The Novel Pimavanserin Derivative ST-2300 with Histamine H3 Receptor Affinity Shows Reduced 5-HT2A Binding, but Maintains Antidepressant- and Anxiolytic-like Properties in Mice

Abstract

The therapy of depression is challenging and still unsatisfactory despite the presence of many antidepressant drugs on the market. Consequently, there is a continuous need to search for new, safer, and more effective antidepressant therapeutics. Previous studies have suggested a potential association of brain histaminergic/serotoninergic signaling and antidepressant- and anxiolytic-like effects. Here, we evaluated the in vivo antidepressant- and anxiolytic-like effects of the newly developed multiple-active ligand ST-2300. ST-2300 was developed from 5-HT_2A/2C inverse agonist pimavanserin (PIM, ACP-103) and incorporates a histamine H₃ receptor (H₃R) antagonist pharmacophore. Despite its parent compound, ST-2300 showed only moderate serotonin 5-HT_2A antagonist/inverse agonist affinity (K_i value of 1302 nM), but excellent H₃R affinity (K_i value of 14 nM). In vivo effects were examined using forced swim test (FST), tail suspension test (TST), and the open field test (OFT) in C57BL/6 mice. Unlike PIM, ST-2300 significantly increased the anxiolytic-like effects in OFT without altering general motor activity. In FST and TST, ST-2300 was able to reduce immobility time similar to fluoxetine (FLX), a recognized antidepressant drug. Importantly, pretreatment with the CNS-penetrant H₃R agonist (R)-α-methylhistamine reversed the antidepressant-like effects of ST-2300 in FST and TST, but failed to reverse the ST-2300-provided anxiolytic effects in OFT. Present findings reveal critical structural features that are useful in a rational multiple-pharmacological approach to target H₃R/5-HT_2A/5-HT_2C.

Keywords: 5-HT2A receptor; ACP-103; antidepressant; anxiolytic; fluoxetine; forced swim test; histamine H3 receptor; open field test; pimavanserin; tail suspension test.

Figure 1

Design of multiple-active compound ST-2300.

Figure 2

The synthesis of compound ST-2300. (a): K₂CO₃, KI (catalytic amount), acetone, reflux, 18 h; (b): Raney-Nickel, NH₃ (saturated in MeOH), H₂, 5 bar, 40 °C, 18 h; (c): Diphosgene, triethylamine, dichloromethane, 0 °C—room temperature, 2 h; (d): CH₃COOH, NaBH(CH₃COO)₃, 1,2-dichloroethane, room temperature 18 h; (e): Tetrahydrofuran, room temperature, 18 h.

Figure 3

Effects of acute ST-2300 pre-treatment on depression-like behavior in FST. Acute ST-2300 dose-dependently decreased the immobility time in the FST. ** Denotes significant differences between drug doses and saline controls (p < 0.01); *** Denotes significant differences between drug doses and controls (p < 0.001). # Denotes significant differences between FLX and ST-2300 (5 and 10 mg/kg, i.p.) as well as PIM (5 and 10 mg/kg, i.p.) (p < 0.05). Vehicle (n = 9); ST-2300 (5 mg/kg, i.p.) (n = 7); ST-2300 (10 mg/kg, i.p.) (n = 6); ST-2300 (15 mg/kg, i.p.) (n = 7); PIM (5 mg/kg, i.p.) (n = 6); PIM (10 mg/kg, i.p.) (n = 7); PIM (15 mg/kg, i.p.) (n = 6); FLX (10 mg/kg, i.p.) (n = 9). Data are shown as the mean ± standard error of the mean. Abbreviations: PIM, pimavanserin; FLX, fluoxetine; FST, forced swim test.

Figure 4

The H₃R agonist RAM reversed the ST-2300-provided antidepressant-like effects in FST. Acute ST-2300 (10 mg/kg, i.p.) significantly decreased the immobility time in the FST. ** Denotes significant differences between ST-2300 (10 mg/kg, i.p.) and saline-treated control mice group (p < 0.01); $ Denotes significant difference when compared to ST-2300 (10 mg/kg, i.p.)-treated mice (p < 0.05). Vehicle (n = 9); ST-2300 (10 mg/kg) (n = 6); ST-2300 (10 mg/kg) co-administered with RAM (10 mg/kg) (n = 6); RAM (10 mg/kg) co-administered with vehicle (n = 9). Data are shown as the mean ± standard error of the mean. Abbreviations: RAM, (R)-α-methyl histamine; FST, forced swim test; SAL, saline.

Figure 5

Effects of acute ST-2300 pre-treatment on depression-like behavior in TST. Acute ST-2300 (15 mg/kg, i.p.) decreased the immobility time in the tail suspension test (TST). * Denotes significant differences between ST-2300 doses and controls (p < 0.05); *** Denotes significant differences between FLX and saline controls (p < 0.001). # Denotes significant differences between FLX and administered drug doses of ST-2223 and PIM (p < 0.05). Vehicle (n = 9); ST-2300 (5 mg/kg, i.p.) (n = 7); ST-2300 (10 mg/kg, i.p.) (n = 6); ST-2300 (15 mg/kg, i.p.) (n = 6); PIM (5 mg/kg, i.p.) (n = 6); PIM (10 mg/kg, i.p.) (n = 7), PIM (15 mg/kg, i.p.) (n = 6); FLX (10 mg/kg, i.p.) (n = 9). Data are shown as the mean ± standard error of the mean. Abbreviations: PIM, pimavanserin; FLX, fluoxetine; TST, tail suspension test.

Figure 6

Effects of the H₃R agonist RAM abrogated ST-2300-provided antidepressant-like effects in TST. Acute ST-2300 (10 mg/kg, i.p.) significantly decreased the immobility time in TST. * Denotes significant difference between ST-2300 (10 mg/kg, i.p.) and saline-treated control mice group (p < 0.05); $ Denotes significant difference when compared to ST-2300 (10 mg)-treated mice (p < 0.05). Vehicle (n = 9); ST-2300 (10 mg/kg, i.p.) (n = 6); ST-2300 (10 mg/kg, i.p.) co-administered with RAM (10 mg/kg, i.p.) (n = 6); RAM (10 mg/kg, i.p.) co-administered with vehicle (n = 9). Data are shown as the mean ± standard error of the mean. Abbreviations: RAM, (R)-α methyl histamine; TST, tail suspension test; SAL, saline.

Figure 7

Effects of acute ST-2300 pre-treatment on anxiety-like behavior and locomotor activity in OFT. In OFT, ST-2300 had no effect on total line crossing (A) but dose-dependently increased the time spent in the center of the arena (B). Pre-treatment with the H₃R agonist RAM did not reverse the anxiolytic effect of ST-2300 (C). * Denotes significant difference between ST-2300 (10 mg/kg, i.p.) and controls (p < 0.05); *** Denotes significant difference between ST-2300 (10 mg/kg, i.p.) or DZP (1 mg/kg, i.p.) and controls (p < 0.001); # Denotes significant differences between DZP and drug doses (p < 0.05). Vehicle (n = 9); ST-2300 (5 mg/kg, i.p.) (n = 7); ST-2300 (10 mg/kg, i.p.) (n = 6); ST-2300 (15 mg/kg, i.p.) (n = 7); PIM (5 mg/kg, i.p.) (n = 6); PIM (10 mg/kg, i.p.) (n = 7); PIM (15 mg/kg, i.p.) (n = 6); DZP (1 mg/kg, i.p.) (n = 9). ST-2300 (10 mg/kg, i.p.) co-administered with RAM (10 mg/kg, i.p.) (n = 6); RAM (10 mg/kg, i.p.) co-administered with vehicle (n = 9). Data are shown as the mean ± standard error of the mean. Abbreviations: PIM, pimavanserin; DZP, diazepam; RAM, (R)-α-methyl histamine; OFT, open field test; SAL, saline.