Antibacterial, antiviral, antiproliferative and apoptosis-inducing properties of Brackenridgea zanguebarica (Ochnaceae)

Brackenridgea zanguebarica is a small tree that is used in traditional African medicine as a type of cure‐all for many diseases, including the treatment of wounds. The yellow bark of B. zanguebarica was used for the preparation of an ethanolic extract, which was tested in various concentrations against eleven bacteria, Herpes simplex virus type 1 (HSV‐1) and different human tumour cell lines. The extract that contains different polyphenolic substances like calodenin B. Cell growth inhibition, assessed via MTT‐assay, was found in all tested human cell lines with IC50 values (concentration of extract that reduced cell viability by 50%) between 33 μg dry extract/mL for HL‐60 human myeloid leukaemia cells and 93 μg dry extract/mL for HaCaT human keratinocytes. Staining with Annexin‐V‐FLUOS and JC‐1 followed by subsequent analysis via flow cytometry revealed significant apoptosis‐inducing properties. Analysis of caspase activity using a fluorogenic caspase‐3 substrate showed a significant caspase activity in Jurkat T‐cells after incubation with the extract. The bark extract had a pronounced activity against free HSV‐1 and a strong antibacterial activity against Gram‐positive strains (MICs: 6–24 μg dry extract/mL), which are often involved in skin infections. Additionally, no irritating properties of the extract could be observed in hen‐egg test chorioallantoic membrane (HET‐CAM) assay. These findings give a rationale for the traditional use of B. zanguebarica and are a basis for further analysis of the plant's components, their biological activity, and its use in modern phytotherapy.