This summer, researchers in the US will apply for permission to test a malaria vaccine that has created excitement and controversy worldwide. Its designer is Manuel Patarroyo, from the Institute of Immunology at the National University of Colombia, in Bogota. More than 30 000 people have already re ceived the vaccine, but many scientists still question its value.
Now Colonel Jerry Sadoff at the Walter Reed Army Institute of Research, and the US National Institutes of Health, are to conduct a joint trial with Patarroyo. ‘We’ll be submitting (the proposal) to the Food and Drug Administration within the next few months,’ says Sadoff.
Of the world’s diseases, malaria is the biggest killer and the situation is worsening as drug-resistant strains of the malarial parasites spread. The drugs that people take to prevent malaria are not completely effective anyway, and most of those at risk do not have access to them. A vaccine that could prevent the suffering is urgently needed.
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Patarroyo’s vaccine is based on four synthetic peptides – copies of surface protein fragments from the parasite Plasmodium falciparum. No one before him has ever managed to synthesise an effective peptide vaccine against malaria. Patarroyo, who trained with Bruce Merrifield, a Nobel prizewinning peptide chemist, says that his vaccine protects up to 85 per cent of the people it has been given to.
Some scientists are impressed, many are sceptical, and a few are jealous. ‘A lot of people want to trip him up,’ says Fred Brown, professor of virology at Yale University, who is familiar with Patarroyo’s work.
Jealous or not, scientists have legitimate questions to ask. Patarroyo publishes infrequently, so it has not been easy for others to evaluate his work. His early trials have been criticised for lacking controls; other laboratories have tried to repeat his animal experiments, and failed; and no one understands how his vaccine can protect against another species of malaria parasite, P. vivax, as he claims. The two species have very different surface proteins.
In 1987 Patarroyo said he had successfully immunised monkeys against malaria with three parasite peptides (Nature, vol 328, p 629). At that time he had two separate designs for the vaccine: in one he coupled the peptides to a carrier molecule. In the other he created a chain or polymer.
At the request of the WHO, a team at the Centers for Disease Control in Atlanta, Georgia, conducted a similar experiment, using both the polymer and the coupled peptides, to see if they could reproduce the Colombian results. Surprisingly, the monkeys they vaccinated with the coupled peptides had an inconsistent immune response and turned out not to be protected against P. falciparum when challenged with the parasites (American Journal of Tropical Medicine, vol 43, p 355).
Patarroyo says he knows why. ‘I had to explain their mistake to them.’ He claims that the team at the CDC unwittingly failed to couple some of the peptides to the carrier molecule, so the monkeys’ immune systems could not ‘see’ them and responded only to a single peptide.
Carlos Campbell, chief of the malaria research laboratory at the CDC, said: ‘I am not sure that the evidence is so clear on that. The point is that there were some distinct differences in the immune responses to some of the peptides.’
Nevertheless, a report from the CDC to the WHO that Patarroyo quoted in a letter to the journal Science last year says: ‘We feel that the most likely explanation for the different serologic responses in animals immunised with the peptide mixtures in Atlanta and Colombia was a problem with the peptide-(carrier molecule) conjugation procedure. This probably also explains the lack of protection in the two groups of animals . . . in Atlanta.’
Whatever the explanation for the failure of the coupled peptides, it does not account for the failure of the polymer to protect the monkeys. Nor can it explain the failure of a second laboratory to repeat the same experiment.
Socrates Herrera, a former student of Patarroyo’s now based at the University of Valle, Cali, near the Colombian coast, tried to replicate Patarroyo’s work. Patarroyo provided the polymer. The animals were not protected – a result that Patarroyo admitted at a recent meeting in Edinburgh.
The final version of the vaccine is a polymer called Spf66. It is a combination of three peptides from the merozoite surface, plus one from the sporozoite . Its first human trials, in a small group of soldier volunteers, were published in Nature in March 1988 (vol 332, p 158). The polymer stimulated antibodies that partially protected the volunteers when they were challenged with live parasites in the bloodstream. It also appeared to suppress and delay the parasites’ invasion of red cells.
Since then thousands of people in Colombia and Venezuela have received the vaccine in field trials. Such trials do not involve deliberately infecting the vaccinees; researchers essentially compare immunised and non-immunised people in a population at risk, to see how many infections occur in each group. A proper field trial must be double-blind and placebo-controlled. This means vaccinating the control group with saline, with none of the doctors or patients involved knowing who is getting which. Patarroyo’s early trials did not follow this design: 16 000 people received vaccine, while 10 000 people received nothing. This puts the findings in question, because both doctors and patients knew who had been vaccinated – a fact that could have biased the results.
‘Here you have a brilliant biochemist doing epidemiology,’ says Michael Hollingdale, an immunologist from the Biomedical Research Institute in Rockville, Maryland. ‘Now you need an epidemiologist to take over.’
Whatever the disagreements over the Colombian vaccine, most scientists are convinced that independent confirmation of Patarroyo’s results is an urgent priority. They also want him to test his vaccine in populations where the burden of malaria is greater than in Colombia. Compared with say Thailand or parts of Africa, Colombia has few new malarial infections each month, so field trials there must involve large numbers of people and go on for many months if they are to show clear trends.
Another reason for doing a trial outside Latin America is the important differences around the world in malarial parasites and, it seems, in the disease they cause. In Africa, there are about 1 million deaths from malaria each year, mostly among children. Here, more than elsewhere, the disease seems to affect the brain.
It was for these reasons that scientists at a unit in the Gambia had hoped to test the vaccine this year. Brian Greenwood and his colleagues at the Medical Research Council’s unit near Banjul applied to the MRC for approval to conduct a placebo-controlled field trial this spring. Their proposal was not approved. The MRC said it needed more information (This Week, 23 February).
Now, even if the council approves the trial after considering additional information, nothing will happen until next year because malaria is seasonal in the Gambia and it takes several months to give the three doses of the vaccine. The trial would have had to begin in January.
Greenwood says the MRC’s questions were reasonable, but he believes the information was available, for example in a report by the Pan American Health Organization. The PAHO is the Washington-based branch of the WHO with responsibility for Latin America. It sent a committee to visit Patarroyo’s laboratory.
Lindsey Martinez, secretary to the Steering Committee on the Immunology of Malaria at the WHO, was disappointed to hear of the Gambian trial hold up. ‘I think it was a pity that a bit of time was lost because there was a special case for getting on.’
The MRC’s questions concerned safety, variability between batches, composition and doses, says Diane McLaren, secretary to the MRC board that considers such applications. The board also wanted substantiation of the claims for the vaccine’s effectiveness. After ¿ìè¶ÌÊÓƵ reported the council’s decision, the MRC wrote to the magazine (Letters, 9 March). Without this information, the MRC said, a vaccine would not be approved for use in Britain. ‘Why should we apply different standards in Africa?’
The PAHO began assessing Patarroyo’s work after five member countries asked for help in introducing the vaccine.
‘Dr Patarroyo opened his lab to a committee of malariologists, immunologists and epidemiologists,’ says Renato Gusmao at PAHO. The committee visited last June and reported its results. They were mixed, highlighting the strengths of Patarroyo’s work but also calling for more research.
The committee was ‘extremely happy’, says Gusmao, with the quality of Patarroyo’s vaccine and its safety. Normally, any authority approving a vaccine has to be sure it is manufactured reliably and that every batch will be the same. In peptide synthesis, it is possible that the polymer will vary in its three-dimensional shape from one batch to another. As a result, one batch may not stimulate exactly the same immune response as another.
Brown from Yale, was a member of the committee. He says Patarroyo showed him six different vaccine preparations that produced the same immune response. ‘I was very impressed by his peptide chemistry,’ says Brown.
Double-blind, placebo-controlled trials involving 6000 people are now under way in Brazil, Venezuela, Colombia, Ecuador and Peru and are due to finish by Christmas.
‘If well-designed trials are now being carried out, this would go a long way to answering the questions about the vaccine’s efficacy,’ says Martinez at the WHO.
Gusmao says the PAHO was ‘extremely excited and anxious to have a product from a research investigator in our region’
The American trial now in the offing will take research on the vaccine back to square one. It will use Patarroyo’s method but, to meet FDA regulations, the peptides will be synthesised by an independent manufacturer in the US. The trial will start by immunising laboratory animals, says Sadoff. Provided the animals produce useful antibodies in response, a small-scale trial in volunteers will then take place. This will seek to confirm the vaccine’s safety and whether it produces a good immune response. If this is successful, further clinical trials will be conducted. By early next year, a field trial may be under way.
Until this and the other studies are complete, there will be no universal confidence in the vaccine and people around the world will be holding their breath.
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A battery of vaccines target the changing parasite
The challenge for the would-be vaccine designers is formidable. The Plasmodium parasites have different phases and it seems impossible to protect against all phases at once. The WHO says it may be necessary to use more than one type of vaccine to obtain maximum protection. ¿ìè¶ÌÊÓƵs around the world are pursuing numerous different approaches.
The first phase in the parasite’s life is a sporozoite, which enters the bloodstream when an infected mosquito bites. Sporozoites travel quickly to the liver where they multiply before breaking out into the body. In this next phase, the parasites, now called merozoites, invade red blood cells where they multiply again through asexual reproduction. This is the stage that causes disease. Some of the merozoites develop into a third stage, gametocytes, which get back into mosquitoes on the next bite and reproduce sexually. The result is more sporozoites.
Several vaccine designers have aimed to disable the sporozoite. Such a potential vaccine developed by Sadoff and his colleagues is on trial in Thailand and Kenya. It is based on a region of the sporozoite’s outer protein. So far, sporozoite proteins have triggered only limited immune response in people, but scientists now think a combination of different fragments will be most effective.
Anti-sporozoite vaccines would prevent the invasion of red blood cells so disease would never develop and any mosquito that bit the individual would fail to become infected. This would sharply reduce the spread of disease as well as protect the individual. But what if the person encountered merozoites, for example by infected blood transfusion? The vaccine would not protect against them.
Another design of vaccine could block the merozoite. This would reduce both illness and infection by stopping reproduction of the parasite in the blood. but the host might remain vulnerable to sporozoites and gametocytes.
The last, and most Machiavellian strategy, is the ‘altruistic vaccine’. This type would stop the gametocytes from sexual reproduction. So it would prevent transmission of the disease via mosquitoes, but would not prevent disease. Good uses for such a vaccine might be in combination with drug therapy or a protective vaccine, according to David Kaslow, a researcher at the National Institutes of Health who is working on gametocyte-blocking vaccines.
As if the designer did not have enough problems with the different stages, each species has many different strains, and surface proteins vary from one to the next. So it is essential to use portions of protein that are common to as many strains as possible. Another risk is that the vaccine will allow those rare strains which can evade it to become suddenly successful in the population.