Pharmacological Studies Supporting the Therapeutic Use of Ginkgo biloba Extract for Alzheimer’s Disease

The standardized Ginkgo biloba extract EGb 761® (definition see editorial) has been shown to produce neuroprotective effects in different in vivo and in vitro models. Since EGb 761® is a complex mixture containing flavonoid glycosides, terpene lactones (non-flavone fraction) and various other constituents, the question arises as to which of these compounds mediates the protective activity of EGb 761®. Previous studies have demonstrated that the non-flavone fraction was responsible for the antihypoxic activity of EGb 761®. Thus, we examined the neuroprotective and anti-apoptotic ability of the main constituents of the non-flavone fraction, the ginkgolides A, B, C, J and bilobalide. In focal cerebral ischemia models, the administration of bilobalide (5 - 20 mg/kg, s. c.) 60 min before ischemia dose-dependently reduced the infarct area in mouse brain and the infarct volume in rat brain 2 days after the onset of the injury. 30 minutes of pretreatment with ginkgolide A (50 mg/kg, s. c.) and ginkgolide B (100 mg/kg, s. c.) reduced the infarct area in the mouse model of focal ischemia. In primary cultures of hippocampal neurons and astrocytes from neonatal rats, ginkgolide B (1 μM) and bilobalide (10 μM) protected the neurons against damage caused by glutamate (1 mM, 1 h) as evaluated by trypan blue staining. In addition, bilobalide (0.1 μM) was able to increase the viability of cultured neurons from chick embryo telencepalon when exposed to cyanide (1 mM, 1h). Furthermore, we attempted to find out whether ginkgolides A, B, and J and bilobalide were also able to inhibit neuronal apoptosis (determined by nuclear staining with Hoechst 33 258 and TUNEL-staining). Ginkgolide B (10 μM), ginkgolide J (100 μM) and bilobalide (1 μM) reduced the apoptotic damage induced by serum deprivation (24h) or treatment with staurosporine (200 nM, 24h) in cultured chick embryonic neurons. Bilobalide (100 μM) rescued cultured rat hippocampal neurons from apoptosis caused by serum deprivation (24h), whereas ginkgolide B (100 μM) and bilobalide (100 μM) reduced apoptotic damage induced by staurosporine (300 nM, 24h). Ginkgolide A failed to affect apoptotic damage neither in serum-deprived nor in staurosporine-treated neurons. The results suggest that some of the constituents of the non-flavone fraction of EGb 761® possess neuroprotective and anti-apoptotic capacity, and that bilobalide is the most potent one. In contrast, ginkgolic acids (100 - 500 μM) induced neuronal death, which showed features of apoptosis as well as of necrosis, but these constituents were removed from EGb 761® below an amount of 0.0005 %. Taking together, there is experimental evidence for a neuroprotective effect of EGb 761® that agrees with clinical studies showing the efficacy of an oral treatment in patients with mild and moderate dementia.

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