Primaquine's gametocytocidal efficacy in malaria asymptomatic carriers treated with dihydroartemisinin-piperaquine

Malaria is a parasitic disease transmitted by mosquitoes of the species Anopheles. The parasite can be found in the human host in two forms, asexual and sexual. The latter does not cause disease but is responsible for the transmission of the infection from the human host to the vector mosquito. Therefore, interventions reducing the transfer of parasite sexual forms, called also gametocytes, from man to mosquito may have a major impact on malaria transmission and hence on the burden of disease. The only available treatment against gametocytes is primaquine, an old drug that has not been extensively used in sub-Saharan Africa because it can cause the destruction of reb blood cells in people with a genetic conditions called glucose-6-phosphate dehydrogenase deficiency, resulting sometimes in life-threathening anemia. However, the risk for anaemia is dose-dependent, i.e. the risk increases with increasing dose. The amount of primaquine needed to eliminate gametocyte has been established in the 60s in a small number of experimentally challenged volunteers. It is unknown if lower dosages of primaquine, which are probably associated with a lower risk for anaemia, could have the same effect than the recommended one. The use of primaquine at lower dosages would open the possibility of using it on a large scale in sub-Saharan Africa, where the prevalence of glucose-6-phosphate dehydrogenase deficiency can be as high as 15%.
The general objective of this proposal is to determine the lowest possible dose of primaquine having similar activity against gametocytes than the recommended one. This will be done by carring out a clinical trial in two rural sites in The Gambia. In the study area, the population will be screened for malaria infection. Those positive, i.e. with a malaria infection but without symptoms, will be given an artemisinin-based combination treatment (dihydroartemisinin-piperaquine) and randomized to receive only this treatment or to have in addition the recommended or two lower dosages of primaquine. The treatment will be given over 3 days, with primaquine given, when required, in association with the last dose of dihydroartemisinin-piperaquine. Study subject will be actively followed up for more than a month, with blood sampling at regular intervals. The blood collected will be used to determine the difference in gametocyte carriage between the different treatment groups. Gametocytes will be searched with molecular methods, which are more sensitive than microscopy. In a subgroup of study subject we will check what is the actual difference in transmission to mosquitoes between individuals having received different treatments. We will collect a blood sample one week after the beginning of the treatment and we will feed laboratory-reared mosquitoes that will be dissected one week after, to check if the have become infected. The results of this study will be used to determine the feasibility of deploying primaquine on a large scale in sub-Saharan Africa, where the malaria burden is the highest, and may contribute to the drive towards malaria pre-elimination/elimination in this continent.

A single dose of primaquine (PQ) can be added for its gametocytocidal properties and for decreasing transmission to an arteminin-based combination treatment (ACT). However, PQ use has been and is still limited by its haematological toxicity (haemolytic anaemia), particularly in individuals with glucose-6-phosphate dehydrogenase deficiency (G6PDd). Such an effect is dose dependent and may be much lower with decreasing doses. We aim at establishing the lowest possible gametocytocidal dose of PQ in asymptomatic malaria carriers. The study will be carried out in 2 rural sites in the Gambia, each with an insectary necessary for the direct membrane feeding assays (DMFA). Asymptomatic malaria carriers will be identified by screening the general population. Individuals fulfilling the inclusion/exclusion criteria will be randomised to one of the 4 arms, i.e. dihydroartemisinin-piperaquine either alone or with PQ at the recommended or at two lower doses. After treatment individuals will be followed at day 3, and then weekly until day 42. Gametocytaemia after treatment will be measured at each scheduled visit by molecular tools. We will measure by DMFA infectiousness to mosquitoes at day 7 in a subgroup of study subjects. Perceptions of: (i) asymptomatic carriage and non-adherence to treatment, and their influence on transmission; (ii) benefits, inconveniences and risks of adherence to PQ treatment among asymptomatic malaria carriers will be assessed by qualitative and quantitative methods. The study has been powered to show a significant difference between the dihydroartemisinin-piperaquine arm alone and that with the recommend PQ dosage. In addition, the study will be able to show non-inferiority (7-8% non-inferiority limit) between the PQ recommended dose and the lower ones.

The proposed research program has the potential of having a large impact on the way malaria control and elimination are tackled in sub-Saharan Africa. It will provide the necessary data for the potential use of a lower-than-recommended dose of primaquine to eliminate gametocytes from malaria infected individuals, hence the potential of reducing transmission. When considering that, despite being recommended as a component of pre-elimination or an elimination program, primaquine is not used in sub-Saharan Africa, for fears of haematological toxicity, providing data on the gametocytocidal activity of lower doses can open the door for its large scale use. The assumption is the risk for haemolysis will be significantly lower with lower doses. This needs to be verified with an additional study in which glucose-6-phospate dehydrogenase deficient individuals will be treated with the new dose. Therefore, beneficiaries will be the populations living in malaria endemic countries, particularly those having achieved a good level of control, e.g. The Gambia, Zanzibar, and Sao Tome. It would be possible to administer primaquine to malaria patients attending health facilities, to asymptomatic carriers identified by systematic screening or to the whole population as a component of a mass drug administration program. The major benefit for subjects treated with primaquine at a lower dose will be the lower risk for serious adverse events, i.e. haemolytic anaemia. Assuming primaquine is deployed on a large scale, its impact on gametocytes and hence transmission will be beneficial for the whole population as it will reduce the risk of malaria.