2.2. Drugs-induced animal models
There are many drugs and their metabolites that are widely used to
develop hepatic injury in animal models (Table 2). These models are used
for analyzing the protection, effectiveness, and development of
hepatoprotective medicines. These are as follows:
2.2.1. Acetaminophen: Also known as paracetamol, it is widely
used as an antipyretic and analgesic and over-the-counter drug. Over
usage of acetaminophen causes liver damage and can be used in
experimental models to induce hepatotoxicity. Acetaminophen-induced
liver cirrhosis occurs at sole dose of 2g/kg given by oral route and
sole dose of 300mg/kg by intraperitoneal injection in rats and mice (45,
46). By microsomal enzymatic action, a metabolite
N-acetyl-p-benzoquinoneimine is produced which is extremely reactive in
nature. The binding of this metabolite with the proteins results in
lipid peroxidation and reduction in glutathione levels. Further, the
mitogen-activated protein kinase (MAPK) and c-Jun N-terminal kinase
(JNK) activation disorient mitochondrial function and elevate oxidative
stress resulting in hepatic fibrosis and necrosis. Chronic
administration of acetaminophen induces autophagy in hepatocytes in the
liver of mice, which might accelerate hepatic cell death (47, 48).
2.2.2. Isoniazid: It is an anti-tuberculosis drug and is also
called isonicotinic acid hydrazide (INH). It induces hepatic fibrosis at
200 mg/kg and 400 mg/kg with oral gavages for the duration of one week
in mice and rats respectively (49, 50). Monoacetyl hydrazine is
generated after metabolism by cytochrome P450, which causes depletion of
ATP in hepatocytes, and an imbalance of free radicals and antioxidant
enzymes. This leads to the development of liver cirrhosis in the animal
model (51). Rifampicin in combination with isoniazid interferes with
bilirubin uptake isoniazid clearance from hepatic sinusoidal cells,
resulting in hyperbilirubinemia, and jaundice without affecting
hepatocytes. When used together, the toxic metabolites i.e. hydrazine
and cytochrome 2E1 production causes oxidative stress and hepatocellular
injury. Therefore, INH (50mg/kg) and rifampicin (100mg/kg) daily dose
for a week can induce hepatotoxicity in rodents (52). Rabbits receiving
this combined dose showed elevation in serum alanine aminotransferase
(ALT) and serum alanine aspartate (AST) measures. The cytochrome P450
activation, co-transportation of sodium taurocholate, and bile formation
indicate that rifampicin enhanced hepatotoxicity caused by isoniazid
(53).
2.2.3. Tetracycline: Tetracycline induces liver injury at a
single dose of 200 mg/kg by intraperitoneal route in mice and rats (54,
55). Tetracycline act mainly via enhanced fatty acid uptake, fatty acid
β-oxidation, triglycerides and cholesterol synthesis, inhibition of
microsomal triglyceride transfer protein activity and hepatic
lipoprotein inhibition (56). It causes micro-vesicular steatosis by
increasing fatty acid infusion, followed by oxidative stress due to
lipid overload and attack on functioning proteins. The excess lipid in
the hepatic cytoplasm activates the long-chain fatty-acid-Coenzyme A
ligase-1 and stimulates the formation of triacylglycerol or fatty acid
transportation into mitochondria. This speeds up β-oxidation and causes
mitochondrial respiration chain stress along with high levels of ROS.
This leads to microvesicular steatosis, and hepatic cirrhosis in rodents
through the breakdown of fatty acids (57).
2.2.4. Doxorubicin: Doxorubicin, an anthracycline derivative
used as an antineoplastic agent in the treatment of leukemia, induces
hepatic fibrosis at 15 mg/kg injection via intraperitoneal route once a
week for 4 weeks in rats (58, 59). It has been reported that doxorubicin
induces hepatotoxicity in off-target cells in individuals who have been
on a therapeutic regimen for cancer. This drug causes oxidative stress
in liver cells leading to reactive oxygen generation, mitochondrial
dysfunction, and hepatic fibrosis and necrosis. The oxidative stress is
mediated via nuclear factor erythroid2-related factor-2/ heme
oxygenase-1(Nrf-2/HO-1) pathway. There is inflammation due to the ROS
generation. There are elevated levels of malondialdehyde via Sirtuin
1/forkhead box protein 1/Nuclear factor-ҡB (SIRT1/FOXO1/NF-κB) signaling
pathway. Mitochondrial dysfunction via deregulation of electron
transport chain, diminished ATP production and slashed hepatic stellate
cells and sinusoidal epithelial cells result in hepatic cellular damage
(60). Doxorubicin is metabolized by the NADPH-cytochrome P450 reductase
by the stimulation of a single-electron initiation reaction. This leads
to the generation of free radicals like hydrogen peroxide and superoxide
which reduces the antioxidant level and elevates the reactive oxygen
species resulting in hepatic vacuolation, bile duct hyperplasia, and
hepatic fibrosis (61).
2.2.5. Cisplatin: Cisplatin is an alkylating anti-neoplastic
agent used conventionally in the treatment of breast, uterine, ovarian,
and neuroblastoma tumors. It acts through membrane peroxidation,
interference in protein synthesis, and DNA damage (62). Simultaneously,
it was reported to cause hepatotoxicity by neutrophil infiltration,
hyperplasia, fibrosis, and hepatocellular necrosis (63). The antioxidant
parameters such as superoxide dismutase (SOD) and catalase (CAT) and
glutathione peroxidase (GSH) have been used for the evaluation of liver
functioning (64, 65).
2.2.6. Tamoxifen: It is an antiestrogen and is used for the
treatment of breast cancer. It induces hepatic injury at 45 mg/kg dose
daily administered by intraperitoneal route for 6 days in rats.
Tamoxifen, after metabolism, forms reactive hydroxyl radicals and lipid
peroxyl radicals that contribute to oxidative stress. Mitochondrial and
cell membrane dysfunction results in hepatic fibrosis (66, 67).
2.2.7. Carbamazepine: Carbamazepine induces hepatic cirrhosis
at dose of 400 mg/kg administered orally for a period of 4 days and a
dose of 800 mg/kg administered orally on the fifth day in mice (68, 69).
It leads to the generation of reactive metabolites by cytochrome P450.
An imbalance between antioxidant enzymes and free radicals causes lipid
peroxidation which stimulates macrophages. Release of pro-inflammatory
chemokines result an inflammatory response leading to liver cirrhosis
(70).
2.2.8. Sodium Valproate: Sodium valproate induces liver
cirrhosis at a dose of 500 mg/kg administered daily by oral gavage for 2
weeks in rats and mice (71, 72). It alters the expression pattern of
genes related to lipid transport leading to change in synthesis of
triacylglycerol, cholesterol and the metabolism of fatty acids. Abnormal
lipid metabolism and interference in the β-oxidation result in
mitochondrial malfunctioning and the release of free radicals. Free
radicals lead to a lipid peroxidation chain reaction and a reduction in
the antioxidant levels, thus inducing hepatic cirrhosis (73).
2.2.9. CNS stimulants: Cannabis, cocaine, heroin,
methamphetamine and anabolic androgenic steroids are other compounds
reported to cause hepatocellular carcinoma in individuals with adenoma
or cirrhosis. These agents successfully induce metabolism-associated
fatty liver disease. The Kava (Piper methysticum) has been reported to
have the highest hepatotoxic potential and can be used in rodents to
evaluate hepatocellular carcinoma (74, 75).