Oral vanoxerine prevents reinduction of atrial tachyarrhythmias: preliminary results.

BACKGROUND: Vanoxerine is a promising, new, investigational antiarrhythmic drug. The purpose of this study was to test the hypothesis that oral dosing of vanoxerine would first terminate induced atrial flutter (AFL) and atrial fibrillation (AF), and then prevent their reinduction. METHODS: In 5 dogs with sterile pericarditis, on the fourth day after creating the pericarditis, we performed electrophysiologic (EP) studies at baseline, measuring atrial excitability, refractoriness (AERP), and conduction time (CT) when pacing from the right atrial appendage, Bachmann's bundle (BB), and the posteroinferior left atrium at cycle lengths (CLs) of 400, 300, and 200 ms. Then, after induction of AFL or AF, all dogs received hourly oral doses of vanoxerine: 90 mg, followed by 180 mg and 270 mg. Blood was obtained to determine plasma vanoxerine concentrations at baseline, every 30 minutes, when neither AFL nor AF were inducible, and, finally, 1 hour after the 270 mg dose. Then we repeated the baseline EP studies. RESULTS: Four dogs had inducible, sustained AFL, and 1 dog only had induced, nonsustained AF. In 4 AFL episodes, oral vanoxerine terminated the AFL and then rendered it noninducible after an average of 111 minutes (range 75-180 minutes) after the first dose was administered. The mean vanoxerine plasma level at the point of noninducibility was 84 ng/mL, with a narrow range of 76-99 ng/mL. In the dog with induced, nonsustained AF, it was no longer inducible at a drug level of 75 ng/mL. Vanoxerine did not significantly (1) prolong the AERP except at BB, and then only at the faster pacing CLs; (2) change atrial excitability thresholds; (3) prolong atrial conduction time, the PR interval, the QRS complex or the QT interval. CONCLUSIONS: Orally administered vanoxerine effectively terminated AFL and rendered it noninducible. It also suppressed inducibility of nonsustained AF. These effects occurred at consistent plasma drug levels. Vanoxerine's insignificant or minimal effects on measured electrophysiologic parameters are consistent with little proarrhythmic risk.

Vanoxerine: cellular mechanism of a new antiarrhythmic.

INTRODUCTION: There remains an unmet need for safe and effective antiarrhythmic drugs, especially for the treatment of atrial fibrillation. Vanoxerine is a drug that is free of adverse cardiac events in normal volunteers, yet is a potent blocker of the hERG (hK(v)11.1) cardiac potassium channel. Consequently,we hypothesized that vanoxerine might also be a potent blocker of cardiac calcium (Ca) and sodium (Na) currents, and would not affect transmural dispersion of repolarization. METHODS: The whole cell patch clamp technique was used to measure currents from cloned ion channels overexpressed in stable cell lines and single ventricular myocytes. We measured intracellular action potentials from canine ventricular wedges and Purkinje fibers using sharp microelectrode technique. RESULTS: We found that vanoxerine was a potent hK(v)11.1 blocker, and at submicromolar concentrations, it blocked Ca and Na currents in a strongly frequency-dependent manner. In the canine ventricular wedge preparation vanoxerine did not significantly affect transmural action potential waveforms, QT interval or transmural dispersion of repolarization. CONCLUSIONS: Vanoxerine (1) is a potent blocker of cardiac hERG, Na and Ca channels; (2) block is strongly frequency-dependent especially for Na and Ca channels; and (3) transmural dispersion of ventricular repolarization is unaffected. The multichannel block and repolarization uniformity resemble the effects of amiodarone, the exemplar atrial fibrillation drug. Vanoxerine is a completely different chemical and has none of amiodarone's toxic effects. Vanoxerine has characteristics of a potentially effective and safe antiarrhythmic.

Vanoxerine, a new drug for terminating atrial fibrillation and flutter.

BACKGROUND: Vanoxerine produces potent block of cardiac hERG, sodium, and L-type calcium channels. Block is strongly frequency dependent, is unassociated with transmural dispersion of repolarization, and occurs at concentrations safe in humans. Therefore, we proposed that vanoxerine might be antiarrhythmic. In these studies, we tested the hypothesis that vanoxerine would terminate induced atrial fibrillation (AF) and atrial flutter (AFL) in dogs with sterile pericarditis (SP). METHODS AND RESULTS: In 9 SP dogs, 11 episodes each of sustained (>10 minutes) AF and AFL were induced. Electrophysiological studies were performed before and after infusion of vanoxerine, which effectively terminated AF and AFL in 19 of 22 episodes. Simultaneous multisite mapping during 3 AF and 3 AFL episodes demonstrated that termination of each arrhythmia occurred with termination of the driver (a reentrant circuit) following an increase in tachycardia CL. Except for conduction in an area of slow conduction in the driver's reentrant circuit, vanoxerine did not significantly affect intraatrial or atrioventricular conduction time, QRS duration, or QT/QTc intervals. Ventricular refractoriness prolonged minimally during ventricular pacing at 400 and 333 ms (176 +/- 16 ms to 182 +/- 16 ms; 173 +/- 11 ms to 178 +/- 18 ms, respectively). Vanoxerine minimally increased (mean 0.7 mA) atrial stimulus threshold for capture. CONCLUSIONS: Vanoxerine effectively terminated induced, sustained AF and AFL in the canine SP model, and produced insignificant or minimal changes in refractoriness, conduction time, or stimulus threshold, consistent with little proarrhythmic risk.

Alfuzosin delays cardiac repolarization by a novel mechanism.

The United States Food and Drug Administration (FDA) uses alfuzosin as an example of a drug having QT risk in humans that was not detected in nonclinical studies. FDA approval required a thorough clinical QT study (TCQS) that was weakly positive at high doses. The FDA has used the clinical/nonclinical discordance as a basis for mandatory TCQS, and this requirement has serious consequences for drug development. For this reason, we re-examined whether nonclinical signals of QT risk for alfuzosin were truly absent. Alfuzosin significantly prolonged action potential duration (APD)(60) in rabbit Purkinje fibers (p < 0.05) and QT in isolated rabbit hearts (p < 0.05) at the clinically relevant concentration of 300 nM. In man, the QT interval corrected with Fridericia's formula increased 7.7 ms, which exceeds the 5.0-ms threshold for a positive TCQS. Effects on hK(v)11.1, hK(v)4.3, and hK(v)7.1/hKCNE1 potassium currents and calcium current were not involved. At 300 nM, approximately 30x C(max), alfuzosin significantly increased whole-cell peak sodium (hNa(v)1.5) current (p < 0.05), increased the probability of late hNa(v)1.5 single-channel openings, and significantly shortened the slow time constant for recovery from inactivation. Alfuzosin also increased hNa(v)1.5 burst duration and number of openings per burst between 2- and 3-fold. Alfuzosin is a rare example of a non-antiarrhythmic drug that delays cardiac repolarization not by blocking hK(v)11.1 potassium current, but by increasing sodium current. Nonclinical studies clearly show that alfuzosin increases plateau potential and prolongs APD and QT, consistent with QT prolongation in man. The results challenge the FDA grounds for the absolute primacy of TCQS based on the claim of a false-negative, nonclinical study on alfuzosin.

Fatty Acid Cysteamine Conjugates as Novel and Potent Autophagy Activators That Enhance the Correction of Misfolded F508del-Cystic Fibrosis Transmembrane Conductance Regulator (CFTR).

A depressed autophagy has previously been reported in cystic fibrosis patients with the common F508del-CFTR mutation. This report describes the synthesis and preliminary biological characterization of a novel series of autophagy activators involving fatty acid cysteamine conjugates. These molecular entities were synthesized by first covalently linking cysteamine to docosahexaenoic acid. The resulting conjugate 1 synergistically activated autophagy in primary homozygous F508del-CFTR human bronchial epithelial (hBE) cells at submicromolar concentrations. When conjugate 1 was used in combination with the corrector lumacaftor and the potentiator ivacaftor, it showed an additive effect, as measured by the increase in the chloride current in a functional assay. In order to obtain a more stable form for oral dosing, the sulfhydryl group in conjugate 1 was converted into a functionalized disulfide moiety. The resulting conjugate 5 is orally bioavailable in the mouse, rat, and dog and allows a sustained delivery of the biologically active conjugate 1.

Variability in the measurement of hERG potassium channel inhibition: effects of temperature and stimulus pattern.

INTRODUCTION: In vitro evaluation of drug effects on hERG K(+) channels is a valuable tool for identifying potential proarrhythmic side effects in drug safety testing. Patch-clamp recording of hERG K(+) current in mammalian cells can accurately evaluate drug effects, but the methodology has not been standardized, and results vary widely. Our objective was to evaluate two potential sources of variability: the temperature at which recordings are performed and the voltage pulse protocol used to activate hERG K(+) channels expressed in HEK293 cells. METHODS: A panel of 15 drugs that spanned a broad range of potency for hERG inhibition and pharmacological class was evaluated at both room and near-physiological temperatures using several patch-clamp voltage protocols. Concentration-response analysis was performed with three stimulus protocols: 0.5- and 2-s step pulses, or a step-ramp pattern. RESULTS: Block by 2 of the 15 drugs tested, d,l-sotalol (antiarrhythmic) and erythromycin (antibiotic), was markedly temperature sensitive. hERG inhibition measured using a 2-s step-pulse protocol underestimated erythromycin potency compared with results obtained with a step-ramp protocol. Using conservative acceptance criteria and the step-ramp protocol, the IC(50) values for hERG block differed by less than twofold for 15 drugs. DISCUSSION: Data obtained at near-physiological temperatures using a step-ramp pattern are highly repeatable and provide a conservative safety evaluation of hERG inhibition.

The action potential and comparative pharmacology of stem cell-derived human cardiomyocytes.

INTRODUCTION: The cardiac action potential (CAP) of stem cell-derived human cardiomyocytes (SC-hCMs) is potentially the most powerful preclinical biomarker for cardiac safety and efficacy in humans. Our experiments tested this hypothesis by examining the CAP and relevant pharmacology of these cells. METHODS: The electrophysiological and pharmacological profiles of SC-hCMs were compared to rabbit and canine Purkinje fibers (PFs). Ventricular SC-hCMs provided the dominant electrophysiological phenotype (approximately 82%) in a population of ventricular, atrial and nodal cardiomyocytes (CMs). The effects of reference compounds were measured in SC-hCMs using perforated patch, current clamp recording. Selective inhibitors of I(Kr), I(Ks), I(Ca,L), and I(Na), and norepinephrine (NE), were tested on SC-hCM action potentials (APs). RESULTS: AP prolongation was observed upon exposure to hERG channel blockers (terfenadine, quinidine, cisapride, sotalol, E-4031 and verapamil), with significantly shorter latencies than in PF assays. For the torsadogenic compounds, terfenadine and quinidine, SC-hCM AP prolongation occurred at significantly lower concentrations than in canine or rabbit PF APs. Moreover, the I(Ks) blocker chromanol 293B prolonged APs from SC-hCMs, whereas both rabbit and canine PF assays are insensitive to I(Ks) blockers in the absence of adrenergic preconditioning. Early afterdepolarizations (EADs) were induced by 100 nM E-4031 and 100 nM cisapride in the SC-hCM assay, but not in the canine or rabbit PF assay. Selective inhibition of I(Na) and I(Ca,L) slowed V(max) and shortened AP duration, respectively. NE prolonged the AP duration of SC-hCMs. DISCUSSION: The CAP of SC-hCMs has been validated as a powerful preclinical biomarker for cardiac safety and efficacy. In addition to its human nature, the SC-hCM AP assay removes diffusion delays, reduces test compound consumption, demonstrates an overall pharmacological sensitivity that is greater than conventional rabbit or canine PF assays, and accurately predicts cardiac risk of known torsadogenic compounds.