In vitro antimycobacterial activity and cytotoxic activity of the acetone extract, fractions and compounds from Oxyanthus speciosus (Rubiaceae)

The acetone extract, fractions and two compounds isolated from Oxyanthus speciosus DC. were screened for their antimycobacterial activity against three non-pathogenic mycobacteria namely: Mycobacterium aurum, M. fortuitum and M. smegmatis and a pathogenic strain, M. tuberculosis (8104) using a microdilution assay. Bioautography was used to determine the presence of antimycobacterial compounds of the plant extracts and fractions. Cytotoxicity was determined using the tetrazolium-based colorimetric cellular assay (MTT) against C3A human liver cells and Vero kidney cells. The selectivity index (SI) values of the extracts were calculated. The extract had significant activity against the 4 tested organisms with minimum inhibitory concentration (MIC) values ranging from 60 – 170 µg/mL. Two fractions out of 11 had significant activity against M. smegmatis with MIC values of 39 µg/mL. Compound 1 had moderate activity against all the tested mycobacterial strains with MIC values ranging from 12.5 – 50 µg/mL while compound 2 had weak activity (MIC value of 100 µg/mL). The acetone leaf extracts of O. speciosus and 4 fractions had relatively low cytotoxicity with LC₅₀ values ranging from 0.160 – 0.383 mg/mL against C3A human liver cells. Compound 1 showed no cytotoxicity against Vero kidney cells even at the highest concentration (200 µg/mL) while compound 2 had an LC₅₀ value of 33.77 µg/mL. The crude extract and all the fractions had selectivity index (SI) values (LC₅₀/MIC) ranging from 0.03 – 3.27. Compound 1 had the best SI value ranging from 4 – 16. The promising activity of compound 1 in vitro suggests its potential as an anti-TB drug candidate. Further work is continuing on structure elucidation and more intensive activity studies. The next phase of this work is to test the activity of promising extracts against M. bovis an important zoonotic pathogen causing large problems in wildlife and bovine management in Africa.