Clozapine (N-oxide), a metabolite of Clozapine, is an agonist for the human muscarinic designer receptors (DREADDs)^[1][2]. Clozapine is a potent antagonist of dopamine. Clozapine is also a potent and selective agonist at the muscarinic M4 receptor (EC₅₀=11 nM)^[3][4]. In Vivo: After a single intraperitoneal (i.p.) injection of Clozapine N-oxide (1 mg/kg) into mice, Clozapine N-oxide (CNO) plasma levels peak at 15 min and are very low after 2 h. Acutely administered CNO can be metabolically converted to Clozapine in other species such as human and guinea-pig. The metabolites that may form after chronic administration of CNO to DREADD-expressing mice (or other species) have not been studied systematically. However, even if back-transformation to Clozapine occurs after chronic CNO administration, it should be noted that Clozapine is a more potent (by ~10-fold) DREADD agonist than CNO itself. Moreover, confounding biological effects of potential CNO metabolites can be easily identified by including both saline- and CNO-treated WT animals in a particular DREADD study. Despite the short plasma half-life of CNO in mice, the biological effects that have been described after acute treatment of DREADD-expressing experimental animals are usually much longer (6-10 h). One possibility is that CNO tends to accumulate in tissues, although other scenarios are also feasible^[1].
Using a general pharmacokinetic model for the interconversion process, the mean total clearances of Clozapine and Clozapine N-oxide (CNO) are 28.45 L/hr and 45.30 L/hr, respectively^[2].