Flavonoids of Ginkgo Biloba

The use of Ginkgo preparations for the treatment of cough, bronchial asthma, irritable bladder and alcohol abuse can be traced back to the origins of Chinese herbal medicine. But the modern use of Ginkgo phytomedicines is not derived from traditional medicine. Research on Ginkgo biloba, done by the pharmaceutical company of Dr. W.Schwabe, Karlsruhe, Germany, led to the introduction of Ginkgo leaf extracts in 1965 for the treatment of circulatory diseases resulting from older age. Today, preparations containing Ginkgo leaf extracts are among the best selling phytopharmaceuticals in Europe, particularly in France and Germany. Compared to the importance of Ginkgo preparations, there is only a very small number of publications dealing with quality control of Ginkgo and Ginkgo preparations. To date, only a few general quality standards for Ginkgo leaves and Ginkgo phytomedicines have been published, although various laboratories are actively involved in the analysis of Ginkgo constituents. It appears that the marketing strategies of most companies which produce Ginkgo preparations do not permit the publication of the company-developed quality control standards. A monograph entitled “Ginkgo folium” is in preparation for the European pharmacopoeia while a similar monograph for the United States Pharmacopeia has been published in draft form.

First of all, standardisation of medicinal plant extracts is used to assure quality of phytomedicines. Standardisation is mostly done on the active compounds, which are in the case of Ginkgo leaves flavonoids and terpene trilactones. In this chapter suitable methods for the analysis of flavonoid glycosides in Ginkgo leaves and Ginkgo preparations will be presented and discussed. Earlier reviews on analytical aspects of Ginkgo flavonoids have been published by Sticher while reviews on the isolation and occurrence have been published by Yoshitama and Hasler.

Flavonoids of Ginkgo Biloba Leaves

The flavonoids of Ginkgo leaves represent a great variety of flavonol glycosides based on kaempferol and quercetin and are found as mono-, di- and triglycosides.

Minor flavonoid compounds are derived from isorhamnetin, myricetin, 3′-methylmyricetin (with flavonol structures) as well as from apigenin and luteolin (with flavone structures). Structurally interesting compounds include flavonol glycosides esterified with p-coumaric acid. These acylated flavonol glycosides are useful lead compounds in the quality control of Ginkgo preparations. In addition, non-glycosidic biflavonoids and proanthocyanidins have been isolated. The flavonol glycosides and the proanthocyanidins contribute to the free radical scavenging activity of Ginkgo extracts.

Standardisation of Ginkgo Extracts and Commercial Products

Two types of Ginkgo extract are available: Ginkgo full or total extracts (inch homeopathic mother tinctures) and enriched or special extracts. Full extracts are mainly produced using ethanol-water for extraction. The result is a complex mixture of polar to fairly non-polar natural compounds consisting of active principles and inert plant constituents. In the case of Ginkgo, 40-60% ethanol is recommended to obtain a high flavonoid yield. An extract of this type generally contains 2 to 4% flavonol glycosides. Commercial products based on total extracts are sometimes standardised to varying concentration levels of flavonol glycosides, and exceptionally also to a minimum terpene trilactone content. They are licensed in a number of European countries for over-the-counter sale.

Enriched extracts require several steps of purification or separation after extraction and thus possess a higher content of flavonol glycosides and terpene trilactones, whereas several other constitutents are largely eliminated. Enriched extracts are obtained using a relatively tedious work-up and purification procedure consisting of various steps such as liquid/liquid extraction, precipitation and concentration. Therefore, high-molecular-weight constituents such as tannins, proteins and polysaccharides, and in addition slightly soluble and lipophilic compounds like biflavones and ginkgolic acids are removed or reduced. These substances are undesirable in some liquid dosage forms because complex formation with other compounds may occur. Furthermore, an up to now not really documented allergenic potential of ginkgolic acid was postulated. The active principles, namely the flavonol glycosides and the terpene trilactones, are thus enriched. Products based on the special extract Ginkgo Biloba Extract 761 or products based on extracts manufactured in a similar way are standardised to a Ginkgo flavonol glycoside content of 16 to 26% (w/w) and a terpene trilactone (ginkgolides and bilobalide) content of 5 to 7%. Considering the available pharmaceutical and clinical evidence, as well as the indications claimed for commercial products based on such special extracts, usually a prescription is required. Generally, the view of therapeutic equivalence should be focused more on the daily dosage rather than on the percentage of active compounds in the extract. For preparations based on Ginkgo Biloba Extract 761 extracts, three dosages of 40mg of extract per day are proposed, corresponding to 28.6mg of Ginkgo flavonol glycosides and 7.2mg of terpene trilactones. With regard to the use of active constituents for standardisation, the quantitative determination of flavonol glycosides and terpene trilactones is not only desirable, but absolutely essential.

In 2010, a meta-analysis of clinical trials has shown Ginkgo to be moderately effective in improving cognition in dementia patients.

The Analysis of Ginkgo Flavonoids

New guidelines for phytopharmaceuticals demand better developed analytical methods for quality control. There is a large number of flavonoids in Ginkgo biloba leaves, mainly derivatives of isorhamnetin, kaempferol and quercetin. After hydrolysis of the flavonoid glycosides determination of the aglycones is performed. The flavonols are commercially available as standards and standardisation of extracts is normally based on this assay. A fingerprint separation for checking identity and stability of Ginkgo extracts is indispensable. The assignment can be done from the on-line UV spectra and elution profile. The analysis of the second group of compounds with therapeutic value, ginkgolides and bilobalide. The originally practiced adjustment of flavonol glycosides no longer satisfies the demand for standardisation of active constituents. Consequently, both the flavonoid and the terpene trilactone (ginkgolides and bilobalide) concentrations should be determined in the future. Unstandardised products should no longer be licensed or allowed to remain on the market.

Otto Sticher, Beat Meier and Andreas Hasler “The Analysis of Ginkgo Flavonoids” (2006)