D-serine added to clozapine for the treatment of schizophrenia.

Objective: D-Serine is a full agonist at the glycine site on the N-methyl-D-aspartate (NMDA) receptor. Previous administration of D-serine to schizophrenic patients taking nonclozapine antipsychotics improved positive, negative, and cognitive symptoms, whereas the partial agonist D-cycloserine improved negative symptoms of patients taking conventional antipsychotics but worsened symptoms in clozapine-treated patients. To study the difference between full and partial agonists at the NMDA receptor glycine site, the clinical effects of adding D-serine to clozapine were assessed.
Method: In a 6-week double-blind trial, 20 schizophrenic patients received placebo or D-serine (30 mg/kg per day) in addition to clozapine. Clinical efficacy, side effects, and serum levels of D-serine were determined every other week.
Results: The patients exhibited no improvement with D-serine, nor did their symptoms worsen, as previously reported with D-cycloserine.
Conclusions: The results suggest either that clozapine may have an agonistic effect on the NMDA system or that clozapine-treated patients do not respond to D-serine.