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).