Chapter 355 Drug- and Toxin-Induced Liver Injury
Hepatic metabolism of drugs and toxins is mediated by a sequence of enzymatic reactions that in large part transform hydrophobic, less-soluble molecules into more nontoxic, hydrophilic compounds that can be readily excreted in urine or bile (Chapter 56). Relative liver size, liver blood flow, and extent of protein binding also influence drug metabolism. Phase 1 of the process involves enzymatic activation of the substrate to reactive intermediates containing a carboxyl, phenol, epoxide, or hydroxyl group. Mixed-function mono-oxygenase, cytochrome-c reductase, various hydrolases, and the cytochrome P450 (CYP) system are involved in this process. Nonspecific induction of these enzymatic pathways, which can occur during intercurrent viral infection, with starvation, and with administration of certain drugs such as anticonvulsants, can alter drug metabolism and increase the potential for hepatotoxicity. A single agent can be metabolized by >1 biochemical reaction. The reactive intermediates that are potentially damaging to the cell are enzymatically conjugated in phase 2 reactions with glucuronic acid, sulfate, acetate, glycine, or glutathione. Some drugs may be directly metabolized by these conjugating reactions without 1st undergoing phase 1 activation. Phase 3 is the energy-dependent excretion of drug metabolites and their conjugates by an array of membrane transporters such as the multiple drug resistant protein 1 (MDR-1).
The pathologic spectrum of drug-induced liver disease is extremely wide, is rarely specific, and can mimic other liver diseases (Table 355-1). Predictable hepatotoxins such as acetaminophen produce centrilobular necrosis of hepatocytes. Steatosis is an important feature of tetracycline (microvesicular) and ethanol (macrovesicular) toxicities. A cholestatic hepatitis can be observed, with injury caused by erythromycin estolate and chlorpromazine. Cholestasis without inflammation may be a toxic effect of estrogens and anabolic steroids. Use of oral contraceptives and androgens has also been associated with benign and malignant liver tumors. Some idiosyncratic drug reactions can produce mixed patterns of injury, with diffuse cholestasis and cell necrosis. Some herbal supplements have been associated with hepatic failure (Table 355-2). Chronic hepatitis has been associated with the use of methyldopa and nitrofurantoin.
Table 355-1 PATTERNS OF HEPATIC DRUG INJURY
DISEASE | DRUG |
---|---|
Centrilobular necrosis | Acetaminophen Halothane |
Microvesicular steatosis | Valproic acid |
Acute hepatitis | Isoniazid |
General hypersensitivity | Sulfonamides Phenytoin |
Fibrosis | Methotrexate |
Cholestasis | Chlorpromazine Erythromycin Estrogens |
Veno-occlusive disease | Irradiation plus busulfan Cyclophosphamide |
Portal and hepatic vein thrombosis | Estrogens Androgens |
Biliary sludge | Ceftriaxone |
Hepatic adenoma or hepatocellular carcinoma | Oral contraceptives Anabolic steroids |
Table 355-2 POTENTIALLY HEPATOTOXIC HERBAL OR DIETARY SUPPLEMENTS
Kava (Kava kava, awa, kew)
Chaparral (creosote bush, greasewood, Larrea tridentata)
Ma huang (Ephedra)
Comfrey leaves (pyrrolizidine alkaloids)
Germander extracts (Trucrium chamaedrys)
Valerian with skullcap
Mushroom (Amanita phalloides, Galerina)
LipoKinetix (phenylpropanolamine, sodium usinate, diiodothyronine, yohimbine, caffeine)
Treatment
Treatment of drug- or toxin-related liver injury is mainly supportive. Contact with the offending agent should be avoided. Corticosteroids might have a role in immune-mediated disease. N-Acetylcysteine therapy, by stimulating glutathione synthesis, is effective in preventing hepatotoxicity when administered within 16 hr after an acute overdose of acetaminophen and appears to improve survival in patients with severe liver injury even up to 36 hr after ingestion (Chapter 58). Intravenous L-carnitine may be of value in treating valproic acid–induced hepatotoxicity. Orthotopic liver transplantation may be required for treatment of drug- or toxin-induced hepatic failure.
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