Dengue vaccine candidates show promise

13 Apr 2010

Tatum Anderson

Source: TropIKA.net

Comments

Chimeric vaccines: four vaccines are created by substituting genes from other serotypes into a dengue backbone [Credit: CDC/Inviragen.]

Dengue fever puts an estimated 2.5 billion people at risk globally. Around 230 million people are infected every year and two million of those – mostly children – develop a severe form of the disease, dengue haemorrhagic fever (DHF), which can be fatal.

Mortality rates are low compared with those from other infectious diseases of poverty. However dengue outbreaks stretch already under-resourced health systems to the limit because patients require hospitalization; there are around half a million hospital visits every year. Outbreaks are increasing in frequency and spreading to new areas of the world – see TropIKA.net News.

The disease cannot be prevented with insecticide-treated bed nets, because the Aedes aegypti mosquito, which spreads dengue, also bites during the day. That is why a vaccine is seen as vital.

It is perhaps surprising that there is no a vaccine against this disease already. After all, dengue fever is related to yellow fever, for which there has been a vaccine for many years. (The dengue virus belongs to the Flavivirus family of viruses; it is closely related to Japanese encephalitis, and more distantly to yellow fever, tick borne encephalitis and hepatitis C. West Nile virus. Approximately 50 other viruses are more distant relatives).

The US armed services, for their part, have been interested in dengue vaccines for decades. That is because dengue has incapacitated so many American troops located all over the world, from the Philippines, Asia and the Western Pacific during World War II to Vietnam and most recently Somalia and Haiti. Stephen Thomas, director of dengue vaccine development at the Armed Forces Research Institute of Medical Sciences (AFRIMS) in central Bangkok, which is supporting various dengue studies, says lack of progress is not due to lack of effort. Dengue is a ferociously complicated disease and much more research is necessary to try and understand it. “It is a really tough nut to crack and it’s not through lack of trying,” he says. “It’s like a ten-thousand piece jigsaw puzzle. I go to meetings and people talk about research into one piece of the puzzle. Slowly but surely we are putting pieces together.”

There are four serotypes of the dengue virus – DEN-1 to DEN-4. In vaccine terms, this means researchers are not just creating one vaccine, but five different ones; one against each type of virus and then a final combination vaccine. Combining the vaccines is not a trivial process because there is significant interference between the strains.

Dr Jean Lang, R&D programme director for travel and endemic vaccines at Sanofi Pasteur, says that previously, during trials of a now abandoned tetravalent candidate, vaccinated patients mounted an immune response against only one or two dengue serotypes. The response to other strains was much lower than seen in the individual vaccines. Managing the interference between strains is a key problem in many dengue vaccines even today he says, although there are some solutions. “We can overcome these interferences by giving at least two doses over several months, plus third dose or booster after one year,” he says.

To make matters worse, there are no animal models for these vaccines. In other words, animals do not seem to exhibit the same clinical signs and conditions that appear in humans. Indeed, the virus does not tend to cause disease in monkeys so it is quite difficult to determine whether it will prevent illness in humans. “You have to do costly human trials,” he says.

But there is also an awful lot of missing research. Nobody really knows yet why some people react so differently to the dengue virus, for instance.Some with dengue never feel ill, some have a slight fever and others experience full-blown dengue fever symptoms, including incapacitating high fever, with severe headache, pain behind the eyes, muscle and joint pain, and rash. That is important because the immunological profile of disease is vital when developing vaccines. Limited understanding of protective immune responses against dengue has hampered advancement of dengue vaccine candidates, according to researchers. As AFRIM’s Thomas puts it: “When you are making a vaccine, you have to try and reproduce the same sort of immunity that people see in nature, without having to go through the illness, but if you really don’t know how that immune profile is generated, it’s tough”.

Luckily, a race is under way create the first dengue vaccines. Groups from Hawaii to France, are racing to create vaccines, many supported by the Pediatric Dengue Virus Initiative (PDVI), a Gates-funded organisation (see below). But it will also be interesting to see how these vaccines will be marketed, priced and distributed in poorer countries. After all, they involve costly trials, use new technologies and require multiple doses too. And importantly, they are a potential money-spinner for the companies involved. One of the companies making a dengue vaccines reckons the market could be worth $1 billion annually. The market for travellers’ jabs alone will be 300-$400 million annually, because visitors make 50 million visits per year to dengue areas. (Over 90% of these revenues would be from American, European and Japanese travellers.)

Researchers say there must also be a more coordinated approach to the different trials and that the sharing of information on these vaccine studies should be coordinated by the WHO. TropIKA.net will follow future progress towards vaccines. Here is our rundown of selected candidates so far.

Sanofi Pasteur
The front-runner is ChimeriVax, a vaccine developed by Sanofi Pasteur, the vaccines division of the sanofi-aventis Group. The vaccine is in Phase IIb clinical trials and is being tested in 4–11 year-old Thai children. Trials should be completed by the second quarter of 2012. The company is working with Thailand’s Mahidol University, the Ministry of Public Health, and PDVI.The candidate is a so-called recombinant live attenuated chimeric or hybrid vaccine. It is called chimeric because researchers have used yellow fever vaccine, which is already on the market, as a backbone and substituted a portion of the dengue virus that will stimulate an immune response. Four backbones, each containing a different dengue serotype portion, are combined to form the vaccine. Despite different regional strains, the company is confident that one vaccine could be created for the entire world.

The technique, bought through the acquisition of a biotech company called Acambis, has already been used to develop a chimeric vaccine against Japanese encephalitis that will be licensed by the end of 2010.

GlaxoSmithKline/Walter Reed Army Institute of Research/National Institutes of Health/Centers for Disease Control (CDC)

This live attenuated tetravalent dengue fever vaccine is in Phase II trials. Here, all four virus strains have been isolated from patients, characterized, purified and weakened. Data collected from Thailand, Puerto Rico and US between 2006 and 2009 are currently being analysed. Results will be available by the end of 2010.

For many years, attenuated viruses have been heralded because they may be able to illicit a better immune response than inactivated or subunit vaccines, which require multiple doses and boosters. (The fact that GSK’s vaccine is still going, is in contrast to sanofi’s first-generation candidate, a live attenuated virus vaccine, which was abandoned after it was found that individual vaccine viruses showed good results when given alone, they performed poorly when injected as a tetravalent mixture.)

Last year, GlaxoSmithKline plc (GSK) announced that it would also develop and manufacture dengue vaccines with Brazil’s Oswaldo Cruz Foundation (Fiocruz).

Hawaii Biotech Inc (HBI)

The HBI vaccine candidate is a recombinant subunit vaccine. This vaccine focuses on delivering just the envelope (E) protein of dengue virus. It is thought that by generating antibodies to envelope protein in particular, the virus will be prevented from latching on to cells and injecting its genetic content into them. The researchers at this Honululu-based company have combined the envelope glycoprotein from all four dengue serotypes to create their tetravalent vaccine.

United States Naval Medical Research Center (NMRC)

Instead of injecting proteins into patients, the NMRC has developed a technique that delivers the genes encoding for pre-membrane (prM) and envelope (E) proteins. The vaccine contains prM and E genes for all four serotypes and mixed with an adjuvant. The idea is that the vaccine will use the patient’s body to generate the antigens which in turn stimulate an immune response. This is called a naked DNA vaccine, because the dengue DNA is administered directly into the body. In the case of dengue, it is administered with a needle-less device that uses high-pressure gas to push DNA directly into cells.A team of researchers at NMRC, led by Captain Kevin Porter, have received funding from Gates Foundation for the DNA vaccine, which is entering Phase I clinical trials.

National Institutes of Health and licensees: Butantan/Panacea/ Biological E/ Vabiotech

Researchers at the National Institute of Allergy and Infectious Diseases (part of the US National Institutes of Health) have managed to create several candidates using a technique called reverse genetics [1]. They have looked closely at the genetic differences between strains that cause different levels of illness. By spotting differences – such as missing nucleotides – between serotype genomes, they have managed to replicate those differences using genetic engineering within strains that would normally cause illness.

Several candidates, each containing a particular mutation, are being tested. One mutation, for instance, has been shown to reduce viraemia in rhesus monkeys and to induce high levels of neutralising antibodies. Another mutation shows improved safety [2]. The group has also looked at chimeric vaccines. However, instead of a yellow fever backbone, this vaccine uses a cloned dengue backbone. The researchers have created vaccine candidates that cause minimal illness – based on DEN-1 and DEN-4. Then, using DEN-4 as a backbone, they have created other vaccines against DEN-2 and DEN-3. The candidates are understood to reduce the level of virus transmission from humans to mosquitoes and limit the virus’ ability to reproduce or replicate, in the mosquito. They may therefore help those not exposed directly to the vaccine, says NIAID. The candidates are being worked on by a number of organisations including Johns Hopkins and the Biological E company in India.

Inviragen/CDC

A similar chimeric strategy has been created by Inviragen and CDC. In this case, however, a tetravalent vaccine, called DENVax, was formed using a DEN-2 backbone. The DEN-2 structural genes, prM and E, have been replaced with those from DEN-1, DEN-3, or DEN-4. So, the resulting vaccine is made up of four DEN-2 viruses, including three that are each inserted with genetic material from one of the remaining serotypes. The viruses therefore express prM and E surface antigens of DEN-1, DEN-3, or DEN-4, but are supported by a safer DEN-2 vaccine backbone. CDC, which developed this candidate, is now working with Inviragen Colorado and the University of Wisconsin to optimise the vaccine. Shantha Biotechnics, from Hyderabad in India, has been contracted to manufacture the vaccine for human clinical trials.

Which of all these candidates will become the first vaccine to enter general use – and exactly when this will be – is hard to predict, but dengue specialists are now optimistic that before too long we will have the long sought-after vaccine. As public concern grows over the continue rise in case numbers, this is good news indeed.

References

1. Blaney JE Jr, Durbin AP, Murphy BR, Whitehead SS (2006). Development of a live attenuated dengue virus vaccine using reverse genetics. Viral Immunol; 19(1):10-32. Available from: http://www.ncbi.nlm.nih.gov/pubmed/16553547

2. Webster DP, Farrar J, Rowland-Jones S (2009). Progress towards a dengue vaccine. Lancet Infect Dis; 9(11):678-687. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19850226