Design, synthesis, and biological evaluation of a new class of MT₂-selective agonists

Abstract

A novel class of chiral 2,3-dihydro-1H-indene derivatives were designed and synthesized as melatonergic ligands. Most of the reported MT₂-selective ligands behave as antagonists. By contrast, our exploration of 2,3-dihydro-1H-indene showed that the introduction of a lipophilic group at the 2- or 3-position of this scaffold could afford highly selective MT₂ agonists. Among all these synthesized molecules, compounds 10b, 12a, 17a, 20a exhibited powerful MT₂ agonistic activity (EC₅₀ < 50 nM) as well as excellent MT₂ selectivity (more than 2200-fold).

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Introduction

Melatonin (5-methoxy-N-acetyltryptamine, MLT, Fig. 1) is a neurohormone synthesized and secreted from the pineal gland of mammals including humans.¹ MLT plays an important role in modulation of the sleep–wake cycle and circadian rhythms in humans.² Other reported effects of MLT include its anti-inflammatory,³ pain modulatory,⁴ retinal,⁵ vascular,⁶ antitumor,^7–9 antioxidant,¹⁰ strokeprotective¹¹ and neuroprotective roles.^12,13 In mammals, MLT exerts its physiological effects mainly through the activation of the high affinity G-protein-coupled receptors MT₁ and MT₂.¹⁴ It is known that MT₁ receptors are expressed in several areas such as brain, especially in the suprachiasmatic nuclei (SCN) as well as the pars tuberalis of pituitary and might be implicated in the sleep promoting effects of MLT and in mediating vasoconstriction, whereas MT₂ receptors are localized in the SCN and retina, and appear to play a major role in the resynchronizing activity of MLT and in mediating vasodilation.

Fig. 1 Representative melatonin receptor ligands.

A large number of studies regarding the treatment of circadian rhythm disorders reported that the efficacy of MLT is always limited by the unfavorable pharmacokinetic profile such as short half-life in the human body.¹⁵ Over the last twenty years, many melatonin receptor agonists with improved properties in comparison to MLT have been published.^16,17 Takeda Pharmaceuticals North America developed ramelteon with a longer half-life, which was approved by FDA for the treatment of insomnia in 2005.^18–20 Agomelatine, a melatonin receptor agonist (MT₁ and MT₂) and selective serotonin receptor antagonist (5-HT_2c),²¹ has been approved by the European Medicines Agency in 2009 for major depressive disorder. It could relieve the symptoms of major depression and meanwhile, it enhances sleep quality in depressed patients.²²

Up to date, most of the melatonin receptor agonists are non-selective and only a few MT₂-selective ligands have been reported.^23–30 However, most of these molecules behave as antagonists, the representative compounds luzindole and 4-phenyl-2-propionamidotetralin (4P-PDOT) block the MLT-mediated phase advances of circadian rhythms in mice. Moreover, an antidepressive effect has been reported for luzindole in a mice model, being ascribed to its selective action at the MT₂ receptor.^31,32 In addition, an accurate characterization of MLT receptors-mediated functions can only be obtained by using subtype-selective ligands. Koike et al. reported the most efficient MT₂-selective agonist 1 as it shows 1200-fold higher affinity for the MT₂ than the MT₁ receptor.³³

Current SAR of indan derivatives disclosed that the chiral centre is essential for melatonin receptor affinity. For example, compound 2 (S-enantiomer) showed over 700 times higher affinity than its R-enantiomer (K_i = 0.041 nM vs. K_i = 30.1 nM).¹⁹ However, the previous reported works are mainly focused on achiral molecules.^17,34,35 More recently we have reported an efficient synthesis of chiral indan derivatives.³⁶ By introducing lipophilic groups to 2- or 3-position of the indan core, we attempted to prepare novel melatonergic ligands with improved MT₂-selectivity and blood–brain barrier permeability.^33,37

Herein we report novel 2,3-dihydro-1H-indene derivatives as highly selective MT₂ agonists. The chiral centre of ramelteon was preserved and a variety of lipophilic groups were introduced on the non-aromatic indan cycle. The preliminary conclusions regarding structure–activity relationships are discussed.

Result and discussion

Chemistry

The general route used for the synthesis of the target indan derivatives is depicted in Scheme 1. Following previously reported procedures, chiral indanone 3 can be easily prepared from (E)-3-(3-methoxyphenyl)acrylaldehyde.³⁶ Subsequently, compound 3 was subjected to hydrolysis, decarboxylation and esterification to afford chiral ester 5 with a global yield of 48%. Treatment of compound 5 with ethane-1,2-dithiol in the presence of SnCl₄ followed by subsequent reduction of the carbonyl group gave indanol 7. Gabriel reaction involving the mesylated form of compound 7 followed by acylation of the corresponding amine 9 in the presence of anhydride furnished chiral amide 10. Then the key ketone intermediate 11 was obtained in an excellent ee value (>99%) after dithiane deprotection with AgNO₃. Finally, the cyclic ketones 11 underwent aldol condensation with the required aldehyde in the presence of MeONa to give the designed molecules 12–20.³⁸

Scheme 1 Synthesis of 2,3-dihydro-1H-indene derivatives. Reagents and conditions: (a) NaOH, EtOH, water, reflux; (b) dioxane, dimethylbenzene, reflux; SOCl₂, EtOH; (c) ethane-1,2-dithiol, SnCl₄, DCM; (d) LiAlH₄, THF; (e) MsCl, triethylamine, DCM; isoindoline-1,3-dione, K₂CO₃, CH₃CN, reflux; (f) 85% hydrazine hydrate, EtOH, reflux; (g) anhydride, triethylamine, DCM; (h) AgNO₃, EtOH; (i) aldehyde, MeONa, MeOH.

Theoretically, the E- and Z-geometric isomers can be equally formed in the above mentioned aldol condensation. However, only the E-isomers were obtained, most probably due to strong steric interaction between the aryl/alkyl and carbonyl groups. The appearance of the “diagnostic” vinyl proton signal in the appropriate range (7.70–7.50 ppm for aryl enones 12–18 and 7.00–6.90 for alkyl enones 19–20) in the ¹H NMR spectra unambiguously corroborated their E configuration.³⁹ This assignment was further confirmed by key NOESY interactions (see ESI† compound 18b) and is consistent with observations made with this reaction in similar substrates.^40–45

Biological evaluation

The intrinsic potency of these compounds was evaluated using HTRF IP-One Terbium-based assay and the results are depicted in Table 1. Compared to full agonist ramelteon and its strict methoxy analogue 2 (EC₅₀ (MT₁) = 0.36 nM; EC₅₀ (MT₂) = 0.97 nM), the oxo derivatives 11a (or 11b) showed moderate potency at both subtypes and slight preference for MT₂ (MT₁/MT₂ = 8–10). It was previously reported that the relative position between the methoxy group and the amide side chain of melatonin is essential for high receptor affinity and intrinsic activity.⁴⁶ Remarkable MT₂ selectivity was achieved by introduction phenyl group to 2-position of indanone core. Compound 12a showed good MT₂ agonist potency and wonderful MT₂ selectivity (EC₅₀ (MT₂) = 20 nM, MT₁/MT₂ > 5000). Replacement of the benzene ring with unsubstituted furan ring had little influence to the MT₂ agonistic activity and selectivity (12a vs. 17a). Substituted aromatic groups exhibited a lower potency which was probably due to the steric effect (12a vs. 14a–16a). Ligands with bulkier aromatic groups lost their agonistic activity at MT₂ subtype (12a vs. 13a and 18a). On the other hand, replacement of the propionylamido group by an acetamido group caused a significant loss of agonistic activity for MT₁ and MT₂ receptors (for example, 11a vs. 11b and 12a vs. 12b). In addition, aliphatic substitution also led to the increase of the MT₂ agonistic activity (19a and 20a). It's worth mentioning that the isopropyl substituted enone 19a displayed the best agonistic activity at both subtypes and slight preference for MT₂. Taken together, these data suggested that the introduction of substituent group to 2-position of indanone core may have an influence on the spatial arrangement of the amide side chain. Surprisingly the dithiane analog with nanomolar to subnanomolar agonist activity showed very high selectivity (MT₁/MT₂ = 2586) in its acetamido form 11b. All these effective indan derivatives behaved as partial agonists, compounds 10b, 12a, 17a, 20a exhibited powerful MT₂ agonistic activity (EC₅₀ < 50 nM) as well as excellent MT₂ selectivity (more than 2200-fold).

Table 1 Agonistic activity of compound 2 and 10–20 on human MT₁ and MT₂ receptors expressed in CHO cells^a

	EC50 MT1 (nM)	Emax (%) MT1	EC50 MT2 (nM)	Emax (%) MT2	MT1/MT2		EC50 MT1 (nM)	Emax (%) MT1	EC50 MT2 (nM)	Emax (%) MT2	MT1/MT2
a Test compound potency was expressed as EC50 (nM), while the ligand selectivity towards the two receptor subtypes was expressed as the MT1/MT2 EC50 ratio. Data reported in the table were means of three or more experiments run at eight different concentrations in triplicates.b NA = no agonist effect detected at 100 μM.
Ramelteon	0.32	100	0.81	100	0.4	2	0.36	102	0.97	101	0.37
10a	6730	52	97.3	88	69	10b	23 200	40	8.97	63	2586
11a	338	68	40.6	52	8	11b	3474	61	342	47	10
12a	NA^b	—	20	46	>5000	12b	NA	—	NA	—	—
13a	NA	—	NA	—	—	13b	NA	—	NA	—	—
14a	NA	—	73	25	>1300	14b	NA	—	NA	—	—
15a	NA	—	92.7	32	>1000	15b	NA	—	NA	—	—
16a	NA	—	222	40	>450	16b	NA	—	NA	—	—
17a	NA	—	43.9	43	>2200	17b	NA	—	NA	—	—
18a	NA	—	NA	—	—	18b	NA	—	NA	—	—
19a	23.95	65	5.08	71	4.7
20a	NA	—	18.6	61	>5300

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Conclusions

In this study, we have described the synthesis and biological evaluation of a novel class of chiral 2,3-dihydro-1H-indene derivatives as melatonergic ligands. Our exploration of indan core showed that the introduction of lipophilic group at the 2- or 3-position of this chiral scaffold could afford highly selective MT₂ partial agonists. Compounds 10b, 12a, 17a, 20a exhibited excellent MT₂ selectivity (more than 2200 times). The present investigation opens up the possibility of further exploring MT₂-specific agonists, which might expand our knowledge about MT₂ receptor in the near future.

Acknowledgements

This work was supported by the State Key Laboratory of Drug Research (no. SIMM1203KF-10), the grants of The National Natural Science Foundation of China (no. 81172936 and no. 21102046), and grants of The Fundamental Research Funds for the Central Universities.

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Footnote

† Electronic supplementary information (ESI) available: Details of experimental procedure, spectral data of all novel compounds. See DOI: 10.1039/c4ra03728f

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