Bioactive alkaloids from medicinal plants of Bhutan

Natural products had been indispensably used by many cultures and traditions in folklore medicines for thousands of years. These traditional medicines cater to about 85% of the world population for their primary health care needs. Natural products have been intensively explored also for their bioactive pharmacophores by modern pharmaceutical companies. In fact they are the skeletal framework of about 60% of the modern drugs that are available today. Of these, about 80% of antibacterial drugs and 90% of antimalarial drugs are natural derivatives. One of the interesting secondary metabolite groups which provides these potent drug leads are the alkaloids. Owing to their broad range of bioactivities, bioprospecting for these natural products are undertaken in many countries. In this study, on the molecular basis of traditional medicine of Bhutan, particularly those with potential antimicrobial components, four alkaloid positive medicinal plants, i.e. Aconitum orochryseum Stapf, Corydalis gerdae Fedde, Rununculus brotherusi Freyn and an Australian naturalized plant Tribulus terrestris Linn, but also used in Bhutan, were selected for phytochemical analysis.\ud \ud Bhutanese traditional medicines use Aconitum orochryseum (whole parts) for the treatment of bilious fever or high fever related to bile disorders including cold and flu, fever of bile (liver), fever caused by malaria, snake bites and for treating blood infections. Corydalis gerdae (whole parts) is used for the treatment of malaria and infections. Ranunculus brotherusi (aerial parts) is used for treating wounds, pus, infections and as antipyretics. Tribulus terrestris (fruit) is used for the treatment of arthritis, kidney diseases and as a diuretic. Though Tribulus terrestris and Ranunculus brotherusi failed to give any alkaloids, a number of new as well as known alkaloids were isolated from Aconitum orochryseum and Corydalis gerdae.\ud \ud Three new hetisane type diterpenoid alkaloids named orochrine, 2-O-acetylorochrine, and lingshinaline, together with two known alkaloids atisinium chloride and virescenine and six other unidentified alkaloids, were isolated from Aconitum orochryseum. The structures of the new alkaloids were elucidated by ¹H-NMR, gCOSY, gNOESY, TOCSY, ¹³C-NMR, DEPT, gHSQC and gHMBC spectral data analysis. Atisinium chloride, a major alkaloid of the plant, was identified by single crystal X-ray crystallography.\ud \ud Four known protopine type and protoberberine type isoquinoline alkaloids (protopine, scoulerine, cheilanthifoline, and stylopine) along with one unidentified alkaloid were isolated from Corydalis gerdae. The protopine was the major alkaloid of this plant. The alkaloids were identified by MS and ¹H-NMR spectral data comparison.\ud \ud The antibacterial testing of crude methanol extracts, crude alkaloids and the major alkaloids (atisinium chloride, orochrine and protopine) of these two plants were found inactive against the bacterium Staphylococcus aureus (MIC = >125 μg/ml) and Vancomycin resistant Enterococcus faecium (MIC = >125 μg/ml). But the in vitro antimalarial assay of crude methanol extracts, crude alkaloids, atisinium chloride, protopine, and cheilanthifoline gave very significant activity against Plasmodium falciparum, TM4 and K1 strains. Crude methanol extract (AO-ME) of Aconitum orochryseum was not that active (IC₅₀ of >10 μg/ml for both the strains). Increased activity was observed for its crude alkaloid (AOCEA) with IC₅₀ values of 20.40 μg/ml against TM4 and 19.20 μg/ml against K1 strains. The activity was further increased for atisinium chloride with IC₅₀ values of 4.02 μM against TM4/8.2 and 3.59 μM against K1CB1 strains.\ud \ud The antimalarial activity was better for the extracts and alkaloids of Corydalis gerdae. Its crude methanol extract (CG-ME) had the IC₅₀ values of 1.00 μg/ml against TM4 strain and 2.56 μg/ml against K1 strain. The activity of crude alkaloid of this plant was far better with IC₅₀ values of 0.33 μg/ml against TM4 strain and 0.63 μg/ml against K1 strain. Protopine had IC₅₀ values of 4.25 μM against TM4/8.2 and 4.29 μM against K1CB1 strains. Cheilanthifoline had the IC₅₀ values of 3.76 μM against K1 and 2.78 μM against TM4 strains.\ud \ud These alkaloids have the potential to become candidates for antimalarial leads. Similar activities may be expected from other alkaloids including new ones isolated from these two plants. Therefore, the combination of these two plant extracts would make the best antimalarial herbal mixture for Bhutanese traditional medicines. The results proved that ethno-directed biorational approach, combined with an alkaloid focus, is an efficient strategy for drug lead discovery. The results also proved for the first time at the molecular level that Bhutanese traditional medicines have clinical efficacy. The medicinal plants of Bhutan thus have potential to provide important new drug leads. Bioprospecting them would benefit local people, pharmaceutical industries and the patients at large