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Piracetam (PIR) and levetiracetam (LEV), an S-enantiomer, are pyrrolidone derivatives that share similar chemical structures but have distinct pharmacological profiles and consequently different clinical uses. Although the mode of action of neither drug has been fully elucidated, they do not interact with inhibitory or excitatory neurotransmission or alter membrane excitability. A brain-specific stereoselective binding site has been identified for which LEV and other S-enantiomers, but not PIR, have high affinity. In preclinical studies, PIR significantly improves learning and memory; in contrast, LEV has less effect but is much more active in preventing seizures. Both drugs have a high therapeutic index and are well tolerated. PIR, a nootropic drug, is used in the therapy of age-related cognitive disturbances and poststroke aphasia. Clinical experience has also shown that at high doses it is effective against cortical myoclonus. LEV is an antiepileptic drug. Clinical trials have confirmed its efficacy in partial seizures and preliminary findings suggest that it is also effective in generalized seizures and myoclonus.