Comments from the Editors
Antioxidants in the treatment of chronic liver diseases: Why is the efficacy evidence so weak in humans?†
Article first published online: 11 AUG 2008
Copyright © 2008 American Association for the Study of Liver Diseases
Volume 48, Issue 5, pages 1359–1361, November 2008
How to Cite
Lu, S. C. (2008), Antioxidants in the treatment of chronic liver diseases: Why is the efficacy evidence so weak in humans?. Hepatology, 48: 1359–1361. doi: 10.1002/hep.22463
Potential conflict of interest: Nothing to report.
- Issue published online: 28 OCT 2008
- Article first published online: 11 AUG 2008
Chronic liver disease is almost always characterized by increased oxidative stress, regardless of the cause of liver disease.1 Oxidative stress, defined as an imbalance between the oxidant and antioxidant systems, occurs due to a number of mechanisms. Activation of molecular oxygen as reactive oxygen species (ROS) is part of aerobic life that regulates diverse cellular functions, including signal transduction pathways, gene expression, growth, and death. ROS can help the organism in many ways—for example, by destroying invading pathogens. However, excessive ROS are toxic to the cell, because they can react with lipids, proteins, and DNA to alter their functions.1–3 In hepatocytes, mitochondria and cytochrome P450 enzymes are the main sources of ROS. In addition, activated Kupffer cells and infiltrating inflammatory cells, in particular neutrophils, are also sources of ROS for the injured and inflammed liver. During inflammation, there is often increased production of nitric oxide. ROS can react with nitric oxide to generate peroxinitrite, a strong oxidizing agent that can interfere with SH-containing enzymes and help propagate lipid peroxidation. Many other reactive nitrogen species can also promote nitrosylation to alter protein structure and function beyond that caused by ROS alone. Nitrosative stress occurs when the ability to neutralize and eliminate reactive nitrogen species is overwhelmed. Major antioxidants within the cell include glutathione (GSH), tocopherol (vitamin E), and vitamin C. Key enzymes that participate in the removal of ROS are superoxide dismutase, GSH peroxidase, and catalase. In acute and chronic liver injury, there is often increased production of ROS coupled with reduced capacity to defend against ROS.1–3
The evidence that ROS are major pathogenetic factors in various liver diseases is overwhelming. ROS participate in the pathogenesis of acute and chronic liver injury in many ways. ROS can modify biomolecules and impair their normal function. Some of these modified biomolecules can stimulate host immune response and cause autoimmune-like diseases. High levels of ROS can lead to cell death by apoptosis and necrosis. Finally, ROS can also promote fibrogenesis and carcinogenesis.1–3 Increased oxidative stress is well documented to occur in most if not all forms of liver diseases, including those caused by alcohol, viruses, drugs, autoimmune disorders, and nonalcoholic steatohepatitis (NASH). In animal models of these different liver diseases, antioxidant therapy is generally protective.1, 4 This evidence has provided the rationale to use antioxidant therapy in patients with these liver diseases.
Agents that have antioxidant properties include not only vitamin E, C, and A, but also complementary and alternative medicine (CAM) agents such as silymarin, S-adenosylmethionine (SAMe), betaine, green tea polyphenols, ginseng, glycyrrhizin (licorice root), and trace metals such as selenium and zinc.4–6 Although all of these agents have multiple effects, such as anti-inflammatory, antiapoptotic, and membrane-stabilizing properties, most experimental data support their ability to lower oxidative stress as a key component of their beneficial effect. Patients with chronic liver diseases are known to use CAM agents commonly; one study in the Unitd States reported that 39% used CAM agents, and a study in Germany reported 65%.6 This number probably underestimates usage, because many surveys show that 31%-40% of patients do not disclose their use of CAM agents to their physicians.5 The appeal of CAM agents is many-fold. These agents are perceived as “natural” and therefore safe; using CAM agents provides patients a sense of control over their own disease and management; and patients generally believe that they are effective and may cost less. The National Center for Complementary and Alternative Medicine was established by Congress in 1998 to promote research on CAM, to provide the public with reliable assessments of safety and efficacy of CAM, and to integrate CAM into conventional medical care.4–6 Over the past 20 years, many clinical trials have been conducted in various chronic liver diseases using CAM agents. Unfortunately, no evidence shows that any of them work, whether used alone or in various combinations.4–9
One of the most widely used CAM agents by patients with liver disease is silymarin (milk thistle). This is the active ingredient extracted from Silybium marianum that includes four flavonoids, the major one being silibinin. Numerous experimental studies in vitro and in vivo have demonstrated that silymarin acts as an antioxidant by increasing the antioxidant defense system and neutralizing ROS. In addition, it modulates cytokine balance (by suppressing proinflammatory cytokines and inducing anti-inflammatory cytokines) and liver fibrosis. Silymarin has been tested in acute and chronic viral hepatitis, alcoholic liver disease, primary biliary cirrhosis and toxic and drug-induced liver injury. The results are inconclusive, and no evidence supports its use in any of these liver diseases. Like many other trials involving CAM agents, many of these trials were small, poorly designed, included heterogeneous patient populations, and had high dropout rates and short follow-up periods. Compounding this, different preparations of silymarin vary greatly in their pharmacokinetics. However, given silymarin's excellent safety record, it will likely remain very popular among patients with liver disease.4–6
Athough silymarin may seem harmless, vitamins E, C, and A are not. Vitamin E has been tested in NASH, alcoholic liver disease, and viral hepatitis with mixed results. Most of these vitamin E trials were small and used different doses for different lengths of time. Although no evidence shows vitamin E helps halt the progression of liver disease, we now know vitamin E increases all-cause mortality in a dose-dependent manner (statistically significant for dosages ≥400 IU/day).10 At high doses, vitamin E can act as a pro-oxidant, inhibit GSH S-transferase and thus impede detoxification, and interfere with coagulation (a statistically significant increased risk for hemorrhagic stroke was reported among patients assigned to vitamin E in the Alpha-Tocopherol, Beta Carotene Cancer Prevention Study).10 Similar to vitamin E, vitamin A at high doses is also a pro-oxidant, and chronic use of high-dose vitamin E and A have both been shown to increase the risk of lung cancer.10, 11 Vitamin C also has the potential to enhance iron-mediated toxicity and should be avoided in those with excess hepatic iron. Several other CAM agents have well-documented side effects. For instance, glycyrrhizin can cause a “pseudoaldosterone effect” and worsen ascites.4, 5 St. John's wort inhibits several major human cytochrome P450 enzymes, such as CYP2D6, CYP2C9, and CYP3A4, which can enhance the likelihood of organ transplant rejection.12 Many other herbal products have also been reported to cause hepatotoxicity.5, 6 Thus, not only is the efficacy in question for these products, they may cause serious adverse side effects.
The rationale to use several CAM agents is based on extensive experimental evidence of their efficacy in animal models. This includes SAMe, betaine, and polyenylphosphatidylcholine (PPC). In alcoholic liver injury, hepatic SAMe and PPC levels are depleted and administration of SAMe, betaine, and PPC is hepatoprotective.4 PPC was particularly attractive for its antioxidant and antifibrotic properties.1 However, a randomized, prospective, double-blind, placebo-controlled clinical trial involving 789 alcoholic liver disease patients failed to show any benefit in fibrosis progression, either biochemically or histologically.13 SAMe and betaine both work in the methionine metabolic pathway, both can increase hepatic SAMe level, and the latter can reduce homocysteine level.4 Most patients with chronic liver disease have impaired hepatic SAMe biosynthesis and homocysteine catabolism, and animals with chronic hepatic SAMe deficiency are more prone to further injury, spontaneous development of NASH, and hepatocellular carcinoma.14 SAMe has been examined in a variety of liver diseases and has been found to be effective in cholestatic liver diseases,15 but its efficacy in alcoholic liver disease remains unproven, according to the most recent meta-analysis performed by the Cochrane Collaboration.8 Only one of nine trials using adequate methodology reported any benefit of using SAMe (1,200 mg/day in three divided doses) in reducing mortality and delaying liver transplantation in those with less advanced alcoholic cirrhosis (Child class A and B).8, 16 However, this was a subgroup analysis that was data-driven, and the results may be misleading. Additionally, how SAMe works remains unclear, because none of these studies included data on liver histology. Quality of life and health economics were also not examined. Whether the dosage used was optimal was also not clear. The Cochrane report urged that larger trials with adequate methodology should be performed. Betaine has only been examined in pilot studies, and its safety and efficacy remain to be determined.
Despite largely negative clinical trials, many patients suffering chronic liver disease will undoubtedly turn to antioxidants and CAM agents. For agents with few harmful side effects, the only drawback is to the patients' bank accounts. For agents with potential adverse effects, proper counseling is important. Still, given lack of effective therapy in most chronic liver diseases, we need to push forward to examine critically the therapeutic role of antioxidants and CAM agents. However, small clinical trials will not help. We need large, well-designed, randomized, double-blind, controlled trials, with well- defined endpoints, different dosages, and proven biomarkers that correlate well with clinical outcome (if not available in human, at least in experimental animals), and ideally also provide insights into the mechanism of their action. Most of the time this will require repeat liver biopsy, which in my view is well justified. Given that the pharmaceutical industry has little incentive to provide funding for these studies, federal funding agencies must support this research, because it impacts the lives of all patients with chronic liver disease and beyond.
- 15Agency for Healthcare Research and Quality. Evidence Report/Technology Assessment No. 64, S-Adenosyl-L-Methionine for Treatments of Depression, Osteoarthritis, and Liver Disease. AHRQ Pub. No. 02-E033, Aug 2002, ISSN: 1530-440X. Available at: www.ahrq.gov/clinic/epcsums/samesum.pdf.