Vascular versus myocardial selectivity of dihydropyridine calcium antagonists as studied in vivo and in vitro

Calcium Antagonists for Perioperative Blood Pressure Control

Calcium entry blockers constitute three major classes of pharmacologic agents: phenylalkylamines (eg, verapa mil), benzothiazepines (eg, diltiazem), and dihydropyri dines (eg, nifedipine). The effectiveness of all types of calcium channel blockers in the prevention and treat ment of coronary artery disease as well as chronic and acute hypertension is undisputable. Their beneficial clinical effects may be due to peripheral and coronary vasodilatation, resulting in reduction in myocardial oxy gen consumption, and an increase in myocardial oxy gen supply in addition to their antispasmodic effect and the ability to prevent intracellular calcium overload. For the management of perioperative hypertension develop ing in patients undergoing cardiac or noncardiac sur gery, the dihydropyridines appear to be especially suit able. Intravenous (IV) formulations of nifedipine, nicardipine, and isradipine have been successfully used in this setting. At the present time, nicardipine is the most widely used IV dihydropyridine. This is due to its potent afterload-reducing activity and relatively short duration of action, although its effect may increase the longer the drug is being infused. The ideal drug for perioperative blood pressure control should be one with the pharmacodynamic profile of the vascular selec tive dihydropyridines, but with an ultrashort duration of action.

Preclinical cardiac safety assessment of drugs

The heart is a frequent site of toxicity of pharmaceutical compounds in humans, and when developing a new drug it is critical to conduct a thorough preclinical evaluation of its possible adverse effects on cardiac structure and function. Changes in cardiac morphology such as myocardial necrosis, hypertrophy or valvulopathy are assessed in regulatory toxicity studies in laboratory animals, although specific models may be needed for a more accurate detection of the risk. The potential proarrhythmic risk of new drugs is a major subject of concern and needs to be fully addressed before treatment of volunteers or patients takes place. In vitro assays are conducted to determine the effects on cardiac ion channels, in particular I(Kr) potassium channel antagonism. Prolongation of the QT interval is assessed in vivo, generally in telemetered dogs. Together, these two tests are considered to detect most arrhythmic drugs. The results of this core battery can be refined by additional studies, in particular assays on isolated cardiac tissues determining changes in cardiac action potential duration, shape and variability over time. Triggering of arrhythmia is assessed in hypokalaemic dogs with artificially created bradycardia, or in vitro in isolated whole hearts. The proarrhythmic risk of the new compound is then evaluated by integrating the results of these different tests. Drug adverse effects on cardiac electrophysiological function, in particular impulse formation and conduction, are evaluated through changes in ECG, generally recorded in dogs, pigs or monkeys. Changes in cardiac contractility occurring either as a primary effect of the drug on cardiac function or as a consequence of cardiac lesions should also be carefully assessed. In telemetered or anaesthetised animals, cardiac contractility is evaluated by measurement of left ventricular pressure and its first derivative over time. Echocardiography allows non-invasive measurement of drug-induced changes in ventricular wall movements and cardiac haemodynamics indicative of effects on contractility. In conclusion, a reliable and accurate evaluation of the cardiac safety of a new pharmaceutical agent is based on the results of in vitro tests, with overall moderate to high throughput, and in vivo experiments assessing the effects of the drug on the heart in its physiological environment. The specific sensitivities of the animals used in these assays to cardiac adverse effects should also be considered. The final evaluation of the cardiac risk is therefore based on an integrated analysis of the results from a battery of tests.

Comparative Effects of Pranidipine with Amlodipine in Rats with Heart Failure

The aim of the present study was to compare the cardioprotective properties of long-acting calcium channel antagonist pranidipine with amlodipine in rat model of heart failure induced by autoimmune myocarditis. Twenty-eight days after immunization the surviving rats were randomized for the oral administration of low-dose amlodipine (1 mg/kg/day), high-dose amlodipine (5 mg/kg/day), pranidipine (0.3 mg/kg/day) or vehicle (0.5% methylcellulose). After oral administration for 1 month, the animals underwent echocardiography and hemodynamic analysis. Histopathology, immunohistochemistry, and Western immunoblotting were carried out in the heart samples. Both pranidipine and high-dose amlodipine increased survival rate. Although the heart rate did not differ among the four groups, left ventricular end-diastolic pressure was significantly decreased and +/-dP/dt was increased in the pranidipine- and high-dose amlodipine-treated rats, but not in low-dose amlodipine-treated rats. In comparison to amlodipine treatment, pranidipine treatment significantly reduced myocyte size and central venous pressure. Furthermore, both pranidipine and high-dose amlodipine treatment significantly reduced myocardial protein levels of atrial natriuretic peptide and inducible nitric oxide synthase, whereas pranidipine only significantly decreased tumor necrosis factor-alpha, and improved sarcoplasmic reticulum Ca2+ ATPase2 protein levels. We conclude that pranidipine ameliorates the progression of left ventricular dysfunction and cardiac remodeling in rats with heart failure after autoimmune myocarditis in a lower dose when compared to amlodipine and which may be a clinically potential therapeutic agent for the treatment of heart failure.

Comparison of amlodipine or nifedipine treatment with developing congestive heart failure: effects on myocyte contractility

BACKGROUND

Past studies have suggested that amlodipine, a dihydropyridine L-type Ca(2+) channel antagonist, may exert useful effects in congestive heart failure (CHF). The present study examined the effects of amlodipine or nifedipine treatment in a model of developing CHF on left ventricular (LV) pump function and myocyte contractility.

METHODS AND RESULTS

Pigs (25 kg) were randomly assigned to 1 of 4 groups: 1) pacing-induced CHF (rapid atrial pacing at 240 bpm) for 3 weeks (n = 9), 2) concomitant Ca(2+) channel blockade with amlodipine (1.5 mg/kg/day) and rapid pacing (n = 7), 3) concomitant Ca(2+) channel blockade with nifedipine (0.7 mg/kg twice daily) and rapid pacing (n = 7), and 4) sham controls (n = 7). LV fractional shortening fell with pacing CHF from baseline values (17% +/- 1% v 42% +/- 1%, P <.05). With rapid pacing and concomitant amlodipine treatment, LV fractional shortening increased from pacing CHF values (24% +/- 1%, P <.05) but was unchanged with concomitant nifedipine treatment (20% +/- 2%, P =.2). LV myocyte velocity of shortening, as measured by high speed videomicroscopy, was reduced with pacing CHF compared with controls (42 +/- 2 microm/s v 87 +/- 9 microm/s, P <.05), and increased from pacing CHF values with amlodipine or nifedipine treatment (62 +/- 8 microm/s, 64 +/- 4 microm/s, respectively; P <.05). Inotropic response to extracellular Ca(2+) (8 mmol/L) was reduced with pacing CHF (94 +/- 5 microm/s v 160 +/- 15 microm/s, P <.05) and increased from CHF values with amlodipine or nifedipine treatment (132 +/- 14 microm/s and 133 +/- 7 microm/s, respectively, P <.05) CONCLUSIONS: These results suggest that the primary mechanism for the effects of amlodipine on myocyte contractility in developing CHF is because of direct Ca(2+) channel blockade.

Cardiac inotropic vs. chronotropic selectivity of isradipine, nifedipine and clevidipine, a new ultrashort-acting dihydropyridine

Cardiac effects of clevidipine, a new ultrashort-acting dihydropyridine Ca2+ channel antagonist were investigated in Langendorff-perfused rat hearts and compared to those of nifedipine and isradipine. The aim was to determine and compare the negative inotropic vs. chronotropic potency of these drugs. The hearts were perfused with oxygenated Krebs-Henseleit buffer at a perfusion pressure of 90 cm H2O. After stabilization, one concentration of each drug was administered for 45 min followed by a higher concentration for an additional 45 min. The concentrations of each drug in this study were 10(-9), 3 x 10(-9), 10(-8), 10(-7), 10(-6.5) and 10(-6) M for clevidipine and nifedipine, and 10(-10), 3 x 10(-10), 10(-9), 10(-8), 10(-7.5) and 10(-7) M for isradipine. Each concentration of each drug was tested in six hearts. Coronary flow, left ventricular dP/dt max, left ventricular systolic pressure and heart rate were recorded when the hearts were beating spontaneously and during pacing at a constant rate for 1 min. Spontaneous heart rate and atrio-ventricular conduction were not affected by clevidipine at any of the concentrations studied, while nifedipine and isradipine caused a concentration-dependent decrease. These two drugs caused atrio-ventricular block at high concentrations. All three compounds reduced cardiac contractility in a concentration-dependent manner. When isradipine was administered, at a given concentration, heart rate and contractility decreased proportionately. When clevidipine or nifedipine was given, at a given concentration, the proportionate reduction in left ventricular dP/dt max was greater than that in heart rate, resulting in a high inotropic vs. chronotropic selectivity. It is concluded that in contrast to nifedipine and isradipine, clevidipine does not impair atrio-ventricular conduction. Like nifedipine, clevidipine is selective for inotropic vs. chronotropic cardiac effects.

Amlodipine therapy in congestive heart failure: hemodynamic and neurohormonal effects at rest and after treadmill exercise

This study examined the acute effects of amlodipine treatment on left ventricular pump function, systemic hemodynamics, neurohormonal status, and regional blood flow distribution in an animal model of congestive heart failure (CHF), both at rest and with treadmill exercise. A total of 14 pigs were studied under control conditions and after the development of pacing-induced CHF (240 beats per minute, 3 weeks, n = 7) or with CHF and acute amlodipine treatment for the last 3 days of pacing (1.5 mg/kg per day, n = 7). Under resting conditions, left ventricular stroke volume (mL) was reduced with CHF compared with the normal state (15+/-2 vs. 31+/-1, p<0.05) and increased with amlodipine treatment (23+/-4, p<0.05). At rest, systemic vascular resistance increased with CHF compared with the normal state (3,078+/-295 vs. 2,131+/-120 dyne x s cm(-5), p<0.05) and was reduced after amlodipine treatment (2,472+/-355 dyne x s cm(-5), p<0.05). With exercise, left ventricular stroke volume remained lower and systemic vascular resistance higher in the CHF group, but was normalized with amlodipine treatment. With exercise, left ventricular myocardial blood flow increased from resting values, but was reduced from the normal state with CHF (normal: 1.69+/-0.12 to 7.62+/-0.74 mL/min per gram vs. CHF: 1.26+/-0.12 to 4.77+/-0.45 mL/min per gram, both p<0.05) and was normalized with acute amlodipine treatment (1.99+/-0.35 to 6.29+/-1.23 mL/min per gram). Resting plasma norepinephrine was increased by >5-fold in the CHF group at rest and was not affected by amlodipine treatment. However, with exercise, amlodipine treatment blunted the increase in plasma norepinephrine by >50% when compared with untreated CHF values. Resting plasma endothelin levels increased with CHF compared with the normal state (10.9+/-0.9 vs. 2.8+/-0.4 fmol/mL, p<0.05) and was reduced with amlodipine treatment (7.5+/-1.5 fmol/mL, p<0.5). In other vascular beds, acute amlodipine treatment with CHF improved pulmonary and renal blood flow both at rest and with exercise; however, there were no effects observed on skeletal muscle blood flow. With the development of CHF, acute amlodipine treatment does not negatively influence left ventricular pump function, but rather may provide favorable hemodynamic and neurohormonal effects.

Chronic amlodipine treatment during the development of heart failure

BACKGROUND

This study examined the effects of chronic amlodipine treatment on left ventricular (LV) pump function, systemic hemodynamics, neurohormonal status, and regional blood flow distribution in an animal model of congestive heart failure (CHF) both at rest and with treadmill exercise. In an additional series of in vitro studies, LV myocyte contractile function was examined.

METHODS AND RESULTS

Sixteen pigs were studied under normal control conditions and after the development of chronic pacing-induced CHF (240 bpm, 3 weeks, n=8) or chronic pacing and amlodipine (1.5 mg . kg-1 . d-1, n=8). Under ambient resting conditions, LV stroke volume (mL) was reduced with CHF compared with the normal control state (16+/-2 versus 31+/-2, P<0.05) and increased with concomitant amlodipine treatment (29+/-2, P<0.05). At rest, systemic and pulmonary vascular resistance (dyne . s-1 . cm-5) increased with CHF compared with the normal control state (3102+/-251 versus 2156+/-66 and 1066+/-140 versus 253+/-24, respectively, both P<0.05) and were reduced with amlodipine treatment (2108+/-199 and 480+/-74, respectively, P<0.05). With CHF, LV stroke volume remained reduced and was associated with a 40% reduction in myocardial blood flow during treadmill exercise, whereas chronic amlodipine treatment normalized LV stroke volume and improved myocardial blood flow. Resting and exercise-induced plasma norepinephrine levels were increased by >5-fold in the CHF group and were reduced by 50% from CHF values with chronic amlodipine treatment. Resting plasma endothelin (fmol/mL) increased with CHF compared with the normal state (10.4+/-0.9 versus 3.1+/-0.3, P<0.05) and was reduced with amlodipine treatment (6.6+/-1.1, P<0.5). With CHF, LV myocyte velocity of shortening ( microm/s) was reduced compared with normal controls (39+/-1 versus 64+/-1, P<0.05) and was increased with chronic amlodipine treatment (52+/-1, P<0.05).

CONCLUSIONS

Chronic amlodipine treatment in this model of developing CHF produced favorable hemodynamic, neurohormonal, and contractile effects in the setting of developing CHF.

Human vascular to cardiac tissue selectivity of L- and T-type calcium channel antagonists

1. Voltage-operated calcium channel (VOCC) antagonists are effective antihypertensive and antianginal agents but they also depress myocardial contractility. 2. We compared four L-type calcium channel antagonists, felodipine, nifedipine, amlodipine and verapamil and a relatively T-type selective calcium channel antagonist, mibefradil, on human and rat isolated tissue assays to determine their functional vascular to cardiac tissue selectivity (V/C) ratio. 3. The V/C ratio was calculated as the ratio of the IC50 value of the antagonist that reduced (by 50%) submaximally contracted (K+ 62 mM) human small arteries from the aortic vasa vasorum (vascular, V) mounted in a myograph and the IC50 value of the antagonist that reduced (-)-isoprenaline (6 nM) submaximally stimulated human right atrial trabeculae muscle (cardiac, C) mounted in organ chambers. 4. The average pIC50 values (-log IC50 M) for the human vascular preparations were felodipine 8.30, nifedipine 7.78, amlodipine 6.64, verapamil 6.26 and mibefradil 6.22. The average pIC50 values for the cardiac muscle were felodipine 7.21, nifedipine 6.95, verapamil 6.91, amlodipine 5.94, and mibefradil 4.61. 5. The V/C ratio calculated as antilog [pIC50V-pIC50C] is thus mibefradil 41, felodipine 12, nifedipine 7, amlodipine 5 and verapamil 0.2. 6. In rat small mesenteric arteries the pIC50 values for the five drugs were similar to the values for human vasa vasorum arteries contracted by K+ 62 mM. However for methoxamine (10 microM) contraction in the rat arteries the pIC50 values were lower for felodipine 7.24 and nifedipine 6.23, but similar for verapamil 6.13, amlodipine 6.28 and mibefradil 5.91. 7. In conclusion. in the human tissue assays, the putative T-channel antagonist mibefradil shows the highest vascular to cardiac selectivity ratio; some 3 fold higher than the dihydropyridine, felodipine, and some 200 fold more vascular selective than the phenylalkylamine, verapamil. This favourable vascular to cardiac selectivity for mibefradil, from a new chemical class of VOCC antagonist, may be explained by its putative T-channel selectivity.

Amlodipine monotherapy, angiotensin-converting enzyme inhibition, and combination therapy with pacing-induced heart failure

In patients with congestive heart failure (CHF) receiving therapy with angiotensin-converting enzyme (ACE) inhibition, institution of calcium channel antagonism with amlodipine provided favorable effects. The goal of the present study was to define potential mechanisms for these effects by measuring left ventricular function, hemodynamics, and neurohormonal system activity in a model of CHF in which amlodipine treatment had been instituted either as a monotherapy or in combination with ACE inhibition. Thirty-two pigs were instrumented to allow measurement of cardiac index, total systemic resistance index, and neurohormonal activity in the conscious state and assigned to one of four groups: (1) rapid atrial pacing (240 bpm) for 3 weeks (n = 8), (2) amlodipine (1.5 mg x kg(-1) x d[-1]) and pacing (n = 8), (3) ACE inhibition (fosinopril 1.0 mg/kg BID) and pacing (n = 8), and (4) amlodipine and ACE inhibition (1.0 mg x kg(-1) x d(-1) and 1.0 mg/kg BID, respectively) and pacing (n = 8). Measurements were obtained in the normal control state and after the completion of the treatment protocols. With rapid pacing, basal resting cardiac index was reduced compared with control values (2.7+/-0.2 versus 4.7+/-0.1 L x min(-1) x m(-2), respectively, P<.05) and increased from rapid pacing-only values with either amlodipine or combination therapy (3.7+/-0.3 and 4.4+/-0.5 L x min(-1) x m(-2), respectively, P<.05). Basal resting total systemic resistance index was higher in the rapid pacing-only group compared with control values (2731+/-263 versus 1721+/-53 dyne x s x cm(-5) x m2, respectively, P<.05), was reduced with either amlodipine treatment or ACE inhibition (2125+/-226 and 2379+/-222 dyne x s x cm(-5) x m2, respectively, P<.05), and was normalized with combination therapy. Plasma catecholamines, renin activity, and endothelin levels were increased threefold with rapid pacing. Amlodipine, either as a monotherapy or in combination with ACE inhibition, did not result in increased plasma catecholamines and renin activity compared with the rapid pacing-only group. Furthermore, combination therapy reduced steady state norepinephrine and normalized epinephrine levels. The results of the present study demonstrated that monotherapy with either amlodipine or ACE inhibition provides beneficial effects in this pacing model of CHF. Combined amlodipine and ACE inhibition provided greater benefit with respect to vascular resistance properties and neurohormonal system activity compared with either monotherapy.

Safety of calcium antagonists in patients with congestive heart failure

Continuing high morbidity and mortality have spurred an ongoing search for new therapeutic agents for patients with congestive heart failure. Calcium antagonists (CAs) have been under active investigation in patients with heart failure since their introduction into clinical medicine, because their anti-ischemic and vasodilator properties were thought to be of potential benefit in this patient population. However, review of published clinical trials of CAs in patients with heart failure reveals that some of these drugs are associated with detrimental effects, including acute hemodynamic deterioration, increased symptoms of heart failure, and increased mortality. The adverse effects of short-acting CAs in patients with heart failure include negative inotropic effects and neurohormonal activation. Long-acting CAs, such as amlodipine and felodipine, had fewer negative inotropic effects, showed less evidence of neurohormonal activation, and were better tolerated in clinical trials. Amlodipine, in combination with an angiotensin-converting enzyme inhibitor, had a neutral effect in patients with ischemic heart failure and an unexplained benefit in a subgroup of patients with non-ischemic cardiomyopathy. Although the preliminary experience with long-acting dihydropyridine CAs in heart failure has been encouraging, safety concerns raised by past trials dictate that no CA can be recommended for the treatment of heart failure at this time.

Vascular selective calcium entry blockers in the treatment of cardiovascular disorders: focus on felodipine

Calcium entry through L-type calcium channels is essential for contraction of both arterial smooth muscle and the myocardium, and is important in cardiac conduction. First-generation calcium entry blockers lack or have a modest degree of vascular selectivity and inhibit cardiac function at doses producing therapeutic arterial dilatation. Such agents may cause deterioration in patients with left ventricular dysfunction, and their combination with a beta-adrenergic blocker may adversely affect cardiac contractility and conduction. Development of newer agents has focused on obtaining a higher degree of vascular selectivity. Felodipine is a highly vascular selective calcium entry blocker, with a vascular selectivity ratio greater than 100, as shown experimentally. Isradipine and nicardipine are also vascularly selective calcium entry blockers. Hemodynamic studies in patients with hypertension, coronary artery disease, congestive heart failure, or in patients receiving beta-adrenergic blockade, show that felodipine can produce profound arteriolar dilatation without the negative effects of left ventricular systolic performance. Furthermore, felodipine alone or when added to a beta-adrenergic blocker does not interfere with cardiac conduction. The primary mechanism that accounts for the efficacy of dihydropyridine calcium entry blockers in hypertension and angina pectoris is arterial dilation, whereas nondihydropyridines may also derive part of their effect from inhibition of cardiac performance. As some of these patients, most commonly the elderly, have concomitant left ventricular dysfunction, it should be advantageous to avoid myocardial depression in the treatment of their primary disease. Preliminary studies in patients with heart failure indicate that felodipine and amlopidine may improve hemodynamics, reduce neurohormonal activation, and increase exercise tolerance, but final conclusions must await the randomized clinical trials now underway.