ISOLATION AND CHARACTERISATION OF SECONDARY METABOLITES WITH ANTIPLASMODIAL ACTIVITY IN SELECTED MEDICINAL PLANTS OBTAINED FROM NIGER STATE, NIGERIA

Abstract

Malaria continues to be a global burden as the efficacy of most anti-malarial drugs have been compromised by the evolution of resistant parasites. Plants present unlimited sources of novel substances with many therapeutic potentials. This study was designed to obtain bioactive compounds with antiplasmodial potential and high safety margin from selected medicinal plants that could be useful against both sensitive and resistant parasites. Eight medicinal plants; namely Agelanthus dodoneifolius, Securidaca longepedunculata, Neocarya macrophylla, Merremia hederacea, Zanthoxylum zanthoxyloides, Leptadenia hastata, Polycarpea linearfolia and Lophira alata collected in Niger State, Nigeria, extracted in methanol were subjected to phytochemical screening and acute toxicity study using standard methods. The plant extracts were screened for antiplasmodial activity in vitro against chloroquine-sensitive (CQS) strain, NF54 and chloroquine-resistant strain (CQR) K1 of Plasmodium falciparum while the in vivo activity was tested against CQS(NPK) strain of Plasmodium berghei at 100, 200 and 400 mg/kg bw. Subchronic toxicological screening of the most active extracts (P. linearfolia and L. hastata) was carried out at 200 mg/kg bw orally for 28 days in rats. Biochemical and haematological parameters were monitored while histopathological examination of the liver, kidney, heart and spleen of test animal and control groups were also undertaken. The most active extracts in vivo were subjected to bioassay guided fractionation. 1H NMR, 13C NMR and HPLC-ESI/MS were used to characterize active compounds from the potent fractions. The plant extracts contained a variety of phytochemicals, except phytosteroids that was absent in A. dodoneifolius. Acute toxicity study revealed P. linearfolia was safest with LD50 value of 4500 mg/kg bw. Z. zanthoxyloides recorded highest in vitro inhibition against CQS and CQR with IC50 of 1.07 and 1.31 µg/ml respectively. In the in vivo activity of the tested extracts, the parasite density of L. hastata (576 parasites/µl) and P. linearfolia (473 parasites/µl) at 400 mg/kg bw were comparable to the standard control (431 parasites/µl). Mean survival time of L. hastata and P. linearfolia treated groups were also comparable to the standard in the range of 31-35 days. Biochemical parameters of the rats for the 2 extracts administered subchronically revealed a significant (p<0.05) increase in creatinine, bilirubin and HDL concentrations when compared to the control. However, the LDL concentrations of the treated groups decreased significantly (p<0.05). Serum glucose concentration decreased while AST activity increased significantly (p<0.05) for P. linearfolia treated group when compared to the control. RBC count, haemoglobin concentration and haematocrit of P. linearfolia treated group decreased significantly(p<0.05) compared to the control. White blood cell indices, of L. hastata group increased significantly(p<0.05) compared to the control. Histopathological result showed normal cell architecture of all the organs analysed except the spleen that showed hyperplastic lymphoid follicles in L. hastata treated group. The six fractions of L. hastata: Lh1, Lh2 and Lh3 had IC50 value higher than10 µg/ml, while Lh4A, Lh4B and Lh5 had IC50 values of 4.24, 8.50 and 7.24 µg/ml. All the six fractions of P. linearfolia, had IC50 higher 10 µg/ml except Tk3 with a value of 2.40 µg/ml. Administration of fractions Lh3 of L. hastata and Tk3 of P. linearfolia resulted in complete parasite clearance on day 14 post infection. Spectral analysis identified two antiplasmodial compounds: 2,2’ –methylene bis(6-tert-butyl-4-methylphenol) and Sclerinone A in P. linearfolia extract, and 2,2’ –methylene bis(6-tert-butyl-4-methylphenol) in L. hastata. These antiplasmodial agents could serve as templates for the synthesis of new antimalarial drugs operating synergistically against both CQ sensitive and resistant strains of Plasmodium falciparum.