Ammonia buildup kills liver cells but can be prevented using existing drug

High levels of ammonia kill liver cells by damaging the mitochondria that power the cells. But this can be prevented using an existing drug due to start clinical trials, finds a new study in mice led by researchers from UCL.

The study, published in Science Advances, is the first to observe that the buildup of ammonia (hyperammonemia) can harm liver cells, and the first to describe how this damage occurs in mouse models that are clinically relevant for humans.

Hyperammonemia is known to cause brain dysfunction in those with liver disease, but a lack of effective treatments for the condition has meant the prognosis for patients is often poor.

Professor Rajiv Jalan, senior author of the study from the UCL Institute for Liver & Digestive Health, said, “Ammonia is a toxin that is usually cleared from the body via the urea cycle, which takes place in the liver. We know that liver disease patients accumulate ammonia and that this can lead to problems, including in the brain. But until now we didn’t understand exactly how this occurred.

“In this study we’ve demonstrated that ammonia kills liver cells by damaging the mitochondria, which—apart from their role in clearing ammonia from the body—also act as the powerhouse of cells. It’s a vicious cycle where the more the mitochondria are damaged, the more ammonia builds up, which snowballs into complete system breakdown.”

Of the 100 million people in the world with cirrhosis (scarring of the liver), about three million are hospitalized with an episode of confusion or coma that is associated with elevated ammonia levels in blood and brain tissues, with 10–15% of these expected to die within three months of the episode.

In promising news for these patients, the research demonstrated that an existing drug, called YAQ-005 (previously known as TAK-242), can halt damage to the mitochondria in liver cells, allowing them to do their job of converting ammonia into urea so that it can be excreted as urine (a process called the urea cycle).

YAQ-005, which has been patented by UCL Business (UCLB), the commercialization company for UCL, and licensed to UCL spinout company Yaqrit, is currently in a phase II clinical trial for acute-on-chronic liver failure, a condition related to cirrhosis.

The authors believe that the drug may also be effective for children with urea-cycle disorders and other genetic diseases that lead to increased levels of ammonia by causing mitochondrial dysfunction (hepatic mitochondriopathies).

In the study, the researchers observed that in two mouse models, elevated levels of ammonia caused an increase in two proteins, called RIPK1 and RIPK3, which leads to mitochondrial damage and a dangerous form of cell death that not only harms the liver but also other organs, including the immune system.

There was also increased activity in the TLR4 signaling pathway, which alerts the immune system when pathogens are detected and is known to induce the production of RIPK1 proteins.

Increases in RIPK1 and RIPK3 corresponded to increased liver scarring and liver cell death, proving that ammonia directly causes liver injury for the first time.

The team then administered two drugs, RIPA-56 to block the RIPK1 pathway and YAQ-005 to prevent the activation of the TLR4 pathway, which led to significant reduction in liver injury and cell death in the mice.

Dr. Annarein Kerbert, first author of the study from UCL Institute for Liver & Digestive Health and Leiden University Medical Centre, said, “Targeted drugs to prevent chronic liver disease progression currently do not exist. In this study, we have shown the potential of the drug YAQ-005 in protecting the liver from the toxic effects of hyperammonemia. We therefore believe that this could be a potential novel therapy for disease progression in cirrhosis. Our aim is to investigate this further in proof-of-concept clinical studies.”

A phase II clinical trial for YAQ-005 is due to begin recruiting patients with liver failure in mid-2025, which will provide the first proof of concept for this treatment in human patients.

Troels Jordansen, CEO of Yaqrit, added, “We are looking forward to advancing this innovative drug, licensed from UCL, into phase II trials in acute-on-chronic liver failure. This is a complicated life-threatening condition and there is an urgent need for new approaches.”