Efficacy & Safety of ALF-5755 in Patients With Nonacetaminophen Severe Acute Hepatitis & Early Stage Acute Liver Failure

Inclusion Criteria: * A signed written informed consent from patient or from patient's next of kin or from an authorized person according to local procedures * Early stage acute liver failure OR severe acute hepatitis defined as: * 15% ≤ PR \< 50% * No hepatic encephalopathy, OR grade I or II encephalopathy (Appendix E) * Presumed acute illness onset of less than 26 weeks * No evidence of underlying chronic liver disease * Patient who can receive first treatment dose within the first 48 hours after biological baseline assessment * Age ≥ 18 and ≤ 65 years * Contraception (only for females of childbearing potential) to be taken throughout the study until D21. Sole mechanic contraceptives, such as condoms, are advised. Note: Oral contraceptives may have contraindications in case of severe acute hepatitis and acute liver failure * Patient affiliated to social security insurance system. Exclusion Criteria: * Acetaminophen-induced hepatitis defined as acetaminophen intake \> 4 g/day, at least once in the 7 days prior to baseline * Shock liver (ischemic hepatopathy) OR HELLP syndrome OR Budd-Chiari syndrome OR intrahepatic malignancy * Serum creatinine ≥ 180 μmol/L * Body Mass Index (BMI) ≥ 35 * Septic shock requiring administration of inotropic drugs * Uncontrolled active bleeding * Patients who received fresh frozen plasma, PPSB (Prothrombin-Proconvertin-Stuart-B), or vitamin K infusion over the last 48 hours * Patient receiving liver support device treatment, including but not exclusively bioartificial liver (BAL), Extracorporeal Liver Assist Device (ELAD), transgenic pig perfusion * Patient receiving hemodialysis, hemofiltration or hemodiafiltration treatment * Intractable arterial hypotension (arterial systolic blood pressure equal to or below 70 mmHg) present or require inotropic drugs at baseline * Human Immunodeficiency Virus (HIV) positive patient * Active cancer * Pregnancy or breast-feeding * Surgery within 4 weeks prior to baseline, or unsolved surgical disease outside liver transplantation. * Patient included in another clinical trial within 4 weeks prior to baseline * Patient with organ or bone-marrow allograft * Absolute contra-indication to liver transplantation.