Cyclophosphamide

Cyclo- phosphamide was discussed earlier in “Sinusoidal Obstruction Syndrome (Hepatic Venoocclusive Syndrome).” Metabolism by CYP2C9 and 3A4 in hepatocytes yields 4-hydroxy-cyclophosphamide, which appears in the circulation. 4-Hydroxycyclophosphamide equilibrates with aldophosphamide, which follows two pathways: spontaneous decomposition to phosphoramide mustard and acrolein and metabolism by aldehyde dehydrogenase 1 (ALD1) to carboxyethyl phosphoramide mustard. Phosphoramide mustard is the putative antineoplastic moiety and acrolein is the proximate toxic metabolite. There is wide interindividual variation in metabolism of intravenously administered cyclophosphamide, which may contribute to the risk of SOS.⁵¹ The presumptive mechanism of toxicity in SOS is through hepatocyte metabolism of cyclophosphamide and formation of acrolein, the proximate toxic metabolite.⁴¹ Acrolein is toxic to endothelial cells in general,^{103, 104} but toxicity is greatest in the sinusoidal endothelial cells because of their proximity to hepatocytes, which metabolize cyclophosphamide.

Cyclophosphamide at standard doses is an uncommon cause of liver toxicity, with few reported cases of liver test abnormalities and rare case reports of clinically significant liver disease with hepatocellular necrosis. SOS is seen almost exclusively at high doses of cyclophosphamide and in conjunction with synergistic agents, such as busulfan, total-body irradiation, or BCNU. The incidence of SOS in high-dose regimens that contain cyclophosphamide is often in the range of 20% to 40%.

Busulfan

Busulfan is a weak alkylating agent. Experimental studies with busulfan have been difficult to interpret as the doses needed in vitro to induce either interstrand cross-linking or toxicity have been significantly above therapeutic plasma concentrations. Busulfan toxicity requires glutathione S-transferase-mediated conjugation to glutathione, which leads to oxidative stress.⁸⁴ In an environment high in glutathione (eg, within the hepatocyte) oxidative stress seems to be a result of the busulfan- glutathione conjugate itself, whereas in a low glutathione environment, the oxidative stress is a result of profound glutathione depletion.

Busulfan significantly depletes whole-liver glutathione levels in vivo at doses comparable to those used in hematopoietic stem cell transplantation.⁸⁴ Glutathione is diminished to a similar degree in hepatocytes and sinusoidal endothelial cell glutathione in vitro. The synergistic effect of busulfan on liver toxicity by conditioning regimens may be caused by glutathione depletion, oxidative stress, or both. Much of the toxicity associated with busulfan is a result of synergism with drugs such as cyclophosphamide or melphalan, which are both glutathione detoxified.

As a single agent, liver toxicity caused by high-dose busulfan is described as cholestatic,⁷⁸ but only two case reports have described cholestatic liver disease at standard chemotherapeutic doses. The literature on high-dose busulfan as a single agent is too limited to comment on whether it can induce SOS at all, but the risk seems to be low,⁷⁸ and it is certainly less than with dimethylbusulfan alone.^{105, 106} This may reflect differences in cellular toxicity: busulfan is equally toxic to hepatocytes and sinusoidal endothelial cells, whereas dimethylbusulfan toxicity is selective for sinusoidal endothelial cells (L.D. DeLeve, unpublished observation).

Dacarbazine

Dacarbazine is an inactive prodrug that can only be activated in the liver by microsomal metabolism. The initial microsomal metabolite transforms spontaneously through several steps to the proximate methyl-donating metabolite, either a methylcarbonium or a methyldiazonium ion. This drug is selectively toxic to sinusoidal endothelial cells, which can metabolically activate it, and it is detoxified by glutathione.⁶³ Dacarbazine-induced SOS has been reported in at least 15 case reports. The disease varies from the usual presentation of SOS in that it is associated with peripheral eosinophilia and thrombosis of the central venules and veins. The peripheral eosinophilia has been seen after the first exposure to dacarbazine, which suggests that this is related to the toxicity rather than implicating a hypersensitivity reaction.

Melphalan

At standard chemotherapeutic doses, melphalan is not hepatotoxic. As a single agent, high-dose melphalan (140 mg/m²) is associated with either mild, transient elevations of serum aminotransferase and bilirubin⁷⁹ or no abnormalities at all.^{80, 81} In high-dose conditioning regimens, it is associated with SOS.

Methotrexate

Concern about methotrexate-induced fibrosis was raised by a much-cited 1960 study of 273 children treated for acute leukemia.¹⁰⁷ The study described a 31% incidence of fibrosis in autopsy cases prior to the use of chemotherapy between 1940 and 1947, and an increase to 78% incidence of fibrosis between 1948 and 1951 when the folic acid antagonists aminopterin and methotrexate were used. This frequency of liver injury has not been reproduced in recent decades. In fact, more recent studies have not found a significant incidence of liver injury (see below).

In high-dose methotrexate therapy for osteosarcoma, childhood acute lymphocytic leukemia, and adult non-Hodgkin's lymphomas, methotrexate may be infused in doses of 3 to 15 g/m². During maintenance therapy, weekly oral doses of low-dose methotrexate may be interspersed with additional infusions of high-dose methotrexate. Thus very large cumulative doses can be achieved. Although high-dose methotrexate has a high incidence of transient elevation of aminotransferase, the current literature suggests that this does not result in chronic liver disease.^{23, 108–110} However, the incidence of fibrosis in this population may be underestimated. Liver tests correlate poorly with histologic abnormalities and clinical studies using high-dose methotrexate have not routinely performed liver biopsies after treatment with significant cumulative doses of methotrexate. Thus, clinically asymptomatic fibrosis likely goes undiagnosed. Given the significant numbers of long-term disease-free survivors, long-term toxicity is a concern. Two case reports of hepatocellular carcinoma following methotrexate-induced fibrosis were reported in 1977 and 1987^{111, 112}; it should be noted that these reports preceded hepatitis C testing in leukemia patients, a population with a high incidence of hepatitis C. In a large meta-analysis, patients with psoriasis and rheumatoid arthritis had a 7% chance of progression of histologic abnormalities on liver biopsy for every gram of methotrexate,¹¹³ although some of the cases of methotrexate toxicity might actually be nonalcoholic steatohepatitis.¹¹⁴ Even if one assumes that asymptomatic fibrosis goes undiagnosed if systematic biopsies are not done, given the very high cumulative doses of methotrexate administered in cancer therapy, the incidence of significant liver injury would seem to be much lower than that suggested by the literature for rheumatoid arthritis and psoriasis.

Thiopurines

Liver injury from 6-mercaptopurine usually presents with jaundice that may be accompanied by pruritus. The injury is most commonly cholestatic, but hepatocellular necrosis may also be present. Although liver injury caused by 6-mercaptopurine has been frequently reported, the actual incidence of toxicity is difficult to estimate, but may be lower than suggested by early studies. In current regimens for acute lymphoblastic leukemia, 6-mercaptopurine is used within multidrug regimens, usually with other potential hepatotoxins such at l-asparaginase, cytarabine, and methotrexate. Liver injury is not singled out as a major toxicity in most of the larger studies, and when it does occur, it cannot be attributed to any particular drug in the regimen. Only one clear-cut case of drug-induced cholestatic hepatitis was found in an 18-year follow-up study of 396 patients with inflammatory bowel disease treated with 6-mercaptopurine (50 mg/d or 1.5 mg/kg).¹¹⁵

An injury described as SOS has been linked to 6-thioguanine, often in combination with cytarabine.^116–119 These cases have most often resembled radiation-induced liver disease (RILD) in some of their clinical and histologic features. Patients present with hepatomegaly that is sometimes described as painful, and ascites, but without jaundice. Bilirubin is often normal or marginally elevated, as is seen in RILD. There are venoocclusive lesions and centrilobular congestion, but centrilobular hepatocyte atrophy is described rather than necrosis. Several of the cases also had underlying cirrhosis. There is also a case report of peliosis hepatis associated with 6-thioguanine plus cytarabine.¹²⁰

It is not surprising that 6-thioguanine would be linked to this group of liver injuries. SOS, nodular regenerative hyperplasia, peliosis hepatis, and sinusoidal dilatation often share similar causes, and in some cases, up to all four have been described within the same liver. Overlap of these forms of liver injury is seen with azathioprine (all four lesions), urethane (peliosis and SOS), heroin (sinusoidal dilatation, sinusoidal and perivenular fibrosis), thorotrast (peliosis, venoocclusive lesions with hepatocyte atrophy), and oral contraceptives (sinusoidal dilatation and peliosis hepatis). In patients with acquired immunodeficiency syndrome infected with Bartonella spp, the initial change in peliosis hepatitis is sinusoidal dilatation caused by damage to sinusoidal endothelial cells. Damage to sinusoidal endothelial cells, and sometimes to hepatic venular endothelial cells, also seems to be the common link in the drug-induced cases of these four types of liver injury.^{64, 100, 121–124}

When 6-mercaptopurine or azathioprine is given orally, it is subject to extensive first-pass metabolism in the intestine and liver by xanthine oxidase, whereas in target tissues the activity is determined by the balance between detoxification by thiopurine S-methyltransferase (TPMT) activity and toxification by hypoxanthine phosphoribosyltransferase. Patients treated with thiopurines who have low TPMT activity are at increased risk for hematologic toxicity (see review in ref. 125). However, a different mechanism may apply for hepatitis caused by oral administration of these thiopurines. Studies in four patients with 6-mercaptopurine hepatitis and in two patients with azathioprine hepatitis,^{125, 126} demonstrated TPMT levels in erythrocytes that were normal (erythrocyte TPMT normally correlates well with activity in the liver). In four cases of hepatitis from oral 6-mercaptopurine in children with acute lymphoblastic leukemia, average peak levels and AUC of 6-mercaptopurine were significantly lower than in controls. This suggests that increased first-pass clearance of orally administered thiopurines by the liver may predispose to accumulation of toxic concentrations in the liver.¹²⁶ First-pass metabolism of oral 6-mercaptopurine is subject to large interindividual variation but the mechanism has not been determined.¹²⁷

Fluorodeoxyuridine

Fluorodeoxyuridine or floxuridine (FUDR) infused into the hepatic artery for chemotherapy of hepatic metastases can cause sclerosing cholangitis. The incidence varies widely in the literature. In a recent study of 32 patients who received an average of 7.3 cycles of chemotherapy, the incidence of liver test abnormalities was 15.6%, and of severe biliary sclerosis was 9.3%.¹²⁸ In another recent study in which 38 patients received two cycles of FUDR plus dexamethasone, 22% had elevations of aspartate aminotransferase and/or alkaline phosphatase, and 7% had bilirubin elevations greater than three times the upper limit of normal.¹²⁹ Concomitant treatment with dexamethasone has been suggested to reduce liver injury from intraarterial FUDR, but this is not well established.^129–131 Cholangiogram often shows a high-grade obstruction of the common hepatic duct, which may extend into the left and right hepatic ducts. Most commonly, there are also multiple strictures of the intrahepatic ducts. The strictures are thought to be a result of ischemia secondary to drug-induced damage to the peribiliary vascular plexus. The histologic changes in the hilar vessels are suggestive of organization of occlusive thrombi. The toxicity may be reversible upon discontinuation of therapy. Patients with rising bilirubin levels, pruritus, or sepsis may be successfully palliated with percutaneous transhepatic biliary drainage.

l-Asparaginase

l-Asparaginase is used in the treatment of acute lymphoblastic leukemia. It is a microbial product derived from Escherichia coli or from Erwinia chrysanthemi. Liver toxicity is thought to be caused by inhibition of protein synthesis because of the asparaginase and glutaminase activity. Interference with hepatic protein synthesis is manifested by decreased serum albumin, coagulation factors, and lipoproteins. In the past a particularly high incidence of liver toxicity was described with elevated alkaline phosphatase and serum aminotransferase and depression of liver synthetic function; 40% to 87% of patients were found to have hepatic steatosis on autopsy, and this could be detected up to 9 months after the last dose of l-asparaginase.^{132, 133} The incidence of frank liver disease has decreased with the use of lower doses, although severe and fatal cases are still reported. In adults, the incidence of transient World Health Organization (WHO) grade I or II liver test (Table 154-5) abnormalities is approximately 50%,¹³⁴ with few cases of significant liver test abnormalities.^{134, 135}

Table 154-5

WHO Grades of Liver Test Abnormalities .

A recent study of 245 patients with acute lymphoblastic leukemia randomized to conventional-dose or high-dose l-asparaginase as part of a multidrug regimen found antithrombin III levels less than 50% in 0% and 2.5%, antithrombin III levels of 50% to 70% in 1.7% and 10.3%, and fibrinogen less than 100 mg/dL in 8.4% and 10.3% of patients, respectively, in the conventional- and high-dose l-asparaginase treatment groups.¹³⁶ Liver test abnormalities and liver injury were not listed among the major toxicity reactions. Another recent study of 377 patients with acute lymphoblastic leukemia treated with a regimen that included l- asparaginase, liver test abnormalities, and liver injury were not listed among the more frequent toxicities; among a subgroup of 43 patients in this study who did not complete the full 30-week treatment regimen, 2% developed hepatitis.¹³⁶

Actinomycin D

Actinomycin D (or dactinomycin) is associated with SOS. There seems to be some degree of synergy with abdominal irradiation and the risk may correlate with the dose of radiation and perhaps also the dose of actinomycin D.¹³⁷ The disease also occurs following chemotherapy with actinomycin D and vincristine in the absence of radiation. The incidence of SOS complicating chemotherapy for left-side Wilms tumors and for rhabdomyosarcoma is low.^{138, 139}

Most of the reported cases of actinomycin D have been in patients with right-sided Wilms tumors.¹³⁹ One series noted that liver toxicity occurred in particularly large right-sided tumors.¹⁴⁰ One must consider the possibility that mass effect from these large right-sided nephroblastomata is impeding hepatic venous outflow. Another contributing factor may be intravascular extension of the nephroblastoma, causing Budd-Chiari syndrome, which may be underdiagnosed in the older literature.¹⁴¹ In one study, intravascular extension of tumor was diagnosed in only 40% of cases by ultrasonography, which the authors attributed to the lesser imaging quality of older ultrasonography equipment in their early cases.¹⁴¹ Budd-Chiari syndrome and SOS share many of the same clinical and histologic features. The unanswered question is whether hepatic venous outflow obstruction acts in concert with actinomycin D-induced SOS or whether the increased incidence of SOS with right-sided Wilms tumors is partially a result of undiagnosed Budd-Chiari syndrome.

Tamoxifen

Tamoxifen is a nonsteroidal drug with both antiestrogenic and estrogenic properties, which is widely used as a chemopreventive for breast cancer. It is metabolized by CYP3A4. Liver injury associated with tamoxifen includes nonalcoholic fatty liver disease,^156–161 peliosis hepatis,¹⁶² hepatic insufficiency,¹⁶³ and, perhaps, hepatocellular cancer.^{164, 165}

Nonalcoholic fatty liver disease is the most common form of liver injury caused by tamoxifen. A Japanese study that screened 105 women on tamoxifen by annual abdominal computed tomography examination found a 38% incidence of fatty liver, which developed during the first 2 years of therapy in 35 of 40 cases (85%).¹⁶¹ Of the patients with fatty liver, 40% (16 of 40) had sustained elevations of aminotransferases. In addition there are three reported cases of cirrhosis in the presence of steatohepatitis by liver biopsy.^{157, 159} Future studies need to determine whether there is significant risk of steatohepatitis in tamoxifen users to warrant routine screening for nonalcoholic fatty liver disease.

Amiodarone, perhexiline maleate, and diethylaminoethoxyhexestrone (coralgil) are three drugs that cause steatohepatitis because of their physicochemical characteristics. Tamoxifen shares these same structural features and likely causes steatohepatitis through the same mechanism.¹⁶⁶ These compounds are cationic amphiphilic compounds that are lipophilic and that have an amine that can become protonated. The unprotonated, lipophilic form easily crosses the mitochondrial outer membrane. In the acidic intermembranous space of the mitochondrion, the amine is protonated and the positively charged form moves across the mitochondrial inner membrane. The compounds are trapped inside the mitochondria and accumulate there. The accumulated drug inhibits mitochondrial β-oxidation (causing steatosis) and blocks electron transfer along the respiratory chain.¹⁶⁶

Rats treated with tamoxifen develop nodular regenerative hyperplasia, hepatic adenomas, and hepatocellular carcinomas.¹⁶⁷ There are substantial differences between rats and humans in the rate of tamoxifen metabolism and in the metabolites formed. To achieve clinically relevant serum concentrations, rats must be given high doses, which result in very high liver concentrations of tamoxifen and its metabolites.^{168, 169} Rats are also more susceptible to liver DNA damage than are women, albeit at liver concentrations that are much higher than those seen in women.^{169, 170} The propensity for hepatocellular cancer in rats may prove to be very species specific, because liver tumors are not found in mice or hamsters. There is currently no evidence in humans of a significant increase in the incidence of liver cancer (see review in ref. 167). Given the expanded indications for use of tamoxifen, future clinical studies will undoubtedly carefully monitor the risk of hepatocellular carcinoma.

Cyproterone Acetate

Cyproterone acetate is a synthetic progesterone derivative with antiandrogenic and progesterone-like activity. It is used in the treatment of advanced prostatic cancer. In a large surveillance study of 1,685 patients receiving cyproterone acetate for indications other than prostatic cancer, elevated liver tests were noted in 10% of patients treated with 50 mg/d and 20% of those receiving > 100 mg/d.¹⁷¹ A retrospective analysis of 78 patients receiving 50 mg/d of cyproterone acetate for advanced prostatic cancer reported elevation of alkaline phosphatase in 14% of patients without known liver involvement and elevated aminotransferases in 2.5%.¹⁷² This study did not report any cases of hepatitis. There are 18 case reports of cyproterone-associated hepatitis with 6 fatalities. In a review article,¹⁷¹ a report was cited of 96 hepatotoxic events with 33 fatalities attributed to cyproterone acetate¹⁷³; however, I have been unable to obtain this report. There has also been one report of cirrhosis in a pediatric patient treated for precocious puberty.¹⁷⁴

Cyproterone acetate is mitogenic, tumorigenic and induces DNA-adducts and DNA-repair synthesis in rat liver (see reviews in refs. 175 and 176). High levels of DNA-adducts are also formed in human hepatocytes from both genders. Formation of the reactive metabolite does not appear to be mediated by P450, but it has been suggested that metabolic activation might be through hydroxysteroid-sulfotransferases. Although long-term exposure to cyproterone acetate at high doses has the potential for inducing hepatocellular carcinomas, it would seem unlikely to be clinically relevant given the current life expectancy of patients with advanced prostate cancer.

Flutamide

In a multicenter study of 905 patients with total androgen blockade, flutamide was discontinued in 0.8% of patients because of abnormal liver tests (elevation greater than four times the upper limit of normal).¹⁷⁷ Based on postmarketing surveillance, severe liver disease caused by flutamide has occurred in 46 patients with 20 fatalities.¹⁷⁸ Based on estimated prescriptions, the rate of serious liver injury is 3 per 10,000 flutamide users. Liver disease presents with marked elevations of bilirubin and a wide range in elevation of serum aminotransferases. The predominant histologic feature on autopsy is the marked to massive hepatic necrosis.

Flutamide is a synthetic nonsteroidal that is a competitive antagonist of the androgen receptor. After oral administration, flutamide undergoes extensive first-pass metabolism with formation of several oxidized metabolites. Metabolism through CYP3A and CYP1A leads to formation of electrophilic metabolites.¹⁷⁹ Experimental studies in rat hepatocytes suggest that the initial toxic event is through formation of electrophilic metabolites, which deplete hepatocyte glutathione.¹⁸⁰ The depletion of glutathione is accompanied by oxidative stress. Flutamide was also shown to be toxic at the mitochondrial level, with depression of mitochondrial respiration and ATP formation, but the study did not report whether mitochondrial toxicity was a result of depletion of mitochondrial glutathione.