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

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.

The pharmacological profile of iloperidone, a novel atypical antipsychotic agent.

Iloperidone (1-[4-[3-[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1- piperidinyl]propoxy]-3-methoxyphenyl]ethanone) demonstrated a potent antipsychotic profile in several in vitro and in vivo animal models. Iloperidone displaced ligand binding at D2 dopamine receptors (IC50 = 0.11 microM) and displayed a high affinity for serotonin (5-HT2) receptors (IC50 = 0.011 microM) and alpha-1 receptors (IC50 = 0.00037 microM). In vivo, iloperidone antagonized apomorphine-induced climbing behavior in mice at low doses with good oral bioavailability, prevented 5-HT-induced head twitch in rats at low doses, and inhibited self-stimulation behavior in rats, pole climb avoidance in rats and continuous Sidman avoidance responding in monkeys. The latter assay also demonstrated a good duration of action. Iloperidone was substantially less active in models of extrapyramidal side effect (EPS) liability, such as preventing apomorphine-induced stereotypy and causing catalepsy in rats. In single dopamine neuron sampling studies, iloperidone demonstrated clozapine-like effects on the number of active midbrain dopamine neurons. Based on the significant increase in the open arm time seen after iloperidone treatment in the elevated plus maze assay and increased interaction score in social interaction, iloperidone may also have favorable effects in the clinic on anxiety and, possibly, negative symptoms. Clinical trials are under way of the use of iloperidone for the treatment of schizophrenia.

3-[[(Aryloxy)alkyl]piperidinyl]-1,2-benzisoxazoles as D2/5-HT2 antagonists with potential atypical antipsychotic activity: antipsychotic profile of iloperidone (HP 873).

A series of 3-[[(aryloxy)alkyl]piperidinyl]-1,2-benzisoxazoles was synthesized and evaluated as potential antipsychotic D2/5-HT2 antagonists. Most of these compounds showed potent antipsychotic-like activity in an apomorphine-induced climbing mouse paradigm, with many also showing preferential mesolimbic activity, as indicated by their weaker effects in an apomorphine-induced stereotypy model. In receptor binding assays, many displayed a moderate affinity for the D2 receptor coupled with a significantly greater affinity for the 5-HT2 receptor: a property that has been suggested as necessary for atypicality. From this series, compound 45, 1-[4-[3-[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1- piperidinyl]propoxy]-3-methoxyphenyl]ethanone (iloperidone, HP 873), was further evaluated in a battery of in vivo and in vitro assays. This compound showed a 300-fold greater potency in inhibition of climbing than in inhibition of stereotypy or induction of catalepsy, and when evaluated chronically in an electrophysiological model, 45 caused a depolarization blockade of dopamine neurons in the A10 area of the rat brain but not in the A9 area. Additionally, it showed positive activity in a social interaction paradigm, suggesting potential efficacy against asociality, a component of the negative symptoms of schizophrenia. In chronic ex vivo studies, 45, similar to clozapine, caused a down regulation of 5-HT2 receptors but had no effect on the number of D2 receptors. Compound 45 is currently undergoing clinical evaluation.

Benzisoxazole- and benzisothiazole-3-carboxamides as potential atypical antipsychotic agents.

A series of benzisoxazole- and benzisothiazole-3-carboxamides has been prepared and tested for potential antipsychotic activity. In general, the compounds showed an affinity for dopamine D2 and serotonin 5HT2A and 5HT1A receptors. Several members of this series have demonstrated activity in animal models predictive of potential antipsychotic activity. In addition, compounds 18, 19, 22, 27, 28, 43, and 44 have also shown a potential for reduced EPS liability as suggested by the ratio of activity seen in mesolimbic-mediated vs nigrostriatal-mediated behavioral assays.

Effect of ritanserin on the interaction of amfonelic acid and neuroleptic-induced striatal dopamine metabolism.

In the present study we evaluated the interaction of amfonelic acid (AFA) with the typical neuroleptic haloperidol and the atypical antipsychotic clozapine on rat striatal dopamine metabolism in the absence or presence of the 5HT2 receptor antagonist ritanserin. In the absence of ritanserin, AFA significantly enhanced haloperidol stimulated 3,4- dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) accumulation by 36% and 37% respectively above that produced by haloperidol alone. This effect is believed to be due to AFA's ability to facilitate dopamine release produced by the potent haloperidol-induced increase in nigrostriatal impulse flow. In contrast, AFA did not potentiate the ability of clozapine to stimulate DOPAC or HVA. This lack of potentiation could be explained by clozapine's known potent 5HT2 receptor blocking activity attenuating its stimulatory effects on impulse flow. To test this, we combined ritanserin with haloperidol and again studied the interaction with AFA on dopamine metabolism. In the presence of ritanserin, AFA failed to potentiate the effects of haloperidol on DOPAC or HVA accumulation; an effect similar to that seen with clozapine. This result extends the idea that 5HT2 receptor blockade modulates nigrostriatal dopaminergic neurotransmission.

Cardiac adenylate cyclase activity, positive chronotropic and inotropic effects of forskolin analogs with either low, medium or high binding site affinity.

A series of in vitro studies were conducted examining the adenylate cyclase stimulation, positive chronotropic and inotropic effects of forskolin and nine analogs which exhibited a range of [3H]forskolin binding site affinities (K1) from 0.020 to 3.174 microM. A significant (P less than .001) linear correlation (r = 0.94) was found between binding site affinity and adenylate cyclase stimulation (EC50) for forskolin and the nine structural analogs. Adenylate cyclase activity was also significantly correlated with the positive chronotropic and inotropic effects of these substances on isolated guinea pig atria. Compounds with K1 values between 0.020 and 1.136 microM produced concentration-dependent increases in heart rate and contractile force in isolated spontaneous and electrically paced guinea pig atria, respectively. In contrast, an analog with a K1 of 3.174 microM caused significant (P less than .05) negative chronotropic and inotropic effects at concentrations above 10 microM. The optimal separation between positive inotropic and chronotropic activity was found with compounds displaying potent [3H]forskolin binding site affinity but moderate adenylate cyclase stimulation, i.e., K1 and EC50 values of approximately 0.05 to 0.10 and 3 microM, respectively. The results of this study show that the forskolin analog, P87-7692 [7-desacetyl-7-(O-propionyl)-hydroxyl amino-carbonyl-forskolin], has marked activity with a wide separation between positive inotropic (248 +/- 41%) and chronotropic effects (43 +/- 13%) at 6.2 microM and may serve as a prototype for a forskolin-based cardiotonic.

Interaction of beta adrenergic agonists and antagonists with brain beta adrenergic receptors in vivo.

There is interest in knowing whether beta adrenergic antagonists or agonists, when administered systemically, can enter the brain to interact with central beta adrenergic receptors. To study this, the reduction in the radioactive content in the brain of rats after administration of (-)-[125I]iodopindolol (IPIN) by systemically administered beta agonists or antagonists was measured. Previous studies show that after the i.v. administration of IPIN the binding in vivo to various areas of the central nervous system has the characteristics expected of binding to beta adrenergic receptors. Of the antagonists tested, pindolol and butylpindolol showed potent interactions with beta receptors in both cortex and cerebellum whereas atenolol and practolol did not interact at doses up to 30 mg/kg. CGP-12177 showed moderate potency in inhibiting IPIN binding in vivo. We have shown previously that propranolol and alprenolol inhibit IPIN binding with high potency in cortex and cerebellum. At high doses, butoxamine, a beta-2 antagonist, reduced the binding of IPIN in the cerebellum but not in the cortex. Of the agonists tested, clenbuterol and prenalterol caused a significant dose-dependent reduction of the binding of IPIN, with clenbuterol being more potent. Isoproterenol, salbutamol, salmefamol and dobutamine had no effect. With the exception of CGP-12177, the affinity of the drugs for central beta adrenergic receptors measured in vitro was correlated significantly with their ability to inhibit IPIN binding in vivo whereas their degree of lipophilicity was not correlated significantly with potency in vivo. The inhibition of IPIN binding in vivo from brain areas can be used to evaluate whether drugs penetrate into brain and interact with central beta adrenergic receptors.

Labeling in vivo of beta adrenergic receptors in the central nervous system of the rat after administration of [125I] iodopindolol.

The amount of radioactivity in vivo in the central nervous system (CNS) of the rat has been studied after tail-vein injections of (-)- [125I] iodopindolol (IPIN). The content of radioactivity in cortex and cerebellum 1 to 4 hr after IPIN administration was significantly reduced in rats pretreated with I-propranolol (1 mg/kg) given i.v. 5 min before IPIN; only a small effect of I-propranolol was seen in brainstem and spinal cord. The maximum reduction in radioactivity caused by I-propranolol was approximately the same in cortex and cerebellum (about 60-65%) and occurred 2 hr after IPIN administration. I-Propranolol was approximately 1500-fold more potent than d-propranolol in reducing radioactivity. Pretreatment of rats with other lipophilic drugs that act at beta receptors was able to reduce the binding of IPIN in vivo; in contrast, pretreatment of rats with drugs which do not have direct agonist or antagonist activity at beta adrenergic receptors (desmethylimipramine, metergoline, diazepam, fluoxetine, phentolamine and haloperidol) had no effect. Experiments using ICI 118, 551, a beta-2 antagonist and betaxolol, a beta-1 antagonist, indicated that the majority of radioactivity in the cortex in vivo was bound specifically to the beta-1 subtype of the receptor whereas in the cerebellum the majority of specific binding was to the beta-2-subtype. When the specific binding of IPIN to beta adrenergic receptors was measured in vitro in seven regions of the CNS, at a ligand concentration of 30 pM, a high correlation was found with the I-propranolol displaceable radioactivity measured in vivo (r = 0.97, P less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)

High- and low-affinity binding components for [3H]imipramine in rat cerebral cortex.

The present study demonstrates that [3H]imipramine binds to both high- and low-affinity imipramine binding components on membranes prepared from rat cerebral cortex. Scatchard and computer analyses of saturation experiments using a wide range of [3H]imipramine concentrations (0.5 nM-50 nM) revealed the presence of two binding components. Inhibition experiments in which membranes were incubated with [3H]imipramine and various concentrations of unlabelled imipramine gave shallow inhibition curves with a Hill coefficient of 0.60 +/- 0.04. When dissociation rates of imipramine were studied, biphasic dissociation curves were obtained with apparent half-times of dissociation of 2.5 +/- 0.4 min and 18.5 +/- 2.5 min. Thus analysis of saturation, competition, and dissociation experiments indicate that [3H]imipramine binds to low as well as high-affinity binding sites in rat cortex.