Mice with "humanized" livers will help in drug development
Researchers at Stanford University Medical School, working under the guidance of Professor Gary Peltz, used genetically modified mice whose liver is partially composed of human cells to study the metabolism of the experimental drug clemizole, intended for the treatment of hepatitis C, the first stage of clinical development of which will begin in the near future. This approach made it possible to evaluate significant aspects of the drug's behavior in the human body, including its interaction with other drugs, as well as the profile of the main cleavage products (metabolites), much more accurately than modern animal testing methods allow.
According to Professor Peltz, experimental drugs that demonstrate excellent results in preclinical studies on animal models often fail miserably already at the first stages of clinical trials. This is due to unforeseen security issues. Often the cause is not even the drug itself, but one of its metabolites.
Another problem that complicates the development of medicines is unforeseen drug interactions. The drug can prolong or enhance the effect of another drug, for example, by influencing its metabolism. Considering that more than 30% of people over the age of 57 take 5 or more medications in parallel, this aspect cannot be neglected.
The analyzed drug clemizole was widely used in the 50-60 years of the last century as an antihistamine. The reason for the rejection of clemizole was the appearance of more effective drugs, as well as its tendency to accumulate in the liver, which is not a virtue for a general-purpose drug. Later, studies showed that clemizole has a very useful quality – it blocks the replication of the currently incurable hepatitis C virus.
Clemizole is safe and is quite inexpensive. However, it was approved even before the modern requirements for testing new drugs came into force, so very little is known about the metabolism of this drug and its interactions with other drugs in the human body.
Currently, before the start of clinical trials, the tolerability of all new drugs and their ability to cause side effects is studied in carefully planned experiments on animals, such as rodents. However, the mouse liver metabolizes chemical compounds differently from the human liver. Cleavage of the same drug by different liver enzyme complexes leads to the appearance of different metabolites or different concentrations of the same metabolites.
Earlier attempts were made to solve this problem by creating so-called chimeric mice with "humanized" livers. To do this, the liver tissue of animals was at least partially replaced with human hepatocytes. The tested methods involved the introduction of toxins or the creation of genetic defects, leading to the partial destruction of the animals' own liver. However, persistent organ dysfunction triggered by such interventions made it difficult for human cells to take root and grow, or raised doubts about the adequacy of the results of studying the metabolism of the drug.
Stanford scientists, together with Japanese colleagues from the Central Institute of Experimental Animals, approached this issue in a different way. In 2011, they created genetically modified mice whose livers can be humanized without triggering an irreversible process of toxic damage. Such animals are injected with a short-acting non-toxic dose of a drug that selectively triggers the processes of self-destruction of liver cells. Human hepatocytes implanted after the drug is removed from the body take root and participate in the restoration of a structurally and functionally complete liver. As a result, the preserved mouse cells continue to produce mouse metabolites, whereas human cells produce human metabolites.
The degree of "humanization" of the liver of such chimeric mice is determined by the concentration of the human version of albumin protein in the blood, and the mathematical algorithm developed by scientists allows us to calculate with a high degree of accuracy which metabolites and in what quantity are produced by mouse and human cells.
The authors studied the metabolism of clemizole in the human body, as well as in the body of mice of several conventional lines. The metabolic profiles of the drug were almost similar for all lines of mice, but they differed significantly from those observed in the blood of 10 people who took part in the study. Whereas in human blood the total amount of clemizole and its metabolites was represented by a single M1 metabolite by more than 50%, in the blood of ordinary mice M1 was detected only in trace amounts.
However, when the drug was administered to chimeric animals, M1 was produced in their body in quantities approximately proportional to the proportion of human cells that make up the liver. Moreover, it turned out that M1 has antiviral activity, which is a component of the total antiviral activity of clemizole in the human body. When studying the activity of the drug on ordinary mice, this fact would inevitably be overlooked.
After that, the researchers tested the suitability of chimeric mice to study the potential interactions of clemizole and other drugs. To do this, they chose the drug ritonavir, which interacts with a metabolic enzyme that plays an important role in the breakdown of many compounds in the human body. First, the chimeric mice were injected with clemizole, and later with a combination of clemizole and ritonavir. It turned out that simultaneous administration of ritonavir increased the concentration of clemizole in the blood of animals and prolonged the period of preservation of its increased concentration. A pilot clinical trial involving three volunteers confirmed the ability of ritonavir to increase the concentration of clemisole in human blood.
The results of the study indicate both the prospects of the new mouse model and the clinical potential of clemizole. The half-life of this drug in the body of ordinary mice is only 15 minutes, so the result of testing it using the traditional method could well be a conclusion about the inexpediency of clinical use of the drug, which must be taken 10-20 times a day. Moreover, if the main metabolite of clemisole M1 was toxic to humans, experiments on mice would not have revealed this side effect. On the other hand, if any of the mouse metabolites of the drug, which is not produced in the human body, had toxicity to animals, this would serve as a reason for refusing its further research.
The results of the work will soon be published in the online version of the journal Annals of Neurology.
Evgeniya Ryabtseva
Portal "Eternal youth" http://vechnayamolodost.ru based on the materials of Stanford University School of Medicine:
Mice with 'humanized' livers improve early drug testing, scientist show.
06.11.2012