Developing a malaria vaccine

There is currently no licensed vaccine to protect people against malaria, a disease caused by infection with the Plasmodium parasite and transmitted by mosquitoes. Therapies are available for malaria patients, but the worsening problem of drug resistance in many parts of the world is making adequate treatment and control of malaria increasingly difficult. In addition, many insecticides are no longer useful against the mosquitoes that transmit the disease.

Crucell is working in collaboration with a number of research groups to develop a safe, effective and affordable vaccine against Plasmodium falciparum, the most lethal of the four species of malaria parasite that infect humans. Crucell’s approach is based on our innovative AdVac^® technology, which uses harmless adenoviruses as vaccine delivery vehicles (vectors).

Towards an AdVac®-based malaria vaccine

Crucell’s first collaboration aimed at malaria vaccine research was born in March 2003, with the Walter Reed Army Institute of Research (WRAIR) and GlaxoSmithKline Biologicals (GSK). In a collaborative preclinical study, we tested a candidate AdVac®-based malaria vaccine as a stand-alone vaccine and in combination with GSK’s RTS,S malaria vaccine candidate. These studies showed that Crucell’s AdVac^® vaccine efficiently primed and/or boosted malaria-specific immune responses.
 
In March 2004, the National Institute of Allergy and Infectious Diseases (NIAID), part of the US National Institutes of Health NIH, agreed to support the development of Crucell’s AdVac^®-based malaria vaccine.

In partnership with the NIAID, Crucell’s candidate malaria vaccine entered a Phase I trial in the USA in the final quarter of 2006. Volunteers were recruited by two centers: Vanderbilt University in Nashville, Tennessee, and Stanford University in Palo Alto, California. Boost vaccinations for the final group of volunteers were completed in December 2009. Analysis of unblinded safety data revealed a good safety profile. Available immunogenicity data indicated that the AdVac®-based vaccine triggers both humoral and cellular immune responses. Extensive (pre-)clinical studies have demonstrated the complementary role and necessity of anti-CS antibody and T-cell mediated immune responses in the ability to protect against malaria. The tested vaccine consists of an adenovirus 35 (Ad35) vector delivering protein from the circumsporozoite (CS) stage of the P. falciparum parasite to the immune system.
 
In July 2009, Crucell announced a new collaboration with the US-based Malaria Vaccine Initiative and the United States Agency for International Development (USAID) Malaria Vaccine Development Program (MVDP) to accelerate development of a promising new type of malaria vaccine. With USAID MVDP funding, the partners are conducting studies to determine the effectiveness of Crucell’s novel prime–boost vaccine approach targeting P. falciparum. The studies explore the efficacy of two different adenovirus vectors, Ad35 and Ad26, as mechanisms for delivering CS protein.
 
In May 2010, the Ad35-CS malaria vaccine candidate Crucell is developing in collaboration with the NIAID, entered a Phase I clinical study in Burkina Faso, Africa. This is the first study evaluating the safety and immunogenicity of this AdVac®-based vaccine in an area where malaria is endemic and the healthy adult population can be assumed to be semi-immune to malaria.

Exploring a combination vaccine strategy

In April 2010, Crucell and GlaxoSmithKline Biologicals (GSK) signed a binding letter of agreement to collaborate on developing a second-generation malaria vaccine candidate. Preclinical data have indicated that immune responses against the malaria parasite—specifically, the circumsporozoite stage of P. falciparum—are significantly enhanced when Crucell’s AdVac® technology and GSK’s RTS,S/AS technology are used in combination, versus either component.

Under the terms of the agreement, Crucell will contribute its recombinant malaria vaccine candidate, Ad35-CS, which is based on Crucell’s AdVac^® technology and PER.C6^® manufacturing platform. GSK will contribute its late-stage malaria vaccine candidate RTS,S/AS, which is based on P. falciparum circumsporozoite surface antigen.

Production of an AdVac® malaria vaccine

Crucell’s AdVac^®-based malaria vaccine is made by inserting selected parts of the malaria parasite into an adenovirus vector, which acts as a ‘vehicle’ for vaccine delivery. AdVac^® technology uses adenoviruses that very rarely infect humans, thereby avoiding the potential problem of pre-existing immunity to the viral vector.

Figure 1: Malaria vaccine production process

The AdVac® vector carrying genetic material from the malaria parasite cannot replicate independently, but it replicates abundantly and rapidly when inoculated into PER.C6^® cells. (Crucell’s PER.C6^® human cell line technology has been developed to facilitate and improve the commercial-scale manufacture of biopharmaceuticals.) The resulting product undergoes extensive purification before use as a vaccine.

This method of vaccine production has significant safety advantages over alternative methods, and results in a vaccine that can trigger a strong immune response against the disease-causing microorganism. The steps used in producing an AdVac^®-based malaria vaccine are outlined in a simplified form in the following diagram.

About malaria

Malaria is a life-threatening parasitic disease transmitted by mosquitoes. It is estimated that malaria started having an impact on human survival about 10,000 years ago. Over the centuries, it spread throughout the world and is thought to have been introduced to the Americas from Europe and Africa during the 16th century. It was once believed that the disease came from fetid marshes, hence the Latin name ‘mal aria’, meaning ‘bad air’. In 1880, however, scientists discovered that malaria is caused by a one-cell parasite called Plasmodium. Researchers later discovered that the parasite is transmitted from person to person through the bite of a female Anopheles mosquito. There are four species of Plasmodium that cause malaria in humans; P. vivax is the most common, but P. falciparum is the most lethal.

Morbidity and mortality
According to the World Health Organization’s latest report, 243 million malaria cases and around 863,000 deaths caused by malaria occurred in 2008. An estimated 89% of these deaths were in Africa. Children and pregnant women are the groups most severely affected. Mortality associated with severe or complicated malaria exceeds 10–30%. Although the overwhelming majority of sickness and death associated with malaria occurs in developing countries, this disease also affects travelers. Each year, approximately 30,000 individuals traveling from industrialized nations to the developing world contract malaria. No licensed vaccine is available to fight malaria.

Geographical distribution
Currently, half of the world’s population, mostly people living in tropical and subtropical regions, is at risk of contracting malaria. The widespread occurrence and high incidence of malaria are a consequence of discontinued malaria control programs, the increasing numbers of drug-resistant parasites, and insecticide-resistant mosquitoes. Other contributing factors include environmental and climatic changes, civil disturbances and increased mobility of populations. The four Plasmodium species that cause malaria in humans are found in all world regions. The Anopheles mosquito occurs in most areas of the globe and around 40 species are important to the spread of malaria.

Figure 1: Global presence of malaria

 
Transmission
Malaria parasites are transmitted to humans by the bite of female Anopheles mosquitoes. Infected mosquitoes inject the malaria parasites into the bloodstream, where they remain for only a few minutes before invading the liver cells. Once in the liver, the parasites replicate and develop for about one week before being released into the bloodstream. There, the parasites invade red blood cells, where they undergo several stages of replication and development before invading new red blood cells. When susceptible mosquitoes ingest infected blood, the parasite completes its maturation inside the insect's gut, finally migrating to the mosquito’s salivary glands. The life cycle of the malaria parasite is perpetuated when the infected mosquito bites a new human host.

Figure 2: Malaria transmission cycle

Symptoms
Malarial symptoms appear about 9–14 days after a person is bitten by an infected mosquito, although this varies with different Plasmodium species. Typically, the symptoms of infection are flu-like and include chills, fever, and sweating, accompanied by headache, nausea, and vomiting. Life-threatening illnesses associated with severe anemia, impaired consciousness, coma, seizures (cerebral malaria), renal failure, and shock may occur in some infected individuals.

Treatment and prevention
Malaria can be prevented through interventions that minimize the number of mosquitoes as well as effective chemotherapeutic agents, such as chloroquine and mefloquine. However, the use of drugs to prevent malaria remains impractical in the majority of the developing world. Furthermore, the number of drug-resistant parasites and insecticide-resistant mosquitoes is increasing. Despite earlier promising results in the 1960s with prototype vaccines, there is no effective vaccine against malaria available today.