ANTIMALARIAL ACTIVITIES OF SOLVENT EXTRACTS AND FRACTIONS FROM Adansonia digitata STEM BARK IN Plasmodium berghei-INFECTED MICE

ADEOYE, AKINWUNMI OLUWASEUN (2017-12)

Thesis

Malaria is caused by a protozoan of the genus Plasmodium and it is an infectious disease of humans and other animals. In the process of studying human malaria parasites, rodent parasites are recognized as valuable model parasites for the investigation of parasite-host interactions, biology of malaria parasites, drug testing and vaccine development. Increase in the resistance of malaria parasite to synthetic drugs has led to the continuous search for alternative treatment approach from plant origin. Majority of these plants have biologically active polyphenol components and phytochemicals which have protective and therapeutic properties. These phytochemicals are found as secondary plant metabolites that are explored for their bioactivities with application in medicine. Adansonia digitata L. Baobab (Bombacaceae) is rich in these secondary metabolites and is used for the treatment of malaria in Nigeria. The aim of this research was to investigate the possible anti-malarial potential of aqueous and methanolic extracts of A. digitata stem bark and its fraction on Plasmodium berghei infected mice. Aqueous and methanolic (AEAD and MEAD) extract was obtained from Adansonia digitata stem bark utilizing solvent extraction and the fractions was obtained through solvent partitioning. The free radical scavenging activity of the extract in vitro was evaluated. The extract was also evaluated for its antimalarial activity using curative test in mice. Severity of malaria was evaluated by determining the serum and tissue alkaline phosphatase (ALP), aspartate amino transferase (AST) activity, alanine amino tranferase (ALT), the serum C-reactive protein (CRP) and tumor necrosis factor-alpha (TNF-α). Mechanisms of action of the extract were investigated by measuring the degree of tissue peroxidation and tissue antioxidant status, mitochondrial swelling and mitochondria ATPase activity. The efficacy of Adansonia digitata stem bark extract in offering protection against experimental malaria was evaluated and its remediation effect when administered after established infection was also examined. The extract fraction was evaluated for its activity against inhibition of β-hematin formation. The results revealed that Adansonia digitata stem bark extracts exhibited DPPH radical scavenging activity, ferric reducing antioxidant power, metal chelating activity and hydroxyl radical scavenging activity comparable (p<0.05) with those of standard antioxidants. The HPLC-DAD analysis showed significant levels of flavonoids and phenolic acids in the extracts and fractions of Adansonia digitata stem bark. The results revealed that the aqueous and methanolic 21 extracts were active against P berghei NK65 in vivo and were not cytotoxic at both 200 and 400 mg/kg body weight. Methanolic extract exhibited higher antimalarial activity causing 70.15%, 90.18% and 100% inhibition in parasite growth at the dose of 400 mg/kg body weight on day 3, 5 and 7 post-inoculation respectively. The extracts showed a significant dose dependent increase in packed cell volume (PCV), at the two doses when administered after established infection compared to control. Moreover, ethylacetate extract fraction exhibited considerable antiplasmodial activity against inhibition of β-hematin formation (IC50 < 50 μg/ml). However, the ability of the extract fraction to inhibit β-hematin formation was significantly lower (P < 0.05) than that of chloroquine and artesunate. The extracts significantly reduced (P < 0.05) malondialdehyde concentrations in all the tissues investigated compared to the infected untreated mice on day 5 and 7 post-inoculation. Administration of the extract after established infections significantly increased (P < 0.05) glutathione concentrations and activities of superoxide dismutase and catalase in a dose-dependent manner compared to the untreated control on day 5 and 7 post-inoculation. The extracts significantly decreased serum creatinine concentrations but significantly increased (P < 0.05) serum urea and uric acid concentrations. Administration of the extract reduced serum concentrations of sodium ion, potassium ion, calcium ion and chloride ion compared to control but significantly increased serum concentrations of magnesium ion when compared to control. A significant increase in the serum concentrations of CRP and TNF-α of the control animals was observed when compared to the baseline (uninfected mice) and the group treated with extract. Significant relationship exists between serum CRP and TNF-α concentration and percentage parasitemia in the control group. Administration of the extracts lowered the CRP and TNF concentrations. Alkaline phosphatase (ALP) activity of the control group increase significantly (P < 0.05) in all the tissues investigated compared to the group that received the extract. However, a significant reduction in ALT and AST activity of the control group were observed compared to the group that received the extract. Ethylacetate fraction of A. digitata stem bark and the phenolics, quercetin and apigenin induced the opening of mitochondrial membrane permeability transition pore in a concentration dependent manner. The ethylacetate extract fraction also increased (P < 0.05) significantly the mitochondrial ATPase activity in a concentration dependent manner. 22 The results suggest that Adansonia digitata protects against Plasmodium berghei induced-malaria, and that administration of the extract after established infection reduced malaria progression. A significant relationship was obtained between antioxidant activity and phenolic content indicating that phenolic compounds contribute significantly to antioxidant and antimalarial activity of the plant. Bioactive compounds from Adansonia digitata influence mitochondria membrane permeability transition by inducing cell death through mitochondria-mediated pathway of apoptosis. Opening of MMPT pore and induction of apoptotic process could likely be one of the mechanisms of action of antimalarial compound.