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  1. The current available drugs for Alzheimer disease (AD) only confer modest benefits. Additional AD therapies are urgently needed. Ginkgo biloba leaf extract, EGb 761, has been used as a dietary supplement in the U.S. for Alzheimer dementia. Several clinical trials have provided evidence of efficacy comparable to donepezil (Mazza et al., 2006) for treatment of mild to moderate AD. The evidence for EGb 761 enhancing learning in healthy humans is inconclusive (LaBar et al., 1997; Solomon, et al., 2002). A well-controlled study supported by the NIH is underway (DeKosky et al., 2006). Although the extract EGb 761 has been substantially studied at all levels of biological systems (Christen, 2002), its neuroprotective mechanisms and pharmacological evaluation of the active components remain puzzling to the scientific community. Two major constituents of EGb 761, flavonoids (24 percent) and terpenoids (6 percent) have been actively investigated (DeFeudis, 1998). The terpenoid fraction primarily contains ginkgolides A, B, C, J, and M, and bilobalide, which are specific to the ginkgo biloba tree (Nakanishi, 2005). The ginkgolide B is known as a potent antagonist of platelet activating factor receptor (PAFR) (Smith et al., 1996). The flavonoids impart antioxidative properties to EGb 761. In this paper, Ottavio Vitolo and colleagues in Michael Shelanski’s laboratory reported a very elegant study demonstrating that a terpene trilactone-enriched EGb 761, P8A, completely blocked Aβ-induced LTP reduction and neuronal cell death, two systems that have been considered to be related to AD. A single constituent of EGb 761, ginkgolide J, exhibited the most potent effect on LTP among other EGb 761 components. This observation is consistent with a previous finding that ginkgolide A and J alleviated Aβ-induced pathological behaviors in a C. elegans model (Wu et al., 2006) and enhanced adult neurogenesis and phosphorylation of CREB in a mouse model of AD (Tchantchou et al., 2007). The novelty of this study is that they employed LTP or long-term potentiation, a well-accepted, sophisticated cellular model for neuroplasticity, to investigate a challenging question of complexity. Their results provided initial identification of the key compounds of the extract that affects synaptic function, and provided the rationale for future medicinal chemistry studies based on the chemical structures derived from the extract. The use of mouse hippocampus slices allows for screening of compounds that target at early synaptic and cognitive changes in relation to AD. Compound(s) identified in this model could implicate potential enhanced efficacy of ginkgolide J to the mammalian model of AD and clinical studies. The unique structure of ginkgolides, their bioavailability, combined with its effects on synaptic dysfunction demonstrated by Vitolo et al. (2007) makes them a promising source of novel therapeutic candidates. Neurotoxicity and the natural resource of ginkgolide J is the next issue to be addressed.

    See also:

    DeFeudis, F.V., Ginkgo biloba extract (EGb 761): from chemistry to clinic. 1998: Publi Ullstein Med. Weisbaden, Germany.

    View all comments by Yuan Luo

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  1. Trials and Treatments: New Results Highlight Would-be AD Medicines