A new dengue fever treatment for victims may not be as far off as you might think. Australian scientists took a massive leap forward following a recent, ground-breaking discovery. The finding that the body reacts similarly to dengue fever and bacterial infections could lead to existing therapies being repurposed as effective drugs, and researchers say clinical trials could begin within 12 months.
The research, conducted at University of Queensland’s School of Chemistry and Molecular Biosciences and the Australian Infectious Disease Research Centre, was published recently in Science Translational Medicine.
The research team was led by virologist and Head of School Professor Paul Young, who has been working on dengue virus for many years. His long-term interest in a viral protein secreted from infected cells, NS1, has now borne fruit as its pivotal role in dengue fever has finally been elucidated.
He says he became interested in dengue virus NS1 back in the late 1980s when he obtained WHO funding to work on projects at the London School of Hygiene and Tropical Medicine. Over the years, Professor Young and his team have characterized the protein, recombinantly expressed it, purified it, and used it as the basis of vaccine candidates, as well as the foundation for a diagnostic assay that was commercialized and is now the gold standard for early diagnosis used around the world.
20-year-old hypothesis unlocks potential dengue fever treatment
The hypothesis that arose nearly 20 years ago was that NS1 was a stimulant for the immune system and would directly activate macrophages (an immune cell type), to release factors promoting inflammation. Dengue typically causes a debilitating fever but this can progress to potentially fatal dengue hemorrhagic fever and dengue shock syndrome.
The team started working in earnest on the project about three years ago, and the original hypothesis has now proven to be correct; the NS1 protein does indeed activate macrophages, and also endothelial cells, the cells that line our blood vessels. This results in the production of proteins by macrophages that lead to inflammation and fever, explains Prof Young.
“Back in the mid-1990s my group identified a possible role for NS1 in exacerbating disease and now our latest findings have shown that this viral protein is, in fact, a viral toxin that can induce leaks in capillaries. The clinical symptoms associated with severe dengue disease, hemorrhage and shock may be directly caused by NS1.”
The UQ team have now carried out early laboratory experiments supporting that assumption as they have been able to block NS1 activity in cellular systems, explains UQ immunologist Professor Kate Stacey.
“We have shown in cell culture systems that NS1 has effects on the endothelial cells consistent with an increase in blood vessel ‘leakiness’. When we treat an intact cell layer with NS1 the junctions between cells are weakened,” she says, explaining that this would be consistent with the hemorrhage and shock that is seen in severe cases of dengue virus infection.
A similar pathway to ‘leakiness’
The pathway that eventually leads to this “leakiness” appears to be similar to the potentially fatal leak caused by bacteria in cases of septic shock but the problem was to identify a cellular receptor for NS1 that was eliciting these responses. The team then found a protein called TLR4, which is responsible for mediating the response to NS1. TLR4 is primarily known as a receptor for lipopolysaccharide (LPS) which is a major bacterial cell wall component.
“This instantly makes immunologists very suspicious of our findings, because LPS is everywhere, and can easily contaminate biological materials. So we had an extensive job to satisfy ourselves and critics that we were not just looking at a response to contaminating LPS,” Professor Stacey explains.
Determining that NS1 is mediating effects through the TLR4 receptor provides a particular advantage in the traditionally slow process of drug development, as this pathway has already intensively studied, particularly for treatment or prevention of bacterial septic shock, says Professor Young.
“As it is the same pathway stimulated by dengue NS1 we believe that we could successfully repurpose these sepsis drugs for treating dengue patients. These compounds have already gone through the many years of clinical trials necessary to establish safety and efficacy. Consequently, we could bypass the normal lengthy early clinical trials process saving many years to application.”
Dramatic effects as target identified
Indeed, Professor Stacey says she was shocked by the dramatic effects observed when the team treated mice infected with dengue virus with an inhibitor of the TLR4 pathway.
“We found this treatment prevented the leakage of fluid into the tissues from the blood. This suggests that TLR4 is a very good target for reducing the symptoms of dengue infection.”
An interesting aspect of this is that receptors like TLR4 have evolved to try and help the immune system identify the presence of invaders and mount defensive responses. But when these responses are too strong, they can become part of the pathology and it is often the body’s response to infection that causes much of the problem. This then leads to the possibility of having drugs that don’t directly target the virus, but instead modify the body’s response to the virus to reduce the disease symptoms.
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Clinical trials possible within a year
Drugs targeting the TLR4 pathway are in various stages of development; one such drug, eritoran, has already been through phase III clinical trials for bacterial sepsis and is also currently under investigation for influenza.
“So if TLR4 is a good target for treating dengue, then we have the advantage of all this prior development, rather than having to make drugs from scratch. We are not at the stage of clinical trials yet and we are just commencing trials of eritoran in mice. If this is successful like our earlier TLR4 inhibitor, then clinical trials could be organized within a year,” states Professor Stacey.
The significance of these findings is not confined to dengue; they have wide-ranging implications throughout the immunology field. Professor Stacey says the research draws interesting and surprising parallels between dengue viral disease and bacterial sepsis.
“The dangerous complications of both involve blood vessel leakiness and shock, leading to organ failure. We would propose that the activation of the TLR4 pathway is common to both these conditions. There are a growing number of viral infections where responses through TLR4 are implicated, including influenza, ebola virus and Japanese encephalitis virus. This is an interesting twist for something that looked for many years like a receptor specific for response to bacteria.”
A drug that lessens the severity of the disease
The hope from this work is for a drug that inhibits the response through TLR4, lessening the severity of the disease and decreasing the length of hospital stays. This is important for countries suffering severe outbreaks of dengue where the total number of hospitalizations can be a substantial strain on the medical system, as well as a significant economic burden. Up to 500,000 cases of dengue hemorrhagic fever are diagnosed each year, with as many as 25,000 deaths.
Ultimately the solution to dengue fever will require control of the transmission cycle, and this approach will not affect that, notes Prof Stacey.
“However, in the medium term, we certainly still need dengue treatments to reduce disease severity.”
A completely new viral player
The researchers will now move quickly to exploit this latest knowledge, says Professor Young.
“We have identified a completely new viral player in the dengue disease process, opening up the opportunity of developing new classes of inhibitors directed at this new target. The next steps are to complete pre-clinical studies to confirm inhibition of the NS1 activation pathway in ameliorating dengue virus disease and to identify clinical trial partners and sites for testing re-purposed compounds.”
For now, he remains extremely positive about the new opportunities his team’s research will open up in terms of dengue fever treatment.
“I’ve been working in the field of dengue research for more than 30 years and I’ve never been more excited about the prospects for successfully treating dengue disease. This is why I got involved – to make a difference.”
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