The present invention concerns a new pharmaceutical composition comprising an antagonist and a co-agonist of the N-methyl-D-aspartate-type glutamate receptor (NMDAR). The inventors found that the co-administration of these two compounds effectively inhibits production of amyloide-² peptide. In patient's suffering from Alzheimer's disease, these peptides are deposited in the extracellular matrix forming neuritic "plaques". Therefore, the present inventors identified a new possibility for prophylaxis and/or treatment of Alzheimer's disease by stopping the continued formation of neuropathological extracellular deposits.