Malaria transmission blocking activity of sesquiterpene lactones from Vernonia amygdalina

Background: Most of the currently available anti-malarial drugs act on asexual stages of the Plasmodium parasite and have limited impact on the sexual stages to block transmission. Search for drugs active against transmissible stages imperative for the development of transmission blocking interventions. This study aimed to assess whether Vernonia amygdalina, a plant used traditionally to treat malaria fever, contains secondary metabolites interfering with the development of early sporogonic stages (ESS): gamete, zygote formation and/or ookinete maturation. Methods: Plasmodium berghei CTRPp.GFP was used to determine in vitro activity of fractions and isolated molecules against ESS. Gametocytemic blood from BALB/c mice was incubated in microplates with the test substances. Zygote and ookinete formation was scored after 40h of incubation under the fluorescent microscope (400x). The fractions, found active on P. berghei, were then examined on P. falciparum field isolates. Gametocytemic blood from volunteers was supplemented with fractions (at 100 ppm) and membrane fed to Anopheles coluzzii mosquitoes. A week later, mosquito midguts were dissected and examined for oocysts. Results: Organic fractions from the methanol extract of V. amygdalina leaves proved to be strongly active against ESS of P. berghei. Fraction 11 (eluted with n-hexane:ethylacetate 1:1 and 7:13) and fraction 13 (eluted with ethylacetate) suppressed ESS development by 98- 100% at a concentration of 50 ppm . In the experiments with P. falciparum field isolates, control mosquitoes displayed an oocyst prevalence ranging from 30 to 50%, whereas the oocyst prevalence in mosquitoes membrane fed with gametocytemic blood treated with fraction 11 and 13 at 100 ppm ranged from 0 to 32% and 0 to 8%, respectively. The oocyst density among oocyst-positive mosquitoes varied from 3.5 to 4.2 per mosquito in controls, compare to 1.8 to 2.4 and 1.3 to 1.8 in fraction 11 and 13, respectively. Subsequent chemical analysis revealed sesquiterpenes vernolide and vernodalol to be the major components of fraction 11 and 13, respectively. The isolated molecules confirmed their effects on P. berghei ESS in vitro, with vernodalol displaying a relatively stronger inhibitory activity than vernolide on the transmissible stages. Conclusion: Vernolide and vernodalol rich fractions from V. amygdalina leaves hold promise for the development of medicines impacting on the transmissible stages of the malaria parasites.