This clinical investigation is a medical device trial to examine the safety and efficacy of TheraSorb® Ig flex adsorber treatment (as an add-on to conventional treatment) used exclusively with the LIFE 18® apheresis system for extracorporeal application for IA therapy (5 treatments) performed over 5 to 8 consecutive days in patients with Idiopathic Pulmonary Arterial Hypertension and WHO functional classification III.

Inclusion Criteria:

• Able to understand and willing to sign the Informed Consent Form
• age > 18 and < 80 years at time of informed consent
• Idiopathic pulmonary arterial hypertension diagnosed by right heart catheter
• WHO functional classification III
• Application of conventional IPAH therapy, which has been stable for the prior 8 weeks
• Able to perform a 6-minute-walk test (MWT)
• Negative pregnancy test (β-HCG) at the start of the trial and appropriate contraception throughout the trial for women with child-bearing potential.

Exclusion Criteria:

• Pregnancy and/or lactation
• PAH of any cause other than permitted in the inclusion criteria, e.g. concomitantly to portal hypertension, complex congenital heart disease, reversed shunt, HIV infection, suspected pulmonary veno-occlusive disease
• Contraindication for right heart catheterization
• Any change in disease-targeted therapy within 8 weeks prior to inclusion
• Walking impairment due to causes other than IPAH that would preclude performance of a 6MWT
• Any patient who has received any investigational medication within 8 weeks prior to the start of this trial or who is scheduled to receive another investigational drug during the course of the trial between visits V1 and V9
• Known intolerance to immunoadsorption in general or to one of the excipients (e.g. sheep antibodies or agarose) or other supporting agents
• Hemoglobin concentration of < 75% below the lower limit of normal
• Systolic blood pressure < 85 mmHg
• Lack of compliance or other similar reason that, in the judgment of the Investigator, precludes satisfactory participation in the trial
• Concurrent severe or uncontrolled medical disease (i.e. uncontrolled diabetes, congestive heart failure, myocardial infarction within 6 months prior to the study, unstable and uncontrolled hypertension, chronic renal disease, or active uncontrolled generalized viral, bacterial and/ or mycotic infection) which by assessment of the treating Investigator could compromise patients safety or participation in the study
• Drug or alcohol abuse within the last 5 years
• Hypercoagulability
• Known severe immunodeficiency (e.g. AIDS)
• Severe lung disease: FEV1/FVC < 50%, total capacity < 60% of normal value
• Indications that prohibit transient anticoagulation using Heparin and/or ACD-A-solutions