Cytotoxic alkaloids from Tabernaemontana johnstonii

Abstract

Tabernaemontana johnstonii is a medium-sized tree native to Africa. Specimens of the hardwood of the plant were procured from the United States Department of Agriculture in cooperation with the cancer chemotherapy program initiated by the National Institutes of Health (NIH). An investigation was undertaken to verify that the plant possessed activity in the anti-tumor bioassay tests conducted by the NIH. Consequently, systematic fractionation of the plant in conjunction with bioassay revealed that the biological activity was concentrated in the alkaloidal portion of the plant material.

Purification of the crude alkaloidal material was accomplished initially by a combination of liquid chromatography on alumina followed by buffer extraction, and subsequently, by liquid chromatography on alumina followed by liquid chromatography on silica gel.

Four crystalline alkaloids were isolated during the purification procedure. Two of these alkaloids, isovoacangine and perivine, were characterized by absorption spectroscopy and mass spectrometry. Their identity was verified by comparison with authentic samples. The remaining two alkaloids were shown by nuclear magnetic resonance and mass spectrometry to be isomeric with the known bisindole alkaloid voacamine. These two alkaloids were subsequently identified as conodurine and conoduramiae, and their structural assignments were verified by comparison with authentic samples.

Each of the isolated alkaloids was submitted for bioassay in the KB in vitro and the PS in vitro test. Only conodurine and conduramine showed KB activity with values of 31 g/ml and 19 g/ml respectively. All four alkaloids showed PS activity arid the following values were obtained: perivine (20 g/ml), isovoacangine (18 g/ml), conodurine (29 g/ml), and conoduraaine (20 g/ml).