Luc Pieters

Correlation between chemical composition and antibacterial activity of essential oils of some aromatic medicinal plants growing in the Democratic Republic of Congo.

The chemical composition of essential oils from 15 aromatic medicinal plant species growing in the Democratic Republic of Congo have been studied. More than 15 constituents in an amount higher than 0.1% were identified in each essential oil. 1,8-cineole, alpha and beta-pinene, p-cymene, myrcene, gamma-terpinene, alpha-terpineol and limonene were prevalent constituents in almost more than 10 selected plant species. Results from the antibacterial testing by the diffusion method indicate that all essential oils (5 microl per disc) inhibited the growth of selected bacteria at different extents. The most active antibacterial essential oils were those of the leaves of Eucalyptus camadulensis and Eucalyptus terticornis (12-30 mm zone diameter of inhibition). They showed particularly a most potent inhibition of Pseudomonas aeruginosa growth (15-16 mm), followed by Eucalyptus robusta (12 mm). Essential oils from the leaves of Eucalyptus alba, Eucalyptus citriodora, Eucalyptus deglupta, Eucalyptus globulus, Eucalyptus saligna, Eucalyptus robusta, Aframomum stipulatum, Cymbopogon citratus, Ocimum americanum and that of the seeds of Monodora myristica showed also a good antibacterial activity (10-18 mm). Eucalyptus propinqua, Eucalyptus urophylla and Ocimum gratissimum essential oils were the less active samples against the selected bacteria. No correlation between the amount of major constituents such as 1,8-cineol, alpha-pinene, p-cymene, cryptone or thymol and the antibacterial activity was observed.

Antimalarial activity of 20 crude extracts from nine African medicinal plants used in Kinshasa, Congo.

Twenty extracts including ten EtOH and ten CH2Cl2 from different parts of nine African medicinal plants used in Congolese traditional medicine for the treatment of malaria, were submitted to a pharmacological test in order to evaluate their effect on P. falciparum growth in vitro. Of these plant species, 14 (70%) extracts including EtOH and CH2Cl2 from Cassia occidentalis leaves, Cryptolepis sanguinolenta root bark, Euphorbia hirta whole plant, Garcinia kola stem bark and seeds, Morinda lucida leaves and Phyllanthus niruri whole plant produced more than 60% inhibition of the parasite growth in vitro at a test concentration of 6 microg/ml. Extracts from E. hirta, C. sanguinolenta and M. morindoides showed a significant chemosuppression of parasitaemia in mice infected with P. berghei berghei at orally given doses of 100-400 mg/kg per day.

Condensed vegetable tannins: Biodiversity in structure and biological activities

Abstract Proanthocyanidins, as an important class of secondary plant metabolites, are in many cases the active principles of the medicinal plants from which they are isolated. The structural complexity and conformational properties of the lower molecular weight oligomers have been investigated thoroughly, while the chemistry of the polymers still remains a difficult topic. Shikimate-derived phenolics like flavonoids and tannins are widely distributed in plant kingdom and are thus not of interest as classificatory tool; oxidation levels however are indicative in the attribution of evolutionary status among phyla and within each phylum. The main biological and pharmacological effects reported for condensed tannins can be classified into antibacterial and antiviral activities, enzyme inhibition, anti-oxidative effects, antimutagenic and antitumoral properties, next to some more specific interactions e.g. with vascular and cardial systems and inflammation processes. Their anticipated interaction with biological systems originates in principle directly from the physical and chemical properties of the polyphenolic skeleton, although prominent individual differences have been observed.

Structure-activity relationship of flavonoids with superoxide scavenging activity.

The superoxide scavenging activities of 12 flavonoids were measured. The superoxide anions were generated by a hypoxanthine-xanthine oxidase system and measured by the nitrite method. The results showed that the scavenging ability enhanced with an increasing number of hydroxyl groups in rings B. Substitution at C3 position with a hydroxyl group increased the activity. Compared to a methoxyl group or a glycoside in this position, a free hydroxyl group showed the highest activity. A saturated C2−C3 bond showed a higher activity than a unsaturated bond. The absence of a carbonyl group at C4 position increased the activity.

New alkaloids from Cryptolepis sanguinolenta

Neocryptolepine and biscryptolepine, two new alkaloids, were isolated from the root bark extract of the African medicinal plant, Cryptolepis sanguinolenta (Lindl.) Schlechter (Periplocaceae), and their structures elucidated on the basis of spectral evidence. The 1H and 13C NMR assignments of cryptoquindoline, a known artefact, were revised.

Antiamoebic and spasmolytic activities of extracts from some antidiarrhoeal traditional preparations used in Kinshasa, Congo.

Three major extracts from some traditional preparations, based on medicinal plants, used as antidiarrhoeal agents were investigated for their putative antiamoebic and spasmolytic activities in vitro. Results indicated that both biological activities are concentrated in the polyphenolic fraction, and not in the saponin or alkaloid containing fractions. The most active polyphenolic extracts were those from Euphorbia hirta whole plant, leaves of Alchornea cordifolia, Crossopteryx febrifuga, Nauclea latifolia, Psidium guajava, Tithonia diversifolia, stem bark of Harungana madagascariensis, Mangifera indica, Maprounea africana and Psidium guajava, inhibiting Entamoeba histolytica growth with MAC < 10 micrograms/ml. The same extracts, at a concentration of 80 micrograms/ml in an organ bath, also exhibited more than 70% inhibition of acetylcholine and/or KCl solution-induced contractions on isolated guinea-pig ileum.

Lignans and neolignans as lead compounds

Many lignans and neolignans have served as lead compounds for the development of new drugs. Perhaps the best known example is podophyllotoxin, an antimitotic compound that binds to tubulin. Etoposide and teniposide are derived from podophyllotoxin, but their antitumoural activity is due to inhibition of topoisomerase II. Combination of both pharmacophores has led to compounds with a dual mechanism of action, such as azatoxin. Dihydrobenzofuran neolignans, based on the natural lead 3′,4-di-O-methylcedrusin, have also been investigated as potential antitumoural agents; the dimerisation product of caffeic acid methyl ester was the most active compound. Here too, he cytotoxic activity was due to inhibition of tubulin polymerisation. In addition, the same compounds showed antiangiogenic activity. Podophyllotoxin, as well as other types of lignans, such as dibenzylbutyrolactones related to arctigenin, dibenzocyclooctadiene-type lignans, and dibenzylbutanes, have been explored as leads for antiviral agents (also including HIV). Synthetic 8.O.4′-neolignans have been evaluated for their antileishmanial and antifungal properties. Detailed study of the antifungal properties of the phenylpropanoid moieties has resulted in the design of highly active arylpropanoid derivatives. Other examples where lignans have been used as lead compounds include enzyme inhibitors of phosphodiesterase IV and V, and 5-lipoxygenase, and for the development of hypolipidemic and antirheumatic agents.

In-vivo antimalarial activity of Cassia occidentalis, Morinda morindoides and Phyllanthus niruri.

The ethanolic, dichloromethane and lyophilized aqueous extracts of Cassia occidentalis root bark, Morinda morindoides leaves and whole plants of Phyllanthus niruri were evaluated for their antimalarial activity in vivo, in 4-day, suppressive assays against Plasmodium berghei ANKA in mice. No toxic effect or mortality was observed in mice treated, orally, with any of the extracts as a single dose, of 500 mg/kg body weight, or as the same dose given twice weekly for 4 weeks (to give a total dose of 4 g/kg). No significant lesions were observed, by eye or during histopathological examinations, in the hearts, lungs, spleens, kidneys, livers, large intestines or brains of any mouse. At doses of 200 mg/kg, all the ethanolic and dichloromethane extracts produced significant chemosuppressions of parasitaemia (of > 60% for C. occidentalis root bark and Ph. niruri whole plant, and of 30% for M. morindoides leaves) when administered orally. The most active ethanolic extract, that of Ph. niruri, reduced parasitaemia by 73%. The dichloromethane extracts of M. morindoides and Ph. niruri produced similar reductions (74% and 72% chemosuppression, respectively), whereas that of C. occidentalis was slightly less active (60% chemosuppression). Each lyophilized aqueous extract was less active than the corresponding ethanolic extract.

Biological screening of traditional preparations from some medicinal plants used as antidiarrhoeal in Kinshasa, Congo

Forty six aqueous extracts from 38 medicinal plant species belonging to different families were selected on the basis of their traditional medicinal use as antidiarrhoeic agents. They were submitted in a broad biological screening including antibacterial, antiamoebic and antispasmodic activities. The results of the testing have indicated that 37 extracts (80.43%), 33 (71.74%) and 32 (69.54%) exhibited some level of antibacterial, antiamoebic and antispasmodic activity respectively. Only 8 plant extracts (17.39%) would act as antidiarrhoeic agents by a triple pronounced antibacterial, antiamoebic and antispasmodic action. They include aqueous extracts from Euphorbia hirta whole plant, leaves of Psidium guajava and Tithonia diversifolia, root bark of Alchornea cordifolia, Heinsia pulchella, Paropsia brazzeana, Rauwolfia obscura and Voacanga africana.

Cytotoxicity and cell cycle effects of the plant alkaloids cryptolepine and neocryptolepine: relation to drug-induced apoptosis

Cryptolepine and neocryptolepine are two indoloquinoline derivatives isolated from the roots of the african plant Cryptolepis sanguinolenta. These two alkaloids, which only differ by the respective orientation of their indole and quinoline rings, display potent cytotoxic activities against tumour cells and present antibacterial and antiparasitic properties. Our previous molecular studies indicated that these two natural products intercalate into DNA and interfere with the catalytic activity of human topoisomerase II. Here we have extended the study of their mechanism of action at the cellular level. Murine and human leukemia cells were used to evaluate the cytotoxicity of the drugs and their effects on the cell cycle were measured by flow cytometry. Cryptolepine, and to a lesser extent neocryptolepine, provoke a massive accumulation of P388 murine leukemia cells in the G2/M phase. With HL-60 human leukemia cells, the treatment with cryptolepine leads to the appearance of a hypo-diploid DNA content peak (sub-G1) characteristic of the apoptotic cell population. With both P388 and HL-60 cells, cryptolepine proved about four times more toxic than its isomer. But the use of the HL-60/MX2 cell line resistant to the anticancer drug mitoxantrone suggests that topoisomerase II may not represent the essential cellular target for the alkaloids, which are both only two times less toxic to the resistant HL-60/MX2 cells compared to the parental cells. The capacity of the drugs to induce apoptosis of HL-60 human leukemia cells was examined by complementary biochemical techniques. Western blotting analysis revealed that cryptolepine, but not neocryptolepine, induces cleavage of poly(ADP-ribose) polymerase but both alkaloids induce the release of cytochrome c from the mitochondria. The cleavage of poly(ADP-ribose) polymerase observed with cryptolepine correlates with the appearance of a marked sub-G1 peak in the cell cycle experiments. The proteolytic activity of Asp-Glu-Val-Asp- or Ile-Glu-Thr-Asp-caspases was found to be enhanced much more strongly with cryptolepine than with its isomer, as expected from their different cytotoxic potential. Despite the activation of the caspase cascade, we did not detect internucleosomal cleavage of DNA in the HL-60 cells treated with the alkaloids. Altogether, the results shed light on the mechanism of action of these two plant alkaloids.