Structure activity relationship of disubstituted piperazine compoundsNK Dadkar, Vaishali N Dadkar, CV Deliwala, UK Sheth
Department of Pharmacology and Pharmacology Research Unit CSIR. Seth G. S. Medical College, Parel, Bombay 400 012, India
The various compounds belonging to the 1-4 disubstituted piperazine series exhibited the adrenolytic, hypotensive and CNS depressant activities. Appropriate substitution could also lead, to a dissociation of these pharmacological effects. It was observed that adrenolytic and CATS depressant but not the hypotensive property of these compounds was greatly susceptible to structural changes in their molecular configuration and a relationship was evident between adrenolytic and hypotensive activity as well as between CNS depressant and hypotensive activity with different structural variations.
Since Bovet and Bovet Nitti  reported the reversal of the epinephrine pressor response by phenylpiperazine and 1-methyl 4-phenylpiperazine compounds, the blockade of the adrenergie receptor has been recognized as being a component of action of certain piperazine compounds. It has been observed by many workers that phenylpiperazine series contain compounds with very potent adrenolytic and hypotensive actions , and even include compounds which show significant sedative tranquillizing activity. ,,,
A series of disubstituted piperazine derivatives General formula: see [Figure 1] synthesized at the Haffkine Institute, Bombay were found to possess the ability to induce central nervous system (CNS) depression , . Further pharmacological studies of these compounds revealed, in some of them, the ability to cause a fall in systemic blood pressure and blockade of alpha adrenergic receptors.
The purpose of this study was an attempt to determine the pharmacological effects produced by modification of the chemical structure in a series of piperazine derivatives. The relation of the changes in the chemical configuration of this series of compounds to their primary pharmacological actions viz. CNS depression, alpha-adrenerrgic blockade and hypotension were determined.
Toxicity studies: The test compounds were administered intraperitoneally (i.p) to albino mice of either sex (Haffkine strain), weighing 20-25 gm in doses of 10, 50, 100, 200, 400, 800 mg/kg; 1% aqueous gum acacia was used as a vehicle. Six animals were used in each group, and the group treated with the vehicle alone served as control. Approximate LD 50 was determined by recording the percentage mortality of each dose level.
Locomotor activity: Spontaneous motor activity studies were carried out in male albino mice (20-25 gm), by photocell method of Dews.  The activity was recorded using actophotometer before and after the treatment with the test compounds (1/10th of LD 50 , i.p.); at the peak of drug action, percentage inhibition was calculated. Three groups (6 mice per group) were studied at each dose level, the animals treated with 1% aqueous gum acacia served as control.
Blood pressure and adrenolytic activity: Hypotensive and adrenergic blocking activity were determined in Mongrel dogs of either sex weighing from 10-16 kg. Animals were anaesthetised with pentobarbital sodium (35.0 mg/kg i.v.), anaesthesia was repeated in small doses when and if necessary. The trachea was cannulated and arterial blood pressure was recorded by means of a mercury manometer. All the animals were maintained on positive artificial ventilation. Drugs were injected through the cannulated femoral vein and the animals were observed for a period of 4-6 hours after the administration of test compounds in doses of 1.0, 2.5, 5.0 and 10 mg/kg i. v. in the form of a suspension using carboxy methyl cellulose (CMC) as a suspending agent. The effect of compounds was assessed on (a) systemic blood pressure, (b) pressor response of epinephrine (2.5 µg/kg), (c) pressure response of nor-epinephrine (2.5 µg/kg) at various time intervals following the administration of test compounds.
Only an actual reversal of the blood pressure response to epinephrine was selected as the criterion of adrenergic blocking activity because mere reduction of the pressor response could result from a number of nonspecific factors as suggested by Nickerson.  Minimal effective dose of the compound producing a reversal of the epinephrine induced pressor response was determined.
In comparative studies of the effect of compounds on blood pressure, the initial blood pressure of the animal was taken as 100 and the per cent change over this initial value was calculated. The fall in blood pressure on administration of compounds lasting for a period of at least 10-15 minutes was considered for comparative studies. When the fall in blood pressure was to an extent of 40% or more, the fall was prolonged, the initial reading could not be attained within a period of 2 to 3 hours.
In order to correlate the effect of changes of structure on biological activity, substitution in different parts (i.e. A, B and C) of the prototype are considered separately [Table 1].
When phenyl group was attached directly at N 4 (I) it showed both adrenolytic and hypotensive activity. The corresponding benzyl (X) and methyl (VIII) compounds had very weak adrenolytic and no hypotensive and no CNS depressant activity. Ortho substituted phenyl compounds showed both adrenolytic and hypotensive activity and varying degree of CNS depressant activity. This CNS depressant activity was most marked in ortho methoxy phenyl (V) and ortho methyl LIII) substitution. With para substitution the compounds possessed very weak adrenolytic and poor hypotensive activity (VI, VII). In the corresponding 2-pyridyl (XI) and 2-pyrimidyl (XII) compounds a dissociation of adrenolytic and hypotensive activity was evident, the former being more marked than the latter. Both groups possessed weak CNS depressant activity.
An increase in chain length at bridge-X-from one carbon atom to three carbon atoms showed increase in adrenolytic, hypotensive and CNS depressant activities. The direct attachment of -CO- at N 1 decreased adrenolytic, hypotensive and CNS depressant activity (XVI-XX). When -CO- was replaced by CH 2 -there was not much change seen in these activities (XXI, XXII). When CO-CH 2 -CH 2 was changed to CH=CH-CO-CH 2 -CH 2 (XXIII-XXVI) an increase in hypotensive and CNS depressant activity occurred, but when the same was changed to CO-O- (CH 2 ) 4 (XXIX-XXXII), there was decrease in CNS depressant activity while adrenolytic and hypotensive activity was unchanged. Increase in adrenolytic and a decrease in CNS depressant activity was seen when CH=CH-CO-CH 2 -CH 2 , was reduced to CH=CH-CHOH-CH 2 -CH 2 (XXVII, XXVIII).
When trimethoxy phenyl moiety was changed to mono methoxy phenyl or simple phenyl, there was increase in adrenolytic activity, decrease in CNS depressant activity, but there was no change in hypotensive activity [Table 2].
The various compounds belonging to the 1-4-disubstituted piperazine series exhibited the adrenolytic, hypotensive and CNS depressant activities. Appropriate substitution could also lead to a dissociation of these pharmacological effects. On this basis, the following conclusions may be drawn.
The direct attachment of phenyl ring to N 4 seemed necessary as the corresponding benzyl and methyl compounds had a weak adrenolytic activity and no hypotensive and CNS depressant activity.
Ortho substitution at the phenyl ring was associated with both the adrenolytic as well as hypotensive activity but variations in CNS depressant activity. The activity was found to be most marked in o-methoxy or o-tolyl substitution but weak for o-chloro and o-fiuoro substitution.
A shift in the substitution from the ortho to the para position in the phenyl ring (at N 4 ) reduced both the adrenolytic as well as hypotensive activity. This finding has been described by Ghouri.  It could possibly be explained by the differences in the geometry of the molecule., In the para and unsubstituted N-phenyl piperazinyl compounds, the phenyl group might be expected to lie in the plane almost parallel to the piperazine ring thus permitting an overlap of the nitrogen lone pair and the π electrons of the phenyl group. However, when there was ortho substituent on N-phenyl part, it would impinge upon an equatorial hydrogen atom in the piperazine ring and would twist the phenyl ring into an almost perpendicular plane as suggested by Anand. 
Structural alterations at bridge -X-. which joins the trimethoxy phenyl moiety to the piperazine nucleus led to the following observations.
The direct attachment of -CO- at N 1 abolished the adrenolytic, hypotensive and CNS depressant properties. This was suggestive that basicity of N 1 was necessary for all these activities.
The adrenolytic, hypotensive and CNS depressant activity was found to increase progressively with an increase in the number of carbon atoms (i.e. from one carbon atom to three carbon atoms) in bridge X. It was observed that the minimum chain length of three carbon atoms was essential for fairly goad adrenolytic, hypotensive and CNS depressant activity.
When the trimethoxy phenyl moiety at `C' was changed to either mono methoxy or simple phenyl moiety, CNS activity was markedly reduced but the adrenolytic and hypotensive activity was not markedly affected. Thus the trimethoxy moiety seemed to be essential for CNS depressant activity as described by Boissier.  This could be due to an electron donation by this group which might be useful in non-effective charge transfer association with no acceptor species on the receptor site.
As a result of this study, it was evident that the adrenolytic and CNS depressant but not the hypotensive property of these compounds was greatly susceptible to structural changes in their molecular configuration. Similar findings have been described by Morphis  in a series of 1-4 disubstituted piperazine compounds.
No firm conclusion could be drawn regarding the degree of effectiveness of these compounds as adrenolytic, hypotensive or CNS depressant agents, as these activities showed varying degrees of susceptibility to the same structural alteration. But a relationship was evident between the adrenolytic and hypotensive activity as well as between the CNS depressant and hypotensive activity with different structural variations. However, a particular mention must be made here in relation to N 4 -2 pyridyl compounds which showed marked adrenolytic activity with minimal hypotensive and CNS depressant activity.
The authors express their appreciation to Dr. Nitya Anand, Director Central Drug Research Institute,, Lucknow, India, for his valuable suggestion.
[Table 1], [Table 2]