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Voacanga,(Apocynaceae), a review of it's taxonomy, phytochemistry, ethnobotany and pharmacology.

Leeuwenberg et al
Agric. Univ. Wagenigen papers #85-3, 1985.

This is a review of the Voacanga genus, which is closely related to both Tabernanthe and Tabernaemontana. Many species from this genus also produce complex indole alkaloids of the same type and structure as these other two genera, some being source material for the isolation and semi-synthesis of medically used alkaloids.

As with the Tabernaemontana there has been a lot of confusion about the species within this genus, many also having various synonyms and type specimens. It is a mostly African and Asian genus, with only one species recorded from nth Qld, Voacanga grandifolia, for which V. papuana is a synonym. This species is recorded from New Guinea, Indonesia and the Phillipines as well. It is described as not only the most widespread Asian species, but also one of the most variable.

There is an extended discussion of the alkaloids found in this genus, including their biogenesis and pharmacology.

In one species ( V. africana ) the alkaloid content has been reported as 5-10% in root bark, 4-5% in trunk bark, 0.3-0.45% in leaves and 1.5% in seeds. From a specimen of V. grandifolia in India some indication of how the alkaloid content varied over the year was recorded, for the root and trunk bark, mar was the minimum, going up to secondary maximum in jun, then falling again in jul and peaking in nov. The leaves and fruit recorded a similar pattern, though the age of the individual leaves affected the alkaloid content. The types of alkaloids recorded was very similar to those found in Tabernanthe and Tabernaemontana. For V. grandifolia the following results of alkaloid analysis are given...

Voacanga grandifolia; synonyms V. papuana

From a cultivated specimen in India;
bark yielded 0.035% (-)-Voacangine, 0.0012% Voacamine, 0.02% Vobtusine, 0.0015% 18-oxovobtusine.
leaves yielded 0.03% Vobtusine, 0.003% 2-deoxyvobtusine, 0.0002% 18-oxovobtusine, amataine.
fruit yielded 0.004% (+)-Akuammidine; 0.0015% Tabersonine, 0.01% Vobtusine.

From New Guinea (as V. papuana);
root bark yielded 0.14% Voacangine, 0.02% Voacamine, 0.44% Vobtusine.
bark yielded 1.74% alkaloid, @ 0.2% Voacamine, 0.006% Vobtusine.
leaves yielded 0.0009% Voacamine, 0.65% Vobtusine.
fruit yielded Voacangine, traces of Voacamine, 0.52% Vobtusine.

Most species have complex mixtures of alkaloids in the leaves, and not so complex mixtures in the bark and root bark, with Voacangine, Vobtusine and Voacamine types predominating. The seeds of many species however display amazing uniformity of alkaloid type across most of the genus, with (-)-Tabersonine comprising almost the whole alkaloid content, sometimes as high as 3.5%. This alkaloid can be used as starting material for the semi-synthesis of (-)-Vincamine, a compound used for geriatric patients in Europe, some 400 tonnes of seeds are exported from the Cameroun annually for this purpose.

In the section on pharmacology there are collected references to studies done on the pharmacology of the alkaloids found in Voacanga.

The total alkaloids of V. africana are reported to be only slightly toxic. They act as CNS depressants and hypotensives.

Dregamine is reported to have local anaesthetic activity, has convulsant and respiratory stimulant properties and is said to inhibit fatigue.

Tabernaemontanine is claimed to be of use when given orally in certain geriatric conditions, (arteriosclerosis, cerebral trauma, headache, vertigo, memory difficulties etc, peripheral circulatory irregularities). A mixture of the HCl salts of this with Vobasine and Ochropamine is stated to have an anti-inflammatory, antipyretic and analgesic effect comparable with acetylsalicyclic acid (aspirin). High doses of Vobasine causes central (inc respiratory) depression.

Tabersonine is only slightly toxic, with a quarter the hypotensive activity of reserpine.

The Ibogan type alkaloids, Coronaridine, Ibogaine, Ibogamine, Iboxygaine, Voacangine and Voacristine exhibit mostly a central stimulant effect. High doses of voacangine produces convulsions and asphyxia. Iboxygaine causes psychomotor(?) effects. Ibogamine , Ibogaine and Iboxygaine are tremorigenic, Coronaridine and Voacangine seem not. Ibogaine is an antagonist to reserpine. Ibogaine is more effective in counteracting electroshock than Voacangine, both these alkaloids lower body temperature. Coronaridine and Voacangine both seem to have local anaesthetic activity. Ibogaine is ‘hallucinogenic'and anti-fatigue agent.

Voacamine has been found to be a useful heart tonic, having a similar action to cardiac glycosides like digitoxin, but without the toxicity associated with these compounds. It's duration of action is also longer, doesn't appear to effect the heart rate much. Voacamine can cause hypertension due to peripheral vasoconstriction in high doses, also a CNS depressant.

Vobtusine is a cardiac depressant, high doses may cause convulsions and death, not considered as particularly useful.

Vincamine was first tried as an anti-hypertensive drug, later studies found it to increase cerebral blood flow, as a result of cerebral vasodilation. It has become popular in Europe, especially for geriatric patients; it ameliorates disturbances of attention, memory and mood. There has been some study of the cerebrovascular effects of this compound. Pretreatment with certain nootropic substances, including Vincamine have been shown to improve performance in animal models of cognitive dysfunction (?). This compound may effect various cerebral enzymes as well as direct vasodilatory effects. This compound has received attention as a ‘smart drug' or ‘nootropic'.

Coronaridine and Voacangine both seem to have some local anaesthetic activity. Coronaridine is reported to be an anti-fertility agent in mice, due to an oestrogenic effect.