Malaria is a devastating disease. It was the cause of 627,000 deaths worldwide in 2020, and malaria-related mortality has increased by more than 10% over the past 3 years. Although there are highly effective treatments, the development of multidrug-resistant strains poses a great threat to the control and elimination of malaria. Moreover, prevention is a better strategy than treatment. Despite several approaches to prevent established infection, no single approach offers complete protection, so a multilayered approach is needed. One potential layer, the monoclonal antibody, is the focus of a trial by Wu and colleagues1 and the editorial by Daily2 in this issue of the Journal. They describe protection conferred by a long-acting, next-generation monoclonal antibody against controlled human malaria infection in healthy persons.
What Does the Antibody Target?
To appreciate the target of the antibody (see Key Concepts), it is important to place it in the context of the life cycle of the malaria parasite, Plasmodium falciparum. This parasite has two hosts — the human and the female mosquito of the genus anopheles, which also acts as a vector. During an initial bite from an infected mosquito (Figure 1), the plasmodium parasite is injected from the insect saliva into the host tissues in the form of a sporozoite, and infection is initiated. The sporozoite passes rapidly into the liver, where it infects the liver cells (hepatocytes). Over the next 5 to 10 days, the sporozoite undergoes asexual division, and the human host has no clinical symptoms.
Next, in the form of merozoites, the parasites are released into the host’s bloodstream. They invade the red cells (erythrocytes), multiplying again before rupturing the red cells and releasing merozoites into the bloodstream. It is the synchronous release of merozoites from erythrocytes that causes the cyclic fevers associated with malaria. Instead of replicating, some of the merozoites develop into sexual forms called gametocytes that circulate in the bloodstream. When a female mosquito bites an infected human, it takes up the gametocytes in its blood meal. These gametocytes develop further inside the mosquito host, and they eventually migrate in the form of sporozoites to the salivary glands of the mosquito. The cycle then recommences.
Monoclonal antibodies can act at three primary points in this life cycle: the pre-erythrocytic stage (i.e., before the infection of the hepatocyte), the asexual erythrocytic stage, and the sexual erythrocytic stage, when gametocytes are formed and are eventually taken up by the mosquito. An initial infectious bite from a mosquito transmits few sporozoites, so the pre-erythrocytic stage of infection is seen as a more tractable point of intervention than the blood stage of infection, when there may be several trillion parasites.5 However, the window within which protection can be provided — between the initial mosquito bite and the invasion of hepatocytes — is less than an hour, and constant circulating levels of antibody are required.
At the pre-erythrocytic stage, the target of choice is the circumsporozoite protein 1 (CSP-1), so called because it is the predominant protein in the sporozoite. It is the target of the malaria vaccine RTS,S/AS01 (Mosquirix, GSK), and thus it is clinically validated. The antibody used in the trial conducted by Wu et al., L9LS, targets a specific part of a highly conserved junctional epitope in CSP-1, and it appears to be at least three times more potent than previous anti–CSP-1 antibodies. (An epitope is the part of the molecule to which the antibody binds.)
Why the Focus on CSP-1?
CSP-1 is an attractive target because it is the major protein of the sporozoite, and there are only a handful of sporozoites during a single cycle of infection. Also, sporozoites are accessible early in infection.
CSP-1 has two conformational states — smooth and adhesive. In its smooth conformation, the protein is folded in such a way that one end of the protein prevents access to a “sticky” region at the other end that is thought to mediate adhesion to human hepatocytes; this adhesion is necessary for the infection of hepatocytes. Migrating sporozoites have this smooth conformation. In the adhesive conformation, the sticky region is exposed. Cleavage of the CSP-1 protein converts CSP-1 from the smooth conformation to the adhesive conformation, which is required for the invasion of hepatocytes by the sporozoites. If the cleavage is prevented, infection of the hepatocytes cannot happen. The L9LS antibody binds to a unique junctional epitope of CSP-1 when it is in the smooth conformation, preventing cleavage and thus formation of the adhesive conformation.
What Is “Next Generation” about L9LS?
L9LS has two advantages over previous antibodies. The first advantage is that it targets a more tightly defined region of the CSP-1 protein, with high affinity. These properties make it three times more potent than previous anti–CSP-1 antibodies.
The second advantage is that the L9LS antibody harbors a key “design feature” (encoded by the LS mutation) that increases neonatal Fc receptor binding, which, in turn, protects the antibody from cellular degradation and thereby increases its half-life in the blood by a factor of almost three — from 21 days to 56 days. The use of monoclonal antibodies that maintain a high level of protection for a long period (e.g., over 3 to 6 months) is a promising approach to protect vulnerable populations such as children, who should ideally receive an injection of L9LS only once per season or once per year.
What about Existing Strategies?
There are two widely used strategies for protection against malaria infection. The first is the use of vector-control methods to prevent initial contact of the infected mosquito with humans. These methods include the use of insecticide-treated bed nets and larvicides.
The second strategy is chemoprevention with small-molecule drugs. Tourists and other travelers generally receive a daily course of the antimalarial drug combination atovaquone–proguanil, which targets the growth of the parasite inside erythrocytes and is highly effective. However, it is expensive and limited to a 60-day regimen, so it is not suitable for extended use. Monthly cycles of a combination regimen, sulfadoxine–pyrimethamine plus amodiaquine, have been widely used for seasonal malaria chemoprevention in African children younger than 5 years of age and are recommended by the World Health Organization (WHO). These drugs are safe, available, and inexpensive (the cost of the drug is $1 per child per season), and they reduce the risk of clinical malaria by almost 75% (risk ratio, 0.27; 95% confidence interval, 0.25 to 0.29).6 Similar chemoprevention programs exist for pregnant women, and more than 60 million monthly courses of treatment are administered each year. This efficacy has helped to frame the thinking about the target efficacy for new interventions, and a target minimal reduction of risk has been set at approximately 80%.
What about the Vaccine?
Last October, after reviewing extensive phase 4 safety studies, the WHO recommended RTS,S/AS01, the first vaccine against malaria caused by P. falciparum. The vaccine efficacy against all episodes of clinical malaria in a per-protocol population was 39 to 50% among children 5 to 17 months of age, but only 23 to 30% among infants 6 to 12 months of age.5 The price of the vaccine is not final, but an assumed price of $5 per dose and a four-dose schedule in children who are 5 months of age or older bring the total cost to $20 per child, plus the program costs of administration.
What about Resistance?
The emergence of drug resistance is the bane of all anti-infectious chemotherapy, including monoclonal antibodies. Testing for the presence of resistance mutations in the P. falciparum genome in vivo and watching for them clinically, as is done for small-molecule drugs, will be important.7 The fact that the antibody targets the sporozoite is reassuring, given that only a handful of sporozoites are present on the initiation of infection (as compared with trillions of merozoites), and they do not undergo sexual division. The use of combination therapies is one traditional approach to address drug resistance. Other antibodies are under development, so it is possible that a combination of antibodies may eventually be developed,8 similar to that against SARS-CoV-2. That said, persons in areas of high malaria transmission are asymptomatic carriers — their immune systems are tolerant to malaria infection and they have no overt symptoms — so the use of any antibody-based approach would require an initial drug treatment to eliminate the reservoir of parasites in such persons.
The effectiveness and cost of any strategy are key factors for consideration. Wu et al. conclude that protection with L9LS was reached at a serum concentration of 9.2 μg per milliliter. The route of delivery is also important. The current trial included intravenous administration, which is appropriate in an experimental setting but of questionable use in the field. The maximum total injection volume with intramuscular or subcutaneous injection is 1 ml in infants, and given antibody solubility and viscosity, the total dose in 1 ml would be approximately 100 mg. Current estimates of the cost of production of antibodies obviously depend on individual antibodies, but estimates of $50 per gram of antibody are certainly feasible.9 These estimates suggest that these antibodies could be in a similar price range as that of current vaccines, but with the potential to be less expensive as technology develops.
It will be important to evaluate the level of protection conferred by L9LS in children in Africa. The assessment of combinations of approaches will be critical, such as testing the antibody in combination with current chemoprevention interventions and, potentially, with the vaccine.
Funding and Disclosures
1. Wu RL, Idris AH, Berkowitz NM, et al. Low-dose subcutaneous or intravenous monoclonal antibody to prevent malaria. N Engl J Med 2022;387:397-407.
2. Daily JP. Monoclonal antibodies — a different approach to combat malaria. N Engl J Med 2022;387:460-461.
3. Phillips MA, Burrows JN, Manyando C, et al. Malaria. Nat Rev Dis Primers 2017;3:17050-17050..
4. Wang LT, Pereira LS, Flores-Garcia Y, et al. A potent anti-malarial human monoclonal antibody targets circumsporozoite protein minor repeats and neutralizes sporozoites in the liver. Immunity 2020;53(4):733-744.
5. Dondorp AM, Desakorn V, Pongtavornpinyo W, et al. Estimation of the total parasite biomass in acute falciparum malaria from plasma PfHRP2. PLoS Med 2005;2(8):e204-e204.
6. Mahmoudi S, Keshavarz H. Efficacy of phase 3 trial of RTS, S/AS01 malaria vaccine: the need for an alternative development plan. Hum Vaccin Immunother 2017;13:2098-2101.
7. Duffey M, Blasco B, Burrows JN, et al. Assessing risks of Plasmodium falciparum resistance to select next-generation antimalarials. Trends Parasitol 2021;37:709-721.
8. O’Brien MP, Forleo-Neto E, Musser BJ, et al. Subcutaneous REGEN-COV antibody combination to prevent Covid-19. N Engl J Med 2021;385:1184-1195.
9. Macintyre F, Ramachandruni H, Burrows JN, et al. Injectable anti-malarials revisited: discovery and development of new agents to protect against malaria. Malar J 2018;17:402-402.
- Breaking the Life Cycle of Plasmodium falciparum.