Malaria, a disease caused by Plasmodium parasites remain one of the deadliest diseases despite progress made with various control measures. Plasmodium parasites are transmitted to humans by the bite of Anopheles mosquitoes. An effective and safe vaccine is the ultimate goal of malaria control and prevention. Recently it was published that a new malaria vaccine, the GAP 3KO is highly protective against malaria. This vaccine which is a triple gene knockout showed no transition to blood stage infection in both humanized mouse model and humans, thus suggesting complete attenuation.  Even for GAP 3KO, safety and risk assessment studies should examine the parasite and host factors/environment that may reverse or modulate the attenuated phenotype.

Malaria remain one of the deadliest diseases despite significant progress made with respect to various malaria control measures. In 2015, World Health Organization (WHO) reported 212 million estimated malaria cases and 429000 deaths due to malaria, and majority of the cases and deaths occurred in sub-Sahara Africa [1].  In humans, six species of Plasmodium parasite are able to cause infection but Plasmodium falciparum remain the major cause of mortality and morbidity. Plasmodium parasites are transmitted to humans by the bite of Anopheles mosquitoes. The development of an effective but safe vaccine is the ultimate goal of malaria control and prevention. In theory, such a vaccine has the potential of eradicating malaria worldwide, just like smallpox was eradicated with the smallpox vaccine. However, the complicated life cycle of Plasmodium parasites involving vertebrate and invertebrate hosts and various stage forms pose significant challenges to the development of an effective and safe vaccine.

Mosquirix^TM (RTS,S), the most advanced malaria subunit vaccine showed only partial efficacy and the immunity wanes significantly three years post immunization [2]. Whole parasite (sporozoites) vaccines have been generated and attenuation was achieved by radiation (radiation attenuated sporozoites, RAS), concurrent administration of un-attenuated sporozoites with antimalarial drugs (chloroquine prophylaxis with sporozoites, CPS), and by gene deletions or genome editing. Sporozoites modified by gene deletions are referred to as first generation genetically attenuated parasites (GAPs) while those modified by genome editing methods are next generation GAPs [3, 4]. The principle of adopting worst-case scenario in malaria vaccine safety and risk assessment studies (pre-clinical and Phase 1 trials) does not imply an examination of all possibilities that may cause harm in the short, medium and long term. Examination of every possibility that may pose a risk (probability that an event may happen multiplied by the consequence should such an event occur) is a miss-interpretation of the concept of the worst-case scenario. Such an interpretation creates an unending loop since it is virtually impossible to falsify or reject all possibilities. In our opinion, a correct interpretation of worst case scenario should be limited to  (i) scenarios that can be rejected or falsified based on existing scientific knowledge and methods, and (ii) scenarios that are not consistent to existing scientific knowledge but the method/s or approach through which the existing knowledge was acquired are imprecise and highly uncertain.

The plausibility of incomplete attenuation in RAS and CAS vaccines should be a major safety concern. Both can result in breakthrough malaria infections. Double gene deleted GAPs (2KO) have also been shown to be incompletely attenuated in human infection resulting in peripheral blood stage parasitemia [5]. Recently, the same group improved the attenuation by deleting another gene, in addition to the ones deleted in the GAP 2KO. This resulted in GAP 3KO which showed no transition to blood stage stage infection in both humanized mouse model and and in human phase 1 clinical trial [6, 7].  Although these results point to complete attenuation of the GAP 3KO vaccine, the results have to be interpreted in the context of the stipulated experimental conditions. In the context of wort case scenario, the following questions are relevant; (i) will complete attenuation hold if animal models and humans are exposed to high sporozite concentration? (ii) Is the GAP 3KO vaccine completely attenuated in immunocompromised individuals especially since it not uncommon to have an individual with both malaria and AIDs? (iii)  Can another Plasmodium falciparium genotype complement or rescue the GAP 3KO vaccine during mixed or co-infection? (iv) Will pre-exposure to Plasmodium antigens as it is in malaria endemic areas affect the attenuation of GAP 3KO? and (v) are their unintended genetic modification in the genome of GAP 3KO compared to the wild type?

Hypothesis derived from these questions can be tested through well-planned safety and risk assessment studies.  Scientific tools and methodologies are available to falsify or reject the hypothesis. Thus, there is no conceivable reason why they should not be addressed in pre-clinical studies in animal models and in phase 1 human trials.

GenØk contact: Malachy Okeke

References

1. Organization WH: World Malaria Report 2016. World Health Organization; 2016.
2. Rts SCTP: Efficacy and safety of RTS,S/AS01 malaria vaccine with or without a booster dose in infants and children in Africa: final results of a phase 3, individually randomised, controlled trial. Lancet, 386:31-45.
3. Hoffman SL, Vekemans J, Richie TL, Duffy PE: The march toward malaria vaccines. Vaccine 2015, 33 Suppl 4:D13-23.
4. Singer M, Frischknecht F: Time for Genome Editing: Next-Generation Attenuated Malaria Parasites. Trends Parasitol 2016.
5. Spring M, Murphy J, Nielsen R, Dowler M, Bennett JW, Zarling S, Williams J, de la Vega P, Ware L, Komisar J, et al: First-in-human evaluation of genetically attenuated Plasmodium falciparum sporozoites administered by bite of Anopheles mosquitoes to adult volunteers. Vaccine 2013, 31:4975-4983.
6. Mikolajczak SA, Lakshmanan V, Fishbaugher M, Camargo N, Harupa A, Kaushansky A, Douglass AN, Baldwin M, Healer J, O’Neill M, et al: A next-generation genetically attenuated Plasmodium falciparum parasite created by triple gene deletion. Mol Ther 2014, 22:1707-1715.
7. Kublin JG, Mikolajczak SA, Sack BK, Fishbaugher ME, Seilie A, Shelton L, VonGoedert T, Firat M, Magee S, Fritzen E, et al: Complete attenuation of genetically engineered Plasmodium falciparum sporozoites in human subjects. Sci Transl Med 2017, 9.

Photo: Adobestock #55068098