atrial fibrillation receiving amiodarone on NOACs vs. warfarin

PURPOSE

Amiodarone is a potent inhibitor of the CYP450:3A4 and inhibitor of the P-glycoprotein, both of which metabolize new oral anticoagulants (NOACs). Patients who are on NOACs and are concomitantly treated with amiodarone may have a higher risk of major bleeding according to recent retrospective trials. Whether this increased risk outweighs the benefits of NOACs compared to warfarin is unknown. We aimed to compare clinical outcomes between NOACs and warfarin in patients with atrial fibrillation (AF) being treated with amiodarone.

METHODS

We performed a systematic review of MEDLINE, Cochrane, and Embase for randomized controlled trials that compared NOACs to warfarin for prophylaxis of ischemic stroke/thromboembolic events (TEs) in patients with AF and reported outcomes on TE, major bleeding, and intracranial bleeding (ICB). Risk ratio (RR) and 95% confidence intervals were measured using the Mantel-Haenszel method. Fixed effects model was used, and if heterogeneity (I2) was > 25%, effects were analyzed using a random model.

RESULTS

A total of four studies comparing NOACs to warfarin were included in the analysis. The total number of patients on amiodarone was 6197. Mean follow up was 23 ± 5 months. No statistically significant difference for TE prevention (RR, 0.73; 95% CI 0.50-1.07), major bleeding (RR, 1.02; 95% CI 0.68-1.53), or ICB outcomes (RR, 0.58; 95% CI 0.22-1.51) between patients on NOACs + amiodarone when compared to patients on warfarin + amiodarone.

CONCLUSION

Among patients with AF taking amiodarone, there is no increased risk of stroke, major bleeding, or ICB with NOACs compared to warfarin.

Efficacy and Safety Outcomes of Direct Oral Anticoagulants and Amiodarone in Patients with Atrial Fibrillation

BACKGROUND

Direct oral anticoagulants (DOACs) and amiodarone are widely used in the treatment of nonvalvular atrial fibrillation. The DOACs are P-glycoprotein (P-gp) and cytochrome p-450 (CYP3A4) substrates. Direct oral anticoagulant levels may be increased by the concomitant use of potent dual P-gp/CYP3A4 inhibitors, such as amiodarone, which can potentially translate into adverse clinical outcomes. We aimed to assess the efficacy and safety of drug-drug interaction by the concomitant use of DOACs and amiodarone.

METHODS

We performed a systematic review of MEDLINE, the Cochrane Central Register of Clinical Trials, and Embase, limiting our search to randomized controlled trials of patients with atrial fibrillation that have compared DOACs versus warfarin for prophylaxis of stroke or systemic embolism, to analyze the impact on stroke or systemic embolism, major bleeding, and intracranial bleeding risk in patients with concomitant use of amiodarone. Risk ratio (RR) 95% confidence intervals were measured using the Mantel-Haenszel method. The fixed effects model was used owing to heterogeneity (I²) < 25%.

RESULTS

Four trials with a total of 71,683 patients were analyzed, from which 5% of patients (n = 3212) were concomitantly taking DOAC and amiodarone. We found no statistically significant difference for any of the clinical outcomes (stroke or systemic embolism [RR 0.85; 95% CI, 0.67-1.06], major bleeding [RR 0.91; 95% CI, 0.77-1.07], or intracranial bleeding [RR 1.10; 95% CI, 0.68-1.78]) among patients taking DOAC and amiodarone versus DOAC without amiodarone.

CONCLUSION

On the basis of the results of this meta-analysis, co-administration of DOACs and amiodarone, a dual P-gp/CYP3A4 inhibitor, does not seem to affect efficacy or safety outcomes in patients with atrial fibrillation.

Are Evidence Standards Different for Genomic- vs. Clinical-Based Precision Medicine? A Quantitative Analysis of Individualized Warfarin Therapy

Evidence requirements for implementation of precision medicine (PM), whether informed by genomic or clinical data, are not well defined. Evidence requirements are driven by uncertainty and its attendant consequences; these aspects can be quantified by a novel technique in health economics: value of information analysis (VOI). We utilized VOI analysis to compare the evidence levels over time for warfarin dosing based on pharmacogenomic vs. amiodarone-warfarin drug-drug interaction information. The primary outcome was the expected value of perfect information (EVPI), which is an estimate of the upper limit of the societal value of conducting future research. Over the past decade, the EVPI for the pharmacogenomic strategy decreased from $1,550 to $140 vs. $1,220 to $280 per patient for the drug-interaction strategy. Evidence levels thus appear to be higher for pharmacogenomic-guided vs. drug-interaction-guided warfarin dosing. Clinical guidelines and reimbursement policies for warfarin PM could be informed by these findings.

The effect of amiodarone on warfarin anticoagulation: a register-based nationwide cohort study involving the Swedish population

Essentials Data on the effect of introducing amiodarone in patients already using warfarin regime are scarce. Information on 754 patients was extracted from three nationwide registers in Sweden. With amiodaron, 37% of patients had an international normalized ratio (INR) over 3.0 To avoid bleeding, the initiation of amiodarone should be accompanied by closer INR monitoring.

SUMMARY

Background Data indicate that the interaction between warfarin and amiodarone results in an increased warfarin effect. There are several large, well-performed studies using genetic and clinical factors such as co-medication to predict an adequate starting dose of warfarin. However, longitudinal data on the effect of introducing amiodarone in patients on an ongoing warfarin regime are more scarce. Objectives An investigation of how initiation of amiodarone affects the anticoagulant effect and dosing of warfarin, using data from three nationwide registries. Patients/Methods In a retrospective cohort study including 754 patients, warfarin doses were compared between two 4-week periods, before and 18-21 weeks after initiating co-treatment with amiodarone. In addition, warfarin doses and international normalized ratio (INR) values were calculated week-by-week after the initiation of amiodarone. Results The initiation of amiodarone increased the mean INR from 2.6 to 3.1. The proportion of patients with a supratherapeutic INR over 3.0 and 4.0 increased from 12% to 37% and 0.9% to 5.5%, respectively. The subsequent mean decrease in warfarin dose was 24.6% (95% confidence interval [CI], 23.5, 25.6). The frequency of INR monitoring within 1 and 2 weeks after initiation of amiodarone was 67% and 90%. Conclusions Although warfarin doses in most patients were within the therapeutic range, more than one in three patients initiating co-treatment with amiodarone were exposed to a supratherapeutic anticoagulative effect within 3 weeks. In order to further avoid severe unnecessary bleeding, the initiation of amiodarone should be accompanied by closer INR monitoring, anticipating an average dose reduction of 25%.

Effects of amiodarone, thyroid hormones and CYP2C9 and VKORC1 polymorphisms on warfarin metabolism: a review of the literature

OBJECTIVE

To review the literature regarding the interaction among amiodarone therapy, thyroid hormone levels, and warfarin metabolism.

METHODS

A 73-year-old male with type 2 after describing an unusual case of amiodarone-induced thyrotoxicosis (AIT) who experienced a severe rise in international normalized ratio (INR) values after initiating warfarin therapy due to an unusual combination of excessive thyroid hormones, amiodarone therapy, and a genetic abnormality affecting warfarin metabolism.

RESULTS

Genetic analysis revealed that the patient was CYP2C9*2 wild-type, CYP2C9*3/*3 homozygous mutant, and VKORC1*3/*3 homozygous mutant. A review of the literature revealed that both mutations can independently affect warfarin metabolism. In addition, amiodarone therapy and the presence of thyrotoxicosis per se can affect warfarin metabolism and reduce the dose needed to maintain INR in the therapeutic range. The association of the 2 genetic polymorphisms in a patient with AIT is extremely rare and strongly impairs warfarin metabolism, exposing the patient to a high risk of overtreatment.

CONCLUSIONS

In patients with AIT, warfarin therapy should be gradually introduced, starting with a very low dose, because of the significant risk of warfarin overtreatment. Whether the genetic analysis of CYP2C9 and VKORC1 polymorphisms should be routinely performed in AIT patients remains conjectural.

Hospitalization for hemorrhage among warfarin recipients prescribed amiodarone

Amiodarone inhibits the hepatic metabolism of warfarin, potentiating its anticoagulant effect. However, the clinical consequences of this are not well established. Our objective in this study was to characterize the risk of hospitalization for a hemorrhage associated with the initiation of amiodarone within a cohort of continuous warfarin users in Ontario. We conducted a population-based retrospective cohort study among Ontario residents aged ≥66 years receiving warfarin. Among patients with at least 6 months of continuous warfarin therapy, we identified those who were newly prescribed amiodarone and an equal number who were not, matching on age, gender, year of cohort entry, and a high-dimensional propensity score. The primary outcome was hospitalization for hemorrhage within 30 days of amiodarone initiation. Between July 1, 1994, and March 31, 2009, we identified 60,497 patients with at least 6 months of continuous warfarin therapy, of whom 11,665 (19%) commenced amiodarone. For 7,124 (61%) of these, we identified a matched control subject who did not receive amiodarone. Overall, 56 (0.8%) amiodarone recipients and 23 (0.3%) control patients were hospitalized for hemorrhage within 30 days of initiating amiodarone (adjusted hazard ratio 2.45; 95% confidence interval, 1.49-4.02). Seven of 56 (12.5%) patients hospitalized for a hemorrhage after starting amiodarone died in hospital. In conclusion, initiation of amiodarone among older patients receiving warfarin is associated with a more than twofold increase in the risk of hospitalization for hemorrhage, with a relatively high fatality rate. Physicians should closely monitor patients who initiate amiodarone while receiving warfarin.

Amiodarone-associated optic neuropathy: a critical review

Although amiodarone is the most commonly prescribed anti-arrhythmic drug, its use is limited by serious toxicities, including optic neuropathy. Current reports of amiodarone-associated optic neuropathy identified from the Food and Drug Administration's Adverse Event Reporting System and published case reports were reviewed. A total of 296 reports were identified: 214 from the Adverse Event Reporting System, 59 from published case reports, and 23 from adverse events reports for patients enrolled in clinical trials. Mean duration of amiodarone therapy before vision loss was 9 months (range 1-84 months). Insidious onset of amiodarone-associated optic neuropathy (44%) was the most common presentation, and nearly one third were asymptomatic. Optic disk edema was present in 85% of cases. Following drug cessation, 58% had improved visual acuity, 21% were unchanged, and 21% had further decreased visual acuity. Legal blindness (<20/200) was noted in at least one eye in 20% of cases. Close ophthalmologic surveillance of patients during the tenure of amiodarone administration is warranted.

Risk factors of vitamin K antagonist overcoagulation

OBJECTIVES

The aims of this prospective study were to identify, in vitamin K antagonist (VKA)-treated patients, factors associated with INR values: (i) greater than 6.0. and (ii) ranging from 4.0 to 6.0 complicated with bleeding. We also assessed VKA-related morbidity in these patients.

METHODS

During a 6-month period, 3090 consecutive patients were referred to our Department of Internal Medicine, including 412 VKA-treated patients. At admission, the medical records of VKA-treated patients were reviewed for type, duration and indication of VKA therapy, previous medical history of VKA-related hemorrhage, comorbidities and concomitant medications.

RESULTS

Forty of the 412 VKA-treated patients (9.7%) exhibited oral anticoagulant related overcoagulation. VKA overcoagulation was associated with high morbidity, leading to major bleeding in 27.5% of cases; moreover, 12.5% of these patients died, death being mainly due to major bleeding. Under multivariate analysis, significant factors for VKA-related overcoagulation were as follows: previous medical history of VKA therapy-related hemorrhage (P=0.00001) and INR levels over therapeutic range (P=0.0006), chronic liver disease (P=0.03), therapy with amiodarone (P=0.009); in contrast, statin therapy was found to be a protective factor of VKA overcoagulation (P=0.008).

CONCLUSIONS

The knowledge of predictive factors of VKA-related overcoagulation seems of utmost importance to improve patients' management. Our study underlines the fact that the potential of drug interaction should be taken into account when choosing amiodarone for patients receiving VKAs. Interestingly, long-term (>6 month) statin therapy may be a protective factor of VKA overcoagulation. Our findings, therefore, suggest that there may be no need to switch long-term users of VKA and statin to a safer alternative therapy.

Analysis of potential interactions between warfarin and prescriptions in Estonian outpatients aged 50 years or more

In Estonia, warfarin is widely prescribed by general practitioners to prevent and treat thromboembolic diseases. To date, there has been no systematic analysis of the potential risk of warfarin interactions with other drugs in the outpatient population.

Objective

The aim of the study was to analyze the incidence of potential interactions in prescription schemes in Estonia in a cohort of outpatients receiving warfarin treatment.

Methods

The retrospective study population included 203,646 outpatients aged 50 years or older of whom 7,175 received warfarin therapy. Patients who had used at least one prescription drug for a minimum period of 7 days concomitantly with warfarin were analyzed. Potential drug interactions were analyzed using Epocrates online, Stockley's Drug Interactions and domestic drug interaction databases.

Results

The average number of drugs used concomitantly with warfarin was 4.8 (SD=1.9) (males: 4.7 SD=2.0, females: 4.9 SD=2.0). No potential interactions in treatment regimens were found in 38% of patients, one potential interaction was observed in 29% and two or more potential interactions were observed in 33% of patients. The mean number of all potential interactions was 1.2 per patient and about the same in men and women. Potential interactions were associated with the number of drugs. Warfarin-related interactions were detected in 57% of patients, and the number of interactions related to warfarin per patient varied from 1 to 5. Most frequent were use of warfarin with NSAIDs (14%), followed by simvastatin (9%) and amiodarone (7%).

Conclusions

This study shows that 57% of outpatients in Estonia receiving warfarin have drugs potentially interacting with warfarin in their treatment schemes. Most interactions (14%) with warfarin are associated with the prescription of NSAIDs.

Dronedarone and vitamin K antagonists: a review of drug-drug interactions

Adverse drug events affect millions of patients each year. An important drug-drug interaction between amiodarone and vitamin K antagonists is encountered frequently in daily clinical practice. Warfarin, a commonly used anticoagulant, is a mixture of 2 optically active isomers (R and S enantiomers). The S enantiomer is approximately 3 times more potent than the R enantiomer and is metabolized primarily by CYP2C9. Inhibition of CYP2C9 by amiodarone and its major metabolite potentiates the anticoagulant effects of warfarin, increasing the risk of serious bleeding. In contrast, dronedarone, a synthetic derivative of amiodarone the structure of which lacks the iodine moiety, is less likely to cause a drug-drug interaction with warfarin. Accordingly, dronedarone may be a particularly attractive antiarrhythmic choice among patients with atrial fibrillation who are also being treated with warfarin.

An evaluation of the early pharmacodynamic response after simultaneous initiation of warfarin and amiodarone

Amiodarone inhibits the metabolism of warfarin. Previous studies characterizing this drug interaction have focused on the effect of adding amiodarone to stable doses of warfarin. The objective of this study was to assess whether simultaneous initiation of warfarin and amiodarone results in early alteration of the international normalized ratio (INR) response to warfarin. Patients initiated on warfarin and amiodarone during the same hospitalization were included in the amiodarone (AMIO) group. Patients initiated on warfarin alone (n = 42) were identified for the CONTROL group. The AMIO and CONTROL groups were matched based on age, gender, and ejection fraction <40% using propensity score matching (final n = 18 patients per group). Total and average daily warfarin dose was lower in the AMIO group, yet INR values were similar on each day between the 2 groups. More patients in the AMIO group had an INR greater than 2 during the 5-day observation period as compared to the CONTROL group. In addition, there were trends toward greater deviation from INR values expected with a 5-mg daily warfarin dose among AMIO group patients. Simultaneous initiation of warfarin and amiodarone leads to an enhanced pharmacodynamic response to warfarin early in therapy. Although these data should be viewed as hypothesis generating, cautious dosing and monitoring with simultaneous initiation of warfarin and amiodarone may be warranted.

Effect of tecarfarin, a novel vitamin K epoxide reductase inhibitor, on coagulation in beagle dogs

BACKGROUND AND PURPOSE

Tecarfarin (ATI-5923) is a novel vitamin K epoxide reductase inhibitor that is metabolized by esterase (mainly human carboxylesterase 2) to a single major metabolite, ATI-5900, in rats, dogs and humans. Tecarfarin is not significantly metabolized by CYP450 enzymes. The objective of this study was to test and compare the efficacy of tecarfarin with that of warfarin, when administered either intravenously or once a day orally, to produce stable anticoagulation in beagle dogs.

EXPERIMENTAL APPROACH

Effects on coagulation were assessed by measuring the activity levels of Factor VII and Factor X and thromboplastin-induced coagulation times, reported as prothrombin time (PT).

KEY RESULTS

Continuous intravenous infusions and oral administration of tecarfarin and warfarin caused a dose-dependent decrease in activity of Factor VII and Factor X, and associated increase in PT. Intravenous fresh frozen canine plasma or subcutaneous vitamin K(1) treatment reversed the anticoagulant effects of orally administered tecarfarin. Consistent with the inhibitory effects of amiodarone on CYP2C9, co-administration of amiodarone significantly increased the anticoagulation effect of warfarin and plasma warfarin concentrations. In contrast, amiodarone had no effect on the anticoagulation induced by tecarfarin or tecarfarin plasma concentrations in this model.

CONCLUSIONS AND IMPLICATIONS

Overall, the data presented herein indicate that tecarfarin, via a vitamin K-dependent mechanism, causes changes in key parameters of haemostasis in beagle dogs that are consistent with effective anticoagulation. Compared to warfarin it has a decreased potential to interact metabolically with drugs that inhibit CYP450 enzymes and, therefore, may offer an improved safety profile for patients.

Significant pharmacokinetic and pharmacodynamic interaction of warfarin with the NO-independent sGC activator HMR1766

HMR1766 is a new nitric oxide (NO)-independent activator of soluble guanylyl cyclase (sGC) in development for the treatment of cardiovascular diseases and chronic heart failure. A significant fraction of patients to be treated with HMR1766 is expected to be maintained on warfarin. Because HMR1766 is an inhibitor and warfarin a substrate of CYP2C9, the authors studied whether warfarin pharmacokinetics and pharmacodynamics are influenced by HMR1766. Eighteen healthy males were to receive a single oral dose of 20 mg warfarin each under steady-state conditions of HMR1766 or placebo. Plasma concentrations of HMR1766, (R)- and (S)-warfarin, and its 7-hydroxy-metabolites were determined using high-performance liquid chromatography and prothrombin time, and the international standardized ratio was determined by the nephelometric method. (S)-Warfarin AUC(inf) and t(1/2) were 106,471 h x microg/L and 82.92 hours versus 33,148 h x microg/L under HMR1766 and 31.72 hours under placebo, and the maximum decrease in prothrombin time values after warfarin dosing was 58.75% versus 39.94%. These data demonstrate a CYP2C9-mediated pharmacokinetic interaction with pharmacodynamic, clinically relevant consequences, which might require warfarin dose adjustment.

The influence of co-treatment with carbamazepine, amiodarone and statins on warfarin metabolism and maintenance dose

AIMS

Warfarin is a frequently used anticoagulant drug with narrow therapeutic index and high interindividual variability in the dose requirement. We have previously shown that warfarin dose is influenced by cytochrome P450 (CYP) 2C9 genotype, age, body weight and co-treatment with drugs that interfere with warfarin metabolism. As, in many patients, drug co-treatment cannot be avoided, we investigated the effect of co-treatment with carbamazepine, amiodarone and statins on warfarin metabolism and maintenance dose.

METHODS

Caucasian patients on stable maintenance warfarin therapy with CYP2C9*1/*1 genotype (n=82) were included in the study. Plasma concentrations of (S)- and (R)-warfarin as well as warfarin hydroxylated metabolites were determined using HPLC assay and corresponding clearances of (S)- and (R)-warfarin were calculated.

RESULTS

Patients co-treated with carbamazepine (n=5) had significantly higher plasma 10-hydroxywarfarin concentrations than patients not taking any interacting drugs (n=54) (median: 0.327 microg/ml vs 0.030 microg/ml, p=0.003). (S)- and (R)-warfarin clearances were also higher in the carbamazepine co-treated group (p=0.003), as were warfarin dose requirements (median: 9.00 mg/day vs 3.86 mg/day, p=0.003). Under the conditions of this study, patients co-treated with amiodarone (n=6) did not differ significantly regarding any measured characteristic from patients with no interacting drug treatment, while patients co-treated with simvastatin or lovastatin (n=17) had lower 10-hydroxywarfarin concentration (p=0.02).

CONCLUSIONS

We confirmed important interaction between carbamazepine and warfarin metabolism which can be of major clinical importance. If treatment with carbamazepine cannot be avoided, patients taking warfarin should be frequently monitored, especially when initiating or stopping carbamazepine therapy.

The use of wireless laptop computers for computer-assisted learning in pharmacokinetics

OBJECTIVE

To implement computer-assisted learning workshops into pharmacokinetics courses in a doctor of pharmacy (PharmD) program.

DESIGN

Workshops were designed for students to utilize computer software programs on laptop computers to build pharmacokinetic models to predict drug concentrations resulting from various dosage regimens. In addition, students were able to visualize through graphing programs how altering different parameters changed drug concentration-time curves. Surveys were conducted to measure students' attitudes toward computer technology before and after implementation. Finally, traditional examinations were used to evaluate student learning.

ASSESSMENT

Doctor of pharmacy students responded favorably to the use of wireless laptop computers in problem-based pharmacokinetic workshops. Eighty-eight percent (n = 61/69) and 82% (n = 55/67) of PharmD students completed surveys before and after computer implementation, respectively. Prior to implementation, 95% of students agreed that computers would enhance learning in pharmacokinetics. After implementation, 98% of students strongly agreed (p < 0.05) that computers enhanced learning. Examination results were significantly higher after computer implementation (89% with computers vs. 84% without computers; p = 0.01).

CONCLUSION

Implementation of wireless laptop computers in a pharmacokinetic course enabled students to construct their own pharmacokinetic models that could respond to changing parameters. Students had greater comprehension and were better able to interpret results and provide appropriate recommendations. Computer-assisted pharmacokinetic techniques can be powerful tools when making decisions about drug therapy.

Effect of Amiodarone on the Serum Concentration/Dose Ratio of Metoprolol in Patients with Cardiac Arrhythmia

Amiodarone has pharmacokinetic interactions with a number of therapeutic drugs, including warfarin, phenytoin, flecainide, and cyclosporine. Metoprolol is mainly metabolized by CYP2D6, and desethylamiodarone, a metabolite of amiodarone, has a markedly greater inhibitory effect on CYP2D6 than amiodarone. Therefore, the goal of this study was to evaluate the effect of amiodarone and desethylamiodarone on the serum concentration/dose ratio (C/D) of metoprolol in 120 inpatients with cardiac arrhythmias that received either metoprolol and amiodarone (MET+AMD group, n=30) or metoprolol alone (MET group, n=90). The ratio of administered metoprolol was compared between the MET and the MET+AMD groups. The dose of metoprolol and patient age were significantly higher in the MET group when compared with the MET+AMD group (1.00+/-0.480 versus 0.767+/-0.418 mg/kg/day, p<0.050; 68.6+/-10.6 versus 57.6+/-14.1 years, p<0.001, respectively), but the C/D ratio was significantly lower in the MET group than in the MET+AMD group (90.8+/-64.0 versus 136+/-97.8, p<0.01). Furthermore, a significant correlation was found between the C/D ratio and desethylamiodarone concentration (n=30, r=0.371, p<0.01). The results suggest that there is a significant interaction between amiodarone and metoprolol via desethylamiodarone-induced inhibition of CYP2D6. Therefore, careful monitoring of metoprolol concentrations/bioactivity of CYP2D6 is required in the context of co-administration of amiodarone and metoprolol.

The influence of ethnicity on warfarin dosage requirement

BACKGROUND

The optimal dose of warfarin varies among individuals, and the prediction of a maintenance dose is difficult. Ethnicity has been reported to influence warfarin dosing.

OBJECTIVE

To quantitate the influence of ethnicity on warfarin dose requirement.

METHODS

We conducted a retrospective cohort study at a university anticoagulation clinic to evaluate the influence of ethnicity on warfarin dose. Inclusion criteria included age > or = 18 years, target international normalized ratio (INR) 2-3, and warfarin management within the clinic for > or = 3 months with a minimum of 5 clinic visits. We collected clinical and demographic data including age, gender, weight, ethnicity, disease states, concomitant medications, indication, weekly warfarin dosage, and INR. To assess potential confounders, multivariate, repeated-measures regression analysis was used to identify and adjust for variables that may influence the maintenance dose of warfarin.

RESULTS

Of the 345 patients who met the inclusion criteria, 27% were Asian American, 6% Hispanic, 54% white, and 14% African American. The adjusted mean (95% CI) weekly warfarin doses for patients with an INR goal of 2 to 3 were Asian Americans 24 mg (21 to 27), Hispanics 31 mg (25 to 37), whites 36 mg (34 to 39), and African Americans 43 mg (39 to 47) (p < 0.001). Additional factors found to influence warfarin dose requirement included age, weight, concomitant use of amiodarone, and diagnosis of venous thromboembolism.

CONCLUSIONS

Warfarin dose requirements vary across ethnic groups even when adjusted for confounding factors, suggesting that genetic variation contributes to interpatient variability.

Pharmacokinetic Characteristics of Amiodarone in Long-Term Oral Therapy in Japanese Population

To evaluate the pharmacokinetic properties and an optimum dose schedule of amiodarone in long-term oral therapy, serum concentrations of amiodarone and its metabolite, desethylamiodarone, were monitored from 345 Japanese inpatients who received amiodarone therapy for a variety of cardiac arrhythmias. Serum amiodarone and desethylamiodarone concentrations were determined by high performance liquid chromatography system. It was observed that the amiodarone and desethylamiodarone concentrations gradually increased with time. The frequency distribution in the amiodarone clearance of 245 subjects, who received fixed maintenance amiodarone therapy for at least 6 months, was nearly a unimodal one. The variation in the ratio of desetylamiodarone to amiodarone concentration in serum was very small. Although no differences in age, dose, dose duration, amiodarone or desethyamiodarone concentration or ratio were observed between men and women: however, the mean amiodarone clearance of women was significantly higer than that of men. The laboratory data were mostly within normal values and no significant relations were observed between serum amiodarone concentration and clinical laboratory data. These results suggest that the individual variation in pharmacokinetics of amiodarone is comparatively small, which might be sufficient to decide that the maintenance dose was the same one (200 mg/d) in long-term oral amiodarone therapy.

Stereoselective Effect of Amiodarone on the Pharmacokinetics of Racemic Carvedilol

We investigated whether there was a stereoselective effect of amiodarone on the pharmacokinetics of carvedilol. Among a series of 106 inpatients with heart failure, 52 received carvedilol monotherapy (carvedilol group) and 54 received carvedilol plus amiodarone (carvedilol+amiodarone group). The serum carvedilol concentration administered/dose ratio was compared between the two groups based on HPLC measurement of the serum levels of carvedilol, amiodarone, and desethylamiodarone. In 6 patients from the carvedilol group, serum carvedilol levels were compared before and after coadministration of amiodarone. There was no significant between-group difference of the serum concentration to dose (C/D ratio) for the R-enantiomer carvedilol, however, the C/D ratio for the S-enantiomer and the serum S-carvedilol to R-carvedilol (S/R) ratio were both significantly lower in the carvedilol group than in the carvedilol+amiodarone group(47.8+/-56.7 versus 95.3+/-105 ng/mg/kg, P=0.0048 and 0.460+/-0.207 versus 0.879+/-0.377 ng/mg/kg, P<0.001), respectively. Furthermore, the mean S-carvedilol concentration over 14 days of coadministration with amiodarone was higher than that before coadministration (6.54+/-1.73 ng/mL versus 3.03+/-0.670 ng/mL, P<0.001). These results suggest that metabolism of S-carvedilol was markedly inhibited by coadministration of amiodarone.

Complex drug-drug-disease interactions between amiodarone, warfarin, and the thyroid gland

Many patients with cardiac arrhythmias require concomitant therapy with warfarin and amiodarone. Beyond the predictable pharmacokinetic drug-drug interaction requiring a significant warfarin dose reduction, the iodine-rich amiodarone affects the thyroid gland, causing overt hypothyroidism or thyrotoxicosis in 14%-18% of cases. In turn, thyroid disorders may affect warfarin sensitivity, with hypothyroidism and thyrotoxicosis resulting in increased or decreased warfarin requirements, respectively. We describe 3 patients on concomitant amiodarone and warfarin who developed amiodarone-induced thyrotoxicosis heralded by a significant decrease in warfarin requirements. We review the literature on the mechanisms of the complex drug-drug and drug-disease interactions within the thyroid gland, warfarin, and amiodarone triad. Given that significant thyroid disorders may be only mildly symptomatic and thus may escape clinical detection, we suggest that thyroid function should be tested in any patient with otherwise unexplained changes in warfarin dose requirements, particularly if concomitantly treated with amiodarone.

Potentially significant drug interactions of class III antiarrhythmic drugs

Class III antiarrhythmic drugs, especially amiodarone (a broad-spectrum antiarrhythmic agent), have gained popularity for use in clinical practice in recent years. Other class III antiarrhythmic drugs include bretylium, dofetilide, ibutilide and sotalol. These agents are effective for the management of various types of cardiac arrhythmias both atrial and ventricular in origin. Class III antiarrhythmic drugs may interact with other drugs by two major processes: pharmacodynamic and pharmacokinetic interactions. The pharmacodynamic interaction occurs when the pharmacological effects of the object drug are stimulated or inhibited by the precipitant drug. Pharmacokinetic interactions can result from the interference of drug absorption, metabolism and/or elimination of the object drug by the precipitant drug. Among the class III antiarrhythmic drugs, amiodarone has been reported to be involved in a significant number of drug interactions. It is mainly metabolised by cytochrome P450 (CYP)3A4 and it is a potent inhibitor of CYP1A2, 2C9, 2D6 and 3A4. In addition, amiodarone may interact with other drugs (such as digoxin) via the inhibition of the P-glycoprotein membrane transporter system, a recently described pharmacokinetic mechanism of drug interactions. Bretylium is not metabolised; it is excreted unchanged in the urine. Therefore the interactions between bretylium and other drugs (including other antiarrhythmic drugs) is primarily through the pharmacodynamic mechanism. Dofetilide is metabolised by CYP3A4 and excreted by the renal cation transport system. Drugs that inhibit CYP3A4 (such as erythromycin) and/or the renal transport system (such as triamterene) may interact with dofetilide. It appears that the potential for pharmacokinetic interactions between ibutilide and other drugs is low. This is because ibutilide is not metabolised by CYP3A4 or CYP2D6. However, ibutilide may significantly interact with other drugs by a pharmacodynamic mechanism. Sotalol is primarily excreted unchanged in the urine. The potential for drug interactions due to hepatic enzyme induction or inhibition appears to be less likely. However, a number of drugs (such as digoxin) have been reported to interact with sotalol pharmacodynamically. If concurrent use of a class III antiarrhythmic agent and another drug cannot be avoided or no published studies for that particular drug interaction are available, caution should be exercised and close monitoring of the patient should be performed in order to avoid or minimise the risks associated with a possible adverse drug interaction.

Amiodarone Interaction Time Differences with Warfarin and Digoxin

Background:

Amiodarone has pharmacokinetic and pharmacodynamic interactions with various therapeutic agents. The mechanism of interaction between warfarin and amiodarone is the inhibition of warfarin metabolism by amiodarone, and that between digoxin and amiodarone is the inhibition of digoxin transport by amiodarone.

Objective:

To investigate the pharmacokinetic magnitude of the time differences between amiodarone–warfarin and amiodarone–digoxin interactions.

Methods:

Amiodarone was administered concomitantly to 79 inpatients who had been receiving fixed-maintenance doses of warfarin or digoxin. Seventy-seven inpatients were prescribed warfarin therapy, and 54 inpatients were prescribed digoxin therapy. To determine serum concentrations of the warfarin enantiomers digoxin, amiodarone, and desethylamiodarone blood samples were obtained with coadministration of amiodarone. Serum S- and R-warfarin, amiodarone, and desethylamiodarone concentrations were measured by HPLC methods, and serum digoxin concentrations were measured by a fluorescence polarization immunoassay.

Results:

A remarkable decrease of S-warfarin clearance was observed within approximately the first 2 weeks after coadministration of amiodarone. Only a small decrease in R-warfarin clearance was observed. Digoxin clearance was gradually decreased with time, and a good reverse correlation was obtained between amiodarone or desethylamiodarone concentrations and digoxin clearance.

Conclusions:

Relatively short-term monitoring of warfarin clearance is required when amiodarone is coadministered. Long-term monitoring of digoxin serum amiodarone and desethylamiodarone concentrations is necessary to detect the amiodarone–digoxin interaction.

Factors affecting warfarin therapy following cardiac valve surgery

OBJECTIVE

To determine the factors that affect the initial response to warfarin therapy in Korean patients after cardiac valve surgery.

METHODS

A retrospective analysis of 127 patients who had undergone cardiac valve surgery at Seoul National University Hospital was performed. On the first day, most patients received warfarin 5 mg, while some received an individualized warfarin dose according to their physician's decision. Doses to be given on the following days were determined based on daily international normalized ratio (INR) and the previous doses. To measure warfarin sensitivity, the warfarin dose index (WDI), defined as the INR divided by the mean warfarin dose administered during the preceding 3 days, was introduced. The effects of age, gender, weight, serum albumin concentration, baseline INR, cardiopulmonary bypass time, and concurrent administration of amiodarone were evaluated.

RESULTS

The patients' weight, initial serum albumin concentration, and baseline INR value influenced their initial response to warfarin. The initial WDI correlated negatively with the initial serum albumin concentration (p < 0.001) and body weight (p < 0.05) and positively with the baseline INR (p < 0.01). The initial WDI of the patients taking amiodarone was significantly higher (mean +/- SD 0.74 +/- 0.34) than that of patients without amiodarone (0.46 +/- 0.22) (p < 0.001). Maintenance doses correlated negatively with the initial warfarin response (p < 0.001) and positively with body weight (p = 0.053).

CONCLUSIONS

The factors associated with an increased initial warfarin response in patients after cardiac valve surgery were high baseline INR, low postoperative serum albumin concentration, and concurrent administration of amiodarone. Thus, patients with any of these factors should receive a smaller initial warfarin dose. Also, to predict the warfarin maintenance dose from the initial response, the effect of transient changes in the sensitivity to warfarin during the initial period should be considered.

Lack of interaction between amiodarone and mexiletine in cardiac arrhythmia patients

Amiodarone has pharmacokinetic interactions with various therapeutic agents, including phenytoin, flecainide, and cyclosporine. Mexiletine is metabolized by CYP2D6 and CYP1A2. The objective of this study is to evaluate the effect of amiodarone on the pharmacokinetics of mexiletine through its inhibition of various cytochrome P450 (CYP) subtypes. In a series of 181 inpatients with supraventricular tachyarrhythmias, 26 inpatients received mexiletine and amiodarone therapy (MEX + AMD group), and the others received mexiletine therapy (MEX group). In 10 inpatients of the MEX + AMD group, the mexiletine clearance (CL(MEX)/F) before and after coadministration of amiodarone was compared. CL(MEX)/F was also compared in the MEX and MEX + AMD groups after the start of amiodarone therapy. Serum mexiletine, amiodarone, and desethylamiodarone concentrations were measured by an HPLC method. The CL(MEX)/F was estimated by the Bayesian method using population pharmacokinetic analysis. There was no significant difference in CL(MEX)/F before and after 1-month coadministration of amiodarone in 10 inpatients of the MEX + AMD group. Although serum amiodarone and desethylamiodarone concentrations gradually increased with time after the start of amiodarone therapy in these patients, CL(MEX)/F showed no change at 3 and 5 months after the start of amiodarone therapy. There was no significant difference in CL(MEX)/F of the MEX group and the MEX + AMD group. The results suggest that the pharmacokinetics of mexiletine is not affected by amiodarone in patients with cardiac arrhythmias.

Clinical observations with the amiodarone/warfarin interaction: dosing relationships with long-term therapy

OBJECTIVES

The interaction between amiodarone and warfarin has only been described in patients being followed up for relatively short time periods. The objectives of this study were to characterize the interaction between these two agents in a clinical situation over a longer period of time in a larger cohort of patients, and to determine the relationship between the maintenance dose of amiodarone and the resultant need to adjust the dose of warfarin.

DESIGN

This was an observational trial of a cohort of patients receiving a stable warfarin regimen in whom oral amiodarone was initiated. Patients received both amiodarone and warfarin for at least 1 year, and the dosage of warfarin was adjusted as clinically necessary to achieve an international normalized ratio of 2 to 3. Data from a total of 43 patients were analyzed.

RESULTS

At baseline, prior to initiation of amiodarone, the warfarin dose was 5.2 +/- 2.6 mg/d. The magnitude of the interaction between these two agents peaked at 7 weeks, which resulted in a 44% mean maximum reduction in the warfarin dose. The warfarin dose inversely correlated with the maintenance dose of amiodarone (r(2) = 0.94, p < 0.005). Minor bleeding episodes occurred in five patients (12%). For patients receiving amiodarone maintenance doses of 400, 300, 200, or 100 mg/d, it is recommended that the daily warfarin dose be reduced by approximately 40%, 35%, 30%, or 25%, respectively.

CONCLUSIONS

The magnitude of the amiodarone/warfarin interaction is highly dependent on the maintenance dose of amiodarone. This relationship can aid clinicians in adjusting the dose of warfarin patients receiving long-term amiodarone treatment.

Cardiovascular drug-drug interactions

The drug-drug interactions discussed in this article have either documented or suspected clinical relevance for patients with cardiovascular disease and the clinician involved in the care of these patients. Oftentimes, drug-drug interactions are difficult, if not impossible, to predict because of the high degree of interpatient variability in drug disposition. Certain drug-drug interactions, however, may be avoided through knowledge and sound clinical judgment. Every clinician should maintain a working knowledge of reported drug-drug interactions and an understanding of basic pharmacokinetic and pharmacodynamic principles to help predict and minimize the incidence and severity of drug-drug interactions.

The stereoselective effects of bucolome on the pharmacokinetics and pharmacodynamics of racemic warfarin

The objective of this study was to investigate the stereoselective influence of bucolome on the pharmacokinetics and pharmacodynamics of warfarin in Japanese inpatients with heart disease. Thirty patients were administered a fixed-maintenance dose of warfarin alone once a day for at least 7 days. The other 25 patients were concomitantly administered warfarin and a 300 mg dose of bucolome once a day, and blood samples were collected on days 1, 4, 7, 14, or 21 after administration of bucolome. Serum concentration of warfarin enantiomers was measured by a chiral reversed-phase HPLC-ultraviolet detection method. The PT-INR was used as a measure of the pharmacodynamic effect of warfarin. Coadministration of bucolome and warfarin had no effect on serum (R)-warfarin concentration and significantly increased serum (S)-warfarin concentration compared with warfarin alone. The PT-INR of warfarin alone was significantly lower with bucolome cotreatment. These results indicate that the augmented anticoagulant effect of warfarin by bucolome is due to inhibition of (S)-warfarin metabolism in vivo. When bucolome is added to a stabilized regimen of warfarin therapy, the dose of warfarin should be reduced by about 30% to 60%, and caution should be exercised during the first 7 days after coadministration of bucolome.

Antiarrhythmic agents: drug interactions of clinical significance

The management of cardiac arrhythmias has grown more complex in recent years. Despite the recent focus on nonpharmacological therapy, most clinical arrhythmias are treated with existing antiarrhythmics. Because of the narrow therapeutic index of antiarrhythmic agents, potential drug interactions with other medications are of major clinical importance. As most antiarrhythmics are metabolised via the cytochrome P450 enzyme system, pharmacokinetic interactions constitute the majority of clinically significant interactions seen with these agents. Antiarrhythmics may be substrates, inducers or inhibitors of cytochrome P450 enzymes, and many of these metabolic interactions have been characterised. However, many potential interactions have not, and knowledge of how antiarrhythmic agents are metabolised by the cytochrome P450 enzyme system may allow clinicians to predict potential interactions. Drug interactions with Vaughn-Williams Class II (beta-blockers) and Class IV (calcium antagonists) agents have previously been reviewed and are not discussed here. Class I agents, which primarily block fast sodium channels and slow conduction velocity, include quinidine, procainamide, disopyramide, lidocaine (lignocaine), mexiletine, flecainide and propafenone. All of these agents except procainamide are metabolised via the cytochrome P450 system and are involved in a number of drug-drug interactions, including over 20 different interactions with quinidine. Quinidine has been observed to inhibit the metabolism of digoxin, tricyclic antidepressants and codeine. Furthermore, cimetidine, azole antifungals and calcium antagonists can significantly inhibit the metabolism of quinidine. Procainamide is excreted via active tubular secretion, which may be inhibited by cimetidine and trimethoprim. Other Class I agents may affect the disposition of warfarin, theophylline and tricyclic antidepressants. Many of these interactions can significantly affect efficacy and/or toxicity. Of the Class III antiarrhythmics, amiodarone is involved in a significant number of interactions since it is a potent inhibitor of several cytochrome P450 enzymes. It can significantly impair the metabolism of digoxin, theophylline and warfarin. Dosages of digoxin and warfarin should empirically be decreased by one-half when amiodarone therapy is added. In addition to pharmacokinetic interactions, many reports describe the use of antiarrhythmic drug combinations for the treatment of arrhythmias. By combining antiarrhythmic drugs and utilising additive electrophysiological/pharmacodynamic effects, antiarrhythmic efficacy may be improved and toxicity reduced. As medication regimens grow more complex with the aging population, knowledge of existing and potential drug-drug interactions becomes vital for clinicians to optimise drug therapy for every patient.

Steady-state interaction between amiodarone and phenytoin in normal subjects

Amiodarone has been reported to increase phenytoin levels. This study was designed to evaluate the pharmacokinetic basis of this interaction at steady-state. Pharmacokinetic parameters for phenytoin were determined after 14 days of oral phenytoin, 2 to 4 mg/kg/day, before and after oral amiodarone, 200 mg daily for 6 weeks in 7 healthy male subjects. During amiodarone therapy, area under the serum concentration time curve for phenytoin was increased from 208 +/- 82.8 (mean +/- standard deviation) to 292 +/- 108 mg.hr/liter (p = 0.015). Both the maximum and 24-hour phenytoin concentrations were increased from 10.75 +/- 3.75 and 6.67 +/- 3.51 micrograms/ml to 14.26 +/- 3.97 (p = 0.016) and 10.27 +/- 4.67 micrograms/ml (p = 0.012), respectively, during concomitant amiodarone treatment. Amiodarone caused a decrease in the oral clearance of phenytoin from 1.29 +/- 0.30 to 0.93 +/- 0.25 liters/hr (p = 0.002). These results were due to a reduction in phenytoin metabolism by amiodarone as evidenced by a decrease in the urinary excretion of the principal metabolite of phenytoin, 5-(p-hydroxyphenyl)-5-phenylhydantoin, 149 +/- 39.7 to 99.3 +/- 40.0 mg (p = 0.041) and no change in the unbound fraction of the total phenytoin concentration expressed as a percentage, 10.3 +/- 2.7 versus 10.7 +/- 2.1% (p = 0.28) during coadministration of amiodarone. The alterations in phenytoin pharmacokinetics suggest that steady-state doses of phenytoin of 2 to 4 mg/kg/day should be reduced at least 25% when amiodarone is concurrently administered. All dosage reductions should be guided by clinical and therapeutic drug monitoring.

Effect of amiodarone on the disposition of acetaminophen in the rat

Amiodarone has been demonstrated to form a cytochrome P-450Fe(II):metabolite complex. The administration of other agents which form this type of complex, such as troleandomycin, has been shown to deplete hepatic glutathione content. Depletion of glutathione will result in an increased synthesis of glutathione and an increased utilization of cysteine. Since inorganic sulfate and glutathione share cysteine as a common precursor, we postulated that amiodarone pretreatment may reduce the sulfation of drugs. To test this hypothesis, the effect of amiodarone pretreatment on acetaminophen disposition was examined in the rat. Acetaminophen (150 mg/kg) was administered to rats pretreated with amiodarone hydrochloride (100 mg/kg/d) or diluent for 5 d. There were no significant differences in the urinary recovery of acetaminophen sulfate or the partial clearance of acetaminophen to the sulfate metabolite between control and amiodarone-pretreated animals. There was a trend toward an increased urinary recovery of acetaminophen glucuronide in animals pretreated with amiodarone, but this did not reach statistical significance. Amiodarone pretreatment had no effect on the renal clearances of acetaminophen or its metabolites. These results suggest that amiodarone pretreatment does not alter the sulfation of drugs and that the formation of an amiodarone P-450Fe(II):metabolite complex is quantitatively insignificant.

A general overview of amiodarone toxicity: its prevention, detection, and management

Although amiodarone is a highly effective antiarrhythmic agent, it has a high incidence of side effects, some of which can be serious or even lethal. With close monitoring, side effects can be found in essentially all patients, but fortunately most of these are mild and well tolerated. Furthermore, many will respond to dosage reduction in a relatively short period of time, ie, days to weeks, which is remarkable considering the long period of time amiodarone has been shown to persist in tissues. There is reasonable evidence that toxicity, particularly the early toxic manifestations with large loading dosages, can be favorably modified by reducing the dosage. Similarly, reducing the maintenance dosage will, in most instances, reduce or eliminate most toxic manifestations. The mechanisms of toxic effects are uncertain, but suggestive evidence exists for and against both an immunologic reaction and an intracellular lysosomal lipoidosis. Principles of use of amiodarone should include individualizing administration of dosages for each patient due to the unusual pharmacokinetic properties of this drug and continuous long-term attempts at using the lowest effective dosage. There are no definite tests that predict amiodarone efficacy or toxicity, but the serum level can be used as a rough guide of absorption and distribution in the attempt to minimize the maintenance dosage. No guidelines regarding screening tests for toxicity can be made at this time since great variability in these tests has been reported, and no evidence exists for their benefit in preventing adverse effects to amiodarone. However, follow-up testing at the intervals noted in the package insert are reasonable and important. The possibility of interactions with drugs already reported and with others not yet reported should always be kept in mind, and appropriate monitoring for clinical evidence of toxicity due to the concomitantly used drugs should be undertaken. Amiodarone can have a tremendous beneficial effect in the proper circumstances, but it is a drug that should command utmost respect because of its side effects and requires constant vigilance from any physician wishing to use it.

Effect of amiodarone on the disposition of procainamide in the rat

We examined the effect of amiodarone on the disposition of procainamide in the rat to determine the mechanism of a reported interaction between amiodarone and procainamide and to determine the effect of amiodarone on drug acetylation. Animals received a 5-d pretreatment with amiodarone hydrochloride (100 mg/kg) or diluent prior to the intravenous administration of 50 mg/kg of procainamide hydrochloride. The plasma clearance, volume of distribution, and half-life of procainamide did not significantly differ between the two groups. The urinary recovery of N-acetylprocainamide was increased by 31% (p less than 0.01) in amiodarone pretreated animals. However, there was no change in the partial clearance of procainamide to N-acetylprocainamide. Neither the renal clearance of procainamide nor N-acetylprocainamide was altered by amiodarone pretreatment. These data suggest that amiodarone interacts with procainamide by reduction of an alternate pathway of elimination, possibly oxidative metabolism.

Amiodarone in the management of cardiac arrhythmias: current concepts

This article reviews current information on the clinical pharmacology, therapeutic utility, and adverse reactions of amiodarone, with emphasis on guidelines for its rational use.

Overview of the clinical pharmacology of antiarrhythmic drugs

Antiarrhythmic drugs have been recognized to possess 1 or more classes of antiarrhythmic action. This classification scheme is useful, but has major limitations because the available drugs and their metabolites have multiple actions. This report presents an overview of the distinguishing features of the most frequently used agents having class I or III actions. Agents with class I actions are local anesthetic agents that depress the fast inward depolarizing sodium current and thereby slow the rate of the rise of the action potential (phase 0). This category is further divided into classes IA, IB, and IC according to the degree of potency as sodium channel inhibitors, and the individual effects of the drug on action potential, conduction velocity and repolarization. Included in the spectrum of agents with class I action are quinidine, procainamide, disopyramide, lidocaine, tocainide, mexiletine, flecainide, amiodarone, encainide and lorcainide. The antiarrhythmic drugs that exert class III action lengthen repolarization and refractoriness; included in this category are amiodarone, quinidine, bretylium and sotalol. Because of the broad range of effects that antiarrhythmic agents may exert, safe and effective therapy requires a thorough familiarity with the pharmacologic profile of each drug administered and a careful evaluation of the presenting condition and the patient history. In some cases, a multiple drug regimen may be most appropriate. Various combinations such as class IA and IB agents, have been shown to slow conduction synergistically and increase refractoriness while keeping adverse effects to a minimum.

Amiodarone pharmacokinetics. I. Acute dose-dependent disposition studies in rats

Single intravenous bolus doses of amiodarone hydrochloride of 30, 60, 90 and 120 mg/kg were administered to male Sprague-Dawley rats to determine the effects of dose on amiodarone pharmacokinetics. Serial blood samples and total urine were collected over 48 hr and assayed for amiodarone and desethylamiodarone by HPLC. The blood amiodarone concentration-time curves for the four doses were best described by a triexponential equation with terminal half-lives (t1/2 gamma) ranging from 17 to 20 hr. Over the dose range studied, no changes in gamma, t1/2 gamma, or central compartment volume (Vc = 1.2-1.4 L/kg) were observed. On the other hand, reductions in amiodarone clearance (CL) and steady-state volume of distribution (Vss) of 44% (17.7 to 10.0 ml/min per kg) and 50% (16.4 to 8.2 L/kg), respectively, were noted as the dose of amiodarone increased. The conversion of amiodarone to desethylamiodarone (fm) was dose-independent and amounted to approximately 10% of each amiodarone dose. No amiodarone or desethylamiodarone was detected in the urine of any of the treated animals. The blood-to-plasma concentration ratio of amiodarone was concentration-independent and therefore did not account for the dose-dependent changes in Vss and CL observed. The data suggested that the dose-dependent changes noted were due to an alteration in the volume (s) of the peripheral tissue compartment(s).