Efficacy and Safety of XEN1101, a Novel Potassium Channel Opener, in Adults With Focal Epilepsy: A Phase 2b Randomized Clinical Trial

Importance

Many patients with focal epilepsy experience seizures despite treatment with currently available antiseizure medications (ASMs) and may benefit from novel therapeutics.

Objective

To evaluate the efficacy and safety of XEN1101, a novel small-molecule selective Kv7.2/Kv7.3 potassium channel opener, in the treatment of focal-onset seizures (FOSs).

Design, Setting, and Participants

This phase 2b, randomized, double-blind, placebo-controlled, parallel-group, dose-ranging adjunctive trial investigated XEN1101 over an 8-week treatment period from January 30, 2019, to September 2, 2021, and included a 6-week safety follow-up. Adults experiencing 4 or more monthly FOSs while receiving stable treatment (1-3 ASMs) were enrolled at 97 sites in North America and Europe.

Interventions

Patients were randomized 2:1:1:2 to receive XEN1101, 25, 20, or 10 mg, or placebo with food once daily for 8 weeks. Dosage titration was not used. On completion of the double-blind phase, patients were offered the option of entering an open-label extension (OLE). Patients not participating in the OLE had follow-up safety visits (1 and 6 weeks after the final dose).

Main Outcomes and Measures

The primary efficacy end point was the median percent change from baseline in monthly FOS frequency. Treatment-emergent adverse events (TEAEs) were recorded and comprehensive laboratory assessments were made. Modified intention-to-treat analysis was conducted.

Results

A total of 325 patients who were randomized and treated were included in the safety analysis; 285 completed the 8-week double-blind phase. In the 325 patients included, mean (SD) age was 40.8 (13.3) years, 168 (51.7%) were female, and 298 (91.7%) identified their race as White. Treatment with XEN1101 was associated with seizure reduction in a robust dose-response manner. The median (IQR) percent reduction from baseline in monthly FOS frequency was 52.8% (P < .001 vs placebo; IQR, -80.4% to -16.9%) for 25 mg, 46.4% (P < .001 vs placebo; IQR, -76.7% to -14.0%) for 20 mg, and 33.2% (P = .04 vs placebo; IQR,  -61.8% to 0.0%) for 10 mg, compared with 18.2% (IQR, -37.3% to 7.0%) for placebo. XEN1101 was generally well tolerated and TEAEs were similar to those of commonly prescribed ASMs, and no TEAEs leading to death were reported.

Conclusions and Relevance

The efficacy and safety findings of this clinical trial support the further clinical development of XEN1101 for the treatment of FOSs.

Trial Registration

ClinicalTrials.gov Identifier: NCT03796962.

Pharmacokinetics of XEN496, a Novel Pediatric Formulation of Ezogabine, Under Fed and Fasted Conditions: A Phase 1 Trial

INTRODUCTION

XEN496 is a novel, granular, immediate-release formulation of ezogabine intended for pediatric use. The objective of this study was to assess the effect of food on the pharmacokinetics (PK) of XEN496 and its N-acetyl metabolite (NAMR) in healthy volunteers.

METHODS

Twenty-four adult subjects were enrolled in this phase 1, single center, open-label, randomized, single-dose, two-way crossover study. Subjects received 400 mg XEN496 as an oral suspension in both fed and fasted states separated by a 6-day washout period. Serial blood samples were collected up to 48 h post-administration. PK parameters evaluated included maximum observed plasma concentration (C_max), time of maximum observed plasma concentration (T_max), and area under the concentration-time curve (AUC_(0-t) and AUC_inf). Safety was assessed by laboratory evaluations, physical exam, and adverse event monitoring.

RESULTS

For XEN496, median T_max was 3 and 2 h in the fed and fasted states, respectively. AUC parameters in the fed and fasted states were equivalent, whereas food decreased C_max of XEN496 by 32% compared to the fasted state. The ratio of geometric means [90% CI] for C_max was 72% [64-82%]. For NAMR, food delayed T_max by 1 h, while C_max and AUC parameters were equivalent in the fed and fasted states. The safety profile of XEN496 in this study appeared comparable to that previously reported for ezogabine tablets.

CONCLUSION

The biopharmaceutical performance of XEN496 in this study was as expected for an immediate-release, granular dosage formulation, and generally comparable to that reported for ezogabine tablets. Future studies are needed to characterize the efficacy, safety, and PK of XEN496 in a pediatric population.

Spontaneous and TMS-related EEG changes as new biomarkers to measure anti-epileptic drug effects

Robust biomarkers for anti-epileptic drugs (AEDs) activity in the human brain are essential to increase the probability of successful drug development. The frequency analysis of electroencephalographic (EEG) activity, either spontaneous or evoked by transcranial magnetic stimulation (TMS-EEG) can provide cortical readouts for AEDs. However, a systematic evaluation of the effect of AEDs on spontaneous oscillations and TMS-related spectral perturbation (TRSP) has not yet been provided. We studied the effects of Lamotrigine, Levetiracetam, and of a novel potassium channel opener (XEN1101) in two groups of healthy volunteers. Levetiracetam suppressed TRSP theta, alpha and beta power, whereas Lamotrigine decreased delta and theta but increased the alpha power. Finally, XEN1101 decreased TRSP delta, theta, alpha and beta power. Resting-state EEG showed a decrease of theta band power after Lamotrigine intake. Levetiracetam increased theta, beta and gamma power, while XEN1101 produced an increase of delta, theta, beta and gamma power. Spontaneous and TMS-related cortical oscillations represent a powerful tool to characterize the effect of AEDs on in vivo brain activity. Spectral fingerprints of specific AEDs should be further investigated to provide robust and objective biomarkers of biological effect in human clinical trials.

TMS as a pharmacodynamic indicator of cortical activity of a novel anti-epileptic drug, XEN1101

Objective

Transcranial magnetic stimulation (TMS) produces characteristic deflections in the EEG signal named TMS‐evoked EEG potentials (TEPs), which can be used to assess drug effects on cortical excitability. TMS can also be used to determine the resting motor threshold (RMT) for eliciting a minimal muscle response, as a biomarker of corticospinal excitability. XEN1101 is a novel potassium channel opener undergoing clinical development for treatment of epilepsy. We used TEPs and RMT to measure the effects of XEN1101 in the human brain, to provide evidence that XEN1101 alters cortical excitability at doses that might be used in future clinical trials.

Methods

TMS measurements were incorporated in this Phase I clinical trial to evaluate the extent to which XEN1101 modulates TMS parameters of cortical and corticospinal excitability. TEPs and RMT were collected before and at 2‐, 4‐, and 6‐hours post drug intake in a double‐blind, placebo‐controlled, randomized, two‐period crossover study of 20 healthy male volunteers.

Results

Consistent with previous TMS investigations of antiepileptic drugs (AEDs) targeting ion channels, the amplitude of TEPs occurring at early (15–55 msec after TMS) and at late (150–250 msec after TMS) latencies were significantly suppressed from baseline by 20 mg of XEN1101. Furthermore, the RMT showed a significant time‐dependent increase that correlated with the XEN1101 plasma concentration.

Interpretation

Changes from baseline in TMS measures provided evidence that 20 mg of XEN1101 suppressed cortical and corticospinal excitability, consistent with the effects of other AEDs. These results support the implementation of TMS as a tool to inform early‐stage clinical trials.

Safety and efficacy of vernakalant for the conversion of atrial fibrillation to sinus rhythm; a phase 3b randomized controlled trial

BACKGROUND

Atrial fibrillation (AF) is a common cardiac arrhythmia that is associated with significant health risks. One strategy to mitigate the risks associated with long-term AF is to convert AF to sinus rhythm (SR). This study assessed the efficacy and safety of vernakalant hydrochloride for the pharmacological conversion of AF to SR.

METHODS

Patients with recent-onset (duration >3 h- ≤ 7 days) symptomatic AF and no evidence or history of congestive heart failure were randomized in a 2:1 ratio to receive vernakalant or placebo. Patients received an infusion of vernakalant (3 mg/kg) or placebo over 10 min, followed by a second infusion of vernakalant (2 mg/kg) or placebo 15 min later if AF had not been terminated. The primary efficacy endpoint was conversion of AF to SR for at least 1 min within 90 min of the start of drug infusion. The primary safety endpoint was a composite of: occurrence of clinically significant hypotension, clinically significant ventricular arrhythmia (including torsades de pointes, ventricular tachycardia or ventricular fibrillation) or death within 2 h of starting the drug infusion.

RESULTS

A total of 217 patients were randomized to receive vernakalant (n = 145) or placebo (n = 72). Of the 129 individuals who received vernakalant, 59 (45.7 %) converted to SR compared with one of the 68 patients (1.5 %) who received placebo (p < 0.0001). Conversion to SR was significantly faster with vernakalant than with placebo (p < 0.0001), and a greater proportion of patients who received vernakalant than those who received placebo reported no AF-related symptoms at 90 min (p = 0.0264). The primary composite safety endpoint was observed in one patient receiving vernakalant and in no patients receiving placebo. In the vernakalant arm, dysgeusia, paraesthesia and sneezing were the most common treatment-emergent adverse events, and three serious adverse events occurred that were considered to be related to study drug.

CONCLUSIONS

Vernakalant resulted in rapid cardioversion of recent-onset AF in almost half of the study population and was generally well tolerated. The safety outcomes affirmed the need for careful selection and management of haemodynamically stable candidates for cardioversion.

TRIAL REGISTRATION

NCT00989001 .

A Randomized, Placebo-Controlled Study of Vernakalant (Oral) for the Prevention of Atrial Fibrillation Recurrence After Cardioversion

Background—

Vernakalant, a relatively atrial-selective antiarrhythmic drug, has previously demonstrated efficacy for the acute conversion of atrial fibrillation (AF) to sinus rhythm. This study was designed to determine the most appropriate oral dose of vernakalant for the prevention of AF recurrence postcardioversion.

Methods and Results—

Patients with nonpermanent AF were randomized to 150, 300, or 500 mg vernakalant or placebo twice daily for up to 90 days. The efficacy analysis was conducted on 605 of 735 patients who entered the maintenance phase on day 3 after cardioversion. The time to the first recurrence of symptomatic sustained AF was significantly longer in the 500 mg vernakalant group, with a median of >90 days versus 29 days in the placebo group (hazard ratio, 0.735; P =0.0275). No significant effect was seen at the lower doses. The percent of patients in sinus rhythm at day 90 was 41%, 39%, and 49% in the 150-mg (n=147), 300-mg (n=148), and 500-mg (n=150) vernakalant groups, respectively, compared with 36% in the placebo group (n=160). There were no vernakalant-related proarrhythmic events. Related serious adverse events occurred in 2 patients in the 150-mg vernakalant group and in 1 patient in each of the other groups.

Conclusions—

Vernakalant, 500 mg twice daily, appears to be effective and safe for the prevention of AF recurrence after cardioversion. The absence of proarrhythmia and favorable safety profile is an important finding for the drug.

Clinical Trial Registration—

URL: http://www.clinicaltrials.gov . Unique identifier: NCT00526136.

The Effect of Vernakalant (RSD1235), an Investigational Antiarrhythmic Agent, on Atrial Electrophysiology in Humans

OBJECTIVES

To determine the acute effects of vernakalant (RSD1235) on electrophysiologic (EP) properties in humans.

BACKGROUND

Vernakalant is an investigational mixed ion channel blocker that can terminate acute atrial fibrillation (AF) in humans at 2 to 5 mg/kg and may be more "atrial-selective" than available agents.

METHODS

Patients (N=19; 53% male; age, 48+/-11 years) underwent EP study before and after 25 minutes of intravenous vernakalant administration: 2 mg/kg over 10 min+0.5 mg/kg/hr for 35 min or 4 mg/kg over 10 min+1 mg/kg/hr for 35 min. EP measurements, including atrial refractory period (AERP) and ventricular refractory period (VERP), were obtained.

RESULTS

The lower dose prolonged AERP at 600, but not at 400 or 300 msec paced cycle length. The higher dose significantly prolonged AERP from 203+/-31 msec to 228+/-24 msec at 600 msec, 182+/-30 msec to 207+/-27 msec at 400 msec, and 172 msec+/-24 to 193+/-21 msec at 300 msec. There was no significant prolongation of VERP at either dose or at any cycle length. There was a small but significant prolongation of AV nodal refractoriness; Wenckebach cycle length prolonged by 18+/-12 msec (from baseline 343+/-54 msec) at the higher dose (P<0.05). Sinus node recovery time also increased by 123+/-158 msec (from baseline 928+/-237 msec) at the higher dose (P<0.05). There was a slight prolongation of QRS duration at the higher dose, during ventricular pacing at CL=400 msec (15+/-15 msec, P=0.0547). QT and HV intervals were unchanged.

CONCLUSIONS

At doses similar to those tested clinically, vernakalant dose-dependently prolonged atrial refractoriness, prolonged AV nodal conduction and refractoriness, and slightly prolonged QRS duration, but it had no effect on ventricular refractoriness.

Synthesis and Biological Studies of Novel 2-Aminoalkylethers as Potential Antiarrhythmic Agents for the Conversion of Atrial Fibrillation

A series of 2-aminoalkylethers prepared as potential antiarrhythmic agents is described. The present compounds are mixed sodium and potassium ion channel blockers and exhibit antiarrhythmic activity in a rat model of ischemia-induced arrhythmias. Structure-activity studies led to the identification of three compounds 5, 18, and 26, which were selected based on their particular in vivo electrophysiological properties, for studies in two canine atrial fibrillation (AF) models. The three compounds converted AF in both models, but only compound 26 was shown to be orally bioavailable. Resolution of the racemate 26 into its corresponding enantiomers 40 and 41 and subsequent biological testing of these enantiomers led to the selection of (1S,2S)-1-(1-naphthalenethoxy)-2-(3-ketopyrrolidinyl)cyclohexane monohydrochloride (41) as a potential atrial selective antiarrhythmic candidate for further development.

RSD1235 blocks late INa and suppresses early afterdepolarizations and torsades de pointes induced by class III agents

OBJECTIVE

RSD1235 is a novel antiarrhythmic drug with atria-selective electrophysiological actions on Na(+) and K(+) currents. The mechanism for its protection of ventricular repolarization was assessed by its action on Purkinje fibers, and by block of late sodium current active during repolarization. Further, RSD1235's ability to reverse the pro-arrhythmic actions of the class III agents dofetilide and clofilium was assessed in isolated Purkinje fibers and an in vivo model of torsades de pointes (TdP).

METHODS

Action potential and early after-depolarization (EAD) recordings were made from in situ and isolated rabbit Purkinje fibers at 37 degrees C using floating sharp microelectrodes; late I(Na) was recorded using a whole-cell patch clamp technique of Nav1.5 expressed in HEK cells at 22 degrees C; In vivo, anesthetized methoxamine-sensitized rabbits were used to test the ability of RSD1235 to suppress clofilium-induced TdP.

RESULTS

RSD1235 (0.5-30 microM) had minor dose-dependent effects on action potential duration (APD) at 50% and 90% repolarization in Purkinje fibers, but pre-treatment significantly attenuated the APD-prolonging effects of dofetilide (300 nM). EADs induced by 300 nM dofetilide were terminated by 30 microM RSD1235 in all experiments (n=7). RSD1235 blocked a late component of Na current (I(Na)), which can produce inward currents contributing to EAD formation. RSD1235 pre-treatment (1 micromol/kg/min) or acute infusions prevented/terminated TdP induced by clofilium in 8 of 9 rabbits, and reduced the duration of TdP episodes from 71 +/- 23 s in control to 17 +/- 7 and 14 +/- 14 s at infusion rates of 0.3 and 1.0 micromol/kg/min, respectively (n = 9, p < 0.001).

CONCLUSION

RSD1235 itself has minor actions on repolarization in Purkinje fibers, but can reverse the AP-prolonging actions of class III agents and terminate arrhythmias in a model of TdP. We suggest that these protective actions of RSD1235 may result, at least in part, from its ability to inhibit late I(Na) during action potential repolarization.

The Mechanism of Atrial Antiarrhythmic Action of RSD1235

INTRODUCTION

RSD1235 is a novel drug recently shown to convert AF rapidly and safely in patients.(1) Its mechanism of action has been investigated in a rat model of ischemic arrhythmia, along with changes in action potential (AP) morphology in isolated rat ventricular myocytes and effects on cloned channels.

METHODS AND RESULTS

Ischemic arrhythmias were inhibited with an ED50 of 1.5 micromol/kg/min, and repolarization times increased with non-significant effects on PR and QRS durations. AP prolongation was observed in rat myocytes at low doses, with plateau elevation and a reduction in the AP overshoot at higher doses. RSD1235 showed selectivity for voltage-gated K+ channels with IC50 values of 13 microM on hKv1.5 (1 Hz) versus 38 and 30 microM on Kv4.2 and Kv4.3, respectively, and 21 microM on hERG channels. RSD1235 did not block IK1 (IC50 > 1 mM) nor ICa,L (IC50= 220 microM) at 1 Hz in guinea pig ventricular myocytes (n = 4-5). The drug displayed mild (IC50= 43 microM at 1 Hz) open-channel blockade of Nav1.5 with rapid recovery kinetics after rate reduction (10-->1 Hz, 75% recovery with tau= 320 msec). Nav1.5 blocking potency increased with stimulus frequency from an IC50= 40 microM at 0.25 Hz, to an IC50= 9 microM at 20 Hz, and with depolarization increasing from 107 microM at -120 mV to 31 microM at -60 mV (1 Hz).

CONCLUSIONS

These data suggest that RSD1235's clinical selectivity and AF conversion efficacy result from block of potassium channels combined with frequency- and voltage-dependent block of INa.

A randomized, controlled trial of RSD1235, a novel anti-arrhythmic agent, in the treatment of recent onset atrial fibrillation

OBJECTIVES

The purpose of this study was to determine the efficacy and safety of intravenous RSD1235 in terminating recent onset atrial fibrillation (AF).

BACKGROUND

Anti-arrhythmic drugs currently available to terminate AF have limited efficacy and safety. RSD1235 is a novel atrial selective anti-arrhythmic drug.

METHODS

This was a phase II, multi-centered, randomized, double-blinded, step-dose, placebo-controlled, parallel group study. Fifty-six patients from 15 U.S. and Canadian sites with AF of 3 to 72 h duration were randomized to one of two RSD1235 dose groups or to placebo. The two RSD1235 groups were RSD-1 (0.5 mg/kg followed by 1 mg/kg) or RSD-2 (2 mg/kg followed by 3 mg/kg), by intravenous infusion over 10 min; a second dose was given only if AF was present. The primary end point was termination of AF during infusion or within 30-min after the last infusion. Secondary end points included the number of patients in sinus rhythm at 0.5, 1, and 24 h post-last infusion and time to conversion to sinus rhythm.

RESULTS

The RSD-2 dose showed significant differences over placebo in: 1) termination of AF (61% vs. 5%, p < 0.0005); 2) patients in sinus rhythm at 30 min (56% vs. 5%, p < 0.001); 3) sinus rhythm at 1 h (53% vs. 5%, p = 0.0014); and 4) median time to conversion to SR (14 vs. 162 min, p = 0.016). There were no serious adverse events related to RSD1235.

CONCLUSIONS

RSD1235, a new atrial-selective anti-arrhythmic agent, appears to be efficacious and safe for converting recent onset AF to sinus rhythm.

RSD1000: a novel antiarrhythmic agent with increased potency under acidic and high-potassium conditions

This study reports the use of a novel agent, RSD1000 [(+/-)-trans-[2-(4-morpholinyl)cyclohexyl]naphthalene-1-acetate mono hydrochloride], to test the hypothesis that a drug with pKa close to the pH found in ischemic tissue may have selective antiarrhythmic actions against ischemia-induced arrhythmias. The antiarrhythmic ED50 for RSD1000 against ischemic arrhythmias was 2.5 +/- 0.1 micromol/kg/min in rats. This value was significantly lower than doses that suppressed electrically induced arrhythmias. In isolated rat hearts, RSD1000 was approximately 40 times more potent in producing ECG changes (i.e., P-R and QRS prolongation) in acid (pHo = 6.4) and high [K+]o (10.8 mM) buffer than in normal buffer (pHo = 7.4; [K+]o = 3.4 mM). In patch-clamped, whole-cell rat cardiac myocytes, inhibition of sodium (INa) currents by RSD1000 was pH- and use-dependent. The IC50 for INa blockade was lower (P <.05) in acid (0.8 +/- 0.1 microM) than in pH 7.3 (2.9 +/- 0.3 microM), respectively, whereas the IC50 for blockade of transient outward potassium current (ITO) at pH = 6.4 and 7.3 was 3.3 +/- 0.4 and 2.8 +/- 0.1 microM, respectively. Mixed ion channel block in ischemic myocardium with minimal effects on normal cardiac tissue, as governed by the low pKa of RSD1000, may account for its antiarrhythmic activity against ischemia-induced arrhythmias.

Kinetics of rate-dependent shortening of action potential duration in guinea-pig ventricle; effects of IK1 and IKr blockade

1. The kinetics of shortening of action potential duration (APD) following an increase in pacing rate, from 2 to 3.3 Hz, was characterized in guinea-pig ventricular preparations. Terikalant (RP62719), an inhibitor of the inwardly rectifying K+ current (IK1), and dofetilide, a specific inhibitor of the rapidly activating delayed-rectifier current (IKr), were applied to determine the effect of inhibition of these ion currents on slow APD shortening. 2. Action potentials were recorded from isolated guinea-pig ventricular myocytes using the perforated-patch patch-clamp technique, and monophasic action potentials were recorded from Langendorff-perfused guinea-pig ventricles using a contact epicardial probe. 3. Under control conditions, after an increase in pacing rate, APD immediately decreased, and then shortened slowly with an exponential time course. In ventricular myocytes, the time constant of this exponential shortening was 28+/-4 s and the amount of slow shortening was 21.9+/-0.9 ms (n=8) for an increase in rate from 2 to 3.3 Hz. Similar values were observed in Langendorff-perfused ventricles. 4. Terikalant dose-dependently increased APD and the increase was enhanced by rapid pacing ('positive' rate-dependence). The drug dose-dependently decreased the time constant of shortening and amount of slow APD shortening. In contrast, dofetilide, an inhibitor of IKr, which shows 'reverse' rate-dependent APD widening, had no significant effect on the time constant or amount of slow shortening. 5. These observations suggest that IK1 plays a role in rate-dependent shortening of APD. The results appear to support the hypothesis that 'reverse' rate-dependent effects of IKr blockers are due to these drugs not affecting the ion current(s) mediating intrinsic rate-dependent slow shortening of APD.

Dofetilide enhances shock-induced extension of refractoriness and lowers defibrillation threshold

OBJECTIVE

To explore the hypothesis that class III antiarrhythmics reduce defibrillation threshold (DFT) by prolonging postshock refractoriness.

ANIMALS AND METHODS

The effect of a new selective potassium channel blocker, dofetilide (DOFlow 2.5 micrograms/kg bolus plus 0.9 microgram/kg/h; DOFmed 10 micrograms/kg bolus plus 3.6 micrograms/kg/h; and DOFhigh 25 micrograms/kg bolus plus 9 micrograms/kg/h), on DFT was compared with that of placebo in anesthetized open chest dogs (n = 6 per group). The effects of dofetilide on refractory period extension (RPE) were assessed by using DFT strength shocks delivered at various stages of repolarization.

RESULTS

DFT was significantly decreased in the DOFhigh group, whether expressed as shock peak voltage or energy (P < 0.05 compared with changes in placebo). At baseline, a shock timing of ventricular effective refractory period of 25 ms resulted in RPE of 100 +/- 24 ms, 80 +/- 11 ms, 91 +/- 14 ms and 90 +/- 20 ms in the placebo, DOFlow, DOFmed, and DOFhigh groups, respectively. After infusion, these RPE values were unchanged in the placebo group but tended to increase in the dofetilide treatment groups. DOFhigh significantly increased RPE by 20 +/- 18 ms (P < 0.05 compared with baseline values and changes in placebo). Dofetilide-induced changes in RPE and DFT were significantly correlated when expressed as voltage (r2 = 0.78, P < 0.01), current (r2 = 0.80, P < 0.01) and energy (r2 = 0.53, P < 0.01).

CONCLUSIONS

These results show that dofetilide prolonged RPE at a plasma level that reduced DFT, thus providing support for the hypothesis that selective prolongation of refractoriness may synergize with shock-induced RPE to decrease the energy requirements for defibrillation.

Development and evaluation of a fully automated monophasic action potential analysis program

A fully automated program has been developed for analysing digitised cardiac monophasic action potentials (MAPs) of several animal species. Under diverse conditions, the program's performance is evaluated by comparison with high-resolution manual analysis of action potential duration, MAP detection, baseline and plateau determination. For each variable, the differences between both forms of analysis are categorised according to species (rabbit, dog or baboon), intervention (control, sematilide or LY-190147), rhythm source (sinus or paced), and rates. They are then examined using ANOVA (null hypothesis: no difference between methods for any category). Intra-group differences are significant for baseline selection (p < 0.05), and treatment effects are significant (p < 0.05) for one species (baboon). However, the differences found are negligible when compared with expected measurement errors. Thus the program reliably detects and accurately analyses MAPs, independent of species, drug or pacing. The program's strength is its use of simple heuristic algorithms to identify and profile MAP signals while rejecting spurious transients. These algorithms focus on the procedural aspects of automatic MAP detection and validation, with minimum use of complex mathematics. The program's estimated throughput exceeds 8200 MAPs min-1 on a 50 MHz 1486 DX machine. With available data format conversion packages, signals from almost any data-acquisition source can be analysed.

Magnesium shifts voltage dependence of activation of delayed rectifier I(K) in guinea pig ventricular myocytes

The sensitivity of the delayed rectifier K+ current (I(K)) to intracellular Mg2+ was investigated in guinea pig ventricular myocytes using the whole cell patch-clamp technique. An increase in free intracellular Mg2+ concentration ([Mg2+]i) led to a dose-dependent decrease in I(K) with a half-maximal effect of approximately 20 nM. Activation of I(K) was shifted toward more positive voltages on increasing [Mg2+]i, but little effect was observed on activation and deactivation kinetics. Isoproterenol increased I(K) and was partially reversible in both control and 100 nM [Mg2+]i. The antiarrhythmic drug dofetilide was used to separate I(K) into its two components, rapidly activating (I(Kr)) and slowly activating (I(Ks)). The magnitude of both components decreased to a similar extent with an increase in [Mg2+]i. As [Mg2+]i was reduced, however, the number of experiments in which the dofetilide-sensitive current I(Kr) displayed inward rectification was reduced. In contrast to results previously reported for frog myocytes, it is unlikely that Mg2+ effects on guinea pig I(K) are mediated by a protein phosphatase.

Rate-dependent effects of sematilide on ventricular monophasic action potential duration and delayed rectifier K+ current in rabbits

Our objective was to define the actions of sematilide in rabbits and to assess the contribution of the delayed rectifier (IK) to rate dependence of action potential duration (APD) in rabbit ventricular myocardium. In studies in vivo, New Zealand White rabbits were used to obtain dose/response curves of the effects of sematilide on APD from contact monophasic action potentials (MAP), ventricular effective refractory period (VERP), and ECG. Sematilide or placebo was administered as an i.v. bolus followed by a 45-min infusion in the following cumulative manner: infusion 1 (1 mg/kg bolus + 8 micrograms/kg/min); infusion 2 (2 mg/kg + 20 micrograms/kg/min); and infusion 3 (7 mg/kg + 68 micrograms/kg/min). At each infusion level, VERP and APD at 75% repolarization (APD75) were measured during cardiac pacing between 200- and 400-ms cycle length (CL). Serum sematilide levels were analyzed by high-performance liquid chromatography (HPLC). In studies in vitro, sematilide's effects on the delayed rectifier were assessed in isolated rabbit ventricular myocytes by using patch-clamp techniques. Sematilide infusion in vivo resulted in stable serum levels of 1.3 +/- 0.5, 3.7 +/- 1.4, and 13.4 +/- 1.8 micrograms/ml during infusions 1, 2, and 3, respectively. Maximal effects occurred at infusion 2, such that at 400 ms CL, sematilide widened predrug APD75 (145 +/- 5 ms) by 27 +/- 4% (p < 0.001 vs. placebo), and at 200-ms CL, sematilide prolonged predrug APD75 (115 +/- 10 ms) by only 18 +/- 4% (p < 0.001 vs. placebo; p < 0.05 vs. 400-ms CL). Similar effects were observed in VERP. Sematilide enhanced rate dependence of APD and produced the same degree of APD prolongation at a given CL, during accommodation to and recovery from rapid pacing. Rabbit ventricular myocytes appeared to have at least two types of delayed rectifier. Sematilide selectively blocked IKr, and block was not relieved by repetitive stimulation. In conclusion, the APD-widening effect of sematilide was independent of previous pacing history. Sematilide had little influence on background processes likely responsible for shortening APD because of rapid repetitive stimulation.

Class III antiarrhythmic effects of LY-190147 on defibrillation threshold

Defibrillation strength shocks delivered within an action potential (AP) delay repolarization. Shock-induced AP duration extension (APDE) may prolong refractoriness and terminate or prevent reinitiation of reentry, favoring defibrillation. This study examined LY-190147 (LY) effects on defibrillation threshold (DFT) in 11 dogs. Ventricular effective refractory period (VERP) and epicardial monophasic AP duration at 75% repolarization (APD75) were recorded at 300-, 400-, 500-, and 600-ms pacing cycle length (CL). APDE was measured as the time to 50% repolarization after a DFT strength shock delivered at 50, 25, and 0 ms before or 25 ms after VERP during pacing at 300 ms CL in 4 of the dogs. We made all recordings before drug administration and after infusions of 0.03, 0.3, and 3.0 mg/kg LY, using 1.5-h dosing intervals. LY lowered DFT in a saturating dose-response manner whether expressed as shock peak voltage (V) or energy. LY decreased DFT-V from 357 +/- 77 V before drug to 331 +/- 60 V (-6 +/- 12%), 290 +/- 43 V (-17 +/- 13%, p < 0.001), and 312 +/- 45 V (-11 +/- 12%, p < 0.05) at 0.03, 0.3, and 3.0 mg/kg, respectively. Similarly, LY treatment decreased defibrillation energy requirements from 6.9 +/- 2.7 J before drug by 7 +/- 25%, 26 +/- 24%, and 12 +/- 25% at the same doses. At 300-600 ms CL, LY prolonged APD75 by an average of 10 +/- 8% at 0.03 mg/kg, 17 +/- 6% at 0.3 mg/kg, and 24 +/- 9% at 3 mg/kg. At these CL, LY prolonged VERP by an average of 4 +/- 6% at 0.03 mg/kg, 15 +/- 10% at 0.3 mg/kg, and 11 +/- 9% at 3 mg/kg. APDE was increased from 62 +/- 9 ms before to 68 +/- 14, 80 +/- 16 (p < 0.001) and 72 +/- 13 ms (p < 0.05) at 0.03, 0.3, and 3.0 mg/kg LY, respectively. Therefore, LY prolonged VERP and APDE and affected DFT in the same saturating dose-response manner. LY may facilitate defibrillation by increasing the duration of postshock refractoriness.

Effects of optical enantiomers CK-4000(S) and CK-4001(R) on defibrillation and enhancement of shock-induced extension of action potential duration

INTRODUCTION

Class III antiarrhythmics have been reported to lower defibrillation threshold (DFT); however, the mechanism(s) of this effect is unknown. Recent evidence suggests that DFT strength DC shocks may terminate reentrant arrhythmias through prolongation of action potential duration and refractory periods. Since Class III antiarrhythmic drugs prolong repolarization, we examined the hypothesis that these drugs enhance shock-induced action potential duration extension (APDE), which might contribute to lowering of DFT.

METHODS AND RESULTS

In order to investigate the specificity of drug effects on action potential repolarization following a shock, an optical enantiomer with mixed beta-blocking and Class III effects (CK-4000) and its enantiomer with "pure" Class III antiarrhythmic effects (CK-4001) were compared against placebo in a canine defibrillation model (n = 8 per group). At the 3 mg/kg dose, the enantiomers nonstereoselectively lowered DFT voltage by 19 +/- 15% (CK-4000, P < 0.05 compared to placebo) and 25 +/- 12% (CK-4001, P < 0.05 compared to placebo), indicating that Class III antiarrhythmic actions alone were sufficient for this effect. Similarly, CK-4000 and CK-4001 at 3 mg/kg enhanced APDE (P < 0.01 compared to placebo) by 20 +/- 11% and 24 +/- 17%, respectively. APDE prolongation significantly correlated with reduction in DFT voltage for both CK-4000 (r = -0.55, P < 0.03) and CK4001 (r = -0.63, P < 0.01). At 3 mg/kg, the enantiomers stereoselectively prolonged action potential duration (APD75) by an average of 37 +/- 14% (CK-4000, P < 0.001) and 23 +/- 14% (CK-4001, P < 0.001), and ventricular effective refractory period (VERP) by 38 +/- 15% (CK-4000, P < 0.01) and 27 +/- 13% (CK-4001, P < 0.05). Prolongations of APD75 and VERP did not correlate with reductions of DFT in individual dogs.

CONCLUSIONS

These results show that Class III antiarrhythmics and DFT strength shocks additively delay repolarization, which suggests that drug enhancement of APDE may contribute to their effects on DFT.

Mexiletine's antifibrillatory actions are limited by the occurrence of convulsions in conscious animals

The antiarrhythmic actions of the racemate and enantiomers of mexiletine were studied in conscious and anaesthetised rats. Racemate or enantiomers, at 20 mg/kg i.v., had little effect on ischaemia-induced ventricular fibrillation in conscious or anaesthetised rats. In conscious rats 20 mg/kg caused convulsions in 78-89% of rats when the plasma concentration of racemate was 20 +/- 2 microM. In anaesthetized animals a higher dose (40 mg/kg) of racemate could be given; this completely prevented ischaemia-induced fibrillation when the plasma concentration was 26 +/- 2 microM. Racemate and enantiomers accumulated in the heart and brain of conscious animals to give tissue: plasma ratios of 7.5 and 23, respectively. With electrical stimulation, both racemate and enantiomers dose dependently (4-32 mg/kg) increased threshold currents for induction of ventricular fibrillation, increased refractory period and minimally changed the ECG; findings expected with a Class Ib antiarrhythmic. The above studies failed to show major differences between racemate or enantiomers except for consistently lower (20-30%) plasma concentrations of R(-) at all dose levels. In conclusion, mexiletine prevented ischaemia-induced ventricular fibrillation in anaesthetised animals but only when given at doses producing convulsions in conscious animals.

Tissue distribution of mexiletine enantiomers in rats

1. The kinetics of distribution of the enantiomers of mexiletine were studied in various tissue (heart, brain, lungs, liver, kidneys and fat) in male Sprague-Dawley rats after administration of a single i.v. dose (10 mg/kg) of racemic mexiletine. 2. The pharmacokinetic parameters calculated from the serum data showed a 32% greater systemic clearance (162 ml/min per kg vs 123 ml/min per kg) and a 22% greater steady-state volume of distribution (9.0 l/kg vs 7.4 l/kg) for R(-)-mexiletine relative to the S(+)-enantiomer. However, the terminal elimination half-lives of the enantiomers (1.4 and 1.3 h for R(-)- and S(+)-mexiletine, respectively) did not exhibit stereoselectivity. 3. Maximum tissue concentrations of the enantiomers were observed at 5 min after dosage in all tissues studied. Stereoselective uptake was evident only in the liver tissue and was 2.4-fold greater for S(+)-mexiletine. High tissue/serum ratios (greater than 20 for both enantiomers) were observed in lungs, brain and kidneys. The cardiac concentrations of R(-)- and S(+)-mexiletine were 8- and 7-fold those of serum, respectively. 4. The results demonstrate that the uptake of mexiletine enantiomers into the target tissue (heart) is not stereoselective. However, the relatively high brain accumulation of the enantiomers may be related to the CNS side-effects commonly associated with mexiletine therapy.

Antiarrhythmic properties of tedisamil (KC8857), a putative transient outward K+ current blocker

1. Rats were used to evaluate the antiarrhythmic properties of tedisamil, a novel agent with the electrophysiological properties of a Class III antiarrhythmic drug. Tedisamil was tested against coronary artery occlusion-induced arrhythmias in conscious animals. 2. The actions of tedisamil on the ECG, as well as responses to electrical stimulation, were compared with those on the configuration of epicardial intracellular action potentials recorded in vivo. 3. Tedisamil (1-4 mg kg-1, i.v.) caused bradycardia, elevated blood pressure and dose-dependently reduced ventricular fibrillation (VF) induced by occlusion of the left anterior descending coronary artery. Other ischaemia-associated arrhythmias were not so well suppressed. Antiarrhythmic activity was greatest when the tedisamil-induced bradycardia was prevented by electrically-pacing the left ventricle. 4. Tedisamil dose-dependently lengthened the effective refractory period and prevented electrically-induced VF. In vivo, tedisamil (0.5-4 mg kg-1, i.v.) prolonged the duration of epicardial intracellular action potentials by up to 400%. 5. Results showed that tedisamil possessed antifibrillatory actions in rats that were related to Class III electrophysiological actions as revealed by electrical stimulation and electrophysiological analyses.

Antiarrhythmic properties of tetrodotoxin against occlusion-induced arrhythmias in the rat: a novel approach to the study of the antiarrhythmic effects of ventricular sodium channel blockade

Blockade of ventricular sodium conductance (gNa) is believed to play an important role in the beneficial antiarrhythmic effects of class I antiarrhythmic agents. The present study was undertaken to examine the importance of ventricular gNa blockade by assessing the antiarrhythmic profile of tetrodotoxin (TTX), a selective sodium channel blocker. Experiments were performed in pentobarbital-anesthetized and artificially ventilated rats. Two doses of TTX were tested for antiarrhythmic action: a low dose (low TTX, 10 micrograms/kg of bolus + infusion of 10 micrograms/kg/hr) which blocked only neuronal activity, and a high dose (TTXh, 50 micrograms/kg of bolus + infusion of 50 micrograms/kg/hr) which also produced signs of ventricular gNa blockade in normal hearts. To control for the decreases in blood pressure and heart rate caused by TTX, hexamethonium, nitroprusside and propranolol were also used. Only TTXh possessed antiarrhythmic activity in rats subjected to myocardial ischemia (produced by ligation of the left anterior descending coronary artery). Arrhythmia scores (mean, n = 9) were: saline, 3.8; hexamethonium, 3.8; nitroprusside, 3.2; nitroprusside + propranolol, 4.3; low TTX, 3.9; and TTXh, 0.9. Only TTXh reduced dV/dt max. of the action potential (recorded in vivo by means of 3 M KCl filled microelectrodes) as well as action potential height, and concomitantly prolonged the P-R and QRS intervals of normal hearts. In conclusion, our study demonstrated that drugs which produced hypotension, bradycardia and loss of autonomic function were not antiarrhythmic. On the other hand, the marked antiarrhythmic activity of TTXh appeared to depend upon ventricular gNa blockade. Thus, TTX provides a useful tool for examining the antiarrhythmic properties of ventricular gNa blockade.

Mechanisms underlying the antiarrhythmic properties of beta-adrenoceptor blockade against ischaemia-induced arrhythmias in acutely prepared rats

1. The mechanism underlying the limited antiarrhythmic effects of beta-adrenoceptor blocking agents against occlusion-induced arrhythmias in acutely prepared, pentobarbitone-anaesthetized rats has been investigated. 2. Atenolol, ICI 111,581 and propranolol were given at low, medium and high doses calculated to shift dose-response curves to exogenous agonists by factors of 10-30, 100-300 and 1000-3000, respectively. 3. Arrhythmias, blood pressure, heart rate, ECG changes and serum K+ were measured. 4. Antiarrhythmic activity was seen with beta-blocker treatment. This was minimal with atenolol (0.1, 1 and 10 mg kg-1) and only statistically significant with the highest dose of ICI 111,581 (5 mg kg-1), and propranolol (10 mg kg-1). 5. Treatment with beta-adrenoceptor blockers elevated serum potassium concentrations, as compared with saline controls, especially when measured at 30 min post-occlusion. 6. Only ICI 111,581 (5 mg kg-1) and propranolol (1 and 10 mg kg-1) prolonged P-R interval. 7. In order to evaluate possible mechanisms of antiarrhythmic action, attempts were made to correlate antiarrhythmic activity with beta-blockade, serum potassium concentrations, and/or with changes in the P-R interval of the ECG. 8. Reductions in arrhythmias did not correlate well with presumed beta-blockade. Better correlation was obtained with elevations of serum potassium concentration, and with prolongation of P-R interval (a presumed Class I antiarrhythmic action). 9. These results suggested that antiarrhythmic effects of adrenoceptor blocking agents in acutely-prepared anaesthetized rats, subjected to occlusion of a coronary artery, are unrelated to cardiac beta-blockade. The limited antiarrhythmic effects which were observed could be attributed to elevations in serum potassium concentration (due to peripheral beta-blockade) and/or possible Class I antiarrhythmic actions.

Ventricular arrhythmias following coronary artery occlusion in the streptozotocin diabetic rat

The following investigation was designed to assess whether or not streptozotocin diabetes has an influence on the number and severity of ventricular arrhythmias following coronary artery occlusion in the conscious rat. In addition, electrocardiogram and haemodynamic data were compared between streptozotocin groups and control. Diabetes was induced in male Sprague-Dawley rats with streptozotocin (55 mg/kg iv) and left anterior descending coronary artery ligation was performed either 6 or 9 weeks later. Rats were allowed to recover from preparative surgery for 1 week prior to ligation. Streptozotocin diabetes (untreated or insulin controlled) appeared to have little influence on the variables tested. When exposed to equivalent degrees of ischaemia (the rat is a microangiopathy-resistant species), the streptozotocin diabetic rat heart was not appreciably more prone to arrhythmias of any type compared with control.