Tacrine is a little bit older. Aricept is newer. What they do, they replace a neurotransmitter in the brain called, acethylcholine. There is a structure in the brain called Nucleus Basalis Meynert, and this is an accumulation of brain cells that send the projections to the brain cortex and supply acethylcholine. In Alzheimer's disease, these cells die very early.
Acethylcholine supply is diminished.
It is known that if we interfere with acethylcholine supply, a memory impairment will ensue. The drugs that I mentioned allow the acethylcholine that is being produced by the few remaining cells have stronger effects. Therefore, it slightly improves the patient's memory. They don't cure the disease. The cells continue to die.