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Antifibrotic therapy in chronic liver disease
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Clinical Gastroenterology and Hepatology (AGA)
Pages 95-107 (February 2005)
Don C. Rockey
Departments of Cell Biology, Duke University Medical Center, Durham, North CarolinaUSA and Medicine, Duke University Medical Center, Durham, North Carolina, USA
Supported by the National Institutes of Health (grants R01 DK 50574 and R01 DK 57830) and the Burroughs Welcome Fund. D.C.R. is the recipient of a Burroughs Welcome Fund Translational Scientist Award.
The response to injury is one of wound healing and, subsequently, fibrosis. This response is generalized, occurring in diverse organ systems. Injury and wounding in the liver ultimately lead to cirrhosis in many patients (although not all patients), and are the result of many different diseases. The fact that various diseases result in cirrhosis suggests a common pathogenesis. Study over the past 2 decades has shed considerable light on the pathogenesis of fibrosis and cirrhosis. A growing body of literature indicates that the hepatic stellate cell is a central component in the fibrogenic process. Stellate cells undergo a transformation during injury that has been termed activation. Activation is complex and multifaceted, but one of its most prominent features is the synthesis of large amounts of extracellular matrix, resulting in deposition of scar or fibrous tissue. The fibrogenic process is dynamic; it is noteworthy that even advanced fibrosis (or cirrhosis) is reversible. The best antifibrotic therapy is treatment of the underlying disease. For example, eradication of hepatitis B or C virus can lead to the reversal of fibrosis. In situations in which treating the underlying process is not possible, specific antifibrotic therapy is desirable. A number of specific antifibrotic therapies have been tried, but have been met with poor or mediocre success. However, elucidation of the mechanisms responsible for fibrogenesis, with particular emphasis on stellate cell biology, has highlighted many putative novel therapies. This article emphasizes mechanisms underlying fibrogenesis, and reviews current antifibrotic therapies as well as potential future approaches.
Chronic injury results in a wound-healing response and, subsequently, fibrosis. The response is a generalized one, with features common among multiple organ systems. In the liver, a variety of different types of injury lead to fibrogenesis-implying a common pathogenesis. Although a number of specific therapies for patients with different liver diseases have been developed, including antiviral therapies for patients with hepatitis B and hepatitis C virus infection, specific and effective antifibrotic therapy remains elusive.
Over the past 2 decades, great advances in the understanding of fibrosis have been made and multiple mechanisms underlying hepatic fibrogenesis have been uncovered. Elucidation of these mechanisms has been of fundamental importance in highlighting novel potential therapies. Indeed, preclinical studies have pointed to a number of putative therapies to abrogate fibrogenesis. This article emphasizes mechanisms underlying fibrogenesis, and reviews the current status of the field with regard to available and future therapeutics.
ARTICLE FROM LA TIMES
Traditional herb may help liver disease
Elena Conis
LA Times
December 19, 2005
The spindly, yellow-flowered Bupleurum chinense and some closely related species are key herbs in traditional Chinese medicine prescribed for mood swings and gastrointestinal conditions. The root of the plant is one of the main ingredients in an herbal formula widely known by its Japanese name, Sho-saiko-to - in Chinese it's known as xiao chai hu tang - that contains ginseng, licorice, ginger and a handful of other herbs in addition to Bupleurum. Sho-saiko-to has recently gained scientific attention for its potential in managing chronic liver disease.
Uses: In traditional Asian medicine, Bupleurum root has been used to treat bloating, colds, fever, malaria and liver diseases, including hepatitis. In the U.S., Bupleurum supplements are commonly marketed for liver health.
Dose: Traditional Chinese herbalists generally recommend 1 to 5 grams of dried Bupleurum root a day, or 5 to 7 grams of Sho-saiko-to. Bupleurum is widely available in health food stores; Sho-saiko-to should be available from some traditional Chinese herbalists and Asian herbal shops.
Precautions: Large doses may cause nausea and vomiting. Sho-saiko-to appears to pose a tiny but measurable risk of pneumonitis (lung inflammation), particularly among patients also taking the drug interferon.
Research: In test tube studies, Bupleurum has displayed antiviral and anti-inflammatory capabilities. In animals, the root has been shown to act like an antihistamine, curbing asthma and other allergy symptoms. It's also shielded the liver from damage and expedited healing in livers already injured. On its own, Bupleurum hasn't been well-studied in humans. Clinical trials on Sho-saiko-to, mostly in Japan, reported reduced symptoms in people with hepatitis B. Perhaps most promising is a decade-old trial showing that in cirrhosis patients, Sho-saiko-to helped prevent liver cancer. But a review published last year in the journal Clinical Gastroenterology and Hepatology concluded that there was still not enough evidence to recommend the concoction for chronic liver disease.
Dietary supplement makers are not required by the U.S. government to demonstrate that their products are safe or effective. Ask your healthcare provider for advice on selecting a brand.
Fibrogenesis and pathophysiology
The fibrogenic process
The response to recurrent injury, in the liver and in other organs, is one of wound healing. The wounding process (in multiple tissues, including the liver) is complicated, but characterized by a typical constellation of features such as increased production of extracellular matrix, secretion of various cytokines and biologically active peptides, and proliferation of a unique population of cells known as myofibroblasts. Inflammation is a common theme in most forms of chronic wound healing. This is true in particular in liver disease, in which inflammation is often a prominent component, and frequently drives the fibrogenic response (it is noteworthy that in some diseases, inflammation may not be as important as in others). Further, it appears that temporal and functional relationships are important with regard to inflammation. For example, chronicity of inflammation is typical and often important in many types of liver disease. Further, the type of inflammation (ie, Th2 vs Th1) and the interplay of inflammation with environmental/metabolic factors and genetic factors appear to be important in fibrogenesis.
Many different types of injury (ie, chronic hepatitis, ethanol, biliary tract disease, iron overload, copper overload, and so forth) lead to hepatocellular injury and ultimately to hepatic fibrosis and cirrhosis. The point that the response to recurrent injury in the liver is similar in multiple different types of liver disease underscores the value of identifying common regulatory components of the fibrotic response because such components theoretically could be targeted without respect to cause of disease. However, it is important to emphasize that, at least in theory, different pathologic patterns of fibrosis (biliary, perisinusoidal, pericentral) can occur and thus may merit different types of therapies.
One of the most remarkable aspects of the wounding response in the liver (as in other tissues) is enhanced extracellular matrix production, or fibrogenesis. Regardless of the specific cause of liver injury (in both experimental models and human cirrhosis), increased content of extracellular matrix constituents occurs after injury. Specific changes in matrix composition are highly similar in all forms of liver injury and hepatic fibrogenesis. Hepatic fibrogenesis is characterized by increases in multiple matrix components, including the interstitial collagens, basement membrane collagens, proteoglycans, and matrix glycoproteins such as laminin, and fibronectin, including its EDA (or cellular fibronectin) isoforms.1 Many-fold increases in collagens (type I > III > IV) are typical, but increases in the other matrix proteins also are prominent. The wounding process is complex and integrated, and moreover is a dynamic one that involves aspects of matrix synthesis and deposition as well as degradation.2 Thus, there are a number of well-defined situations in which fibrosis clearly is reversible.3-6 In addition, in some instances, advanced cirrhosis may be reversible.7,8
Hepatic stellate cells and their activation in fibrogenesis
Although multiple liver cell types, including periportal and pericentral fibroblasts, myofibroblasts, and even bile duct epithelial cells and endothelial cells, play a role in fibrogenesis, stellate cells (also known as lipocytes, Ito cells, and perisinusoidal cells) have attracted great attention. Abundant evidence points mechanistically to a critical role for perisinusoidal stellate cells in the pathogenesis of hepatic fibrosis. Stellate cells, which are distributed throughout the hepatic lobule, serve as the principal storage site for retinoids (vitamin A metabolites) and are well known for their vitamin A handling capacity.9 It is important to emphasize that the identification and isolation of this cell type represents a major advance in understanding the pathogenesis of hepatic fibrogenesis because it has allowed careful characterization of its biology.10
A central feature of the fibrogenic response is the transformation of stellate cells (lipocytes, Ito, and perisinusoidal cells) from quiescent (normal) to an activated (injured liver) state (Figure 1).11 Although simple in concept, the activation process is remarkably complex and consists of many important cellular changes. Characteristic features of this transition include loss of vitamin A, acquisition of stress bundles, and development of prominent rough endoplasmic reticulum. Among the more prominent features of activation is a striking increase in secretion of extracellular matrix proteins, including types I, III, and IV collagens, fibronectin, laminin, and proteoglycans, some of which are increased by greater than 50-fold, consistent with the position that stellate cells are the cellular source of the enhanced extracellular matrix production at the whole organ level.12 A further critical feature of activation is de novo expression of smooth muscle-specific proteins, such as smooth muscle a actin.13 This feature identifies stellate cells as liver-specific myofibroblasts, and has important implications for their contractile properties.
Although the most prominent feature of activation is enhanced extracellular matrix production, activation also is associated with other important cellular phenotypes including proliferation, contractility, release of proinflammatory cytokines, and release of matrix degrading enzymes and their inhibitors.2,11,14 It is important to emphasize that each of these features of activation (and fibrogenesis) represent a potential target for therapy. Key pathogenic events in stellate cell activation are related intricately and are interdependent (ie, in fibrogenesis, proliferation, contractility, and so forth); several important components of the activation process are highlighted later.
Stellate cell fibrogenesis and activation: regulatory factors
Multiple factors play a key pathogenic role in stellate cell fibrogenesis. Prominent among these factors are cytokines, small peptides, and the extracellular matrix itself. Transforming growth factor-B-1 (TGF-B1) appears to be the most profibrogenic cytokine present in the liver.15-17 TGF-B1 is produced in a paracrine manner by Kupffer cells, sinusoidal endothelial cells, bile duct epithelial cells, hepatocytes, or by stellate cells themselves.11,18 TGF-B1 production by stellate cells is important, and thus points to this cytokine as a classic autocrine factor.11,18 When overexpressed in the liver, it leads to fibrosis,15 and when inhibited during experimental liver injury, fibrosis is decreased.19 TGF-B1 appears to act via direct (and to a lesser extent, indirect) stimulation of extracellular matrix production in stellate cells. A number of other cytokines and peptides appear to exhibit profibrogenic properties toward stellate cells (Table 1); however, none is as potent as TGF-B1. Finally, cytokines and growth factors that drive stellate cell proliferation are important in the fibrogenic cascade because they help expand the total number of fibrogenic (stellate) cells. Included in this group are platelet-derived growth factor (PDGF), monocyte chemotactic factor, insulin-like growth factors-1 and -2, interleukin-6, and, possibly, hepatocyte growth factor.11
A body of literature indicates that vasoactive peptides including endothelin-1 and angiotensin II, each of which have pleiotrophic cell biologic and molecular effects, are important in hepatic fibrogenesis.20-22 Additionally, the inheritance of single nucleotide polymorphism for angiotensinogen correlated with fibrosis progression in patients with chronic hepatitis C,23 raising the possibility of a genetic role for angiotensin II in fibrogenesis. Because these compounds also have vasoactive properties including presumably in portal hypertension, the data have opened an entirely new therapeutic area (ie, targeting both fibrogenesis and portal hypertension). Other biologically active peptides (including unidentified compounds) also are important in mediating hepatic fibrogenesis. Included in this group are compounds involved in adrenergic signaling (ie, norepinephrine), which appear to be profibrogenic.24,25 For example, the exposure of rats undergoing liver injury to 6-hydroxydopamine, a toxin that destroys noradrenergic fibers, significantly decreased fibrosis.24 Additionally, dopamine B-hydroxylase-deficient mice, which cannot make norepinephrine, are resistant to fibrogenesis.25
A number of cytokines and peptides appear to have anti-activation or antifibrogenic properties toward stellate cells. Included in this group are interferon r,26 interferon a,27 stellate cell activation-associated protein,28 and, possibly, adiponectin29 and hepatocyte growth factor.30
Finally, although cytokines, growth factors, and other soluble substances are important components of fibrogenesis, it is clear that the matrix itself modulates stellate cell activation. For example, culture of stellate cells on a basement membrane mimicking the normal basement membrane inhibits stellate cell activation and matrix synthesis,31 whereas culture of stellate cells on abnormal substrates such as the EDA isoform of fibronectin leads to increased activation of stellate cells.32 Recent data suggest that stellate cells sense their surrounding environment and can respond to cell-matrix tension.33 Finally, integrins, which link the extracellular matrix to stellate (and other cells), also may play an important role in transmitting fibrogenic signals.34
A prominent feature of liver fibrosis is extracellular matrix turnover, including not only its synthesis, but also its degradation.14 During fibrosis progression there is increased expression of matrix metalloproteinases (MMPs) and in particular their tissue inhibitors (TIMPs). Available data indicate that increases in expression of MMP-2 and membrane type 1 MMP as well as TIMP-1 and TIMP-2 are prominent during fibrogenesis,14,35-37 and that the overexpression of the TIMPs in particular contributes to the profibrogenic phenotype.14 Interestingly, overexpression of MMP8 led to partial reversal of fibrosis, providing proof of concept for a therapeutic role for overexpression of MMPs.38
Nonfibrogenic features of stellate activation
An increase in stellate cell number is typical after both experimental and human liver injury.11 Indeed, proliferation is an important component of the activation cascade because it amplifies the stellate cell-mediated response to injury. A number of mitogens appear to be important in stimulation of stellate cell proliferation and include PDGF, epidermal growth factor, fibroblast growth factor, endothelin-1, angiotensin II, insulin-like growth factor, thrombin, peroxisomal proliferator activated receptor agonists, and TGF-a to name several.11 The major mitogen driving cellular proliferation appears to be PDGF, a cytokine that also plays a key role in cellular proliferation during other forms of injury and wounding. Studies have shown further that increased responsiveness to PDGF accompanies stellate cell activation through up-regulation of its receptors.39 Thus, neutralization of PDGF activity, by either competitive antagonists or receptor blockade, is an important putative therapeutic approach.
Importantly, not only is stellate cell proliferation important in liver fibrosis, but it recently has been shown that during spontaneous recovery of experimental liver fibrosis, stellate cell apoptosis (ie, programmed cell death) is prominent.3 These data suggest that apoptosis of activated stellate cells may play a role in resolution of fibrosis, and imply that a balance between cell proliferation and death is important in determining the dynamics of the total overall stellate cell population in the liver. Indeed, based on these data, stimulation of stellate cell apoptosis could be an attractive therapeutic approach.40 Although apoptosis of activated stellate cells may well be critical in fibrosis resolution, other work suggests that activated stellate cell apoptosis may stimulate activation, and thus could be detrimental with regard to fibrogenesis.41
Activation of stellate cells is accompanied by a marked increase in proteins that are characteristic of contractile cells (ie, such as smooth muscle a actin and smooth muscle myosins13,42). Stellate cell contraction is important in the injured liver because it may contribute to the collapse and shrunken state of cirrhotic livers, and because it also appears to play a role in portal hypertension.22 Thus, stellate cell contractility, although not directly related to fibrosis, is an important physiologic target.
Approach to therapy for fibrosis
Although obvious in principle, it is important to emphasize that the most effective antifibrotic therapies are likely to be those that target or remove the underlying stimulus to fibrogenesis (Table 2). For example, eradication or inhibition of hepatitis B virus4,5 or hepatitis C virus (HCV)6 leads to reversion of fibrosis, even in some patients with histologic cirrhosis. Additionally, fibrosis (and cirrhosis) in patients with autoimmune hepatitis who respond to medical treatment (prednisone or the equivalent) is reversible.8 Fibrosis may improve in patients with alcohol-induced liver disease who respond to anti-inflammatory therapy such as corticosteroids.43,44 Fibrosis reverts in patients with hemochromatosis during iron depletion45,46 and after relief of bile duct obstruction.47 Finally, in a preliminary report in patients with non-alcohol-induced steatohepatitis treated with the peroxisomal proliferator active receptor-r agonist, rosiglitazone decreased both steatosis and fibrosis.48
Preclinical studies have highlighted a number of therapies that specifically could abrogate fibrogenesis. Such therapies have been targeted at inhibition of collagen synthesis, matrix deposition, modulation of stellate cell activation, stimulation of matrix degradation, or stimulation of stellate cell death. A number of these preclinical approaches have been transitioned to clinical trials in humans, which are highlighted later and in Table 3. The summary presented later indicates that as of the current writing, a specific antifibrotic that fits the profile of an ideal agent-one that is potent, safe, orally bioavailable, and inexpensive-is not yet available.
Specific antifibrotic therapies
Colchicine
Colchicine, a plant alkaloid, inhibits polymerization of microtubules, a process that is believed to be required for collagen secretion and thus has been touted as an antifibrotic compound. Further, abundant evidence supports the antifibrotic properties of colchicine in experimental animal models,49 and thus this compound has been studied in a number of clinical trials,50-53 including in primary biliary cirrhosis, alcohol-induced cirrhosis, as well as in miscellaneous other liver diseases.51 In a double-blind, randomized, controlled trial examining colchicine in primary biliary cirrhosis, improvements were noted in a number of biochemical markers, but colchicine failed to decrease fibrosis.50 In a double-blind, randomized, controlled trial of colchicine vs. placebo in patients with various liver diseases, colchicine led to improved fibrosis as well as a dramatic improvement in survival.51 However, this study has been criticized because of methodologic concerns because many patients were lost to follow-up evaluation and because there was substantial unexplained excess mortality in the control group from causes unrelated to liver disease. A meta-analysis including 1138 subjects found that colchicine had no effect on fibrosis or mortality.53 In a recent multicenter study involving 549 patients comparing colchicine (0.6 mg orally twice daily) with placebo in patients with alcohol-induced liver disease, there was no effect of active treatment on survival (histologic data were not obtained).52 The aggregate data led to the conclusion that colchicine is safe and may lead to improvement in markers of liver disease and even mortality from liver disease. However, the failure of colchicine to clearly decrease hepatic fibrosis makes recommendation of this drug as an antifibrotic problematic.
Polyenylphosphatidylcholine
Polyenylphosphatidylcholine, a mixture of polyunsaturated phosphatidylcholines, is extracted from soybeans. This compound has gained interest as an antifibrotic agent, particularly in alcohol-induced liver injury, because this disease often is associated with oxidative stress. Oxidative stress in turn leads to lipid peroxidation, cellular injury, inflammation, and a wounding response. Thus, it has been proposed that because phosphatidylcholine is a prominent component of cell membranes, that supplementation of it should protect cell membranes and might lead to decreased cellular injury and fibrogenesis. Experimental data support this notion.54 Given the available experimental data and the apparent safety of polyenylphosphatidylcholine, a Veterans Affairs cooperative clinical trial examining its effect in patients with alcohol-induced hepatitis was performed.55 This multicenter, prospective, randomized, double-blind, placebo-controlled trial study examined 789 alcoholic subjects (average alcohol intake of 16 drinks/day). Study subjects were randomized to either polyenylphosphatidylcholine or placebo for 2 years. Although the majority of subjects substantially decreased their ethanol consumption during the trial (which was felt to result in improvement in fibrosis in the control group), polyenylphosphatidylcholine failed to lead to a significant improvement in fibrosis.
Interleukin-10
Interleukin-10 has both anti-inflammatory and immunomodulatory effects. Interleukin-10 can down-regulate production of proinflammatory cytokines, such as tumor necrosis factor-a, interleukin-1, interferon r, and interleukin-2 from T cells. Endogenous interleukin-10 appears to decrease the intrahepatic inflammatory response and decrease fibrosis in several models of liver injury.56 Notably, a direct antifibrotic effect for interleukin-10 has not been established. Nonetheless, it has been postulated that in vivo administration of interleukin-10 to patients with HCV infection may shift the intrahepatic immunologic balance away from Th1 cytokine predominance, and thus have an anti-inflammatory and subsequent antifibrotic effect.57 Thirty patients with advanced fibrosis who had failed typical current antiviral therapy were enrolled in a 12-month treatment trial of subcutaneous interleukin-10 given daily or 3 times a week. In these patients, the hepatic inflammation score decreased by at least 2 points (Ishak) in 13 of 28 patients, and 11 of 28 patients had a decrease in fibrosis score (mean change from 5.0 ± 0.2 to 4.5 ± 0.3, P < .05). However, serum HCV-RNA levels increased during therapy (mean HCV-RNA level at day 0: 12.3 ± 3.0 mEq/mL; at 12 months: 38 mEq/mL; P < .05), and returned to baseline at the end of the follow-up period. The changes in liver histology and HCV-RNA levels were accompanied by an apparent shift in lymphocyte response toward a Th2-predominant phenotype. Thus, long-term therapy with interleukin-10 decreased hepatic inflammatory activity and fibrosis, but led to increased HCV viral levels by virtue of interleukin-10-induced immunologic modifications. Thus, although potentially antifibrotic, interleukin-10 may have detrimental effects on human HCV biology, and thus has not been pursued.
Interferon r
The interferons consist of a family of 3 major isoforms including a, B, and r. Each of these isoforms is unique, not only in terms of protein structure, but also with regard to their biologic actions. There are many different interferon a subtypes, whereas there appear to be only single interferon B and interferon r species. Interferon a and B bind to the same receptor and therefore share many common signaling properties. Interferon a has much more potent antiviral effects than does interferon r. However, interferon r has been shown specifically to inhibit extracellular matrix synthesis in fibroblasts.58
Several studies have indicated that interferon r has potent effects on stellate cells, inhibiting multiple aspects of stellate cell activation.26,59 With regard to use of interferon r in patients with hepatic fibrogenesis, there has been concern about its use because its overexpression in the liver leads to chronic hepatitis,60 and because of potential long-term side effects related to its profound immunomodulatory effects. However, a recent report in patients with chronic hepatitis C infection and fibrosis suggested that the compound is safe and well tolerated, and that a subgroup of patients may have an antifibrotic response.61 Although this pilot study provides a foundation for the potential use of interferon r in patients, larger studies will be required to document its therapeutic potential.
Silymarin
Silymarin extract, derived from the milk thistle Silybum marianum (the major active component of which is silybinin), decreases lipid peroxidation and inhibits fibrogenesis in animal models,62,63 including in baboons.64 It has been tested in several carefully performed trials, although fibrosis was not used as an outcome. The compound has been found to be safe, but had mixed effects.65,66 In one study,65 a benefit on mortality was shown specifically in the subgroup of alcoholic patients. Those with early stages of cirrhosis also appeared to benefit. However, in another study focused solely on alcoholic patients, no survival benefit could be identified.66 Thus, although silymarin is safe, data supporting its use are lacking.
Ursodeoxycholic acid
Ursodeoxycholic acid binds to hepatocyte membranes and presumably is cytoprotective, thereby reducing inflammation and thus fibrogenesis.67 Although neither experimental data nor human studies indicate a primary antifibrotic effect of ursodeoxycholic acid in the liver, a number of studies have examined its overall effects.68-76 Ursodeoxycholic acid has been studied in patients with cystic fibrosis, primary biliary injury (primary biliary cirrhosis, primary sclerosing cholangitis, and progressive familial intrahepatic cholestasis), and miscellaneous liver diseases. The data regarding the use of ursodeoxycholic acid in these conditions are controversial. Both symptomatic and biochemical improvement have been observed in these diseases, but data on histologic improvement (and survival) are mixed. For example, in one study, survival was improved in ursodeoxycholic acid-treated patients, but fibrogenesis was no different than in controls.73 In a randomized controlled trial of ursodeoxycholic acid in primary biliary cirrhosis, active treatment led to decreased fibrosis in those with mild liver disease, but had no effect on those with severe disease.69 Further, in a histopathologic study of 54 patients with primary biliary cirrhosis and paired liver biopsy examinations, 4 years of ursodeoxycholic acid therapy was associated with a significant decrease in the prevalence of florid interlobular bile duct lesions, lobular inflammation, and necrosis. Worsening of fibrosis was observed in 14 patients (the majority had only a 1-grade progression in fibrosis score), whereas stabilization was noted in the 40 remaining patients.74 Although the results of meta-analyses have been mixed, and largely have reported that ursodeoxycholic acid is not effective in primary biliary cirrhosis,72 a recent combined analysis of the histologic effect of ursodeoxycholic acid on paired liver biopsy examinations (a total of 367 patients-200 ursodeoxycholic acid and 167 placebo), revealed that when patients with initial stages I-II were considered, ursodeoxycholic acid significantly delayed histologic stage progression (P < .03).75 The aggregate data suggest that ursodeoxycholic acid may impede progression of fibrosis in primary biliary cirrhosis via effects on bile ductal inflammation, particularly if given early in the disease course. Ursodeoxycholic acid is safe, and although expensive, it is this author's belief that the available data justify its use in patients with primary biliary cirrhosis as an antifibrotic.
A beneficial effect of ursodeoxycholic acid on fibrogenesis was shown in a small number of children with progressive familial intrahepatic cholestasis.70 Further, a case series indicated that 7 of 10 patients with cystic fibrosis treated with ursodeoxycholic acid had a decrease in liver fibrosis.71 It should be emphasized that although these effects are promising, the number of patients studied has been small. Finally, in a large, randomized, controlled trial of the effect of a 2-year course of ursodeoxycholic acid in patients with nonalcoholic steatohepatitis, including 107 subjects who had paired biopsy data, there was no improvement in fibrosis.76
Herbal medicines
A number of herbal medicines have been shown to have antifibrotic properties in animal models, and in some, specific mechanisms have been identified.77-79 Herbal medicines with putative antiviral, anti-inflammatory, and antifibrotic effects are being used extensively in the far East in patients with a variety of liver diseases.80 Medications containing herbs of the Salvia genus have been popular in particular as antifibrotics.80 Although human trials have suggested effectiveness of specific herbal medicines in some studies,80 data in peer-reviewed Western journals remain lacking. Because it is well appreciated that such herbal medicines may have significant toxicity, including hepatotoxicity,81 these medications should be used with caution.
Miscellaneous
Given the role of oxidative stress in injury and in stellate cell activation and stimulation of extracellular matrix production, antioxidants have received considerable attention as antifibrotics. Vitamin E has been examined in animal models82 as well as in humans.83-86 The vitamin E precursor, d-a-tocopherol (1200 IU/day for 8 weeks) was tested in 6 patients with HCV infection who failed to respond to interferon therapy,83 resulting in inhibition of parameters of stellate cell activation. However, it did not affect fibrosis. A randomized controlled trial examined vitamin E in patients with mild to moderate alcohol-induced hepatitis and found that vitamin E decreased serum hyaluronic acid levels, but did not lead to a change in type III collagen.85 Antioxidant therapy, including vitamin E, in patients with severe alcohol-induced hepatitis, had no effect on outcome, although fibrosis was not addressed specifically.86
Malotilate appears to be cytoprotective, perhaps via inhibition of cytochrome P450 2E1, and additionally may have anti-inflammatory effects. Unfortunately, in patients with primary biliary cirrhosis, although it was found to decrease plasma cell and lymphocytic infiltrate and piece-meal necrosis, it had no significant effect on fibrogenesis.87
The heavy metal chelating properties of penicillamine have been proposed to contribute to its anti-inflammatory and antifibrogenic effects88; however, this compound was ineffective in mitigating fibrogenesis in patients with primary biliary cirrhosis.89,90
Methotrexate also is considered to be an anti-inflammatory compound, and typically has been considered to be profibrogenic,91 although it is noteworthy that the risk for fibrosis progression may be less prominent than classically believed.91,92 Nonetheless, because of its anti-inflammatory properties, it has been assessed as a potential therapeutic agent in patients with primary biliary cirrhosis; although improvement in disease and fibrosis have been reported, including reversion of fibrosis,93 the majority of the data on methotrexate are either negative94,95 or show that its effects are marginal, either alone,94 or in combination with colchicine.96 If methotrexate is used to treat patients with primary biliary cirrhosis, an experienced hepatologist should manage its use (with caution).
S-adenosylmethionine is important in the synthesis of the antioxidant, glutathione. Since it was proposed to have antioxidant properties in the liver, and decreased expression of the enzyme responsible for its synthesis (methionine adenosyltransferase) has been found after liver injury,97S-adenosylmethionine has been tested in a large randomized trial in patients with alcohol-induced cirrhosis. Histologic assessment of fibrosis was not measured specifically as an outcome, although there was an improvement in overall mortality/need for liver transplantation in the treatment arm, especially in patients with Child's A/B cirrhosis, raising the possibility fibrosis may have improved.98
Propylthiouracil, an antithyroid drug that reacts with some of the oxidizing species derived from the respiratory burst, may be protective in alcohol-induced liver disease, a disease in which an increase in hepatic oxygen consumption may predispose the liver to ischemic injury. Thus, propylthiouracil has been tested in randomized clinical trials in patients with alcohol-induced liver disease. Unfortunately, a systematic review and meta-analysis found that propylthiouracil led to no benefit in fibrosis (or other outcome variables).99
Anabolic-androgenic steroids such as oxandrolone have been tested in patients with alcohol-induced liver disease, but have not been found to have significant effects on fibrosis (or other outcomes).100
Several recent pilot studies have examined the effect of anti-tumor necrosis factor-a compounds in patients with alcohol-induced liver disease.101-104 The rationale for these therapies is that tumor necrosis factor-a is up-regulated in the liver injured by alcohol, and thus these compounds should decrease inflammation, and is the stimulus for fibrosis. Although there are little data on the effect of these interventions on hepatic fibrosis, preliminary analyses suggest an improvement in inflammation and acute injury (which presumably precedes fibrosis in this disease).103 However, the use of these compounds will require great caution because they may increase the risk for serious infection.105
It is important to emphasize that for many human studies (ie, involving S-adenosylmethionine, propylthiouracil, androgenic steroids, and anti-tumor necrosis factor-a, and so forth) subjects with alcohol-induced hepatitis and liver injury were examined (see Table 3), and in these studies fibrosis typically was not measured as a specific outcome. Thus, it is not entirely appropriate to consider these agents as primary antifibrotics, but rather as compounds that could have secondary effects on fibrogenesis owing to other properties.
Ones to watch
There is scientific rationale for the use of a number of other compounds, many of which have been studied in experimental models and have been shown to have antifibrotic effects (Table 4). For example, TGF-B plays a central role in the fibrogenic cascade and therefore is an important therapeutic target. Several approaches to inhibit the action of TGF-B have been proposed and include the use of molecules such as decorin, the protein core component of proteoglycan, which binds and inactivates TGF-B,106 antibodies directed against TGF-B1, and soluble receptors that typically encode for sequences that bind active TGF-B and prevent it from binding to cognate receptors.19,107 The concept has been well established experimentally; indeed, the effect of inhibition of TGF-B in animal models of liver injury and fibrogenesis has been striking.19,107 Additionally, stellate cells express angiotensin II and endothelin receptors and stimulation of these receptors with their cognate ligands leads to prominent stellate cell effects.22 Inhibition of endothelin signaling leads to decreased fibrogenesis21; likewise, the blockade of angiotensin II function (ie, with angiotensin enzyme inhibitors, angiotensin 2 receptor antagonists) in vivo also inhibited stellate cell activation and fibrosis.108 Thus, inhibition of their binding in human liver disease may be beneficial clinically. Among others, compounds such as pirfenidone,109 peroxisomal proliferator-activated receptor-r ligands,110 pentoxifylline,111 halofuginone,112 and 5'-lipoxygenase inhibitors113 appear to have direct effects on stellate cells and/or in vivo effects in hepatic fibrogenesis. A recent report suggests that the adipocytokine, adiponectin, inhibits PDGF-induced proliferation and attenuates the effect of TGF-B1 in stellate cells, and thereby leads to inhibition of fibrogenesis.29
Table 4.Potential Antifibrotic Therapies
HGF, hepatocyte growth factor; STAP, stellate cell activation-associated protein; PAR, protease activated receptor; PPAR, peroxisomal proliferator activated receptor.
a-Also angiotensin converting enzyme inhibitors.
Diagnosis and monitoring of hepatic fibrosis and cirrhosis
Liver biopsy examination is considered to be the gold standard for determining the extent of fibrosis. Liver biopsy examination also is used to assess fibrosis progression. Connective tissue stains, including reticulin, Masson's trichrome, and sirius red, readily identify extracellular matrix within tissue sections. A quantitative measure of collagen content can be made by colorimetric assay of sirius red in liver tissue or by image analytic quantitation of collagen-containing tissue.21 Additionally, scoring systems have been developed114-116 to quantitate fibrosis and to help standardize the interpretation of biopsy examinations among different centers; such systems are most useful for standardization and comparison of fibrosis in studies. For individual patients, direct comparison of biopsy specimens over time is most useful. It is important to emphasize that although histology is helpful for judging the presence and degree of fibrosis, clinical tools such as Child-Pugh classification are most useful for assessing overall disease severity.
Liver biopsy examination, although considered the gold standard tool to assess fibrosis, is inexact. Not only is liver biopsy examination subject to interobserver variability, but sampling error may be important, as evidenced by studies examining liver samples from different regions of the liver.117 Additionally, liver biopsy examination also is associated with significant potential morbidity, including a significant risk for death.118 Thus, noninvasive measures that can monitor fibrogenesis would be ideal. Noninvasive tools used to assess fibrosis include radiographic tests,119 combinations of routine laboratory tests,120,121 and specific serum markers.122,123 In particular, serum marker panels, including several that use mathematic algorithms,120,121 have recently been emphasized. However, as of this writing, they have proven to be of limited clinical use.
Summary and future directions
Understanding of the basis of hepatic fibrogenesis has advanced significantly in the past 2 decades, and with it, a field dedicated to therapeutic antifibrotics has emerged. The central event in fibrogenesis appears to be activation of hepatic stellate cells. Stellate cell activation is characterized by a number of important features including enhanced matrix synthesis and a prominent contractile phenotype-processes that each undoubtedly contribute to physical distortion and dysfunction of the liver in advanced disease. It is important to emphasize that factors controlling activation are multifactorial and complex, and thus multiple potential therapeutic interventions are possible. A further critical concept is that the fibrogenic lesion, in particular, the extracellular matrix, is dynamic and reversible. Even advanced fibrosis may be reversible under the appropriate conditions. Currently, effective therapy for hepatic fibrogenesis exists for several diseases-in the form of removal of the underlying disease process. As for specific therapy directed only at the fibrotic lesion, the most effective therapies most likely will be directed at the stellate cell. Additionally, approaches that address matrix remodeling (ie, by enhancing matrix degradation or inhibiting factors that prevent matrix breakdown) will be attractive. Thus, although specific, effective, safe, and inexpensive antifibrotic therapies do not yet exist, multiple potential targets have been identified, and it is highly likely that candidates will emerge.
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