Larvicidal compounds from the plant Zanthoxylum gilletii against malaria vector Anopheles gambiae sensu stricto

Abstract

Malaria is a serious health problem in many African countries. The Anopheles gambiae mosquito, which is the major vector for this disease, has developed resistance against synthetic pyrethroids, which are the main stay of insecticide treated bed nets. The development of insecticide resistance and side effects associated with synthetic pesticides has triggered intense research efforts towards natural products (for vector control) such as essential oils and the nonvolailes because of their efficacy and safety. In this study, the larvicidal potential of essential oil and secofuroquinoline alkaloids isolated from the leaves of Zanthoxylum gilletii was studied against larvae of the malaria vector An. gambiae s.s. The essential oil was extracted by hydrodistillation and its chemical compositions determined by GC - MS. The oil was dominated by monoterpenes and sesquiterpenes which accounted for 34.00 % and 38.30 %, respectively. For the larvicidal assay 20 third instar larvae were used for each concentration of the solution. The oil recorded LC50 and LC90 values of 57.73 and 140.24 ppm after 24 h exposure period, respectively. Methanol extract (LC50 = 497.62 ppm), ethyl acetate (LC50 = 155.65 ppm) and hexane extract (LC50 = 274 ppm) were all active against the third instar larvae of An. Gambiae s.s. Bioassay-guided column fractionation of the ethyl acetate crude extract (6:4 ethyl acetate/hexane as eluting solvent mixture) afforded four fractions F1, F2, F3 and F4. All the four fractions were active against the larvae of An. gambiae s.s, F1, (LC50 = 705.24 ppm), F2 (LC50 = 542.33 ppm), F3 (LC50 = 146.80 ppm) and F4 (LC50 = 83.59 ppm). Further purification of F3 and F4 by Preparative Thin Layer Chromatography (PTLC) afforded mixture Z. Purification of mixture Z by Preparative High Performance Liquid Chromatography (PHPLC) afforded a pair of isomeric secofuroquinoline alkaloids Z-Dimethylrhoifolinate (43) and E-Dimethylrhoifolinate (44). Identification of compound 43 and 44 was done by the use of Mass Spectrometry, 1D and 2D NMR. Mixture Z exhibited LC50 value of 110.31 ppm and LC90 value of 216.31 ppm when tested against third instar larvae of An. gambiae. Compound 43 and 44 were isolated for the first time from Z. gilletii. The results obtained show that both the essential oil and the Secofuroquinoline alkaloids 43 and 44 isolated from the leaves of Z. gilletii have larvicidal effects and potentially can be used in malaria vector control.