(Q)SAR modeling and safety assessment in regulatory review.

The ability to predict clinical safety based on chemical structures is becoming an increasingly important part of regulatory decision making. (Quantitative) structure-activity relationship ((Q)SAR) models are currently used to evaluate late-arising safety concerns and possible nonclinical effects of a drug and its related compounds when adequate safety data are absent or equivocal. Regulatory use will likely increase with the standardization of analytical approaches, more complete and reliable data collection methods, and a better understanding of toxicity mechanisms.

What happens when cardiac Na channel function is compromised? 2. Numerical studies of the vulnerable period in tissue altered by drugs.

OBJECTIVE: The fate of an impulse arising from stimulation is determined by the ability of the wave front to recruit sufficient Na channels from adjacent cells. Previous numerical studies of mutant Na channels revealed both the velocity of a conditioning wave and the recruiting capacity of the front as determinants of the vulnerable period (VP), an interval within which excitation results in unidirectional conduction. Drugs that block excitatory Na channels in a voltage dependent manner, such as antiarrhythmics, abused substances and antidepressants, slow the restoration of Na conductance trailing an action potential and are associated with proarrhythmia and sudden cardiac death. We hypothesized that drug-induced slowing of Na conductance recovery would flatten the Na conductance restoration gradient thereby reducing the recruiting capacity of a front, extending the VP and increasing the probability of unidirectional propagation. METHODS: In a cable of ventricular cells, we explored the sensitivity of the VP to voltage-dependent blockade. While varying the unbinding time constant from 100 ms to 5 s, we measured the Na conductance restoration gradient, the liminal length, the refractory period (RP) and the VP. RESULTS: Reducing the rate of drug unbinding flattened the restoration gradient, diminished the recruiting capacity of a premature impulse and extended the liminal length, RP and the VP. The VP was linearly dependent on the drug unbinding time constant. Rapidly unbinding drugs (time constant <1 s) reduced the liminal length below that of a quiescent cable. CONCLUSIONS: Slowing the unbinding rate of voltage-dependent drug block of Na channels extended the RP and the VP. Drugs with unbinding time constants greater than 1 s dramatically increased the probability of unidirectional propagation, reflecting increases in both the RP and the VP. This study provides a new mechanism linking Na channel function, compromised by voltage-dependent Na channel drug block, with proarrhythmic conditions that can lead to sudden cardiac death following premature stimulation.

Comparison of the potassium channel openers, WAY-133537, ZD6169, and celikalim on isolated bladder tissue and In vivo bladder instability in rat.

The effects of the ATP-dependent potassium channel agonists ZD6169, celikalim, and WAY-133537 on bladder contractile function were examined in vitro on isolated bladder strips and in vivo on spontaneous bladder contractions. All three compounds produced a concentration-dependent relaxation of isolated rat detrusor strips (IC50 values = 0.93, 0.03, and 0.09 microM, respectively for ZD6169, celikalim, and WAY-133537. Contractile inhibition by all three compounds was fully reversed by 6 microM glyburide. These compounds also effectively inhibited spontaneous bladder contractions in the rat hypertrophied bladder model of detrusor instability. We also examined the electrophysiological properties of WAY-133537 on isolated rat bladder detrusor myocytes. Myocytes had an average resting membrane potential of -40 mV. Under patch current-clamp conditions, WAY-133537 (0.3 and 1.0 microM, n = 4-5) produced a significant hyperpolarization of 21 and 26 mV, respectively. Hyperpolarization was reversed by the addition of 5 microM glyburide. In patch voltage-clamp studies, WAY-133537 (0.3 microM, n = 3) significantly increased outward current in response to both voltage step and ramp protocols consistent with activation of the ATP-dependent potassium channel. In the detrusor instability model, WAY-133537 and celikalim had similar oral potencies (ED50 = 0.13 and 0.3 mg/kg, respectively), whereas ZD6169 was less potent (ED50 = 2.4 mg/kg). The antihypertensive agent celikalim exerted effects on the bladder at doses that significantly reduced systemic blood pressure. In contrast, both WAY-133537 and ZD6169 inhibited bladder hyperactivity at doses that produced minimal changes in both mean arterial blood pressure and heart rate. These data suggest that both WAY-133537 and ZD6169 may be useful in the treatment of bladder instability at doses associated with minimal hemodynamic side effects.

Inhibition of cardiac delayed rectifier K+ current by overexpression of the long-QT syndrome HERG G628S mutation in transgenic mice.

Mutations in the HERG gene are linked to the LQT2 form of the inherited long-QT syndrome. Transgenic mice were generated expressing high myocardial levels of a particularly severe form of LQT2-associated HERG mutation (G628S). Hearts from G628S mice appeared normal except for a modest enlargement seen only in females. Ventricular myocytes isolated from adult wild-type hearts consistently exhibited an inwardly rectifying E-4031-sensitive K+ current resembling the rapidly activating cardiac delayed rectifier K+ current (Ikr) in its time and voltage dependence; this current was not found in cells isolated from G628S mice. Action potential duration was significantly prolonged in single myocytes from G628S ventricle (cycle length=1 second, 26 degrees C) but not in recordings from intact ventricular strips studied at more physiological rates and temperature (200 to 400 bpm, 37 degrees C). ECG intervals, including QT duration, were unchanged, although minor aberrancies were noted in 20% (16/80) of the G628S mice studied, primarily involving the QRS complex and, more rarely, T-wave morphology. The aberrations were more commonly observed in females than males but could not be correlated with sex-based differences in action potential duration. These results establish the presence of IKr in the adult mouse ventricle and demonstrate the ability of the G628S mutation to exert a dominant negative effect on endogenous IKr in vivo, leading to the expected LQT2 phenotype of prolonged repolarization at the single cell level but not QT prolongation in the intact animal. The model may be useful in dissecting repolarization currents in the mouse heart and as a means of examining the mechanism(s) by which the G628S mutation exerts its dominant negative effect on native cardiac cells in vivo.

Long QT syndrome patients with mutations of the SCN5A and HERG genes have differential responses to Na+ channel blockade and to increases in heart rate. Implications for gene-specific therapy.

BACKGROUND: The genes for the long QT syndrome (LQTS) linked to chromosomes 3 (LQT3) and 7 (LQT2) were identified as SCN5A, the cardiac Na+ channel gene, and as HERG, a K+ channel gene. These findings opened the possibility of attempting gene-specific control of ventricular repolarization. We tested the hypothesis that the QT interval would shorten more in LQT3 than in LQT2 patients in response to mexiletine and also in response to increases in heart rate. METHODS AND RESULTS: Fifteen LQTS patients were studied. Six LQT3 and 7 LQT2 patients were treated with mexiletine, and its effects on QT and QTc were measured. Mexiletine significantly shortened the QT interval among LQT3 patients (QTc from 535 +/- 32 to 445 +/- 31 ms, P < .005) but not among LQT2 patients (QTc from 530 +/- 79 to 503 +/- 60 ms, P = NS). LQT3 patients (n = 7) shortened their QT interval in response to increases in heart rate much more than LQT2 patients (n = 4) and also more than 18 healthy control subjects (9.45 +/- 3.3 versus 3.95 +/- 1.97 and 2.83 +/- 1.33, P < .05; data expressed as percent reduction in QT per 100-ms shortening in RR). Among these patients, there is also a trend for LQT2 patients to have syncope or cardiac arrest under emotional or physical stress and for LQT3 patients to have cardiac events either at rest or during sleep. CONCLUSIONS: This is the first study to demonstrate differential responses of LQTS patients to interventions targeted to their specific genetic defect. These findings also suggest that LQT3 patients may be more likely to benefit from Na+ channel blockers and from cardiac pacing because they would be at higher risk of arrhythmia at slow heart rates. Conversely, LQT2 patients may be at higher risk to develop syncope under stressful conditions because of the combined arrhythmogenic effect of catecholamines with the insufficient adaptation of their QT interval when heart rate increases.

Analysis of the potassium channel openers celikalim, pinacidil and cromakalim in platelet models of thrombosis.

The antihypertensive agents pinacidil, cromakalim and celikalim lower blood pressure by opening potassium channels in vascular smooth muscle. The role of these compounds in inhibiting human platelet aggregation and preventing white thrombus formation in a rabbit arteriovenous shunt model was examined. None of these agents (100 microM), substantially inhibited platelet aggregation induced by epinephrine or arachidonic acid. Only celikalim (100 microM) inhibited collagen (45%), ADP (56%), or serotonin (61%) induced platelet aggregation and ADP- (41%) or epinephrine-potentiated (61%) serotonin-induced platelet aggregation. Celikalim inhibited white thrombus formation at i.v. doses of 0.25 mg/kg (46% inhibition) but not 0.1 mg/kg; (14.6%); equihypotensive doses of pinacidil (0.5 mg/kg; 21.7%) and cromakalim (0.2 mg/kg, 7.5%; 0.4 mg/kg, 33%) were less effective. Glyburide (i.v. dose of 0.5 mg/kg) inhibited the antithrombotic activity of celikalim and to a lesser extent cromakalim. The greater antithrombotic activity of celikalim in vivo may be related to beneficial effects on blood rheology and reduced red blood cell deformability.

Cellular electrophysiology of WAY-123,398, a new class III antiarrhythmic agent: specificity of IK block and lack of reverse use dependence in cat ventricular myocytes.

OBJECTIVE: The objectives were (a) to evaluate the effects of WAY-123,398, a new class III antiarrhythmic agent, on the action potential of canine Purkinje fibres in comparison with dofetilide, E-4031, and dl-sotalol, and (b) to characterise the mechanism of the class III action by studying its effects on several ionic currents in isolated cat myocytes. METHODS: Transmembrane potentials in Purkinje fibres were studied with standard microelectrodes filled with 3M KCl. Myocytes were isolated by enzymatic disaggregation with collagenase and current recordings were obtained by voltage clamp with either the nystatin perforated patch technique or the usual whole cell configuration. RESULTS: WAY-123,398 prolonged action potential duration (APD) in Purkinje fibres and in cat ventricular myocytes without altering other variables of the action potential; in Purkinje fibres the concentration producing a 20% prolongation of APD-60 mV at a basic cycle length of 1000 ms was 0.2 microM. After depolarising voltage steps, the delayed rectifier (IK) peak tail currents in cat myocytes were blocked with IC50 = 0.1 microM. The block was unaffected by varying the duration (200 to 500 ms) or the frequency (0.4 to 2.5 Hz) of the depolarising steps. A much higher concentration of WAY-123,398 (10 microM) did not have effects on the L type Ca current (ICa-L), and on the inward rectifier (IK1) and transient outward (I(to)) K currents. CONCLUSIONS: The results indicate that WAY-123,398 is an effective and specific class III agent devoid of class I activity, and suggest that WAY-123,398 prolongs cardiac repolarisation by specifically blocking the delayed rectifier current (IK). The block was unchanged over a range of frequencies and duration of depolarisation, showing no evidence of "reverse use dependence" of block.

Effects of WAY-123,398, a new class III antiarrhythmic agent, on cardiac refractoriness and ventricular fibrillation threshold in anesthetized dogs: a comparison with UK-68798, E-4031, and dl-sotalol.

Previous studies in isolated ventricular myocytes showed that WAY-123,398 is a selective blocker of the delayed rectifier K+ current (IK). In this report, we studied the electrophysiological and hemodynamic effects of WAY-123,398 in open-chest anesthetized dogs. WAY-123,398 prolonged atrial and ventricular refractoriness without affecting conduction; WAY-123,398 was as effective as UK-68798, E-4031, and dl-sotalol, but less potent than UK-68798 and E-4031. The increase in atrial refractoriness was approximately twice as large as the ventricular increase with all compounds. The hemodynamic effects of WAY-123,398 were similar to those of UK-68798; at the ED20 for increasing ventricular refractoriness, WAY-123,398 did not affect the mean arterial pressure and decreased the heart rate by 20%. In a different series of experiments, all four compounds produced large and comparable increases in the ventricular fibrillation threshold in anesthetized dogs; WAY-123,398 and UK-68798 induced defibrillation and restoration of sinus rhythm in two of six dogs each and E-4031 in one of six dogs. No episodes of drug-induced restoration to sinus rhythm were observed in dogs treated with sotalol or vehicle. In conclusion, WAY-123,398 is an effective Class III agent without Class I actions and with a favorable hemodynamic profile.

Differential block of cardiac delayed rectifier current by class Ic antiarrhythmic drugs: evidence for open channel block and unblock.

OBJECTIVE: The aim was to compare the effects of the class Ic antiarrhythmic drugs flecainide, encainide, and recainam on the delayed rectifier current, IK. METHODS: Membrane currents were studied using the single suction pipette voltage clamp technique in freshly dissociated cat ventricular myocytes bathed in HEPES buffered physiological saline at 32 degrees C. RESULTS: Flecainide and encainide decreased IK with IC50 values of 2.1 microM and 6 microM, respectively. Recainam (100 microM) reduced IK by only 7 (SEM 3)% after 20-30 min exposure and by 19% after an 80 min exposure (IC50 > 400 microM). None of the compounds blocked the inward rectifier, IK1. Block of IK by flecainide and encainide increased with depolarisation following a voltage dependence similar to that describing channel activation. Flecainide and encainide also slowed the time course of the IK tail currents, consistent with drug dissociating from open channels. CONCLUSIONS: The observed voltage dependence for IK block by flecainide and encainide resembles the interaction reported between these agents and the excitatory sodium channel, ie, depolarisation enhances block while repolarisation leads to removal of block. The results further suggest that the electrophysiological profile of class Ic agents can have a markedly different ionic basis, ie, K+ channel block by flecainide and encainide is balanced by a potent block of sodium channels, while recainam appears to be a weak but relatively specific blocker of sodium channels only. These differences are not readily accommodated by the current Harrison-Vaughan-Williams classification scheme, and suggest the possibility that potentially important drug specific differences can exist within the same antiarrhythmic drug class.

Electromechanical effects of the putative potassium channel activator celikalim (WAY-120,491) on feline atrial and ventricular muscle.

Celikalim (WAY-120,491) is a putative potassium channel activator that has been shown to lower blood pressure in animal models and humans. In the present study, we have examined the effects of celikalim on contractility and ionic currents in feline cardiac muscle. Celikalim was found to decrease contractility in electrically stimulated (2 Hz frequency) left atrial and right ventricular papillary muscle preparations with IC50 values of 0.95 +/- 0.12 microM (n = 6) and 0.29 +/- 0.07 microM (n = 5), respectively. Glyburide (1 microM) reversed the celikalim-induced negative inotropy (left atrial halves). Celikalim was also shown to activate a glyburide-sensitive current in voltage-clamped isolated ventricular myocytes that reversed close to the calculated value of the potassium equilibrium potential (n = 4 cells). In addition, celikalim was found to inhibit voltage-activated calcium current (L-type) in isolated ventricular myocytes (51 +/- 2% inhibition at 1 microM; n = 4 cells). We conclude that celikalim is a potassium channel activator and hypothesize that both the negative inotropy and the glyburide-sensitive current evoked by this drug are mediated by ATP-regulated potassium channels. Inhibition of voltage-activated calcium channels by celikalim may also contribute to the negative inotropy induced by this drug.

Potassium channel activators cromakalim and celikalim (WAY-120,491) fail to decrease myocardial infarct size in the anesthetized canine.

The cardioprotective effects of the K channel activator drugs celikalim (WAY-120,491) and cromakalim were studied in a canine model of myocardial infarction consisting of 90 min of ischemia and 5 h of reperfusion. Intracoronary infusion of cromakalim and celikalim at 0.2 microgram/kg/min beginning 10 min before occlusion of the left circumflex coronary artery and continuing throughout the duration of the reperfusion period appeared to exacerbate ischemic injury. Infarct size (percent of risk area) was 27.7 +/- 5.6% in vehicle control animals (n = 5), 40.3 +/- 6.2% for cromakalim (n = 5) and 55.7 +/- 6.4% (p less than 0.05 vs. vehicle) for celikalim-treated animals (n = 5). When these compounds were administered intravenously, using doses shown to increase total coronary flow in nonoccluded control animals, no exacerbation of ischemic injury was observed. Anatomic infarct size was 32.8 +/- 7.1% for vehicle animals (n = 5) and 32.6 +/- 13.3 and 30.9 +/- 9.8% for cromakalim- (n = 6) and celikalim-treated (n = 5) animals, respectively. Intravenous diltiazem decreased myocardial infarct size to 16.3 +/- 7.3% (n = 5) of area at risk (p = NS vs. vehicle). The anatomic area at risk was similar in all three treatment groups, and no significant differences in rate-pressure product were observed. Results of this study suggest that K-channel-activating drugs such as cromakalim and celikalim may not be effective agents in the acute therapeutic management of myocardial ischemic injury.

Modulation of the delayed rectifier, IK, by cadmium in cat ventricular myocytes.

The effects of cadmium on the delayed outward potassium current (IK) were investigated in isolated cat ventricular myocytes using the single suction pipette voltage-clamp technique. IK activation was examined using peak tail currents elicited after 750-ms voltage-clamp steps to selected membrane potentials from a holding potential of -40 mV. In the presence of Cd2+ (0.2 mM), peak tail currents increased from a control value of 85 +/- 12 to 125 +/- 18 pA (n = 4). Activation curves constructed from the average peak tail-current measurements in all experiments showed that Cd2+ shifted the voltage dependence of activation to more positive potentials by 16.4 +/- 2.0 mV and increased the slope factor of the activation curve from 6.1 +/- 0.2 to 6.9 +/- 0.2 mV. In the absence of Cd2+, increases in holding potential from -30 to -70 mV had no effect on the magnitude of the peak tail currents, suggesting that the Cd(2+)-induced increase was not the result of a voltage-dependent increase in the number of available K+ channels at the holding potential. Slow voltage ramps from -70 to +70 mV revealed that Cd2+ increased the outward current at membrane potentials positive to +20 mV and shifted the voltage range in which IK inwardly rectified to more positive potentials. The fully activated current-voltage relationship was also shifted to more positive potentials by Cd2+. Cd2+ did not alter channel selectivity for K+.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative antithrombotic activities of the phosphodiesterase inhibitors pelrinone (AY-26,768), AY-31,390 and milrinone.

The phosphodiesterase (PDE) inhibitors AY-31,390, milrinone and pelrinone (AY-28,768) were analyzed in human platelet aggregatory systems and in a rabbit arteriovenous shunt model to delineate their activity. AY-31,390 showed a remarkably potent capacity to inhibit human antithrombotic platelet aggregation. AY-31,390 inhibited arachidonic acid, U46619, collagen, epinephrine (second phase) and adenosine diphosphate (second phase) induced platelet aggregation (PA) with IC50 values of 0.18, 0.21, 0.54, 0.43 and 0.20 microM, respectively. Milrinone, although less potent than AY-31,390, inhibited PA with IC50 values of 2.1, 2.0, 5.4, 3.7 and 4.1 microM and pelrinone's IC50 values were 2.8, 6.6, 13.3, 18.6 and 11.8 microM, respectively. Platelets which were incubated with AY-31,390, milrinone or pelrinone, washed with Hanks' balanced salt solution and then resuspended in platelet poor plasma, lost their inhibitory activity in collagen and arachidonic acid PA systems. These results suggested that AY-31,390, milrinone and pelrinone did not bind tightly to cAMP PDE. If human platelet-rich plasma was pretreated with adenosine deaminase, an enzyme that degrades adenosine, the inhibitory effect of milrinone and to a lesser extent pelrinone was reversed. AY-31,390 did not produce a loss of activity with adenosine deaminase in the arachidonic acid system and only a small loss in the collagen system. Adenosine did not appear to be a meaningful factor in AY-31,390's inhibitory activity. Pelrinone, milrinone to a greater extent, and AY-31,390 to the greatest extent were effective inhibitors of white thrombus formation in the in vivo rabbit arteriovenous shunt model. These PDE III inhibitors were potent deterrants of platelet aggregation and white thrombus formation; these agents would be expected to be efficacious therapeutic antithrombotics.

The effects of the putative potassium channel activator WAY-120,491 on 86Rb efflux from the rabbit aorta.

WAY-120,491 [(-)-(3S-trans)-2-[3,4-dihydro-3-hydroxy-2,2-dimethyl-6-(trifluoromet hox y)- 2H-1-benzopyran-4-yl]-2,3-dihydro-1H-isoindol-1-one] is a novel antihypertensive agent. We have investigated the effects of this compound on contractile force and 86Rb efflux, using the rabbit aorta, in order to assess its K channel activator properties. K channel blockers and ionic conditions thought to modulate specific K channel types have been used to provide insight into the K channel(s) affected by this compound. WAY-120,491 evoked relaxation of precontracted rabbit aortic rings and increased the rate of 86Rb efflux from strips of rabbit aorta; both effects occurring in a concentration-dependent manner. The WAY-120,491 (1 microM)-induced 86Rb efflux was inhibited by tetraethylammonium (IC50 = 0.38 mM), indicating that the increased efflux was mediated by K channels. Glyburide completely blocked the WAY-120,491 (1 microM)-evoked 86Rb efflux with 50% block occurring at a concentration of 0.48 microM. Glyburide also antagonized the WAY-120,491-induced relaxation of aortic rings. Omission of Ca from the solution bathing the aorta did not inhibit the WAY-120,491 induced 86Rb efflux but rather caused an augmentation of the response. It is concluded that WAY-120,491 may be classified as a K channel opener. Furthermore, the K channel upon which WAY-120,491 acts exhibits some characteristics normally associated with the ATP regulated K channel although the involvement of other K channel types has not been ruled out.

Potentiation of phosphodiesterase inhibitor antithrombotic activity with alpha-2 adrenergic blockade.

The antithrombotic activity of pelrinone, a phosphodiesterase III inhibitor was examined in a canine model of coronary thrombosis that uses electrical current to injure the coronary endothelium. Ninety percent of vehicle treated animals developed complete coronary occlusion and thrombus mass was 32.0 +/- 5.8 mg. In a group of animals treated with zomepirac, 10 mg/kg i.v., included as a positive control, thrombus mass was decreased to 10.3 +/- 3.3 mg and incidence of occlusion was reduced to 37.5%. Pelrinone, 5.0 mg/kg i.v. decreased the incidence of occlusion to 50%, thrombus mass to 21.3 +/- 8.3 mg and inhibited platelet aggregation to collagen, ADP and arachidonic acid by 80%, 54% and 87% of baseline, respectively. When yohimbine, an alpha 2-adrenergic antagonist, was co-administered (2.0 mg/kg at the beginning of the experiment +0.5 mg/kg halfway through the experiment) with the same dose of pelrinone, thrombus mass was decreased to 1.0 +/- 0.5 mg and none of the animals developed coronary occlusion. Yohimbine administration by itself at 2.0-3.0 mg/kg showed no evidence of antithrombotic activity (thrombus mass = 32.8 +/- 8.0 mg, incidence of occlusion = 100%). This dose of yohimbine inhibited significantly ADP-induced aggregation in the presence of epinephrine. These results demonstrate that, even though this dose of pelrinone elicited near maximal inhibition of platelet aggregation, the concurrent administration of an alpha 2-adrenergic antagonist was able to potentiate markedly the phosphodiesterase inhibitor antithrombotic activity.

Channel specificity in antiarrhythmic drug action. Mechanism of potassium channel block and its role in suppressing and aggravating cardiac arrhythmias.

Although work on class III antiarrhythmics remains at an early stage, these agents still appear to possess greater efficacy and less proarrhythmia than conventional class I agents in those experimental arrhythmia models considered to be most representative of the clinical situation. Although prolongation of repolarization carries with its own tendency for pause-dependent arrhythmogenesis (i.e., torsade de pointes), available data suggest that this may be a function of nonspecificity in potassium channel block rather than a general characteristic of class III activity. The availability of new and more selective blockers of specific cardiac potassium channels under development as class III agents have already helped to clarify basic questions about the ionic mechanism of repolarization in the heart, and one hopes that a growing clinical data base will eventually determine the relative safety and efficacy of these agents in preventing symptomatic and life-threatening arrhythmias.