Synthesis, computational studies and preliminary pharmacological evaluation of 2-[4-(aryl substituted) piperazin-1-yl]-N-benzylacetamides as potential antipsychotics

2.2.1 Synthesis of 2-chloro-N-benzylacetamide (2)

Benzylamine 1 (4.37 ml, 0.04 mol) in 2N aqueous sodium hydroxide (150 ml) was treated with chloroacetylchloride (3.18 ml, 0.04 mol) as a solution in dichloromethane (100 ml) at 0 °C. After1 h, the layers were separated and the aqueous phases extracted with an additional portion of dichloromethane. The organic phases were combined, washed with an aqueous solution of 1N HCl, saturated NaHCO₃, dried (Na₂SO₄), and concentrated to afford 2. Yield: 78%; m.p. 94–96 °C; IR (KBr, cm⁻¹): 3268, 3017, 2945, 1669, 1220, 1066, 769, 689. ¹H NMR (300 MHz; CDCl₃ δ): 4.11 (s, 2H); 4.49–4.51 (d, J = 6, 2H); 6.88 (br s, NH); 7.26–7.39 (m, 5H, Ar-H); Anal. Calcd. for C₉H₁₀ClNO: C, 58.86; H, 5.49; N, 7.63; Found: C, 58.81; H, 5.43; N, 7.58.

2.2.2 General procedure for the synthesis of 3a–j

2-Chloro-N-benzylacetamide 2 (0.91 g, 0.005 mol) was dissolved in 100 ml of acetonitrile in a 250 ml round bottomed flask. Anhydrous K₂CO₃ (0.69 g, 0.005 mol) and catalytic amount of potassium iodide and 0.005 mol of appropriate arylpiperazine were added to the above solution. The above mixture was allowed to reflux with continuous stirring on a magnetic stirrer for 12 h. After completion of reaction the solvent was removed by vacuum distillation and residue was dissolved in chloroform and water. The organic layer was washed with brine and dried over MgSO₄, removal of the solvent afforded target compounds 3a–j.

2.3.1 Computation of physiochemical properties

A set of molecular parameters were computed for the target compounds as well as three standard drugs clozapine, ketanserin and risperidone using Chem 3D Ultra version, 11.0, and Chem Silico online free software. The important molecular parameters for antipsychotics are blood-brain barrier (BBB), log P and topological polar surface area. The literature review suggests that TPSA is a measure of a molecule's hydrogen bonding capacity and its value should not a exceed certain limit if the compound is intended to be CNS active. Two differing limits have been proposed: van de Waterbeemed et al. (1998) suggested a limit of 90 A², where, Kelder et al. (1999) suggested 60–70 A².

2.3.2 Similarity calculations

The physicochemical similarity of the target compounds was calculated with respect to the standard drugs (Bali et al., 2010).

Firstly, the distance d_i of a particular target compound j to drug molecules e.g., clozapine was calculated by the formula: $$d_i^2={\displaystyle \sum _{j=1}^n}{(1-X_{i\text{,}j}/X_{i\text{,}\mathit{std}})}^2/n$$ where X_i,j is the value of molecular parameter ‘i’ for compound ‘j’, X_i,std is the value of the same molecular parameter for the standard drug, e.g., clozapine, ketanserin and risperidone. Then, the similarity of compound ‘j’ to the standard drug was calculated as $$\text{Similarity}(\%)=(1-R)\times 100$$ where R = √d² is the quadratic mean (root mean square), a measure of central tendency.

2.4.1 Apomorphine induced mesh climbing behavior

Swiss albino mice (six mice in each group) of either sex (24–26 g) were habituated by individually placing in a circular cage made of wire mesh of diameter 13 cm and height 14 cm. Mice in the test, control and standard groups were injected, respectively, with test compound, normal saline and clozapine intraperitoneally and returned to the home cage. After a gap of 10 min, apomorphine (2.5 mg/kg) was injected intraperitoneally. Mesh climbing behavior was noted at 5 min intervals for up to 20 min, starting 10 min after the apomorphine administration using the following scoring system: 0 – no paws on the cage, 1 – one paw on the cage, 2 – two paws on the cage, 3 – three paws on the cage, 4 – four paws on the cage (Fig. 1). The score recorded for each animal was based on the position of the animal at the moment it was first observed. A score of 20 is the maximum possible. Recording was done by an observer who was unaware of the specific drug treatments.

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Figure 1

The effect of synthesized compounds (3a–j) on the apomorphine induced climbing behavior. Each column represents the mean ± SEM of total climbing score for group of six mice assessed at 5-min intervals for 20 min, starting 10 min after apomorphine treatment. All values are statistically significant with respect to control at p < 0.05.

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2.4.2 5-Hydoxytryptophan (5-HTP) induced head twitches behavior

Swiss albino mice in the control group (n = 6) was injected with pargyline (75 mg/kg, i.p.) in order to prevent the rapid degradation of 5-HTP. Thirty minutes later, the test compound was administered. After a further 30 min, the mice received 5-HTP (50 mg/kg, i.p.). The mice were returned to the test cages and then head twitches were assessed at 10 min intervals for 30 min, starting 20 min after the 5-HTP treatment. Head twitches were monitored using the following scoring system: 0 – absent, 1 – moderate, 2 – marked (Fig. 2). A score of 20 is the maximum possible. An observer made all observations unaware of the specific drug treatments.

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Figure 2

The effect of synthesized compounds (3a–j) on the 5-HTP induced head twitches behavior. Each column represents the mean ± SEM of total head twitches score for group of six mice assessed at 10-min intervals for 30 min, starting 20 after the 5-HTP treatment. All values are statistically significant with respect to control at p < 0.05.

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2.4.3 Catalepsy

Catalepsy was induced in albino mice (n = 6) with haloperidol (1.0 mg/kg, i.p.) and was assessed at 30 min intervals until 120 min and at the end of 240 min by means of a standard bar test. Catalepsy was assessed in terms of the time (s) for which the mouse maintained an imposed position with both front limbs extended and resting on a 4 cm high wooden bar (1.0 cm diameter). The endpoint of catalepsy was considered to occur when both front paws were removed from the bar or if the animal moved its head in an exploratory manner. The cataleptic behavior was scored as 1 if the imposed posture was maintained for at least 20 s and every additional 20 s one extra point was given for severity of catalepsy (Fig. 3). A cut-off time of 1100 s was applied during the recording of observations. The animals were returned to their individual home cages in between determinations. All observations were made between 10.00 and 16.00 h in a quiet room at 23–25 °C. The animals in the test group were administered with test drugs instead of haloperidol and the remaining procedure for assessment of catalepsy was same as mentioned above.

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Figure 3

Induction of catalepsy of synthesized compounds (3a–j). Results are expressed as the mean ± SEM. (n = 6). All values are statistically significant with respect to control at p < 0.05.

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