Gary L. Stump

Atrial Antifibrillatory Effects of Structurally Distinct IKur Blockers 3-[(Dimethylamino)methyl]-6-methoxy-2-methyl-4-phenylisoquinolin-1(2H)-one and 2-Phenyl-1,1-dipyridin-3-yl-2-pyrrolidin-1-yl-ethanol in Dogs with Underlying Heart Failure

Drug discovery efforts have focused recently on atrial-selective targets, including the Kv1.5 channel, which underlies the ultrarapid delayed rectifier current, IKur, to develop novel treatments for atrial fibrillation (AF). Two structurally distinct compounds, a triarylethanolamine TAEA and an isoquinolinone 3-[(dimethylamino)-methyl]-6-methoxy-2-methyl-4-phenylisoquinolin-1(2H)-one (ISQ-1), blocked IKur in Chinese hamster ovary cells expressing human Kv1.5 with IC50 values of 238 and 324 nM, respectively. In anesthetized dogs, i.v. infusions of TAEA and ISQ-1 elicited comparable 16% increases in atrial refractory period, with no effect on ventricular refractory period or QTc interval. Plasma concentrations at end infusion for TAEA and ISQ-1 were 58.5 ± 23.6 and 330.3 ± 43.5 nM, respectively. The abilities of TAEA and ISQ-1 to terminate AF, with comparison to the rapidly activating component of delayed rectifier potassium current blocker (+)-N-[1′-(6-cyano-1,2,3,4-tetrahydro-2(R)-naphthalenyl)-3,4-dihydro-4(R)-hydroxyspiro(2H-1-benzopyran-2,4′-piperidin)-6-yl]methanesulfonamide] monohydrochloride (MK-499) and the class IC 1-[2-[2-hydroxy-3-(propylamino)-propoxy]phenyl]-3-phenyl-1-propanone (propafenone), were assessed in conscious dogs with heart failure and inducible AF (entry criterion). All test agents administered in i.v. bolus regimens terminated AF in at least half of animals tested; conversely no agent was universally effective. MK-499, ISQ-1, TAEA, and propafenone terminated AF in five of six, four of seven, four of six, and five of six animals at plasma concentrations of 32.6 ± 18.7, 817 ± 274, 714 ± 622, and 816 ± 240 nM, respectively. Directed cardiac electrophysiologic studies in anesthetized dogs using i.v. bolus (consistent with AF studies) plus infusion regimens with TAEA and ISQ-1 demonstrated significant increases in atrial refractory period (12–15%), A-H and P-A intervals, but no effects on ventricular refractory period, H-V, and HEG intervals. The demonstration of AF termination with TAEA and ISQ-1 in the dog heart failure model extends the profile of antiarrhythmic efficacy of Kv1.5 blockade.

MK-0448, a specific Kv1.5 inhibitor: safety, pharmacokinetics, and pharmacodynamic electrophysiology in experimental animal models and humans.

Background— We evaluated the viability of IKur as a target for maintenance of sinus rhythm in patients with a history of atrial fibrillation through the testing of MK-0448, a novel IKur inhibitor. Methods and Results— In vitro MK-0448 studies demonstrated strong inhibition of IKur with minimal off-target activity. In vivo MK-0448 studies in normal anesthetized dogs demonstrated significant prolongation of the atrial refractory period compared with vehicle controls without affecting the ventricular refractory period. In studies of a conscious dog heart failure model, sustained atrial fibrillation was terminated with bolus intravenous MK-0448 doses of 0.03 and 0.1 mg/kg. These data led to a 2-part first-in-human study: Part I evaluated safety and pharmacokinetics, and part II was an invasive electrophysiological study in healthy subjects. MK-0448 was well-tolerated with mild adverse experiences, most commonly irritation at the injection site. During the electrophysiological study, ascending doses of MK-0448 were administered, but no increases in atrial or ventricular refractoriness were detected, despite achieving plasma concentrations in excess of 2 &mgr;mol/L. Follow-up studies in normal anesthetized dogs designed to assess the influence of autonomic tone demonstrated that prolongation of atrial refractoriness with MK-0448 was markedly attenuated in the presence of vagal nerve simulation, suggesting that the effects of IKur blockade on atrial repolarization may be negated by enhanced parasympathetic neural tone. Conclusions— The contribution of IKur to human atrial electrophysiology is less prominent than in preclinical models and therefore is likely to be of limited therapeutic value for the prevention of atrial fibrillation.Background— We evaluated the viability of IKur as a target for maintenance of sinus rhythm in patients with a history of atrial fibrillation through the testing of MK-0448, a novel IKur inhibitor. Methods and Results— In vitro MK-0448 studies demonstrated strong inhibition of IKur with minimal off-target activity. In vivo MK-0448 studies in normal anesthetized dogs demonstrated significant prolongation of the atrial refractory period compared with vehicle controls without affecting the ventricular refractory period. In studies of a conscious dog heart failure model, sustained atrial fibrillation was terminated with bolus intravenous MK-0448 doses of 0.03 and 0.1 mg/kg. These data led to a 2-part first-in-human study: Part I evaluated safety and pharmacokinetics, and part II was an invasive electrophysiological study in healthy subjects. MK-0448 was well-tolerated with mild adverse experiences, most commonly irritation at the injection site. During the electrophysiological study, ascending doses of MK-0448 were administered, but no increases in atrial or ventricular refractoriness were detected, despite achieving plasma concentrations in excess of 2 μmol/L. Follow-up studies in normal anesthetized dogs designed to assess the influence of autonomic tone demonstrated that prolongation of atrial refractoriness with MK-0448 was markedly attenuated in the presence of vagal nerve simulation, suggesting that the effects of IKur blockade on atrial repolarization may be negated by enhanced parasympathetic neural tone. Conclusions— The contribution of IKur to human atrial electrophysiology is less prominent than in preclinical models and therefore is likely to be of limited therapeutic value for the prevention of atrial fibrillation.

In vivo cardiac electrophysiologic and antiarrhythmic effects of an isoquinoline IKur blocker, ISQ-1, in rat, dog, and nonhuman primate.

The cardiac electrophysiologic effects of ISQ-1, an isoquinolinone IKur blocker, were characterized in vivo. In rat, ISQ-1 elicited maximal 33% to 36% increases in atrial and ventricular refractoriness at a plasma concentration of 11.5 μM. In African green monkey, ISQ-1 increased atrial refractory period (maximal 17% at plasma concentration up to 20 μM) with no effect on ventricular refractory period or ECG QTc. Likewise in dog, ISQ-1 increased atrial refractory period (maximal 16% at plasma concentration up to 2 μM) with no effect on ventricular refractory period or QTc. In contrast, studies with ibutilide in nonhuman primate and dog demonstrated concomitant increases in atrial and ventricular refractoriness and QTc. Additionally, in a dog model of atrial flutter, ISQ-1 terminated ongoing flutter at doses (2.5 ± 0.5 mg/kg IV) that selectively prolonged atrial refractoriness (13% increase), whereas flutter termination with ibutilide occurred at doses that increased both atrial and ventricular refractoriness as well as QTc. Of note, the cardiac electrophysiologic profiles displayed by ISQ-1 in these species were similar to those reported previously by our lab with a structurally distinct IKur blocker. Taken together, these results further support the inhibition of IKur as an approach to terminate atrial arrhythmia.

Calcitonin Gene-Related Peptide Receptor Antagonism Does Not Affect the Severity of Myocardial Ischemia during Atrial Pacing in Dogs with Coronary Artery Stenosis

Calcitonin gene-related peptide (CGRP) is a sensory neuropeptide that also has potent vasodilator activity. There are conflicting preclinical reports regarding the effect of CGRP receptor antagonism in the setting of myocardial ischemia. The present study was conducted in a canine model in which regional myocardial ischemia was reproducibly evoked by serial periods of atrial pacing (80 beats per min above baseline rate) in the presence of a 40% stenosis of the left anterior descending (LAD) coronary artery. Ischemia severity was quantitated by changes in unipolar epicardial electrograms (EG) recorded in the area of ischemia. In validation studies, the calcium entry blocker diltiazem reduced ischemia severity (before versus after treatment: ΔEG, 1.92 ± 0.23 versus 0.54 ± 0.24 mV; p < 0.05) and tended to increase LAD flow (7.7 ± 0.7 versus 9.4 ± 1.4 ml/min; p = 0.10), whereas the coronary constrictor serotonin increased ischemia severity (before versus after treatment: ΔEG, 2.11 ± 0.44 versus 4.90 ± 1.46 mV; p < 0.05) concomitant with a reduction in LAD flow (9.1 ± 1.1 versus 5.4 ± 1.5 ml/min; p < 0.05). A 30 μg/kg/min i.v. infusion test dose of the CGRP receptor antagonist CGRP(8-37) was validated by demonstrating complete block of the depressor effects of exogenous i.v. 0.03 to 0.3 μg/kg CGRP. This dose of CGRP(8-37), administered either intravenously or intra-atrially, had no effect on ischemia severity or paced LAD flow, indicating no intrinsic effect of CGRP receptor antagonism on the severity of acute myocardial ischemia. Likewise, the administration of a hemodynamically active dosing regimen of CGRP (0.03 μg/kg/min i.v.) had no effect on paced coronary flow or ischemia severity, suggesting no major role of CGRP in regulating ischemic blood flow.

In vivo canine cardiac electrophysiologic profile of 1,4-benzodiazepine IKs blockers.

&NA; Previous cardiac electrophysiologic studies of blockers of the slowly activating delayed rectifier (IKs) current have focused primarily on ventricular repolarization. This report summarizes an extensive in vivo cardiac electrophysiologic profile of four 1,4‐ benzodiazepine IKs blocker analogues (L‐761334, L‐763540, L‐761710, and L‐768673) in dogs. At 3.0 mg/kg intravenously, all four analogues elicited 14.5%‐21.4% increases in ventricular refractoriness and 19.2%‐22.6% increases in QTc interval. Concomitant 11.1%‐13.5% increases in atrial refractoriness were noted with all four analogues. Decreases in sinus heart rate of 8.4%‐17.3% were noted with all four compounds. No effects on atrial, His Purkinje, ventricular conduction or atrial and ventricular excitation were observed. One analogue, L‐761710, significantly delayed atrioventricular (AV) nodal conduction (40.7 ± 17.4% increase in atrial‐to‐His interval) and increased the AV conduction system functional refractory period 19.9 ± 6.2%. The lack of effect of the other three 1,4‐ benzodiazepine IKs blockers on AV nodal function at dosages producing comparable effects on atrial and ventricular refractoriness suggest that the AV nodal effects of L‐761710 were unrelated to IKs blockade. These findings indicate IKs plays important roles in both atrial and ventricular refractoriness as well as pacemaker function in the dog heart, suggesting potential utility for IKs blockers in the treatment of atrial and ventricular arrhythmias.

Cardiovascular profile of RWJ 29009, a new potassium channel activator, in anesthetized and conscious dogs

RWJ-29009, (6S)-trans(-)-1-(6,7-dihydro-6-hydroxy-5,5-dimethyl-2-nitro-5 H-thieno[3,2-b]pyran-7-yi)-2-piperidinone, is a structurally novel and extremely potent potassium channel activator that may be useful for treatment of hypertension and ischemic heart disease. We assessed the cardiovascular profile of RWJ 29009 in anesthetized and conscious dogs. RWJ 29009 (0.1–2 μg/kg intravenously, i.v.) dose-relatedly increased coronary blood flow (CBF) and decreased arterial pressure in anesthetized dogs. Total peripheral resistance and coronary vascular resistance were concurrently reduced without significant changes in heart rate (HR) or cardiac output (CO). Left ventricular (LV) dP/dtmax and myocardial contractile force were decreased only at the highest dose of 10 μg/kg. Cromakalim (3–100 μg/kg), although much less potent, had a qualitatively similar profile. Glyburide pre-treatment (5 mg/kg i.v.) shifted the dose response of RWJ 29009 for increasing CBF and decreasing arterial pressure to the right. The dose responses of cromakalim were similarly shifted to the right, whereas the effects of nifedipine on CBF and arterial pressure were not affected by gly-buride. RWJ 29009 (0.3 and 1 μg/kg) had no effect on myocardial O2 consumption (MVO2) except for a transient increase immediately after administration of 1 μg/kg. MVO2 returned to control 15 min after dosing, although CBF remained significantly increased. In conscious dogs, RWJ 29009 (0.3–10 μg/kg, i.v. and orally, p.o.) produced dose-related increases in CBF and decreases in arterial pressure similar to those produced in anesthetized dogs, except that HR was increased concurrently. The i.v. and p.o. potency of RWJ 29009 were comparable, indicating high oral bioavailability. Thus, RWJ 29009 is an extremely potent coronary and peripheral vasodilator with a cardiovascular profile similar to that of other potassium channel activators. Like those of other potassium channel activators, its mechanism of action appears to involve activation of ATP-regulated potassium channels.

The Prototype Serotonin 5-HT1B/1D Agonist Sumatriptan Increases the Severity of Myocardial Ischemia During Atrial Pacing in Dogs With Coronary Artery Stenosis

The triptans, serotonin 5-HT1B/1D agonists exemplified by sumatriptan, are an effective class of migraine therapy but have class labeling contraindicating their use in patients with coronary artery disease. Triptans have been shown to constrict human coronary artery in vitro, and there have been case reports of myocardial infarction in patients using sumatriptan. However, preclinical in vivo studies with sumatriptan in normal dogs have failed to demonstrate an effect on coronary flow. The present studies were conducted in a canine model in which regional myocardial ischemia was evoked by atrial pacing in the presence of a 40% stenosis of the left anterior descending coronary artery. Ischemic severity was quantified by changes in local epicardial electrograms (EGs) recorded in the ischemic zone. The intra-atrial administration of 10 μg·kg−1·min−1 sumatriptan variably but not significantly increased the severity of regional ischemia (pre- vs. posttreatment: ΔEG: 2.00 ± 0.17 vs. 3.05 ± 1.15 mV). Sumatriptan at 30 μg·kg−1·min−1 significantly increased ischemic severity (ΔEG: 1.88 ± 0.19 vs. 3.32 ± 0.58 mV, P < 0.05) concomitant with a significant reduction in coronary blood flow (8.9 ± 0.5 vs. 7.2 ± 0.8 mL/min, P < 0.05). These results demonstrate that a reduction in coronary flow with proischemic consequence can be modeled preclinically with sumatriptan in a canine model of cardiac stress.

Hemodynamic and cardiac neurotransmitter-releasing effects in conscious dogs of attention- and wake-promoting agents: a comparison of d-amphetamine, atomoxetine, modafinil, and a novel quinazolinone H3 inverse agonist.

Conscious coronary sinus-cannulated dogs were used to assess the hemodynamic effects and local cardiac norepinephrine (NE) and histamine (HA) release of 4 mechanistically diverse agents either clinically approved or representing a potential novel mechanism for the promotion of wakefulness or attention. Dosing regimens were based on reported or concurrently determined wake-promoting activities in canine models. The central nervous system stimulant, d-amphetamine [0.1 mg·kg−1·10 min−1 intravenous (IV)], significantly elevated mean arterial pressure (+30%) and increased coronary sinus and peripheral venous NE concentrations, indicative of cardiac neurotransmitter release. The selective NE reuptake inhibitor atomoxetine (2.0 mg·kg−1·10 min−1 IV) and modafinil (30.0 mg·kg−1·10 min−1 IV) also significantly elevated mean arterial pressure (+15% and +30%, respectively), but with no effect on coronary sinus or peripheral NE concentration, suggesting central mechanisms underlying the hemodynamic effects. The preclinical demonstrations of pressor effects with d-amphetamine, atomoxetine, and modafinil are consistent with clinically reported hemodynamic effects with these agents. The quinazolinone HA receptor subtype H3 inverse agonist 5r (0.3 mg·kg−1·10 min−1 IV) displayed no effect on hemodynamics or on coronary sinus or peripheral NE and HA concentrations. These data suggest the potential for therapeutic effect with the latter mechanism in the absence of peripheral cardiac neurotransmitter release or obvious changes in cardiovascular function.

In vivo cardiac electrophysiologic effects of RWJ 29009, a new potassium-channel activator, in comparison to cromakalim and nicardipine.

RWJ 29009 is a new potassium channel activator with prominent coronary and peripheral vasodilating actions. Because of the potential direct cardiac electrophysiologic actions of increased potassium conductance in myocardium, we evaluated the effects of RWJ 29009 on cardiac conduction and refractoriness in comparison to its vasodilator activity in anesthetized, open-chest dogs. We assessed effects during both intrinsic sinus rhythm and during constant atrial pacing. RWJ 29009 markedly increased coronary blood flow and decreased mean arterial blood pressure (MAP) dose dependently (0.3–10 u.g/kg intravenously, i.v.). RWJ 29009 had no effect on PR interval but decreased AV-nodal conduction time (AH) and Wenkebach cycle length slightly. RWJ 29009 decreased QT interval, left ventricular (LV) monophasic action potential duration (APD), and ventricular and atrial refractory period. These effects were consistent with shortening of cardiac repolarization. RWJ 29009 had no effect on QRS or His-Purkinje conduction time. Cromakalim had a qualitatively similar profile but was much less potent (3–300 μg/kg i.v.). In addition, the effects of cromakalim on repolarization parameters were somewhat less marked than those of RWJ 29009. Nicardipine also markedly increased coronary blood flow and decreased arterial pressure (10–300 μg/kg i.v.). Unlike the potassium channel activators, nicardipine (100–300 μg/ kg), did not affect cardiac repolarization, but increased PR and AH interval, and Wenkebach cycle length (WENK) and reduced heart rate (HR) consistent with calcium channel blockade. These results indicate that RWJ 29009, like cromakalim, increases coronary blood flow at low doses without substantial electrophysiologic effects. Electrophysiologic effects observed at higher doses indicated a shortening of repolarization, expectedly produced by potassium channel activation in cardiac tissue.

Discovery of MK-1832, a Kv1.5 inhibitor with improved selectivity and pharmacokinetics.

Selective inhibition of Kv1.5, which underlies the ultra-rapid delayed rectifier current, IKur, has been pursued as a treatment for atrial fibrillation. Here we describe the discovery of MK-1832, a Kv1.5 inhibitor with improved selectivity versus the off-target current IKs, whose inhibition has been associated with ventricular proarrhythmia. MK-1832 exhibits improved selectivity for IKur over IKs (>3000-fold versus 70-fold for MK-0448), consistent with an observed larger window between atrial and ventricular effects in vivo (>1800-fold versus 210-fold for MK-0448). MK-1832 also exhibits an improved preclinical pharmacokinetic profile consistent with projected once daily dosing in humans.