Ursodeoxycholic Acid in Cholestasis: Potential Mechanisms of Action and Therapeutic Applications

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1 Concise Review Ursodeoxycholic Acid in Cholestasis: Potential Mechanisms of Action and Therapeutic Applications ULRICH BEUERS, 1 JAMES L. BOYER, 2 AND GUSTAV PAUMGARTNER 1 The dihydroxy bile acid, ursodeoxycholic acid (UDCA), is increasingly used for the treatment of chronic cholestatic liver diseases. First reports of a beneficial effect on serum liver tests in cholestatic disorders were published in the 1980s in the Western literature, 1-3 but similar observations had been made previously in Japan. 4,5 Today, the therapeutic efficacy of UDCA has been shown for only one cholestatic disorder, primary biliary cirrhosis (PBC). 6 The mechanism of action of UDCA in cholestasis has also not yet been clearly identified. Therefore, the aim of this article is to summarize clinical and experimental work that may help to clarify possible mechanisms of action of UDCA in cholestasis and provide a clearer rationale for the administration of UDCA in special cholestatic disorders. PHYSICOCHEMICAL AND PHARMACOLOGICAL PROPERTIES OF UDCA UDCA (3,7 -dihydroxy-5 -cholanic acid) was first identified as a major constituent of dried bile of the Chinese black bear 7 and has been used for centuries as a remedy for liver diseases in Chinese medicine. 8 UDCA represents up to 3% of the bile acid pool in humans. It is more hydrophilic than the two major dihydroxy bile acids, chenodeoxycholic acid (CDCA) and deoxycholic acid. Comparison with the trihydroxy bile acid, cholic acid, shows differing results depending on the analytical method used. 9 After oral administration, UDCA is passively absorbed in the small intestine. It is mainly conjugated with glycine and taurine in the human liver and secreted into bile, where it subsequently undergoes an enterohepatic circulation caused by active reabsorption of UDCA amides in the terminal ileum. 9 Although administration of UDCA inhibits the reabsorption of endogenous bile acids in the small intestine, the pool sizes of the primary bile acids, cholic acid and CDCA are not significantly diminished by UDCA treatment in patients with PBC and PSC, and the pool size of the hydrophobic deoxycholic Abbreviations: UDCA, ursodeoxycholic acid; PBC, primary biliary cirrhosis; CDCA, chenodeoxycholic acid; PSC, primary sclerosing cholangitis; PKC, protein kinase C; TUDCA, tauroursodeoxycholic acid; ICP, intrahepatic cholestasis of pregnancy; CF, cystic fibrosis; GVHD, graft-versus-host disease. From the 1 Department of Medicine II, Klinikum Grosshadern, University of Munich, Munich, Germany; and the 2 Liver Center, Yale University School of Medicine, New Haven, CT. Received August 4, 1998; accepted August 14, Address reprint requests to: Ulrich Beuers, M.D., Department of Medicine II, Klinikum Grosshadern, University of Munich, Munich, Germany. Fax: ; beuers@med2.med.uni-muenchen.de. Copyright 1998 by the American Association for the Study of Liver Diseases /98/ $3.00/0 acid is low in many patients with chronic cholestasis. 13 UDCA becomes the predominant bile acid in serum and bile when administered at doses of 13 to 15 mg/kg/d in patients with cholestatic liver disease. 13,16-19 The rate of intestinal absorption of UDCA depends on the presence of endogenous bile acids in the intestine and on the intestinal ph. 9 HEPATOTOXICITY OF ENDOGENOUS BILE ACIDS AND RATIONALE FOR THE USE OF UDCA IN CHOLESTASIS The pathogenesis of cholestatic liver diseases like PBC involves a series of events in which injury of bile duct epithelial cells and hepatocytes by hydrophobic bile acids may be essential (Fig. 1). Indeed, in experimental animals hydrophobic bile acids can (1) cause hepatocellular cholestasis and damage and (2) aggravate existing bile duct lesions. 23 Both types of cholestatic injury may serve as experimental models to study the protective effects of UDCA. Hepatic retention of hydrophobic bile acids has been implicated as a major cause of liver damage for more than three decades when it was shown that lithocholic acid induced cirrhosis 24 and that its taurine conjugate caused cholestasis in experimental animals. 20 In addition, hepatic levels of hydrophobic dihydroxy bile acids, CDCA and deoxycholic acid, correlate with the degree of liver damage However, the molecular mechanisms of bile acid induced liver cell damage are not yet known, and a number of possibilities have been proposed UDCA conjugates prevent liver damage and cholestasis caused by conjugates of cholic acid 32 and other more hydrophobic bile acids including CDCA, 21,22,33,34 deoxycholic acid, 22 and lithocholic acid 35,36 in experimental animals, the isolated perfused liver, or isolated hepatocytes. Amidation of UDCA seems to be a prerequisite for its protective effect. 34,36,37 UDCA was first administered to patients with progressive cholestatic disorders such as PBC with the rationale that changes in the composition of the endogenous bile acid pool induced by long-term ingestion of UDCA might be beneficial PUTATIVE MECHANISMS AND SITES OF ACTION OF UDCA IN CHRONIC CHOLESTATIC LIVER DISEASE Increasing experimental evidence suggests two major mechanisms and sites of action of UDCA in cholestatic disorders (Fig. 1): Protection Against Injury of Bile Ducts by Hydrophobic Bile Acids The mdr2-knockout mouse lacks the ability to secrete phospholipids into bile and to buffer potential membrane damaging effects of hydrophobic bile acids. As a consequence, it develops a chronic nonsuppurative cholangitis. 38 In this experimental model, UDCA feeding impedes the development of chronic cholestatic liver disease suggesting an effect of UDCA conjugates at the level of the biliary tree. 23 In vitro studies show that UDCA conjugates have a membrane stabilizing effect when artificial or biomembranes are exposed to millimolar levels of hydrophobic bile acids. 3,34,39-41 These effects may result in part from effects of UDCA on the formation and structure of simple and mixed micelles rather than from a direct membrane interaction. 42

2 1450 BEUERS, BOYER, AND PAUMGARTNER HEPATOLOGY December 1998 Stimulation of Hepatocellular Secretion of Hydrophobic Bile Acids and Other Potential Hepatotoxins The hepatic accumulation of potential hepatotoxins such as hydrophobic bile acids is assumed to play a pivotal role in cholestasis-associated liver damage Increasing experimental and clinical evidence supports the hypothesis first suggested by Kitani that the cytoprotective effect [of UDCA] is most probably exerted in the excretory pathway from the hepatocyte to the biliary canaliculus. 8 UDCA amides stimulate secretion of bile acids in patients with PBC and PSC 43,44 as well as in the bile fistula rat 21,22 and isolated hepatocytes. 45 Consistent with these observations, UDCA lowers serum levels of bilirubin in patients with PBC and PSC and of CDCA in patients with PBC. 19 UDCA also stimulates the biliary secretory capacity as determined by hepatobiliary scintigraphy in patients with cystic fibrosis (CF)-associated liver disease. 52 What is the molecular mechanism by which UDCA induces stimulation of the biliary secretion of cholephils? Adenosine triphosphate dependent carrier-mediated transport across the canalicular membrane is the rate-limiting step in hepatocellular secretion of numerous substrates including bile acids, bilirubin, and other organic anions and cations. 53 Experimental evidence indicates that canalicular membrane transport activity is regulated by the insertion (and retrieval?) of transport protein carrying vesicles into this domain, a process that is defective in experimental cholestasis. 57,58 It has FIG. 1. Pathogenesis of cholestatic disorders and potential sites of action of UDCA: Depending on the etiology of the cholestatic disorder, the biliary tree (e.g., T-cell mediated destruction of interlobular bile ducts in PBC) or the hepatocyte (e.g., drug-induced hepatocellular cholestasis) may be the first target of attack, which subsequently may lead to cholestatic liver disease. The therapeutic effect of UDCA may be explained by (1) protection against injury of bile ducts by hydrophobic bile acids and (2) stimulation of hepatocellular secretion of hydrophobic bile acids and other potential hepatotoxins (for details, see text). been suggested that UDCA conjugates enhance the impaired secretion of cholestatic hepatocytes by stimulating exocytosis and, thereby, insertion of transport proteins into the canalicular membrane Vesicle fusion or exocytosis is modulated by a complex network of signals including cytosolic-free calcium [Ca 2 ] i and protein kinase C (PKC). 64,65 Tauroursodeoxycholic acid (TUDCA) induces a sustained elevation of [Ca 2 ] i at physiological bile acid concentrations in isolated hepatocytes 59,66,67 via depletion of inositol 1,4,5-trisphosphate (IP 3 )-sensitive endoplasmic Ca 2 stores independently of IP 3 and stimulation of Ca 2 inflow across the plasma membrane. 59,66,67 The [Ca 2 ] i -enhancing effect of TUDCA was three orders of magnitude greater in hepatocytes than in neutrophils 68 suggesting that selective carrier-mediated uptake of TUDCA across the hepatocellular membrane is a prerequisite for its pronounced effect on [Ca 2 ] i in liver cells. In the perfused rat liver, TUDCA, but not taurocholic acid and taurochenodeoxycholic acid induce a sustained stimulation of hepatocellular exocytosis by Ca 2 -dependent mechanisms. 59 In addition, TUDCA induces translocation of the -isoform of PKC to the hepatocellular membrane, 62,69,70 elevates s,n-1,2-diacylglycerol levels (the intracellular activator of PKC), and activates membrane-associated PKC in isolated hepatocytes. 62 Thus, because TUDCA stimulates Ca 2 - and putatively -PKC dependent hepatocellular exocytosis in the experimental model of the isolated perfused rat liver, it may also enhance the excretory capacity of the liver cell. 59,62 TUDCA also induces activation of the mitogen-activated protein kinases extracellular signal-regulated kinase-1 and -2, the activation of which is apparently needed for TUDCA-induced stimulation of bile acid secretion. 63 Therefore, TUDCA may enhance the excretory capacity of the cholestatic liver cell by activating a complex signaling network in which [Ca 2 ] i, -PKC, and mitogen-activated protein kinases are closely linked. The clinical significance of other in vitro phenomena induced by UDCA is unclear. These effects include stimulation of cholangiocellular bicarbonate secretion via Ca 2 - dependent activation of apical cholangiocellular Cl channels in cholestasis 71 and UDCA-induced hypercholeresis at high pharmacological doses in vitro. 9 Immunomodulating effects of UDCA, such as reduction of aberrant major histocompatibility complex class I expression on hepatocytes under different cholestatic conditions, 50,72-74 may be regarded as a consequence rather than a cause of the anticholestatic effect of UDCA and are, therefore, not discussed here in detail. In summary, experimental and clinical evidence at present suggests that the anticholestatic effects of UDCA in patients with chronic cholestatic liver diseases can be best explained by protection of cell membranes at the level of the biliary tree and stimulation of impaired hepatocellular secretion of potentially toxic cholephils. THERAPEUTIC APPLICATIONS OF UDCA IN CHOLESTATIC SYNDROMES UDCA has been administered to patients with a large variety of cholestatic disorders. The drug is well tolerated. To date, its therapeutic efficacy has only been shown for PBC as defined by prolongation of survival, improvement of prognostic markers such as histological progression or serum levels of bilirubin, and relief of symptoms such as pruritus in randomized, double-blind, placebo-controlled trials.

3 HEPATOLOGY Vol. 28, No. 6, 1998 BEUERS, BOYER, AND PAUMGARTNER 1451 Proven Efficacy Primary Biliary Cirrhosis. UDCA markedly improves serum liver tests including bilirubin, a potent prognostic marker in PBC, 75,76 as shown by four large randomized, double-blind, placebo-controlled trials that included a total of 699 patients treated with doses of 10 to 15 mg/kg/d for 2 years UDCA improved pruritus 46,49 and histological features 46,47,49 in some patients. Most importantly, combined analysis of data from three of the trials showed a significant prolongation of transplant-free survival after 4 years of UDCA treatment. 6 Thus, UDCA is recommended for treatment of PBC at a dose of 13 to 15 mg/kg/d. Justifiable Administration Primary Sclerosing Cholangitis. UDCA improves serum liver tests in PSC as shown by randomized, placebo-controlled trials. 50,51,77 In particular, serum bilirubin, an important prognostic marker of PSC, 78,79 was improved in patients treated with UDCA for 1 to 2 years. 50,51 UDCA did not significantly affect symptoms of PSC, 50,51 but improved histological features in a small group of patients. 50 So far, no effect on transplant-free survival has been noted. 51 However, the number of patients and the median follow-up period has been limited in PSC compared with observations in PBC patients. 6 In addition, endoscopic treatment of dominant strictures may add to the improvement of prognosis in patients with PSC. 80 Thus, administration of UDCA for treatment of PSC at doses of 13 to 15 mg/kg/d is justified, although prolongation of survival has not been proven and the optimal dose remains to be determined. Intrahepatic Cholestasis of Pregnancy (ICP). ICP is a rare disorder of late pregnancy characterized by pruritus and jaundice in the mother and an increased risk of premature delivery and stillbirth. One small randomized, double-blind, placebo-controlled trial 81 showed improvement of pruritus and serum liver tests (bilirubin, aspartate transaminase, and alanine transaminase) during UDCA treatment in pregnant women. More importantly, all babies delivered of mothers after UDCA treatment were born at or near term, whereas five of seven babies of mothers treated with placebo were delivered before week 36 and one of them was a stillbirth. 81 No side effects of UDCA have been reported in children of women treated during pregnancy. Therefore, UDCA may be considered a safe treatment of ICP in the third trimenon. However, further controlled trials are needed before UDCA treatment of ICP can be generally recommended. Liver Disease in CF. In a randomized, double-blind, placebocontrolled trial, patients with CF were treated with UDCA or placebo for 1 year. 82 A significantly higher Shwachman- Kulczycki score was observed with UDCA as a measure of nutritional status, pulmonary involvement, and the patient s general condition. These effects were accompanied by improvement of biochemical markers of cholestasis. However, the prognostic significance of these findings remains unclear. 82 Improvement of histological features has also been reported during UDCA treatment. 83 Studies suggest that a high dose of UDCA (20 mg/kg/d) induces a better response than a lower dose (5-15 mg/kg/d) possibly because of impaired intestinal absorption caused by pancreatic insufficiency in most patients with CF. 52,84 Therefore, UDCA may be considered a promising and safe treatment option in patients with CF-associated liver disease, although its ability to affect the natural course of the disease has yet to be proven in controlled trials. Other Pediatric Liver Diseases. In open-label uncontrolled trials, beneficial effects of UDCA on clinical symptoms and/or biochemical markers of cholestasis have been reported at doses of at least 15 mg/kg/d in patients with progressive familial intrahepatic cholestasis, 85,86 syndromic paucity of the intrahepatic bile ducts (Alagille syndrome), 86 biliary atresia, 86,87 and total parenteral nutrition associated liver disease. 88 Chronic Graft-Versus-Host Disease (GVHD) of the Liver. In an open-label study, marked improvement of serum liver tests was observed in patients with GVHD of the liver, a disorder with features similar to PBC. 89 Although further data are desirable, UDCA may be considered for the treatment of GVHD of the liver. Questionable Efficacy Acute Rejection After Liver Transplantation. The effect of UDCA on the rate of acute rejections after liver transplantation has been studied in three randomized, placebocontrolled trials Two trials could not detect an effect of UDCA on the incidence of acute rejection, including one with the largest study population (n 102) and the highest dose of UDCA (15 mg/kg/d). 91 In the third study, the number of rejection-free patients did not differ; however, fewer patients in the UDCA group had multiple rejections. 92 Thus, the data do not support administration of UDCA to most patients after liver transplantation. Alcoholic and Nonalcoholic Steatohepatitis and Drug-Induced Hepatopathies. Data of randomized, controlled trials with endpoints beyond serum liver tests are to be awaited. Administration Not Recommended Chronic Viral Hepatitis. UDCA has been studied in a number of randomized, placebo-controlled trials for the treatment of chronic hepatitis C. 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