Joachim E. Roehr

A comparison of the receptor binding and HERG channel affinities for a series of antipsychotic drugs

Many antipsychotic drugs produce QT interval prolongation on the electrocardiogram (ECG). Blockade of the human cardiac K(+) channel known as human ether-a-go-go-related gene (HERG) often underlies such clinical findings. In fact, HERG channel inhibition is now commonly used as a screen to predict the ability of a drug to prolong QT interval. However, the exact relationship between HERG channel blockade, target receptor binding affinity and clinical QT prolongation is not known. Using patch-clamp electrophysiology, we examined a series of seven antipsychotic drugs for their ability to block HERG, and determined their IC(50) values. We then compared these results to their binding affinities (K(i) values) for the dopamine D(2) receptor, the 5-HT(2A) receptor and, where available, to clinical QT prolongation data. We found that sertindole, pimozide and thioridazine displayed little (<10-fold) or no selectivity for dopamine D(2) or 5-HT(2A) receptors relative to their HERG channel affinities. This lack of selectivity likely underlies the significant QT interval prolongation observed with administration of these drugs. Of the other drugs tested (ziprasidone, quetiapine, risperidone and olanzapine), olanzapine displayed the greatest selectivity for dopamine D(2) and 5-HT(2A) receptor binding (100-1000-fold) compared to its HERG channel IC(50). We also compared these HERG channel IC(50) values to QT interval prolongation and plasma drug levels obtained in a recent clinical study. We found that the ratio of total plasma drug concentration to HERG IC(50) value was indicative of the degree of QT prolongation observed. Target receptor affinity and expected clinical plasma levels are important parameters to consider for the interpretation of HERG channel data.

Iloperidone binding to human and rat dopamine and 5-HT receptors.

Iloperidone (HP 873; 1-[4-[3-[4-(6-fluoro-1,2-benzisoxazol-3-yl)-1-piperidinyl]propoxy] -3- methoxyphenyl]ethanone) is a compound currently in clinical trials for the treatment of schizophrenia. Iloperidone displays affinity for dopamine D2 receptors and for 5-HT2A receptors and has a variety of in vivo activities suggestive of an atypical antipsychotic. Here we present an examination of the affinity of iloperidone to a variety of human and rat homologs of dopamine and 5-HT receptor subtypes. We employed receptor binding assays using membranes from cells stably expressing human dopamine D1, D2S, D2L, D3, D4 and D5 and 5-HT2A and 5-HT2C receptors and rat 5-HT6 and 5-HT7 receptors. Iloperidone displayed higher affinity for the dopamine D3 receptor (Ki = 7.1 nM) than for the dopamine D4 receptor (Ki = 25 nM). Iloperidone displayed high affinity for the 5-HT6 and 5-HT7 receptors (Ki = 42.7 and 21.6 nM, respectively), and was found to have higher affinity for the 5-HT2A (Ki = 5.6 nM) than for the 5-HT2C receptor (Ki = 42.8 nM). The potential implications of this receptor binding profile are discussed in comparison with data for other antipsychotic compounds.

Changes in paroxetine binding in the cerebral cortex of polydipsic rats.

Schedule-induced polydipsia was induced when food-deprived rats were subjected to a fixed-time (60 s) feeding schedule for 150 min daily for 3 weeks (training period). Subsequent chronic administration of the serotonin reuptake inhibitor fluoxetine reduces schedule-induced polydipsia over 2-4 weeks. We asked whether changes in the serotonin reuptake carrier occur following the development of schedule-induced polydipsia and its reduction by fluoxetine. Using [3H]paroxetine binding, we found a 40% increase in Kd and a 50% decrease in Bmax in polydipsic rats; both were reversed by fluoxetine. Food deprivation alone did not affect these parameters. These observations suggest that changes in the serotonin reuptake carrier correlate with the development and reversal of schedule-induced polydipsia.

Serotonergic activity of HP 184: does spontaneous release have a role?

Examination of HP 184, [N-n-propyl)-N-(3-fluoro-4-pyridinyl)-1H-3-methylindodel-1-amine hydrochloride], in a variety of tests for serotonergic activity revealed some unique properties of this compound. We report here that 100 μM HP 184 enhanced spontaneous release of [3H]serotonin (5-HT) from rat hippocampal slices. This release was independent of the uptake carrier. In vivo assays confirmed that HP 184 (20 mg/kg, i.p.) lacked significant interactions at the norepinephrine (NE) or 5-HT uptake carrier itself. Notably, HP 184 (15 mg/kg, i.p.) reduced drinking behavior in schedule-induced polydipsic (SIP) rats. We previously reported that some selective 5-HT reuptake inhibitors decrease SIP 30–40% after a 14–21 day treatment. In the current study, HP 184 decreased SIP beginning with the first treatment, and this reduction (30%) was maintained for 28 days. We further investigated HP 184 and serotonin metabolite levels. One hour after i.p. administration of 30 mg/kg HP 184, the ratio of whole brain 5-hydroxyindolacetic acid (5-HIAA) to 5-HT was increased, suggesting serotonergic activation. Under these conditions, the brain: plasma ratio of HP 184 was approximately 2∶1, with brain concentrations of 1.6 μg/gram. We speculate that the spontaneous release effects of HP 184 may be responsible for the behavioral effects observed.

Design and synthesis of D1 agonist/D2 antagonist for treatment of schizophrenia

A series of tetrahydroisoquinolines were designed, synthesized and evaluated as the first non-natural product type of compounds with dual D(1) receptor (D(1)R) agonism and D(2) receptor (D(2)R) antagonism properties for treatment of schizophrenia. The initial SAR of the series was explored. The lead in the series, 3g, exhibited high affinity and good potency. Compound 3g displayed 95% of D(1)R occupancy (10 mg/kg, sc) and 75% of D(2)R occupancy (10 mg/kg, sc) in the striatum of male CD-1 mice. The series exhibited unique pharmacology and merit as tool compounds for target validation and future optimizations.