Review
Assessing African medicinal plants for efficacy and safety: pharmacological screening and toxicology

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Abstract

This paper reviews progress in establishing the scientific rationale for and safety of traditional medicine use in Africa. Selected plants were screened for antibacterial, antifungal, anthelmintic, anti-amoebic, antischistosomal, antimalarial, anti-inflammatory and antioxidant activity, as well as psychotropic and neurotropic activity using appropriate in vitro tests. Isolation of active compounds, in almost all cases, provided scientific validation for the use of the plants in traditional medicine. Although plants used medicinally are widely assumed to be safe, many are potentially toxic. Where poisoning from traditional medicines has been reported, it is usually because the plants used have been misidentified in the form in which they are sold, or incorrectly prepared and administered by inadequately trained personnel. The issue of quality control may, in the interim, be addressed using chromatographic techniques.

Introduction

Herbal medicines are an important part of the culture and traditions of African people. Today, most of the population in urban South Africa, as well as smaller rural communities, is reliant on herbal medicines for their health care needs. Apart from their cultural significance, this is because herbal medicines are generally more accessible and affordable (Mander, 1998). As a consequence, there is an increasing trend, worldwide, to integrate traditional medicine with primary health care.

Renewed interest in traditional pharmacopoeias has meant that researchers are concerned not only with determining the scientific rationale for the plant’s usage, but also with the discovery of novel compounds of pharmaceutical value. Instead of relying on trial and error, as in random screening procedures, traditional knowledge helps scientists to target plants that may be medicinally useful (Cox and Balick, 1994). Already an estimated 122 drugs from 94 plant species have been discovered through ethnobotanical leads (Fabricant and Farnsworth, 2001). Various assays can be used to test for biological activity, firstly in vitro and later, for promising natural products, in vivo. Crude or fractionated extracts and sometimes individual compounds reported here were screened for antibacterial, anti-inflammatory, antioxidant, anthelmintic, anti-amoebic, antischistosomal and antimalarial activity, as well as psychotropic and neurotropic properties. In testing for biological activity in vitro, a standard drug is always included in the test system to ensure that the assay is working effectively. The activity of an extract can then also be compared between different assays, although not with pure standards, as crude extracts contain a myriad of compounds that may be acting synergistically.

The prescription and use of traditional medicine in South Africa is currently not regulated, with the result that there is always the danger of misadministration, especially of toxic plants. The potential genotoxic effects that follow prolonged use of some of the more popular herbal remedies, are also cause for alarm.

This review brings together work conducted in the Research Centre for Plant Growth and Development, and elsewhere in Africa, in screening traditional herbal medicines for biological activity and toxicity. The effects of agricultural and storage practices on the biological activity of medicinal plants are reviewed elsewhere (Fennell et al., 2004).

Section snippets

Antibacterial activity

Sanitation and hygiene levels for the majority of people in Africa are not comparable to those of First World countries. This exposes African people to a wider array of microbial pathogens, which increases their susceptibility to bacterial infections. Local and indigenous plants are often the only available means of treating such infections (Taylor et al., 2001).

Plants were tested in three types of antibacterial bioassays: diffusion, dilution and bioautographic. In order to screen plant

Toxic plants

Plants commonly used in traditional medicine are assumed to be safe. This safety is based on their long usage in the treatment of diseases according to knowledge accumulated over centuries. However, recent scientific research has shown that many plants used as food or in traditional medicine are potentially toxic, mutagenic and carcinogenic (Schimmer et al., 1988, Schimmer et al., 1994, Higashimoto et al., 1993, Kassie et al., 1996, De Sa Ferrira and Ferrao Vargas, 1999).

Recently our group

Conclusions

In an extensive screening programme of plants used in traditional medicine, researchers provided scientific evidence for their rational use in treating infections and diseases, inflammation, and disorders of the central nervous system. Using the ethnobotanical approach and bioassay-guided fractionation, several compounds having biological activity were isolated and identified. Genotoxicity studies also showed that several plants used for medicinal purposes cause damage to the genetic material

Acknowledgements

The National Research Foundation (Pretoria, South Africa) and University of Natal Research Fund are thanked for their financial support.

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