Inclusion Criteria:

1. Age between 40-85 years old. 2. A diagnosis of IPF that fulfills current American Thoracic Society (ATS) Consensus Criteria. 3. IPF duration <5 years, based on the date of definitive diagnosis. 4. Ability and willingness to give informed consent and adhere to study requirements. 5. Ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1/FVC) >70% of predicted values.

Exclusion Criteria:

1. Diagnosis of major comorbidities expected to interfere with study participation. 2. History of malignancy, excluding basal or squamous cell skin cancer and low-risk prostate cancer, the latter defined as stage T1 or T2a, with prostate specific antigen <10 ng/dl. NOX inhibition is not known to promote cancer, and these criteria are within current guidelines. 3. The occurrence of any acute infection requiring systemic antibiotic therapy within 2 weeks prior to Screening (Visit 1). 4. Treatment for >14 days within the preceding month with >20 mg. prednisone (or equivalent) or any treatment during the last month with a cellular immunosuppressant (e.g., cyclophosphamide, methotrexate, calcineurin inhibitors, etc.), given increased risks of opportunistic infections. 5. Treatment with any investigational agent within 4 weeks of Screening (Visit 1) or 5 half-lives of the investigational medicinal product (whichever is longer). 6. Fertile women who do not agree to contraception or abstinence, or who are breast feeding. IPF is a disease of older adults, and male predominant, so this will not be a frequent consideration. 7. Subjects with known hypersensitivity to GKT137831 or its excipients (e.g. capsule "bulking" agents). 8. A history of bone marrow disorder including aplastic anemia, or marked anemia defined as hemoglobin < 10.0 g/dL (or 6.2 mmol/L). 9. Severe cardiovascular disease, defined as any of the following within the preceding 12 weeks: acute myocardial infarction or unstable angina, a coronary revascularization procedure, congestive heart failure (NYHA Class III or IV), or stroke, including a transient ischemic attack. 10. Evidence of cardiac conducting abnormalities, defined as second or third degree atrial-ventricular (AV) block not successfully treated with a pacemaker, or a personal or family history of long QT syndrome (QTc interval >450 msec for males or 470 msec for females). 11. End-stage renal disease requiring dialysis. 12. Undergoing transplantation evaluation, or listed with the United Network for Organ Sharing (UNOS) as a lung transplantation candidate at the time of enrollment in this trial. 13. Liver function tests (transaminases, alkaline phosphatase, direct and total bilirubin) >3x upper limit of normal values

Arms

Experimental: GKT137831

GKT137831 will be administered orally, at a dose of 400 mg twice daily, for a total of 24 weeks.

Placebo Comparator: Placebo Oral Tablet

Identically-appearing placebo oral tablets will be administered orally, twice daily, for a total of 24 weeks.

Interventions

Other: - Placebo Oral Tablet

see Arm/Group description

Drug: - GKT137831

GKT137831 is a NOX enzyme inhibitor