Supplementary information

Other drug targets under exploration for Heart failure

  1. Soluble guanylate cyclase activators

Soluble guanylate cyclase (sGC) is an enzyme present in the cytosol which is activated by nitric oxide. Although sGC is present in three forms namely a reduced (Fe2+)-heme bound form, an oxidized (Fe 3+) heme-bound form, and an oxidized (Fe3+) heme-free form, it is only the reduced-heme bound form of sGC that increases cGMP. In conditions associated with increased oxidative states or decreased nitric oxide production as in endothelial dysfunction in the setting of heart failure, the sGC system is inhibited. Cinaciguat is potent activator of sGC that activates sGC in an NO- independent manner. In a study performed in 60 patients with acute decompensated heart failure, intravenous infusion of cinaciguat was demonstrated to have significant reduction in the pulmonary capillary wedge pressure. There was also a substantial reduction in blood pressure, pulmonary artery pressure, right atrial pressure, systemic vascular resistance, and pulmonary vascular resistance with an increase of heart rate and cardiac output [1]. A larger phase IIb study that was done in 139 patients also appeared to show similar results, but the increased incidence of hypotension seen in the study subjects receiving cinaciguat over placebo led to premature termination of the study [2]. Two other trials namely COMPOSE 1 and COMPOSE EARLY had to be abruptly terminated due to the high risk of hypotension among patients on cinaciguat [3]. Although cinaciguat did not see further development, another heme –dependent sGC stimulator named riociguat showed favorable results in the therapy of pulmonary hypertension that led to its approval by US FDA in 2013[4]. A study by Bonderman et al showed that riociguat improved cardiac index, pulmonary and systemic vascular resistance and was well tolerated in patients with pulmonary hypertension caused by left ventricular systolic dysfunction [5].

  1. Improvement of mitochondrial biogenesis:

Improvement of the energy production by the failing myocardium can be used to meet the rising energy demand in patients with heart failure. One strategy to augment the energy production is to increase the mitochondrial biogenesis- the production of new mitochondria [6]. Mitochondrial biogenesis can be activated by a complex interplay of events that is coordinated by peroxisome proliferator–activated receptor gamma coactivator 1α (PGC1α), a nuclear encoded protein. Several lines of evidence seem to strongly suggest that mitochondrial dysfunction may lead to the development of heart failure [7-9]. Although there are currently no drugs that directly target mitochondrial biogenesis, this process can be augmented by AMPK and eNOS pathways. AMPK has been documented to be an important modulator of mitochondrial biogenesis through its interaction with PGC1α and other key factors such as NRF1, Tfam and ERR α [10]. Two compounds in pre-clinical development that target AMPK are A769662 and PT1. A769662 was found to reduce infarct size in animal models. The development of A769662 had to be curtailed owing to its propensity to cause cell cycle arrest by other AMPK independent mechanisms. PT1 is expected to have actions on several AMPK subtypes and its exact actions on the heart have not been elucidated yet.

There is substantial evidence to suggest that eNOS/ NO/cGMP pathway is an important activator of mitochondrial biogenesis. Treatment of adipocytes with nitric oxide donor activates mitochondrial biogenesis. The mechanism by which eNOS pathway stimulates mitochondrial biogenesis is not known. Phosphodiesterase inhibitors can also stimulate the mitochondrial biogenesis through PGC1α upregulation. A literature search on the utility of sildenafil in chronic heart failure was carried out which showed that sildenafilwas associated with improvements in cardiac index, right ventricular ejection fraction, and other markers of cardiovascular function, as well as reduced pulmonary arterial pressure [11]. In a pilot study that was carried out in 45 heart failure patients, sildenafil was found to show favorable improvement in the LV diastolic function and cardiac geometry [12]. However sildenafil did not show any significant improvement in exercise capacity among patients who are in heart failure with normal ejection fraction, when given for a period of 24 weeks [13].

  1. Inhibition of adenylyl cyclase type 5

Adenylyl cyclase is a transmembrane protein that converts ATP to cAMP whenever stimulated by G protein coupled receptors such as beta adrenergic receptors. Adenylyl cyclase exists as several isoforms, of which type 5 and type 6 isoform are the ones which are predominantly expressed in the cardiomyocyte. Low calcium concentration can inhibit the AC type 5 isoforms [14]. Disruption of type 5 adenylyl cyclase gene was found to protect against the pressure overload caused by thoracic banding of aorta in mice. The LVEF reduction was more pronounced in mice which did not have a disruption of type 5 adenylyl cyclase [15]. A mouse model was developed with genetic disruption of AC type 5 which lived one third longer than the wild type mice. As these mice were protected from age induced/pressure overload induced and catecholamine induced stress than the wild type mice, inhibition of AC type 5 could emerge as a strong contender as a drug target for heart failure [16,17].

  1. Inhibition of matrix metalloproteinase

The matrix metalloproteinase enzymes are involved in the complex myocardial remodeling that occurs in patients with myocardial infarction and congestive heart failure. These enzymes are involved in the degradation of proteins in the extracellular milieu of the myocardium. The tissue inhibitors of matrix metalloproteinase are locally synthesized proteins that bind to MMP and thus serve to control the MMP activity. An imbalance between MMP and TIMP in patients with heart failure could tilt towards increased LV remodeling [18]. Plasma MMP-9 was identified as a novel predictor of increased mortality among patients with coronary artery disease [19]. In another study done in MI patients, higher left ventricular volumes were associated with lower plasma MMP-2 concentrations [20]. MMP-9 levels were also found to predict adverse cardiac remodeling after acute MI. The antibiotic doxycycline is the only MMP inhibitor that is currently approved by the US FDA. Although animal studies have shown reduction in adverse LV remodeling with doxycycline along with improvement in LV function, these findings have not been translated successfully in human population as of date [21]. PG-116800 is an inhibitor of MMP-2, -3, -8, -9, -13, and -14. A study was carried out in which patients with acute MI were randomized to receive PG-116800 or placebo. At 90 days of follow up, there was no significant on the LV remodeling [22]. The lack of benefit with MMP inhibition is partly explained by the biphasic profile of MMP-9 in which early inhibition appeared to have greater benefit than prolonged inhibition of MMP-9 as higher plateau levels of MMP-9 were associated with better preservation of LV function [23,24].

  1. Caspase - 3 inhibition

Caspase-3 is one of the key enzymes involved in apoptosis of cardiomyocyte. Caspase-3 is activated in the myocardium of end stage heart failure. There is strong evidence for caspase 3 playing an incriminatory role in the pathophysiology of heart failure. Over expression of caspase -3 in animal models is associated with destruction of the myofibrillar structure [25]. Activation of caspase-3 in myocardium, results in cleavage of contractile proteins leading to reduced cardiac contractile function [26]. Infusion of Z-Asp-2,6DCBMk, a cell-permeable, broad-spectrum caspase inhibitor, in rats was observed to preserves myocardial contractile proteins, reduces systolic dysfunction, and attenuates ventricular remodeling [27]. Although caspase -3 appears to be a promising target for heart failure, there are no drug candidates that are currently being explored to modulate caspase-3 activity in heart failure patients.

  1. Aldosterone synthase inhibitors: Aldosterone synthase or CYP11B2 has caught the imagination of several researchers in the last few years as a therapeutic target for hypertension, heart failure and renal disorders [28]. It is now proven beyond doubt that aldosterone is a major determinant of adverse remodelling of the heart produced by cardiac fibrosis. Aldosterone escape is a common phenomenon that occurs with use of ACE-inhibitors. Even aldosterone antagonists such as spironolactone do not have the ability to impede the ischemia-induced deterioration of myocardial contractile and metabolic functions or the negative inotropic effect in human trabeculae brought about by aldosterone [29, 30]. Thus blocking the synthesis of aldosterone by inhibition of aldosterone synthase would help in countering these actions that are insensitive to aldosterone antagonists. In a study done in rat model of CHF, long-term of an aldosterone synthase inhibitor FAD286, was able to improve the cardiac hemodynamics and prevent LV remodelling [31]. Although this study showed the drug to be superior to spironolactone, it is not known if this could translate clinically into better survival outcomes.

References

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4. Press Announcements > FDA approves Adempas to treat pulmonary hypertension . March 2014

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29. Chai W, Garrelds IM, de Vries R, Batenburg WW, van Kats JP, Danser AH (2005)Nongenomiceffects of aldosterone in the human heart: interaction with angiotensin II.Hypertension46:701–706.

30. Fujita M, Minamino T, Asanuma H, et al (2005) Aldosterone nongenomically worsens ischemia via protein kinase C-dependentpathways in hypoperfused canine hearts. Hypertension 46:113–117.

31. Mulder P, Mellin V, Favre J, et al (2008) Aldosterone synthase inhibition improves cardiovascular function and structure in rats with heart failure: a comparison with spironolactone. Eur Heart J 29:2171–9.