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Formula | C26H38N2O4 |
Molar mass | 442.589 g/mol g·mol−1 |
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Alnespirone (S-20,499) is a selective 5-HT1A receptor full agonist of the azapirone chemical class.[1][2][3] It has antidepressant, anti-aggression and anxiolytic effects in animal models [1]. Phase II trials have been performed in Europe, but development was discontinued [4] [5].
History
editDiscovery of Alnespirone’s (S-20499) selective agonistic binding to 5-HT1A began with studies of chroman derivative 5-methoxy-3-(di-n-propylamino)chroman, which was found to act as an agonist on 5-HT1A receptors [6]. This subsequently led to synthesis of other chroman derivatives such as S-20244, which has both high affinity and selectivity for 5-HT1A receptors [6]. Amongst racemic (+/-)-S-20244 and its enantiomers (+)-S-20499 and (-)-S-20500, (+)-S-20499 yields the highest affinity for 5-HT1A [6].
Pharmacokinetics
editAlnespirone is an aminochroman derivative of the azapirone family [6]. It contains a piperazine nucleus, which is commonly found in other antidepressant and anxiolytic drugs such as gepipirone, ipsapirone and buspirone which also act on 5HT1A receptors [7].
Alnespirone shares structural similarities to other 5-HT receptor agonists 8-OH-DPAT and buspirone [8] [3] [9]. Because it does not metabolize into 1-(2-pyrimidinyl)-piperazine, which antagonizes α2-adrenoceptors, Alnespirone does not exert any affect on catecholaminergic neurons [10] [3]. The drug is lipophilic with a partition coefficient of log P > 2, so can easily pass through the blood brain barrier [9].
Anxiolytic activity is produced at lower doses (<1 mg/kg/day in Wistar rats) and antidepressant activity at midrange doses (1-5 mg/kg/day in Wistar rats) [11]. Alnespirone also has an ED50 of 0.3 mg kg-1 in the DRN, MRN, STR, FC, DHPC and VHPC [10]. It has a longer half-life than fellow 5-HT1A agonist 8-OH-DPAT [10].
From animal model studies, Alnespirone is usually administered intraperitoneally or subcutaneously in rats, and intramuscularly in monkeys [11] [10] [8] [12].
In a clinical trial, Alnespirone was administered orally [13].
Pharmacodynamics
editIn vitro, the drug acts as a selective full presynaptic and postsynaptic agonist, and in vivo, full presynaptic and partial postsynaptic agonist [11]. If acting on the 5-HT1A autoreceptors, Alnespirone has anti-aggressive effects as the drug inhibits firing of 5-HT neurons and 5-HT release [14]. Conversely, when acting on postsynaptic receptors, Alnespirone promotes 5-HT release at doses that otherwise reduce aggression when acting presynaptically [15] [14].
The drug displays low affinity for other 5-HT receptor binding sites such as 5-HT1B (IC50 = 5-10 uM) and 5-HT2 (IC50 = 1-5 uM) [6].
Rat studies demonstrate that Alnespirone is regionally selective, reducing extracellular 5-HT in the striatum (maximal reduction to 23% of baseline), frontal cortex (29%), dorsal and ventral hippocampus (65%), median raphe nucleus (30%) and dorsal raphe nucleus (60%) [10] [8].
Desensitization and Down Regulation of 5-HT1A
editAlnespirone is capable of desensitizing the somatodendritic 5-HT1A receptors in the dorsal raphe nucleus and frontal cortex [8] [16]. In rats, Alnespirone is capable of decreasing the potency of 8-OH-DPAT inhibition in DRN HT1A autoreceptors, but this Alnespirone-mediated desensitization takes longer to develop that that mediated by agonists ipsapirone and depirone [16].
Autoradiographic experiments in rats also show a reduction in the somatodendritic 5-HT1A receptors, but no such down regulation is observed in the postsynaptic receptors when treated with Alnespirone[8].
Potency
editAlnespirone has a Ki = 0.19 nM at 5-HT1A hippocampal receptors [6], and its radioligand [3H] Alnespirone has a Kd = 0.36 nM in rat hippocampal membrane [17]. The drug has an anti-aggressive ID50 of 1.24 mg/kg [18].
Efficacy
editThe drug has an inverted U-shaped dose dependent relationship in the Chronic Mild Stress Model of Depression as demonstrated in rats, in which there was a lack of significant effect for low doses (0.5 mg/kg/day) and high doses (10, 20 mg/kg/day) [11].
Treatment
editStudies primarily using animal models have demonstrated how Alnespirone could serve as a potential drug to treat symptoms of depression, aggression, and anxiety.
Depression
edit5-HT1A agonists such as Alnespirone are able to produce anti-depressive effects, but there is still debate on whether the drug acts on presynaptic or postsynaptic receptors [11].
Other studies demonstrate Alnespirone’s anti-depressive properties through the drug’s indirect regulation of certain receptors. β1-adrenoceptors are reportedly more abundant in depressed patients, and upon antidepressant treatment usually decrease in number [19]. Similarly, 5-HT2 receptors have been found to increase in density in the cortex of depressed suicide victims [19]. Anti-depressive activity was demonstrated in chronic treatment of Alnespirone in rats, which produced a slight decrease in β1-adrenoceptor density, and significantly reduced 5-HT2 density in the cortex [12].
Anti-Aggression
editDrugs that target the 5-HT1A receptors have been found to alter aggressive behavior [18]. Studies using a resident-intruder agonistic paradigm in Alnespirone treated rats showed a dose-dependent reduction in aggressive actions (less threat, attacks and pursuit towards an intruder) [15] [18].
The 5-HT syndrome is often induced in rats by drugs that act as agonists on 5-HT1A receptors [9]. Anti-aggressive studies on Alnespirone show that the drug produces hypothermia, but unlike other 5-HT1A agonists 8-OH-DPAT, buspirone, ipsapirone and 5-MeODMT, Alnespirone does not yield the other sedative side effects characterized by the syndrome [15] [9]. Instead, the reduction in anti-aggressive behavior induced by Alnespirone also produced significant increases in social exploration [14]. Research suggests that this may be because the drug does not affect postsynaptic 5-HT1A receptors at anti-aggressive doses [14], or may selectively act on subtypes of the 5-HT1A receptors [9]. Action at subtypes of 5-HT1A was proposed as Alnespirone does not decrease or induce forepaw treading in rats unlike other 5-HT1A agonists [9]. Because the dose of Alnespirone (75-120 mg/kg) in forepaw treading studies is much larger than the EC50 hypothermia-inducing dose, Alnespirone may preferentially bind to the 5-HT1A subtype mediating hypothermia [9].
Anxiety
editAgonistic effects mediated by Alnespirone on presynaptic 5-HT1A receptors appear to produce anxiolytic properties [19]. Mice treated with 4-63 mg/kg i.p of Alnespirone experienced symptoms of sedation such as hypoactivity and decrease in traction and muscle tone [20]. At 4 mg/kg, there was an increase in number of shocks taken in the Vogel conflict test and at 16 mg/kg, greater immobility was found in the tail suspension test, indicative of increased anxiolytic activity [20].
In a clinical trial, Alnespirone was able to induce dose-dependent sedation in healthy volunteers as demonstrated through EEG mapping and psychometric measures [13].
Side Effects
editAlnespirone has been found to increase melatonin production at night, suggesting how its effects on 5-HT1A receptors may influence the sleep/wake cycle [21].
In addition, studies using animal models demonstrate how Alnespirone increases levels of oxytocin, ACTH and dopamine in rats [17] [22] [23]. The drug maximally stimulated oxytocin secretion as early as 15 minutes after administration (5 mg/kg, IP), with effects lasting for 30 minutes [17]. This may be due to action at postsynaptic 5-HT1A receptors; somatodendritic receptors have a lower ED50 compared to those on the postsynaptic, and oxytocin stimulation was observed at high but not low doses [17]. This poses implications of how the drug may promote prosocial behaviors such as trust, which is exhibited in humans with increased levels of oxytocin [24]. Alnespirone dose dependently increased ACTH levels (at 0.01-20 mg/kg i.p), which may consequently produce downstream effects in the hypothalamic-pituitary-adrenal axis, [25] and also indirectly enhanced dopamine turnover (at 32 mg kg-1) [17]. The indirect effect of the drug on dopamine is speculated to be due to activation of 5-HT1A and subsequent 5-HT reduction that the receptor mediates [17].
Alnespirone induces dose-dependent hypothermia at 2.5-20 mg/kg in rats, but does not produce other symptoms of the 5-HT syndrome [15] .
There is also debate on Alnespirone’s effects on alcohol consumption in animal models. Studies reveal how alcohol drinking and serotonin levels share an inverse relationship, so increase in 5-HT neurotransmission results in decreased alcohol intake [12]. A significant dose-dependent decrease in alcohol intake and increased resting duration was found in adult Squirrel Monkeys treated with Alnespirone [12]. The drug also caused emesis at 0.01-0.1 mg/kg prior to the alcohol self-administration. However, results are contradictory with behavior in Long-Evans rats, in which there were significant increases in alcohol consumption after administration of Alnespirone at low doses [12]. This suggests that low doses do not potentiate the 5-HT inhibition caused by 5-HT1A autoreceptors, but may also alternatively show how 5-HT1A receptors have different effects in rats and monkeys [12].
See also
editReferences
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Category:Serotonin receptor agonists Category:Imides Category:Phenol ethers Category:Amines Category:Azapirones Category:Chromanes Category:Lactams