Cytotoxic stilbenes from Cajanus cajan (L.) Millsp. leaves

Cancer remains one of the leading causes of death in the world especially in the developed countries [1]. We now report a biologically monitored phytochemical separation of the leaves of Cajanus cajan against a panel of human cancer and non- cancer cell lines in vitro. Cajanus cajan, commonly called Pigeon pea, is grown for food and medicinal purposes in the tropics, especially Nigeria. The air dried leaves were extracted with methanol continuously for five days. The extract was then concentrated under reduced pressure. 60g of the extract was adsorbed on Silica Gel GF₂₅₄ and separated into fractions by vacuum liquid chromatography (VLC). The SRB assay was used to evaluate the cytotoxicity of the extracts and the isolated compounds. The dichloromethane (DCM) fraction of the leaves exhibited modest cytotoxicity against human amelanotic melanoma – C32, human breast adenocarcinoma – MCF-7 and human large cell lung carcinoma cell lines – COR-L23 and human fetal lung fibroblast – MRC-5 (IC₅₀ =12.0, 10.0, 10.0 and 15.0µg/mL, respectively). This finding prompted further activity-guided fractionation of the DCM fraction by flash chromatography and subsequent purification on preparative thin-layer chromatography which led to the identification of two prenylated stilbenes, longistylin A and C. These compounds have previously been reported to have antiplasmodial activity [2]. This is also the first time that these prenylated stilbenes are shown to exhibit in vitro cytotoxic activity against human amelanotic melanoma, C32, human breast adenocarcinoma, MCF-7, and human large cell lung carcinoma, COR-L23, cell lines. The IC₅₀ of the compounds ranges between 20 and 35µM. These compounds could explain the rational inclusion of Cajanus cajan in traditional herbal medicines used for the treatment of cancer in south- western Nigeria. Further study to establish the mechanism of action of these two stilbenes is in progress.

Acknowledgement: JSA thanks the Association of Commonwealth Universities, UK, for financial support.

References: 1. Anonymous, http://health.yahoo.com/news/58569. 2. Duker-Eshun, G, et al. (2004), Phytotherapy Research, 18:128–130.