Partial agonist efficacy of EMD386088, a 5-HT6 receptor ligand, in functional in vitro assays

EMD386088 (5 mg/kg) has no effect on latent inhibition shown to both light and noise stimuli

Activation of 5-hydroxytyptamine₆ (5-HT₆) receptors stimulates attentional switching and 5-HT₆ receptor antagonists are putative drugs for psychosis. Latent inhibition (LI) provides a pre-clinical model of attentional switching and 'antipsychotic-like' action and is known to be modulated by 5-hydroxytyptamine. In the present study, LI was shown in a fear conditioning procedure that measured suppression of drinking after conditioning with footshock. In two experiments (each n = 48) it was shown that pre-exposure to both light- and noise-conditioned stimuli reduced conditioned suppression relative to the corresponding non-pre-exposed control. However, counter to prediction, LI was intact after treatment with the 5-HT₆ agonist EMD386088 (5 mg/kg).

Anti-Alzheimer's multitarget-directed ligands with serotonin 5-HT6 antagonist, butyrylcholinesterase inhibitory, and antioxidant activity

Serotonin 5-HT₆ receptors, butyrylcholinesterase (BuChE) and oxidative stress are related to the pathophysiology of Alzheimer's disease. Inhibition of BuChE provides symptomatic treatment of the disease and the same effect was demonstrated for 5-HT ₆ antagonists in clinical trials. Oxidative stress is regarded as a major and primary factor contributing to the development of Alzheimer's disease; therefore, antioxidant agents may provide a disease-modifying effect. Combining BuChE inhibition, 5-HT ₆ antagonism, and antioxidant properties may result in multitarget-directed ligands providing cognition-enhancing properties with neuroprotective activity. On the basis of the screening of the library of 5-HT ₆ antagonists against BuChE, we selected two compounds and designed their structural modifications that could lead to improved BuChE inhibitory activity. We synthesized two series of compounds and tested their affinity and functional activity at 5-HT ₆ receptors, BuChE inhibitory activity and antioxidant properties. Compound 12 with K _i and K _b values against 5-HT ₆ receptors of 41.8 and 74 nM, respectively, an IC ₅₀ value of 5 µM against BuChE and antioxidant properties exceeding the activity of ascorbic acid is a promising lead structure for further development of anti-Alzheimer's agents.

Dorsomedial prefrontal cortex 5-HT6 receptors regulate anxiety-like behavior

The dorsomedial prefrontal cortex (dmPFC) plays a very important role in decision-related and anxiety-related information processing. It has enriched 5-HT6 receptors; however, the precise role of dmPFC 5-HT6 receptors in anxiety remains to be fully investigated. In this study, we injected dmPFC with the 5-HT6 receptor agonist EMD 386088 and antagonist SB 271046 using stereotactic technology. 5-HT6 receptor activation in mice increased time spent in the center area on the open-field test, increased exploration of the open arms on the elevated plus maze test, and increased ratio on the social interaction test. 5-HT6 receptor inactivation induced the opposite effects. In brain slices, EMD 386088 decreased both spontaneous inhibitory postsynaptic currents (sIPSC) and spontaneous excitatory postsynaptic currents (sEPSC), while SB 271046 only increased sEPSC. These effects of EMD 386088 and SB 271046 could be reversed by the GABA_A receptor antagonist bicuculline (BMI) and positive allosteric modulator clonazepam (CLZ), respectively. Our results suggest that neurotransmission in the dmPFC by 5-HT6 receptor activation and inhibition may play an important role in anxiety-like behavior, and may provide new insight into the pathological mechanism and potential target of anxiety disorders.

Chronic antidepressant-like effect of EMD386088, a partial 5-HT6 receptor agonist, in olfactory bulbectomy model may be connected with BDNF and/or CREB signalling pathway

BACKGROUND

The removal of the olfactory bulbs has been attributed to behavioral changes and neuroplasticity manifesting themselves among others like increases in brain neurotrophin expression and neurogenesis. Earlier data presented that EMD386088, a 5-HT6 receptor partial agonist, exerts antidepressant-like properties after chronic administration in olfactory bulbectomy (OB) model as was it compared with amitriptyline (AMI). The aim of this study was to compare acute and chronic biochemical effects of EMD386088, administered in its antidepressant active (2.5mg/kg) and non-active (1.25mg/kg) doses, found in the open field test in OB rats, with those of AMI (10mg/kg). The levels of 5-HT6 receptor protein and selected neurotrophins in prefrontal cortex (PFC) and hippocampus (Hp) of rats have been examined.

METHODS

5-HT6 receptor protein and selected neurotrophins: brain-derived neurotrophic factor (BDNF), cAMP-response element binding protein (CREB), the product of the immediate early gene c-fos (cFos) protein levels were assessed using a Western blot analysis in PFC and Hp of bulbectomized rats after acute or chronic (14-day) EMD386088 or AMI intraperitoneal (ip) treatment.

RESULTS

The acute treatment with EMD386088 caused significant increases in CREB and BDNF protein levels in PFC, and an increase in BDNF in Hp of OB rats, while AMI injection decreased CREB and did not change BDNF levels. After the chronic administration of EMD386088, the increasing levels of BDNF and CREB were still observed in PFC and Hp.

CONCLUSIONS

The antidepressant-like effect of EMD386088 may be associated with the neuroplasticity activation in PFC and Hp in rats.

Study on the effect of EMD386088, a 5-HT6 receptor partial agonist, in enhancing the anti-immobility action of some antidepressants in rats

The effect of some antidepressants co-administered with EMD386088 in the modified forced swim test in rats was investigated. Additionally, the pharmacokinetics, metabolic stability, and the effect of EMD386088 on P450 cytochromes were determined. Intraperitoneal (i.p.) coadministration of EMD386088 (2.5 mg/kg) and imipramine (15 mg/kg), reboxetine (5 mg/kg), moclobemide (10 mg/kg), or bupropion (10 mg/kg) evoked significant antidepressant-like activity, whereas no effect was observed after joint administration of EMD386088 with s-citalopram (10 mg/kg). Pharmacokinetic in vivo investigation showed a rapid absorption of EMD386088 (2.5 and 5 mg/kg) with t_1/2 = 67 min (t_max = 5 min). Large volume of distribution (V_d/F = 102 L/kg) indicated its penetration into peripheral compartments. The most active coadministration of EMD386088 (2.5 mg/kg) with imipramine (15 mg/kg) resulted in slower absorption of the compound (C_max = 60 min) and decrease in the volume of distribution (V_d/F = 32.2 L/kg). EMD386088 penetrates the blood–brain barrier with a high brain/plasma ratio of about 19 (2.5 mg/kg) and 7.5 for coadministration with imipramine. The in silico and in vitro studies on EMD386088 metabolic stability showed the dehydrogenation of tetrahydropyridine moiety as its main metabolic pathway. EMD386088 did not influence on CYP3A4 activity, and it has been classified as a very weak CYP2D6 inhibitor (IC₅₀ = 2.25 μM). The results obtained from the forced swim test in rats indicate that an activation of 5HT₆ receptor may facilitate antidepressant-like activity of some antidepressants. The pharmacokinetic results suggest that the interaction between EMD386088 and imipramine could not have been pharmacokinetic in nature.

Metabolic and Cardiovascular Benefits and Risks of EMD386088-A 5-HT6 Receptor Partial Agonist and Dopamine Transporter Inhibitor

Since 5-HT₆ receptors play role in controlling feeding and satiety and dopamine is essential for normal feeding behavior, we evaluated the ability of EMD 386088-5-HT₆ receptor partial agonist and dopamine transporter inhibitor-to reduce body weight in obese rats, as well as its anorectic properties (calorie intake reduction) in rat model of excessive eating and the influence on metabolism (plasma glucose and glycerol levels). We also determined the effect of the studied compound on pica behavior in rats and its influence on blood pressure after single administration. EMD 386088 reduced body weight in obese rats fed high-fat diet and decreased calorie intake in both models applied (rat model of obesity and of excessive eating). In both models EMD 386088 regulated plasma glucose and increased plasma glycerol levels. The latter proves that the compound reduced body fat. We think that it might have increased lipolysis, but this requires further studies. The reduction in glucose levels is the first symptom of metabolic disorders compensation. EMD 386088 did not cause pica behavior in rats but increased blood pressure after single administration. We think that partial 5-HT₆ agonists might have potential in the treatment of obesity. Thus, EMD 386088 requires extended studies.

Serotonin 5-HT6 Receptor Antagonists in Alzheimer's Disease: Therapeutic Rationale and Current Development Status

Alzheimer's disease (AD) is the most common cause of dementia in elderly people. Because of the lack of effective treatments for this illness, research focused on identifying compounds that restore cognition and functional impairments in patients with AD is a very active field. Since its discovery in 1993, the serotonin 5-HT₆ receptor has received increasing attention, and a growing number of studies supported 5-HT₆ receptor antagonism as a target for improving cognitive dysfunction in AD. This article reviews the rationale behind investigations into the targeting of 5-HT₆ receptors as a symptomatic treatment for cognitive and/or behavioral symptoms of AD. In addition to describing the available clinical evidence, this article also describes the purported biochemical and neurochemical mechanisms of action by which 5-HT₆ receptor antagonists could influence cognition, and the preclinical data supporting this therapeutic approach to AD. A large number of publications describing the development of ligands for this receptor have come to light and preclinical data indicate the procognitive efficacy of 5-HT₆ receptor antagonists. Subsequently, the number of patents protecting 5-HT₆ chemical entities has continuously grown. Some of these compounds have successfully undergone phase I clinical studies and have been further evaluated in clinical phase II trials with variable success. Phase II studies have also revealed the potential of combining 5-HT₆ receptor antagonism and cholinesterase inhibition. Two of these antagonists, idalopirdine and RVT-101, have been further developed into ongoing phase III clinical trials. Overall, 5-HT₆ receptor antagonists can reasonably be regarded as potential drug candidates for the treatment of AD.

Study of a mechanism responsible for potential antidepressant activity of EMD 386088, a 5-HT6 partial agonist in rats

It was shown that 5-HT₆ receptor agonists can exert pharmacological activity due to various modifications in monoamines’ level and metabolism activity in rats’ brain structures. This finding was correlated with antidepressant- or anxiolytic-like properties of these compounds. The study was designed to establish a possible mechanism of the antidepressant-like activity of the partial 5-HT₆ receptor agonist EMD386088 (5-chloro-2-methyl-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole hydrochloride) in rats. The concentrations of monoamines (dopamine (DA), noradrenaline (NA), and serotonin (5-HT)) and the rate of their metabolism were measured ex vivo in the brain structures (hippocampus, nucleus accumbens, striatum) using high-performance liquid chromatography (HPLC). The rats were killed after the forced swim test (FST); the collected tissue samples were used to ex vivo experiments. The potency of EMD386088 to blockade dopamine transporter (DAT) was tested in a functional in vitro study. FST was used to assess the involvement of D₁- and D₂-like receptor subfamilies in antidepressant-like properties of EMD386088. Neurochemical data from ex vivo experiments showed that antiimmobility activity of EMD386088 may be connected with the activation of dopaminergic system, while neither noradrenergic nor serotonergic ones are involved in its effect. EMD386088 also possesses a significant affinity for DAT which may be a mechanism in the abovementioned effect. Behavioral data seem to confirm the importance of dopaminergic system activation in antidepressant-like activity of EMD386088, since this effect, observed in the FST, was abolished by the preferential D₁- and D₂-like receptor subfamily antagonists SCH23390 and sulpiride, respectively. Dopaminergic system is involved in antidepressant-like activity of EMD386088.

Idalopirdine - a small molecule antagonist of 5-HT6 with therapeutic potential against obesity

5HT6 receptor antagonists offer the potential for safe and effective drugs against obesity, because they can reduce weight without causing serious side effects in the cardiovascular system. Also, their anorexic effect is associated with reduced food intake via an enhancement of satiety. In the present study we investigated the anorexic effect of idalopirdine (LuAE58054) in a model of obesity induced by high-fat diet. To induce obesity in rats, the animals were treated with feed with a fat content of 40 %. Body weight was controlled and the amount of food and water consumed was determined. The influence of the test compound on the lipid profile and glucose level was measured, as well as locomotor activity in home cages on the 20th day of the treatment. LuAE58054, at 5 mg kg(-1)/day i.p., was significantly anorectic in this model of obesity. Animals treated with LuAE58054 weighed 8 and 9.2 % less than the control obese animals on the 12th and 21st days, respectively. It significantly reduced food intake and the amount of peritoneal fat in animals, and reduced the level of triglycerides in plasma. LuAE58054 did not have a statistically significant effect on the spontaneous activity of diet-induced obese rats. The present study clearly demonstrates the effectiveness of LuAE58054 in reducing body weight. This compound is in phase III of clinical trials for the treatment of cognitive deficits associated with Alzheimer's disease and schizophrenia. It is a 5HT6 receptor antagonist and is, therefore, free of those unacceptable side effects that preclude chronic use of anti-obesity drugs with other mechanisms of action. The search for an effective and safe anti-obesity drug is essential for an increasingly obese population; therefore, the anorectic action of LuAE58054 is important and there is a need for more research in this direction.