Administration Of Dofetilide Research Articles

Evaluation of a Fully Automatic Measurement of Short-Term Variability of Repolarization on Intracardiac Electrograms in the Chronic Atrioventricular Block Dog

Background: Short-term variability (STV) of repolarization of the monophasic action potential duration (MAPD) or activation recovery interval (ARI) on the intracardiac electrogram (EGM) increases abruptly prior to the occurrence of ventricular arrhythmias in the chronic AV-block (CAVB) dog model. Therefore, this parameter might be suitable for continuous monitoring of imminent arrhythmias using the EGM stored on an implanted device. However, 24/7 monitoring would require automatic STVARI measurement by the device.Objective: To evaluate a newly developed automatic measurement of STVARI for prediction of dofetilide-induced torsade de pointes (TdP) arrhythmias in the CAVB-dog.Methods: Two retrospective analyses were done on data from recently performed dog experiments. (1) In seven anesthetized CAVB-dogs, the new automatic STVARI method was compared with the gold standard STVMAPD at baseline and after dofetilide administration (0.025 mg/kg in 5 min). (2) The predictive value of the automatic method was compared to currently used STVARI methods, i.e., slope method and fiducial segment averaging (FSA) method, in 11 inducible (≥3 TdP arrhythmias) and 10 non-inducible CAVB-dogs.Results: (1) The automatic measurement of STVARI had good correlation with STVMAPD (r2 = 0.89; p < 0.001). Bland-Altman analysis showed a small bias of 0.06 ms with limits of agreement between −0.63 and 0.76 ms. (2) STVARI of all three methods was significantly different between inducible and non-inducible dogs after dofetilide. The automatic method showed the highest predictive performance with an area under the ROC-curve of 0.93, compared to 0.85 and 0.87 of the slope and FSA methods, respectively. With a threshold of STV set at 1.69 ms, STVARI measured with the automatic method had a sensitivity of 0.91 and specificity of 0.90 in differentiating inducible from non-inducible subjects.Conclusion: We developed a fully-automatic method for measurement of STVARI on the intracardiac EGM that can accurately predict the occurrence of ventricular arrhythmias in the CAVB-dog. Future integration of this method into implantable devices could provide the opportunity for 24/7 monitoring of arrhythmic risk.

Different effects of amiodarone and dofetilide on the dispersion of repolarization between well-coupled ventricular and Purkinje fibers1.

Increased transmural dispersion of repolarization is an established contributing factor to ventricular tachyarrhythmias. In this study, we evaluated the effect of chronic amiodarone treatment and acute administration of dofetilide in canine cardiac preparations containing electrotonically coupled Purkinje fibers (PFs) and ventricular muscle (VM) and compared the effects to those in uncoupled PF and VM preparations using the conventional microelectrode technique. Dispersion between PFs and VM was inferred from the difference in the respective action potential durations (APDs). In coupled preparations, amiodarone decreased the difference in APDs between PFs and VM, thus decreasing dispersion. In the same preparations, dofetilide increased the dispersion by causing a more pronounced prolongation in PFs. This prolongation was even more emphasized in uncoupled PF preparations, while the effect in VM was the same. In uncoupled preparations, amiodarone elicited no change on the difference in APDs. In conclusion, amiodarone decreased the dispersion between PFs and VM, while dofetilide increased it. The measured difference in APD between cardiac regions may be the affected by electrotonic coupling; thus, studying PFs and VM separately may lead to an over- or underestimation of dispersion.

Expression of Kv11.1 Channels in Lungs and Role in Pulmonary Hypertension

BackgroundWhile Kv11.1 potassium selective channels are well known for their contribution to the heart repolarization little is known about their importance for the lung function. Here we show that Kv11.1 channels are abundantly expressed in the human and rat lung tissues.Methods &amp; ResultsWe found that, in the lungs of healthy humans and rats, Kv11.1 channel expression was limited to the large pulmonary arteries (PAs). In the lungs of human patients with chronic obstructive pulmonary disease (COPD) and of rats with pulmonary arterial hypertension (PAH), Kv11.1 channels were expressed not only in the large PAs, but also in the small PAs with the diameter &lt;100 micrometers. The increase in the Kv11.1 channel expression followed the time course of the development of pulmonary vascular remodeling in PAH rats. Administration of dofetilide, a Kv11.1 blocker, to rats with PAH inhibited PAH‐associated pulmonary vascular remodeling.ConclusionsOur study indicates that Kv11.11 channels are expressed in the lung and that these channels mediate the development of pulmonary vascular remodeling. Dofetilide is an FDA approved drug used for treatment of atrial fibrillation. Thus, our findings raise a possibility of repurposing this antiarrhythmic drug for the treatment of pulmonary hypertension.Support or Funding InformationSupported by NIH

Renal Tubular Secretion of Dofetilide is Dependent on MATE1 Function

BackgroundDofetilide is a delayed rectifier potassium channel inhibitor used to treat patients with atrial fibrillation and flutter. The clinical use of this drug is associated with increases in QTc interval, which can exacerbate cardiac arrhythmias. The elimination of dofetilide is predominantly mediated by renal tubular secretion, yet the mechanism by which this occurs remains poorly understood. Previously reported drug‐drug interaction (DDI) studies of dofetilide with cimetidine, ketoconazole, and megestrol suggest the involvement of organic cation transporters. Here, we investigate the contribution of organic cation transporter 2 (OCT2; SLC22A2) and multidrug and toxin extrusion protein 1 (MATE1; SLC47A1) to the pharmacokinetics (PK) of dofetilide in examine these mechanisms in the drug’s elimination and its narrow therapeutic window.MethodsIn vitro transport kinetics of tetraethylammonium (TEA), and dofetilide were determined in HEK293 cells stably‐transfected with OCT2 or MATE1. In vivo PK, DDIs, and mass balance studies were performed in wild‐type, OCT1,2‐, and MATE1‐deficient mice receiving a single dose of dofetilide (5 mg/kg, p.o.; 2.5 mg/kg, i.v.), and contraindicated drugs. Concentrations of dofetilide in plasma and urine were determined by UPLC‐MS/MS. PK parameters were calculated using Phoenix® WinNonlin®, while urinary excretion was calculated as a ratio of dofetilide recovery to the dose administered. Cardiomyocytes from wild‐type and MATE1‐deficient mice were isolated using a Langendorff perfusion system, and used to examine ex vivo cardiomyocytes uptake.ResultsIn vitro studies show that dofetilide is a substrate of MATE1 (4.3‐fold increase over vector control). Oral administration of dofetilide increased the plasma concentration (Cmax) and area under the curve (AUC) in MATE1‐deficient mice with low plasma clearance. However, MATE1‐deficiency in mice was associated with significant 3‐ and 5‐fold reduction of urinary excretion of dofetilide in females and males, respectively, whereas no noticeable differences were observed in OCT2‐deficient animals. Accumulation of dofetilide in cardiomyocytes was higher by 2‐fold in female mice having MATE1‐deficiency than males, while addition of an inhibitor, e.g., cimetidine significantly reduced dofetilide uptake. DDI studies with ketoconazole (50 mg/kg, p.o.) elevated the Cmax in wild‐type, OCT1,2‐, and MATE1‐deficient mice by 181%, 80%, and 54%, respectively, which suggest that transport inhibition is the major mechanism for ketoconazole‐induced increases in dofetilide exposure. Several contraindicated drugs listed in the dofetilide prescribing information inhibited MATE1‐mediated dofetilide transport by &gt;75%.ConclusionThis study suggests that the tubular secretion of dofetilide is mediated by MATE1 and occurs independently of OCT2, and is sensitive to inhibition by widely used prescription drugs. Further research is required to identify the transmembrane processes involved in dofetilide uptake into proximal renal tubular cells. Based on the preliminary data, deficiency of MATE1 not only reduces urinary excretion of dofetilide but also increases accumulation in the heart which may contribute to individual variation in response.Support or Funding InformationNIH grant: R01CA215802Urinary excretion‐time profile of dofetilide after an intravenous (2.5 mg/kg) administration in FVB wildtype, OCT1,2‐ and MATE1‐deficient mice (mean±SEM, n=5).Figure 1In vivo drug‐drug interaction studies of dofetilide (5 mg/kg; p.o.) with ketoconazole (50 mg/kg; p.o.) in FVB wild‐type, OCT1,2‐ and MATE1‐deficient mice (mean±SEM, n=5).Figure 2

Assessment of spatial heterogeneity of ventricular repolarization after multi-channel blocker drugs in healthy subjects

Background and ObjectivesIn contrast to potassium channel blockers, drugs affecting multiple channels seem to reduce torsadogenic risks. However, their effect on spatial heterogeneity of ventricular repolarization (SHVR) is still matter of investigation. Aim of this work is to assess the effect of four drugs blocking the human ether-à-go-go-related gene (hERG) potassium channel, alone or in combination with other ionic channel blocks, on SHVR, as estimated by the V-index on short triplicate 10 s ECG. MethodsThe V-index is an estimate of the standard deviation of the repolarization times of the myocytes across the entire myocardium, obtained from multi-lead surface electrocardiograms. Twenty-two healthy subjects received a pure hERG potassium channel blocker (dofetilide) and 3 other drugs with additional varying degrees of sodium and calcium (L‐type) channel block (quinidine, ranolazine, and verapamil), as well as placebo. A one-way repeated-measures Friedman test was performed to compare the V-index over time. ResultsComputer simulations and Bland-Altman analysis supported the reliability of the estimates of V-index on triplicate 10 s ECG. Ranolazine, verapamil and placebo did not affect the V-index. On the contrary, after quinidine and dofetilide administration, an increase of V-index from predose to its peak value was observed (ΔΔV-index values were 19 ms and 27 ms, respectively, p < 0.05). ConclusionsHigh torsadogenic drugs (dofetilide and quinidine) affected significantly the SHVR, as quantified by the V-index. The metric has therefore a potential in assessing drug arrhythmogenicity.

Classification of drug-induced hERG potassium-channel block from electrocardiographic T-wave features using artificial neural networks.

Human ether-à-go-go-related gene (hERG) potassium-channel block represents a harmful side effect of drug therapy that may cause torsade de pointes (TdP). Analysis of ventricular repolarization through electrocardiographic T-wave features represents a noninvasive way to accurately evaluate the TdP risk in drug-safety studies. This study proposes an artificial neural network (ANN) for noninvasive electrocardiography-based classification of the hERG potassium-channel block. The data were taken from the "ECG Effects of Ranolazine, Dofetilide, Verapamil, and Quinidine in Healthy Subjects" Physionet database; they consisted of median vector magnitude (VM) beats of 22 healthy subjects receiving a single 500μg dose of dofetilide. Fourteen VM beats were considered for each subject, relative to time-points ranging from 0.5hr before to 14.0hr after dofetilide administration. For each VM, changes in two indexes accounting for the early and the late phases of repolarization, ΔERD30% and ΔTS/A , respectively, were computed as difference between values at each postdose time-point and the predose time-point. Thus, the dataset contained 286 ΔERD30% -ΔTS/A pairs, partitioned into training, validation, and test sets (114, 29, and 143 pairs, respectively) and used as inputs of a two-layer feedforward ANN with two target classes: high block (HB) and low block (LB). Optimal ANN (OANN) was identified using the training and validation sets and tested on the test set. Test set area under the receiver operating characteristic was 0.91; sensitivity, specificity, accuracy, and precision were 0.93, 0.83, 0.92, and 0.96, respectively. OANN represents a reliable tool for noninvasive assessment of the hERG potassium-channel block.

Dofetilide-Induced Microvolt T-Wave Alternans.

Dofetilide is an antiarrhythmic drug that selectively inhibits the rapid component of the delayed rectifier potassium current. The administration of dofetilide may cause ventricular arrhythmias and torsade de pointes. Electrocardiographic (ECG) microvolt T-wave alternans (TWA), an electrophysiologic phenomenon consisting in the beat-to-beat alternation of the T-wave amplitude requiring computerized algorithms to be detected, has also been associated to malignant ventricular arrhythmias. Aim of the present study was to evaluate if dofetilide induces TWA during the 24 hours following administration. The study population consisted of 22 healthy subjects ("ECG Effects of Ranolazine, Dofetilide, Verapamil, and Quinidine in Healthy Subjects" database by Physionet) to whom a 500 μg-dose of dofetilide was administered. For each subject, 10 s ECG were acquired at baseline (0.5 hour before dofetilide administration) and at 15 time points during the 24 hours following the drug administration. ECG were then processed for automatic TWA detection by correlation method. In 21 subjects out of 22, after dofetilide administration, TWA significantly increased to a peak value (median TWA values went from 6 μV at baseline to a max 32 μV; p<; 0.05), on average after 5 hours, to then come back to values closer to baseline. Thus, in healthy subjects, dofetilide increases occurrence and levels (6 times baseline value on average) of TWA in the hours following its administration.

Comparison of one- and three-lead ECG to measure cardiac intervals and differentiate drug-induced multi-channel block

IntroductionFDA has established initiatives to characterize clinical and non-clinical biomarkers to enable more precise prediction of proarrhythmia risk based upon knowledge of drug effect on multiple cardiac ion channels (Colatsky et al., 2016). The FDA has recently demonstrated superiority of early ventricular repolarization interval (JTp) in differentiating pure hERG block from multi-channel block in human subjects. Preclinical studies often acquire a single lead ECG, whereas FDA measurements of JTp were derived ​from a spatial vectorcardiogram computed using multiple leads. This study compares QT subintervals derived from single lead vs. spatial magnitude (SM) ECG and contrasts information obtained from multilead and single lead ECGs in the canine model. MethodsFour beagle dogs were instrumented with 3-lead Holter monitors to acquire continuous surface ECG recordings for three consecutive days. A 24-h baseline recording was obtained on day 1 followed by administration of dofetilide on day 2 and atropine and dofetilide on day 3. Lead II and SM ECGs were automatically analyzed using the AE-1010 Rhythm Express™ (RE) software (VivaQuant, St. Paul, MN USA) without manual intervention or editing of the results (auto). Five-minute averages of beat-to-beat intervals measured on each lead were compared for agreement assessed by Bland-Altman (BA) statistics and consistency measured as the repeatability standard deviation (SD) from 5-min intervals. The fully automated results were screened by an operator (semi-automated) and compared to automated results. ResultsJTp and TpTe measured using SM lead are less sensitive to changes in posture and respiration related changes in T-wave morphology. The 24-h repeatability SD of 5-min subintervals for JTp and TpTe over the three days was improved by 15.4% and 15.5% respectively with the highest improvements of 23.3% for JTp on day 2 and 25.3% for TpTe on day 3. Drug induced changes in QTcV, QRS, RR, and PR intervals were qualitatively similar between the SM lead and Lead II and in close agreement based on BA statistics. Semi-automated and automated measurements from SM Lead were in close agreement based on BA statistics. DiscussionSingle lead ECG is adequate for PR, RR, QRS, and QT, but produces different and more variable results when assessing QT subintervals relative to the SM lead. Close agreement between automated and semi-automated measurements demonstrates Rhythm Express accuracy and the potential to streamline interval analysis.

Electrophysiological measurements that can explain and guide temporary accelerated pacing to avert (re)occurrence of torsade de pointes arrhythmias in the canine chronic atrioventricular block model

Pacing at higher rates is known to suppress torsade de pointes (TdP) arrhythmias. Nevertheless, exact application and mechanism need further clarification. In the anesthetized canine chronic atrioventricular block model, ventricular remodeling is responsible for a high and reproducible incidence of TdP upon a challenge with dofetilide. We used this model to investigate by what mechanism accelerated pacing averts TdP and what repolarization parameter could be used to guide temporary accelerated pacing (TAP). Ten dogs with repetitive TdP after administration of dofetilide when paced at 60 beats/min were selected. In a serial experiment, TAP was initiated at 100 beats/min after the first ectopic beat. Electrocardiogram and right and left ventricular (LV) monophasic action potential durations (MAPDs) were recorded. In a subset, vertical dispersion was determined with a duodecapolar catheter. Temporal dispersion was quantified as short-term variability (STV). Arrhythmias were quantified with the arrhythmia score. The increase in repolarization parameters observed after administration of dofetilide was counteracted by TAP (eg, LV MAPD from 381 ± 94 ms back to 310 ± 17 ms; P < .05). Temporal dispersion (STVLVMAPD) increased from 0.69 ± 0.37 to 2.59 ± 0.96 ms (P< .05) after administration of dofetilide and back to 1.15±0.54 ms (P < .05) with TAP. This was accompanied by suppression of recurrent TdP in 7 of 10 dogs (P<.05) and a trend toward reduction in vertical (spatial) dispersion from 56 ± 25 to 31 ± 4 ms (P = .06). In those dogs, seconds after capture of TAP, almost all ectopy disappeared, causing a decrease in arrhythmia score from 21 ± 12 to 4 ± 3 (P < .05). TAP is effective in averting TdP by decreasing spatial and temporal measures of repolarization. Increase in temporal dispersion (STV) can guide TAP.

Torsade de pointes arrhythmias arise at the site of maximal heterogeneity of repolarization in the chronic complete atrioventricular block dog.

The chronic complete atrioventricular block (CAVB) dog is highly sensitive for drug-induced torsade de pointes (TdP) arrhythmias. Focal mechanisms have been suggested as trigger for TdP onset; however, its exact mechanism remains unclear. In this study, detailed mapping of the ventricles was performed to assess intraventricular heterogeneity of repolarization in relation to the initiation of TdP. In 8 CAVB animals, 56 needles, each containing 4 electrodes, were inserted in the ventricles. During right ventricular apex pacing (cycle length: 1000-1500 ms), local unipolar electrograms were recorded before and after administration of dofetilide to determine activation and repolarization times (RTs). Maximal RT differences were calculated in the left ventricle (LV) within adjacent electrodes in different orientations (transmural, vertical, and horizontal) and within a square of four needles (cubic dispersion). Dofetilide induced TdP in five out of eight animals. Right ventricle-LV was similar between inducible and non-inducible dogs at baseline (327 ± 30 vs. 345 ± 17 ms) and after dofetilide administration (525 ± 95 vs. 508 ± 15 ms). All measurements of intraventricular dispersion were not different at baseline, but this changed for horizontal (206 ± 20 vs. 142 ± 34 ms) and cubic dispersion (272 ± 29 vs. 176 ± 48 ms) after dofetilide: significantly higher values in inducible animals. Single ectopic beats and the first TdP beat arose consistently from a subendocardially located electrode terminal with the shortest RT in the region with largest RT differences. Chronic complete atrioventricular block dogs susceptible for TdP demonstrate higher RT differences. Torsade de pointes arises from a region with maximal heterogeneity of repolarization suggesting that a minimal gradient is required in order to initiate TdP.

Increased Late Sodium Current Contributes to the Electrophysiological Effects of Chronic, but Not Acute, Dofetilide Administration.

Drugs are screened for delayed rectifier potassium current (IKr) blockade to predict long QT syndrome prolongation and arrhythmogenesis. However, single-cell studies have shown that chronic (hours) exposure to some IKr blockers (eg, dofetilide) prolongs repolarization additionally by increasing late sodium current (INa-L) via inhibition of phosphoinositide 3-kinase. We hypothesized that chronic dofetilide administration to intact dogs prolongs repolarization by blocking IKr and increasing INa-L. We continuously infused dofetilide (6-9 μg/kg bolus+6-9 μg/kg per hour IV infusion) into anesthetized dogs for 7 hours, maintaining plasma levels within the therapeutic range. In separate experiments, myocardial biopsies were taken before and during 6-hour intravenous dofetide infusion, and the level of phospho-Akt was determined. Acute and chronic dofetilide effects on action potential duration (APD) were studied in canine left ventricular subendocardial slabs using microelectrode techniques. Dofetilide monotonically increased QTc and APD throughout 6.5-hour exposure. Dofetilide infusion during ≥210 minutes inhibited Akt phosphorylation. INa-L block with lidocaine shortened QTc and APD more at 6.5 hours than at 50 minutes (QTc) or 30 minutes (APD) dofetilide administration. In comparison, moxifloxacin, an IKr blocker with no effects on phosphoinositide 3-kinase and INa-L prolonged APD acutely but no additional prolongation occurred on chronic superfusion. Lidocaine shortened APD equally during acute and chronic moxifloxacin superfusion. Increased INa-L contributes to chronic dofetilide effects in vivo. These data emphasize the need to include time and INa-L in evaluating the phosphoinositide 3-kinase inhibition-derived proarrhythmic potential of drugs and provide a mechanism for benefit from lidocaine administration in clinical acquired long QT syndrome.

Abstract 13875: Torsade de Pointes Arrhythmias Arise at the Site of Maximal Heterogeneity of Repolarization in the Chronic Complete AV-block Dog

Introduction: The chronic complete atrioventricular block (CAVB) dog model is highly sensitive for drug-induced Torsade de Pointes (TdP) arrhythmias. Focal mechanisms have been considered as trigger for the onset of TdP, however, its exact mechanism remains unclear. In this study, detailed mapping of the heart was performed to assess intraventricular heterogeneity of repolarization in relation to the initation of TdP. Methods: In 6 animals, 48 needles, each containing 4 electrodes, were inserted in the ventricles. During right ventricular apex pacing (cycle length: 1000-1500ms) local unipolar electrograms were recorded before and after administration of dofetilide. Activation- and repolarization times (RT) were measured at baseline and before occurrence of the first TdP or at 5 minutes after start of dofetilide infusion in inducible and non-inducible dogs. Maximal RT differences were calculated in the left ventricle 1) within adjacent electrodes in different planes (transmural, apex-base, anterior-posterior) and 2) within a square of 4 needles (3D dispersion). Results: TdP could be induced in 3 out of 6 animals. No difference in mean RT was observed between inducible and non-inducible dogs both at baseline (343±30 vs. 345±17ms) and after dofetilide administration (582±73 vs. 508±15ms). Maximal dispersion of RT was not different between both groups at baseline: 36±32 vs. 32±7ms (transmural), 50±20 vs. 42±9ms (apex-base), 71±28 vs. 47±13ms (anterior-posterior), 83±19 vs. 70±6ms (3D dispersion). After dofetilide infusion anterior-posterior (203±28 vs. 142±34ms, p=0.04) and 3D dispersion (268±41 vs. 176±48ms, p=0.03) showed a higher RT difference in the inducible animals. No differences were observed for transmural (124±14 vs. 116±18ms, p=0.557) and apex-base dispersion (142±12 vs. 121±23ms, p=0.229). The initiating beat of the TdP arose consistently from the region with the largest local spatial dispersion of repolarization moments. Conclusion: TdP arises from a region with maximal heterogeneity of repolarization and animals susceptible for TdP demonstrate higher RT differences. These results suggest that a certain gradient of repolarization moments is required in order to initiate TdP in CAVB dogs.

Ventricular remodelling is a prerequisite for the induction of dofetilide-induced torsade de pointes arrhythmias in the anaesthetized, complete atrio-ventricular-block dog

A number of predisposing factors have been suggested to be contributing to drug-induced torsade de pointes (TdP) arrhythmias: short-long-short (SLS) sequence, bradycardia, timing of drug administration, anaesthesia, ventricular remodelling, and altered ventricular activation due to ventricular ectopic beats (SLS) or idioventricular rhythm (IVR). Chronic atrio-ventricular (AV)-block (CAVB) dogs are susceptible to dofetilide-induced TdP. In 32 anaesthetized animals, the relevance of ventricular remodelling for TdP susceptibility was studied by dofetilide [0.025 mg/kg/5 min intravenously (iv)] during bradycardia in the presence (CAVB, n= 18) or absence [acute atrio-ventricular block (AVB), n= 32] of ventricular remodelling. In sub-protocols, the possible pro-arrhythmic effects of timing of dofetilide administration: prior to (n= 11), or after creation of AVB (n= 9) and relevance of SLS pacing (n= 17) was investigated during IVR. Dofetilide was also given after AVB when the activation of the ventricles was normal: pacing (1000 ms) from the high septum (n= 7) or abnormal but fixed from the left ventricular apex (n= 5). Torsade de pointes inducibility was defined as reproducible (≥ 3 times) occurrence. In acute AV block (AAVB), dofetilide did not induce TdP spontaneously (0 of 32), whereas TdP was seen in 10 out of 18 serially tested dogs in CAVB (P< 0.001). The other factors: timing of dofetilide (0 of 11 vs. 0 of 9), SLS pacing (0 of 17 vs. 1 of 17), or ventricular activation (0 of 7 vs. 0 of 5) did not increase TdP susceptibility. Beat-to-beat variability of repolarization increased after ventricular remodelling and was highest prior to TdP induction. In AAVB dogs, TdP is not spontaneously seen, whereas it is present in CAVB. This implies that ventricular remodelling is a prerequisite for TdP induction in this model.

Incidence of Atrial Fibrillation in Patients with either Heart Failure or Acute Myocardial Infarction and Left Ventricular Dysfunction: A Cohort Study

BackgroundWe examined the incidence of new-onset atrial fibrillation in patients with left ventricular dysfunction. Patients either had a recent myocardial infarction (with or without clinical heart failure) or symptomatic heart failure (without a recent MI). Patients were with and without treatment with the class III antiarrhythmic drug dofetilide over 36 months.MethodsThe Danish Investigations of Arrhythmia and Mortality ON Dofetilide (DIAMOND) studies included 2627 patients without atrial fibrillation at baseline, who were randomised to treatment with either dofetilide or placebo.ResultsThe competing risk analyses estimated the cumulative incidences of atrial fibrillation during the 42 months of follow-up to be 9.6% in the placebo-treated heart failure-group, and 2.9% in the placebo-treated myocardial infarction-group.Cox proportional hazard regression found a 42% significant reduction in the incidence of new-onset AF when assigned to dofetilide compared to placebo (hazard ratio 0.58, 95% confidence interval 0.40-0.82) and there was no interaction with study (p = 0.89).In the heart failure-group, the incidence of atrial fibrillation was significantly reduced to 5.6% in the dofetilide-treated patients (hazard ratio 0.57, 95% confidence interval 0.38-0.86).In the myocardial infarction-group the incidence of atrial fibrillation was reduced to 1.7% with the administration of dofetilide. This reduction was however not significant (hazard ratio 0.61, 95% confidence interval 0.30-1.24).ConclusionIn patients with left ventricular dysfunction the incidence of AF in 42 months was 9.6% in patients with heart failure and 2.9% in patients with a recent MI. Dofetilide significantly reduced the risk of developing atrial fibrillation compared to placebo in the entire study group and in the subgroup of patients with heart failure. The reduction in the subgroup with recent MI was not statistically significant, but the hazard ratio was similar to the hazard ratio for the heart failure patients, and there was no difference between the effect in the two studies (p = 0.89 for interaction).

Biomarkers and endogenous determinants of dofetilide-induced torsades de pointes in α(1) -adrenoceptor-stimulated, anaesthetized rabbits.

Torsades de pointes (TdP) liability is a stochastic event, which indicates that unidentified factors have an important role in facilitating the initiation of TdP by increasing the probability of TdP occurrence. We sought to identify factors that facilitate drug-induced TdP. We studied dofetilide-induced TdP in pentobarbital-anaesthetized, phenylephrine-sensitized rabbits, seeking biomarkers that discriminated between the animals that experienced TdP ('TdP+' animals) and those that did not ('TdP-' animals). As novel variables, the beat-to-beat variability and instability of ECG intervals were measured at preset times, irrespective of whether or not hearts were in stable sinus rhythm ('absolute' variability and instability). Autonomic activity was also determined. Dofetilide delayed repolarization and induced arrhythmias prior to TdP. The variability of the coupling interval and shape of arrhythmic beats before TdP were significantly greater in the 'TdP+' group than in the 'TdP-' group. Accordingly, the 'absolute' variability and instability of the ECG intervals were significantly elevated in the 'TdP+' group. Phenylephrine increased significantly the up-baroreflex sensitivity in the 'TdP+' group before dofetilide administration. 'Preceding' arrhythmias have characteristics that permit prediction of TdP occurrence: the more chaotic the ventricular rhythm, the greater the probability of TdP initiation. This suggests that complexity of the arrhythmic beats may play an important mechanistic role in TdP genesis. The electrical instability quantified by the novel 'absolute' variability and instability parameters correlates with the probability of TdP occurrence. Baroreflex may contribute to TdP genesis in vivo.

A Novel Computational Model of the Human Ventricular Action Potential and Ca transient

We have developed a detailed mathematical model for Ca handling and ionic currents in the human ventricular myocyte. Our objective was to implement a model that: 1) accurately reflects Ca-dependent Ca release; 2) uses repolarizing K currents with realistic amplitude; 3) comes to steady state; 4) simulates basic excitation-contraction coupling phenomena; and 5) runs on a normal desktop computer. The model relies on the framework of the rabbit myocyte model developed by Shannon et al. in our laboratory, and includes a subsarcolemmal compartment (in addition to the other two commonly formulated cytosolic compartments, junctional and bulk) where the ion channels sense ion concentrations that differ from the bulk. Ion channels and transporters have been modeled on the basis of the most recent experimental data obtained in our group and from the literature. In particular, novel formulations of the rapidly and slowly inactivating components of Ito have been implemented and utilized to differentiate between endocardial and epicardial myocytes. The model has been validated against a wide set of experimental data including action potential adaptation and restitution properties, frequency dependent inotropy and intracellular sodium staircase. It also correctly predicts the effect of pharmacological intervention on K currents (e.g. chromanol 293 B and dofetilide administration) on ventricular repolarization. We conclude that this model is more robust than previously existing models and provides a useful framework to explore excitation-contraction coupling mechanisms and repolarization abnormalities at the single cell level. To overcome the substantial limitations to experimental studies involving human cardiac tissue, due to its low computational cost this model is suitable to be integrated into multi-scale models of tissue and/or heart.

Potassium Channel Subunit Remodeling in Rabbits Exposed to Long-Term Bradycardia or Tachycardia

Sustained heart rate abnormalities produce electrical remodeling and susceptibility to arrhythmia. Uncontrolled tachycardia produces heart failure and ventricular tachyarrhythmia susceptibility, whereas bradycardia promotes spontaneous torsade de pointes (TdP). This study compared arrhythmic phenotypes and molecular electrophysiological remodeling produced by tachycardia versus bradycardia in rabbits. We evaluated mRNA and protein expression of subunits underlying rapid (IKr) and slow (IKs) delayed-rectifier and transient-outward K+ currents in ventricular tissues from sinus rhythm control rabbits and rabbits with AV block submitted to 3-week ventricular pacing either at 60 to 90 bpm (bradypaced) or at 350 to 370 bpm (tachypaced). QT intervals at matched ventricular pacing rates were longer in bradypaced than tachypaced rabbits (eg, by approximately 50% at 60 bpm; P<0.01). KvLQT1 and minK mRNA and protein levels were downregulated in both bradypaced and tachypaced rabbits, whereas ERG was significantly downregulated in bradypaced rabbits only. Kv4.3 and Kv1.4 were downregulated by tachypacing only. Patch-clamp experiments showed that IKs was reduced in both but IKr was decreased in bradypaced rabbits only. Continuous monitoring revealed spontaneous TdP in 75% of bradypaced but only isolated ventricular ectopy in tachypaced rabbits. Administration of dofetilide (0.02 mg/kg) to mimic IKr downregulation produced ultimately lethal TdP in all tachypaced rabbits. Sustained tachycardia and bradycardia downregulate IKs subunits, but bradycardia also suppresses ERG/IKr, causing prominent repolarization delays and spontaneous TdP. Susceptibility of tachycardia/heart failure rabbits to malignant tachyarrhythmias is induced by exposure to IKr blockers. These results point to a crucial role for delayed-rectifier subunit remodeling in TdP susceptibility associated with rate-related cardiac remodeling.

A new method to calculate the beat-to-beat instability of QT duration in drug-induced long QT in anesthetized dogs

Introduction: Instability of QT duration is a marker to predict Torsade de Pointes (TdP) associated with both congenital and drug-induced long QT syndrome. We describe a new method for the quantification of instability of repolarization. Methods: Female, adult beagle dogs anesthetized with a potent morphinomimetic were treated with either solvent ( n = 7) or dofetilide ( n = 7). Poincaré plots with QT n versus QT n+1 were constructed to visualize the beat-to-beat variation in QT intervals from the lead II ECG. Short-term instability (STI), long-term instability (LTI) and total instability (TI) were quantified by calculating the distances of 30 consecutive data-points from the x and y-coordinate to the “centre of gravity” of the data cluster. Dofetilide at 0.0025 to 0.04 mg/kg i.v. (plasma concentrations of 4 ± 0.6 to 41 ± 2.7 ng/ml), dose-dependently prolonged QT and QTcV (at 0.04 mg/kg i.v.: QT: 280 ± ms versus 236 ± 5 ms with solvent; p < 0.05 and QTcV: 290 ± 9 ms versus 252 ± 4 ms with solvent; p < 0.05). Concomitantly, the compound induced an increase in the instability parameters in a similar dose-dependent manner (at 0.04 mg/kg i.v.: TI: 6.8 ± 0.9 ms versus 1.7 ± 0.3 ms; p < 0.05, LTI: 3.6 ± 0.5 ms versus 1.0 ± 0.2 ms; p < 0.05 and STI: 4.2 ± 0.6 ms versus 1.0 ± 0.2 ms; p < 0.05). The increases induced by dofetilide were associated with a high incidence of early afterdepolarizations (EADs) in the endocardial monophasic action potential (in 6 out of the 7 compound-treated animals versus 0 out of the 7 solvent animals; p < 0.05). Conclusion: Quantification of beat-to-beat QT instability by our method clearly detects changes in short-term, long-term and total instability induced by dofetilide, already at pre-arrhythmic doses. Dofetilide administration to anesthetized dogs prolongs ventricular repolarization, concomitantly increases beat-to-beat QT instability and induces early after depolarizations (EADs). As such, the use of these parameters in this in vivo model shows clear potential for risk identification in cardiovascular safety assessment.

A pharmacokinetic-pharmacodynamic model for the quantitative prediction of dofetilide clinical QT prolongation from human ether-a-go-go-related gene current inhibition data

QT prolongation is an important biomarker of the arrhythmia torsades de pointes and appears to be related mainly to blockade of delayed inward cardiac rectifier potassium currents. The aim of this study was to quantify the relationship between in vitro human ether-a-go-go-related gene (hERG) potassium channel blockade and the magnitude of QT prolongation in humans for the class III antiarrhythmic dofetilide. The in vitro affinity and activity of dofetilide were determined in recombinant cell cultures expressing the hERG channel, and the QT-prolonging effect of dofetilide was assessed in 5 clinical studies (80 healthy volunteers and 17 patients with ischemic heart disease). A population pharmacokinetic-pharmacodynamic analysis of the in vitro and in vivo data was performed in NONMEM by use of the operational model of pharmacologic agonism to estimate the efficiency of transduction from ion channel binding to Fridericia-corrected QT response. A 3-compartment pharmacokinetic model with first-order absorption characterized the time course of dofetilide concentrations. On the basis of an in vitro potency of 5.13 ng/mL for potassium current inhibition and predicted unbound dofetilide concentrations, the estimated transducer ratio (tau) of 6.2 suggests that the QT response plateaus before currents are fully blocked. In our study population, 10% hERG blockade corresponds to a QT prolongation of 20 ms (95% confidence interval, 12-32 ms). With long-term dofetilide administration, tolerance develops with a half-life of 4.7 days. The current mechanism-based pharmacokinetic-pharmacodynamic model quantified the relationship between in vitro hERG channel blockade and clinical QT prolongation for dofetilide. This model may prove valuable for assessing the risk of QT prolongation in humans for other drugs that selectively block the hERG channel on the basis of in vitro assays and pharmacokinetic properties.

Ventricular arrhythmias triggered by alerting stimuli in conscious rabbits pre-treated with dofetilide.

We tested whether normally benign alerting/arousing stimuli provoke cardiac arrhythmias in conscious rabbits with electrically unstable myocardium. Alerting stimuli (loud sound, tapping and moving the cage, pinprick, inhalation of formaldehyde vapour) were presented before and after administration of dofetilide to conscious unrestrained rabbits (New Zealand White). Dofetilide (0.28-3.0 mg/kg i. v.) caused prolongation of QT interval (from 131 +/- 9 to 217 +/- 11 ms; p < 0.01, n = 6) and Tpeak-Tend interval (from 34 +/- 5 to 81 +/- 9 ms; p < 0.01, n = 6), altered ventricular conductance, and caused appearance of spontaneous ventricular ectopic beats. Alerting stimuli elicited ventricular ectopic beats in 18/30 trials in all dofetilide-treated animals, with a short latency (3.1 +/- 0.4 s). Formaldehyde vapour, in addition, elicited profound bradycardia, and precipitated non-sustained polymorphic ventricular tachycardia (torsades de points) lasting 0.6-8.5 s in 5/6 animals. These arrhythmias occurred also with a short latency (mean 8.7 +/- 1.6 s). Betaadrenergic blockade with propranolol (1.5 mg/kg i. v.) abolished spontaneous ventricular ectopy, suppressed torsades de points precipitated by formaldehyde, and significantly (p < 0.05) reduced the number of ventricular ectopic beats triggered by alerting stimuli. In predisposed hearts, alerting stimuli precipitate arrhythmias by producing transient increases in sympathetic discharge in the ventricular myocardium. Vagally induced bradycardia with concurrent ventricular beta-adrenoreceptor activation may underlie development of torsades de points in patients with long QT syndrome precipitated by swimming, diving or facial immersion.