Experimental Challenge of Volunteers with Malaria

As physicians sworn to “do no harm” and clinical investigators working to develop malaria vaccines and drugs, my colleagues and I have had to repeatedly ask ourselves whether it is ethical to conduct experimental studies of malaria challenge in volunteers. We have discussed that question with members of human trial committees, volunteers, colleagues, and friends and family, many of whom have initially been incredulous to hear that persons volunteer to be infected with the parasites that cause malaria.

Because malaria is a major threat to U.S. troops [1] and persons who travel abroad, the Department of Defense provides funding for us to develop vaccines and drugs to protect nonimmune persons from malaria. As tropical infectious disease specialists, we are also committed to developing vaccines and drugs to reduce the impact of a disease estimated to affect 300 to 500 million persons and kill 1.5 to 2.7 million persons annually [2]. In other words, every 2 to 4 years, malaria kills more persons than AIDS has killed since it was recognized. Given the magnitude of the problem, the fact that the studies have been considered safe and crucial to development of vaccines and drugs, and the fact that that the volunteers have understood the risks and provided informed consent, there has been consensus that infecting volunteers with malaria organisms is acceptable.

Understanding the safety and importance of these studies depends on understanding the life cycle of Plasmodium falciparum, P. vivax, P. malariae, and P. ovale, the parasites that cause malaria in humans. Anopheles species mosquitoes transmit malaria by inoculating small numbers of sporozoites into the recipient's bloodstream. The sporozoites rapidly enter the liver, where they generally develop in hepatocytes during a period of 5 to 10 days; after that time they rupture, releasing tens of thousands of merozoites into the bloodstream, each of which can invade an erythrocyte. In 48 hours, P. falciparum merozoites (an average of 16 per erythrocyte) develop into mature parasites. They then rupture from the erythrocytes. Each can reinvade another erythrocyte, thereby initiating the cycle of invasion, development, and rupture responsible for human disease. The sporozoite and liver stages are not associated with pathologic conditions or clinical manifestations. Some of the intraerythrocytic parasites develop into sexual stages that are consumed by feeding mosquitoes; once in the mosquito, these parasites develop into sporozoites in approximately 14 days.

In 1899, Giovanni Grassi used experimentally infected volunteers to establish that P. falciparum is transmitted to humans by infected mosquitoes [3]. Fifty years later, volunteers were used to demonstrate that P. falciparum sporozoites enter the bloodstream while infected mosquitoes are feeding and remain in the bloodstream for approximately 30 minutes [4].

Before being used widely in research, challenge with malaria parasites had found a place in medicine as therapy for neurosyphilis [5]. On the basis of the clinical experience with patients who had neurosyphilis, experimental challenge with malaria was introduced and accepted as a method for evaluating the efficacy of antimalarial drugs. From the 1930s through the early 1980s, volunteers were infected by the bite of mosquitoes that had been infected by feeding on infected volunteers, injection of sporozoites obtained by dissection of the salivary glands from infected mosquitoes, or direct inoculation of infected blood. In many studies, volunteers were allowed to have symptoms for as long as could be tolerated, and some patients acquired parasite densities greater than 10⁵/µL of blood before treatment was initiated. These experimental challenges were critical to the development of several antimalarial drugs, including chloroquine, a drug that has saved millions of lives.

In the mid-1980s, the development of malaria vaccines began in earnest. By this time, it was possible to culture P. falciparum and to infect mosquitoes by having them feed on infected blood in culture. For clinical trials of vaccines that required experimental challenge of volunteers, this method was superior to older methods, which depended on obtaining parasites from volunteers who had been experimentally infected. One advantage of the newer technique was that volunteers were not needed as donors of infected blood. A second advantage was that production of parasites in culture became standardized and was accomplished with well-characterized reagents previously screened for infectious agents, such as viruses. The consistency of the procedure has recently been increased by the production, characterization, storage, and use of a master cell bank of infected erythrocytes in conformance with guidelines of the U.S. Food and Drug Administration.

Two of the principal approaches to the creation of malaria vaccines are preerythrocytic-stage and erythrocytic-stage vaccine development [6]. The goal of preerythrocytic-stage vaccination is to induce immune responses that prevent infection of hepatocytes or parasite development in hepatocytes, thereby preventing establishment of the erythrocytic stage that is responsible for the pathologic conditions and clinical manifestations. The goal of erythrocytic-stage vaccine development is to induce immunity against the erythrocytic stage and either eliminate all parasites or modulate the infection to the extent that the parasite burden never becomes high enough to cause severe disease and death.

Testing the efficacy of preerythrocytic vaccines or chemoprophylactic drugs in volunteers is straightforward. Interventions are considered to have failed if they do not prevent or delay the development of an erythrocytic-stage infection. Blood films from volunteers are examined daily and at the time of symptoms. Volunteers are treated when parasites are first noted, regardless of the concentration of parasites or presence of symptoms. This has proved to be a safe and reproducible procedure, and more than 200 civilian and military volunteers have been infected since this method was introduced [7, 8]. Data from 118 volunteers infected in this manner from 1985 to 1992 were recently analyzed [9]. Ninety-seven percent of volunteers developed symptoms, but the illnesses were generally moderate, short lived (median duration, 3 days), and diagnosed at low levels of parasitemia (geometric mean, 46 parasites/microL). Treatment was uncomplicated because the drug sensitivities of the P. falciparum strains were known, and the parasite densities were far below those associated with severe disease. These studies have been fundamental to the incremental development of preerythrocytic-stage malaria vaccines [10-16] and chemoprophylactic drugs [17, 18].

Testing the efficacy of erythrocytic-stage vaccines in volunteers may be more difficult. Whether such vaccines prevent or delay the onset of parasitemia can be determined by using the same end points as previously described, as well as methods that measure parasite growth rates before the onset of disease [19]. However, erythrocytic-stage vaccines can also be designed to induce immune responses that are not optimally effective until they are either boosted by a P. falciparum infection or have neutralized toxic parasite products but not the parasite. In fact, reduction in the mortality rate, not prevention of infection, is the primary goal of most malaria vaccine development efforts for Africa. Thus, if the vaccine is designed simply to limit the infection to 10⁴ parasites/µL or to neutralize parasite toxins, it may not be possible to establish efficacy if volunteers are treated at the first sign of parasitemia (when the parasite density is almost always < 10² parasites/microL). In the past, volunteers participating in drug studies were frequently followed for days after onset of symptoms and were allowed to develop parasitemia greater than 10⁴ parasites/µL. Today, many investigators believe that this practice is not acceptable because it exposes volunteers to high levels of parasites and is potentially lethal. Thus, repeated experimental challenges or field trials are required to establish that an infection is controlled by vaccine-induced immunity of the type that is boosted by the infection. However, it should be noted that symptomatic, experimentally challenged volunteers with relatively high parasitemia served in the development of the experimental malaria vaccine, SPf66 [20].

In summary, although not always optimal for assessing interventions, experimental challenge of volunteers with malaria organisms is one critical component of the malaria vaccine and drug development process. Bypassing these experimental studies and going directly to field trials may slow the process of testing and increase its cost, with a consequent reduction in the number of candidate vaccines and drugs that can be evaluated. Furthermore, it is logistically and, I believe, ethically warranted to establish efficacy of a candidate vaccine or drug, whenever possible, in a safe, controlled setting, before assessing it under the inherently more variable conditions of field testing.

• Copyright ©2004 by the American College of Physicians