Abstract
Chromatographic separation of the n-hexane extract of the aerial part of Plectranthus barbatus led to the isolation of five abietane-type diterpenes: dehydroabietane (1); 5,6-didehydro-7-hydroxy-taxodone (2); taxodione (3); 20-deoxocarnosol (4) and 6α,11,12,-trihydroxy-7β,20-epoxy-8,11,13-abietatriene (5). The structures were determined using spectroscopic methods including one- and two-dimensional NMR methods. Compounds (1)–(3) and (5) are isolated here for the first time from the genus Plectranthus. The isolated abietane-type diterpenes tested in vitro for their antiprotozoal activity against erythrocytic schizonts of Plasmodium falciparum, intracellular amastigotes of Leishmania infantum and Trypanosoma cruzi and free trypomastigotes of T. brucei. Cytotoxicity was determined against fibroblast cell line MRC-5. Compound (2) 5,6-didehydro-7-hydroxy-taxodone showed remarkable activity with acceptable selectivity against P. falciparum (IC50 9.2 μM, SI 10.4) and T. brucei (IC50 1.9 μM, SI 50.5). Compounds (3)–(5) exhibited non-specific antiprotozoal activity due to high cytotoxicity. Compound (1) dehydroabietane showed no antiprotozoal potential.
Highlights
The genus Plectranthus (Labiatae) represents a large and widespread group of species with a diversity of traditional medicinal uses
During our previous research for compounds with antiprotozoal activities from medicinal plants [6,7,8], we found that the extract from P. barbatus revealed antiplasmodial, antileishmanial and antitrypanosomal potential [8]
The results suggest that the antiprotozoal activity depends on oxygenated and dehydrogenated chromophoric systems through rings B and C since dehydroabietane (1), without oxygen functions in ring B and C as well as dehydrogenations in ring B, had no activity against all protozoal strains
Summary
The genus Plectranthus (Labiatae) represents a large and widespread group of species with a diversity of traditional medicinal uses. The genus comprises a group of around 300 species, distributed in tropical and subtropical areas of Africa, Asia and Australia [1]. One of the most interesting species of this genus is Plectranthus barbatus Andr., which is well-known for the treatment of various ailments. A diversity of traditional medicinal uses of P. barbatus in India (Hindu and Ayurvedic medicine), East and Central Africa, China, and Brazil have been reported [2]. In previous work [6,7,8], we screened around 70 medicinal plants from the Arabian Peninsular region (Yemen and Saudi Arabia) for their antiplasmodial, antileishmanial and antitrypanosomal properties. Plectranthus barbatus represented one of the more interesting plants for its in vitro antiprotozoal effects [8]. The present study on P. barbatus deals with bio-guided fractionation, isolation and structural elucidation of abietane diterpenoid constituents
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