This trial is a multi-centre, open-label, single-arm phase 2 trial investigating the safety, efficacy and pharmacokinetics of C21 in subjects with idiopathic pulmonary fibrosis.

Inclusion Criteria:

1. Written informed consent, consistent with ICH-GCP R2 and local laws, obtained before the initiation of any trial related procedure

2. A diagnosis of IPF within 5 years prior to Visit 1, as per either ATS/ERS/JRS/ATLAT/Fleischner guidelines

3. Age ≥40 years

4. Forced vital capacity (FVC) ≥60% predicted at Visit 1 (specifically for UK: FVC ≥80% predicted at Visit 1)

5. Forced expiratory volume in the first sec (FEV1)/FVC ratio ≥0.7 prebronchodilator at Visit 1

6. Oxygen saturation (SpO2) >85% by pulse oximetry while breathing ambient air at rest at Visit 1

7. High-resolution computed tomography (HRCT) within 36 months prior to Visit 1 with central reading demonstrating either a or b, and c:

   a. A pattern consistent with usual interstitial pneumonitis (UIP) according to ATS/ERS/JRS/ALAT or Fleischner guidelines i. UIP ii. Probable UIP or b. A pattern indeterminate for UIP according to either ATS/ERS/JRS/ALAT or Fleischner guidelines and a historical biopsy consistent with IPF c. Extent of fibrosis > extent of emphysema

8. Fully vaccinated against COVID-19 prior to screening (Visit 1). Subjects are considered fully vaccinated for COVID-19 ≥14 days after they have received vaccination dose(s) according to local label

Exclusion Criteria:

1. Previous use of antifibrotic treatment for an interstitial lung disease (e.g. nintedanib or pirfenidone) for > 6 months

2. Smoking (including e-cigarettes) within 6 months prior to Visit 1

3. Body mass index (BMI) >35 or <18

4. IPF exacerbation within 3 months prior to Visit 1:

   • Acute worsening or development of dyspnoea typically <1 month duration
   • Computed tomography with new bilateral ground-glass opacity and/or consolidation superimposed on a background pattern consistent with usual interstitial pneumonia pattern (if no previous computed tomography is available, the qualifier "new" can be dropped)
   • Deterioration not fully explained by cardiac failure or fluid overload

5. Concurrent serious medical condition with special attention to cardiac or ophthalmic conditions (e.g. contraindications to cataract surgery) which in the opinion of the investigator makes the subject inappropriate for this trial

6. Malignancy within the past 5 years with the exception of in situ removal of basal cell carcinoma and cervical intraepithelial neoplasia grade I

7. Treatment with any of the medications listed below within 4 weeks prior to Visit 1:

   • Cytochrome p450 (CYP) 3A4 inducers (e.g. rifampicin, phenytoin, St. John's Wort)
   • CYP3A4 inhibitors (e.g. clarithromycin, ketoconazole, nefazodone, itraconazole, ritonavir)
   • Medicines that are substrates of CYP1A2, CYP3A4 or CYP2C9 with a narrow therapeutic range
   • Experimental drugs
   • Any systemic immunosuppressive therapies other than:
   • Inhaled corticosteroids which can be used throughout the trial period provided the dose is kept stable
   • Corticosteroids for the treatment of acute exacerbations
   • The continuation of stable doses of ≤15 mg daily doses of prednisolone

8. Treatment with any of the medications listed below within 2 weeks prior to Visit 1:

   • Proton pump inhibitors (PPI's) more than once daily
   • Histamine H2 receptor antagonists (H2RA's)
   • Sulphasalazine and rosuvastatin
   • High dose breast cancer resistance protein sensitive substrates (other than sulphasalazine or rosuvastatin)

9. Any of the following findings at Visit 1:

   • Prolonged QTcF (QT interval with Fridericia's correction) (>450 ms), cardiac arrhythmias or any clinically significant abnormalities in the resting ECG, as judged by the Investigator
   • Positive results for hepatitis B surface antigen (HBsAg), hepatitis C virus antibody (HCVAb) or human immunodeficiency virus 1+2 antigen/antibody (HIV 1+2 Ag/Ab)
   • Positive serum pregnancy test (minimum sensitivity 25 IU/L or equivalent units of human chorionic gonadotropin)

10. Inability to generate lung function data at Visit 1 meeting the minimum standards of the ATS/ERS 2005 guideline, as determined by central review

11. Clinically significant abnormal laboratory value at Visit 1 indicating a potential risk for the subject if enrolled in the trial as evaluated by the investigator

12. Pregnant or breast-feeding female subjects

13. Female subjects of childbearing potential not willing to use contraceptive methods

14. Male subjects not willing to use contraceptive methods

15. Subjects not willing to adhere to dietary restrictions during the trial period

16. Participation in any other interventional trial during the trial period

17. Subjects known or suspected of not being able to comply with this trial protocol (e.g. due to alcoholism, drug dependency or psychological disorder)

18. Discontinuation or change of previous antifibrotic treatment (e.g. nintedanib or pirfenidone) due to disease progression