Animal studies reactive metabolites

The mechanisms underlying hepatotoxicity from halothane remain unclear, but studies in animals have implicated the formation of reactive metabolites that either cause direct hepatocellular damage (eg, free radical intermediates) or initiate immune-mediated responses. With regard to the latter mechanism, serum from patients with halothane hepatitis contains a variety of autoantibodies against hepatic proteins, many of which are in a trifluoroacetylated form. These trifluoroacetylated proteins could be formed in the hepatocyte during the biotransformation of halothane by liver drug-metabolizing enzymes. However, TFA proteins have also been identified in the sera of patients who did not develop hepatitis after halothane anesthesia. 

At high doses the interaction with protein can cause immediate damage to the liver (acute hepatitis). At lower doses interaction with DNA will lead to mutations in the genetic code which can cause cancer. Laboratory studies in experimental animals like rats have revealed that aflatoxin Bi can cause liver cancer, and the products of the reactive metabolites of aflatoxin can be detected in the blood of these animals. Furthermore the same metabolites have also been detected in the blood and urine of humans eating fungus-contaminated food in China, for example. Most recently a correlation has been shown between exposure to these fungal toxins, as indicated by metabolites of aflatoxins bound to protein in blood samples, and liver cancer in humans in China. 

The hepatotoxicity of germander has been well documented. An animal study suggests that the hepatotoxicity resides in one or more reactive metabolites of its furano-diterpenoids (19). 

Acetaminophen provides another example of an analogous in vitro multi-hit process. Previously, most of the acetaminophen-induced hepatotoxicity research focused on the reactive metabolite of acetaminophen, which led to covalent protein modification (reviewed in Reference 76). Recently, it was realized that mitochondrial damage and oxidative stress may be the second-hit leading to acetaminophen-induced liver injury.7778 This is substantiated by animal studies where partial knockdown of SOD2 in rats, as well as mice heterozygous to SOD2 were more susceptible to acetaminophen-induced liver injury than wild type animals.79-80... 

A number of chemicals with demonstrable suppression of immune function produce this action via indirect effects. By and large, the approach that has been most frequently used to support an indirect mechanism of action is to show immune suppression after in vivo exposure but no immune suppression after in vitro exposure to relevant concentrations. One of the most often cited mechanisms for an indirect action is centered around the limited metabolic capabilities of immunocompetent cells and tissues. A number of chemicals have caused immune suppression when administered to animals but were essentially devoid of any potency when added directly to suspensions of lymphocytes and macrophages. Many of these chemicals are capable of being metabolized to reactive metabolites, including dime-thylnitrosamine, aflatoxin Bi, and carbon tetrachloride. Interestingly, a similar profile of activity (i.e., suppression after in vivo exposure but no activity after in vitro exposure) has been demonstrated with the potent immunosuppressive drug cyclophosphamide. With the exception of the PAHs, few chemicals have been demonstrated to be metabolized when added directly to immunocompetent cells in culture. A primary role for a reactive intermediate in the immune suppression by dimethylnitrosamine, aflatoxin Bi, carbon tetrachloride, and cyclophosphamide has been confirmed in studies in which these xenobiotics were incubated with suspensions of immunocompetent cells in the presence of metabolic activation systems (MASs). Examples of MASs include primary hepatocytes, liver microsomes, and liver homogenates. In most cases, confirmation of a primary role for a reactive metabolite has been provided by in vivo studies in which the metabolic capability was either enhanced or suppressed by the administration of an enzyme inducer or a metabolic inhibitor, respectively. 

Although formaldehyde is a normal intermediary cellular metabolite involved in the biosynthesis of purines, thymidine, and several amino acids, it is a highly reactive molecule that can be directly irritating to tissues with which it comes into contact. Human and animal studies indicate that formaldehyde, at appropriate exposure levels, can be irritating to the upper respiratory tract and eyes with inhalation exposure, to the skin with dermal exposure, and to die gastrointestinal tract with oral exposure. Reports of allergic dermal sensitization to fonnaldehyde are widespread and supported by results from animal studies, but the evidence that formaldehyde sensitizes the respiratory tract is less convincing. 

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