Liver Part 1: Viral Hepatitis and Acute Liver Failure

Direct-acting antivirals target three essential enzymes in the hepatitis C virus replication cycle. NS3/4A protease inhibitors (ending in "-previr") block the viral serine protease responsible for cleaving the polyprotein precursor into functional viral proteins; examples include glecaprevir, voxilaprevir, and grazoprevir. NS5B polymerase inhibitors (nucleoside analogue: sofosbuvir) block the RNA-dependent RNA polymerase responsible for viral genome replication; sofosbuvir is a prodrug activated intracellularly to its triphosphate form and acts as a chain terminator. NS5A inhibitors (ending in "-asvir") target the NS5A protein required for viral replication complex assembly and virion assembly; examples include ledipasvir, velpatasvir, pibrentasvir, and elbasvir.

Current treatment guidelines favor pan-genotypic regimens that achieve cure — defined as sustained virologic response at 12 weeks after completing therapy, which is equivalent to undetectable hepatitis C virus RNA 12 weeks post-treatment — without requiring genotype testing. The two dominant pan-genotypic regimens are sofosbuvir plus velpatasvir (Epclusa) for 12 weeks and glecaprevir plus pibrentasvir (Mavyret) for 8 weeks in treatment-naive patients without cirrhosis. Both achieve sustained virologic response rates exceeding 95 percent across all genotypes. Patients with compensated cirrhosis or prior treatment experience typically receive extended courses of 12 to 16 weeks. Patients with decompensated cirrhosis cannot receive NS3/4A protease inhibitors, as these agents can worsen hepatic function; sofosbuvir plus velpatasvir is the regimen of choice in decompensated cirrhosis.

Direct-acting antivirals have clinically important drug interactions mediated primarily through cytochrome P450 3A4 and P-glycoprotein. NS3/4A protease inhibitors are substrates and inhibitors of cytochrome P450 3A4 and P-glycoprotein; co-administration with strong cytochrome P450 3A4 inducers such as rifampin, carbamazepine, and phenytoin substantially reduces protease inhibitor plasma concentrations and risks treatment failure. Sofosbuvir is a P-glycoprotein substrate; P-glycoprotein inducers reduce its absorption. Amiodarone combined with sofosbuvir-containing regimens has caused serious and fatal bradycardia, including complete heart block; amiodarone is contraindicated with sofosbuvir-containing regimens. The NS5A inhibitor ledipasvir requires an acidic environment for absorption; proton pump inhibitors significantly reduce ledipasvir exposure and should be avoided or given at the lowest effective dose with ledipasvir-containing regimens.

Acetaminophen is normally metabolized primarily by glucuronidation and sulfation to non-toxic conjugates. A small fraction undergoes cytochrome P450 2E1 and cytochrome P450 3A4 metabolism to N-acetyl-para-benzoquinoneimine, a reactive electrophilic metabolite. In therapeutic doses, N-acetyl-para-benzoquinoneimine is immediately detoxified by conjugation with glutathione. In overdose, glucuronidation and sulfation pathways become saturated, shunting a larger fraction through cytochrome P450 metabolism; N-acetyl-para-benzoquinoneimine accumulates faster than glutathione can neutralize it, covalently binding hepatocyte macromolecules and causing centrilobular necrosis. Cytochrome P450 2E1 is induced by chronic alcohol use and fasting, which is why chronic alcoholics and malnourished patients are at higher risk of hepatotoxicity at lower acetaminophen doses than the general population.

N-acetylcysteine is the antidote for acetaminophen hepatotoxicity. It replenishes hepatic glutathione by serving as a cysteine precursor — the rate-limiting substrate for glutathione synthesis — enabling continued detoxification of N-acetyl-para-benzoquinoneimine. Intravenous N-acetylcysteine is given as a 21-hour loading and maintenance infusion protocol. The decision to treat is guided by the Rumack-Matthew nomogram, which plots serum acetaminophen concentration against time since ingestion to define the probable, possible, and treatment-not-indicated risk zones; treatment is initiated when concentration falls in the probable hepatotoxicity zone or above the possible hepatotoxicity line as a precaution. Treatment initiated within 8 hours of ingestion virtually eliminates the risk of severe hepatotoxicity; efficacy decreases progressively with delayed presentation, but intravenous N-acetylcysteine has demonstrated benefit even in patients presenting more than 24 hours post-ingestion with established acute liver failure, through mechanisms that may include improved hepatic microcirculation and antioxidant effects beyond glutathione replenishment.

N-acetylcysteine is also used in non-acetaminophen acute liver failure — most commonly indeterminate etiology — where it may improve transplant-free survival, though the evidence is strongest for acetaminophen-related injury. The principal adverse effect of intravenous N-acetylcysteine is anaphylactoid reactions — flushing, urticaria, bronchospasm — that occur in approximately 10 to 20 percent of patients, are most common during the loading infusion, and are managed by temporarily stopping the infusion and administering antihistamines; N-acetylcysteine is restarted at a slower rate after symptoms resolve.

The King's College Criteria identify patients with acute liver failure who are unlikely to survive without liver transplantation. For acetaminophen-related acute liver failure, the criteria are: arterial pH below 7.3 after adequate resuscitation, or the combination of all three of prothrombin time greater than 100 seconds, serum creatinine greater than 300 micromoles per liter, and grade III or IV hepatic encephalopathy. For non-acetaminophen acute liver failure, the criteria are: prothrombin time greater than 100 seconds alone, or any three of five factors including unfavorable etiology, age extremes, jaundice-to-encephalopathy interval greater than 7 days, bilirubin above 300 micromoles per liter, and prothrombin time greater than 50 seconds. Patients meeting King's College Criteria should be urgently evaluated for liver transplantation.

Supportive management includes aggressive management of hepatic encephalopathy with lactulose and rifaximin, correction of coagulopathy only if active bleeding or invasive procedures are planned (not prophylactically), renal replacement therapy for hepatorenal syndrome, and vasopressors for hemodynamic instability. Prophylactic antibiotic use in acute liver failure reduces the high infection risk but has not been shown to improve survival in randomized trials.