Ethnobotanical survey and cytotoxicity testing of plants of South-western Nigeria used to treat cancer, with isolation of cytotoxic constituents from Cajanus cajan Millsp. leaves

Ethnopharmacological relevance: There is only scant literature on the anticancer components of medicinal plants from Nigeria, yet traditional healers in the area under study claim to have been managing the disease in their patients with some success using the species studied. Aim of study: To document plants commonly used to treat cancer in South-western Nigeria and to test the scientific basis of the claims using in vitro cytotoxicity tests. Methods: Structured questionnaires were used to explore the ethnobotanical practices amongst the traditional healers. Methanol extracts of the most common species cited were screened for cytotoxicity using the sulforhodamine B (SRB) assay in both exposure and recovery experiments. Three cancer cell lines (human breast adenocarcinoma cell line MCF-7, human large cell lung carcinoma cell line COR-L23 and human amelanotic melanoma C32) and one normal cell line (normal human keratinocytes SVK-14) were used for the screening of the extracts and the fractions obtained. The extract of Cajanus cajan showed considerable activity and was further partitioned and the dichloromethane fraction was subjected to preparative chomatography to yield six compounds: hexadecanoic acid methyl ester, α-amyrin, β-sitosterol, pinostrobin, longistylin A and longistylin C. Pinostrobin and longistylins A and C were tested for cytotoxicity on the cancer cell lines. In addition, an adriamycin-sensitive acute T-lymphoblastic leukaemia cell line (CCRF-CEM) and its multidrug-resistant sub-line (CEM/ADR5000) were used in an XTT assay to evaluate the activity of the pure compounds obtained. Results: A total of 30 healers from S W Nigeria were involved in the study. 45 species were recorded with their local names with parts used in the traditional therapeutic preparations. Cytotoxicity (IC₅₀ values less than 50μg/mL) was observed in 5 species (Acanthospermum hispidum, Cajanus cajan, Morinda lucida, Nymphaea lotus and Pycnanthus angolensis). Acanthospermum hispidum and Cajanus cajan were the most active. The dichloromethane fraction of Cajanus cajan had IC₅₀ value 5-10μg/mL, with the two constituent stilbenes, longistylins A and C, being primarily responsible, with IC₅₀ values of 0.7-14.7μM against the range of cancer cell lines. Conclusions: Most of the species tested had some cytotoxic effect on the cancer cell lines, which to some extent supports their traditional inclusion in herbal preparations for treatment of cancer. However, little selectivity for cancer cells was observed, which raises concerns over their safety and efficacy in traditional treatment. The longistylins A and C appear to be responsible for much of the activity of Cajanus cajan extract.