This trial is a randomized, double-blind, placebo-controlled, parallel-group, 2-arm, multicenter trial to evaluate the efficacy and safety of C21 versus placebo as add-on to standard of care (SoC) in adult subjects with COVID-19.

The trial planned to enroll a total of maximum 300 randomized subjects, 150 per arm (oral C21 100 mg twice a day (BID) or placebo for 14 days) according to the 1:1 randomization.

The trial consists of 3 consecutive periods: a screening period of up to 48 hours, a 2-week investigational medicinal product (IMP) treatment period and a follow-up period of up to 7 weeks after last IMP intake.

All subjects went through a series of efficacy, safety, and laboratory assessments. Safety laboratory tests and samples for future exploratory analysis were obtained at the screening visit.

The trial duration for an individual subject was not exceeded 9 weeks.

Inclusion Criteria:

1. Age ≥18 years or the legal age of consent in the jurisdiction in which the trial was taking place at the time of signing the informed consent (Specific for India; Age ≥18 to ≤65 years at the time of signing the informed consent)

2. Hospitalized due to SARS-CoV-2 infection confirmed by a hospital-approved polymerase chain reaction (PCR) test, documented by either of the following:

   1. PCR positive in sample collected <72 hours prior to randomization (Visit 2); OR
   2. PCR positive in sample collected ≥72 hours and ≤7 days prior to randomization, documented inability to obtain a repeat sample AND progressive disease suggestive of ongoing SARS-CoV-2 infection

3. A score of 5 or 6 on the 8-point ordinal scale:

   1. Score 5: Hospitalized, requiring supplemental oxygen
   2. Score 6: Hospitalized, on non-invasive ventilation or high-flow oxygen device

4. Contraceptive use by men and women of childbearing potential consistent with local regulations regarding the methods of contraception for those participating in clinical studies and according to Appendix 3 in the Protocol (see Section 10.3)

5. Written informed consent, consistent with International Council for Harmonization Good Clinical Practice Revision 2 and local laws, obtained before the initiation of any trial-related procedure

6. Capable of giving signed informed consent as described in Appendix 1 in the Protocol (see Section 10.1.3) which includes compliance with the requirements and restrictions listed in the informed consent form and in this Protocol

   Country specific:

7. Specific for India: For subjects with an ordinal scale score of 5, moderate to severe COVID-19 disease confirmed by at an SpO2≤93 % or a respiratory rate≥24/min on room air. Note: If a subject was on supplemental oxygen with SpO2>93% and respiratory rate<24/min, but desaturation to ≤93 % or increase of respiratory rate to ≥24/min on lower supplemental oxygen or room air is documented during screening, the inclusion criterion was considered to be met.

Exclusion Criteria:

1. Concurrent serious medical condition which in the opinion of the investigator constituted a risk or a contraindication for the participation in the trial or that could interfere with the trial objectives, conduct or evaluation

2. Known, active tuberculosis, active hepatitis B, C, or human immunodeficiency virus (HIV) infection (i.e., HIV with a CD4 count<500 cells/mm³).

3. Moderate or severe impairment of hepatic function (e.g., Child-Pugh class B or C where alterations in the score components were not due to another underlying disease (see Section 8.4.5 in the Protocol)).

4. Severe renal impairment (i.e., estimated glomerular filtration rate (eGFR)≤30 ml/min/1.73m2)

5. COVID-19 symptom onset >21 days prior to screening (Visit 1).

6. Hospitalized due to COVID-19 for >72 hours at screening (Visit 1).

7. Invasive mechanical ventilation or ECMO within 72 hours of screening (Visit 1).

8. Expected need for invasive mechanical ventilation or ECMO in <48 hours in the opinion of the investigator

9. Moderate to severe ARDS (e.g., same-day PaO2/FiO2 ≤200 mmHg; or SpO2/FiO2 ≤232 if arterial blood gas test is not available), if on non-invasive mechanical ventilation or high-flow oxygen.

10. Pregnant or breast-feeding female subjects

11. Any previous and concurrent experimental treatment for COVID-19 that was not considered local SoC.

12. Treatment with the medications listed below within 1 week prior to screening (Visit 1) or anticipated need for such medication during the participation in this trial:

    1. Strong Cytochrome P450 (CYP) 3A4 inducers.
    2. P-glycoprotein (P-gp) substrates with narrow therapeutic index.
    3. High dose Breast Cancer Resistance Protein (BCRP) sensitive substrates.
    4. Warfarin.
    5. Sulphasalazine or rosuvastatin.

13. Current or previous participation in any other clinical trial where the subject had received a dose of IMP within 1 month or 5 half-lives of the IMP, whichever was longest, prior to screening (Visit 1).

14. Positive pregnancy test (see Section 8.4.6 in the Protocol).

15. Abnormal laboratory value at screening (Visit 1) indicating a potential risk for the subject if enrolled in the trial as evaluated by the investigator