Inclusion Criteria

1. Males and females of non-childbearing potential confirmed at screening by fulfilling one of the following criteria:
2. Postmenopausal defined as amenorrhoea for at least 12 months or more following cessation of all exogenous hormonal treatments and follicle-stimulating hormone (FSH) levels in the postmenopausal range
3. Documentation of irreversible surgical sterilisation by hysterectomy, bilateral oophorectomy or bilateral salpingectomy but not tubal ligation
4. Age ≥ 30 years
5. Symptoms of dyspnea (II-IV) at the time of screening
6. EF ≥ 50% as determined on imaging study within 12 months of enrollment
7. Catheterization documented elevated filling pressures at rest (PCWP ≥15) or with exercise (PCWP ≥25)

Exclusion Criteria

1. Use of nitrates, phosphodiesterase 5 inhibitors or other NO-providing therapy in the past 24 hours of screening
2. Significant valvular disease (>moderate left-sided regurgitation, >mild stenosis)
3. Requirement of intravenous heparin at the start of case
4. Severe pulmonary parenchymal disease
5. Acute coronary syndrome or coronary disease requiring revascularization in the judgement of investigators
6. Resting systolic blood pressure < 100 mmHg
7. Constrictive pericarditis
8. Infiltrative, restrictive, or hypertrophic obstructive cardiomyopathies
9. Previous anaphylaxis to any drug
10. Pregnancy or breastfeeding mothers
11. High Output heart failure
12. Active thyroid disease
13. Treatment with a new chemical entity (defined as a compound which has not been approved for marketing) within the preceding 3 months
14. Patients with any prior allergy to propylthiouracil