Specificity of effects

It is important for preclinical studies to use the same extract that would be used in humans. A plant extract as active ingredient is defined by both the origin, quality, and composition of the raw material and the production process. Leaves harvested from dedicated plantations in the USA, France and other regions and subjected to more than 20 standardized process steps result in the specific extract EGb 761^®. Its composition differs considerably from dried Ginkgo leaves, as relevant constituents like terpene lactones and flavone glycosides are enriched while undesired compounds like ginkgolic acids, biflavones, certain proanthocyanidines, lipophilic compounds and contaminants are removed. Therefore, results from studies with EGb 761^® cannot be extrapolated to other Ginkgo extract preparations and vice versa.

Replication of significant EGb 761^® findings with single fractions or single compounds of the extract would facilitate interpretation of the data. In a molecular biology context, attribution of specific effects to specific compounds supports the rationale for use of a multi-target compound like EGb 761^®. Improved availability of dopamine and acetylcholine in the prefrontal cortex could already be attributed to Ginkgo-specific acylated flavonols (Kehr et al., Reference Kehr, Yoshitake, Ijiri, Koch, Nöldner and Yoshitake2012), neuroplasticity partly to isorhamnetine (Müller et al., Reference Müller, Heiser and Leuner2012) while mitochondrial effects have been observed with flavonoids, bilobalide, and the ginkgolides B and J (Eckert, Reference Eckert2012).

Mitochondrial function as a key mode of action?

While the flavonoids definitively show radical-scavenging properties, the properties of EGb 761^® go beyond that simple mechanism and clearly indicate that it is not just an antioxidant. Mitochondrial protection and functional improvement can explain some of the preclinical and clinical effects of EGb 761^® in AD and age-related cognitive decline. Protection of mitochondrial function would not only explain the enhanced production of adenosine triphosphate (ATP) but also the anti-apoptotic properties, because mitochondria are also relevant for apoptosis. Mitochondrial function also contributes to improved neuroplasticity, as neuritic outgrowth and synaptic plasticity are highly energy demanding processes, and Aß-regulation, as reactive oxygen species from dysfunctional mitochondria upregulate amyloidogenic Aß-turnover.

Some findings, however, such as improved neurotransmission (Kehr et al., Reference Kehr, Yoshitake, Ijiri, Koch, Nöldner and Yoshitake2012) or effects on protein clearance (Behl et al., unpublished data) cannot be explained by improvements in mitochondrial function. Therefore, EGb 761 can really be conceptualized as a multi-target compound with demonstrated activity on distinct pathophysiological pathways in AD and age-related cognitive decline (Hampel, Reference Hampel2012).

Dose finding for disease modification

Current clinical dosing recommendations for symptomatic treatment with EGb 761 have been developed over five decades of clinical research guidelines. Safety, tolerability, and efficacy of the 240 mg daily dose have been demonstrated by a series of clinical trials in thousands of patients (Ihl, Reference Ihl2012; Kasper, Reference Kasper2012). As poor compliance has been associated with lack of efficacy in dementia prevention trials using 240 mg/day dosing (Kasper, Reference Kasper2012), a lower dose is unlikely to produce disease-modifying effects. Good tolerability has been reported for higher doses in small single-dose trials, but a maximum tolerated dose has never been established for chronic dosing. Therefore the optimal dose when looking for disease modifying effects has not been rigorously established.

Can pharmacokinetics contribute to dosing recommendations? The kinetics of constituents like terpene lactones in animals and humans are known. Substantial plasma levels can be reached by clinical doses, and animal data demonstrate substantial brain penetration by major ingredients of EGb761^®. The concentrations seen for single constituents in the brain are rather small, but they demonstrate efficacy in cognition, so the concentrations needed are probably in a low range. However, because (1) a plant extract like EGb 761 contains multiple active ingredients with the extract being more active than the sum of major constituents in preclinical models, (2) EGb 761 requires chronic dosing and has no relevant acute single dose activity in animal models or in humans, and (3) multiple pathophysiologic alterations finally lead to cognitive declines and dementia and there is no magic bullet target structure, blood or brain kinetics of single constituents are of limited value for the determination of clinically active dosing regimens. Long-term studies or biomarker trials are probably the best way to gather more knowledge and it would be interesting to see what doses could be optimal in the disease-modifying setting.

Bridging the link between pre-clinical and clinical research

Various types of pathophysiology may contribute to the clinical picture of Alzheimer's disease (AD). From a clinician's perspective, it is important for experimental studies to be conducted as closely as possible to the clinical situation. The task is how to identify mechanisms that are relevant for a disease that develops over 40 years. Although it is interesting to identify a potential pathophysiological mechanism through studies in cell cultures, Caenorhabditis elegans or transgenic animals, clinicians are interested in its effect on memory problems. Bridging work is therefore needed for Gingko biloba as for any other multi-target compounds for which experimental doses cannot be transferred directly to humans to close the gap between pre-clinical research and the clinical situation. This might be more feasible now with the emergence of sensitive functional imaging methods that allow detailed imaging of the brain under standardized conditions and at relatively little expense (Hampel, Reference Hampel2012).

In most animal experiments, the effects of EGb 761^® are clearly dose dependent, and doses higher than those used in clinical practice are frequently administered. For example, in rodents the most usual dose is 100 mg/kg/day, whereas the human dose is 240 mg/day. It is important to consider to what extent effects observed with such doses translate to humans, as different concepts are involved in calculating the adequate allometric dose.

The problem of translating doses from animals to humans is not specific to Ginkgo biloba or plant extracts but is a general problem for all drugs. Combining pharmacokinetic and pharmacodynamic research can be helpful when interpreting plasma levels of a synthetic drug as this can provide some correlation between the levels found in the plasma of animals and what would be expected in humans. For plant extracts, this approach is of limited utility for the reasons outlined above.

Suitable clinical trial designs

Interpretation of completed dementia prevention trials with EGb 761^® can be complicated by poor drug compliance and unexpectedly low dementia incidence rates (Kasper, Reference Kasper2012). As available data give a promising signal for the disease-modifying effect of this well-tolerated compound if taken regularly and long enough, adequate trial design of early intervention trials deserves special attention.

Biomarker-based enriched samples

An attractive approach is to enrich trials with participants at increased risk of disease progression with well-defined target mechanisms in order to increase the likelihood of positive outcomes with an intervention. This approach has gained merit in heterogeneous diseases, such as cancer, but may not be so appropriate in chronic diseases such as AD. Indeed, large-scale studies of cancer and prevention of cardiovascular disease often include 100,000 people or more, but study populations of this size are impractical when it is necessary to perform regular lumbar punctures, carbon 11-Pittsburgh compound B (PiB)-positron emission tomography (PET) and magnetic resonance imaging (MRI) and when specialists need to see patients every six months to determine whether participants are converting to mild cognitive impairment (MCI). In this setting, therefore, different design options should be considered.

Exploratory experimental epidemiologic design

An alternative approach would be an exploratory experimental epidemiologic study, in which exposure to the product under investigation would be randomized from the start, but entry criteria would be very broad. Such a trial would include a large number of participants (2,000–3,000) with subjective memory complaints and follow them for a period of two to four years. The outcome for a trial designed in such a way would be repeated measures of cognition. All covariates other than cognitive complaints, such as genetic profile and biomarkers, would be determined at baseline and regular intervals but would not be an entry criteria. Such a study could be conducted in the community, except for frail elderly participants who might have to attend tertiary care medical centers. Depending on the regulations of the country where the trial is conducted, the study would not necessarily need a doctor or nurse; instead, psychometricians and phlebotomists could measure the covariates and obtain blood, with repeated measures of cognition assessed on the telephone. Patients could be recruited in the community at a low entry threshold. With a carefully planned study design that includes sufficient covariates and a large enough population, a standard epidemiologic approach could then be used to identify characteristics of patients who would benefit most from an intervention with EGb 761^®. In this way, a number of research questions could be approached in parallel rather than sequentially, with all of the relevant questions built into a single protocol without the need for a series of studies.