Clinical pharmacokinetics of drugs for Alzheimer's disease

Pharmacological treatment of patients with Alzheimer's disease is becoming more important, as evidenced by the number of drugs being developed in different countries. It has been shown in the majority of clinical trials that cholinesterase inhibitors, such as tacrine (tetrahydroaminoacridine), are able to induce beneficial effects in cognition and memory. Tacrine, like most of the other oral antidementia agents, is rapidly absorbed from the gastrointestinal tract. It is excreted mainly through the kidney, with a terminal elimination half-life of about 3 hours. Tacrine has nonlinear pharmacokinetics and there are large interindividual differences in pharmacokinetic parameters after oral, intravenous and rectal administration. A positive relationship between cognitive changes and plasma tacrine concentrations has been recently described. Similarly, velnacrine exhibits evidence of nonlinearity in some pharmacokinetic parameters, but renal excretion is a minor route of elimination for this drug. Pharmacokinetic data pertaining to eptastigmine, a third cholinesterase inhibitor, is more limited. However, the drug is rapidly distributed to the tissues after oral administration and readily enters the central nervous system, where it can be expected to effectively inhibit acetylcholinesterase in the brain for a prolonged period. Pharmacokinetic data for the nootropic agents are more limited. However, of the 3 agents reviewed only pramiracetam penetrates the central nervous system (CNS) poorly. Indeed, oxiracetam crosses the blood-brain barrier and persists for longer in the CNS than in the serum. Selegiline (deprenyl), a neuroprotective agent, is readily absorbed from gastrointestinal tract. It is metabolised mainly in the liver, and to a minimal extent in the lung or kidneys. The steady-state concentrations of metabolites in the cerebrospinal fluid (CSF) and serum are very similar, reflecting their easy penetration into the CNS. Idebenone, another neuroprotective agent, likewise is rapidly absorbed and achieves peak concentrations in the brain comparable to those in plasma. Similarly, CSF concentrations of metabolites of ST 200 (acetyl-L-carnitine) parallel those in plasma, suggesting that they easily cross the blood-brain-barrier. Gangliosides (GM1) can be given intramuscularly or subcutaneously, but the latter route of administration provides a concentration 50% higher both in the serum and the ganglioside fraction. However, because of its longer elimination, the intramuscular route is the best form of administration when the brain is the target organ for the treatment. Absorption of nimodipine is quite rapid. The pharmacokinetics of nimodipine during multiple-dose treatment have not been studied extensively; however, the drug does not appear to accumulate during repeated administration of standard doses.(ABSTRACT TRUNCATED AT 400 WORDS)