Combination therapy for pulmonary hypertension using racecadotril

INTERVENTION: Racecadotril and Placebo capsule(s). Placebo is matched in appearance to active. Step 1a: 1 racecadotril 100mg capsule OR 1 placebo capsule administered once Step 1b: 1 or 2 racecadotri 100mg capsules OR 1 or 2 placebo capsules (depending on Independent Data Monitoring review, administered once) Step 2: 1 or 2 racecadotril 100mg capsules OR 1 or 2 placebo capsules (depending on Independent Data Monitoring review, administered 3 x a day for 12 ‐14 days) Study Entry : Randomisation ?patients may be randomised more than once if they consent to both steps CONDITION: Topic: Cardiovascular, Respiratory; Subtopic: Cardiovascular (all Subtopics), Respiratory (all Subtopics); Disease: Respiratory, Congenital Heart Disease and Pulmonary Hypertension ; Circulatory System ; Primary pulmonary hypertension PRIMARY OUTCOME: The maximum percentage change from baseline in plasma atrial natriuretic peptide (ANP) concentration; Timepoint(s): Step 1: Day 0 baseline & 1, 2, 3 and 6 hours post IMP; Step 2: Day 0 baseline & at 12‐14 days of IMP SECONDARY OUTCOME: Step 1:; 1. Additional biomarkers in the blood; timepoints: Day 0 baseline & at 1,2,3 & 6 hours post IMP ; 2. Pulmonary haemodynamic assessments; timepoints: Day 0 baseline and post IMP at 1.5 hours and 2 hours; 3. Systemic haemodynamic assessments; timepoints: Day 0 baseline and over 6 hours, at half hourly intervals post IMP; 4. Safety assessment; timepoints: Day 0 baseline and over 6 hours, at half hour post IMP ; ; Step 2:; 1. 6 minute walk test; timepoints: Day 0 baseline and at 12‐14 days post treatment with IMP. ; 2. Additional biomarkers in the blood; timepoints: Day 0 baseline and at 12‐14 days post treatment with IMP; 3. Systemic haemodynamic assessments; timepoints: Day 0 baseline and at 12‐14 days post treatment with IMP.; 4. Safety assessment timepoints: At 7 days of IMP treatment and at 12‐14 days post treatment with IMP INCLUSION CRITERIA: 1. WHO Group I pulmonary hypertension (i.e. idiopathic, familial or associated with connective tissue diseases 2. 18‐80 years old 3. Technically satisfactory routine right heart catheterisation (Step 1 only)* 4. On sildenafil (20‐100 mg; t.i.d.) 5. Six minute walk distance of >150 m 6. No changes to PH specific therapies for 1 month 7. Not pregnant (women only). Women of child‐bearing potential must have a negative pregnancy test within seven days prior to randomisation they must be willing to use an effective method of contraception (including hormonal or barrier method of birth control) from the time consent is signed until six weeks after treatment discontinuation. 8. Able to provide consent for the trial * Technically satisfactory means that the PI or delegated physician is content that the patient is able to have the catheter in for longer and that the necessary trial information is able to be extracted. 9. Target Gender: Male & Fe