Anti-mycobacterial and cytotoxic activities of some selected medicinal plants and pharmacognostic studies of the most active plant

Abstract

Buruli ulcer (BU), caused by Mycobacterium ulcerans, is the third most important mycobacteriosis disease after tuberculosis and leprosy. The disease usually affects the skin and subcutaneous tissues. It is dominant in poor rural settlements and in rural endemic settings of Africa where topical application of herbs and oral consumption of plant extracts are used as first-line remedy instead of the WHO approved rifampicinstreptomycin (SR) for 8 weeks with surgery to correct deformities if necessary. Medicinal plants used need to be validated and proven to be a source of antimycobacterial agents. This study was undertaken to determine the antimycobacterial properties and toxicity of some selected medicinal plants used for the managing and/or treating of Buruli ulcer disease against M. ulcerans. The dried extracts of 7 medicinal plants; Alstonia boonei, Cryptolepis sanguinolenta, Heterotis rotundifolia, Nicotiana tobacum, Periploca nigrescens, Pycnanthus angolensis, and Spathodea campanulata were subjected to in vitro antimycobacterial activity against M. ulcerans using the Resazurin Microtiter Assay (REMA) and subsequently their cytotoxicity was assayed on normal Chang liver cells via the Alamar Blue assay method. Results showed that out of the 7 extracts, only four showed considerable antimycobacterial activity; C. sanguinolenta P. angolensis, N. tobacum, and S. campanulata. The susceptibility of the M. ulcerans varied between extract concentrations ranging from 64 to 512 µg/mL. Pycnanthus angolensis and Cryptolepis sanguinolenta showed marked antimycobacterial activity against the acid-fast bacilli with the latter recording a high MIC of 64 µg/mL. Its active constituent, cryptolepine, showed higher activity with MIC of 32 µg/mL even though this was lower than that of the reference drugs, Streptomycin and Rifampicin which recorded MICs of 0.25 µg/mL and 0.125 µg/mL, respectively. All the antimycobacterial extracts showed 50% cytotoxicity (CC50) ranging from 94 to 384 µg/mL compared to the reference agent curcumin (CC50 value of 6.9 µg/mL). This study has provided the first report on the antimycobacterial activity of P. angolensis, C. sanguinolenta extracts and the compound cryptolepine against M. ulcerans. The results substantiate the use of some of these plants for Buruli ulcer management and also a potential source of antimycobacterial agents. However, care is to be taken when using them due to toxicity levels. Pharmacognostic studies were conducted on the most active plant against M. ulcerans and with less cytotoxicity against Chang liver cell line. This revealed thevi surface data determinations, physicochemical parameters, extractives and phytochemical screening of P. angolensis’ leaf and stem bark. These features may be useful as a partial monograph for the quality control and correct identification of P. angolensis.