Positioning Imatinib for Pulmonary Arterial Hypertension

What does the study involve? The study involves treatment of PAH patients with imatinib (study drug) for up to 24 weeks, and clinical assessments and tests to assess the drug's safety and tolerability. PAH patients will be seen at their local hospital by the PAH clinical research team. Before someone can start study, the study doctor (or clinical study team) will describe the clinical trial in detail. If a potential subject decides to participate, he/she will be asked to sign the informed consent form before any study procedures are done. Participants will be asked to come to their local hospital for clinical appointments. This includes a screening visit, a baseline visit, three clinical assessments and an end-of-study visit. In between, and at the very end of these, there will be six tele-visits (assessments over the phone). Each clinical appointment will be on a weekday morning or afternoon. No major lifestyle restrictions are required for these appointments. Participants will undergo clinical examinations and tests to monitor the severity of PAH and the response to the study drug. Clinical procedures include: * Questions about demographics, medical and medication history * Physical examination and record of vital signs (blood pressure, temperature, heart and respiratory rate) * Questionnaire about quality of life, * Assessments of PAH severity (WHO Functional Class, six-minute walk test, and Borg dyspnoea index) * Right heart catheterisation to assess response to the drug * Haematology and clinical chemistry blood tests to ensure safety * Serum pregnancy test and urine pregnancy tests (if applicable) to exclude pregnancy * Blood samples to measure the levels of the study drug in the circulation * Additional blood samples for future research on PAH and/or the mechanism of action of the drug * Electrocardiogram (ECG), and echocardiogram to assess the size, shape, pumping action and the extent of any damage to the heart * Brain MRI scan (or CT scan if MR is not indicated/tolerated) to exclude bleeding in the brain