Novel use of a short-acting intravenous beta blocker in combination with inotropic therapy as a bridge to chronic oral beta blockade in patients with advanced heart failure

Novel role of phosphodiesterase inhibitors in the management of end-stage heart failure

In advanced heart failure (HF), chronic inotropic therapy with intravenous milrinone, a phosphodiesterase III inhibitor, is used as a bridge to advanced management that includes transplantation, ventricular assist device implantation, or palliation. This is especially true when repeated attempts to wean off inotropic support result in symptomatic hypotension, worsened symptoms, and/or progressive organ dysfunction. Unfortunately, patients in this clinical predicament are considered hemodynamically labile and may escape the benefits of guideline-directed HF therapy. In this scenario, chronic milrinone infusion may be beneficial as a bridge to introduction of evidence based HF therapy. However, this strategy is not well studied, and in general, chronic inotropic infusion is discouraged due to potential cardiotoxicity that accelerates disease progression and proarrhythmic effects that increase sudden death. Alternatively, chronic inotropic support with milrinone infusion is a unique opportunity in advanced HF. This review discusses evidence that long-term intravenous milrinone support may allow introduction of beta blocker (BB) therapy. When used together, milrinone does not attenuate the clinical benefits of BB therapy while BB mitigates cardiotoxic effects of milrinone. In addition, BB therapy decreases the risk of adverse arrhythmias associated with milrinone. We propose that advanced HF patients who are intolerant to BB therapy may benefit from a trial of intravenous milrinone as a bridge to BB initiation. The discussed clinical scenarios demonstrate that concomitant treatment with milrinone infusion and BB therapy does not adversely impact standard HF therapy and may improve left ventricular function and morbidity associated with advanced HF.

Inotropic therapy: an important role in the treatment of advanced symptomatic heart failure

Inotropic therapy remains an option in the management of patients with advanced heart failure symptoms from systolic dysfunction who do not respond to conventional therapies. The decision to use this class is largely predicated on an accurate evaluation of the patient's fluid and perfusion status. Selection of the appropriate agent and dosing regimens requires an understanding of the underlying pathophysiology of heart failure and concomitant therapy. Most important, the goals of care should be stated clearly, given inherent risks associated with this class of drug.

Effects of cilostazol in the heart

Cilostazol is a selective phosphodiesterase 3 (PDE3) inhibitor approved by the Food and Drug Administration for treatment of intermittent claudication. It has also been used in bradyarrhythmic patients to increase heart rates. Recently, cilostazol has been shown to prevent ventricular fibrillation in patients with Brugada syndrome. Cilostazol is hypothesized to suppress transient outward potassium (Ito) current and increase inward calcium current, thus, maintaining the dome (phase 2) of action potential, decreasing transmural dispersion of repolarization and preventing ventricular fibrillation. Although many PDE3 inhibitors have been shown to increase cardiac arrhythmia in heart failure, cilostazol has presented effects that are different from other PDE3 inhibitors, especially adenosine uptake inhibition. Owing to this effect, cilostazol could be an effective cardioprotective drug, with its beneficial effects in preventing arrhythmia. In this review, the cardiac electrophysiological effects of cilostazol are presented and its possible cardioprotective effects, particularly in preventing ventricular fibrillation, are discussed, with emphasis on the need to further verify its clinical benefits.

Myocardial phosphodiesterases and regulation of cardiac contractility in health and cardiac disease

Phosphodiesterase (PDE) inhibitors are potent cardiotonic agents used for parenteral inotropic support in heart failure. Contractile effects of these agents are mediated through cAMP-protein kinase A-induced stimulation of I (Ca2+) which ultimately results in increased Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. A number of additional effects such as increases in sarcoplasmic reticulum Ca(2+) stores, stimulation of reverse mode Na(+)-Ca(2+) exchange, direct or cAMP-mediated effects on sarcoplasmic reticulum ryanodine receptor, stimulation of the voltage-sensitive sarcoplasmic reticulum Ca(2+) release mechanism, as well as A(1) adenosine receptor blockade could contribute to positive inotropic responses to PDE inhibitors. Moreover, some PDE inhibitors exhibit Ca(2+) sensitizer properties as they could increase the affinity of troponin C Ca(2+)-binding sites as well as reduce Ca(2+) threshold for thin myofilament sliding and facilitate cross-bridge cycling. Inotropic responses to PDE inhibitors are significantly reduced in cardiac disease, an effect largely attributed to downregulation of cAMP-mediated signalling due to sustained sympathetic activation. Four PDE isoenzymes (PDE1, PDE2, PDE3 and PDE4) are present in myocardial tissue of various mammalian species, of which PDE3 and PDE4 are particularly involved in regulation of cardiac myocyte contraction. PDE cAMP-hydrolysing activity is preserved in compensated cardiac hypertrophy but significantly reduced in animal models of heart failure. However, clinical studies have not revealed any changes in distribution profile as well as kinetic and regulatory properties of myocardial PDEs in failing human hearts. A reduction of PDE inhibitors-induced contractile responses in heart failure has therefore been ascribed to reduced cAMP synthesis due to uncoupling of adenylyl cyclase from beta-adrenoreceptor. In cardiac myocytes, PDEs are targeted to distinct subcellular compartments by scaffolding proteins such as myomegalin, mAKAP and beta-arrestins. Over subcellular microdomains, cAMP hydrolysis by PDE3 and PDE4 allows to control the activity of local pools of protein kinase A and therefore the extent of protein kinase A-mediated phosphorylation of cellular proteins.

Inhibitors of cyclic nucleotide phosphodiesterase 3 and 5 as therapeutic agents in heart failure

Cyclic nucleotide phosphodiesterases (PDE) 3 and 5 regulate cAMP and cGMP signalling in cardiac and smooth muscle myocytes. Important advances in the understanding of the roles of these enzymes have recently been made. PDE3 inhibitors have inotropic and vasodilatory properties, and although they acutely improve haemodynamics in patients with heart failure, they do not improve long-term morbidity and mortality. Although combination therapy with beta-adrenergic receptor antagonists or selective inhibition of specific PDE3 isoforms might result in a more favourable long-term outcome, more clinical data are needed to test this proposition. The role of PDE5 inhibitors in the treatment of cardiac disease is evolving. PDE5 inhibitors cause pulmonary and systemic vasodilation. How these drugs will compare with other vasodilators in terms of long-term outcomes in patients with heart failure is unknown. Recent studies also suggest that PDE5 inhibitors may have antihypertropic effects, exerted through increased myocardial cGMP signalling, that could be of additional benefit in patients with heart failure.

Alterations in cAMP-mediated signaling and their role in the pathophysiology of dilated cardiomyopathy

Dilated cardiomyopathy is a disease characterized by enlargement of the chambers of the heart and a decrease in contractility of the heart muscle. The process involves several alterations in proteins involved in cyclic adenosine monophosphate (cAMP) generation that result in a decrease in intracellular cAMP content per unit of adrenergic stimulation in cardiac myocytes. A fundamental question is whether these changes constitute a pathologic mechanism that contributes to chamber enlargement and hypocontractility or a compensatory adaptation that protects the heart from the adverse effects of increased catecholamine stimulation. Clinical studies in humans suggest that the latter effect may be more important. Studies in animal models, however, make the picture more complex: changes in cAMP-mediated signaling can have different effects depending on the specific protein whose expression or function is altered and the setting in which the alteration occurs. It may be that dilated cardiomyopathy represents a collection of different diseases in which alterations in cAMP-mediated signaling have different roles in the pathophysiology of the disease, and, furthermore, that changes in the phosphorylation of individual substrates of cAMP-dependent protein kinase may be either beneficial or harmful. Identifying differences among patients with dilated cardiomyopathy with respect to the role of altered cAMP-mediated signaling in their pathology, and identifying the "good" and "bad" substrates of cAMP-dependent protein kinase, are important areas for further research.

PDE3 inhibition in dilated cardiomyopathy: reasons to reconsider

BACKGROUND

PDE3 cyclic nucleotide phosphodiesterases have important roles in regulating cAMP- and cGMP-mediated signaling. Drugs that inhibit these enzymes raise cAMP and cGMP content in cardiac and vascular smooth muscle and increase the phosphorylation of proteins by cAMP- and cGMP-dependent protein kinases (PK-A and PK-G), thereby eliciting inotropic and vasodilatory responses.

METHODS

Although these actions are beneficial acutely in patients with dilated cardiomyopathy, long-term use of these agents was shown in several clinical trials to increase mortality. Several new clinical studies, however, suggest PDE3 inhibitors may be safe and effective when used in conjunction with beta-adrenergic receptor antagonists, whereas new studies at the cellular and molecular levels indicate that there are several isoforms of these enzymes in cardiac and vascular myocytes that are likely to regulate cAMP content in different intracellular compartments.

CONCLUSIONS

Both sets of observations suggest that PDE3 inhibition may be refined to allow more selective effects on phosphorylation of PK-A substrates, possibly allowing the beneficial effects of PDE3 inhibition to be separated from the adverse long-term consequences of their use.

Inhibitors of cyclic nucleotide phosphodiesterase PDE3 as adjunct therapy for dilated cardiomyopathy

PDE3 cyclic nucleotide phosphodiesterases are important in cyclic AMP (cAMP) and possibly cyclic GMP-mediated signalling in cardiac and vascular smooth muscle myocytes. Drugs that inhibit these enzymes have inotropic and vasodilatory actions that have proven useful in the short-term treatment of contractile failure and pulmonary hypertension in dilated cardiomyopathy (both ischaemic and idiopathic). With long-term usage, however, these drugs appear to increase mortality in treated patients through an as yet undetermined mechanism that is in some way attributable to an increase in intracellular cAMP content in cardiac myocytes. Several recent clinical trials have raised the possibility that these drugs may be used to advantage in dilated cardiomyopathy when they are administered in combination with beta-adrenoceptor antagonists, which act to lower intracellular cAMP content. In this review, the relevant basic and clinical data are examined and the possible justification for the combination of two therapies with seemingly opposite effects on intracellular cAMP content is considered.