A series of potent 5-hydroxytryptamine 7 (5-HT 7) ligands has been synthesized that contain a 1,3-dihydro-2 H-indol-2-one (oxindole) skeleton. The binding of these compounds to the 5-HT 7 and 5-HT 1A receptors was measured. Despite the structural similarity of these two serotonin receptor subtypes, several derivatives exhibited a high selectivity to the 5-HT 7 receptor. According to the structure-activity relationship observations, compounds unsubstituted at the oxindole nitrogen atom and containing a tetramethylene spacer between the oxindole skeleton and the basic nitrogen atom are the most potent ligands. Concerning the basic group, besides the moieties of the 4-phenylpiperazine type, halophenyl-1,2,3,6-tetrahydropyridines also proved to be 5-HT 7 receptor-ligands. Because of halogen substitution on the aromatic rings, good metabolic stability could be achieved. A representative of the family, 3-{4-[4-(4-chlorophenyl)-piperazin-1-yl]-butyl}-3-ethyl-6-fluoro-1,3-dihydro-2 H-indol-2-one ( 9e') exhibited selective 5-HT 7 antagonist activity ( K i = 0.79 nM). The in vivo pharmacological potencies of these 5-HT 7 receptor-ligands were estimated by the conflict drinking (Vogel) and the light-dark anxiolytic tests.

A series of (arylpiperazinylbutyl)oxindoles as highly potent 5-HT(7) receptor antagonists has been studied for their selectivity toward the 5-HT(1A) receptor and α(1)-adrenoceptor. Several derivatives exhibited high 5-HT(7)/5-HT(1A) selectivity, and the key structural factors for reducing undesired α(1)-adrenergic receptor binding have also been identified. Rapid metabolism, a common problem within this family of compounds, could be circumvented with appropriate substitution patterns on the oxindole carbocycle. Contrary to expectations, none of the compounds produced an antidepressant-like action in the forced swimming test in mice despite sufficiently high brain concentrations. On the other hand, certain analogues showed significant anxiolytic activity in two different animal models: the Vogel conflict drinking test in rats and the light-dark test in mice.

In the mammalian central nervous system (CNS) GABA receptors (GABARs) mediate neuronal inhibition and are important therapeutic targets. GABARs are composed of 5 subunits, drawn from 19 proteins, underpinning expression of 20-30 GABAR subtypes. In the CNS these isoforms are heterogeneously expressed and exhibit distinct physiological and pharmacological properties. We report the discovery of S44819, a novel tricyclic oxazolo-2,3-benzodiazepine-derivative, that selectively inhibits α5-subunit-containing GABARs (α5-GABARs). Current α5-GABAR inhibitors bind to the "benzodiazepine site". However, in HEK293 cells expressing recombinant α5-GABARs, S44819 had no effect on H-flumazenil binding, but displaced the GABAR agonist H-muscimol and competitively inhibited the GABA-induced responses. Importantly, we reveal that the α5-subunit selectivity is uniquely governed by amino acid residues within the α-subunit F-loop, a region associated with GABA binding. In mouse hippocampal CA1 neurons, S44819 enhanced long-term potentiation (LTP), blocked a tonic current mediated by extrasynaptic α5-GABARs, but had no effect on synaptic GABARs. In mouse thalamic neurons, S44819 had no effect on the tonic current mediated by δ-GABARs, or on synaptic (α1β2γ2) GABARs. In rats, S44819 enhanced object recognition memory and reversed scopolamine-induced impairment of working memory in the eight-arm radial maze. In conclusion, S44819 is a first in class compound that uniquely acts as a potent, competitive, selective antagonist of recombinant and native α5-GABARs. Consequently, S44819 enhances hippocampal synaptic plasticity and exhibits pro-cognitive efficacy. Given this profile, S44819 may improve cognitive function in neurodegenerative disorders and facilitate post-stroke recovery.

ABSTRACT:The most recently discovered serotonin (5-HT) receptor subtype, 5-HT 7 , is considered to be associated with several CNS disorders. Noninvasive in vivo positron emission tomography (PET) studies of cerebral 5-HT 7 receptors could provide a significant advance in the understanding of the neurobiology and eventual dysfunctions of the 5-HT 7 receptor. To date, no appropriate 5-HT 7 receptor PET ligand has been developed. Here, we modified known 5-HT 7 selective phenylpiperazinyl-butyloxindole derivatives so that they may be labeled either with carbon-11 or fluorine-18. A set of potential 5-HT 7 ligands for PET molecular imaging was successfully synthesized. Two compounds (10 and 14) were tested against a range of targets. Both compounds display a promising in vitro profile with respect to PET imaging of the 5-HT 7 receptor in thalamic regions. KEYWORDS: Oxindole, 5-HT 7 receptor distribution, PET T he relatively recently discovered G-protein coupled 5-HT 7 receptor has been implicated in various central nervous system (CNS) disorders such as schizophrenia, depression, epilepsy, migraine, and in the control of circadian rhythm. 1 For example, the atypical antipsychotic drug amisulpride has antidepressant effects, 2,3 and a study in 5-HT 7 receptor knockout mice supports that the 5-HT 7 receptor antagonism of amisulpride alleviates depression symptoms. 4 Other atypical antipsychotics also have relatively high affinity for the 5-HT 7 receptor, but their involvement in alleviating depressive symptoms through blocking the 5-HT 7 receptor remains to be investigated. 5,6 In vivo studies of cerebral 5-HT 7 receptor binding in humans would thus provide a significant advance in the understanding of the above-mentioned physiology and pathophysiology. Positron emission tomography (PET) is used to quantify neuroreceptor binding in vivo, and the availability of an appropriate PET radiotracer for the 5-HT 7 receptor would be of particular interest.Previous attempts of other groups to develop a 5-HT 7 receptor selective PET tracer have not been convincingly successful. 7,8 Most recently, 18 F-labeled SB-269970 derivatives were synthesized and evaluated in vivo in cats, 9,10 but in the absence of a validated reference region or an arterial input function it was not possible to fully evaluate the validity of those radiolabeled compounds. 11Several lead structures of 5-HT 7 receptor ligands have been identified within various structural classes. 12 Among these structures, phenylpiperazinyl-butyloxindoles display an interesting selectivity profile (Figure 1). 13,14 Some oxindoles showed inhibition constants (K i ) 2000-fold lower for the 5-HT 7 receptor than for the 5-HT 1A receptor. This large difference in K i is necessary because of low brain tissue 5-HT 7 receptor density (B max ) compared to 5-HT 1A receptor values in, e.g., hippocampus and cortical areas. 15,16 Received: July 31, 2012 Accepted: August 31, 2012 Published: August 31, 2012