Alzheimer's disease: beware of interactions with cholinesterase inhibitors

(1) Cholinesterase inhibitors such as donepezil, galantamine and rivastigmine, are not very effective in slowing the cognitive decline associated with Alzheimer's disease. Memantine, which is no more effective, has dopaminergic and atropinic effects but is not a cholinesterase inhibitor. (2) Cholinesterase inhibitors have mainly cholinergic adverse effects, causing gastrointestinal, neurological, cardiovascular and urinary disorders (incontinence). (3) Increased mortality, mainly due to cardiovascular events, was observed in placebo-controlled trials of galantamine. In one trial there were more deaths in patients on donepezil than on placebo. (4) Atropinic drugs tend to aggravate cognitive disorders that are treated with cholinesterase inhibitors. (5) Cholinesterase inhibitor + neuroleptic combinations are associated with an increased risk of extrapyramidal adverse effects. An increase in mortality was reported during trials of neuroleptics involving patients with dementia, and also during trials of cholinesterase inhibitors. (6) Combining cholinesterase inhibitors with drugs that reduce the heart rate, depress cardiac conduction, or induce torsades de pointes increases the risk of arrhythmias and cardiac conduction disorders. (7) Donepezil and galantamine are metabolised by cytochrome P450 isoenzymes 3A4 and 2D6, creating a strong potential for pharmacokinetic interactions with inhibitors and inducers of these isoenzymes. Rivastigmine is mainly metabolised by cholinesterases, and binds poorly to cytochrome P450 isoenzymes. (8) Cholinesterase inhibitors inhibit the metabolism of suxamethonium and thereby augment and prolong the neuromuscular blockade induced by this curare. (9) In practice, caregivers should be aware of the potential adverse effects of cholinesterase inhibitors, which often resemble symptoms of Alzheimer's disease and may be due to drug-drug interactions or to antagonist effects with other drugs, such as those with atropinic effects. Additionally, the many adverse effects associated with the use of cholinesterase inhibitors highlights the need for regular re-evaluation of the use of these medicines and of the balance of benefit versus risk in individual patients.