Antimicrobial compounds from tropical rainforest plants

Abstract

Tropical rainforest plants represent a potential source of novel antimicrobial drugs since they have adapted to their humid, septic environment by synthesising such compounds as a means of self defence. Similarly, people who inhabit rainforests have adopted certain species to treat infectious diseases, and by investigating such plant medicines it may be possible to find novel compounds with therapeutic potential. With this is mind, ethnobotanical fieldwork was carried out in New Guinea, an area of the world little investigated for indigenous plant medicines and rich in endemic flora. An antibacterial field assay kit was developed using freeze-dried strains of S. epidermidis and E. coli which allowed plants used in traditional medicines to be screened in situ without having to take them back to a laboratory. This approach identified Lunasia amara (Blanco) as a candidate species; the use of its bark by tribes of the Whitman Range to treat tropical ulcers, supported by clear zones of inhibition with S. aureus. Samples of the bark were collected for analysis and through activity-guided fractionation, the anti- S. aureus activity of the bark extract was pinned down to a single well resolved HPLC peak (MIC S. aureus NCTC 6571 64μg/ml) which subsequent NMR analysis revealed to be the quinoline alkaloid lunacridine; 2'-<9-trifluoroacetyl lunacridine was found to be a more stable derivative however. Lunacridine's planar cationic structure suggested it might act as a DNA intercalator; 220μM giving 50% binding in an ethidium bromide displacement assay. This in turn suggested DNA topoisomerase II as a likely target for the compound which was confirmed with a kDNA decatenation assay revealing complete inhibition of the enzyme at 5μM. Cell viability assays with MRC-5, H226 and HELA cells showed the compound to be cytotoxic in a time dependent manner producing non-linear dose response curves indicative of a topoisomerase poison mode of action. Activation of the apoptosis pathway enzymes caspase 3/7 was also detected, reaching maximal activity between 24 and 48 hours for the H226 cell line. Thus, lunacridine does not represent a selective antibiotic but with the right structural modifications could be developed as an antineoplastic agent.