All marketed antipsychotics act by blocking dopamine D 2 receptors. Fast dissociation from D 2 receptors may be one of the elements contributing to the lower incidence of extrapyramidal symptoms (EPS) exhibited by newer antipsychotics. Therefore, we screened for specific D 2 receptor blockers with a fast rate of dissociation. Radioligand binding experiments identified N-[1-(3,4-difluorobenzyl)piperidin-4-yl]-6-(trifluoromethyl)pyridazin-3-amine (JNJ-37822681) as a fast-dissociating D 2 ligand. Its D 2 receptor specificity was high compared with atypical antipsychotics, with little activity at receptors associated with unwanted effects [␣ 1 , ␣ 2 , H 1 , muscarinic, and 5-hydroxytryptamine (5-HT) type 2C] and for receptors that may interfere with the effects of D 2 antagonism (D 1 , D 3 , and 5-HT 2A ). JNJ-37822681 occupied D 2 receptors in rat brain at relatively low doses (ED 50 0.39 mg/kg) and was effective in animal models of psychosis (e.g., inhibition of apomorphine-induced stereotypy or D-amphetamine/phencyclidine-induced hyperlocomotion). Prolactin levels increased from an ED 50 (0.17 mg/kg, peripheral D 2 receptors) close to the ED 50 required for apomorphine antagonism (0.19 mg/kg, central D 2 receptors), suggesting excellent brain disposition and minimal prolactin release at therapeutic doses. JNJ-37822681 induced catalepsy and inhibited avoidance behavior, but with a specificity margin relative to apomorphine antagonism that was larger than that obtained for haloperidol and similar to that obtained for olanzapine. This larger specificity margin (compared with haloperidol) may reflect lower EPS liability and less behavioral suppression after JNJ-37822681. JNJ-37822681 is a novel, potent, specific, centrally active, fast-dissociating D 2 antagonist with optimal brain disposition, and it is the first compound that allows the evaluation of the potential value of fast D 2 antagonism for the treatment of schizophrenia and bipolar disorder.

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