What's happening at Natural Products.
So far three commercial plant products have been examined under a continuing program in the Natural Products Section led by Dennis V.C. Awang, FCIC. These three products exemplify different aspects of the complex and varied issues which confront the effective regulation of plant products. (Taheebo Lapacho/Pau d'Arco/Ipe Roxo) is a traditional South American remedy of panacea proportions. Commercial products are purported to be the inner bark of huge trees (hardly herbs!) of the Tabebuia genus. Extravagant therapeutic claims made on behalf of this product led, in 1984, to seizure of tabeebo products which were being promoted as a cure for cancer and other serious diseases - in contravention of Schedule A of the Food and Drugs Act.
Therapeutic claims were generally based on content of the naphthoquinone lapachol, a known constituent of Tabebuia, which had shown promise as an anti-tumor agent, but was abandoned because of unfavourable dose-toxicity characteristics. Examination of tabeebo products purchased in Canada revealed that only two of 13 contained even a trace of lapachol and related lapachones. Furthermore, preparation of tea according to package instructions yielded an aqueous solution containing no detectable amount of the sparingly soluble naphthoquinones. However, detection of lapachones in organic extracts supported Tabebuia origin, but did not allow discrimination between species.
The laboratory effort in this study as well as that of Symphythum, described later, was conducted by Michel Girard, MCIC, and Daryl Kindack. Assurance of identity of plant species is one of the most vexing questions facing the purchaser of plant products. Every purchase is quite literally an act of faith, since one has no assurance that the contents of the package correspond to the labeled identity. No identity tests apply to plant products as they do for conventional pharmaceuticals. Numerous examples exist of gross contamination leading to poisoning of unsuspecting consumers, as in the case of belladonna poisoning of two Torontonians in 1981 and 1984. In England, feverfew importers were supplied chamomile and common tansy by professional Eastern European growers.
Efficacy as well as toxicity can vary significantly between species of the same plant genus. An investigation of commercial comfrey (Symphytum spp.) was prompted by a world-wide concern over the toxicity of constituent pryrolizidine alkaloids (PA) which are notoriously hepatotoxic, some also having been shown to be carcinogenic in laboratory animals and mutagenic in in vitro testing. Common comfrey (S. officinale) can be distinguished from hybrids with Prickly comfrey (S. asperum) by the presence in the latter of the PA, echimidine, arguably the most toxic of Symphytum PA. The form of comfrey usually encountered in the UK is the officinale X asperum hybrid, Russian comfrey (S. X uplandicum). All the commercial Canadian comfrey products obtained were labeled either simply comfrey or Symphytum officinale. However, six of 13 samples examined were found to contain echimidine and therefore not to be Symphytum officinale (common comfrey), including three of six specifically so labeled.
The advent of medicinal plants legitimized by orthodox clinical trials impresses the need for strict standards to ensure identity and efficacy - safety considerations apart. Criteria must be established to ensure that marketed products are the right stuff and are of quality comparable to the material used in published trials. For the migraine prophylactic feverfew (Tanacetum parthenium), the first in line of these modern medicinals, the challenge is viewed as first establishing a means of ensuring plant species identity, and secondly a basis for judging efficacy comparability. The material used in the British trials was encapsulated dried leaf, in which the sesquiterpene lactone, parthenolide, was overwhelmingly the major biologically active secondary plant metabolite. Studies of the mechanism of feverfew prophylaxis of migraine so far support the theory of inhibition of serotonin release from blood platelets, principally effected by parthenolide. In view of all these considerations, it seems therefore reasonable to require a minimum level of parthenolide in marketed products; a level of 0.2% is proposed, roughly half the average content (0.42%) of London-grown feverfew used in the trial conducted at the City of London Migraine Clinic.
For every new medicinal plant product seeking government approval for sale making therapeutic claims, there should be a clinical trial, ideally, a randomized double-blind placebo-controlled crossover study of appropriate statistical power. A chemical profile should also be established for the material used in such a trial, particularly in terms of active constituent(s) responsible for the particular therapeutic effect. Reasonable levels for such constituents may then be set on the basis of current chemical - biochemical understanding. The question of toxicity standards, particularly relating to long-term and often cryptic toxic effects, requires serious further consideration.
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|Title Annotation:||Natural Products Section of Canada's Bureau of Drug Research|
|Author:||Awang, Dennis V.C.|
|Publication:||Canadian Chemical News|
|Date:||Nov 1, 1989|
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