UDCA for Drug-Induced Liver Disease: Clinical and Pathophysiological Basis

Abstract

Drug-induced liver injury (DILI) is an adverse reaction to medications and other xenobiotics that leads to liver dysfunction. Based on differential clinical patterns of injury, DILI is classified into hepatocellular, cholestatic, and mixed types; although hepatocellular DILI is associated with inflammation, necrosis, and apoptosis, cholestatic DILI is associated with bile plugs and bile duct paucity. Ursodeoxycholic acid (UDCA) has been empirically used as a supportive drug mainly in cholestatic DILI, but both curative and prophylactic beneficial effects have been observed for hepatocellular DILI as well, according to preliminary clinical studies. This could reflect the fact that UDCA has a plethora of beneficial effects potentially useful to treat the wide range of injuries with different etiologies and pathomechanisms occurring in both types of DILI, including anticholestatic, antioxidant, anti-inflammatory, antiapoptotic, antinecrotic, mitoprotective, endoplasmic reticulum stress alleviating, and immunomodulatory properties. In this review, a revision of the literature has been performed to evaluate the efficacy of UDCA across the whole DILI spectrum, and these findings were associated with the multiple mechanisms of UDCA hepatoprotection. This should help better rationalize and systematize the use of this versatile and safe hepatoprotector in each type of DILI scenarios.

 

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Keywords

ursodeoxycholic acid - hepatotoxicity - cholestatic DILI - hepatocellular DILI

Lay Summary

The anticholestatic agent UDCA has been empirically used as a supportive drug mainly in cholestatic DILI. However, preliminary data in the literature suggest beneficial effects of UDCA in both cholestatic and hepatocellular DILI forms, and in both therapeutic and prophylactic treatments. UDCA has a plethora of hepatoprotective mechanisms potentially useful in all DILI scenarios, including anticholestatic, antioxidant, anti-inflammatory, antiapoptotic, antinecrotic, mitoprotective, endoplasmic reticulum stress alleviating, and immunomodulatory ones. To help better rationalize and systematize the use of this medication in each type of DILI, we are providing here a comprehensive, integrated, and updated review article that links the current evidence of UDCA efficacy in the clinical DILI context with the multiple mechanisms of action of this compound in the context of DILI physiopathology.

Drug-induced liver injury (DILI) is defined as a liver injury induced by xenobiotics, such as medications, herbs, and dietary supplements, which leads to liver dysfunction in the context of no other identifiable etiologies.[1] DILI is a fairly common adverse drug reaction,[2] and one of the major causes of acute liver failure and liver transplantation.[3]

DILI can be classified into intrinsic and idiosyncratic types, depending on whether it is dose-dependent or not, respectively; in the latter case, the adverse effects result from genetic and non-genetic risk factors.[4]

DILI can also be classified into hepatocellular, cholestatic, and mixed forms, based on the biochemical pattern of liver enzyme elevations.[5] This classification reflects different histological injury patterns; although hepatocellular DILI is more often associated with severe inflammation, necrosis, and apoptosis, cholestatic DILI is usually associated with bile plugs and bile duct paucity.[6]

There is no established treatment for DILI other than discontinuation of the culprit drug and reexposure avoidance[7]; following drug withdrawal, therapy is mostly supportive, waiting for spontaneous remission. However, if the altered liver function persists, therapeutic approaches are in order. One of the medications commonly used for this purpose is ursodeoxycholic acid (UDCA), either alone or in combination with other medications (e.g., glucocorticoids in serious immune-allergic cases).[8]

UDCA is a hydrophilic and hence harmless bile acid recommended by international liver societies and U.S. Food and Drug Administration (FDA) for the treatment of primary biliary cholangitis (PBC) and pregnancy-induced cholestasis.[9] However, UDCA has been empirically used in virtually all forms of cholestatic liver diseases, and even in non-cholestatic ones,[10] as well as in several extrahepatic disorders.[11] [12] As for DILI, bibliographical evidence of UDCA effectiveness consists mostly of case reports and small case series. However, a recent systematic review by Robles-Díaz et al[13] suggested that UDCA would have some beneficial effects in DILI prevention and cure, although a firm conclusion could not be drawn due to design shortcomings of the published studies. These studies reported benefits of UDCA therapy in shortening the time required for a reduction or normalization of total bilirubin (TBL) and serum liver enzymes to be achieved, or in avoiding the increase in aminotransferase levels when UDCA had been used in a preventive manner.[13] Surprisingly, considering that UDCA is better recognized as an anticholestatic drug, no difference in UDCA beneficial response was found between “hepatocellular” and “cholestatic” DILI types.[13] This perhaps reflects the fact that UDCA bears multiple hepatoprotective mechanisms that far exceed the anticholestatic ones, which can be beneficial in all kinds of DILI scenarios.

In this review, a revision of the literature has been performed to evaluate the clinical effectiveness of UDCA across the whole DILI spectrum. In addition, we correlate this information with the multiple mechanisms of UDCA hepatoprotection, with emphasis in linking them to the different pathomechanisms involved in DILI. This should help better rationalize and systematize the eventual use of this versatile and safe hepatoprotector in each type of DILI scenario.

 

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Clinical Impact of UDCA in DILI Treatment

Although it has been claimed that UDCA may be helpful for DILI treatment, there is a lack of international recommendations supporting UDCA for this purpose. This is mainly due to the fact that no randomized, controlled trials in patients with DILI have been performed yet, despite reports on the successful use of UDCA in patients with idiosyncratic DILI date back to more than 20 years. In spite of this limitation, some international guidelines encourage its use in selected DILI cases, both as an antipruritic agent and as a possible accelerator of DILI recovery.[14] [15] Indeed, UDCA is usually added to the list of the very few agents with therapeutic effects in DILI ([Table 1]), even when controlled trials are lacking to confirm this role, and to determine the dose and duration of the treatment.[14] The same holds true for the remaining medications for DILI shown in [Table 1]. Indeed, except for N-acetylcysteine in acetaminophen-induced DILI, only moderate or very limited evidence of benefits has been shown for the remaining treatment options.

Table 1 Main targeted therapies for specific forms of DILI

Drug	Indication/Comments
UDCA[14] [153]	Improved pruritus and cholestasis Shortened disease time
Corticosteroids[24]	Hypersensitivity features Eosinophilia and systemic symptoms (DRESS) Autoimmune hepatitis induced by drug
Cholesteramine[154]	Leflunomide-induced DILI
L-carnitine[155]	Improved survival in children with severe liver injury associated with valproate therapy
N-acetycisteine[156]	Paracetamol and non–paracetamol-induced acute liver failure (randomized placebo-controlled trial)

Abbreviations: DILI, drug-induced liver injury; DRESS, drug reaction with eosinophilia and systemic symptoms.

As for the use of UDCA in preventive treatments, there is currently no recommendation for this medication as a prophylactic tool, although some studies have suggested its efficacy to prevent transaminase elevations in different DILI scenarios[16] [17] (see section “Role of UDCA as a Preventing Tool in DILI”).

There are also no recommendations on the use of biomarkers of therapeutic response to UDCA in DILI. MicroRNA-122 (miR-122) has been suggested to be a potential early biomarker of DILI, according to a recent metabolomics investigation.[18] Interestingly, Kim et al[19] showed that UDCA reduces miR-122 levels in healthy volunteers, thus suggesting that miR-122 may be a valuable biomarker to monitor its therapeutic effectiveness.

 

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Bibliographic Search Results of UDCA Efficacy in DILI

A Medline search of all studies showing beneficial effects of UDCA on DILI, using both therapeutic and preventive approaches, was performed in the literature between 1995 and 2022. We retrieved a total of 30 publications, including 24 case reports and 6 clinical studies, with observational, prospective, and retrospective designs ([Table 2]).

Table 2 Therapeutic and preventive clinical studies in which UDCA was beneficial as DILI treatment

		Patient		Drug		DILI			UDCA treatment
Study/number	Author/Year	Sex	Age	Drug	Treatment duration	Symptoms	DILI pattern	Histological findings	Doses of UDCA	Duration	Outcome
1	Piotrowicz et al (1995)[157]	Male	57 y	Flucloxacillin	−	Jaundice	Cholestatic	Canalicular cholestasis, portal eosinophils	750 mg/d	16 d	Improvement 7 d after starting UDCA, with no evidence of cholestasis 6 wk later
2		Male	78 y	Flucloxacillin	−	Jaundice	Cholestatic	Canalicular cholestasis, portal eosinophils	750 mg/d	21 d	Improvement after 11 d of UDCA treatment
3	Cicognani et al (1996)[158]	Male	83 y	Flutamide	30 d	Pruritus, jaundice	Hepatocellular	−	12 mg/kg/d	−	Clinical improvement after 1 mo of UDCA treatment
4	Singh et al (1996)[20]	Male	24 y	Anabolic androgenic steroid	120 d	Pruritus, jaundice	Mixed	−	900 mg/d	180 d	59% decrease in TBL after 1 mo of UDCA treatment
5	Katsinelos et al (2000)[159]	Male	71 y	Amoxicillin/clavulanate	7 d	Pruritus, jaundice	Cholestatic	Centrilobular canalicular cholestasis	750 mg/d	30 d	Improvement 10 d after starting UDCA and normalization after 30 d of UDCA treatment
6		Male	81 y	Amoxicillin/clavulanate	7 d	Pruritus, jaundice	Cholestatic	Centrilobular canalicular cholestasis	750 mg/d	60 d	Improvement 10 d after starting UDCA, with no evidence of cholestasis 4 wk later
7	Salmon et al (2001)[16]	Male (5), female (11)	70 − 86 y	Tacrine	105 d	−	Cholestatic	−	14 patients received preventive UDCA (13 mg/kg/d)	105 d	Significant reduction of moderate hepatotoxicity (1 ULN < ALT < 3 ULN) in the UDCA-treated group (p = 0.036)
8	Kojima et al (2002)[21]	−	74 ± 8 y	Flutamide	260 ± 220 d	−	Hepatocellular	−	70 patients received preventive UDCA, 375 mg/d	−	Lower proportion of patients with transaminase increases in the UDCA group, compared with patients without UDCA
9	Agca et al (2004)[160]	Female	56 y	Terbinafine	60 d	Jaundice, vomiting, anorexia, pruritus	Hepatocellular	Cholestatic hepatitis	15 mg/kg/d	105 d	Clinical and analytical improvements, 2 wk after UDCA treatment
10	Smith et al (2005)[161]	Male	10 y	Amoxicillin/clavulanate	7 d	Vomiting, abdominal pain, pruritus	Cholestatic	Canalicular cholestasis and bile duct proliferation	15 − 45 mg/kg/d	75 d	2 wk after the increase in UDCA to 45 mg/kg, a significant decrease in TBL was observed, and clinical improvement occurred 4 mo after the onset of symptoms
11	Jorge and Jorge (2005)[162]	Female	61 y	Asiatic spark	30 d	Right upper quadrant pain, jaundice	Hepatocellular	Acute granulomatous hepatitis with eosinophilic infiltrate	10 mg/kg/d	60 d	Clinical and analytical improvement, with negativization of antibodies
12		Female	52 y	Asiatic spark	21 d	Pruritus, jaundice	Hepatocellular	Intense cholestasis on cirrhotic liver Granulomas and eosinophilic infiltrate	10 mg/kg/d	60 d	Clinical and biochemical improvement
13		Female	49 y	Asiatic spark	60 d	Abdominal pain, jaundice	Hepatocellular	Granulomatous hepatitis	10 mg/kg/d	30 d	Clinical and biochemical improvement
14	Sánchez-Osorio et al (2008)[163]	Male	29 y	Anabolic androgenic steroid	105 d	Pruritus, jaundice, abdominal pain	Cholestatic	Canalicular cholestasis	15 mg/kg/d	120 d	50% decrease in TBL, after 1 mo of UDCA treatment
15	Gallelli et al (2009)[164]	Male	54 y	Methimazole	14 d	Fever, rash, jaundice, right upper abdominal pain	Cholestatic	Intracanalicular cholestasis	Not specified	−	Clinical improvement and normal laboratory parameters after 5 d of UDCA treatment
16	Wree et al (2011)[50]	−	−	Anabolic steroids	−	−	Cholestatic	Liver biopsy (n = 13) Cholestasis (n = 7) Hepatocellular (n = 2) Mixed (n = 4)	12 patients received UDCA (750–1,500 mg/d) + corticosteroids	28 − 70 d	Improvement after 8 d of UDCA treatment
17	Studniarz et al (2012)[165]	Male	8 y	Amoxicillin/ clavulanate	14 d	Jaundice	Cholestatic	Mild intrahepatic cholestasis	UDCA (20 mg/kg/d) + corticosteroids	84 d	Resolution of the symptoms after 12 wk of UDCA treatment
18	Mohammed Saif et al (2012)[22]	−	2 − 18 y	Methotrexate.	180 d	–	Hepatocellular	−	19 patients received preventive UDCA (10 − 15 mg/kg/d)	180 d	There was a trend toward decreased transaminase levels, compared with the control group
19	Goossens et al (2013)[166]	Female	61 y	Ibandronate	120 d	Asthenia	Hepatocellular	First biopsy: autoimmune hepatitis Second biopsy: cholangitis and biliary dystrophy	UDCA (10 mg/kg/d) + corticosteroids	140 d	Partial biochemical improvement with corticosteroids, and normalization 1 y after UDCA treatment
20	Ito et al (2014)[23]	Female	64 y	Bosentan	30 d	−	Cholestatic	−	Preventive UDCA (300 − 600 mg/d)	−	Liver function did not show any abnormalities after 2 y of bosentan–UDCA combined therapy
21		Female	69 y	Bosentan	21 d	−	Cholestatic	−	Preventive UDCA 300 − 600 mg/d	−	Liver function did not show any abnormalities after 2 y of bosentan–UDCA combined therapy
22	Asgarshirazi et al (2015)[167]	−	3 mo − 3 y	Valproic acid	−	−	Hepatocellular	−	22 patients received UDCA (10 − 15 mg/kg/d)	180 d	Significant decrease in transaminase levels
23	Li et al (2019)[168]	Male	6 y	Amoxicillin/clavulanate	−	Jaundice, Stevens-Johnson syndrome	Hepatocellular	Vanishing bile duct syndrome	UDCA (15 − 40 mg/kg/d) + corticosteroids, and 12-h plasma exchange	150 d	Clinical and biochemical improvement after 5 mo of UDCA treatment
24	Lang et al (2019)[153]	Female (11), male (16)	19 − 90 y	Rifampicin, isoniazid, and pyrazinamide	−	Asymptomatic	22 patients linked to cholestatic pattern	−	27 patients received treatment with UDCA (250 − 500 mg, every 8 h, with decrease)	−	21 patients showed normalization of liver enzymes, 5 patients showed a significant reduction in liver enzymes, and no change was observed in 1 patient
25	Kurokawa et al (2019)[169]	Male	48 y	Pembrolizumab	1 d	Fever, asthenia	Cholestatic	Interlobular bile duct damage	UDCA (900 mg/d) + corticosteroids	50 d	70% decrease in ALP and GGT, after 2 wk of UDCA treatment
26	Fernandes et al (2019)[170]	Male	52 y	Kratom	Months	Jaundice	Cholestatic	Acute cholestatic hepatitis	1,800 mg/d	60 d	TBL dropped to 4 mg/dL, after 2 wk of UDCA treatment
27	Onishi et al (2020)[171]	Male	68 y	Nivolumab	Four cycles of nivolumab (240 mg, every 2 wk), prednisolone	Diarrhea	Hepatocellular	Hepatocellular injury	UDCA (600 mg/d) + corticosteroids	90 d	Rapid decline after 40 d of UDCA treatment
28	Ahmed et al (2020)[172]	Female	20 y	Amoxicillin/clavulanate	10 d	Vomiting, epigastric pain, jaundice	Cholestatic	Hepatocellular and cholestatic pattern	UDCA (900 mg/d) + corticosteroids 4 mo after antibiotic treatment	120 d	Clinical and analytical improvements
29	Teixeira et al (2020)[173]	Male	72 y	Flucloxacillin	−	Asthenia, abdominal pain, fever, jaundice, pruritus	Hepatocellular	Bridging necrosis. Bile-duct proliferation	1,500 mg/d	−	After 4 mo of UDCA treatment, there was clinical and biochemical improvement, with normalization 18 mo later
30	Ireland et al (2021)[174]	Female	39 y	Ashwagandha root	42 d	Jaundice, pruritus, abdominal pain	Hepatocellular	Acute cholestatic hepatitis associated with confluent necrosis	750 mg/d	−	Clinical and biochemical improvement after 2 wk of UDCA treatment

Abbreviations: ALP, alkaline phosphatase; ALT, alanine aminotransferase; AST, aspartate aminotransferase; DILI, drug-induced liver injury; GGT; gamma-glutamyl transferase; TBL, total bilirubin; UDCA, ursodeoxycholic acid; ULN, upper limit of normal.

 

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UDCA Role According to the Clinical Pattern of DILI

UDCA was initially tested in cholestatic forms of DILI due to the beneficial effects shown in other types of cholestatic hepatopathies. However, UDCA has a plethora of hepatoprotective mechanisms that far exceed those that can be useful in the cholestatic context. In addition, the cholestatic mechanisms associated with DILI can induce hepatocellular patterns of liver injury as well, and they could also be efficiently counteracted by UDCA. For example, intrahepatic retention of bile salts in cholestasis is the main causal event involved in liver hepatic damage by triggering secondarily inflammation and hepatocellular death. This can lead to a predominant hepatocellular rather than a cholestatic pattern of liver injury, as has been shown in DILI associated with bile salt export pump (BSEP) inhibition, which can occur with high levels of alanine aminotransferase (ALT) and aspartate aminotransferase (AST), as compared with those of alkaline phosphatase (ALP).[13] Therefore, those cases of hepatocellular DILI underlying a cholestatic cause can benefit of UDCA as well, not only for the UDCA capability to counteract the cholestatic failure itself but also to counteract its secondary consequences.

We will describe next the protective effects of UDCA according to the different DILI patterns of injury (cholestatic, mixed, and hepatocellular).

Cholestatic and Mixed DILI

As shown in [Table 2], cholestatic and mixed forms of DILI injury treated with UDCA were reported in 14 case reports (studies 1, 2, 5, 6, 10, 14, 15, 16, 17, 21, 25, 26, and 28), and were predominantly observed in men (12/13). More than half of the patients were older than 50 years (8/13), and the drugs most frequently implicated in DILI were amoxicillin/clavulanate (n = 5), flucloxacillin (n = 2), bosentan (n = 2), and anabolic steroids (n = 2). Jaundice, pruritus, and abdominal pain were the most frequent signs and symptoms observed at presentation. Twelve out of 14 patients were biopsied, and a pattern of canalicular cholestasis was observed in 11 of them. The dose of UDCA used to treat these cholestatic forms ranged from 15 to 45 mg/kg/day, while the therapy duration ranged from 16 to 120 days. Clinical and laboratory improvements were observed in most cases between 7 and 12 weeks after the treatment initiation.

Singh et al[20] (study 4) reported a case of a 24-year-old man with a mixed pattern of DILI induced by anabolic androgenic steroids used for 120 days. Jaundice and pruritus were observed at presentation, and UDCA was prescribed at the dose of 900 mg/day for 180 days. The patient showed a 60% decrease in TBL levels 1 month after UDCA therapy.

 

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Hepatocellular DILI

As depicted in [Table 2], 10 case reports were associated with the hepatocellular form of DILI (studies 3, 9, 11, 12, 13, 19, 23, 27, 29, and 30), with women accounting for the majority of cases (6/10). More than half of the patients (8/13) were older than 50 years. The culprit agent most frequently implicated was Asiatic spark, with three cases described so far. Two cases were associated with flutamide, while single cases were reported for terbinafine, ibandronate, amoxicillin/clavulanate, nivolumab, flucloxacillin, and ashwagandha root.

The most frequent signs and symptoms were right quadrant abdominal pain, asthenia, diarrhea, vomiting, jaundice, and pruritus. Liver biopsy was performed in 9 out of 13 patients, and cholestatic hepatitis, granulomas, Steve-Johnson syndrome, granulomatous hepatitis, bridging necrosis, vanishing bile duct syndrome, and hepatitis autoimmune-like were the most conspicuous histologic features. All patients received UDCA at doses ranging from 10 to 40 mg/kg/day, and total or partial clinical and biochemical improvements were observed in all patients between 2 weeks and 5 months of UDCA administration.

 

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Role of UDCA as a Preventing Tool in DILI

We retrieved four articles (three series of patients and a publication with two case reports) describing the use of UDCA to prevent DILI (studies 7, 8, 18, and 20), in which the improvement in hepatic profile or no increase of liver enzymes was observed after prophylactic treatment ([Table 2]).

Salmon et al[16] studied the preventive role of UDCA in tacrine-induced hepatotoxicity. Fourteen patients diagnosed with Alzheimer's disease received both tacrine and UDCA (13 mg/kg/day) for 105 days, and the results were compared with those of 100 patients who had been treated with tacrine alone. In UDCA-treated patients, serum ALT levels were elevated in 93% of patients versus 69% in the control group. Moderate hepatotoxicity (ALT < 3 upper limit of normal [ULN]) did not occur in UDCA-treated patients, while it was present in 25% of controls (p = 0.036). These findings suggest that UDCA prevents tacrine-induced moderate hepatotoxicity.

Kojima et al[21] studied 181 prostate cancer patients treated with flutamide, from which 70 of them received prophylactically UDCA and 111 did not. The incidence of hepatotoxicity was 11% (8/70) in patients on UDCA and 32% (36/111) in those without UDCA treatment (p < 0.05). The DILI-free rates were also significantly higher in patients treated with UDCA (88%, after 1 year of flutamide administration), compared with those in patients who did not receive UDCA (60%; p < 0.005). These results suggest that UDCA also has a prophylactic effect against flutamide-induced DILI.

Mohammed Saif et al[22] recruited 39 children with acute lymphoblastic leukemia, which were randomized to receive methotrexate together with UDCA for 6 months. This schema was discontinued, and the patients were followed up for 3 months (UDCA group, n = 19). The other arm of this study received only chemotherapy, and was followed up for 9 months (control group, n = 20). The ALT levels were significantly higher in the control group versus the UDCA group (p = 0.013). No significant differences in serum AST and TBL levels were observed between both groups.

Ito et al[23] studied a 64-year-old woman with systemic sclerosis and pulmonary hypertension, which was first treated with bosentan (125 mg/day). Since she presented altered liver profile after 1 month of treatment, bosentan was discontinued. After spontaneous normalization of liver enzymes, bosentan (62.5 mg/day) and UDCA (300 mg/day) were administered simultaneously for exertional dyspnea, without any further liver function test alterations. After 2 years of this combined therapy, the doses of UDCA and bosentan were increased to 600 and 125 mg/day, respectively, and no abnormal liver function tests were recorded during the 31-month follow-up. In another case, a 69-year-old woman with systemic sclerosis and pulmonary hypertension was treated with bosentan (125 mg/day), and alterations in liver function tests were observed after 3 weeks of treatment. Bosentan was reduced to 62.5 mg/day and UDCA was simultaneously administered at 300 mg/day. Her liver function abnormalities normalized soon after initiation of the combined therapy. UDCA was increased to 600 mg/day and bosentan was increased to the initial dose of 125 mg/day, and no liver function test abnormalities were observed during the further 24 months of follow-up.

 

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Limitations of Clinical Research Evidence

The clinical studies reviewed earlier have certain common limitations that are important to consider. Most of them, whether they were prospective or retrospective in nature, lacked a control group in their designs. Therefore, it is difficult to draw convincing conclusions about the effectiveness of UDCA in patients with DILI. Indeed, there is large number of anecdotal case reports where UDCA beneficial effects might be ascribed to drug cessation alone, and the majority of the case series have significant methodological flaws. Furthermore, potential hepatoprotective agents are frequently administered concurrently with UDCA.

However, if we take into account all the circumstantial evidence described earlier in favor of the use of UDCA in DILI, its empirical prescription could be justified due to its good safety profile, and the reduction of transaminase and TBL levels often seen in both therapeutic and preventive studies. This has prompted some international guides to suggest that a UDCA course in certain cases of cholestatic DILI may improve both pruritus and biochemical alterations,[14] [15] although other guides remain inconclusive on this matter, and rather suggests a case-by-case decision.[24] [25]

These data lay a solid foundation for the development of comprehensive clinical research studies examining its effectiveness in curing and preventing DILI. Given the current state of knowledge, it should be mandatory to investigate additional UDCA mechanisms in clinical studies based on the substantial preclinical research where therapeutic efficacy of UDCA in DILI has been demonstrated. The ideal clinical study design to address this knowledge gap would require multicenter, randomized, double-blind, placebo-controlled clinical trials, aimed to conclusively demonstrate the effectiveness of UDCA in both prevention and therapeutic schemes. The study should also be long enough to enroll an appropriated number of patients, so that to achieve the sufficient power to show differences in the variables of interest, a task that most likely requires international research collaboration. According to a 2011 Expert International Consensus Meeting, the groups should be equally distributed by DILI severity (mild, moderate, severe), as well as by type of liver injury (hepatocellular, cholestatic, and mixed).[26] The primary outcome measures should include the number of patients achieving at least a 50% reduction in their baseline liver function tests (transaminases, ALP, and TBL), the period of time needed for liver function tests to fully normalize, the survival rate, the rate of DILI relapse upon UDCA removal, and the safety profile of the treatment (rate of adverse events). The inclusion criteria should be limited to patients having a clinical diagnosis of DILI defined as ALT ≥ 5 × ULN, ALP ≥ 2 × ULN or ALT ≥ 3 × ULN + TBL > 2 × ULN,[26] and with causality scores greater than possible (RUCAM score ≥ 3).[27]

Several conclusions arise from our analysis: (1) UDCA was free of serious adverse events at the wide range of doses administered; (2) this agent has not only been shown to be useful in cholestatic and mixed forms of DILI, but also in hepatocellular DILI; (3) beneficial effects have also been shown when UDCA was used to prevent potential DILI, although we do not have the same data regarding the indication of UDCA as a tool to shorten the course of the liver disease; and (4) well-designed clinical trials should be performed to confirm the so far preliminary clinical evidence on the benefits of this medication in all clinical patterns of DILI (hepatocellular, cholestatic, and mixed).

 

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Mechanistic Basis for the Hepatoprotective Mechanisms of UDCA in DILI

UDCA has pleiotropic beneficial effects in liver. The greater hydrophilicity of UDCA due to the β-orientation of its –OH in position 7 explains its low capacity to induce detergent-like harmful effects, as most endogenous human bile acids do.[28] In turn, it retains many of the beneficial and regulatory properties of endogenous bile acids (e.g., anti-inflammatory properties, activation of intracellular regulatory signaling pathways, and the triggering of adaptive hepatic responses to bile acid overload),[29] while it adds several beneficial effects exclusive to itself, such as antioxidant and antiapoptotic properties.[30]

 

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Protective Mechanisms of UDCA in “Hepatocellular” DILI

Hepatocellular DILI often involves lack of hepatocyte integrity, leading eventually to apoptotic and/or necrotic cell death, depending on the severity of the injury.[6] This pattern of liver injury is usually ischemic-, toxic-, or immune-mediated in nature. Although these three pathological processes differ in the triggering mechanisms and the downstream pathways involved, the final effectors causing hepatocellular death are common, often involving oxidative stress, mitochondrial dysfunction, endoplasmic reticulum (ER) stress, and immune-mediated attack. These factors are, in turn, intertwined, and influence one another to produce several vicious circles of liver damage.

UDCA can counteract several of these deleterious vicious circles that lead to liver injury of hepatocellular type ([Fig. 1]).

Fig. 1 Pathogenesis of hepatocellular DILI, and putative protective mechanisms of UDCA. Hepatocyte may be injured by the original drug or its reactive metabolites via multiple mechanisms. They include direct or drug-evoked immune-mediated attack, with the latter being mediated by both innate and adaptive immune responses. Mitochondrial impairment due to direct drug-mediated chemical insult or ROS generation, with further mitochondrial pore formation, triggers the intrinsic (mitochondrial) pathways of cell death, which can result in apoptosis or necrosis, depending on the ATP depletion level associated with the severity of the mitochondrial injury. Alternatively, drug reactive metabolites can act as haptens, and evoke the adaptive immune response by binding covalently to proteins that can be presented by dendritic cells (DC) to naive T helper (Th) and T cytotoxic (Tc) lymphocytes in association with MHC class I or II molecules, respectively, and convert these lymphocytes into drug-specific, effector lymphocytes; these activated Tc can interact with death cell receptors to trigger the extrinsic apoptosis pathway. Drug-reactive metabolites or ROS mitochondrial elevations can also affect endoplasmic reticulum (ER) functional integrity, leading to ER stress, which can trigger, in turn, proapoptotic signals from this organelle to reinforce hepatocyte apoptosis. In turn, loss of membrane integrity associated with necrosis leads to DAMP release from various intracellular compartments, which evokes the innate immune-mediated injury, via toll-like receptor (TRL)-mediated activation of Kupffer cells and further release of cytokines; they recruit and activate several immune cells, such as neutrophils (N), natural killer (NK), and macrophages, which trigger hepatocyte death via different harmful mediators.

UDCA Antioxidant Properties

Generation of radical oxygen species (ROS) in DILI is triggered by (1) uncoupling of the enzymatic cycle of cytochrome P450 (CYP)-mediated, phase I metabolism of drugs[31]; (2) drug-induced inhibition of mitochondrial electron-transport chain proteins, with exacerbated leakage of electrons that react with oxygen to form ROS[32]; and (3) ER stress, associated with the covalent modification of ER-resident proteins by the formation of adducts with drug-reactive metabolites formed in the ER by CYP-mediated reactions[33]; the third leads to elevated misfolded protein production, whose refolding demands disulfide-bond formation, a ROS-generating process.[34]

UDCA attenuates oxidative stress through the following mechanisms: (1) UDCA is a ROS scavenger itself, bearing an efficiency to neutralize hydroxyl-free radicals one order of magnitude greater than that of mannitol, a typical pharmacological “scavenger,” or than that of glucose and histidine, two physiological ROS scavengers[35]; (2) UDCA induces both in murines[36] [37] [38] and humans[39] the expression of nuclear factor-E2-related factor-2 (Nrf2), a master redox-sensitive transcription factor that increases the synthesis of antioxidant enzymes; (3) UDCA increases the synthesis of the main endogenous antioxidant, glutathione (GSH),[38] [40] via upregulation of the rate-limiting step enzyme involved in its synthesis, glutamate-cysteine ligase,[40] and of its precursor, N-acetyl-L-cysteine[41]; (4) UDCA induces the hydroxyl–radical–scavenger protein, metallothionein IIA.[37]

 

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UDCA Anti-inflammatory Properties

Acute hepatitis is a fairly common feature in both idiosyncratic and intrinsic DILI. Liver histology reveals necrosis, apoptosis, and inflammatory infiltrates composed of lymphocytes, mononuclear cells, neutrophils, and eosinophils.[6] Immune-mediated DILI patients have autoantibodies as well as signs and symptoms typical of immunoallergic reactions.[42]

One possible mechanism underlying this hypersensitivity reaction in idiosyncratic DILI is the formation of reactive metabolites during phase-I and II biotransformation reactions,[43] which can covalently bind to cellular proteins (e.g., CYP enzymes involved in their activation), thus forming drug-protein adducts that function as immunogenic haptens. They can be presented by antigen-presenting cells (APCs) via MHC II molecules, thus triggering the adaptive immune response via the sequential activation of both CD4 + , helper T lymphocytes (HTLs), and CD8 + , cytotoxic T lymphocytes (CTLs). Activated CTLs interact with hepatocytes via MHC I molecules and express cytokines such as tumor necrosis factor-α (TNF-α) and Fas ligand (FasL), which mediate hepatocyte cell death via interaction with death receptors profusely expressed on the hepatocyte surface.[44]

With certain drugs causing idiosyncratic DILI, such as halothane, CD4 + , helper T cells can also activate antibody-producing B cells of the IgE and IgG types to drug-protein adducts, which may promote antibody-dependent cellular cytotoxicity.[45] On the other hand, in intrinsic DILI, inflammation rather involves the innate immune system, and it is more likely to be triggered by the direct injurious effects of drugs on hepatic cells, as occurs in acetaminophen hepatotoxicity.[46] In this case, intracellular damage-associated molecular patterns (DAMPs) released from injured hepatocytes activate Kupffer cells (KCs) and neutrophils via toll-like receptors (TLRs), and activated KCs secrete inflammatory cytokines and chemokines, which trigger accumulation of monocytes and neutrophils into necrotic areas.

UDCA has a plethora of immunomodulatory effects that can be beneficial against this exacerbated immune response ([Fig. 2]). UDCA can translocate to the hepatocyte nucleus and activate the glucocorticoid receptor (GR) in a ligand-independent manner[47]; UDCA promotes dissociation of GR from its molecular chaperone, hsp90, thus inducing nuclear GR translocation in the absence of specific ligands.[48] UDCA is also translocated to the nucleus through a GR-dependent mechanism, where it promotes DNA binding of GR through interaction with its ligand-binding domain.[49] UDCA can also enhance GR-dependent gene expression in the presence of coactivators, as well as promote GR-responsive gene expression induced by dexamethasone[48]; this perhaps explains why combination of UDCA and dexamethasone can be more effective than each of them alone in severe DILI cases.[50] Through these GR-dependent mechanisms, UDCA counteracts transcriptional activation of activator protein-1 (AP-1)[51] and represses nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB) transcription activity, which in turn blocks NF-κB-dependent transcription via interaction with its p65 subunit.[49] Signaling pathways upstream of AP-1 and NF-κB can also be inhibited by UDCA. Activation by phosphorylation of mitogen-activated protein kinases (MAPKs) leading to AP-1 activation and activation of the NF-κB inhibitor-α (IκBα) leading to NF-κB activation were both blocked by UDCA in lipopolysaccharide-stimulated macrophages.[52] UDCA also inhibits the release of certain proinflammatory cytokines produced by mononuclear cells (IL-2, IL-4, and IFN-γ), which induce the proliferation and activation of CTLs and NK cells.[53] [54] In addition, TNF-α-induced release by activated macrophages of IL-8, a potent neutrophil chemoattractant, is also counteracted by UDCA.[55] Finally, UDCA suppresses IgM, IgG, and IgA production by B lymphocytes exposed to bacteria,[53] and perhaps MHC I expression in hepatocytes,[56] presumably via activation of GR.[45]

Fig. 2 Immunological adaptive mechanisms of hepatocellular and cholangiocellular damage in DILI, and their attenuation by UDCA. (1) The drug, or its reactive metabolites, may act as a hapten by binding covalently with cellular proteins, so that it can be recognized as a neoantigen that can be perceived as a foreign protein by the adaptive immune system. (2) Dendritic cells (DCs) degrade the antigenic proteins and present these neo-antigens via major histocompatibility complex class II molecules (MHC-II). (3) The hapten–MHC-II complex is recognized by helper T lymphocytes (Th), and this triggers the autocrine activation of these lymphocytes via interleukin-2 (IL-2) release and their further clonal proliferation via autocrine binding of IL-2 to its receptor, IL-2R. (4) Th induces clonal expansion and/or activation of cytotoxic T lymphocytes (Tc) via release of IL-2. (5) Activated Tc induces cholangiocyte apoptosis by releasing perforin and granzymes; binding of Tc to bile ducts is facilitated by cholangiocyte expression of both MHC class I antigen (MHC-I) and adhesion molecules, such as intracellular adhesion molecule 1 (ICAM-1). (6) Hepatocytes also bind to Tc via MHC-I, thus inducing the release of perforin and granzymes, and the consequent hepatocyte death. (7) Activated Th can also prompt B lymphocytes (B) to produce drug-specific antibodies, although there is no compelling evidence yet to support that these antibodies play a role in DILI. Due to its multiple anti-inflammatory and immunomodulatory mechanisms, UDCA can counteract several steps in DILI-activated adaptive immunological response. For example, UDCA may counteract the production of antibodies by B cells, and the production of IL-2 by Th cells, thus preventing activation of both B and Tc cells. In addition, it may repress the bile salt–induced overexpression of MHC I and MHC II in hepatocytes and cholangiocytes, respectively.

 

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UDCA Antiapoptotic Properties

Apoptosis is a key histological finding in “hepatocellular” DILI. It can be caused by the direct deleterious effect of the drug or its reactive metabolites, or by the exacerbated immunological response triggered by the drug.[57] Apoptosis in DILI involves activation of several interrelated pathways, namely, (1) the intrinsic or mitochondrial pathway, triggered by the release of mitochondrial proapoptotic factors after mitochondrial membrane pore formation, (2) the extrinsic pathway, initiated by the activation of death receptors located on the surface of the hepatocellular plasma membrane that trigger signals that affect mitochondrial integrity, and (3) apoptosis by ER stress, induced by activation of caspases and proapoptotic nuclear receptors in this organelle.[58]

UDCA is a powerful antiapoptotic agent capable of inhibiting key processes in these three apoptotic pathways ([Fig. 3]).

Fig. 3 Mechanisms of production of cell death in cholestasis, and its protection by UDCA. Pro-apoptotic pathways are shown in red (inhibited by UDCA, in dashed line), and antiapoptotic pathways are shown in green. In DILI, three apoptotic pathways are activated, namely (1) the intrinsic or mitochondrial apoptosis pathway, which depends on the formation of mitochondrial permeability transition pores (PTPM), as well as on the balance between the expression/activity of mitochondrial pore-forming proapoptotic mitochondrial proteins (e.g., Bax) and antiapoptotic proteins that sequester the previous ones (e.g., Bcl-2, Bcl-XL); (2) the extrinsic apoptosis pathway, mediated by cell death receptors (TNFR, Fas, CD40, TRAILR-2) activated by their respective proapoptotic cytokines (TNF-α, Fas-L, CD154, and TRAIL-2), which induces apoptosis by truncating Bid, with the subsequent insertion of truncated Bid (tBid) into the mitochondrial outer membrane or, for TNF-α, by phosphorylating and promoting translocation to the nucleus of the proapoptotic transcription factor AP-1; (3) the endoplasmic reticulum (ER) stress apoptosis pathway, which results in Ca²⁺-dependent activation of caspase-12 and release of CHOP, an AP-1 activator. UDCA prevents the activation of all these three apoptotic pathways by acting either directly as an inhibitor of key processes involved in its generation or, indirectly, by stimulating antiapoptotic signaling pathways (e.g., Erk, PI3K/Akt), via binding of UDCA to the epidermal growth factor receptor (EGFR). In addition, UDCA counteracts the nuclear translocation of p53, another transcription factor that, like AP-1, induces proapoptotic Bcl2 family proteins and represses antiapoptotic ones. Finally, UDCA blocks the recruitment to plasma membrane of endocytosed cell death receptors, awaiting for demand. Many of the proapoptotic mechanisms described earlier are triggered by reactive oxygen species (ROS) generated by the release of electrons (e⁻) from damaged mitochondria, and UDCA counteracts them due to its antioxidant properties.

The Intrinsic or Mitochondrial Apoptosis Pathway

This pathway involves disruption of mitochondrial membrane integrity by:

1. Formation of mitochondrial permeability transition (MPT) pores (MPTPs), generated by interactions between inner and outer membrane proteins[59]; this facilitates entry of small-size solutes, further mitochondrial swelling, and, eventually, outer-membrane rupture.[60]

2. Mitochondrial outer membrane permeabilization (MOMP), due to the mobilization to the outer membrane of proapoptotic proteins of the Bcl-2 family, Bax and Bak, which form mitochondrial pores by homo-oligomerization. Bax and Bak can be activated by proapoptotic members of the BH3-only protein family, such as Bid and Bim, or sequestered and hence inhibited by antiapoptotic proteins of the BCL-2 family, such as Bcl-2, Bcl-xL, and Mcl-1. Bax and Bak can also be indirectly activated by Bid, Bad, Bim, Puma, and Noxa, via inhibition of these antiapoptotic proteins.[61]

Both MPT and MOMP trigger the release of cytochrome c from the intermembrane space into the cytosol, where it binds to caspase-9 and the scaffold protein, apoptosis protease-activating factor-1 (APAF1), to form the so-called apoptosome; this complex triggers apoptosis by activating the executioner caspases 3, 6, and 7.[58]

Oxidative stress is a key factor in the activation of both apoptotic mechanisms. Oxidation of respiratory proteins impairs mitochondrial respiration, which triggers MPTP opening and cytochrome c release, disruption of mitochondrial membrane potential, and, eventually, suppression of ATP synthesis[62]; this may lead to apoptosis or necrosis, depending on the severity of ATP depletion.[63] Drug-induced oxidative stress also leads to apoptosis via JNK activation, and the further MOMP generation[64]; this involves ROS-mediated glycogen synthase kinase-3β (GSK-3β) and further mixed-lineage kinase-3 (MLK3) activation, followed by ROS-mediated activation of apoptosis signal-regulating kinase-1 (ASK1).[65] Activated JNK translocates to the outer mitochondrial membrane and triggers cytochrome c release via Bcl-xL phosphorylation.[66] JNK also activates others proapoptotic proteins through direct phosphorylation (e.g., Bim and Bad), and inhibits proapoptotic ones (e.g., Bcl-2).[67] Finally, JNK upregulates proapoptotic genes via phosphorylation of specific transcription factors, such as p53,[68] and the AP-1 components, c-Jun[69] and c-Fos[70]; AP-1 upregulates proapoptotic proteins, such as Bax, Bak, and Bim,[71] [72] whereas p53 upregulates proapoptotic proteins, such as Bax, Noxa, and Puma, and downregulates antiapoptotic ones, such as Bcl-2 and Bcl-xL.[73] The JNK/c-Jun/p53 pathway is potentiated by the activation of the miR-34a/sirtuin 1 (SIRT1)/p53 signaling pathway, a proapoptotic positive feedback loop by which p53 induces miR-34a, and miR-34a further activates p53 via blockage of SIRT1, a deacetylase that antagonizes p53.[74]

UDCA counteracts many of these proapoptotic mechanisms. It prevents the MPTP generation induced by multiple stimulus,[75] [76] [77] by counteracting mitochondrial transmembrane-potential impairment and mitochondrial-derived ROS.[78] Direct UDCA antioxidant effects on mitochondria are likely,[79] since UDCA colocalizes with mitochondrial-membrane structures.[80]

UDCA also counteracts apoptosis via MOMP at multiple levels. UDCA translocates to the nucleus bound to GR, where it inhibits the transcription factor E2F-1, and the further p53-dependent Bax activation.[81] Furthermore, the major UDCA metabolite, tauroursodeoxycholate (TUDCA), prevents Bax from binding to the mitochondrial outer membrane.[82] UDCA also inactivates Bad by binding to the epidermic growth factor receptor (EGFR), which activates the antiapoptotic signaling pathways mediated by phosphoinositide 3-kinase (PI3K) and extracellular signal-regulated kinase (ERK)[83] [84]; both PI3K (via Akt)[85] and ERK[86] phosphorylate Bad, thus leading to Bcl-xL dissociation from Bad, and the further Bcl-xL-mediated prevention of Bax/Bak pore formation. ERK also activates antiapoptotic transcription factors of the ternary complex factor (TCF) family, which bind to serum-response factor (SRF) to form a complex that upregulates c-Fos promoter activity to induce antiapoptotic proteins, such as Bcl-2[87] and Mcl-1.[88] Finally, UDCA interferes with JNK activation by inhibiting its regulatory upstream kinases, MAPK kinases 4/7.[89] UDCA also counteracts JNK-mediated upregulation of the AP-1 components, c-Fos and JunB, and the resulting increase in AP-1 transcriptional activity,[51] and also impairs AP-1 binding activity, leading to p53 downregulation.[90] In addition, UDCA downregulates the miR-34a/SIRT1/p53 proapoptotic pathway,[91] by hindering miR-34a expression, by inducing SIRT1 expression, and by inhibiting p53 acetylation, which is required for p53 transcriptional activity.[91] Finally, UDCA reduces p53 DNA-binding activity, by stabilizing the association of p53 with the oncogenic E3 ligase, MDM-2,[92] and by enhancing MDM-2-dependent ubiquitination and further p53 degradation.[93]

 

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The Extrinsic Apoptosis Pathway

This pathway is driven by inflammation, with CTLs being the main primary effectors. Active CTLs interact with hepatocytes via MHC I molecules and express both membrane and soluble cytokines, such as TNF-α, FasL, CD40, and TRAIL 1/2. These proteins bind to the death receptors of the TNF receptor superfamily, TNFR1, Fas, TRAILR 1/2, and CD40L, respectively, thus triggering apoptosis via downstream activation of initiator caspases 8 and 10, which further activate the effector caspases 3, 6, and 7.[44] Alternatively, initiator caspases trigger proteolysis of Bcl-2 interacting domain (Bid), and the resulting truncated Bid (tBid) inserts into mitochondria and recruits Bax and Bak,[94] leading to MOMP-mediated apoptosis. Activation of TNFR1 by TNF-α also induces apoptosis by recruiting TRAF2 and ASK1, which activate AP-1 by phosphorylation.[95]

TUDCA can inhibit the extrinsic pathway by binding to Bax,[96] thus preventing tBid-dependent translocation of Bax to mitochondria.[82] UDCA also inhibits the TNF-α-induced, JNK-mediated phosphorylation of AP-1.[51] Finally, UDCA attenuated FasL-induced apoptosis in mouse hepatocytes co-cultured with fibroblasts expressing FasL, by interfering with apoptotic mechanisms downstream of Fas.[80]

 

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ER Stress-Induced Apoptosis

Excessive accumulation of defective proteins in DILI generates ER stress.[64] [97] The so-called adaptive unfolded protein response is then activated, based on the overexpression of chaperones (e.g., Bip) and the activation of transcription factors that induce enzymes involved in the assembly of newly synthetized proteins and the proteasomal degradation of unfolded/misfolded ones, such as inositol-requiring enzyme 1 (IRE1), protein kinase RNA-like endoplasmic reticulum kinase (PERK), and activating transcription factor 6 (ATF6).[98] This adaptive response may, however, be insufficient when ER stress is overwhelming, leading to apoptosis to eliminate the damaged cells.[99] This occurs because IRE1, PERK, and ATF6 also activate C/EBP-homologous protein (CHOP), a transcription factor that inhibits the expression of antiapoptotic Bcl-2 proteins[100] and induces TRAILR2.[101] In addition, IRE1 recruits TRAF2 and ASK1 to the ER membrane, which activates JNK,[102] and further MOMP generation. Finally, Ca²⁺ release by the stressed ER triggers the sequential activation of Ca²⁺-dependent calpains, calpain-dependent caspase 12 activation (an executer caspase itself), and caspase-12-mediated activation of the executer caspases 9 and 3.[103]

TUDCA acts as a chemical chaperone that attenuates ER stress. This involves stabilization of conformations of de novo synthesized proteins that facilitates their folding, enhancement of ER protein-assembly capacity, and acceleration of the trafficking of incorrectly localized and/or aggregated dysfunctional proteins, to restore proper destination.[104] [105] [106] Protein folding capacity is further improved by TUDCA via ATF6 activation.[107] Finally, UDCA counteracts caspase 12 activation.[108]

 

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UDCA Antinecrotic Properties

Necrosis is characterized by loss of ion homeostasis causing massive cell swelling and plasma membrane rupture (oncosis), which promotes ion imbalance, mitochondrial dysfunction with massive MPT generation and ATP depletion, as well as severe inflammation due to DAMP release.[57]

Since MPT is a common mechanism responsible for both necrosis and apoptosis, the multiple mechanisms of UDCA protection against MPT described above (see “The Intrinsic or Mitochondrial Apoptosis Pathway”) could be protective against DILI-induced necrosis. Furthermore, UDCA is expected to induce a switch from necrosis to apoptosis, thus attenuating the DAMP-mediated inflammatory response, as shown in oxaliplatin-induced necrosis in HepG2 cells.[109] Finally, UDCA downregulates the expression of receptor-interacting protein kinase 3 (RIP3), a critical kinases that mediate TNF-α-dependent necroptosis, a programmed form of necrosis.[110] Compelling evidence that this occurs in human DILI is awaited.

 

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Protective Mechanisms of UDCA in “Cholestatic” DILI

In “cholestatic” DILI, two possible etiological agents should be considered: (1) the drug itself or its reactive metabolites and (2) the damaging effect of secondarily accumulated bile acids. The drug can cause either hepato- or cholangiocellular cholestatic dysfunction, depending on whether its deleterious mechanisms occur before or after biliary excretion. Toxicity of bile acids could lead to necrosis, due to direct plasma-membrane damage when present at very high concentrations, or more likely to apoptosis, at the levels usually found in cholestatic hepatopathies.[111] Therefore, UDCA protective mechanism that attenuate the cholestatic effects of the drug itself or the damaging effects of the accumulated bile acids should be beneficial in cholestatic DILI, as discussed next.

 

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Replacement of Cytotoxic Endogenous Bile Acids in the Bile Acid Pool

UDCA administration at the usual doses of 13 to 15 mg/kg/day increases from 4% to 40–60% the UDCA proportion in the total endogenous bile-acid pool,[112] without changes in serum total bile acid levels.[113] This occurs because UDCA competes with the absorption at the terminal ileum of the two major hydrophobic, harmful bile acids, deoxycholic, and chenodeoxycholic acids.[114]

Apart from being enriched in serum, UDCA and its conjugates are enriched in bile, accounting for 19 to 64% of total bile acids, depending on the dose.[115] This is a significant protective factor for the biliary tree in obstructive DILI-associated cholangiopathies. Liver parenchyma is also proportionally enriched in UDCA, which also makes UDCA beneficial in cholestatic processes with defective canalicular bile acid excretion, for example, pharmacological inhibition of BSEP.[116]

 

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Improved Clearance of Hepatotoxic Biliary Constituents Accumulated in Cholestasis

UDCA improves the body's ability to depurate hepatotoxic endo- and xenobiotics by modulating the expression of transporters and biotransformation enzymes in different detoxifying organs, such as liver, kidney, and intestine. This is critical in cholestatic liver disease, as there are pathologic increments in the systemic levels of toxic compounds otherwise cleared by bile, including bile acids and bilirubin, as well as the cholestatic drug itself. UDCA reduces hepatocellular levels of these compounds by inhibiting their uptake and by accelerating their reflux into sinusoidal blood, thus favoring renal excretion ([Fig. 4]).

Fig. 4 Effect of UDCA administration to rodents on the expression of hepatic and renal transporters, as well as on hepatic enzymes involved in the metabolism of endogenous bile salts (BS⁻). UDCA stimulates the expression and/or activity of canalicular transporters involved in bile formation (e.g., Bsep, Mrp2). UDCA also favors the reflux to plasma of anionic, deprotonated BS⁻ and glucuronidated bilirubin (Br-G⁻) accumulated in cholestasis, by stimulating the expression of export pumps in the sinusoidal membrane (e.g., Mrp3, Mrp4), and by inhibiting BS⁻ uptake transporters (e.g., Ntcp). Both uptake inhibition and exacerbated sinusoidal efflux allow toxic, accumulated substances to be diverted to the kidney, for their further urinary excretion via glomerular filtration and/or renal tubular secretion; the latter process is stimulated by UDCA via induction of apical transporters (Mrp2, Mrp4). Finally, UDCA represses hepatocellular endogenous BS⁻ synthesis and facilitates BS⁻ hydroxylation, leading to a lower level of BS⁻ with attenuated toxicity.

Oral administration of UDCA to normal mice reduces the transcriptional expression of Oatp1, a sinusoidal bile-acid transporter.[117] In addition, UDCA induces the basolateral bilirubin glucuronide and bile-acid export pumps Mrp3[118] and Mrp4.[119] Finally, at the canalicular pole, oral UDCA administration to rodents upregulates Bsep.[117] [118] UDCA also induces Mrp2,[117] [118] which transports into bile bilirubin glucuronides, glutathione, and glucuronidated or sulfated bile acids.

UDCA also stimulates renal expression of the apical export pumps Mrp2[118] and Mrp4,[119] which facilitates urinary excretion of bilirubin glucuronides (via Mrp2) and bile acids (via Mrp4). The urinary excretion of bile acids is further enhanced by the repressed expression of apical sodium-dependent bile-salt transporter (Asbt), involved in tubular reabsorption.[119]

In addition, UDCA modulates the expression of hepatic enzymes that metabolize bile acids. In rodents, UDCA represses the expression of the enzyme cyp7a1, the rate-limiting step of the classical bile acid biosynthetic pathway.[119] Furthermore, it stimulates sterol hydroxylases responsible for bile acid hydroxylation, such as Cyp3a11 and Cyp2b10 (in rodents) and CYP3A4 (in humans),[120] thus rendering bile acids less toxic.[28]

These findings in rodents must be cautiously extrapolated to the clinical setting, since only some of these UDCA effects have been replicated in humans. For example, healthy patients receiving UDCA showed increased levels of BSEP and MRP4, but not of MRP2, MRP3, and OATP1 in liver.[121]

 

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Protection against Bile-Acid-Induced Hepatocellular Cell Death

It has been proposed that cytotoxic bile acids can induce necrosis or apoptosis depending on the cholestasis severity.[122] UDCA possesses numerous specific mechanisms of cell protection against bile-acid-induced apoptosis and necrosis, which are summarized below.

Protection against Bile Acid–Induced Hepatocellular Necrosis

UDCA protection against necrosis has been demonstrated in isolated rat hepatocytes[123] [124] [125] and whole rats.[126] Hydrophobic bile acids can induce necrosis by both plasma membrane lipid peroxidation[127] and solubilization by detergent action,[128] and both pathomechanisms are efficiently counteracted by UDCA. Antioxidant UDCA properties have been described in detail previously (see section “UDCA Antioxidant Properties”). Regarding its ability to counteract detergent action of bile acids, UDCA directly neutralizes the plasma membrane disorganizing effect of surfactants, as shown in liposomes[129] and in isolated hepatocellular plasma membrane.[125] This effect seems to involve formation of a complex between cholesterol and UDCA, which enhances the stabilizing effect that cholesterol has per se on lipid bilayers.[129] As for the anionic form of TUDCA (main form at physiological pH), it would electrostatically repel surface-active, negatively charged bile acids.[129] [130] One limitation of these findings is that millimolar concentrations of UDCA are required to exert membrane stabilizing effects, but UDCA achieves only micromolar levels in systemic circulation under normal therapeutic regimens. However, millimolar concentrations can be easily reached within the biliary lumen, and protective effects can be exerted from there, where endogenous bile acids also reach cytotoxic concentrations.

 

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Protection against Bile Acid–Induced Hepatocellular Apoptosis

Bile acids induce the three main mechanisms of apoptosis (i.e., intrinsic [mitochondrial], extrinsic, and RE-mediated forms of apoptosis), and UDCA has specific antiapoptotic mechanisms to counteract all of them.

MPTP generation is triggered by hydrophobic bile acids via uncoupling of the respiratory chain.[75] [76] [123] UDCA has protective effects against this pathomechanism,[75] [78] presumably by exerting direct antioxidant effects on mitochondria.[38]

Hydrophobic bile acids can also activate the extrinsic pathway of apoptosis by ligand-independent and ligand-dependent mechanisms. The ligand-independent pathway involves stimulation of vesicular trafficking to plasma membrane of both Fas[131] and TRAILR2,[132] and their subsequent oligomerization and activation by auto-phosphorylation. This is mediated by activation of NADPH oxidase (NOX).[133] This redox, membrane-bound enzyme produces hydroxyl-free radicals that activate Yes, a tyrosine kinase of the Src family that, in turn, phosphorylates and activates EGFR, which associates with Fas to trigger apoptosis.[133] [134] The increased density of Fas at the plasma membrane makes hepatocytes more susceptible to ligand-dependent Fas-mediated apoptosis as well.[131]

UDCA inhibits the bile-acid–induced extrinsic pathway of apoptosis by counteracting NOX-mediated signaling, due to its ROS-scavenging properties and, downstream of Fas, by counteracting the action of tBid on mitochondria[80] and bile-acid–induced AP-1 phosphorylation.[51]

Finally, bile acid induces ER stress and the resulting apoptosis via elevation of intracellular Ca²⁺ and NOX-derived ROS generation, in a hydrophobicity-dependent manner,[135] and TUDCA can counteract this deleterious effect due to its chaperone activity (see “ER Stress-Induced Apoptosis”).

 

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Protection against Drug-Induced Cholangiocyte Death

Drug-induced bile-duct injury is a cholestatic or mixed type of biliary disease, with features of cholestasis persistent over time, despite drug withdrawal.[136] Drugs may induce bile-duct injury mostly by (1) direct toxic effects on cholangiocytes, (2) immune-mediated cholangiocyte attack as a hepatic manifestation of a T cell-mediated hypersensitivity reaction, and (3) biliary obstruction, leading to sustained and/or exacerbated exposure of cholangiocytes to toxic bile salts.[136]

Attenuation of Direct Drug-Induced Cholangiocyte Toxicity

Cholangiocytes are expected to be highly susceptible to oxidant insults, due to their far lower content in GSH, compared with hepatocytes.[137] This may be aggravated further by GSH-depleting drugs. Furthermore, GSH depletion is associated with decreased BCL-2 expression and increased apoptosis in biliary cells.[138] The earlier-discussed UDCA antioxidant and antiapoptotic mechanisms in hepatocytes are expected to apply also for cholangiocytes, since they are not cell-type dependent. Actually, the UDCA metabolite, glycoursodeoxycholic acid, protected against cytochrome c release in a human cholangiocyte cell line exposed to the pro-oxidant and Ca²⁺-elevating agent, beauvericin.[139] Clearly, more studies are required to confirm UDCA antiapoptotic effects in this type of ductopenic DILI.

 

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Attenuation of Drug-Induced Cholangiocyte Immunological Attack

The general mechanisms of UDCA immunosuppressive and immunomodulatory effects discussed earlier (see “UDCA Anti-inflammatory Properties”) would attenuate the cellular immune response by inhibiting the release of mononuclear cell-released cytokines, such as IL-2, IL-4, and IFN-α; this was confirmed for IL-2 in a cholangitis experimental model.[140]

 

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Protection against Bile Acid–Induced Toxicity on Bile Ducts

Bile contains high concentrations of toxic bile acids with potential capacity to cause cholangiocyte death by apoptosis or necrosis.[141] However, this is attenuated by phospholipids, whose excretion is mediated by the phospholipid floppase, multidrug-resistant protein 3 (MDR3; mdr2 in murines). Phospholipids form mixed micelles with cholesterol and bile acids, thus lowering the levels of highly toxic bile-acid monomers. HCO₃ ⁻-rich ductular bile production via AE2 via anion exchanger 2 (AE2) also helps reduce biliary monomeric bile-acid concentration by dilution.[142] This secretion also forms the so-called HCO₃ ⁻ umbrella, an alkaline ductular fluid layer that helps maintain bile acids in their non-diffusible, anionic forms, thus preventing their simple passive diffusion into cholangiocytes as neutral molecules.[143]

As for phospholipids, mutations in the gene that codifies MDR3 have been associated with ductopenia,[144] and therefore, this may be a predisposing factor in drug-induced ductopenia, particularly when combined with drugs known to inhibit MDR3.[145] [146] UDCA induces the expression of MDR3 in normal individuals by post-transcriptional mechanisms, so it could increase the excretion of phospholipids into bile.[121] In addition, unconjugated UDCA induces HCO₃ ⁻-rich hypercholeresis associated with its cholehepatic recirculation and post-transcriptional stimulation of the ductular HCO₃ ⁻ transporter AE2[147] [148] ([Fig. 5]); via this mechanism, UDCA reduced portal inflammation, bile duct proliferation, and fibrosis in the mdr2-knockout mice.[149] UDCA, when administered together with dexamethasone, induces AE2 expression in cells of cholangiocellular and hepatocellular lineages.[150] This occurs through the binding of UDCA and dexamethasone to GR, and the further interaction of GR with the transcription factor liver-enriched hepatocyte nuclear factor 1 (HNF-1) to increase the transcriptional activity of the AE2 promoter.[150] Perhaps, this may help explain the better results obtained when combing UDCA with corticoid therapy in certain cases of drug-induced cholestasis.[50]

Fig. 5 Mechanisms of UDCA-induced HCO₃ ⁻-rich choleresis. UDCA cholehepatic shunting involves cholangiocyte absorption of the non-conjugated, protonated (uncharged) form of this bile acid by non-ionic diffusion, followed by its transport to the hepatic sinusoids via the peribiliary plexus, and its return to cholangiocytes via hepatocellular biliary re-excretion in its anionic form, UDC⁻; further UDC⁻ protonation renders a bicarbonate (HCO₃ ⁻) molecule in the biliary lumen each time UDCA suffers a cycling event, which acts as an osmotic driving force for ductular bile formation, and the consequent dilution of potentially toxic bile acids. In addition, UDCA activates AE2, the main transporter involved in ductular HCO₃ ⁻ excretion, via both transcriptional and post-transcriptional signaling mechanisms; the latter process involves stimulation of ATP release by both hepatocytes and cholangiocytes into bile, and the further ATP-mediated activation of purinergic 2Y receptors (P2YR) in cholangiocytes. These receptors stimulate AE2 via an increase in cytosolic Ca²⁺, and the further activation of Ca²⁺-dependent PKC isoforms (cPKC), which in turn activates Cl⁻ channels present in the apical cholangiocyte membrane required for the AE2-mediated HCO₃ ⁻ exchange.

 

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Conclusions

Lack of strategies properly validated to prevent and treat DILI is alarming, since this condition is a frequent cause of acute liver failure in clinical practice.[24] Therefore, efforts should be made to provide new therapeutic and prophylactic approaches for this condition, and to validate those that show promise or that are widely used in an empirical manner. The rationale behind each of these approaches is also essential, and efforts should be made to link the increasing knowledge on underlying beneficial mechanisms of potentially useful compounds with the multiple causative mechanisms of DILI, having in mind that they are usually multifactorial in nature, and even different from one patient to another.

The unique beneficial pleiotropic effects UDCA has due to its anticholestatic, antioxidant, anti-inflammatory, antiapoptotic, antinecrotic, mitoprotective, ER-stress alleviating, and immunomodulatory properties make this compound a priori a highly versatile medicine to treat the wide range of injuries with different pathomechanisms of damage occurring in the context of DILI, and this review was aimed to summarize them to prompt researchers to explore its iatric potential in old and new DILI scenarios. Whether such a consorted, beneficial actions of UDCA will actually occur in patients with DILI remains to be ascertained, and this will represent a challenge for clinical researchers in their effort to develop new therapeutic alternatives for this condition.

UDCA is a well-tolerated drug and has an excellent safety profile at the doses recommended to treat DILI (10–15 mg/kg/day).[14] Indeed, no serious adverse effects have been reported in controlled clinical trials in UDCA-treated patients with gallstone disease (10–12 mg/kg/day), as well as in large-scale, long-term, placebo-controlled trials in patients with PBC (13–15 mg/kg/day).[151] This explains its generalized use in the clinical practice, far beyond the reduced number of hepatopathies where it has been sufficiently validated, and DILI is not the exception. For example, UDCA is often given in DILI cholestatic conditions where pruritus is a common feature, due to its alleged antipruritic effects in other cholestatic hepatopathies,[152] but the hope also exists that the drug aids to improve the natural history of the disease. A case-by-case decision should be therefore often made, and a clear understanding of the multiple hepatoprotective mechanisms of UDCA should help to better support this decision.

UDCA innocuousness makes this compound also ideal to test this compound in prophylactic approaches when a potentially harmful drug well known to induce DILI is going to be administered, particularly in patients where a superimposed DILI would be detrimental for their underlying condition or those without alternative treatment options to cure their diseases. There is some preliminary examples in the literature using this prophylactic approach, as discussed earlier (see section “Role of UDCA as a Preventing Tool in DILI”), showing encouraging results in DILI with different patterns of liver damage, including hepatocellular (e.g., tacrine, flutamide) and cholestatic (e.g., bosentan) DILI types. It should be kept in mind that preventing damage caused by a drug is more feasible than reversing an already established injury. Consequently, the likelihood of success is anticipated to be greater in the former scenario.

We hope the exponential advances in cell and molecular biology applied to the understanding of the mechanisms of DILI and UDCA hepatoprotection will fuel a growing feedback between basic research and applied therapeutics, aimed to envisage new indications of UDCA in DILI, based on increasing rational bases.

 

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Conflict of Interest

None declared.

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Artikel online veröffentlicht:
20. Februar 2024