The Calabar bean and physostigmine: from African ethno-jurisprudence to medicinal discovery and modern pharmacotherapeutics

Abstract

Physostigmine, or eserine, is an alkaloid found in the Calabar bean (eséré), Physostigma venenosum (Balfour). The scientific discovery of physostigmine played a pivotal role in our understanding of chemical neurotransmission and the emergence of pharmacology as a science. The Efik people of Old Calabar, in present day Cross River State of Nigeria, used the bean as an ordeal poison to determine if persons accused of certain allegations such as witchcraft were guilty or innocent. Scottish missionaries working in Old Calabar witnessed eséré ordeal trials and attended to patients inadvertently poisoned by the plant. Perplexed by the ordeal trials and concerned for their patients, the missionaries sent reports of their observations and samples of the beans to scientists back in Britain for study. Evaluations of the plant material for its chemical constituents and biological activity led to the discovery of its active principle, physostigmine. Further explorations with physostigmine led to the discovery of acetylcholine and chemical neurotransmission, and the Nobel Prize for Physiology or Medicine in 1936. From a classic practice of ethno-jurisprudence, the world came to understand the chemical basis of synaptic transmission, and the nature of substrate-enzyme and ligand-receptor interactions as underlying principles in biochemistry and pharmacology. Numerous medicines including physostigmine have been developed based on these principles and are being used in current pharmacotherapy. The rich history of eséré plant and its physostigmine alkaloid anchors a compelling story of the role natural products have played in the discovery of modern therapeutic agents. Moreover, the story highlights the reality that probably many more medicinal plants in Africa remain to be explored for their chemical constituents as potential leads in breakthrough drug discovery.