In vivo PET study of cerebral [11C] methyl- tetrahydroaminoacridine distribution and kinetics in healthy human subjects

Acetylcholine esterase activity in mild cognitive impairment and Alzheimer's disease

PURPOSE

Impairment of cholinergic neurotransmission is a well-established fact in Alzheimer's disease (AD), but there is controversy about its relevance at the early stages of the disease and in mild cognitive impairment (MCI).

METHODS

In vivo positron emission tomography imaging of cortical acetylcholine esterase (AChE) activity as a marker of cholinergic innervation that is expressed by cholinergic axons and cholinoceptive neurons has demonstrated a reduction of this enzyme activity in manifest AD. The technique is also useful to measure the inhibition of cerebral AChE induced by cholinesterase inhibitors for treatment of dementia symptoms.

RESULTS

A reduction of cortical AchE activity was found consistently in all studies of AD and in few cases of MCI who later concerted to AD.

CONCLUSION

The in vivo findings in MCI and very mild AD are still preliminary, and studies seem to suggest that cholinergic innervation and AChE as the main degrading enzyme are both reduced, which might result in partial compensation of their effect.

Deficits of the cholinergic system in early AD

Impairment of cholinergic neurotransmission is a well-established fact in Alzheimer's disease (AD) but there is controversy about its relevance at the early stages of the disease. In the recent years new techniques for in vivo imaging of key components of the cholinergic system in humans have developed. They are beginning to be applied to the very early stages of AD. Preliminary results suggest that there is early impairment of presynaptic receptors and acetylcholine esterase, the main degrading enzyme for acetylcholine, in cerebral cortex. The relation of these findings to neuronal function and post-mortem findings is being discussed.

Presence or absence of at least one epsilon 4 allele and gender are not predictive for the response to donepezil treatment in Alzheimer's disease

The objective was to evaluate the effects of the apolipoprotein E (ApoE) genotype and gender on the response to donepezil treatment in Alzheimer's disease. ApoE genotyping was performed on 117 patients with mild to moderate Alzheimer's disease who were included in a 36-week open label trial of donepezil therapy. Of these 117 patients, who constituted the intent-to-treat population (ITT), 80 completed the trial, and constituted the evaluable population. Patients were treated blindly in relation to ApoE genotype. Outcome measures were Alzheimer's disease Assessment Scale-Cognitive Component (ADAS-Cog), the Mini Mental State Examination, Instrumental Activities of Daily Living, and the Caregiver-rated Clinical Global Impression of Change. ITT analysis did not reveal significant differences between the responses of epsilon 4- and epsilon 4+ carriers according to the ADAS-Cog (P = 0.28). No differences were found either between the responses of men and women (P = 0.81), and there was no significant interaction between genotype and gender (P = 0.09). Other outcome measures all exhibited similar patterns of change to those seen using the ADAS-Cog. Consequently, these results do not support the hypothesis that the ApoE phenotype and gender are predictors of the response to donepezil in Alzheimer's disease patients.

Imaging brain cholinergic activity with positron emission tomography: its role in the evaluation of cholinergic treatments in Alzheimer's dementia

One of the strategies in the treatment of Alzheimer's disease is the use of drugs that enhance cholinergic brain function, since it is believed that cholinergic dysfunction is one of the factors that contributes to cognitive deterioration. Positron emission tomography is a medical imaging method that can be used to measure the concentration, kinetics, and distribution of cholinergic-enhancing drugs directly in the human brain and assess the effects of the drugs at markers of cholinergic cell viability (vesicular transporters, acetylcholinesterase), at muscarininc and nicotinic receptors, at extracellular acetylcholine, at markers of brain function (glucose metabolism and blood flow), and on amyloid plaque burden in vivo in the brains of patients with Alzheimer's disease. In addition, these measures can be applied to assess the drugs' pharmacokinetic and pharmacodynamic properties in the human brain. Since the studies are done in living human subjects, positron emission tomography can evaluate the relationship between the drugs' biological, behavioral, and cognitive effects; monitor changes in brain function in response to chronic treatment; and determine if pharmacologic interventions are neuroprotective. Moreover, because positron emission tomography has the potential to identify Alzheimer's disease during early disease, it can be used to establish whether early interventions can prevent or delay further development.