Neprilysin (neutral endopeptidase 24.11) is an enzyme responsible for the breakdown of natriuretic peptides and has long been considered a target for hypertension. Indeed, inhibition of neprilysin increases natriuretic peptide levels, resulting in natriuresis, vasodilation, and functional inhibition of the RAS. However, any blood-pressure-lowering effect of neprilysin inhibition is offset, since this enzyme also degrades peptides such as endothelin-1 and Ang II that cause vasoconstriction. Indeed, combined neprilysin and ACE inhibition, achieved using omapatrilat, evoked greater anti-hypertensive effects than did the ACE inhibitor enalapril alone ²¹ . However, the higher rate of angioedema noted by omapatrilat (most likely involving raised bradykinin levels) in large-scale heart failure trials has led to the discontinuation of this therapeutic strategy, despite clinical efficacy ²² . Given that AT ₁ receptor antagonists, unlike ACE inhibitors, do not inhibit bradykinin metabolism, other combinations of RAS and neprilysin inhibition have been considered. Indeed, there is much interest in a new combination of equal proportions of the AT ₁ receptor antagonist valsartan and a neprilysin inhibitor sacubitril, known as LCZ696.

Clinical trial data for LCZ696 demonstrated significantly greater blood pressure reductions compared to valsartan in patients with mild-to-moderate hypertension ²³ . Anti-hypertensive efficacy of LCZ696 was confirmed in Asian populations with minimal safety and tolerability issues ²⁴ . The effect of LCZ696 has also been examined on biomarkers in heart failure patients with preserved ejection fraction (PARAMOUNT) ²⁵ and on clinical outcomes in heart failure patients with reduced ejection fraction (PARADIGM-HF) ^{26, 27} . The latter trial was terminated early since there was a 20% reduction in cardiac death compared with an ACE inhibitor. Interestingly, in all trials, blood pressure reductions were greater than the RAS inhibitor used as the comparator. The beneficial effects of the valsartan/sacubitril combination are described in detail elsewhere ^{20, 28, 29} . Moreover, their striking effects in systolic heart failure patients and the prospects of benefits in future outcome trials for heart failure patients with preserved ejection fraction ^{28, 29} begs the question of the potential role of such vasopeptidase inhibition in resistant hypertension.

In addition, a dual vasopeptidase inhibitor called daglutril, combining neprilysin and endothelin-converting enzyme inhibition, has been developed and was reported to lower blood pressure in hypertensive type 2 diabetics who were already receiving losartan ³⁰ , although its current status is not clear.

The regulatory requirement to obtain robust cardiovascular safety data to approve new diabetes drugs has increased the number of trials focused on a lack of cardiovascular toxicity rather than examining longer-term studies on the effects of glucose lowering on cardiovascular outcome data ³¹ . However, data are emerging showing that new anti-diabetic drugs can lower blood pressure, perhaps contributing to their therapeutic effects to improve glycaemic control and cardiovascular outcomes. The glucagon-like peptide 1 (GLP-1) analogue liraglutide reduced blood pressure in hypertensive diabetic patients ³² . There was a small reduction in blood pressure in the recent LEADER trial, studying over 9,300 patients, in which liraglutide improved cardiovascular outcome (composite of death and non-fatal myocardial infarction or stroke) assessed over 3.5–5 years ³³ . Inhibitors of sodium-glucose cotransporter 2, such as empagliflozin, are another new class of anti-diabetic compound that impairs renal glucose reabsorption. In high-risk cardiovascular patients with type 2 diabetes, empagliflozin, added to standard care, lowered the rate of the primary composite cardiovascular outcome and of death from any cause ³⁴ and reduced blood pressure versus placebo in patients with type 2 diabetes and hypertension ³⁵ . Thus, while these compounds are not strictly anti-hypertensive agents, even modest blood pressure reductions are likely to contribute to other cardiovascular protective mechanisms evoked by these novel anti-diabetic agents ^{33, 36} .

Aminopeptidases are involved in the metabolism of Ang II into shorter Ang peptide fragments. Aminopeptidase A (APA) converts Ang II into Ang III by removing N-terminal aspartate, which can also activate AT ₁ or type 2 (AT ₂) receptors. Given the ubiquitous AT ₁ receptor expression, Ang II and Ang III both evoke predominant AT ₁ receptor-mediated peripheral vasoconstriction and pressor responses. Interestingly, preclinical studies have shown that centrally administered Ang III more effectively raised blood pressure than did Ang II (involving centrally mediated sympathetic nerve stimulation and vasopressin release) ^{37, 38} . Such studies provided the rationale for inhibition of APA as a potential therapeutic strategy to treat hypertension ^{38, 39} . RB150 is a dimer of EC33, which is a selective APA inhibitor that inhibits the conversion of Ang II to Ang III ^{38, 39} . While the clinical efficacy of RB150, also known as QGC001, in the treatment of hypertension is yet to be evaluated, it was well tolerated in healthy male volunteers and did not significantly change heart rate or blood pressure ⁴⁰ . However, it remains to be seen if any centrally mediated anti-hypertensive effect of QGC001, due to decreased bioavailability of Ang III in the brain, is offset by an increase in peripheral Ang II accumulation and consequent vasoconstrictor potential in hypertensive patients. It is also less common nowadays for the development of centrally acting anti-hypertensive drugs because of central side effects.

In addition to the ACE/AT ₁ receptor arm of the RAS, there are two separate but related “protective” arms of the RAS, so called because their activation generally opposes AT ₁ receptor-mediated cardiovascular effects. These include the Ang III/AT ₂ receptor arm ^{41, 42} and the ACE2/Ang (1–7)/Mas receptor arm ^{41, 43– 45} . Within the RAS, there are a number of bioactive Ang peptides in addition to the main effector Ang II. For example, Ang III can also exert more subtle depressor effects and natriuretic responses in preclinical studies that are mediated by AT ₂ receptor stimulation ^{41, 42} . A number of selective AT ₂ receptor agonists have been developed ⁴⁵ , of which compound 21 (C21) has been the best studied in preclinical hypertension-related models. Indeed, selective AT ₂ receptor stimulation reduces hypertension-induced target organ damage, even in the absence of blood pressure reduction in most but not all instances ^{42, 46} . The AT ₂ receptor field has been hampered by a lack of selective ligands, but there is great interest in the clinical effects of AT ₂ receptor activation and the potential for additive effects of AT ₂ receptor agonists with conventional RAS blockade. However, C21 has only entered phase I testing in healthy volunteers at this stage, so this field will be watched with interest.

ACE2 is a carboxypeptidase that differs to ACE in that it cleaves one (not two) amino acids from the C-terminal of peptides. In the context of RAS, Ang I and Ang II can be converted by ACE2 to Ang (1–9) and Ang (1–7), respectively. In particular, there is much preclinical evidence to indicate that Ang (1–7) can protect against hypertension-related target organ damage via another G-protein-coupled receptor known as the Mas receptor ^{43, 44, 47} . Given the ability of ACE2 activation to facilitate conversion to the “cardioprotective” Ang (1–7) at the expense of (decreasing) levels of the substrate Ang II, research has focused on developing compounds that activate these RAS pathways. Indeed, there are a number of Mas receptor and AT ₂ receptor agonists ^{20, 45} that have not yet progressed beyond preclinical studies. Moreover, there are compounds that activate ACE2, such as diminazene aceturate (known as DIZI). Interestingly, this ACE2 activator, structurally related to xanthenone, is used clinically for the treatment of the parasitic disease African trypanosomiasis ⁴⁷ , so there is already translational potential. Similarly, recombinant ACE2 (rACE2) has been used in preclinical studies where it has been reported to lower blood pressure and exert cardioprotective effects ^{43, 47} . Human rACE2 was tested in phase I studies in which single intravenous injections of varying doses were given to healthy subjects ⁴⁸ . Human rACE2 was well tolerated and increased plasma ACE2 without affecting blood pressure and evoked a prolonged, marked suppression of plasma Ang II levels ⁴⁸ . Clearly, additional clinical trial data are required to assess the veracity of this form of therapy.