2.1 Chemical-based animal models
Various chemicals can induce liver cirrhosis (Table 1) and are used to study the safety and efficacy of hepatoprotective drugs. These chemicals initiate liver cirrhosis in less time when given through the intraperitoneal route as compared to the oral and inhalation routes (26). The chemicals used to inducer liver cirrhosis in animals are:
2.1.1. Carbon tetrachloride: It is also known as tetrachloromethane, is used in dry cleaning, manufacturing of refrigerants and fire extinguishers. Carbon tetrachloride combined with maize oil at a ratio of 1:1 (v/v) produces liver cirrhosis in rats when given intraperitoneally two to three times per week for four to six weeks at a dose of 1ml/kg body weight. This solution can also be given at two distinct doses of 2 ml/kg in mice and 4 ml/kg in rats for a period of 10 weeks orally (27). Cytochrome P450 mono-oxygenase specifically CYP2E1 metabolizes carbon tetrachloride to trichloromethyl radical. It is very reactive and alkylates the lipids, proteins, and nucleic acid further causing damage to mitochondria and producing oxidative stress. Further, trichloromethyl radical leads to the formation of trichloromethyl peroxy radicals by reacting with molecular oxygen that initiates lipid peroxidation chain reaction damaging cell membranes and induces cirrhosis by causing liver injury in the animal models (28).
2.1.2. Ethanol: Ethanol-induced cirrhosis can be caused by 7.9 g/kg dose of ethanol for six weeks with oral gavages for mice and rats (29). Ethanol is metabolized by the microsomal enzyme CYP2E1, which causes change in the composition of the phospholipid membranes by enhancing lipid peroxidation. Due to free radical formation, there are reduced levels of antioxidant enzymes resulting in increase in glutathione level in the liver (30). This causes damage to the structure of the phospholipids membrane and apoptosis of hepatic cells, leading to cirrhosis (31, 32).
2.1.3. Thioacetamide: Thioacetamide-induced cirrhosis occurs at a dose of 200 mg/kg thrice a week through intraperitoneal injection for a period of 8 weeks in mice and rats. It is metabolized by the cytochrome P450 to sulfine leading to an increase in both volume of the nucleus and intracellular concentration of calcium which modifies cell permeability and causes dysfunction of mitochondria. Cellular oxidative stress is caused by an imbalance between antioxidant enzymes resulting from the enhanced generation of reactive oxygen species in cells or tissues. Eventually, excessive oxygen radicals damage DNA, proteins, and membranes by altering enzyme activity that induces cirrhosis in rodents (33-35).
2.1.4. Dimethyl nitrosamine: Dimethyl nitrosamine leads to hepatic fibrosis at a dose of 10 mg/kg by intraperitoneal route twice a week for a period of 4 weeks in mice and rats (36, 37). After the metabolism by microsomal enzymes, there is production of methylation moieties which attach with nucleic acids causing DNA damage and protein denaturation. It induces fibrosis in animal models by stimulating Kupffer cells, hepatic stellate cells, and profibrotic cytokines to change into myofibroblasts, which results to collagen deposition and create nodules in the liver (38).
2.1.5. Lead nitrate: Lead nitrate induces hepatotoxicity at the dose of 50 mg/kg each day by oral administration for 40 days in mice and at the dose of 5 mg/kg through intraperitoneal administration for 30 days in rats (39, 40). It leads to liver injury by initiation of lipid peroxidation that targets the sheath of cells in hepatocytes. This results in depletion of antioxidant enzymes and generates free radicals which finally causes liver cirrhosis in animals (41).
2.1.6. D-galactosamine: D-galactosamine induces liver fibrosis at 500 mg/kg when given thrice a week through the intraperitoneal route for 12 weeks in mice. The rats treated with 0.5ml of normal saline daily for 3 weeks and 800 mg/kg of single dose of D-galactosamine on the 21st day developed hepatic cirrhosis in rodents (42, 43). It leads to the reduction of uridine triphosphate (UTP) which ceases the synthesis of proteins. There is production of reactive oxygen species which causes increased oxidative stress along with the apoptosis of hepatocytes. As a result, there is stimulation of cycloxygenase-2 and nitric oxide synthase 1 which promote the release of inflammatory mediators, chemokines and prostaglandins. Imbalance of antioxidant enzymes increases the permeability of cells. In addition high free radicals levels result in degeneration of cells and collagen formation. This leads to liver fibrosis in animals (44).