Abstract
Malaria is still the most dangerous parasitic infectious disease causing millions of deaths every year. In many countries where Malaria is endemic the traditional medical methods constitute a strong part in the public health care system. Phytochemical research on medicinal plants traditionally used as antimalarials are urgently needed. In this context we looked at several Central American and West African medicinal plants, which are traditionally used against malaria and/or symptoms of the disease. Analysis of all plant species was based on literature and/or ethnobotanical studies. We produced plant extracts of different polarity from various plant parts of eight species from El Slavador and eleven species from Ghana, which are still used in the Central American and West African traditional phytotherapy, All extracts were tested for their in vitro actvity against a Chloroquine-sensitive strain PoW and a Chloroquine-resistent clone Dd2 of Plasmodium falciparum. The stem bark from Exostema mexicanum Gray (Rubiaceae), the leaves from Calea tenuifolia Kunth (Asteraceae), and the aerial parts from Microglossa pyrifolia (Lam.) Kuntze (Asteraceae) exhibited a promising antiplasmodial activity. Thus bioactivity-guided fractionations were carried out in order to isolate and characterize the major antiprotozoan principles of this species.
Further purification of the active lipophilic extract (petrol ether/ methanol 1:1) and the methanolic extract of E. mexicanum led to the isolation of seven 4-phenylcoumarines, with 4',5,7,8-tetramethoxy-4-phenylcoumarin [O-methyl-exostemin], 4',8-dihydroxy-5,7-dimethoxy-4-phenylcoumarin [exomexin A], and 3',4'-dihydroxy-5,7,8-trimethoxy-4-phenylcoumarin [exomexin B] as new natural compounds. O-Methyl-exostemin showed a significant activity against P. falciparum (IC50-values: 10.5 µM [PoW] and 4.7 µM [Dd2]). Additionally, of all active compounds the cytotoxic effect on the human endothelium cell line (ECV 304) in vitro were assessed. Here, all tested 4-phenylcoumarines exhibited a significant cytotoxic activity with IC50-values between 1,2 µM and 10,0 µM. Given this result, the unrestricted use of the E. mexicanum as an ethnomedicinal substance cannot be recommended based.
Bioactivity-guided fractionation of the lipophilic leaf extract of C. tenuifolia led to the isolation of five flavones and two unsaturated fatty acids. All compounds showed activities against P. falciparum, with 7-methylacacetin exhibiting the strongest inhibition of the parasite (IC50-values: 20.1 µM [PoW] and 17.1 µM [Dd2]). All seven compounds were also tested on the human endothelium cell line (ECV 304). Based on our results, we cannot completely assess the antiplasmodial potential of this plant species. However our findings may represent a rational explanation for a potential antimalarial effect of the leaves of C. tenuifolia.
Bioactivity-guided fractionation of the aerial parts of M. pyrifolia lead to the isolation of 18 natural compounds, e.g. different furanoditerpenes and geranylgeraniol derivatives. Sinapyl diangelate, 1-acetyl-6E, 10E, 14E-geranylgeraniol-19-oic acid and 19-oxo-6E, 10E, 14E-geranylgeraniol were identified as new natural compounds. The strongest inhibition of P. falciparum could be observed for 2E- phytol and 6E, 10E, 14E-geranylgeraniol-19-oic acid with IC50-values ranging from 8.4 µM [PoW] to 16.3 µM [Dd2]. All active compounds showed only a weak cytotoxic effect on ECV 304 with IC50-values between 95.6 µM and 200.2 µM, or they were not cytotoxic. From several active fractions we isolated 6E, 10E, 14E-geranylgeraniol-19-oic acid. This diterpene was also detectable in the aqueous extract despite of its rather lipophilic character. Therefore, we assume that the aliphatic unsaturated compounds are representing the antiprotozoan principle of this species, and 6E, 10E, 14E -geranylgeraniol-19-oic acid supposed to be mainly responsible for a moderate antiplasmodial activity of a traditional plant preparation of M. pyrifolia.
During another study, the three monomeric ergolines terguride, festuclavine, pergolide, and their N, N'-spacer-linked oligomeric derivatives have been investigated for their in vitro activity against P. falciparum. All monomers have shown only a weak antiplasmodial effect, but either N-1, N-1'- or N-6, N-6'-spacer-linked dimerization are able to enhance their antiparasitic activity. A N-6, N-6?-spacer-linked depropylpergolide dimer exhibited the highest activities with IC50-values of 0.14 µM and 0.13 µM against PoW and Dd2, respectively. Additionally from all tested compounds the cytotoxic effects against mouse fibroblasts (NIH 3T3) in vitro were evaluated. For further studies a N-1, N-1'-spacer-linked festuclavine trimer and a N-1, N-1'-spacer-linked terguride tetramer are interesting. Both compounds were not cytotoxic and possessed good aniplasmodial activities with IC50-values between 0.54 µM and 1.53 µM against Dd2. |
