Prior to reperfusion therapy, the major therapeutic breakthrough in myocardial infarction was the demonstration that ACE inhibitors or ARBs, given to prevent adverse "remodelling" (progressive dilatation and decline in systolic function) in high risk patients, reduced the likelihood of developing heart failure and the risk of death. The neurohumoral systems which are activated in patients after myocardial infarction (and in heart failure) are not all harmful and some endogenous systems may be protective. The best recognised of these is the natriuretic peptide system.

A- and B-type natriuretic peptides are secreted by the heart when it is stressed and these peptides promote vasodilation (reducing left ventricular wall stress), stimulate renal sodium and water excretion (i.e. antagonising the retention of salt and water characterising heart failure) and inhibit pathological growth i.e. hypertrophy and fibrosis (key components of the adverse left ventricular remodelling that occurs after infarction and in heart failure).The augmentation of plasma levels of endogenous natriuretic peptides can be achieved through inhibition of neutral endopeptidase, also known as neprilysin (NEP), which is responsible for the breakdown of natriuretic peptides. Recently, the addition of neprilysin inhibition to blockade of the RAAS (using sacubitril/valsartan), compared with RAAS blockade alone, reduced the risk of heart failure hospitalisation and death in patients with HF-REF. These exciting findings may lead to a new approach to the treatment of heart failure, with an angiotensin receptor neprilysin inhibitor (ARNI) replacing an ACE inhibitor as one of the fundamental treatments for this condition. We believe that the same approach may be beneficial in highrisk survivors of myocardial infarction. Recently, sacubitril/valsartan was shown to ameliorate adverse left ventricular remodelling in an experimental model of acute myocardial infarction. The objective of the present proposal is to gather "proof-ofconcept", mechanistic, evidence in humans to support adoption of this new approach in patients at high risk after myocardial infarction as a result of residual left ventricular systolic dysfunction.

The objective of the present proposal is to obtain information, which is currently not available, on the cardiac effects of sacubitril/valsartan in patients with LVSD, better characterise the neurohumoral actions of sacubitril/valsartan and gather "proof-ofconcept", mechanistic, evidence in humans to support adoption of this new treatment in patients at high risk after myocardial infarction as a result of residual LVSD.

Surprisingly, there is currently limited evidence about how sacubitril/valsartan works in humans. PARADIGM-HF was a large pragmatic mortality/morbidity trial with no mechanistic sub-studies and this is also true of a ongoing trial (PARADISE-MI) in acute myocardial infarction. Moreover, both trials either used or will use an ACE inhibitor (enalapril and ramipril, respectively), rather than an ARB as the active comparator for sacubitril/valsartan; use of valsartan in our study will allow us to precisely define the effects of neprilysin inhibition. A-type (or atrial) natriuretic peptide (ANP), C-type natriuretic peptide (CNP) and adrenomedullin are substrates for neprilysin and may play a role in the action of sacubitril/valsartan but have not been measured in existing clinical trials (in part because of the instability of these peptides and unfeasibility of measuring them in multi-centre, multi-national trials).

Indeed, ANP and CNP are more specific substrates for neprilysin than BNP. As has been mentioned above, cardiac fibrosis appears to be important in the process of LV remodelling in patients with asymptomatic LVSD and the development of HF-REF and is reflected in circulating biomarkers which may be influenced by sacubitril/valsartan

Inclusion Criteria:

• Acute myocardial infarction (AMI) at least 3 months prior to recruitment

  • Left ventricular ejection ≤40% as measured by transthoracic echocardiography
  • Ability to provide written, informed consent
  • Age ≥18 years
  • Tolerance of a minimum dose of ACE inhibitor/ARB (ramipril 2.5mg BD or equivalent)
  • Treatment with a beta-blocker unless not tolerated or contraindicated.

Exclusion Criteria:

• Contraindication to CMR (ferrous prosthesis, implantable cardiac device or severe claustrophobia)

  • Clinical and/or radiological heart failure (NYHA≥2)
  • Symptomatic hypotension and/or systolic blood pressure <100mmHg
  • eGFR < 30 mL/min/1.73m2 and/or serum potassium >5.2mmol/L
  • Persistent/permanent atrial fibrillation
  • History of AMI within last 3 months
  • History of hypersensitivity or allergy to ACE-inhibitors/ARB
  • History of angioedema
  • Known hypersensitivity to the active study drug substances, contrast media or any of the excipients
  • Obesity (where body girth exceeds MRI scanner diameter)
  • Pregnancy, planning pregnancy, or breast feeding
  • Inability to give informed consent or comply with study protocol
  • Evidence of hepatic disease as determined by any one of the following: AST or ALT values exceeding 2 x ULN at Visit 1, history of hepatic encephalopathy, history of oesophageal varices, or history of portacaval shunt
  • History of biliary cirrhosis and cholestasis
  • Active treatment with cholestyramine or colestipol resins
  • Active treatment with lithium or direct renin inhibitor
  • Participation in another intervention study involving a drug or device within the past 90 days (co-enrolment in observational studies is permitted)

1 year of study follow up whilst taking study drug and an additional 30 days following the final study visit.

Double blind