By Rebecca Ram
This month marks World Liver Day (held every year on 19 Apr) to raise awareness about diseases of the second largest organ in the body. After the brain, the liver is the most complex organ and is vital to the efficient control of the gastrointestinal (digestive) system.
Liver diseases account for approximately 2 million deaths per year worldwide, including cirrhosis, viral hepatitis and hepatocellular carcinoma. Cirrhosis and liver cancer are among the global leading causes of fatality, responsible for 3.5% of all deaths worldwide. According to the British Liver Trust, liver disease is expected to overtake heart disease as the biggest cause of premature death in the next few years and there is an urgent need for new treatments.
In addition to liver disease, specific liver damage as a result of medication continues to be a long-term patient safety problem. The symptoms of Drug Induced Liver Injury, or DILI, can vary from mild to severe, but in the worst cases DILI can lead to permanent liver failure and/ or death. DILI is associated with many drugs currently on the market, accounting for approximately 14 cases per 100,000 patients and is a leading cause of drug failure, both during clinical trials and post-market withdrawal.
‘In vivo’ (animal) models are routinely used to try to replicate characteristics of liver damage, for example the use of carbon tetrachloride is a common procedure to induce liver injury in rodents. Attempts to artificially recreate human lifestyle factors which can result in liver disease such as chronic alcohol consumption or a high sugar/fat diet are also repeatedly used in animals, which have often been genetically modified (GM). However, there is widespread concern over the limitations of animals to mimic the complexities of liver conditions in humans. Despite such concern, animal studies continue to be cited (mainly by those that use them) as ‘essential’ for gaining a better understanding of liver diseases, whilst at the same time scientists acknowledge how little is known despite decades of animal use. Systematic reviews (SR) of animal models of liver disease have also raised questions over the ability of animal models to replicate the human response.
Furthermore, as many liver adverse effects are idiosyncratic in humans, they are not adequately predicted by animal studies, with species differences being described as ‘drastic’, for example when comparing the liver responses of dogs to humans. Again, despite this problem being widely recognised, animal studies are still the conventional approach for testing for potential DILI.
Between January and June 2020, UK research projects related to liver disease were approved which will use over a quarter of a million animals (279,325) in the UK over the following five years. The majority of projects involve mice and rats, as well as many other species, including beagle dogs, rabbits and guinea pigs. Projects range from direct liver disease or DILI projects, to other projects where liver injury is involved or the toxicity testing of new drugs. Just one study for example to investigate DILI will use over 40,000 mice. Of further grave concern is that only 25% of these projects will be independently assessed afterwards to see if they met their scientific objectives. In ‘lay summaries’ describing these projects it is often stated that there is a ‘liver disease crisis’ and that there are ‘urgent unmet clinical needs’, yet these sit alongside claims such as ‘these animal models have been used for years’.
We cannot afford to continue testing unrealistically high experimental doses of drugs or creating artificial diseases in GM animals. New, human-relevant methods are available, including in-vitro approaches using human tissues and cells, e.g. primary human liver cells, or hepatocytes; 3D spheroids (which act as ‘mini-organs’ to predict human responses) and advances in liver on a chip or ‘microphysiological’ systems. These are small but sophisticated devices containing micro-sized channels which are supplied with human liver cells or tissue, to which potential new drugs can be applied to test for human safety, or used as models of human liver diseases. Recent systematic reviews of mechanistic in-vitro assays have shown them to effectively predict DILI for several drugs where preclinical (animal) and other studies previously failed.
Other methods include in-silico (or computational) methods and advanced artificial intelligence ‘deep learning’ models to predict liver toxicity, as well as many other sources of human-derived data. No single method can be used alone, instead different method types can be combined as bespoke, new approach methodologies (or NAMs) to reveal pathways of events right through from a molecular level to physical effects as they manifest in patients.
Due to the enormous expense and time-consuming nature of using animal models, the number of substances that can be effectively tested using in vivo liver toxicity assays is limited. The development of human-based approaches (e.g., in vitro methods to test for liver injury (or hepatotoxicity) allow many substances to be tested more cheaply and quickly. Indeed, cost reductions of over 90% have been estimated when compared to many typical animal tests.
For further insight, follow this link to see a recent presentation on ‘In vitro models for assessment of DILI’ by Safer Medicines Trust’s Pharmaceutical Director Dr. Gerry Kenna.