Treatment of cognitive dysfunction associated with Alzheimer's disease with cholinergic precursors. Ineffective treatments or inappropriate approaches?

The observations of the loss of cholinergic function in neocortex and hippocampus in Alzheimer's disease (AD) developed the hypothesis that replacement of cholinergic function may be of therapeutic benefit to AD patients. The different approaches proposed or tested included intervention with acetylcholine (ACh) precursors, stimulation of ACh release, use of muscarinic or nicotinic receptor agonists and acetylcholinesterase (AChE) or cholinesterase (ChE) inhibition. Inhibition of endogenous ACh degradation through ChE inhibitors and precursor loading were treatments more largely investigated in clinical trials. Of the numerous compounds in development for the treatment of AD, AChE and ChE inhibitors are the most clinically advanced, although clinical trials conducted to date did not always confirm a significant benefit of these drugs on all symptom domains of AD. The first attempts in the treatment of AD with cholinergic precursors did not confirm a clinical utility of this class of compounds in well controlled clinical trials. However, cholinergic precursors most largely used such as choline and phosphatidylcholine (lecithin) were probably not suitable for enhancing brain levels of ACh. Other phospholipids involved in choline biosynthetic pathways such as CDP-choline, choline alphoscerate and phosphatidylserine clearly enhanced ACh availability or release and provided a modest improvement of cognitive dysfunction in AD, these effects being more pronounced with choline alphoscerate. Although some positive results cannot be generalized due to the small numbers of patients studied, they probably would justify reconsideration of the most promising molecules in larger carefully controlled trials.