Duncan W. Thomas, M. Boyd, J. Cardellina
2008
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Economic Botany
Abstract
Sustainable Harvest of Ancistrocladus korupensis (Ancistrocladaceae) Leaf Litter for Research on HIV. -Leaves from a new species of rainforest liana, Ancistrocladus korupensis, are the only known substantial source of michellamine B, a novel naphthyl isoquinoline alkaloid with antiviral activity against the AIDS virus. This alkaloid may be needed in large quantities by the U.S. National Cancer Institute (NCI) for ant i -HIV drug development and clinical trials. This note describes how the leaves are being sustainably harvested from the Korup National Park in Cameroon with minimal damage to the environment. An ant i -HIV screening program organized at NCI (1) led to interest in A. korupensis. Extracts of this tropical plant material tested in vitro were active against HIV, and bioassay-guided fract ionation led to the discovery of michellamine B. The compound showed essentially equivalent activity against HIV1 and HIV-2 (2) and is currently undergoing advanced preclinical development, including toxicity testing in animals. The original Ancistrocladus leaf sample from which michellamine B was first isolated, together with a sterile herbarium voucher, was first collected by Duncan Thomas in 1987 in the Korup National Park, in Cameroon 's Southwest Province. The herbarium voucher for the active sample proved difficult to identify; it was first thought to be A. abbreviatus Airy Shaw, a species widespread in west and central Africa. Further collections ofA. abbreviatus, however, d id not contain michellamine B. When michellamine-containing plants were finally recollected by Thomas at the original collection site in 1991, they were determined to belong to an undescribed A ncistrocladus species. After flowers and fruits were collected and examined, the plant was named A. korupensis D. W. Thomas & Gereau (3). The mature plant is a tall, woody liana, with stems growing to 20 m or more in length. Most of the leaf production therefore takes place in the inaccessible forest canopy. Although there are abundant saplings and small lianas in the understory, the leaf production of the juvenile population is insignificant. Leaf flushes in the canopy appear to be more or less annual. The youngest leaves are red, changing to yellow and pale green and finally becoming dark green. During the second year, the leaves develop a coating of epiphyllae, then die and fall off. Leaves of different ages were tested at NCI for michellamine B content, which reached highest concentrations in mature leaves (Y. Hallock et al., NCI, unpublished). Prel iminary exploration and forest inventories carried out in 1992 and 1993 showed that the species probably has a very l imited distribution, and that most of the known population grows inside the Korup National Park. Within its distr ibution area, the vine is fairly common; average densities may exceed one large vine per hectare over considerable areas. Harvesting leaves, even in the relatively small quantities needed for research purposes, presented some serious problems: first, the species is very local in its occurrence; second, most of the populat ion is in a national park; and third, the leaves are inaccessible in the canopy. In Apri l 1992, leaf harvesting trials were conducted in a small area of unreserved forest at the edge o f the Ndian oil palm estate, where shifting agriculture was encroaching into the forest. For four vines, the support trees were felled without breaking the vines ' main stems, and the leaf rosettes were then stripped. Five other vines had all accessible leaf rosettes stripped in situ by a local tree-climber. By March 1993, all treated vines had produced new leaf flushes, but none appeared ready for a second harvest, suggesting that an interval of at least two years between harvests would be needed. Because these techniques are fairly destructive, harvest could not take place within the national park, which is strictly protected under Cameroon law. Only the small population outside the Korup National Park could be exploited. It became apparent that i f only these methods were used, the annual harvest of available A. korupensis leaves would be small. Consequently, in March 1993 (at the end of the dry season), 10 small samples ofA. korupensis leaf litter were collected from underneath canopy vines, air dried, and tested at NCI for michellamine content. Surprisingly, all samples proved to have a high amount of michellamine B. This discovery meant there was an easy, sustainable method of harvesting leaves. Although