A Multicenter Trial to Evaluate the Efficacy, Safety and Tolerability of HZN-825 in Subjects With Idiopathic Pulmonary Fibrosis

Key Inclusion Criteria in Core Phase: 1. Male or female ≥18 years of age at Screening. 2. Current diagnosis of IPF, as defined by American Thoracic Society (ATS)/European Respiratory Society (ERS)/Japanese Respiratory Society (JRS)/Latin American Thoracic Society (ALAT) guidelines and determined by central review; the date of initial diagnosis of IPF should be ≤7 years prior to Screening. 3. No recent changes or planned changes to the dose or regimen for IPF therapy, defined as: * Receiving a stable dose of IPF-approved therapy (i.e., nintedanib or pirfenidone) for a minimum of 3 months prior to Day 1 with no plans to change the background regimen during trial participation, or * Not currently receiving background IPF-approved therapy at Screening (either naïve to IPF-approved therapy or previously discontinued any IPF-approved therapy at least 4 weeks prior to Day 1 or drug-specific, 5 half-lives elimination period if longer than 4 weeks), and with no current plans to restart treatment during trial participation * Participants receiving any additional agent for IPF therapy must be on a stable regimen for at least 3 months prior to Day 1 with no current plans to change the treatment regimen during trial participation. Any previously discontinued therapy used to treat IPF must have been discontinued at least 4 weeks prior to Day 1 or 5 half-lives for that specific therapy must have elapsed, whichever is longer, with no plans to restart the therapy during trial participation. 4. Lung high-resolution computed tomography (HRCT) historically performed within 6 months prior to the Screening Visit and according to the minimum requirements for IPF diagnosis by central review based on participant's HRCT. If an evaluable HRCT is not available within 6 months prior to Screening, an HRCT will be performed at Screening to determine eligibility, according to the same requirements as the historical HRCT. 5. HRCT shows ≥10% to \<50% parenchymal fibrosis (reticulation) and the extent of fibrotic changes is greater than the extent of emphysema on the most recent HRCT scan (central reviewer determined). 6. Meets all of the following criteria during the Screening Period: 1. FVC ≥45% predicted of normal 2. forced expiratory volume in 1 second (FEV1)/FVC ≥0.7 3. Diffusing capacity of the lungs for carbon monoxide (DLCO) corrected for hemoglobin is ≥25% and ≤90% predicted of normal 7. Estimated minimum life expectancy of ≥30 months for non-IPF-related disease, in the opinion of the Investigator. 8. Vaccinations are up to date given age, comorbidities and local availability prior to trial drug dosing. 9. Willing and able to comply with the prescribed treatment protocol and evaluations for the duration of the trial. Key Inclusion Criteria in Extension Phase: 1. Completed the Double-blind Treatment Period (Week 52) of the Core Phase of the trial; subjects prematurely discontinued from trial drug in the Core Phase of the trial for reasons other than safety or tolerability may be included at the discretion of the Investigator after completing scheduled visits, including Week 52 assessments. 2. Willing and able to comply with the prescribed treatment protocol and evaluations for the duration of the Extension Phase of the trial. Key Exclusion Criteria Core Phase: 1. Any of the following cardiovascular diseases: 1. uncontrolled, severe hypertension (≥160/100 mmHg), within 6 months of Screening 2. myocardial infarction within 6 months of Screening 3. unstable cardiac angina within 6 months of Screening 2. Interstitial lung disease (ILD) associated with known primary diseases (e.g., sarcoidosis, amyloidosis and coronavirus disease 2019 \[COVID-19\]), connective tissue disorders (e.g., rheumatoid arthritis, systemic lupus erythematosus, Sjogren's, dermatomyositis, scleroderma), exposures (e.g., radiation, silica, asbestos and coal dust) or drugs (e.g., amiodarone). 3. Known active bacterial, viral, fungal, mycobacterial or other infection, including tuberculosis or atypical mycobacterial disease (fungal infections of nail beds are allowed). The participant must be 3 months beyond any acute infection with COVID-19 if there has been a prior infection. 4. Clinically significant pulmonary hypertension requiring chronic medical therapy. 5. Use of any of the following therapies within 4 weeks prior to Screening, during the Screening Period or planned during the trial: prednisone at steady dose \>10 mg/day or equivalent or cyclosporine. Change in regimen or dosage of any immunosuppressant during the Screening Period through the end of trial participation will require consultation with and approval by the trial Medical Monitor. 6. Use of rifampin within 2 weeks prior to Day 1 or planned during the trial. 7. Malignant condition in the past 5 years (except successfully treated basal/squamous cell carcinoma of the skin or cervical cancer in situ). 8. Women of childbearing potential (WOCBP) or male subjects not agreeing to use highly effective method(s) of birth control throughout the trial and for 4 weeks after last dose of trial drug. Females must refrain from egg/ova donation for 4 weeks after the last dose of trial drug and males must refrain from sperm donation for 3 months after the last dose of trial drug. 9. Pregnant or lactating women and women who plan to become pregnant or breast feed during the trial and within 4 weeks after the last dose of trial drug. 10. Current drug or alcohol abuse or history of either within the previous 2 years, in the opinion of the Investigator or as reported by the subject. 11. Previous enrollment in this trial or participation in a prior HZN-825 or SAR100842 clinical trial. 12. Known history of positive test for human immunodeficiency virus (HIV). 13. Active hepatitis (hepatitis B: positive hepatitis B surface antigen and positive anti-hepatitis B core antibody \[anti-HBcAb\] and negative hepatitis B surface antibody \[HBsAb\] or positive for HBcAb with a positive test for HBsAb and with presence of hepatitis B virus DNA at Screening; hepatitis C: positive anti-hepatitis C virus \[anti-HCV\] and positive RNA HCV). 14. Current alcoholic liver disease, primary biliary cirrhosis or primary sclerosing cholangitis. 15. Previous organ transplant (including allogeneic and autologous marrow transplant). 16. International normalized ratio \>2, prolonged prothrombin time \>1.5 × the upper limit of normal (ULN) or partial thromboplastin time \>1.5 × ULN at Screening. 17. Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \>2.0 × ULN. 18. Estimated glomerular filtration rate \<30 mL/min/1.73 m\^2 at Screening. 19. Total bilirubin \>1.5 × ULN. Subjects with documented diagnosis of Gilbert's syndrome may be enrolled if their total bilirubin is ≤3.0 mg/dL. 20. Moderate (Child-Pugh B) to severe (Child-Pugh C) hepatic impairment according to the Child-Pugh scoring system. 21. Any confirmed Grade 3 or higher laboratory abnormality. 22. Any laboratory abnormality at Screening that, in the opinion of the Investigator, would preclude the participant's entry in the trial. 23. Exposure to an experimental drug (with the exception of HZN-825) or experimental vaccine within either 30 days, 5 half-lives of the test agent, or twice the duration of the biological effect of the test agent, whichever is the longest, prior to Day 1. 24. Any other condition that, in the opinion of the Investigator, would preclude enrollment in the trial. Key Exclusion Criteria Extension Phase: 1. Anticipated use of another investigational agent for any condition during the course of the trial. 2. New diagnosis of malignant condition after enrolling in Trial HZNP-HZN-825-303 (except successfully treated basal/squamous cell carcinoma of the skin or cervical cancer in situ). 3. Estimated minimum life expectancy ≤18 months, in the opinion of the Investigator.