ChemMedChem. 2016 Oct 19;11(20):2287-2298.

The Versatile 2-Substituted Imidazoline Nucleus as a Structural Motif of Ligands Directed to the Serotonin 5-HT1A Receptor 

Del Bello F1, Cilia A2, Carrieri A3, Fasano DC3, Ghelardini C4, Di Cesare Mannelli L4, Micheli L4, Santini C1, Diamanti E1, Giannella M1,*, Giorgioni G1, Mammoli V1, Paoletti CD1, Petrelli R1, Piergentili A1, Quaglia W1, Pigini M1

Dedicated to Prof. Maria Pigini, who passed away on February 8, 2016

1School of Pharmacy, Medicinal Chemistry Unit, University of Camerino, Via S. Agostino 1, 62032 Camerino, Italy

2Recordati S.p.A., Drug Discovery, via Civitali 1, 20148 Milano, Italy

3Department of Pharmacy-Drug Science, University of Bari “Aldo Moro”, Via E. Orabona 4, 70125 Bari, Italy

4Department of Neuroscience, Psychology, Drug Research and Child Health – Neurofarba – Pharmacology and Toxicology Section, University of Florence, Viale Pieraccini 6, 50039 Firenze, Italy.

Running title: 2-Substituted imidazoline nucleus in serotonin 5-HT1A receptor ligands

To whom correspondence should be addressed: Prof. Mario Giannella, School of Pharmacy, Medicinal Chemistry Unit, University of Camerino, via S. Agostino 1, 62032 Camerino (Italy), Telephone (039) 0737 402257; FAX (039) 0737 637345; E-mail: mario.giannella@unicam.it

 

Abstract

The involvement of the serotonin 5-HT1A-receptor (5-HT1A-R) in the antidepressant effect of allyphenyline and its analogues indicated that ligands bearing the 2-substituted imidazoline nucleus as a structural motif interacted with 5-HT1A-R. Therefore, we examined the 5-HT1A-R profile of several imidazoline molecules endowed with a common scaffold consisting of an aromatic moiety linked to the 2-position of an imidazoline nucleus by a biatomic bridge. Our aim was to discover other ligands targeting 5-HT1A-R and to identify the structural features favoring 5-HT1A-R interaction. Structure-activity relationships, supported by modeling studies, suggested that some structural cliché such as a polar function and a methyl group in the bridge, as well as proper steric hindrance in the aromatic area of the above scaffold, favored 5-HT1A-R recognition and activation. We also highlighted the potent antidepressant-like effect (mouse forced swimming test) of (S)-(+)-19 [(S)-(+)-naphtyline] at very low dose (0.01 mg/kg). This effect was clearly mediated by 5-HT1A, as it was significantly reduced by pretreatment with the 5-HT1A antagonist WAY100635.

KEY WORDS: antidepressant agents; drug design; nitrogen heterocycles; receptors; serotonin receptor agonists

PMID: 27690321; DOI: 10.1002/cmdc.201600383

 

Supplement:

Our research group, involved in the field of Receptor Chemistry, has being engaged in the design and synthesis of novel biologically active ligands bearing the 2-substituted imidazoline as a structural motif. The privileged nature of this scaffold (I, Figure 1) has been widely confirmed. Structure-activity relationship studies show that the linker (X) and the aromatic moiety (Ar), forming the substituent in position 2 of the imidazoline nucleus, display different functions. Indeed, minor chemical modifications of the linker affect the recognition of one or more specific biological targets, whereas suitable decorations in the aromatic region are generally responsible for the ligand functional behaviour. Among the most interesting compounds, homoazanicotine (1) retains affinity and antinociceptive potency of nicotine [1]. In this case, the N-methyl substitution plays a critical role whereas an ethylenic chain is also compatible with the nicotinic α4β2-type interaction. Constraining the linker in the cyclic structure of furan leads to amifuraline (2), able to significantly inhibit liver but not brain MAO-A [2]. An ethylene linker is suitable for I2-IBS interaction: indeed, phenyzoline (3) and its ortho-phenyl analogue diphenyzoline (4) show a very high selectivity for I2-Imidazoline Binding Sites (I2-IBS) over both α2-adrenergic receptors (α2-ARs) and I1-IBS [3]. These two compounds display an interesting “positive” and “negative” morphine analgesia modulatory effect. In fact, 3 enhances morphine analgesia, whereas 4 decreases it. On the contrary, the -CH2CH(CH3)- chain favors the I1-IBS selectivity as demonstrated by the potent I1-IBS agonist carbomethyline (5), endowed with significant antihypertensive activity [4]. The isosteric -OCH(CH3)- chain preferentially orients towards α2-ARs interaction: biphenyline (6), the most representative member of this series of compounds, behaves as an agonist at both α2A- and α2C-AR subtypes. Its eutomer (S)-(-)-6 displays potent and long-lasting antinociceptive effect completely blocked by the selective α2-AR antagonist RX 821002 [5]. An important modulation of the pharmacological profile of the ligands is provided by the ortho phenyl substituent. In particular, substituents with steric bulk comparable to that of the phenyl ring (i. e. cyclopentyl or cyclohexyl) lead to compounds behaving similarly to 6, whereas substituents with lower steric bulk lead to compounds that mantain α2C-AR agonism but display α2A-AR antagonism. Allyphenyline (7), for example, at low dose (0.05 mg/Kg) enhances morphine analgesia (due to its α2C-AR agonism), prevents and reverses morphine tolerance and dependence without sedative side effects (due to its α2A-AR antagonism) [6]. Further studies demonstrate that 7 exerts a potent antidepressant like effect in the mouse forced swimming test partially due to the additional serotonin 5-HT1A receptor activation [7]. This multifunctional profile accounts the ability of 7 to reduce hyperanxiety-like behavior after alcohol intoxication and hyperlocomotor activity associated with alcohol withdrawal [8] and to induce gastroprotection and to inhibit fundic and colonic contractility [9]. An even more efficacious antidepressant-like effect is exerted by naphtyline (8) whose (S)-(+)-enantiomer is active at the very low dose of 0.01 mg/Kg [10].

The isosteric substitution of the O in the linker with an NH function, together with the insertion of suitable substituents in the ortho position of the aromatic moiety also led to multitarget compounds. Indeed, the -NH-CH2– chain proved to be suitable for ligands simultaneously interacting with I1-, I2-IBS and α2-ARs. Among these, 9 might be considered a promising tool in managing opioid addiction, since it demonstrated to be effective in reducing expression and acquisition of morphine dependence in mice at the dose of 5 mg/kg [11].

Finally, the scaffold I has been recently inserted in a set of N-phenethylimidazoline compounds that proved to be selective for dopamine D4 receptor [12]. In particolar, the 3-methoxy-substituted derivative 10 endowed with the highest D4 affinity value behaves as a partial agonist with low intrinsic efficacy and as competitive D4 antagonist when tested in the presence of the D2-like receptor agonist quinpirole.

The versatility of the scaffold I and the multifunctionality of the obtained ligands suggest to extend the study to further potential applications. In particular, considering the pivotal role of the α2-AR and 5-HT1A systems in the inhibitory descending pain pathway, the combination of α2-AR and 5-HT1A activation might be fruitful for neuropathic pain relief.

 

 

 

Figure 1. The 2-substituted imidazoline scaffold and some of the most important compounds contain this structural motif.

 

 

Figure 2. The receptor chemistry group together with Robert Lefkowitz (awarded with the Nobel Prize in Chemistry for 2012) during the 33rd Camerino-Cyprus Symposium “Receptor Chemistry; Reality and Vision” (Camerino May 15-19; 2016).

From left: Alessandro Piergentili, Piero Angeli, Riccardo Petrelli, Robert Lefkowitz, Mario Giannella, Gianfabio Giorgioni, Giovanni Caprioli, Ajiroghene Thomas, Wilma Quaglia, Fabio Del Bello.

 

 

 

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