Ginko

Ginkgo

Common names: maidenhair, yin-hsing (Chinese name), silver apricot, duck foot tree, fossil tree

Background

Ginkgo is derived from the leaf of Ginkgo biloba, also known as the maidenhair or fossil tree. It is the oldest living species of tree in existence today, and fossils of the Ginkgo tree date as far back as 200 million years. An individual tree can live as long as 1000 years. In traditional Chinese medicine, Ginkgo biloba was used to make medicinal teas.

Uses

Ginkgo is primarily used to treat cognitive impairment, particularly Alzheimer's disease. A multi-center, randomized, placebo-controlled, double-blinded trial showed that ginkgo extract (EGb 761) stabilized or improved cognitive function in patients with Alzheimer's disease and multi-infarct dementia. In a meta-analysis of studies investigating the use of gingko for dementia or cognitive impairment, researchers concluded that ginkgo had a small but significant effect on objective measures of cognitive function in patients with Alzheimer's disease. Whether ginkgo can improve cognitive function in healthy people is under active investigation; although results are mixed, preliminary evidence is generally promising.

The use of ginkgo has also been advocated for the treatment of peripheral vascular disease, age-related macular degeneration, vertigo, tinnitus, sexual dysfunction, and altitude sickness.

Phytochemistry and pharmacology

Ginkgo contains a number of active compounds. Those believed to be responsible for its pharmacological effects are the terpenoids and flavonoids. The terpenoids include the sesquiterpene bilobalide and ginkgolides A, B, C and J. The flavonoids are ginko-flavone glycosides that include kaempferol, quercetin, and isorhamnetin derivatives.

Ginkgo appears to alter vasoregulation, act as an antioxidant, modulate neurotransmitter and receptor activity,, and inhibit platelet-activating factor (PAF). The antioxidant and free radical scavenging effects have been attributed to the flavonoids because they inhibit the expression of inducible nitric oxide synthase. Ginkgolide B was a potent inhibitor of PAF in laboratory animals and humans. PAF is an ether-linked phospholipid that mediates a diverse number of processes including stimulation of the inflammatory response, induction of platelet aggregation, and modulation of neuronal function. Inhibition of PAF protects against hypoxia-induced neuronal injury. Ginkgo may also have a non-PAF-mediated inhibitory effect on platelet aggregation in stressed laboratory animals, a finding that if confirmed in humans, may be significant. Ginkgo inhibited monoamine oxidase in laboratory animals,, but not in humans.

Bilobalide and ginkgolides A and B are highly bioavailable when administered orally. Glucuronidation appears to be part of the metabolism of the flavonoids. Elimination half-lives of ginkgolides A and B and bilobalide after oral administration are 4.5, 10.6, and 3.2 hours, respectively.

Safety

Although serious adverse effects have not been reported in clinical trials in relatively small numbers of patients, the anticoagulant effects of ginkgo have been associated with bleeding complications. There are four reported cases of spontaneous intracranial bleeding,- one case of spontaneous hyphema, and one case of postoperative bleeding associated with ginkgo use.

Hypersensitivity to ginkgo preparations is possible. Use during pregnancy or by nursing mothers is not recommended.

Preparations and dosage

Ginkgo is usually prepared as a dried leaf extract. The two ginkgo extracts used in clinical trials, EGb 761 and LI 1370, undergo extensive processing and are standardized to ginkgo-flavone glycoside and terpenoid content. The recommended dosage of ginkgo extract is 120-240 mg per day in two or three divided doses.