The metabolites N-desmethylclozapine and N-desmethylolanzapine produce cross-tolerance to the discriminative stimulus of the atypical antipsychotic clozapine in C57BL/6 mice.

It has been previously shown that cross-tolerance to the discriminative stimulus properties of clozapine can be demonstrated with the drug discrimination paradigm. This study examined the ability of N-desmethylclozapine and N-desmethylolanzapine (metabolites of the atypical antipsychotic drugs clozapine and olanzapine, respectively) to induce cross-tolerance to the discriminative stimulus effects of clozapine. After C57BL/6 mice were trained to reliably discriminate 2.5 mg/kg clozapine from vehicle, a clozapine generalization curve was generated. Next, training was suspended and the mice received a maintenance dosing regimen in which they were injected twice daily with 10 mg/kg N-desmethylclozapine for 10 days. Then a second clozapine generalization curve was generated. This was followed by a 10-day washout period during which the mice did not receive drug injections or discrimination training. Finally, a third clozapine generalization curve was generated. These same procedures were followed for N-desmethylolanzapine (10 mg/kg twice daily during maintenance dosing). Both N-desmethylclozapine and N-desmethylolanzapine produced significant rightward shifts in the clozapine generalization curve indicating cross-tolerance between N-desmethylclozapine and clozapine and between N-desmethylolanzapine and clozapine. After a washout period with no training or drug administration this cross-tolerance effect was lost for both metabolites. This cross-tolerance drug discrimination procedure demonstrated in-vivo similarities between these two metabolites and clozapine and suggests that common underlying pharmacological mechanisms were responsible for the cross-tolerance that was observed. These findings also demonstrated that this procedure may be useful for identifying drugs with therapeutic efficacy similar to the atypical antipsychotic clozapine under repeated dosing conditions.