Design,Synthesis and Evaluation of Antidepressant Activity of Novel 2‐Methoxy 1, 8 Naphthyridine 3‐Carboxamides as 5‐HT3 Receptor Antagonists

A series of novel 1,8‐naphthyridine‐3‐carboxamides as 5‐HT₃ receptor antagonists were synthesized with an intention to explore the antidepressant activity of these compounds. The title carboxamides were designed using ligand‐based approach keeping in consideration the structural requirement of the pharmacophore of 5‐HT₃ receptor antagonists. The compounds were synthesized using appropriate synthetic route from the starting material nicotinamide. 5‐HT₃ receptor antagonism of all the compounds, which was denoted in the form of pA₂ value, was determined in longitudinal muscle myenteric plexus preparation from guinea‐pig ileum against 5‐HT₃ agonist, 2‐methyl‐5‐HT. Compound 8g (2‐methoxy‐1, 8‐naphthyridin‐3‐yl) (2‐methoxy phenyl piperazine‐1‐yl) methanone was identified as the most active compound, which expressed a pA₂ value of 7.67. The antidepressant activity of all the compounds was examined in mice model of forced swim test (FST); importantly, none of the compounds was found to cause any significant changes in the locomotor activity of mice at the tested dose levels. In FST, the compounds with considerably higher pA₂ value exhibited promising antidepressant‐like activity, whereas compounds with lower pA₂ value did not show antidepressant‐like activity as compared to the control group.     

In Search of Potent 5‐HT6 Receptor Inverse Agonists

A series of non‐sulfonamide/non‐sulfone derived potent 5‐HT₆ receptor inverse agonists has been disclosed. Representative compound 9 ( K _i = 14 nm ) displayed selectivity against a set of family members as well as brain permeability 6 h post‐oral administration. In addition, the separated enantiomers of compound 9 displayed difference in activity indicating the influence of chirality on potency.     

Improved Anti‐Emetic Efficacy of 5‐HT3 Receptor Antagonists in Cancer Patients with Genetic Polymorphisms of ABCB1 (MDR1) Drug Transporter

Chemotherapy‐induced nausea and vomiting (CINV) remains a major adverse effect decreasing quality of life in patients with cancer. Genetic variations among patients may be responsible for part of the lack of efficacy of anti‐emetic drugs. The aim of this study was to investigate how the genetic variants of the drug transporter ABCB1 (MDR1) gene affect anti‐emetic treatment with 5‐HT₃ receptor antagonists. Patients (n = 239) receiving moderately or highly emetogenic chemotherapy and ondansetron or granisetron were included in the study. Anti‐emetic responses were recorded daily. The primary end‐point of the assessment was the total control rates of CINV in the acute and delayed phases after chemotherapy. Genotyping was performed by PCR‐RFLP. In the acute phase, patients with ABCB13435TT, 1236TT or 2677TT genotypes had a higher control rate of CINV than other genotype groups: (64.7% in 3435TT versus 45.7% in 3435CC+CT, p = 0.016; 65.1% in 1236TT versus 46.4% in 1236CC+CT, p = 0.027; 66.7% in 2677TT versus 46.5% in other genotypes, p = 0.021). Subjects carrying homozygous variant alleles together (TT‐TT‐TT) showed a significantly higher protection from nausea and vomiting (67.7% in TT‐TT‐TT versus 47.1% in other genotypes, p = 0.032). After the logistic regression analysis with adjustment for other known covariates, the total control rate was significantly higher in the 3435TT genotype group during the acute phase (p = 0.021). No significant change was found between the total control rates among genotypes in the delayed phase. Each of three 3435TT, C1236TT, 2677TT genotypes of ABCB1 and their combination was associated with about 50% higher anti‐emetic response to 5‐HT₃ receptor antagonists in the acute phase of chemotherapy in patients with cancer receiving moderately or highly emetogenic chemotherapy. ABCB1 (MDR1) genotypes may contribute to predict the anti‐emetic efficacy of 5‐HT₃ antagonists.     

New Arylpiperazines with Flexible versus Partly Constrained Linker as Serotonin 5‐HT1A/5‐HT7 Receptor Ligands

A series of new long‐chain arylpiperazine (LCAP) derivatives with flexible and partly constrained alkyl linker were synthesized and investigated in vitro as potential serotonin 5‐HT_1A and 5‐HT₇ receptor ligands. The compounds were prepared by a two‐step procedure using naphthalimide and 2H‐1,3‐benzoxazine‐2,4(3H)‐dione as imides, and 1‐(2‐methoxyphenyl)piperazine (o‐OMe‐PhP) and 1,2,3,4‐tetrahydroisoquinoline (THIQ) as amine pharmacophores. Modifications of the spacer structure included introduction of flexible penta‐ and hexamethylene chains as well as partly constrained m‐ and p‐xylyl moieties. In general, the new compounds were more active at the 5‐HT_1A than at the 5‐HT₇ receptor, and the o‐OMe‐PhP derivatives displayed higher affinities than their respective THIQ analogs. The spacer modifications had little effect on the observed in vitro activities. Within the o‐OMe‐PhP series, except for a small binding reduction for ligands containing the m‐xylyl moiety, there was no substantial change in the compounds' potency at both receptors, while for the THIQ derivatives a clear structure–activity relationship was visible only for the interaction of the compounds with the 5‐HT₇ receptor, which strongly favored flexible analogs.     

Quinolinesulfonamides of Aryloxy‐/Arylthio‐ethyl Piperidines: Influence of an Arylether Fragment on 5‐HT1A/5‐HT7 Receptor Selectivity

The solid‐phase synthesis of a new series of 19 biomimetics of long‐chain arylpiperazines, namely flexible quinoline sulfonamides of aryl(heteroaryl)oxy‐/heteroarylthio‐ethyl 4‐aminomethylpiperidines, is reported. Various structural modifications applied followed by biological evaluation for 5‐HT_1A, 5‐HT₆, and 5‐HT₇ receptors gave further support of a possible replacement of arylpiperazine with aryloxy‐/arylthio‐ethyl derivatives of alicyclic amines and control of receptor selectivity upon diversification in the aryl(heteroaryl)oxy‐/heteroarylthio‐ethyl fragment.     

Tolterodine and its Active 5‐Hydroxymethyl Metabolite: Pure Muscarinic Receptor Antagonists

Abstract:Tolterodine and its major active 5‐hydroxymethyl metabolite (5‐HM) are potent muscarinic receptor antagonists that show selectivity for the urinary bladder over salivary glands in vivo. This tissue selectivity cannot be attributed to muscarinic receptor subtype selectivity, since both compounds are non‐selective with respect to the M₁–M₅ receptor subtypes. The aim of the present in vitro study was to determine the specificity of tolterodine and 5‐HM for muscarinic receptors compared to other potential cellular targets. Carbachol‐induced contractions of isolated guinea pig bladder were effectively inhibited by tolterodine (IC₅₀ 14 nM) and 5‐HM (IC₅₀ 5.7 nM). Tolterodine and 5‐HM were weak inhibitors of effects mediated by α‐adrenergic receptors (rat portal vein), histamine receptors (guinea pig ileum) and calcium channels (guinea pig potassium‐depolarised urinary bladder, spontaneously beating right atria and electrically‐driven right papillary muscle). The IC₅₀ values were in the μM range and the antimuscarinic potency of tolterodine was 27, 200 and 370–485 times higher, respectively, than its potency in blocking histamine receptors, α‐adrenoceptors and calcium channels. The active metabolite, 5‐HM, was >900 times less potent at these sites than at bladder muscarinic receptors. Radioligand binding data on 54 different receptors and binding sites showed that tolterodine and 5‐HM bind with significant affinity only at muscarinic receptors. In conclusion, the results of the present study indicate that both tolterodine and 5‐HM are specific muscarinic receptor antagonists. The tissue selectivity of these agents in vivo cannot therefore be explained by secondary pharmacological actions.     

Solid‐Supported Synthesis and 5‐HT7/5‐HT1A Receptor Affinity of Arylpiperazinylbutyl Derivatives of 4,5‐dihydro‐1,2,4‐triazine‐6‐(1H)‐one

A series of arylpiperazinylbutyl derivatives of 4,5‐dihydro‐1,2,4‐triazine‐6(1H)‐ones was designed and synthesized according to the new solid‐supported methodology. In this approach, triazinone scaffold was constructed from the Fmoc‐protected glycine. The library representatives showed different levels of affinity for 5‐HT₇ and 5‐HT_1A receptors; compounds 13 , 14 and 18 – 20 were classified as dual 5‐HT₇/5‐HT_1A receptors ligands. The structure–affinity relationship analysis revealed that the receptor affinity and selectivity of the tested compounds depended on the kind of substituent in position 3 of triazinone fragment as well as substitution pattern of the phenylpiperazine moiety.     

α‐Adrenoceptor‐Mediated Modulation of 5‐HT2 Receptor Agonist Induced Impulsive Responding in a 5‐Choice Serial Reaction Time Task

Abstract: The activation of 5‐HT_2A receptors has been shown to enhance the probability of premature responding, regarded as a form of motor impulsive behaviour. At the behavioural level, the interaction of α‐adrenoceptors and 5‐HT₂ receptors has been linked to head twitch behaviour, regarded as an experimental model of compulsive behaviour. The aim was to determine whether the probability of premature responding induced by an excess activation of 5‐HT_2A receptors can be modulated by the blockade of α₁‐ or α₂‐ adrenoceptors. In the experiments, the 5‐choice serial reaction time task was used to measure attention and response control of the rats. The experiments assessed the effects of (±)‐1‐(2,5‐dimethoxy‐4‐iodophenyl)‐2‐aminopropane hydrochloride (DOI) 0.1–0.2 mg/kg subcutaneously, a 5‐HT_2A/2C agonist, and prazosin, an α₁–adrenoceptor antagonist, alone or in combination, on the performance of rats. In an additional experiment to examine the possible role of the α₂–adrenoceptors, a potent, selective and specific α₂–adrenoceptor antagonist, atipamezole, was given alone or in combination with DOI. Results showed that DOI increased the probability of premature responses, but it did not affect the choice accuracy. Prazosin (0.1 or 0.3 mg/kg, subcutaneously), given on its own had no effects on probability of responding prematurely, but prazosin (0.3 mg/kg.) was able to attenuate the DOI‐induced responding. Atipamezole (0.1 mg/kg, s.c.) did not attenuate the effect of DOI on probability of premature responding. When given at lower doses, DOI (0.03 mg/kg) and atipamezole (0.03 mg/kg) synergistically increased the probability of premature responding, whereas a higher dose of atipamezole (0.3 mg/kg) on its own increased the probability of responding prematurely, but this effect was not additive to that of 0.1 mg/kg DOI. These data indicate that 5‐HT₂ receptor activation enhances impulsive responding and this effect can be diminished by the blockade of α₁‐adrenoceptors. Atipamezole, an α₂‐antagonist, enhances the probability of premature responding and shares the mechanism of action with the 5‐HT₂ agonist in this respect. These results provide evidence for an interaction between the serotonergic 5‐HT₂ receptors and α‐adrenoceptors in the modulation of response control to the motor impulsivity type of behaviour (premature responding) in addition to that of compulsory behaviour (head shakes) found previously.     

Novel Mannich Bases, 5‐Arylimidazolidine‐2,4‐dione Derivatives with Dual 5‐HT1A Receptor and Serotonin Transporter Affinity

A computer aided ligand design study of imidazolidine‐2,4‐dione derivatives was conducted in order to obtain compounds with dual 5‐HT_1A receptor and serotonin transporter (SERT) affinity. According to molecular modeling results, series of Mannich bases were chosen and synthesized. Investigated compounds were tested for 5‐HT_1A, 5‐HT_2A, α₁ and SERT affinity. Two selected compounds ( 5 , 9 ) were characterized in functional experiments and possessed a pharmacological profile which may enhance SERT blocking efficacy – 5‐HT_1A partial agonism and 5‐HT_2A antagonism in one molecule. Furthermore these compounds displayed satisfactory selectivity over adrenergic α₁ receptors. The most promising compounds, 5‐arylimidazolidine‐2,4‐dione derivatives with 4‐(3‐chlorophenyl)piperazinylmethyl moiety were tested for antidepressant and anxiolytic activity. In particular, compound 5 (5‐(2‐methoxyphenyl)‐3‐{1‐[4‐(3‐chlorophenyl)piperazin‐1‐yl]methyl}‐imidazolidine‐2,4‐dione), tested in the forced swim test in mice, exhibited a favorable antidepressant‐like profile without affecting spontaneous locomotor activity.     

New 1‐Benzyl‐4‐hydroxypiperidine Derivatives as Non‐imidazole Histamine H3 Receptor Antagonists

A series of 1‐benzyl‐4‐(3‐aminopropyloxy)piperidine and 1‐benzyl‐4‐(5‐aminopentyloxy)piperidine derivatives has been prepared. The 1‐benzyl‐4‐hydroxypiperidine derivatives obtained were evaluated for their affinities at recombinant human histamine H₃ receptor, stably expressed in HEK 293T cells. All compounds investigated show moderate to pronounced in‐vitro affinities. The most potent antagonists in this series 9b2 (hH₃R, pK_i = 7.09), 9b1 (hH₃R, pK_i = 6.78), 9b5 (hH₃R, pK_i = 6.99), and 9b6 (hH₃R, pK_i = 6.97) were also tested in vitro as H₃ receptor antagonists – the electrically evoked contraction of the guinea‐pig jejunum. The histaminergic H₁ antagonism of selected compounds 9b1 , 9b2 , and 9b4 – 9b6 was established on the isolated guinea‐pig ileum by conventional methods; the pA₂ values were compared with the potency of pyrilamine. The compounds did not show any H₁ antagonistic activity (pA₂ < 4; for pyrilamine pA₂ = 9.53).     

Involvement of 5‐HT2A Receptors in the Serotonin (5‐HT) Syndrome caused by Excessive 5‐HT Efflux in Rat Brain

Abstract: Previous studies have demonstrated that serotonin (5‐HT) syndromes, particularly for the malignant cases, can be alleviated by ice water mists, cooling blankets and many other external cooling measures. In this study, we tested the hypothesis that external cooling measures reduce the responsivity of 5‐HT_2A receptors to excessive 5‐HT efflux, which may be a possible mechanism underlying the treatment of serotonin syndrome. To test this, rat experiments were carried out in the standard and cool ambient temperature (T_amb) by administration of the 5‐HT precursor 5‐hydroxy‐l ‐tryptophan combined with the monoamine oxidase inhibitor clorgyline. The first set of experiments was to assess severity of the syndromes by measuring body temperature responses. Consistent with the hypothesis, we found that the syndrome was malignant at the standard T_amb of 22°C but alleviated at 12 or 6°C, these results being similar to those in rats pre‐treated with the 5‐HT_2A receptor antagonist ketanserin. The second set of experiments was to utilize microdialysis to determine the relationship between the syndrome severity and 5‐HT levels at the above‐mentioned T_amb. We found that excessive 5‐HT efflux consisted of primary and secondary components through two distinct mechanisms. Furthermore, the secondary component efflux, which can be ascribed to 5‐HT_2A receptor activation, was proportionally reduced at the cool T_amb of 12 and 6°C. In conclusion, results of this study support the hypothesis that cooling T_amb reduces the functional activity of 5‐HT_2A receptors, thus alleviating the malignant syndrome.     

Anxiolytic Potential of 5-HT3 Receptor Antagonists

Abstract: The 5-HT₃ receptor antagonists such as ondansetron have been shown to have anxiolytic-like activity in a wide range of preclinical tests: in the mouse black: white test, the rat social interaction test, the rat elevated X-maze and the marmoset human threat test. Ondansetron, like other 5-HT₃ receptor antagonists appears to have important advantages over existing anxiolytic therapies. Thus, the 5-HT₃ receptor antagonists are not sedative, they do not have addictive liabilities, there are no problems of withdrawing from chronic treatment and they can be used following benzodiazepine withdrawal. Such compounds even inhibit the behavioural syndrome associated with withdrawal from drugs of abuse, nicotine, alcohol, cocaine, opiates. The preclinical data, therefore, indicates the 5-HT₃ receptor antagonists as novel anxiolytics of the future, and there is new clinical work in support of this suggestion.     

5‐HT modulates the rat mesenteric vasopressor outflow by 5‐HT1D sympatholytic receptors

5‐hydroxytryptamine (5‐HT) modulates noradrenergic activity in different cardiovascular territories, but its effect on the mesenteric vasopressor outflow has not yet been clarified. This study investigated the in vivo serotonergic influence, characterizing 5‐HT receptors implicated, in sympathetic innervation of mesenteric vasculature. Wistar rats were anaesthetised and prepared for the in situ autoperfused rat mesentery, monitoring systemic blood pressure (SBP), heart rate (HR) and mesenteric perfusion pressure (MPP). Electrical stimulation of mesenteric sympathetic nerves resulted in frequency‐dependent increases in MPP (9 ± 1.6, 25.7 ± 3.9 and 60.2 ± 5 mmHg for 2, 4 and 8 Hz, respectively), without altering SBP or HR. 5‐HT (1‐25 μg/kg), 5‐carboxamidotryptamine (5‐HT_1/7 agonist; 25 μg/kg) or L‐694,247 (5‐HT_1D agonist; 1‐25 μg/kg) i.a. bolus inhibited vasopressor responses by mesenteric nerves electrical stimulation, unlike i.a. bolus of agonists 8‐OH‐DPAT (5‐HT_1A), CGS‐12066B (5‐HT_1B), BRL 54443 (5‐HT_1e/1F), α‐methyl‐5‐HT (5‐HT₂), 1‐PBG (5‐HT₃), cisapride (5‐HT₄) or AS‐19 (5‐HT₇) (25 μg/kg each). Interestingly, i.a. L‐694,247 (25 μg/kg) also reduced the exogenous norepinephrine‐induced vasoconstrictions. Pretreatment with selective 5‐HT_1D receptor antagonist, LY310762 (1 mg/kg, i.v.), completely abolished L‐694,247‐ and 5‐HT‐induced mesenteric sympathoinhibition. Furthermore, ELISA analysis confirmed 5‐HT_1D receptors expression in mesenteric artery. These findings suggest that serotonergic mechanisms‐induced sympathoinhibition of mesenteric noradrenergic outflow is mediated by pre and/or postjunctional 5‐HT_1D receptors.     

Effects of 5‐HT4 Receptor Agonists,Cisapride and Mosapride Citrate on Electrocardiogram in Anaesthetized Rats and Guinea‐Pigs and Conscious Cats

Abstract: The purpose of this study was to examine the arrhythmogenic potential of 5‐HT₄ receptor agonists, cisapride and mosapride citrate (mosapride) in vivo. In anaesthetized rats, cisapride at intravenous infusion of 10 and 30 mg/kg/hr for 1 hr prolonged the electrocardiographic RR and QT intervals, whereas at 3 mg/kg/hr, it prolonged the RR interval without affecting the QT interval. Mosapride at 30 mg/kg/hr for 1 hr slightly, but not significantly, prolonged the QT interval. In anaesthetized guinea‐pigs, cisapride at intravenous infusion of 0.3, 1 and 3 mg/kg over 15 min. prolonged the RR interval (18–44%), QT interval (18–42%) and the corrected QT interval (QTc; 8–19%). Mosapride at 3, 10 and 30 mg/kg over 15 min. little affected the QTc although at 30 mg/kg, it slightly prolonged the RR and QT intervals. With repeated oral administrations of 30 mg/kg twice a day for 7 days, cisapride prolonged the QT interval (11–35%) and QTc (11–32%) at the 3rd and 7th days in conscious cats. In addition, cisapride depressed the ST segment in two out of five cats. Mosapride at 60 mg/kg twice a day for 7 days did not affect the QT interval or QTc in cats. The maximal plasma concentrations of mosapride and its main metabolite (a des‐4‐fluorobenzyl‐mosapride) at the 7th day in cats were 9.4±2.8 μM and 2.5±0.3 μM, respectively, being 100 and 30–60 times higher than those in man given therapeutic doses (Sakashita et al. 1993a&b). These results indicate that mosapride has little arrhythmogenic potential.