Vaccines have increased life expectancy and quality of life around the globe but HIV and malaria continue to outwit the drug developers. Dr Sarah Houlton reports on vaccine research that may provide the right approach to challenge HIV and malaria in future
Vaccines have been used to prevent disease since Edward Jenner first used cowpox to immunise against smallpox in the late 18th century. Since then, many diseases that used to pose huge public health problems (from measles to typhoid) have become preventable by the use of vaccines and smallpox has been eradicated. And in the past couple of years, two vaccines to prevent human papilloma virus have been introduced, with the hope that they will dramatically reduce the number of cases of cervical cancer in future years.
While research continues into better vaccines against childhood diseases and improved formulations that will mean fewer jabs need to be given, it is in the diseases that affect mainly the developing world where successful vaccines could have the biggest impact on human health. However, these pose huge discovery and development problems, as the targets are not simple.
HIV is one infection where a vaccine is very much needed - indeed, a vaccine is perhaps the only way the virus will ever be eradicated or even the pandemic in sub-Saharan Africa halted. But it poses incredibly difficult problems for researchers, and some doubt whether a safe, effective vaccine against HIV will ever be possible, regardless of how much research effort and money is thrown at it.
The WHO and UNAIDS estimate that more than 33 million people were living with the virus at the end of last year, with 2.5 million having become newly infected in 2007, and 2.1 million dying from AIDS in that year. More than 95% of all infections are in developing countries, and more than 28 million people have the virus in sub-Saharan Africa alone. It is now the leading cause of death in Africa.
The biggest barrier to a safe, effective HIV vaccine is the high genetic variability of the virus. The standard approach to vaccine creation is to prime the immune system to recognise viral envelope proteins, but this failed with HIV because its epitopes are too variable. This, combined with a lack of knowledge of immune correlates of protection and the lack of relevant and predictive animal models, plus the complexity of running the trials in developing countries, mean that a vaccine remains a long way off. The first Phase I trial on a potential vaccine was carried out more than 20 years ago, and more than 80 Phase I/II trials on more than 30 candidate vaccines have taken place since then in a total of more than 10,000 human volunteers. But still there is no vaccine - and there have been high profile failures.
Numerous different approaches can be used to create candidate vaccines that produce either antibodies or cytotoxic T-cells. Peptide vaccines are perhaps the simplest, as they are made from small pieces of HIV proteins. Larger protein fragments are involved in recombinant subunit protein vaccines. These are found on the surface of the virus, and examples include gp120, gp140 and gp160, which can be made by genetic engineering techniques.
Another approach is to use a live vector vaccine, where other viruses are engineered to carry genes that encode for HIV proteins. These are then inserted into another vector, which takes them into the cells in the body where they produce proteins that are found on the surface of the virus. A similar approach is used in the smallpox vaccine.
A pseudovirion vaccine is a non-infectious agent that has at least one of the HIV proteins, but not all of them. DNA vaccines are also possible. Here, a few HIV genes are inserted into
plasmids - pieces of DNA - which then produce proteins similar to those that the HIV virus itself makes. And, of course, more than one of these strategies could be used sequentially as a "prime-boost" that creates a stronger immune response than either vaccine alone.
Hopes that a vaccine could be getting close were hit late last year when Merck stopped trials on the experimental vaccine V520. Preliminary results of a proof of concept study in 3,000 subjects proved it didn't work, and Merck immediately stopped enrolment into a second Phase II trial, plus two more Phase I trials that were already under way.
The vaccine contained three synthetic HIV genes carried by a weakened adenovirus. This was meant to stimulate an immune response that would programme CD8 T-cells to kill cells that had been infected with the virus. While initial trials were promising, initial analysis of the large-scale trial in healthy volunteers who were at high risk of infection showed that more of those given the vaccine had contracted HIV than those given placebo. It also had no effect on the amount of virus circulating in the bloodstream of infected subjects.
US company GeoVax specialises in the search for an HIV vaccine, and it recently reported its plans to move a candidate vaccine product into Phase II trials. This will involve 225 healthy volunteers in the US and South America. The preventive vaccine candidate combines two separate vaccines - one a DNA priming vaccine, and the other designed to boost MVA (the smallpox vaccine developed for use in immuno-compromised humans) - with the combined effect being the stimulation of T-cells to destroy HIV-1 viruses when they appear in the body.
First, the immune response is primed with a DNA vaccine, and then the rMVA live virus vector booster is injected. This enhances the immune response by expressing larger amounts of antigens than the DNA vaccine can on its own, and also by the infection stimulating a pro-inflammatory response that enhances immunity.
In Phase I trials, both 1/10th and full doses of the vaccine elicited anti-HIV T-cells, and the full dose was needed to create good frequencies of antibody to the HIV envelope glycoprotein. But even if the Phase II trial succeeds, it would still be several years before it reaches the market.
The International AIDS Vaccine Initiative remains optimistic that a vaccine will be possible. At this August's International AIDS Conference in Mexico City, it launched an "AIDS Vaccine Blueprint" with the aim of resetting expectations in the search for a vaccine.
"In the wake of the failure of a leading AIDS vaccine candidate nearly a year ago, some have questioned whether we will ever have an AIDS vaccine," said IAVI's president and chief executive, Seth Berkley. "To these sceptics, I say that developing an AIDS vaccine may take more time and innovation than we might have once imagined, but we are confident that science will prevail."
Its senior vice president of r&d, Wayne Koff, believes that there is strong scientific evidence in both humans and animal models that suggest it will be possible. "The challenge we face now is how to translate advances made in our understanding of the virus and the human immune responses to it into promising vaccine candidates as quickly and safely as possible," he said.
The blueprint sets out four priorities for future research, two of them scientific. The first, solving the neutralising antibody problem, will involve creating immunogens that generate similar antibodies to the neutralising antibodies against HIV that have already been identified.
The cell mediated immunity problem also needs solving. This is one of the lessons learnt from the failed Merck trial - inducing effective CMI responses will be more difficult than expected. The answer could lie in the handful of individuals who are infected with the virus but do not develop the disease.
IAVI also believes that the pipeline of candidates has to be trimmed and improved, allowing resources to be focused on those that look most promising. It thinks that accelerating the development of replicating vector-based vaccines should be a major focus, as the success of live attenuated simian immunodeficiency viruses (SIV) in conferring protection against SIV infection in non-human primates is believed to be at least in part because of its replicative nature. And finally, the effort must be sustained. "We will keep an AIDS vaccine on the centre stage as a long-term solution to this devastating public health problem," said Berkley.
Malaria is another disease where a vaccine could solve a major public health problem. However, it is caused by a parasite rather than a virus, which provides a whole new challenge for developing a vaccine. There are four Plasmodium parasites that cause infection: P. falciparum, P. vivax, P. malariae and P. ovale. The first two of these are the most common, and P. falciparum is the most lethal.
According to the WHO, about 40% of the world's population are at risk of catching malaria, most of them living in the poorest countries. At least half a billion people suffer from the disease every year, and if it is not promptly treated with effective drugs, it is often fatal.
As the parasite mutates to develop resistance against drugs, the medicines lose their effectiveness. And because most patients are in poor countries that cannot pay commercial rates for the drugs, there is little financial incentive for large companies to develop new treatments themselves, so most of the research is carried out either in the public sector, or by agencies such as the Wellcome Trust in partnership with big pharma companies.
If a vaccine to prevent the disease were possible, then this could provide a solution - if the use of the drugs could be reduced, then the rate at which resistance develops would drop too, allowing the few drugs that are developed to retain their effectiveness for longer.
The Malaria Vaccine Initiative is leading the search for a potential vaccine. The ideal malaria vaccine would be safe, easy to manufacture and administer, and - importantly - when given to infants would confer life-long immunity against all forms of the disease. However, it concedes that a vaccine that prevents all malaria infection by priming the immune system to destroy all parasites, whether circulating in the bloodstream, in the liver, or even within the red blood cells, might be difficult to achieve, if not impossible.
The focus has therefore been on creating a vaccine that would limit the parasite's ability to infect large numbers of red blood cells. While this would not prevent infection completely, it should reduce the disease's severity and help prevent death.
The size and genetic complexity of the infectious parasite means that more than 40 different antigens have already been identified that are potential targets for a vaccine. The fact that it progresses through different life stages in the human host doesn't help either, and further challenges are posed by its ability to hide, confuse and misdirect the human immune system.
However, MVI is optimistic that a vaccine will be possible. It has been shown that human volunteers can be protected from malaria infection by repeatedly injecting them with radiation-attenuated sporozoites - the minute active entities into which sporozoa parasites like P. falciparum divide, each of which develops into a new organism.
This is the approach being taken by US company Sanaria, building on initial research carried out some years ago where live mosquitos were used to deliver the sporozoites. All bar one of 14 subjects were completely protected against malaria; protection lasted for at least 12 months, and was effective against several different strains of P. falciparum.
Sanaria has recently opened a clinical manufacturing facility to produce attenuated sporozoites using mosquitoes. Live, aseptically produced mosquitoes will be fed blood containing the malaria parasite. They will then be irradiated to weaken the parasites, before harvesting them from the salivary glands. An initial clinical trial with these sporozoites is planned for next year.
The furthest advanced malaria vaccine candidate, RTS,S/AS02A, is being developed by GlaxoSmithKline Biologicals and the Walter Reed Army Institute of Research, in collaboration with the PATH Malaria Vaccine Initiative. It is reported to be ready to enter Phase III trials.
The vaccine fuses two surface proteins - one that is found when the malaria parasite is in its infectious stage, and the second from hepatitis B, which means it would also protect against hepatitis B infection. The hope is that this will increase its ability to stimulate an effective immune response. The fusion protein is then combined with an o/w emulsion of a fat particle derived from the cell walls of salmonella bacteria and a second compound of plant origin, which acts to boost the immune system.
Nearly 1,500 children aged four and under in sub-Saharan Africa were given three doses of the vaccine, and after 18 months the number of clinical malaria episodes was cut by a third, and severe malaria was halved. If Phase III trials are successful, the vaccine could be ready for licensing by 2011.
Austrian company Intercell is also working on the malaria vaccine problem - it has created adjuvants that are being used in combination with recombinant malaria antigens from the US National Institutes of Health. Its adjuvant IC31 is being investigated in animal studies to see whether the combination will trigger an immune response as it can in other infectious diseases. It induces both T-cell and B-cell responses, and requires nothing more complicated than mixing with the antigens. Initial results are expected towards the end of this year.
Other strategies are also being investigated. For example, a team at the Radboud University in Nijmegen, the Netherlands, and the University of Edinburgh has been looking at using vaccines based on viral carriers. These vectors are immunogenic without the need for adjuvants, and are relatively straightforward to produce. Two influenza virosomal vaccines are already registered for use in humans, one to prevent influenza itself and the other for hepatitis A, and the team thought this same vector might be applicable to malaria.
The idea was to use it as a carrier for a P. falciparum vaccine targeting both the exo-erythrocytic and erythrocytic stages of malaria infection. Disappointingly, it showed no protection against the disease in a Phase IIa trial, but they believe that the flexibility of the platform for delivering antigens means they warrant further investigation.
This is just one of various strategies that have not as yet succeeded. A recent opinion piece published in Trends in Parasitology by Danish scientists Lars Hviid and Lea Barford details some of the issues, but they believe that recent technological advances could help overcome some of these hurdles. "We hope that the rate of progress will increase as a result of recent advances in molecular and antibody technologies," they conclude.