Part A, B & C 1. History or presence of significant cardiovascular, hepatic, renal, haematological, gastrointestinal, endocrine, immunologic, dermatologic, neurological or psychiatric disease, including any acute illness or surgery within the past three months determined by the Investigator to be clinically relevant. 2. History or presence of obstructive pneumonia, severe pulmonary interstitial fibrosis, alveolar proteinosis and allergic alveolitis caused by lung tumour. 3. Any history or presence of: a. Coagulation abnormalities b. Significant bleeding disorder 4. Known allergy to low molecular weight heparin (LMWH) or heparin, or heparin-induced thrombocytopenia. 5. Positive heparin-induced thrombocytopenia Platelet Factor 4 (PF4) antibody test at the screening visit. 6. Known allergy or sensitivity to N-acetylcysteine (NAC)(Part C only). 7. Females who are currently breastfeeding. 8. Positive serum pregnancy test for women of child-bearing potential at the screening visit or positive urine pregnancy test with confirmatory serum pregnancy test on Day -1. 9. Liver function test results (i.e., aspartate aminotransferase [AST], alanine aminotransferase [ALT], and gamma-glutamyl transferase [GGT]) and total bilirubin elevated >1.2 fold above the normal limits at the screening visit. 10. Positive testing for active human immunodeficiency virus (HIV), hepatitis B surface antigen (HBsAg) or hepatitis C virus (HCV) antibodies at the screening visit. 11. Positive testing for SARS-CoV-2 viral RNA prior to check-in on Day -1 (the test should be performed between Day -3 and Day -2, with the results to be reviewed by the PI prior to check-in. 12. Donation of blood or plasma within 30 days prior to randomisation, or loss of whole blood of more than 500 mL within 30 days prior to randomisation, or receipt of a blood transfusion within 1 year of study enrolment. 13. Participation in another investigational clinical trial within 30 days or 5 half-lives (whichever is longer) prior to the first drug administration. 14. Any other condition or prior therapy, which, in the opinion of the Investigator, would make the volunteer unsuitable for this study, including unable to cooperate fully with the requirements of the study protocol or likely to be non-compliant with any study requirements. Part D 1. Participation in any other clinical trial of an experimental treatment for COVID-19, with the exception of emergency access treatments. 2. Concurrent treatment with other agents with actual or possible direct acting antiviral activity against SARS-CoV-2 < 24 hours prior to study drug dosing except for remdesivir and azithromycin.. 3. Uncontrolled hypertension (systolic blood pressure (BP) > 150 mm Hg and/or diastolic BP > 100 mmHg), unstable angina, congestive heart failure (CHF) of any New York Heart Association (NYHA) classification, serious cardiac arrhythmia requiring treatment (exceptions: atrial fibrillation, paroxysmal supraventricular tachycardia), history of myocardial infarction within 12 months of enrolment. 4. Hypotension requiring vasoactive peptides, such as dopamine, norepinephrine, epinephrine, or dobutamine. 5. Renal function impairment with Creatinine >2 mg/dL. 6. Liver function impairment with Bilirubin >2 mg/dL. 7. Platelet count <50,000/µL. 8. Multi-organ failure. 9. Documented active infection with a bacterial pathogen requiring parenteral systemic antibiotics. 10. Bacterial or fungal sepsis.

Part A, B & C 1. History or presence of significant cardiovascular, hepatic, renal, haematological, gastrointestinal, endocrine, immunologic, dermatologic, neurological or psychiatric disease, including any acute illness or surgery within the past three months determined by the Investigator to be clinically relevant. 2. History or presence of obstructive pneumonia, severe pulmonary interstitial fibrosis, alveolar proteinosis and allergic alveolitis caused by lung tumour. 3. Any history or presence of: a. Coagulation abnormalities b. Significant bleeding disorder 4. Known allergy to low molecular weight heparin (LMWH) or heparin, or heparin-induced thrombocytopenia. 5. Positive heparin-induced thrombocytopenia Platelet Factor 4 (PF4) antibody test at the screening visit. 6. Known allergy or sensitivity to N-acetylcysteine (NAC)(Part C only). 7. Females who are currently breastfeeding. 8. Positive serum pregnancy test for women of child-bearing potential at the screening visit or positive urine pregnancy test with confirmatory serum pregnancy test on Day -1. 9. Liver function test results (i.e., aspartate aminotransferase [AST], alanine aminotransferase [ALT], and gamma-glutamyl transferase [GGT]) and total bilirubin elevated >1.2 fold above the normal limits at the screening visit. 10. Positive testing for active human immunodeficiency virus (HIV), hepatitis B surface antigen (HBsAg) or hepatitis C virus (HCV) antibodies at the screening visit. 11. Positive testing for SARS-CoV-2 viral RNA prior to check-in on Day -1 (the test should be performed between Day -3 and Day -2, with the results to be reviewed by the PI prior to check-in. 12. Donation of blood or plasma within 30 days prior to randomisation, or loss of whole blood of more than 500 mL within 30 days prior to randomisation, or receipt of a blood transfusion within 1 year of study enrolment. 13. Participation in another investigational clinical trial within 30 days or 5 half-lives (whichever is longer) prior to the first drug administration. 14. Any other condition or prior therapy, which, in the opinion of the Investigator, would make the volunteer unsuitable for this study, including unable to cooperate fully with the requirements of the study protocol or likely to be non-compliant with any study requirements. Part D 1. Participation in any other clinical trial of an experimental treatment for COVID-19, with the exception of emergency access treatments. 2. Concurrent treatment with other agents with actual or possible direct acting antiviral activity against SARS-CoV-2 < 24 hours prior to study drug dosing except for remdesivir and azithromycin.. 3. Uncontrolled hypertension (systolic blood pressure (BP) > 150 mm Hg and/or diastolic BP > 100 mmHg), unstable angina, congestive heart failure (CHF) of any New York Heart Association (NYHA) classification, serious cardiac arrhythmia requiring treatment (exceptions: atrial fibrillation, paroxysmal supraventricular tachycardia), history of myocardial infarction within 12 months of enrolment. 4. Hypotension requiring vasoactive peptides, such as dopamine, norepinephrine, epinephrine, or dobutamine. 5. Renal function impairment with Creatinine >2 mg/dL. 6. Liver function impairment with Bilirubin >2 mg/dL. 7. Platelet count <50,000/µL. 8. Multi-organ failure. 9. Documented active infection with a bacterial pathogen requiring parenteral systemic antibiotics. 10. Bacterial or fungal sepsis.