Future Prospects Currently Under Investigation

By | 2011-12-04

Medical Therapy

Search for a Safe Positive Inotropic Agent

A class of drugs currently receiving attention in the management of congestive heart failure is the calcium sensitizers. These agents can augment systolic performance by enhancing calcium binding to troponin C or by affecting the cross-bridge turnover kinetics without increasing cytosolic calcium levels. A potential drawback to clinical use of pure calcium sensitizers would be the enhanced reactivity of troponin C with diastolic cytosolic calcium levels, which would slow the process of myocardial relaxation. In an attempt to avoid this potential diastolic dysfunction, most of the agents under investigation today use a combination of phosphodiesterase (PDE) inhibition and calcium sensitization. Because these drugs produce both calcium sensitization and phosphodiesterase inhibition, it becomes difficult to determine whether the positive inotropic action stems from enhanced reactivity of troponin C and Ca2+ or cAMP-mediated phosphorylation of phospho-proteins, or a combination of the two.

In an effort to elucidate the mechanisms of the positive inotropic effects of pimobendan, EMD 53998, levosimendan, and OR-1896, a group of investigators sought to determine whether these drugs were capable of increasing left ventricular contractility (+dP/dt) without altering the myoplasmic Ca2+ transient. Each agent was administered at variable concentrations, and subsequently +dP/dt, myocardial calcium transients, phosphorylation of key intracellular phosphoproteins, and myocardial cyclic adenosine monophosphate levels were measured. The investigators found that low concentrations of levosimendan and OR-1896 were capable of increasing left ventricular contractility without affecting Ca2+ transients or myocardial cyclic adenosine monophosphate levels and without promoting significant phosphorylation of troponin I and C proteins. Administration of low concentrations of pimobendan or EMD 53998 produced more pronounced increases in Ca2+ transients compared with increases in left ventricular contractility. As the administration dose increased, all of the agents caused an increase in +dP/dt that was associated with further increases in myocardial cyclic adenosine monophosphate levels and phosphorylation of intracellular phosphoproteins. The investigators therefore concluded that pimobendan, EMD 53998, and higher doses of levosimendan and OR-1896 exerted positive inotropic properties through phosphodiesterase inhibition-mediated responses rather than through calcium sensitization. These researchers felt that their results lent support to the premise that low-dose levosimendan and OR-1896 were capable of augmenting left ventricular contractility by increasing the response of the myofilaments to the prevailing calcium concentrations.

It should be noted that these conclusions are far from universally accepted. Other investigators, including Endoh, have found that pimobendan can elicit a positive inotropic effect even in the presence of carbachol, a muscarinic receptor agonist that is useful for differentiating cAMP-mediated effects from those classified as calcium sensitization. Endoh’s report found that although levosimendan appears to have calcium-sensitizing properties, it must rely on simultaneous activation of cAMP-mediated signaling processes.

The varying results of these studies show that the precise mechanisms by which the calcium sensitizers exert a positive inotropic effect is uncertain and that long-term controlled studies are required to evaluate the effects of these drugs on the heart rate, arrhythmogenesis, and sudden death.

Pimobendan Despite its questionable calcium-sensitizing properties, pimobendan currendy is the most investigated agent in this drug class. Similar to other agents in this class, pimobendan combines calcium sensitization and phosphodiesterase inhibition to circumvent any diastolic dysfunction. However, this same property carries the potential for cAMP-dependent increases in arrhythmogenesis. Despite its predominandy cAMP-mediated inotropic effect, pimobendan can increase cardiac contractility to a degree similar to that of dobutamine but at a lower oxygen cost for contractility. Although pimobendan appears to lower myocardial oxygen consumption, the stigma associated with positive inotropic agents, especially those that increase cyclic adenosine monophosphate concentrations, may have doused early enthusiasm for pimobendan in the United States. Although not available in the United States, pimobendan currendy is available in Canada, Europe, and Japan and has undergone several small to medium-sized investigations in humans and dogs.

Multiple pimobendan trials have been completed in humans with heart failure. The results of these trials have been variable, pointing out both apparent beneficial effects and questionable, undesirable side effects.

A recent placebo-controlled study has been published that evaluated pimobendan therapy (0.3 to 0.6 mg/kg/day) in a small number of Doberman pinschers and cocker spaniels with dilated cardiomyopathy. The addition of pimobendan to standard therapy with digoxin, enalapril, and furosemide was associated with a significant improvement in the heart failure class. The cocker spaniels allocated to placebo and to pimobendan showed statistically similar median survival times (537 days versus 1037 days, P = 0.77), whereas the Doberman pinschers allocated to placebo had significandy shorter median survival times than those given pimobendan (50 days versus 329 days, P < 0.02).

This small study highlighted several findings. The first interesting finding is that cocker spaniels with dilated cardiomyopathy appear to have the potential for long-term survival. At the conclusion of the 4-year study, six of the 10 cocker spaniels were still alive (two in the pimobendan group and four in the placebo group). Three of the four dogs that had died (one in the placebo group and two in the pimobendan group) were euthanized for noncardiac disease; the fourth dog, allocated to pimobendan, died suddenly within 1 month of diagnosis. The study was too small to allow assessment of the propensity of pimobendan to exacerbate arrhythmias, but the drug may have contributed to the cocker spaniel’s sudden death.

Another interesting aspect of this study is that although positive inotropic therapy may prolong survival in Doberman pinschers with DCM, it may be prudent to randomize patients with atrial fibrillation (AF) separately from those without this condition. Historically Doberman pinschers with atrial fibrillation have had an abysmal 2.9-week median survival time. The pimobendan randomization happened to allocate three Doberman pinschers with AF into the placebo group and only one into the pimobendan group. This may have negatively affected survival in the placebo group, but it should be noted that the Doberman with atrial fibrillation that was treated with pimobendan survived for 37 weeks.

The observed reduction in heart failure class associated with pimobendan therapy likely will prompt further investigations into this agent. Whether pimobendan administration to dogs produces a trend toward increased mortality, as identified in the Pimobendan in Congestive Heart Failure (PICO) trial, is uncertain. Whether the combination of low-dose pimobendan and a beta blocker would have synergistic properties in dogs with dilated cardiomyopathy is uncertain but is an interesting concept. The study by Mallery et al. continues to pose the question of whether a small but significandy increased incidence of sudden death in dogs should preclude the availability of an agent with the potential to enhance their quality of life.

Angiotensin II Receptor Antagonists

The beneficial effects of angiotensin-converting enzyme inhibitors have largely been attributed to these drugs’ ability to decrease the formation of angiotensin II and increase circulating levels of bradykinin. However, alternative, ACE-independent pathways for the production of angiotensin II, including tissue plasminogen activator, cathepsin G, tonin, and chymase, may enable AT II “escape” even with administration of angiotensin-converting enzyme inhibitors. A study of patients treated with a variety of angiotensin-converting enzyme inhibitors identified angiotensin II “reactivation” in 15% of patients. This process occurred in the face of both high and low levels of angiotensin-converting enzyme activity, which suggests that alternate pathways were responsible for the production of angiotensin II. As a potential strategy for offsetting these alternate angiotensin II-generating pathways, agents capable of directly antagonizing the AT II receptors were developed. Two subtypes of angiotensin II receptors have been identified in humans: angiotensin II type 1 (AT1) and angiotensin II type 2 (AT2). Although these receptors belong to the same family, some researchers have suggested that their biologic activities differ markedly. AT, receptors are found primarily in the adrenal glands, vascular smooth muscle cells, kidney, and heart, where they mediate almost exclusively all the known actions of angiotensin II on blood pressure and osmoregulation. AT2 receptors are expressed in high density during fetal development; in adults they are found less abundandy in the adrenal medulla, uterus, ovary, and vascular endothelium and in areas of the brain. AT2 receptors appear to mediate biologic processes that counteract the trophic, AT|-mediated responses. Agents capable of blocking the AT, receptor subtype were developed with the aim of achieving more complete angiotensin II antagonism than is obtained with angiotensin-converting enzyme inhibitors. Also, selectively targeting the AT, receptor subtype would still allow for the potentially beneficial actions mediated by the AT2 receptors and avoid common side effects, predominandy coughing, associated with angiotensin-converting enzyme inhibitor therapy in humans. It is believed that the increased circulating levels of bradykinin promoted by angiotensin-converting enzyme inhibitors contribute to these drugs’ side effects, but it must be noted that this same property, which is lacking in AT II receptor antagonists, may contribute to the angiotensin-converting enzyme inhibitors’ success in the management of heart failure.

Efforts to determine whether angiotensin-converting enzyme inhibitors or angiotensin II receptor antagonists were superior or noninferior quickly followed the development of the latter agents. An early study, the Evaluation of Losartan in the Elderly (ELITE) trial, lent support to the premise that ATII receptor blockade may yield mortality benefits compared with angiotensin-converting enzyme inhibitors. Other studies that examined various subgroups followed. Mortality and morbidity end-points were examined to determine whether AT II inhibitors are preferable to angiotensin-converting enzyme inhibitors.

The results of ELITE II and the Valsartan Heart Failure Trial (Val-HeFT) suggest better tolerability of AT II inhibitors compared with angiotensin-converting enzyme inhibitors, but these studies provide no data to support the superiority of AT II inhibitors in the management of heart failure subsequent to systolic dysfunction. The proposed significant benefit behind combination therapy with AT II inhibitors and angiotensin-converting enzyme inhibitors identified in Val-HeFT is limited after the angiotensin-converting enzyme inhibitor-naive subgroup is removed from the analysis. Of additional concern was the fact that valsartan, compared to placebo, had an adverse effect on mortality (P = 0.009) in patients previously treated with angiotensin-converting enzyme inhibitors and beta blockers. Although these results cannot be extrapolated to veterinary medicine and because clinical trials have not been performed in dogs or cats with naturally occurring heart failure, the current utility of angiotensin II receptor antagonists appears limited.

Neutral Endopeptidase Inhibitors

With the success of the angiotensin-converting enzyme inhibitors established, research interests quickly shifted to additional, potentially beneficial neurohormonal pathways. Based on the recognition that atrial natriuretic peptide can promote natriuresis and diuresis and can directly inhibit the release of renin and aldosterone, new agents were developed in an effort to increase ANP’s circulating concentration. Neutral endopeptidase (NEP) is an enzyme that inactivates several substrates, including natriuretic peptides, angiotensins, and bradykinins. It was hypothesized that drugs capable of inhibiting NEP may prove beneficial in the management of congestive heart failure as single agents or in combination therapy. Early studies of the NEP inhibitor ecadotril given to dogs with pacing-induced heart failure showed increased urine output, sodium clearance, and renal sodium excretion compared with dogs given placebo. Another short-term study of ecadotril identified a dose- and time-dependent reduction in left ventricular end-diastolic pressures in dogs with experimental heart failure produced by repeated coronary embolization. Long-term administration of ecadotril (3 months) in dogs with left ventricular dysfunction produced by sequential intracoronary microembolizations was recendy reported to attenuate left ventricular remodeling and progressive left ventricular dysfunction compared with dogs receiving placebo.

Despite these findings, NEP inhibitors administered as single agents appear to have fallen out of the research arena. A communication on behalf of the International Ecadotril Multi-Centre Dose-Ranging Study reported findings from a study of 279 patients with chronic heart failure randomized to placebo or one of four doses of ecadotril. The study’s primary aim was to evaluate the safety and tolerability of ecadotril at doses of 50-400 mg twice daily. Two patients randomized to the highest dose of ecadotril developed pancytopenia at 47 and 53 days, and died rapidly of sepsis. The presence of a thioester group within the compound suggested an idiosyncratic ecadotril-induced aplastic anemia that was potentially dose related. Lower doses of ecadotril were not associated with adverse reactions. Although the study was not powered to exclude symptomatic benefit, patient-reported symptoms and quality of life-scores were unable to reveal any overall symptomatic benefit during the administration of ecadotril. A similar, smaller pilot study from the United States evaluating 50 patients randomized to placebo or ecadotril, 50 to 400 mg twice daily, did not identify any adverse sequela but again there were no changes identified in signs and symptoms of heart failure, NYHA class, or patient self-assessment of symptoms.

Vasopeptidase Inhibitors

Although it is difficult to determine the reasons for the relative lack of efficacy seen in the small pilot studies of ecadotril, the problem may have been that the drug increased not only natriuretic peptide levels but also the levels of circulating angiotensin II. More recently a group of drugs that can inhibit NEP and ACE, the so-called vasopeptidase inhibitors, has been developed. Omapatrilat has been the most rigorously evaluated vasopeptidase inhibitor in several trials for the management of congestive heart failure No significant difference was seen in the primary end-point of combined risk of death or hospitalization for heart failure requiring intravenous treatment (P = 0.187) between omapatrilat and angiotensin-converting enzyme inhibitors. To date, the inability of vasopeptidase inhibitors to provide substantial benefit compared with the proven efficacy of angiotensin-converting enzyme inhibitors may limit their clinical utility.

Endothelin Antagonists

Endothelin, a peptide released from endothelial cells, has powerful vasoconstrictive activity. Currently three peptides, endothelin-1, endothelin-2, and endothelin-3, and two receptor subtypes, ETA and ETB, have been identified. In principle, endothelin-1 promotes vasoconstriction by complexing with the ETA receptor and vasodilatation by binding to the ETB receptor, but this is somewhat species dependent. In addition to its vasodilative properties, the ETB receptor may promote vasoconstriction in some regional blood vessels, including the mesenteric and coronary vasculature.

Although endothelin’s name suggests that it is produced only in vascular endothelial cells, the genes that encode the three peptides have been identified in varying patterns in vascular smooth muscle cells, cardiac myocytes, renal tubular epithelial cells, bronchia] epithelial cells, glial cells, pituitary cells, macrophages, and mast cells. Therefore the importance of endothelin may extend well beyond simple vasoconstriction and include additional pleiotropic effects on many nonvascular tissues. Although endothelin is suspected of being linked to systemic hypertension, pulmonary hypertension, vascular remodeling, and acute renal failure, the emphasis of this discussion is on potential applications of endothelin to the progression of heart failure

Heart failure, through increased filling pressures and decreased peripheral perfusion, promotes activation of numerous neurohormonal reflexes to maintain cardiac output and circulatory homeostasis. Activation of the renin-angiotensin-aldosterone system and SNS and the release of arginine vasopressin are recognized to increase the concentration of circulating vasoconstrictors (i.e., angiotensin II, norepinephrine, and vasopressin) and to promote maladaptive remodeling. Furthermore, it has been shown that these compensatory mechanisms enhance the production of endothelin. Increased endothelin binding to the ETA receptors promotes vasoconstriction and smooth muscle cell proliferation, thereby further diminishing cardiac performance through afterload mismatch. Studies have also shown that the ETB receptors become upregulated and can promote vasoconstriction and cardiac fibrosis in the presence of heart failure. These findings have raised the possibility of administration of ETA receptor antagonists or a combination of ETA/ETB receptor antagonists to combat heart failure.

Chronic administration of endothelin antagonists to laboratory animals with experimental heart failure and acute administration to humans with heart failure resulted in beneficial hemodynamic profiles. In rats with heart failure induced by coronary artery ligation, the combination of an angiotensin-converting enzyme inhibitor and an endothelin (ETA) antagonist lowered the systolic blood pressure and the left ventricular end-systolic and left ventricular end-diastolic pressures significantly more than the angiotensin-converting enzyme inhibitor or endothelin antagonist alone. However, the drug combination did not significantly improve survival or reduce left ventricular weight and collagen density compared with rats randomized to an angiotensin-converting enzyme inhibitor alone. Endothelin antagonism without the benefit of angiotensin-converting enzyme inhibition did not result in a survival benefit compared with untreated rats with heart failure.

Despite these promising hemodynamic profiles, early evidence suggests that chronic nonselective and ETA-selective antagonism of endothelin receptors is of limited benefit in patients with severe heart failure. The Endothelin Antagonist Bosentan for Lowering Cardiac Events in Heart Failure (ENABLE) trial evaluated the nonselective endothelin antagonist bosentan, which has proved beneficial in pulmonary hypertension, for the management of humans with NYHA class IIIb-IV heart failure and an ejection fraction of less than 35%. The patients randomized to Bosentan appeared to have an increased early risk of worsening heart failure requiring hospitalization.

These studies appear to highlight the fact that hemodynamic benefit may not equate with clinical benefit. Whether lower doses of the endothelin antagonists and slow uptitration might prove beneficial is unresolved at this time; however, the results of studies to date may have curtailed the momentum for the use of endothelin antagonists in the management of chronic heart failure.

Interventional Therapy

Passive Ventricular Constraint

In the early 1990s an experimental “ventricular assist” technique was developed in which a patient’s native skeletal muscle was wrapped around the failing ventricles to provide cardiac support. The latissimus dorsi, connected to a synchronizable burst stimulator, was used in an effort to increase cardiac contractility and hence functional status. The utility of dynamic cardiomyoplasty held promise because it alleviated the need for cardiac bypass, donor organs, and immunosuppression, which are required for cardiac transplantation. The feasibility of dynamic cardiomyoplasty in dogs was subsequendy documented, and researchers found a potential for improved contractile function even during contractions that were not assisted by myostimulation of the latissimus dorsi.

Although dynamic cardiomyoplasty has failed to gain prominence in the management of heart failure, the potential for limiting ventricular dilatation by means of passive constraint has prompted further study. The premise is that passive ventricular constraint may significandy slow or stabilize the remodeling process that accompanies heart failure. Because the constraint is passive and does not require myostimulation, a synthetic wrap could be used, simplifying the surgical procedure.

Despite the potential benefit of passive ventricular constraint in these models, the fact remains that veterinary patients tend to be presented for evaluation after severe cardiac dysfunction has already developed. New diagnostic modalities may allow for earlier detection of occult cardiac disease, but the variable nature of disease progression could still complicate the determination of which patients should be fitted with a cardiac support device. At this time, further studies are required to evaluate the efficacy of passive ventricular constraint in severe cardiac dysfunction, and diagnostic tests allowing early detection of cardiac disease must be developed.

Ventricular Resynchronization

Although systolic dysfunction classically is believed to be a myocardial phenomenon, the important role the cardiac conduction system plays in maintaining optimal cardiac performance is often forgotten. The normal conduction pathway modulates the contraction rate, the mechanical efficacy of atrial systole, and the co-ordination of the ventricular chambers. In humans the presence of an intraventricular conduction delay, typically in a left bundle branch block pattern, promotes disco-ordinate contraction, with early activation of the septal wall followed by delayed iateral contraction at higher stress. The presence of this conduction defect has been associated with a 60% to 70% higher risk of all-cause mortality and has remained an independent risk factor after adjustment for age, underlying cardiac disease, severity of heart failure, and treatment with angiotensin-converting enzyme inhibitors or beta blockers. The problem of ventricular discordance has long been recognized in patients with artificial ventricular pacemakers, and efforts to approximate the ventricular activation sequence more closely (i.e., by pacing the right ventricular outflow tract rather than the apex) have been shown to increase cardiac output.

Recendy a similar approach, called cardiac resynchronization therapy (CRT), has been used in patients with congestive heart failure and intraventricular conduction delay. Biventricular pacing or univentricular pacing of the left ventricular free wall has improved the contractile index +dP/dTmax and arterial pulse pressure within a single beat of commencement of pacing. The proposed benefits of CRT include a reduction in end-systolic and end-diastolic volumes and the ability to promote reverse remodeling. Ventricular resynchronization has been associated with increased systolic function despite a decline in myocardial oxygen consumption. This contrasts with traditional cAMP-dependent inotropic therapy and may explain the ability of biventricular pacing to improve functional patient status while promoting mortality benefits. Unfortunately, the ability of CRT to improve heart failure class and reduce mortality in veterinary medicine may be limited by the low prevalence of intraventricular conduction disturbances in dogs with dilated cardiomyopathy.

Stem Cells and Cellular Transplantation

Historically it has been recognized that cardiac and neural tissues lack the large postnatal regenerative capacity seen in the epithelial layers of the skin, intestinal and pulmonary mucosal linings, and connective tissues. Therefore the ability to repopulate areas of ischemic myocardium with cells capable of promoting angiogenesis or areas of fibrosis with cells that contribute to contractile function could prove curative.

Numerous cell types have been identified that may serve as potential sources for tissue grafting, including skeletal myoblasts, fetal cardiomyocytes, smooth muscle cells, embryonic stem cells and bone marrow-derived stromal and hematopoietic stem cells. Because of the complex electrophysiologic, structural, and contractile properties of myocardial cells, cardiomyocytes appear to be the ideal donor cells. The difficulty lies in harvesting and transplanting enough cells to mitigate the degree of cardiac dysfunction. The isolation of pluripotent stem cells from mouse blastocysts in 1981 may have provided the key to an endless vault of viable, transplantable cardiomyocytes.

The promise of stem cells lies in their capacity for prolonged self-renewal and their potential to differentiate into one or more cell types. Embryonic stem (ES) cells display wide-ranging plasticity, which allows them to form derivatives of all three germ layers. Their ability to undergo near endless cell doublings while retaining the capacity to differentiate into various cell types further advances their therapeutic potential. However, beyond the ethical considerations involved in the use of human embryonic stem cells (which veterinary practitioners may not face), many other hurdles must be overcome. One of the most important obstacles is defining a methodology to generate reproducible, spontaneous cardiomyocyte-differentiating stem cells with sufficient purity for clinical purposes. Emerging evidence suggests that an unknown combination of growth factors, transcription factors, feeder layers, and physical properties is involved in early cardiomyocyte differentiation. Delivery systems such as intracoronary catheterization techniques, intramyocardial injection, transendocardial delivery by means of catheter-based systems, and intravenous administration are currently being evaluated and refined to optimize the long-term survival of the grafted cells, to ensure the delivery of a critical mass of viable cells, and to promote the appropriate alignment with the host cells. These steps are vital to the functional and structural integration of stem cells in the host tissue. The immunogenicity of embryonic stem cells poses yet another challenge.

Stem cells are not confined to embryos; rather, small numbers of stem cells can be harvested from adult bone marrow. These adult mesenchymal stem (MS) cells maintain an undifferentiated phenotype in culture and appear to have the capacity for multilineage differentiation. The potential benefits of autologous adult mesenchymal stem cells, including ease of attainability, lack of immunogenicity, and absence of ethical objections, have prompted widespread research into their use in the management of cardiovascular disease. It remains to be seen whether repopulation of the myocardium with these cells is feasible in veterinary medicine and whether a sufficient number of viable cells to enhance systolic function can be delivered.