Exploring the Use of Humanized Mouse Models in Drug Safety Evaluation

The purpose of drug development is to find new safe, effective drugs of high quality and stability to cure diseases and save lives. Developing a new drug is a very complex process, but it can be divided into preclinical research and clinical studies of a new drug. 

Preclinical research is the first challenge in drug development, and preclinical toxicity studies of new medicines are an essential part of the journey from the laboratory to the clinic. Since many studies closely related to the safety of subjects, such as drug distribution, reproductive toxicity, and carcinogenicity studies, cannot be performed on issues in clinical trials, animal experiments have an irreplaceable role. 

However, there are differences between animals and humans, safety studies cannot be conducted on animal models alone, and normal animals do not respond to drugs the same way as patients. The humanized mouse model solves this challenge to some extent.

Preclinical studies of new drugs include starting from compound screening to conducting various animal experiments, which generally involve pharmacological experiments, acute toxicity experiments, long-term toxicity experiments, pharmacokinetic experiments, teratogenic experiments, carcinogenic experiments, allergic experiments, etc. 

The test data that can be obtained in animals are completed before implementing human trials. These animal experiments are done on small animals, such as mice and rats, and large animals, such as beagle dogs and rhesus monkeys. Animal experimental data have to be sent to the State Food and Drug Administration for strict approval before entering clinical research is possible. 

The success rate from Compound Screening to entering clinical studies is only a few percent. Generic or modified drugs have a higher success rate but are subject to intellectual property restrictions. Medicilon is a preclinical CRO that provides pharmacokinetics, preclinical studies, compound screening services, medicinal chemistry services, drug safety evaluation, toxicology studies, compound activity screening, and other services.

Due to the significant differences between human and animal physiology, experimental results obtained using animal models sometimes cannot be applied to humans. Humanized mice are chimeric mice carrying human genes, cells, or tissues. Since these mice are chimeric with specific human genes, they overcome the drawback that conventional mice cannot replicate certain human diseases and are good tools for human disease model research. Therefore, the human-derived tumor tissue xenograft PDX mouse model can realize human-related safety and efficacy assessment of new drugs in animals, simulating human clinical trials.

1， Humanized mice for evaluating drug genotoxicity

Evaluation of drug genotoxicity is an essential part of drug toxicology research. 2-Amino-1-methyl-6-phenylimidazo［4,5-b］pyridine (PhIP) is a heterocyclic amine carcinogen, mainly activated by CYP1A2 metabolism, and its metabolites can form adducts with DNA and produce carcinogenicity. There are significant differences in the metabolic disposition of PhIP between experimental animals and humans. The murine family metabolizes it mainly to 4-OHPhIP, which relieves its toxicity, but in humans, it is primarily metabolized to the genotoxic N-OH PhIP. 

Researchers established CYP1A1 and CYP1A2 transgenic mouse models in 2005 to study the metabolism of PhIP. By detecting the metabolites of PhIP in mouse urine, it was found that PhIP was preferentially metabolized to N-OH PhIP in CYP1A2 transgenic mice and mainly to 4-OH PhIP in WT mice. Therefore, compared to experimental mice, CYP1A2 transgenic mice can more accurately assess the genotoxicity of heterocyclic amines such as PhIP to humans.

2， Humanized mice for evaluation of drug hepatotoxicity

In addition to genotoxicity, drug-induced hepatotoxicity is essential to drug development. Many drugs are eventually withdrawn from the market due to their hepatotoxicity, such as the anti-diabetic drug troglitazone, which was removed from the market because it caused severe liver damage. However, animal studies have not found hepatotoxicity, presumably due to genetic differences. Using a human liver chimeric mouse model, researchers have successfully reconstructed a model of liver injury caused by troglitazone and used it to study the mechanism of liver injury, which provides a new animal model for studying liver injury caused by other idiosyncratic drugs.

The hepatotoxicity of acetaminophen (APAP) in human liver chimeric mice was also investigated. The effects of APAP on protein expression and endogenous substance metabolism in the liver were systematically analyzed using proteomic and metabolomic approaches. Alterations were found in the face of proteins involved in fatty acid metabolism, glycolysis, and energy synthesis processes, as well as changes in the levels of endogenous metabolites involved in the tricarboxylic acid cycle in urine and plasma, which are consistent with the results of previous hepatotoxicity experiments in normal mice. Thus, the use of histological methods to investigate drug-induced hepatotoxicity in human liver chimeric mice is an essential reference for explaining the toxicological mechanisms of drugs. Although human liver chimeric mice do not fully express the full functions of the human liver, establishing this mouse model still has important implications for drug excretion and toxicological studies.

The humanized mouse model overcomes the species differences in drug metabolism studies and is essential for preclinical studies of new drugs in drug metabolism and toxicology studies. Humanized transgenic mouse models can specifically express drug-metabolizing enzymes in the liver and small intestine, allowing one to study better the role of a particular enzyme in the metabolism and toxicity of a drug. However, since only one or two enzymes can be expressed, the complex metabolism of drugs in vivo cannot be reflected at an overall level.

The human liver chimeric mouse liver has the properties of the human liver. It can express multiple drug-metabolizing enzymes at the same time, which can mimic the disposition of drugs in the human body and help to predict the in vivo essence of drugs. 

Human liver chimeric mice can be produced on a large scale overseas, and their liver replacement rate can reach 90%. Still, the data obtained cannot be used for new drug approval yet and must be fully recognized by official authorities such as the FDA. However, they are still crucial for drug research and can make predictions and references for drug development.

However, the human liver chimeric mouse model has some drawbacks and limitations, such as the background effect of residual hepatocytes on drug metabolism and the interaction between non-hepatic parenchymal cells of the mouse's liver and human hepatocytes are still unclear, the functional impact of the mouse's endocrine hormones on human hepatocytes and the bile secretion in the chimeric mouse liver need to be further investigated. However, these problems are believed to be solved in future studies so that the humanized mouse model can better serve human drug development.