| The primary research on
Ginkgo biloba was first conducted by the W. Schwabe Co. of Karlsruhe, Germany,
producer of the proprietary extract EGb 761. Research does not support the clinical
benefits of other dosage forms of the crude ginkgo leaf or low-concentration
extracts made from the leaf. The development process involves drying and milling
the leaves, followed by extraction of the leaves with an acetone-water mixture
under partial vacuum. The organic solvent is then removed and the extract
processed, dried, and standardized. The preparation/extract ratio is 35-67:1, on
average 50:1.
Ginkgo compounds
- Flavonoids (0.5-1.8%)
- Biflavonoides (0.4-1.9%)
- Proanthocyanidins (8-12%)
- Trilactonic diterpenes: ginkgolide A, B, C
(0.06-0.23%)
- Trilactonic sesquiterpene bilabolids (0.04-0.2%)
Extract composition
Flavonone glycosides
(22-27%)
- Quercetin
and kaempferol, including isorhamnetin
Terpene lactones
(5-7%)
- Ginkgolides A, B and
C (2.8-3.4%)
- Bilobalide (2.6-3.2%)
Proposed mechanisms
Flavonoides
- Help to prevent cell membrane lipid peroxidation
- Reduce oxidative damage to erythrocytes
- Protect neurons and retinal tissue from oxidative
stress and ischemic injury
Ginkgolides
Inhibit PAF binding at its membrane receptors,
which in turn decreases platelet aggregation
- This
inhibits PAF-induced bronchoconstriction, airway hyperactivity, cytokine
production and T-lymphocyte proliferation.
Decreases phagocyte chemotaxis
Smooth muscle relaxation
- Ginkgo has a
direct effect on the alpha-adrenoceptors and smooth muscle through the signal
transduction pathway, intracellular camp, antagonism of the adrenergic nervous
system and hyperpolarization.
Prevents degranulation of neutrophils
Decreases phagocyte chemotaxis and glycine production
May increase cardiac contractility and coronary blood
flow
- Ginkgo
restores the balance between prostacyclin and thromboxane A2, improving
vasoregulation. It may do this by inhibiting phosphodiesterase, and
increasing cAMP levels and catecholamine release.
Absorption and distribution
The pharmacokinetics of ginkgo
biloba extract have been studied in rats using radiolabeled extracts. After oral
administration, at least 60% of the radiolabeled extract was absorbed by the
gut. Blood levels peaked after 1.5 hours. The level of radioactivity was
highest in organs rich in connective tissue such as the aorta, eyes, skin, and
lungs, indicating an affinity by the flavonoids for connective tissue. After 72
hours, the hippocampus and the striated bodies showed radioactivity five times
greater than that of the blood. Other areas of the brain such as the cerebral
cortex, brain stem, and cerebellum do not show such high levels of
radioactivity.
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