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Inclusion Criteria:

1. Written informed consent.
2. Male or female; aged 40 years or older.
3. HF, defined as one symptom present at the time of screening, and one sign present at the time of screening or in the previous 12 months. Symptoms and signs are defined as:

   Symptoms: dyspnoea on exertion, orthopnoea or paroxysmal nocturnal dyspnoea Signs: peripheral oedema, crackles on chest auscultation post-cough, raised jugular venous pressure or chest x-ray demonstrating pleural effusion, pulmonary congestion, or cardiomegaly
4. Left Ventricular Ejection Fraction (LVEF) \> 45% at Visit 0, (any local LVEF measurement made using echocardiography or CMR).
5. BNP ≥ 100 pg/ml or NTproBNP ≥ 300 pg/ml recorded at Visit 0. For patients in atrial fibrillation on Visit 0 ECG, BNP \> 300pg/ml or NTproBNP \> 900 pg/ml at Visit 0.
6. Myocardial fibrosis, defined as Extracellular Matrix (ECM) volume \> 27% by CMR at Visit 0.

Exclusion Criteria:

1. Myocardial infarction, coronary artery bypass graft surgery or percutaneous coronary intervention within the previous 6 months.
2. Probable alternative cause of patient's HF symptoms that in the opinion of the investigator primarily accounts for patient's dyspnoea such as significant pulmonary disease, anaemia or obesity. Specifically, patients with the below are excluded:

   1. Severe chronic obstructive pulmonary disease (COPD) (i.e., requiring home oxygen, chronic nebuliser therapy, or chronic oral steroid therapy), or
   2. Haemoglobin \< 9 g/dl, or
   3. Body mass index (BMI) \> 55 kg/m2.
3. Known pericardial constriction, genetic hypertrophic cardiomyopathy, or infiltrative cardiomyopathy.
4. Clinically significant congenital heart disease.
5. Presence of severe valvular heart disease.
6. Atrial fibrillation or flutter with a resting ventricular rate \> 100 bpm.
7. Any medical condition, which in the opinion of the Investigator, may place the patient at higher risk from his/her participation in the study, or is likely to prevent the patient from complying with the requirements of the study or completing the study.
8. Severe renal dysfunction at Visit 0, defined as estimated Glomerular Filtration Rate (eGFR) \<30 mL/min (using Chronic Kidney Disease Epidemiology Collaboration Equation (CKD-EPI) calculation), or end-stage renal disease requiring dialysis.
9. History of severe hepatic impairment or liver dysfunction at Visit 0, defined as total bilirubin above the upper limit of normal (ULN) (excluding patients with Gilbert's syndrome), aspartate aminotransferase (AST) or alanine transaminase (ALT) \>3 times the ULN or alkaline phosphatase \>2.5 times the ULN.
10. Prolonged corrected QT interval, defined as a corrected QT interval \>500 msec on ECG using Bazett formula.
11. Known hypersensitivity to any of the components of the investigational medicinal product (IMP).
12. Use of other investigational drugs at the time of enrolment, or within 30 days or 5 half-lives of enrolment, whichever is longer.
13. Fluvoxamine use within 28 days of Visit 0.
14. Contraindication to MRI scanning or gadolinium-based contrast agent
15. Pregnancy, lactation or planning pregnancy. Women of childbearing capacity are required to have a negative serum pregnancy test before treatment, must agree to pregnancy tests at study visits as defined in the Section 7.2.5 and must agree to maintain highly effective contraception during the study and for 3 months thereafter. Similarly male participants with female partners of childbearing potential must agree to maintain highly effective contraception during the study and for 3 months thereafter.

• Manchester, Greater Manchester, United Kingdom