Ex vivo studies with iloperidone (HP 873), a potential atypical antipsychotic with dopamine D2/5-hydroxytryptamine2...

Iloperidone {HP 873: 1-[4-[3-[4-(6-fluoro-1, 2-benzisoxazol-3-yl)-1-piperidinyl]propoxy] -3-methoxyphenyl]ethanone} is a dopamine (D2)/serotonin (5-HT2) receptor antagonist with the preclinical profile of an atypical antipsychotic based on biochemical studies in rats. Iloperidone significantly increased dopa accumulation, an index of dopamine turnover in response to D2 receptor blockade, at doses from 0.3 to 10 mg/kg i.p. in the striatum and from 1 to 10 mg/kg in the nucleus accumbens. Blockade of dopaminergic presynaptic autoreceptors was measured by the reversal of apomorphine-inhibition of gamma-butyrolactone-induced dopa synthesis. Iloperidone did not significantly reverse the apomorphine inhibition of gamma-butyrolactone-induced dopa synthesis at any of the doses tested (0.3-10 mg/kg i.p.). In ex vivo receptor autoradiography studies, a 30-min pretreatment with iloperidone (2.5-20 mg/kg i.p.) inhibited the binding of [3H]spiperone to cortical and subcortical 5-HT2 receptors by 42 to 94%, in contrast to only 1 to 15% inhibition of [3H]spiperone binding to D2 receptors in the nucleus accumbens and striatum. Iloperidone, at 2.5 mg/kg i.p., inhibited 5-HT2 receptor binding by 54 to 62% at 4-hr post-treatment, whereas there was negligible inhibition of D2 receptors. Chronic treatment with 5 mg/kg i.p. of iloperidone for 19 days significantly decreased the number of 5-HT2 receptors in the frontal cortex with no change in receptor affinity. D2 receptor number and affinity were unchanged in the nucleus accumbens and six regions of the striatum. In summary, iloperidone is a 5-HT and dopamine receptor antagonist with weak activity at presynaptic dopamine autoreceptors. Potent 5-HT2 receptor antagonism may be an important component in the preclinical profile of iloperidone as a potential atypical antipsychotic.