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Liver-side of inflammatory bowel diseases: Hepatobiliary and drug-induced disorders

Hepatobiliary disorders are among the most common extraintestinal manifestations in inflammatory bowel diseases (IBD), both in Crohn’s disease and ulcerative colitis (UC), and therefore represent a diagnostic challenge. Immune-mediated conditions include primary sclerosing cholangitis (PSC) as the main form, variant forms of PSC (namely small-duct PSC, PSC-autoimmune hepatitis overlap syndrome and IgG4-related sclerosing cholangitis) and granulomatous hepatitis. PSC is by far the most common, presenting in up to 8% of IBD patients, more frequently in UC. Several genetic foci have been identified, but environmental factors are preponderant on disease pathogenesis. The course of the two diseases is typically independent. PSC diagnosis is based mostly on typical radiological findings and exclusion of secondary cholangiopathies. Risk of cholangiocarcinoma is significantly increased in PSC, as well as the risk of colorectal cancer in patients with PSC and IBD-related colitis. No disease-modifying drugs are approved to date. Thus, PSC management is directed against symptoms and complications and includes medical therapies for pruritus, endoscopic treatment of biliary stenosis and liver transplant for end-stage liver disease. Other non-immune-mediated hepatobiliary disorders are gallstone disease, whose incidence is higher in IBD and reported in up to one third of IBD patients, non-alcoholic fatty liver disease, pyogenic liver abscess and portal vein thrombosis. Drug-induced liver injury (DILI) is an important issue in IBD, since most IBD therapies may cause liver toxicity; however, the incidence of serious adverse events is low. Thiopurines and methotrexate are the most associated with DILI, while the risk related to anti-tumor necrosis factor-α and anti-integrins is low. Data on hepatotoxicity of newer drugs approved for IBD, like anti-interleukin 12/23 and tofacitinib, are still scarce, but the evidence from other rheumatic diseases is reassuring. Hepatitis B reactivation during immunosuppressive therapy is a major concern in IBD, and adequate screening and vaccination is warranted. On the other hand, hepatitis C reactivation does not seem to be a real risk, and hepatitis C antiviral treatment does not influence IBD natural history. The approach to an IBD patient with abnormal liver function tests is complex due to the wide range of differential diagnosis, but it is of paramount importance to make a quick and accurate diagnosis, as it may influence the therapeutic management.


INTRODUCTION
Hepatobiliary disorders are common extraintestinal manifestations of inflammatory bowel diseases (IBD) and may occur in both Crohn's disease (CD) and ulcerative colitis (UC). The range of IBD-associated hepatobiliary disorders is wide and can underlie different pathogenetic mechanisms. They include diseases with immune-mediated pathogenesis, which typically have a course independent of intestinal activity, the most common being primary sclerosing cholangitis (PSC); variant form of PSC, like small-duct PSC, must also be considered. Other non-immune-mediated conditions include gallstone disease, whose incidence is increased in IBD patients, non-alcoholic fatty liver disease (NAFLD), pyogenic liver abscess and portal vein thrombosis. Druginduced liver diseases is another important chapter, since several drugs used in IBD, mainly thiopurines, methotrexate and anti-tumor necrosis factor-α (anti-TNF) may induce liver toxicity. Concomitant viral hepatitis B and C in IBD is also a relevant issue, particularly hepatitis B reactivation under immunosuppressive therapy; however, the recent introduction of potent antiviral drugs for both the infections and the spread of the anti-hepatitis B virus vaccine (HBV) contributed to significantly lower the risk. The diagnosis of such hepatobiliary conditions is of great importance, since they may influence the management and therapeutic approach to IBD, contraindicate the use of some therapies, or prevent the evolution towards the end stage of liver disease. The main hepatobiliary disorders, which are discussed in this review, are summarized in Table 1. A proposed practical approach to abnormal liver function tests (LFT) in a patient with IBD is presented in Figure 1. progressive disease, leading to liver biliary cirrhosis and portal hypertension.
Epidemiology: According to a recent systematic review, the incidence and prevalence rates of PSC range from 0 to 1.3 per 100.000 inhabitants/year and from 0 to 16.2 per 100.000 inhabitants, respectively. There is a 2:1 male predominance and a peak of incidence between 30 to 40 years old[1]. PSC is commonly associated with IBD, with about 50%-80% of patients with PSC having concomitant IBD, more frequently UC [2], and about 2%-8% of patients with IBD having PSC [3]. PSC diagnosis usually precedes that of IBD, although PSC may be diagnosed many years after proctocolectomy for colitis [4].

AIH
AIH is an immune-mediated chronic liver disease characterized by hepatocellular inflammation, necrosis and progression to cirrhosis. The clinical presentation varies from persistent mild elevation of AST and ALT to fulminant forms of acute hepatitis. Mean age at presentation shows a bimodal pattern with one peak during childhood/teenage years and another between the 4 th and 6 th decade of life. The diagnosis of AIH must be suspected in presence of autoantibodies (mainly antinuclear, smooth muscle, soluble liver antigen/liver pancreas and liver/kidney microsomal type 1 antibodies), IgG elevation, consistent liver histology and exclusion of other forms of hepatitis [52]. Despite most of the data about AIH/IBD coexistence comes from studies focusing on PSC and AIH/PSC overlap syndrome, a higher prevalence of AIH has been found in patients with IBD, compared to subjects without IBD.

Pyogenic liver abscess
Pyogenic liver abscesses are rarely seen in IBD, with only a few cases reported in literature, mainly in CD. A nationwide case-cohort study from Taiwan reported an incidence of pyogenic liver abscess in IBD patients of 6.7 cases/10000 persons/year, which was significantly higher compared to controls without IBD

DRUG-INDUCED LIVER DISEASE IN IBD
The therapeutic armamentarium for the treatment of IBD is gradually expanding. This certainly offers greater potential for therapeutic benefit, but the risk of hepatotoxicity is a concern. Although the overall risk of serious adverse events is low, cases of druginduced liver injury (DILI) have been reported for most drugs used in IBD, and some therapies carry a significant risk of liver toxicity. DILI induced by IBD drugs can be allergic/idiosyncratic (dose-independent) or related to hepatotoxins (typically dosedependent). In addition, some drugs can cause hepatotoxicity with more than one pathogenic mechanism. According to EASL guidelines, the exclusion of other causes of hepatotoxicity is necessary for the diagnosis of DILI, and recovery after drug discontinuation is an important criterion for the causality assessment[101] (Figure 1). The following paragraphs will describe the association between the main drugs used in IBD and the risk and type of DILI.

Aminosalicylates
Sulfasalazine was the first aminosalicylate approved for the induction and maintenance of remission in mild-to-moderate UC. Within the bowel, sulfasalazine is cleaved into sulfapyridine and 5-aminosalicylic acid, most called mesalamine. Sulfapyridine, a sulfa-containing antibacterial agent, is then absorbed from the colon into the bloodstream, transported to the liver, and acetylated; acetylation was reported to be genetically programmed, with slow acetylators having higher levels of free sulfasalazine and more drug-induced adverse events [102]. Mesalamine is minimally absorbed and largely excreted in the stools and is primarily responsible for the antiinflammatory effect on the colon. Thiopurine-related DILI has been related to thiopurine metabolites. After absorption, AZA is metabolized in the liver to 6-MP, which undergo a complex metabolization by three enzymes; one of them is the thiopurine S-methyltransferase (TPMT), that lead to 6-methylmercaptopurine (6-MMP) formation. 6-MMP is a non-effective metabolite which is important in hepatotoxicity development [109]. Approximately 15%-20% of IBD patients treated with thiopurines demonstrate hypermethylation (or shunting), a phenomenon due to a high TPMT activity that leads to preferential methylation of 6-MP to 6-MMP over bioactivation to thioguanine nucleotides (TGNs); the usual definition of hypermethylation is a ratio of 6-MMP to TGNs of > 11. Subtherapeutic TGNs level results in a poor response to therapy, while a high 6-MMP level (> 5700 pmol/8 × 10 8 erythrocytes) has been correlated with a 3-fold increased risk of liver toxicity [115]. Allopurinol is a xanthine oxidase inhibitor that prevents the breakdown of thiopurines into thiouric acid (TUA), thus increasing the bioavailability of 6-MP. Several studies have demonstrated that the combination of low dose thiopurine, i.e. 25%-50% of the standard dose, with 100 mg of allopurinol corrects hypermethylation in patients who have experienced thiopurines-induced hepatotoxicity or who have had a poor response to thiopurines treatment [116,117]. However, Shaye et al [118] showed that about 90% of patients with 6-MMP > 5700 pmol/8 × 10 8 erythrocytes have no hepatotoxicity and almost 40% of subjects with hepatotoxicity had 6-MMP levels below this cut-off [118]. Moreover, a recent case-control study and a meta-analysis failed to demonstrate any correlations between TPMT gene polymorphisms and hepatic adverse events in IBD patients [119,120]. The reported frequency of thiopurinerelated hepatotoxicity varies widely among studies, ranging from to 3% to 17%[108, 115,121,122]; a systematic review by Gisbert et al [113] reported a mean prevalence of thiopurine-induced liver injury of 3%, with a mean annual rate of 1.4% [113]. In a prospective cohort study, abnormal liver function (defined by ALT or ALP levels > 50% the upper normal limit) occurred in 13% of patients, while hepatotoxicity (defined by ALT or ALP levels greater than twice the upper normal limit) developed in 10% [111]. CD, liver steatosis and concomitant steroid therapy are reported risk factors for liver injury during thiopurine therapy [108,111,123]. It has been shown that most cases of thiopurine-induced liver injury completely resolved after dose reduction, while the need to discontinue therapy only occurred in about 3%-4% of cases[111, 118,124]. Switching from AZA to 6-MP in the case of AZA-induced DILI is a possible strategy, which is effective in resolving the liver toxicity in 71%-87% of cases [114,125]. Despite an optimal frequency has not yet been established, regular monitoring of blood tests should be performed for the entire duration of thiopurine treatment, more frequently in the first 3 mo of therapy[113, 126,127]. British Society of Gastroenterology (BSG) guidelines on IBD recommend the monitoring of full blood count and LFT at 2, 4, 8 and 12 wk of thiopurine therapy, and every 12 wk thereafter[127].

Methotrexate
Methotrexate (MTX) is a folic acid analogue with inhibitory activity against many enzymes in the metabolic pathway of folic acid. MTX inhibits production of thymidylate, purines, and methionine and leads to accumulation of adenosine, which has a potent anti-inflammatory activity. These actions inhibit cellular proliferation and tissue migration, and decrease production of inflammatory mediators [128]. MTX is currently indicated for the maintenance of remission in steroid-dependent CD [129], while its role in UC is still controversial [130]. The hepatotoxic potential of MTX is well known. A meta-analysis of clinical trials on IBD patients treated with MTX reported a pooled incidence rate of abnormal hepatic aminotransferase levels, which the author defined as up to a 2-fold increase over the upper limit of the normal, of 1.4 per 100 person-months. The rate of hepatotoxicity, defined as aminotransferase levels greater than a 2-fold over the upper normal limit, was 0.9 per 100 person-months. The rate of withdrawal of MTX due to these abnormalities was 0.8 per 100 person-months [112,131]. Alcohol intake is a main risk factor for MTX-induced hepatotoxicity and should be strictly avoided. Other potential risk factors are obesity, diabetes mellitus and chronic viral hepatitis [112,131,132]. Folic acid supplementation has been correlated with reduction of methotrexate-induced hepatic adverse events and is therefore recommended [133]. Regular liver chemistry tests are recommended for the monitoring of hepatotoxicity, every 2 wk for the first 2 mo and then every 2-3 mo [134]; the drug should be stopped if transaminases exceed twice the upper normal limit [127]. Although liver biopsy was previously indicated after an MTX-treatment cumulative dose ≥ 1.5 g, this practice is no longer recommended by current rheumatologic guidelines [134], this is based on recent evidence that show a low incidence of liver injury in patients receiving a chronic low dose of MTX [112]. In a retrospective study on 87 IBD patients with a mean MTX cumulative dose of 1813 mg, 76% of patients maintained normal liver chemistry tests throughout MTX therapy; a liver biopsy was performed in 11 patients after a cumulative dose ≥ 1.5 g and found no case of moderate or severe fibrosis [112]. Another study evaluating 20 liver biopsies after a cumulative methotrexate dose of ≥ 1.5 g (mean dose 2.6 g) found mild histological abnormalities in 95% of patients; abnormal liver chemistry tests were present in 30% of patients and did not correlate with histological toxicity [135]. However, liver biopsy should be performed in cases of persistent alteration of transaminases, especially in case of no reduction after lowering the drug dose. Transient elastography is a promising tool for the monitoring of liver fibrosis in MTX-treated patients and can be useful in selecting patients for liver biopsy [136].

Biological agents
Anti-TNF-α: Since its introduction in the 1990s, anti-TNF-α antibody therapy has revolutionized the treatment of IBD. Anti-TNFs, which include infliximab, adalimumab, golimumab and certolizumab pegol, are approved for the treatment of moderate-to-severe CD and UC and demonstrated high efficacy in the induction and maintenance of both clinical and endoscopic remission [91,103]. Several types of anti-TNF-related adverse events have been reported, mostly of infectious, auto-immune and tumoral types. DILI caused by anti-TNF is uncommon, mostly mild and related to infliximab. However, cases of liver failure requiring transplantation has rarely been reported [137][138][139]. Shelton et al [140] evaluated the incidence of liver enzyme elevation in a large cohort of IBD patients treated with anti-TNF: Only 102 out of 1753 patients (6%) developed ALT elevation, and in about half of cases this could clearly be linked to an alternative etiology. Infliximab was the involved anti-TNF in 96% cases. Compared to a control population of anti-TNF-treated patients without liver enzyme elevation, no difference in concomitant immunomodulator therapy, body max index, age and gender was found. The majority of patients with ALT elevation continued anti-TNF, most of them normalizing the liver enzyme during the follow-up. In 10 patients switching to a second anti-TNF was performed, without recurrence of liver injury [140]. Ghabril et al [141] identified 34 cases of DILI related to anti-TNF used for a variety of auto-immune conditions from a review of the United States DILI Network database and PubMed research. The drug presumed to have caused DILI was infliximab in 76% of cases. The liver injury was scored as mild-to-moderate in 93% of cases. Fifteen of the 17 patients undergoing liver biopsy showed clear features of autoimmunity. All patients improved after discontinuation of the anti-TNF [141]. The mechanism underlying liver toxicity remains to be elucidated. Infliximab-related hepatitis seems to be sustained by an immune-mediated mechanism, mimicking the characteristics of AIH type I, although a direct liver damage cannot be ruled out [112]. Currently, Food and Drug Administration classifies infliximab as a Most-DILI-concern drug, adalimumab as a Less-DILI-concern drug, and golimumab and certolizumab as Ambiguous-DILI-concern drug [142]. The current consensus recommends the use of infliximab in selected cases of patients with significant liver disease, and that treatment should be discontinued or avoided in patients with transaminases above three times the upper limit of normal [143]. December 27, 2021 Volume 13 Issue 12 Anti-integrins: Natalizumab is a monoclonal antibody that antagonizes both the integrins α4β1 and α4β7, which are necessary for the homing of lymphocytes to brain and gut, respectively. Natalizumab is therefore approved for the treatment of multiple sclerosis, and has been tested with good results in CD; however, the risk of JC virusassociated progressive multifocal leukoencephalopathy (PML) has limited its use in IBD [144][145][146]. Vedolizumab is a humanized monoclonal antibody which, unlike natalizumab, specifically inhibits α4β7 integrin, thus eliminating the risk of PML [147]. Vedolizumab is approved for the treatment of both moderate to severely active CD and UC since 2014 [91,103]. To date, only sporadic cases of liver injury during vedolizumab therapy have been reported [148,149]. In the prelicensure trials, three patients developed hepatitis, although it is unclear whether the increase in transaminases indicated drug-induced or autoimmune etiology [150]. In the GEMINI-1 and GEMINI-2 phase III trials, no differences in LFT were found compared to placebo [151,152]. Therefore, vedolizumab is considered almost free from liver toxicity.
Anti-interleukin 12/23: Ustekinumab is a human monoclonal antibody directed against the p40 subunit, which is a component of both interleukin (IL)-12 and IL-23, allowing this drug to simultaneously inhibit both these cytokines. Ustekinumab has been recently approved as a second line therapy for moderate-to-severe CD and UC since 2016 and 2019, respectively [153,154]. Although current data are limited, liver injury related to ustekinumab seems to be very uncommon. In the phase III trial on CD, a similar rate of adverse events compared to placebo was reported, with no mention of hepatotoxicity [153]. According to the Clinical and Research Information on DILI database, mild-to-moderate serum aminotransferase elevation was reported in 0.5% to 1.4% of patients during ustekinumab therapy. However, this event was no more frequent than placebo and resolved without discontinuing the drug [155]. Risankizumab is a monoclonal antibody directed against p19 subunit of IL-23 and therefore selectively inhibit this cytokine. Phase II and III trials in IBD are ongoing and safety data are still limited [156].

Tofacitinib
Tofacitinib is an oral Janus kinase inhibitor and is the first drug of this class approved for the treatment of IBD, specifically UC since May 2018 [157], while others are currently being tested in phase II and III trials [158]. Tofacitinib is indicated for the treatment of adult patients with moderately to severely active UC, who have had an inadequate response or who are intolerant to anti-TNF [127,158]. Data about hepatotoxic effects of tofacitinib mainly derive from rheumatoid arthritis, where a slight ALT elevation was reported in about 30% of patients, but elevation above 3 times the upper normal limit occurred in 1%-2% of patients [159][160][161]. Data regarding tofacitinib-induced liver toxicity in IBD are still limited. However, no increased incidence of liver injury has been reported either in the pivotal trial or in subsequent real-life studies [162][163][164][165].

IBD AND VIRAL HEPATITIS B AND C
In literature, the reported prevalence of hepatitis B surface antigen (HBsAg) and anti-HBc positivity in IBD patients ranges from 0.6%-5.7% and from 1.6%-41.6%, respectively, depending on the geographic area considered [166]. Despite previous studies reported a higher prevalence of HBV positivity in IBD patients compared to the general population, more recent studies indicated an equal or lower prevalence which tends to decrease over time, suggesting that preventive measures like vaccination, use of disposable materials and implementation of transfusion safety programs are effective [166,167]. The risk of viral reactivation is a major concern in HBV patients treated with immunosuppressants. This event is closely related both to the stage of the infection and the type of immunosuppressive drug used. HBV reactivation, defined as the increase in HBV viremia of more than 1 Log10 IU/mL, is characterized by a broad spectrum of clinical manifestations, that range from viremia without clinically relevant manifestations to fulminant life-threatening hepatitis [168]. For this reason, both ECCO and BSG guidelines recommend hepatitis B screening immediately after diagnosis of IBD, checking for HBsAg, anti-HBs, and anti-HBc [127,169]. If screening was not performed at the time of diagnosis, it should be performed before immunosuppressive therapy initiation [127,169]. HBsAg-positive/anti-HBc-positive patients carry the higher risk of reactivation, and should receive potent anti-viral agents (nucleoside/nucleotide analogues with high barrier to resistance) such as tenofovir and entecavir. Prophylactic treatment should be started 3-4 wk before immunosuppressive therapy and continued until at least 12 mo after the end of treatment [169]. HBsAg-negative/anti-HBc-positive patients are considered to have occult infection and viral reactivation is rare in this group with types of immunosuppressants used in IBD; in this case, HBV viremia (HBV-DNA) should be checked every 2-3 mo during the treatment and antiviral treatment started when HBV-DNA is detected [169]. Hepatitis B vaccination in all seronegative patients at IBD diagnosis is recommended by ECCO guidelines [169], while BSG guidelines indicate vaccination in high-risk groups [127]. Anti-HBs level should be measured after vaccination to confirm response; however, a reduction in vaccination during immunosuppressive therapy (mainly immunomodulators and anti-TNF) has emerged from several studies [170] and a recent meta-analysis [171]. Indications for the management of the IBD patient undergoing immunosuppressive therapy according to HBV status are summarized in Table 2. Hepatitis C prevalence in IBD is similar to the general population [168]. The risk of HCV reactivation under immunosuppressive therapies used in IBD is low [172,173]. Small case series reported successful treatment of hepatitis C with direct-acting antiviral (DAA) in patients on anti-TNF therapy [170] and no drug-drug interaction between DAA and anti-TNF has emerged[174]; thus, concomitant treatment with DAA and anti-TNF seems to be safe, although more studies specifically addressing this setting are needed.

CONCLUSION
Hepatobiliary disorders are frequently seen in IBD, and PSC represents the most common of them. A broad spectrum of pathogenic mechanisms may underlie the disorders, ranging from autoimmune conditions, metabolic diseases, infections up to drug toxicity, and two or more diseases can co-exist in the same patient. Moreover, liver disease severity can range from mild, which only requires monitoring over time, to liver failure, that may require LT. A step-by-step approach to the IBD patient with abnormal LFTs is extremely important to make the correct diagnosis, prevent complications, and identify those cases that warrant early and aggressive treatment. Finally, the diagnostic complexity often requires a multidisciplinary management involving gastroenterologist and hepatologist.