With the increase in recent years in the prevalence of malaria, and in drug resistance of Plasmodium falciparum, there has been much interest in natural plant products for new antimalarials with novel modes of action against Plasmodium. Artemisinin or Qinghaosu is one such antimalarial isolated from a Chinese herb, Anemisia annua (Asteraceae) and it is currently undergoing phase I and II clinical trials. The Southern African species, Artemisia afra (African wormwood, wildeals, lengana) is commonly used by local traditional healers for symptoms of malaria, in particular fever. Thus it seemed appropriate to investigate this species for antimalarial activity. Crude petroleum ether soxhlet extracts of Anemisia afra had demonstrated antimalarial activity against Plasmodium falciparum, FCR-3, cultured in vitro. The IC₅₀ values ranged from 5-13μg/ml. The extract from leaves and flowers was then screened against D10 (chloroquine-sensitive) and FAC8 (chloroquineresistant) P. falciparum, in vitro, with IC₅₀ values of 1.03μg/ml and l.5μg/ml respectively. This extract was fractionated by column chromatography using silica gel-60 and the fractions obtained were screened for antimalarial activity. The most active fraction had an IC₅₀ of 0.5μg/ml against D10 and FAC8. Using TLC and HPLC-UV analysis with pure artemisinin as a standard, no artemisinin could be detected in this fraction. This result was confirmed by thermospray LC-MS analyses. Purification of this fraction yielded ultimately a single pure compound; a clear colourless oil identified by MS and NMR analyses as hydroxydavanone. The compound was screened against a variety of P. falciparum strains with varying degrees of sensitivity and resistance to both chloroquine and mefloquine. Their sensitivity against artemisinin was also established. IC₅₀ values obtained for the isolated pure compound against P. falciparum ranged from 0.87 to 2.54μg/ml. The IC₅₀ values obtained for general cytotoxicity of the crude extract and isolated pure compound against RAT-I fibroblast cells were 34.78 ± 8.23 and 6.29 ± 0.95 μg/ml (n=4) respectively. Thus the crude extract and isolated pure compound exhibited a greater antimalarial than cytotoxic effect. Hence, there are implications for A. afra to be used as a phytomedicine for the treatment of malaria. In vivo studies are recommended for hydroxydavanone in order to fully assess its potential for clinical use.

No / New antimalarial drugs are urgently needed to combat emerging multidrug resistant strains of malaria parasites. This Highlight focuses on plant-derived natural products that are of interest as potential leads towards new antimalarial drugs including synthetic analogues of natural compounds, with the exception of artemisinin derivatives, which are not included due to limited space. Since effective antimalarial treatment is often unavailable or unaffordable to many of those who need it, there is increasing interest in the development of locally produced herbal medicines; recent progress in this area will also be reviewed in this Highlight.

Malaria is one of the three most deadly diseases in Africa. Although there are available treatments, their efficacy has been greatly reduced over the past two decades due to the development of resistance to currently available drugs. This has necessitated the search for new and effective antimalarial agents. This project approached the search for new antimalarial compounds in two ways: (i) by screening natural products isolated from marine algae against the Plasmodium parasite and (ii) by modification of selected isolated active compounds to target 1-deoxY-đ-xylulose 5-phosphate reductoisomerase (DXR), an enzyme found in the nonmevalonate isoprenoid biosynthetic pathway of Plasmodium Jalciparum. It was envisaged that such a compound would exhibit dual action on the Plasmodium parasite. Extracts obtained from 22 marine algae were prefractionated by solvent partitioning and were screened for anti plasmodial activity against the chloroquine sensitive (CQS) P. Jalciparum D 10 strain. Overall, 50% of the algae screened produced at least one crude fraction with activity against P. Jalciparum. Extracts of the algae Sargassum heterophyllum, Plocamium cornutum, Amphiroa ephedrea and Pterosiphonia cloiophylla gave the most promising results. Fractionation of S. heterophyllum afforded three tetraprenyltoluquinols (3.1, 3.2 and 3.5) and an all-trans-fucoxanthin (3.6). Three new compounds (4.5, 4.6 and 4.7) and two known halogenated monoterpenes (4.1 and 4.4) were isolated from P. cornutum. Each of the isolated compounds from both S. heterophyllum and P. cornutum showed antiplasmodial activity with IC₅₀ values ranging from 2.0 - 15.3 μM for S. heterophyllum and 13 - 230 μM for P. cornutum. Attempts to synthetically modify halogenated monoterpene 4.4 by dihydroxylation and phosphorylation in order to inhibit the DXR enzyme was unsuccessful. However, the hemiterpene analogue (5.42) of the halogenated monoterpenes was successfully phosphorylated and dihydroxylated to give compound 5.45 which showed promising activity against DXR. The result obtained indicated that the proposed phosphorylation and dihydroxylation of the halogenated monoterpene 4.4 would result in the synthesis of a potent DXR inhibitor and therefore a potential antimalarial agent with dual mode of action on the Plasmodium parasite.

by Ng Wang Kit. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (leaves 131-139). / Abstracts in English and Chinese. / Acknowledgements --- p.iii / Abstract --- p.v / List of Tables --- p.ix / List of Figures --- p.x / List of Abbreviations --- p.xiii / Table of Contents --- p.xv / Chapter Chapter 1: --- General Introduction --- p.1 / Chapter Chapter 2: --- Literature Review --- p.3 / Chapter 2.1 --- Malaria --- p.3 / Chapter 2.1.1 --- Global picture --- p.3 / Chapter 2.1.2 --- Malaria mechanics --- p.4 / Chapter 2.1.3 --- Life cycle of malaria parasite --- p.4 / Chapter 2.2 --- Treatment of malaria ´ؤ malaria drugs --- p.5 / Chapter 2.2.1 --- Antimalarial drugs --- p.5 / Chapter 2.2.2 --- Drug resistance --- p.6 / Chapter 2.3 --- Treatment of malaria - malarial vaccines --- p.7 / Chapter 2.3.1 --- Malarial vaccine developments --- p.7 / Chapter 2.3.2 --- Transmission blocking vaccines --- p.7 / Chapter 2.3.3 --- Pre-erythrocytic vaccines --- p.9 / Chapter 2.3.4 --- Blood stage vaccines --- p.10 / Chapter 2.4 --- The major merozoite protein - gpl95 --- p.11 / Chapter 2.5 --- Plants as bioreactors --- p.12 / Chapter 2.5.1 --- Products of transgenic plants --- p.13 / Chapter 2.6 --- Transgenic plants for production of subunit vaccines --- p.14 / Chapter 2.6.1 --- Norwalk virus capsid protein production --- p.15 / Chapter 2.6.2 --- Hepatitis B surface antigen production --- p.15 / Chapter 2.7 --- Tobacco and Arabidopsis as model plants --- p.16 / Chapter 2.7.1 --- Arabidopsis --- p.16 / Chapter 2.7.2 --- Tobacco --- p.17 / Chapter 2.8 --- Transformation methods --- p.17 / Chapter 2.8.1 --- Direct DNA uptake --- p.17 / Chapter 2.8.1.1 --- Plant protoplast transformation --- p.17 / Chapter 2.8.1.2 --- Biolistic transformation --- p.18 / Chapter 2.8.2 --- Agrobacterium-mediated transformation --- p.18 / Chapter 2.8.2.1 --- Leaf-disc technique --- p.18 / Chapter 2.8.2.2 --- In planta transformation --- p.19 / Chapter 2.9 --- Phaseolin --- p.20 / Chapter 2.10 --- Detection and purification of recombinant products - Histidine tag --- p.21 / Chapter 2.11 --- Aims of study and hypotheses --- p.22 / Chapter Chapter 3: --- Materials and Methods --- p.24 / Chapter 3.1 --- Introduction --- p.24 / Chapter 3.2 --- Chemicals --- p.24 / Chapter 3.3 --- Expression in tobacco system --- p.24 / Chapter 3.3.1 --- Plant materials --- p.24 / Chapter 3.3.2 --- Bacterial strains --- p.25 / Chapter 3.3.3 --- Chimeric gene construction for tobacco transformation --- p.25 / Chapter 3.3.3.1 --- The cloning of pTZPhasp/flgp42-His/Phast (F1) --- p.26 / Chapter 3.3.3.2 --- The cloning of pBKPhasp-sp/flgp42-His/Phast (P9) --- p.30 / Chapter 3.3.3.3 --- The cloning of pHM2Ubip/flgp42-His/Nost (C2) --- p.30 / Chapter 3.3.4 --- Confirmation of sequence fidelity of chimeric gene by DNA sequencing --- p.33 / Chapter 3.3.5 --- Cloning of chimeric gene into binary vector --- p.34 / Chapter 3.3.6 --- Triparental mating of Agrobacterium tumefaciens LBA4404/pAL4404 --- p.35 / Chapter 3.3.7 --- Tobacco transformation and regeneration --- p.36 / Chapter 3.3.8 --- GUS assay --- p.37 / Chapter 3.3.9 --- Genomic DNA isolation --- p.37 / Chapter 3.3.10 --- PCR amplification and detection of transgene --- p.38 / Chapter 3.3.11 --- Southern blot analysis --- p.38 / Chapter 3.3.12 --- Total seeds RNA isolation --- p.39 / Chapter 3.3.13 --- RT-PCR --- p.39 / Chapter 3.3.14 --- Northern blot analysis --- p.40 / Chapter 3.3.15 --- Protein extraction and SDS-PAGE --- p.40 / Chapter 3.3.16 --- Western blot analysis --- p.41 / Chapter 3.4 --- Expression in Arabidopsis system --- p.42 / Chapter 3.4.1 --- Plant materials --- p.42 / Chapter 3.4.2 --- Bacterial strains --- p.42 / Chapter 3.4.3 --- Chimeric gene construction --- p.42 / Chapter 3.4.3.1 --- The cloning of pBKPhasp-sp/His/EK/p42/Phast (DH) --- p.43 / Chapter 3.4.3.2 --- The cloning of pTZPhaSp/His/EK/p42/Phast (EH) --- p.45 / Chapter 3.4.3.3 --- The cloning of pBKPhasp-sp/His/EK/flgp42/Phast (DHF) and pTZPhasp/His/EK/flgp42/Phast (EHF) --- p.45 / Chapter 3.4.4 --- Confirmation of sequence fidelity of chimeric genes --- p.45 / Chapter 3.4.5 --- Cloning of chimeric gene into Agrobacterium binary vector --- p.49 / Chapter 3.4.6 --- Transformation of Agrobacterium tumefaciens GV3101/pMP90 with chimeric gene constructs --- p.49 / Chapter 3.4.7 --- Arabidopsis Transformation --- p.49 / Chapter 3.4.8 --- Vacuum infiltration transformation --- p.50 / Chapter 3.4.9 --- Selection of successful transformants --- p.51 / Chapter 3.4.10 --- Selection for homozygous plants with single gene insertion --- p.51 / Chapter 3.4.11 --- GUS assay --- p.52 / Chapter 3.4.12 --- Genomic DNA isolation --- p.52 / Chapter 3.4.13 --- PCR amplification and detection of transgenes --- p.52 / Chapter 3.4.14 --- Southern Blot analysis --- p.52 / Chapter 3.4.15 --- Total siliques RNA isolation --- p.53 / Chapter 3.4.16 --- RT-PCR --- p.53 / Chapter 3.4.17 --- Northern blot analysis --- p.53 / Chapter 3.4.17 --- Protein extraction and SDS-PAGE --- p.54 / Chapter 3.4.18 --- Western blot analysis --- p.54 / Chapter 3.5 --- In vitro transcription and translation --- p.54 / Chapter 3.5.1 --- In vitro transcription --- p.54 / Chapter 3.5.2 --- In vitro translation --- p.55 / Chapter 3.6 --- Particle bombardment of GUS fusion gene --- p.56 / Chapter 3.6.1 --- Chimeric gene constructs --- p.56 / Chapter 3.6.2 --- Particle bombardment using snow bean cotyledon --- p.61 / Chapter Chapter 4: --- Results --- p.63 / Chapter 4.1 --- Tobacco system --- p.63 / Chapter 4.1.1 --- Chimeric gene constructs --- p.63 / Chapter 4.1.2 --- Tobacco transformation and regeneration --- p.65 / Chapter 4.1.3 --- GUS activity assay --- p.67 / Chapter 4.1.4 --- Molecular analysis of transgene integration --- p.68 / Chapter 4.1.4.1 --- Genomic DNA extraction and PCR --- p.68 / Chapter 4.1.4.2 --- Southern blot analysis --- p.70 / Chapter 4.1.5 --- Molecular analysis of transgene expression --- p.72 / Chapter 4.1.5.1 --- Total RNA isolation and RT-PCR --- p.72 / Chapter 4.1.5.2 --- Northern blot analysis --- p.75 / Chapter 4.1.6 --- Genomic PCR to confirm whole gene transfer --- p.76 / Chapter 4.1.7 --- Biochemical analysis of transgene expression --- p.78 / Chapter 4.1.7.1 --- Protein extraction and SDS-PAGE --- p.78 / Chapter 4.1.7.2 --- Western blot analysis --- p.78 / Chapter 4.2 --- Arabidopsis system --- p.83 / Chapter 4.2.1 --- Chimeric gene constructs --- p.83 / Chapter 4.2.2 --- Arabidopsis transformation and selection --- p.85 / Chapter 4.2.3 --- Selection of transgenic plants --- p.87 / Chapter 4.2.4 --- Assay of GUS activity --- p.91 / Chapter 4.2.5 --- Molecular analysis of transgene integration --- p.92 / Chapter 4.2.5.1 --- Genomic DNA extraction and PCR --- p.92 / Chapter 4.2.5.2 --- Southern blot analysis --- p.96 / Chapter 4.2.6 --- Molecular analysis of transgene expression --- p.99 / Chapter 4.2.6.1 --- Total RNA isolation and RT-PCR --- p.99 / Chapter 4.2.6.2 --- Northern blot analysis --- p.106 / Chapter 4.2.7 --- Genomic PCR for confirmation of whole gene transfer --- p.107 / Chapter 4.2.8 --- Biochemical analysis of transgene expression --- p.108 / Chapter 4.2.8.1 --- Protein extraction and SDS-PAGE --- p.108 / Chapter 4.2.8.2 --- Western blot analysis --- p.108 / Chapter 4.3 --- In vitro transcription and translation --- p.112 / Chapter 4.4 --- Particle bombardment of p42/ GUS fusion gene --- p.115 / Chapter Chapter 5: --- Discussion and Future perspectives --- p.117 / Chapter 5.1 --- Failure in detecting transgene expression --- p.117 / Chapter 5.2 --- Poor transgene expression --- p.120 / Chapter 5.2.1 --- Bacillus thuringiensis toxin and green fluorescent protein --- p.120 / Chapter 5.2.2 --- AT-richness --- p.121 / Chapter 5.2.3 --- Deleterious sequence - AUUUA --- p.123 / Chapter 5.2.4 --- Presence of AAUAAA or AAUAAA-like motifs --- p.125 / Chapter 5.2.5 --- Codon usage --- p.126 / Chapter 5.3 --- Future perspectives --- p.127 / Chapter Chapter 6: --- Conclusion --- p.129 / References --- p.131