Novel antagonists of serotonin-4 receptors: Synthesis and biological evaluation of pyrrolothienopyrazines

doi:10.1016/j.bmc.2008.11.045

Bioorganic & Medicinal Chemistry

Volume 17, Issue 6, 15 March 2009, Pages 2607-2622

https://doi.org/10.1016/j.bmc.2008.11.045 Get rights and content

Abstract

Based on the definition of a 5-HT₄ receptor antagonist pharmacophore, a series of pyrrolo[1,2-a]thieno[3,2-e] and pyrrolo[1,2-a]thieno[2,3-e] pyrazine derivatives were designed, prepared, and evaluated to determine the properties necessary for high-affinity binding to 5-HT₄ receptors. The compounds were synthesized by substituting the chlorine atom of the pyrazine ring with various N-alkyl-4-piperidinylmethanolates. They were evaluated in binding assays with [³H]GR113808 (1) as the 5-HT₄ receptor radioligand. The affinity values (K_i or inhibition percentages) were affected by both the substituent on the aromatic ring and the substituent on the lateral piperidine chain. A methyl group on the tricyclic ring produced a marked increase in affinity while an N-propyl or N-butyl group gave compounds with nanomolar affinities. Among the most potent ligands, 34d was selected for further pharmacological studies and evaluated in vivo. This compound acts as an antagonist/weak partial agonist in COS-7 cells stably expressing the 5-HT_4(a) receptor and is of great interest as a peripheral antinociceptive agent.

Graphical abstract

Introduction

In the last decade, much effort has been directed toward understanding the functions¹ of the various receptor subtypes of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT), with emphasis on the most recently discovered binding sites, that is, the 5-HT₄, 5-HT₅, 5-HT₆ and 5-HT₇ receptors.2, 3, 4, 5, 6, 7 Among these receptors, which are linked to stimulation of cAMP production, the 5-HT₄ subtypes have sparked the interest of scientists and physicians⁸ because of their functional and physiological significance. Indeed, 5-HT₄ receptors have been demonstrated to be modulators of neurotransmitter release in various neuronal populations in the central nervous system, including basalocortical cholinergic,9, 10 striatal dopaminergic11, 12 and hippocampal serotoninergic¹³ cells. In parallel, the 5-HT₄ receptor has been implicated in cognitive performance,14, 15, 16, 17, 18, 19, 20, 21, 22 making it a potential therapeutic target for treatment of the cognitive deficits associated with Alzheimer’s disease. Additionally, though with some inconsistencies,²³ the observations that 5-HT₄ receptor antagonists such as 1-methyl-1H-indole-3-carboxylic acid 1-(2-methanesulfonylamino-ethyl)-piperidin-4-ylmethyl ester GR113808 (1), 1-butyl-4-piperidinylmethyl 8-amino-7-chloro-1,4-benzodioxan-5-carboxylate SB204070 (2), and N-[(1-butyl-4-piperidinyl)methyl]-3,4-dihydro-2H-[1,3]-oxazino[3,2-a]indole-10-carboxamide SB207266 (3) have shown anxiolytic-like action in various models of anxiety in the rat,24, 25 suggest another possible therapeutic application of 5-HT₄ receptor ligands.

Considering the distribution (e.g., atrium, gut) and the roles of 5-HT₄ peripheral receptors, various cardiac or gastrointestinal effects can also be anticipated for selective ligands.26, 27, 28, 29 Indeed, 5-HT₄ receptor partial agonists such as tegaserod (4) and prucalopride (5)³⁰ are being developed for the management of irritable bowel syndrome. However, there exist only a limited number of high-affinity ligands selective for 5-HT₄ receptors³¹ (Chart 1). When we began this work, very little was known about structure–activity relationships (SAR) of the 5-HT₄ receptor ligands. Among the 5-HT₄ receptor antagonists (Chart 2), tropisetron³² (6) was the first compound available; it exhibits low affinity for 5-HT₄ receptors (pK_i = 6.5) and high affinity for 5-HT₃ receptors (pK_i = 10). The second generation of ligands consisted of compounds such as endo-8-methyl-8-azabicyclo[3.2.1]oct-3-y1-2,3-dihydro-6-methoxy-2-oxo-1H-benzimidazole-1-carboxylate DAU6285 (7) possessing an equivalent affinity for 5-HT₃ and 5-HT₄ receptors.³¹ For compounds of the third generation, such as 2-diethylaminoethyl-[2-methoxy-4-amino-5-chloro] benzoate SDZ205557 (8), (1-S,8-S)-N-[(hexahydro-1H-pyrrolizin-1-yl)methyl]-6-chloroimidazo[1,2-a]pyridine-8-carboxamide33, 34 SC53606 (9), or 2-piperidinopropyl 4-amino-5-chloro-2-methoxybenzoate35, 36 RS23597 (10), the selectivity ratio toward 5-HT₄ was greatly improved compared to 5-HT₃ receptors, but these compounds remained insufficiently selective toward other receptors to be used as references. Finally, compounds 1 and 2 emerged as the first selective and high-affinity 5-HT₄ receptor antagonists37, 38 (e.g., for 1, pK_i = 9.5 toward 5-HT₄ receptors; pK_i < 6 toward 5-HT₃ receptors).

The search for potent and selective 5-HT ligands has been ongoing in our laboratory for several years. The 5-HT₃ receptor ligands have caused a huge interest in this receptor due to their potential therapeutic applications in a number of areas: emesis, anxiety, psychotic disorders, drug abuse, depression, migraine, pain, irritable bowel syndrome.³⁹ We previously reported the synthesis, receptor binding profiles, and in vitro and in vivo pharmacological evaluation of several tricyclic series of piperazinopyrrolothienopyrazine, piperazinopyrido-pyrrolopyrazine, piperazinopyrroloquinoxaline and piperazinopyridopyrroloquinoxaline derivatives having high affinity, good selectivity, and partial agonist activity toward 5-HT₃ receptors.40, 41 A 3D-QSAR study led to a precise definition of the pharmacophore for these partial 5-HT₃ agonists⁴² (Fig. 1). The hypothesis that these compounds possess several characteristics in common with the 5-HT₃ antagonists and also with 5-hydroxytryptamine itself, in terms of functional groups, was validated.

Furthermore, we developed a definition of a pharmacophore for the 5-HT₄ receptor antagonists (Fig. 2) by considering a 3D-QSAR study starting from 15 antagonists described in the literature.⁴³ In light of these two studies, and after a comparison between the two pharmacophores (partial 5-HT₃ agonists and 5-HT₄ antagonists), we formulated the hypothesis that it should be possible to transform a 5-HT₃ ligand into a 5-HT₄ ligand. This was the basis of our study of pharmacomodulation⁴³ starting from 5-(4-benzylpiperazin-1-yl)pyrrolo[1,2-a]thieno[3,2-e]pyrazine³⁹ S21007 (11). We also successfully used this pharmacophore to design benzo[h][1,6]naphthyridine and azepino[3,2-c]quinoline derivatives⁴⁴ (12) by considering the bioisosteric replacement of the ester function [the hydrogen bond acceptor (HBA) component of the 5-HT₄ pharmacophore] by a cyclic iminoether (see Chart 3).

We are now reporting in this paper the most recent results we obtained for the pharmacomodulation of the pyrrolothienopyrazine core by considering the 5-HT₄ receptor antagonist pharmacophore as the starting point. On the basis of the potential therapeutic interest associated to selective 5-HT₄ receptor ligands but also to bipotent 5-HT₃/5-HT₄ receptor ligands, the 5-HT₃/5-HT₄ receptor selectivity of our best derivatives will be assessed.

Section snippets

Chemistry

The new 5-substituted pyrrolo[1,2-a]thieno[3,2-e]pyrazines 31a–u, 32a–32n, 5-substituted pyrrolo[1,2-a]thieno[2,3-e]pyrazines 33a–f and 1-methyl-5-substituted pyrrolo[1,2-a]thieno[2,3-e]pyrazines 34a–k are shown in Table 1, Table 2, Table 3, Table 4. The general synthetic procedures used for their preparation are illustrated in Scheme 1. These compounds were obtained⁴⁰ from 5-chloropyrrolo[1,2-a]thieno[3,2-e]pyrazine (28), 5-chloropyrrolo[1,2-a]thieno[2,3-e]pyrazine (29), and

Conclusion

We described a new family of compounds, pyrrolothienopyrazine derivatives, with high affinity for 5-HT₄ receptors. We have demonstrated that it is possible to transform 5-HT₃ ligands into 5-HT₄ ligands by the means of a slight pharmacomodulation. Thus, these compounds allowed us to validate the 5-HT₄ receptor antagonist pharmacophore and provided some insights into the selectivity toward 5-HT₃/5-HT₄ receptors. However, we have not yet found all the structural features in our series which could

Chemistry

Each compound was characterized by elemental analysis, IR spectra and ¹H and ¹³C NMR spectra. ¹H NMR and ¹³C NMR spectra were recorded on a JEOL Lambda 400 MHz spectrometer. The values of chemical shifts (d) are reported in parts per million (ppm) while coupling constants are given in Hertz (Hz) (these data are reported only for the compounds tested in the pharmacological study). Melting points were determined on a Köfler bank. IR spectra were taken with a Genesis Series FTIR spectrophotometer.

Acknowledgments

We thank the CRIHAN (Centre de Ressources Informatiques de Haute Normandie) and the European Community (FEDER) for the molecular modeling software. The authors wish to thank Béatrice Rouzaire Dubois (Laboratoire de Neurobiologie Cellulaire et Moléculaire UPR 9040—CNRS, Gif sur Yvette, France) for providing us with the NG108-15 cells. For the financial support, we thank the “Conseil Régional de Basse-Normandie”.

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^†: Present address: Cognition et impulsivité, EA3917, Université de Caen, 14032 Caen.

^‡: Present address: GMPC, EA4259, Université de Caen, 5 rue vaubénard, 14032 Caen.

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Novel antagonists of serotonin-4 receptors: Synthesis and biological evaluation of pyrrolothienopyrazines

Abstract

Graphical abstract

Introduction

Section snippets

Chemistry

Conclusion

Chemistry

Acknowledgments

Neuropharmacology

Biochem. Biophys. Res. Commun.

Exp. Neurol.

Neuropharmacology

Neuropharmacology

Neuropharmacology

Pharmacol. Biochem. Behav.

Neuropharmacology

Neuropharmacology

Exp. Neurol.

Neuropharmacology

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Neuropharmacology

Bioorg. Med. Chem. Lett.

Bioorg. Med. Chem. Lett.

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FEBS Lett.

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Proc. Natl. Acad. Sci. U.S.A.

Prog. Drug Res.

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J. Neurochem.

J. Pharmacol. Exp. Ther.

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J. Pharmacol. Exp. Ther.

Naunyn-Schmiedeberg’s Arch. Pharmacol.