Heart failure (HF) is a debilitating syndrome characterized by symptoms of fluid overload and dyspnea.9 It can be categorized as heart failure with reduced ejection fraction (HFrEF) or preserved ejection fraction (HFpEF).8 HFrEF is defined as left ventricular dysfunction as demonstrated by a left ventricular ejection fraction (LVEF) of < 40%.8 During acute phases of decompensation, compensatory mechanisms are active in an attempt to maintain cardiac output and preserve organ perfusion. Although HF increases the risk of death, nonfatal worsening of symptoms is the most common problem encountered by patients, who experience progressive impairment of functional capacity and quality of life.4 Nonfatal worsening may require intensification of oral medications or can necessitate emergent treatment, including hospitalization, intensive care, or expensive medical or surgical interventions.5 Two pivotal compensatory mechanisms are the sympathetic nervous system and the renin-angiotensin-aldosterone system (RAAS); these are responsible for many of the symptoms experienced by patients, including fluid overload, dyspnea and tachycardia.9
Historically, the treatment of HF was to prescribe an angiotensin converting enzyme inhibitor (ACEi), or an angiotensin receptor blocker (ARB). An ACEi had traditionally been used first and exchanged for an ARB if the patient had the inconvenient side effect of a dry, hacking cough, or the dangerous side effect of angioedema. A third drug was developed to treat HF that combined the benefits of an ARB without the side effects of an ACEi, along with the action of inhibiting the membrane-bound enzyme neprilysin.
Renin is produced and stored in the kidneys and is the first component of the RAAS. Renin is secreted by the kidney in response to one or all of the following conditions:
Angiotensinogen is produced and stored in the liver and is converted to angiotensin I in the presence of renin. Angiotensin I is a fairly inert decapeptide that is converted to angiotensin II when exposed to an angiotensin converting enzyme (ACE) found mostly in the lungs. Angiotensin II is an active pressor substance that increases vasopressin production, vasoconstricts the arteries and veins, and increases aldosterone secretion from the renal cortex. Increased aldosterone levels promote natriuresis and increased circulating volume through decreased diuresis.
ACE also inactivates the vasodilator peptide, bradykinin, which contributes to its pressor action, also giving the enzyme its alternate name kininase II.3 Bradykinins are peptides that cause vasculature dilation.
With the administration of an ACEi, bradykinin levels are elevated, along with an increase in plasma and urinary prostaglandins, which are also vasodilators.3 The effect of lowering blood pressure through the vasodilatory act of bradykinin and prostaglandins makes ACEi a first-line therapeutic treatment for heart failure.
A negative side-effect of elevated bradykinin and prostaglandins is an annoying dry cough. This occurs in 10%-15% of patients being treated with an ACEi.3 A life-threatening upper airway obstruction named angioedema that occurs in 0.1%-0.2% of Caucasians and larger percentages in African Americans is also the result of increased bradykinin and prostaglandins.3
The actions of angiotensin II are mediated by two angiotensin (AT) receptors—AT1, which exerts the detrimental effects, and AT2, which triggers some beneficial effects.11 Angiotensin receptor blockers (ARBs) were developed with the rationale that angiotensin II production continues in the presence of ACE inhibition, driven through alternative enzyme pathways.13 ARBs primarily work by blocking angiotensin II at the AT1 receptor. By directly blocking this receptor, all angiotensin II is inhibited, and this allows the beneficial effects of AT2 receptor activation to dominate.10 The effect of angiotensin II blockade is similar to ACE inhibition and includes reduction in preload and afterload, as well as delaying the progression of ventricular dysfunction as mediated by hypertrophy and dilation.9
Similar to ACEis, ARBs are potent antihypertensive agents and are indicated as both first-line drugs in both hypertension and HFrEF (heart failure with reduced ejection fraction).3 However, a major difference between ACEis and ARBs is the greater specificity of the latter.3 Because ARBs do not block the degradation of kinins, angioedema and persistent cough are quite uncommon with their use.3
Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) are members of the natriuretic peptide family responsible for body fluid homeostasis and blood pressure control. These three peptides form the natriuretic peptide system (NPS).
Distention of the atria and ventricles, as occurs in cardiac injury and/or overload, ventricular dysfunction, and HF, results in significant increases in the expression of ANP and BNP.3 The concentrations of circulating BNP and of its inactive precursor, N-terminal fragment of the pro-B-type natriuretic peptide (NT-proBNP), have become extremely valuable in the recognition of hemodynamic overload, ventricular dysfunction, and HF.6 Measurements of these peptides are also useful in assessing prognosis and monitoring therapy, and appears to be therapeutic targets as well.12
Neprilysin (NEP) catalyzes the degradation of several vasodilator peptides, including natriuretic peptides (ANP/BNP), angiotensin II, bradykinin, substance P, adrenomedullin, and endothelin-1.2 Therefore, inhibiting NEP (NEPi) will augment the naturally occurring natriuretic peptides, which promote natriuresis, induce vasodilation, and reduce cardiac hypertrophy and fibrosis.2 NEPi also increases the amount of the circulating vasopressors angiotensin II and endothelin I.7 These two opposing actions—inhibition of degradation of both vasoconstrictor and vasodilator peptides—neutralize each other and, as a consequence, NEPis alone have little effect on blood pressure or HF.1
Drugs were developed that would inhibit both ACE and NEP (duel inhibitors). The duel inhibitor was referred to as a vasopeptidase inhibitor and would be called angiotensin receptor-neprilysin inhibitors.
ARNIs combine both effective actions of decreasing the RAAS and inhibiting NEP. Because ACEi have the unwanted effect of bradykinin degradation with its accompanying cough and possible angioedema, an ARB was used in combination with a NEPi to produce an ARNI.
The introduction of an angiotensin receptor–neprilysin inhibitor (ARNI) (valsartan/sacubitril) and a sinoatrial node modulator (ivabradine), when applied judiciously, complements established pharmacological and device based therapies and represents a milestone in the evolution of care for patients with HF. 13