Comparative anticonvulsant study of epoxycarvone stereoisomers

Article abstract of DOI:10.3390/molecules201119649

Stereoisomers of the monoterpene epoxycarvone (EC), namely (+)-cis-EC, (-)-cis-EC, (+)-Trans-EC, and (-)-Trans-EC, were comparatively evaluated for anticonvulsant activity in specific methodologies. In the pentylenetetrazole (PTZ)-induced anticonvulsant test, all of the stereoisomers (at 300 mg/kg) increased the latency to seizure onset, and afforded 100% protection against the death of the animals. In the maximal electroshock-induced seizures (MES) test, prevention of tonic seizures was also verified for all of the isomers tested. However, the isomeric forms (+) and (-)-Trans-EC showed 25% and 12.5% inhibition of convulsions, respectively. In the pilocarpine-induced seizures test, all stereoisomers demonstrated an anticonvulsant profile, yet the stereoisomers (+) and (-)-Trans-EC (at 300 mg/kg) showed a more pronounced effect. A strychnine-induced anticonvulsant test was performed, and none of the stereoisomers significantly increased the latency to onset of convulsions; the stereoisomers probably do not act in this pathway. However, the stereoisomers (+)-cis-EC and (+)-Trans-EC greatly increased the latency to death of the animals, thus presenting some protection. The four EC stereoisomers show promise for anticonvulsant activity, an effect emphasized in the isomers (+)-cis-EC, (+)-Trans-EC, and (-)-Trans-EC for certain parameters of the tested methodologies. These results serve as support for further research and development of antiepileptic drugs from monoterpenes.

Full text of DOI:10.3390/molecules201119649

Article
Comparative Anticonvulsant Study of
Epoxycarvone Stereoisomers
Paula Regina Rodrigues Salgado 1, Diogo Vilar da Fonsêca 1, Renan Marinho Braga 1,
Cynthia Germoglio Farias de Melo 1, Luciana Nalone Andrade 2,
Reinaldo Nóbrega de Almeida 1,3 and Damião Pergentino de Sousa 1,4,
*
Received: 27 August 2015 ; Accepted: 20 October 2015 ; Published: 29 October 2015
Academic Editor: Katalin Prokai-Tatrai
1
Instituto de Pesquisa em Fármacos e Medicamentos, Universidade Federal da Paraíba, CP 5009, João
Pessoa, CEP 58051-900, PB, Brazil; paulasalgado87@gmail.com (P.R.R.S.); divilar@hotmail.com (D.V.F.);
renan_braga123@hotmail.com (R.M.B.); cynthia_fariasm@hotmail.com (C.G.F.M.);
reinaldoan@uol.com.br (R.N.A.)
2
Departamento de Farmácia, Universidade Federal de Sergipe, São Cristóvão-SE, CEP 49100-000, Brazil;
lulisynalone@yahoo.com.br
Departamento de Fisiologia e Patologia, Universidade Federal da Paraíba, CP 5009, João Pessoa,
3
CEP 58051-900, PB, Brazil
Departamento de Ciências Farmacêuticas, Universidade Federal da Paraíba, CP 5009, João Pessoa,
4
CEP 58051-900, PB, Brazil
*
Correspondence: sousadam@yahoo.com or damiao_desousa@yahoo.com.br; Tel.: +55-83-3216-7347;
Fax: +55-83-3216-7156
Abstract: Stereoisomers of the monoterpene epoxycarvone (EC), namely (+)-cis-EC, (´)-cis-EC,
(+)-trans-EC, and (´)-trans-EC, were comparatively evaluated for anticonvulsant activity in specific
methodologies. In the pentylenetetrazole (PTZ)-induced anticonvulsant test, all of the stereoisomers
(at 300 mg/kg) increased the latency to seizure onset, and afforded 100% protection against the death
of the animals. In the maximal electroshock-induced seizures (MES) test, prevention of tonic seizures
was also verified for all of the isomers tested. However, the isomeric forms (+) and (´)-trans-EC
showed 25% and 12.5% inhibition of convulsions, respectively. In the pilocarpine-induced
seizures test, all stereoisomers demonstrated an anticonvulsant profile, yet the stereoisomers
(+) and (´)-trans-EC (at 300 mg/kg) showed a more pronounced effect. A strychnine-induced
anticonvulsant test was performed, and none of the stereoisomers significantly increased the latency
to onset of convulsions; the stereoisomers probably do not act in this pathway. However, the
stereoisomers (+)-cis-EC and (+)-trans-EC greatly increased the latency to death of the animals, thus
presenting some protection. The four EC stereoisomers show promise for anticonvulsant activity, an
effect emphasized in the isomers (+)-cis-EC, (+)-trans-EC, and (´)-trans-EC for certain parameters of
the tested methodologies. These results serve as support for further research and development of
antiepileptic drugs from monoterpenes.
Keywords: anticonvulsant; carvone; stereoisomers; terpene; natural products; essential oils;
pentylenetetrazole; seizures; para-menthanes; enantiomers
Molecules 2015, 20, 19660–19673; doi:10.3390/molecules201119649
www.mdpi.com/journal/molecules

Molecules 2015, 20, 19660–19673
1. Introduction
Epilepsy has been characterized as a brain disease characterized by certain conditions: at least
two unprovoked seizures in a range greater than 24 h; an unprovoked seizure and the likelihood
of further recurrent crises after two unprovoked seizures, continuation over 10 years; and diagnosis
of epileptic syndrome [1]. Epileptic patients present recurrent spontaneous seizures [2], and some
30% of them are considered resistant to drug therapy, without adequate response to treatment.
Often, surgical removal of the epileptic focus is the only option that actually controls the emergence
of seizures [3]. Degradation of cognitive functions (due to factors such as seizure etiology, and
type), psychosocial problems [4], and possible psychomotor impairment [5] are clinically observed
in these patients.
It is known that the existing clinical anti-epileptic drugs are not good anti-epileptogenics, since
they only increase the onset threshold for seizure [6]. Many patients have seizures despite access to
the complete pharmacological arsenal [7]. When considering the development of new antiepileptic
drugs from 1981–2002, it is important to emphasize the role of natural products, since most of the
synthetic drugs developed have had a natural product as their model. This reinforces the role of
nature as a source for anticonvulsant agents [8].
Essential oils and their constituents are natural products known for their biological
effects [9], including anxiolytic [10], spasmolytic [11], antinociceptive [12], and anticonvulsant [13]
activities. Structural diversity in the constituents of essential oils is responsible for the variety
of biological effects already observed; this is especially true for the monoterpene class, which
has shown interesting anticonvulsant activity in animal models [14]. Besides their functional
groups, monoterpenes have a range of optical isomers of specific compounds; these isomers may
have different properties involving differing pathways [15], with synergistic and/or antagonistic
actions [16], and membrane models [17].
Epoxycarvone is a monoterpene found in essential oils of plants such as Carum carvi [18],
Kaempferia galanga [19], and others [20], or it is obtained by organic synthesis [21]. Epoxycarvone
has demonstrable antimicrobial activity against Staphylococcus aureus and Candida albicans [22],
and pharmacological effects on the central nervous system [23], with antinociceptive [24], and
anticonvulsant activities [25]. The compound has four isomers, whose psychopharmacological
stereo-selectivity is still unknown. These stereoisomers can be obtained using the enantiomers
of carvone as starting materials; they also have differences in anticonvulsant activity due to the
influence of their stereogenic center configuration [26]. This study aims to comparatively verify the
anticonvulsant potential of the four stereoisomers of epoxycarvone in animal models of chemically
and electrically induced seizures.
2. Results and Discussion
2.1. Preparation of Epoxycarvone Stereoisomers
The enantiomer trans-carvone oxides 9 and 10 were prepared via stereoselective reduction
of the carbonyl group of carvones 1 and 2, stereospecific hydroxyl-assisted epoxidation of the
allylic alcohols 3 and 4 and oxidative regeneration of the carbonyl group from epoxy alcohols 5
and 6 (Scheme 1).
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O
HO
HO
c
5
3
d
b
O
O
O
O
O
a
7
10
1
O
O
O
O
O
a
8
9
2
d
b
O
HO
HO
c
6
4
Scheme 1. Preparation of epoxycarvone stereoisomers. Reagents and Conditions: a. H2O2 30%, KOH,
.
MeOH, 4 h, 0 ˝C; b. NaBH4, CeCl3 7 H2O, MeOH, 5 min., 20 ˝C; c. m-CPBA in CH2Cl2, 6 h, 0 ˝C;
d. PCC, Pyr, 30 h, r.t.
Alkaline epoxidation of carvones 1 and 2 formed enantiomer cis-carvone oxides 7 and 8.
1
The 13C- and H-NMR, IR, polarimetric analysis, and comparison of the data with that found in
the literature [27–30] confirmed the chemical structures and stereochemistry of the carvone oxides
and intermediates.
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2.2. Pentylenetetrazole-Induced Seizure Test
In the acute test of pentylenetetrazole-induced seizures, (+)-cis-EC (7): 900.0 ˘ 0.0 s, (´)-cis-EC
(8): 763.3 ˘ 69.2 s, (+)-trans-EC (9): 791.1 ˘ 108.9 s and (´)-trans-EC (10): 743.0 ˘ 83.7 s, significantly
increased the latency to onset of first seizure, compared with the control group (109.9 ˘ 13.0 s)
(Table 1). Similar response to the groups treated with the isomers was seen in animals treated with
diazepam (4 mg/kg) (900.0 ˘ 0.0 s). Table 1 presents data related to the first seizure latency to onset,
and to the score for dead animals after treatments. No deaths were recorded in the group treated with
epoxycarvone stereoisomers (300 mg/kg) and the standard antiepileptic drug, diazepam (4 mg/kg),
in contrast to what was observed for the control group.
Table 1. Epoxycarvone stereoisomers effect on pentylenetetrazole-induced seizures test.
Experimental Groups
Latency (s)
Mortality (%)
Control (Tween 80 5%)
Diazepam (standard)
(+)-cis-EC (7)
109.9 ˘ 13.0
900.0 ˘ 0.0 a
900.0 ˘ 0.0 a
763.3 ˘ 69.3 a
791.1 ˘ 108.9 a
743.0 ˘ 83.7 a
a
50
0
0
0
0
(´)-cis-EC (8)
(+)-trans-EC (9)
(´)-trans-EC (10)
0
The values represent the mean ˘ SEM (n = 8 in each group). p < 0.001 for the epoxycarvone stereoisomers
group (300 mg/kg) as compared to the control group (ANOVA One way/Tukey’s test). Fisher’s exact test was
used to analyze the mortality rate.
2.3. Maximal Electroshock-Induced Seizure (MES)
The seizure durations were decreased (Table 2) in the groups treated with (7: 9.8 ˘ 0.5 s),
(8: 8.3 ˘ 1.4 s), (9: 0.7 ˘ 0.7 s) and (10: 0.0 ˘ 0.0 s) compared to the control group (16.3 ˘ 0.9 s).
As expected, animals that received the standard drug phenytoin (25 mg/kg) also presented seizures
inhibition of (0.0 ˘ 0.0 s).
Table 2. Epoxycarvone stereoisomers effect on maximal electroshock-induced seizures test (MES).
Experimental Groups
Seizures Duration (s)
Tonic Seizures (%)
Mortality (%)
Control (Tween 80 5%)
Phenytoin (standard)
(+)-cis-EC (7)
16.3 ˘ 0.9
0.0 ˘ 0.0 a
9.8 ˘ 0.5 a
8.3 ˘ 1.4 a
0.7 ˘ 0.7 a
0.0 ˘ 0.0 a
100
0
100
87.5
25
37.5
0
0
0
0
0
(´)-cis-EC (8)
(+)-trans-EC (9)
(´)-trans-EC (10)
12.5
a
The values represent the mean ˘ SEM (n = 8 in each group). p < 0.001 for the epoxycarvone stereoisomers
group (300 mg/kg) as compared to the control group (ANOVA One way/Tukey’s test). Fisher’s exact test was
used to analyze the percentages of animals with seizures and the mortality rate.
In Table 2, the tonic convulsions percentage was significant in groups treated with (7: 100%) and
(8: 87.5%). The groups which received (9: 25%), and (10: 12.5%) presented fewer tonic convulsions.
The control (100%) and standard (0%) groups had the expected responses. No group treated with
either the isomers or phenytoin showed mortality, the control group showed 37.5%.
2.4. Pilocarpine-Induced Seizures Test
The results for the behavioral changes induced by administration of pilocarpine (400 mg/kg) in
animals treated with isomers (300 mg/kg), vehicle, and diazepam (4 mg/kg) are shown in Table 3.
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Table 3. Epoxycarvone stereoisomers effect on pilocarpine-induced seizures test.
Peripheral
Cholinergic
Signs (%)
Stereotyped
Movements
(%)
Status
Epilepticus
(%)
Latency to
Convulsions (s)
Latency to
Death (s)
Tremors Seizures
Mortality
(%)
Experimental Group
(%)
(%)
Control (Tween 80 5%)
Diazepam (standard)
(+)-cis-EC (7)
429.3 ˘ 25.4
3600.0 ˘ 0.0 b
862.6 ˘ 36.6 b
890.8 ˘ 76.2 b
1044.0 ˘ 49.1 b
888.4 ˘ 78.3 b
817.6 ˘ 22.3
3600.0 ˘ 0.0 b
1845.9 ˘ 453.1
1135.9 ˘ 68.1
2345.0 ˘ 477.9 a
100
100
100
100
50
100
0
50
100
100
100
100
50
100
100
50
100
12.5
100
100
100
100
100
0
0
50
50
50
100
0
75
100
50
50
(´)-cis-EC (8)
2895.7 ˘ 343.3 b
(+)-trans-EC (9)
(´)-trans-EC (10)
50
50
a
b
The values represent the mean ˘ SEM (n = 8 in each group). p < 0.05; p < 0.001 for the epoxycarvone
stereoisomers group (300 mg/kg) as compared to the control group (ANOVA One way/Tukey’s test). Fisher’s
exact test was used to analyze the percentages of animals with peripheral cholinergic signs, stereotyped
movements, tremors, seizures, status epilepticus and the mortality rate.
All groups showed peripheral cholinergic signs, but the animals treated with (9: 50%) and
(10: 50%), both 300 mg/kg, showed fewer percentages of animals with these signs. The control and
experimental groups had convulsions (100%), while the standard group showed a lower percentage
for this parameter (12.5%). The death percentage was highest in animals treated with vehicle
(100%) and (8: 100%). The animals that received diazepam (4 mg/kg) did not die during the
experiment. All of the animals had tremors, but in animals treated with (9: 50%) and (10: 50%),
the parameter was smaller. Regarding emergence of status epilepticus, the group treated with
(7: 0%) did not score, while 100% of the control groups did. Pilocarpine (400 mg/kg) induced
stereotypical movements in all groups, except the standard group, which received diazepam (0%).
Latency to the first seizure (Table 3) increased significantly in animals that received (7: 862.6 ˘ 36.6 s),
(8: 890.8 ˘ 76.2 s), (9: 1044.0 ˘ 49.1 s), (10: 888.4 ˘ 78.3 s) compared to the control group (429.3 ˘ 25.4 s).
A similar result was observed for latency to death in the animals (Table 3), where those treated with
(9: 2895.7 ˘ 343.3 s) and (10: 2345.0 ˘ 478.0 s), both at 300 mg/kg, increased latency compared to
the control group (817.6 ˘ 22.3 s). There was a significant difference between the groups treated with
(8: 1135.9 ˘ 68.1 s) and (9: 2895.7 ˘ 343.3 s), and no significant increase for this parameter in animals
from groups 7 and 8 (300 mg/kg).
2.5. Strychnine-Induced Seizure Test
No significant differences were observed in latency to the first seizure in animals treated with
the EC isomers (300 mg/kg) or the vehicle, in the strychnine-induced seizures model. However,
there was a significant increase in latency to death of the animals treated (7: 306.5 ˘ 55.3 s) and
(9: 396.5 ˘ 61.5 s), both 300 mg/kg, as compared to the control group (53.0 ˘ 0.6 s). All animals had
seizures after strychnine administration, and there were no survivors except for the group treated
with diazepam (4 mg/kg) (Table 4).
Table 4. EC isomers effect on strychnine-induced seizures test.
Latency to
Convulsions (s)
Latency to Death
(s)
Seizures
(%)
Mortality
(%)
Experimental Group
Control (Tween 80 5%)
Diazepam (standard)
(+)-cis-EC (7)
51.1 ˘ 0.5
144.3 ˘ 29.4 a
79.2 ˘ 20.8
89.2 ˘ 9.1
53.0 ˘ 0.6
435.8 ˘ 93.2 b
306.5 ˘ 55.3
169.3 ˘ 17.8
396.5 ˘ 61.5 c
273.8 ˘ 51.4
a
100
100
100
100
100
100
c
100
0
100
100
100
100
(´)-cis-EC (8)
(+)-trans-EC (9)
111.8 ˘ 21.8
65.0 ˘ 4.9
(´)-trans-EC (10)
b
The values represent the mean ˘ SEM (n = 8 in each group). p < 0.001; p < 0.01; p < 0.05 regarding
the epoxycarvone stereoisomers group as compared to the control group (ANOVA One way/Tukey’s test).
Fisher's exact test was used to analyze the percentages of animals with seizures, and the mortality rate.
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In this study, the anticonvulsant activity of the stereoisomers (+)-cis-EC (7), (´)-cis-EC (8),
(+)-trans-EC (9), and (´)-trans-EC (10) were analyzed comparatively in four methodologies that
induced seizures chemically and electrically. Pentylenetetrazole-induced seizure test is one of
the standard methods of anticonvulsant screening [31]. In this experiment, the isomers 7, 8,
9, and 10 (300 mg/kg) increased the latency to onset of seizure. This finding emphasizes the
anticonvulsant potential attributed to the 4 isomers. The protection also extended to the animal
mortality rate; there were no deaths in groups treated with the isomers, or in the standard
group (diazepam). Anticonvulsants such as diazepam and phenobarbital have the ability to
open GABAA channels, inhibiting seizures induced by PTZ [32], a substance that acts as an
antagonist of the GABAA receptor [33]. The activation of GABAergic neurotransmission inhibits
or attenuates seizures, while inhibition of this neurotransmission increases convulsions [34].
Idris and collaborators [35] evaluated the anticonvulsant profile of three isomers (30 mg/kg)
of N-benzyl-3-[(methylphenyl)amino]propanamide, and all demonstrated excellent anti-convulsant
properties in the pentylenetetrazole-induced seizure test.
Another study by Wieland [36]
compared the effect of two neuro-active endogenous steroid isomers, 3α-hydroxy-5α-pregnan-20-one
and 3α-hydroxy-5β-pregnan-20-one, for possible behavioral differences in this methodology.
The resulting anticonvulsant activity was similar, which reinforces the findings of the present
study. Seizure inhibition in the PTZ test by all four isomers, suggests possible interference in
GABAergic neuro-transmission.
Maximal electroshock-induced seizures (MES) testing was also performed. This model is
widely used to study anticonvulsant agents [37]. In the MES test, drugs that suppress tonic-clonic
seizures have the ability to prevent the spread of seizure discharge through neuronal tissue, and
thus increase the seizure threshold [38]. There were differences in the EC isomers effects, although
all of the isomers significantly reduced the duration of the seizures compared to the control group.
Substances 9 and 10 showed greater reductions. In other studies, constitutional isomers of valproic
acid amide derivatives were comparatively tested for anticonvulsant activity in the MES test, with
negligible modulations occurring [39]. trans-1-Propenyl-2,4,5-trimethoxy-benzene, also known as
α-asarone, showed anticonvulsant property in the above test [40]. As previously reported, the
monoterpene α,β-epoxycarvone prevented tonic seizures induced in this test [25], as was seen with
the four isomers. Certain enantiomers of neurosteroids [41] also show remarkable anticonvulsant
effects in the MES test, supporting the results of this research.
In both the PTZ and MES test all of the isomers showed significant protection (increased time
to onset of convulsions). These data emphasize their anticonvulsant potential, and suggests that
further studies regarding the possible participation of pathways involving the GABAA receptor, must
be performed. A study by Almeida and collaborators [25] showed that the anticonvulsant effect
of monoterpene α,β-epoxycarvone was not reversed by pretreatment with flumazenil, a selective
GABAA receptor antagonist. Since α,β-epoxycarvone was the isomer analyzed in the comparative
study and knowing this information about its mechanism of action, it was suggested that the possible
role in GABAergic receptors might occur in other subunits involved with epileptogenesis in both
animal models [42] and in humans [43].
Epilepsy pathophysiology has been understood using animal models, which induce seizures.
One model that reproduces human epilepsy components is the pilocarpine-induced seizures test [44].
Pilocarpine is a non-selective agonist of muscarinic acetylcholine receptors; and brain structures with
a high density of muscarinic receptors play an important role in the emergence of seizures induced
by this chemical agent [45]. Following the seizures induced by pilocarpine administration there is a
latency period preceding spontaneous and recurrent focal seizures. Neuropathological changes, such
as hippocampal sclerosis occur with status epilepticus (induced by pilocarpine), similar to the temporal
lobe epilepsy in humans [46]. The animals treated with the EC isomers (300 mg/kg) showed signs
of peripheral cholinergic activity; however, greater protection was afforded by isomers 9 and 10,
reducing data percentage of cholinergic signs. Regarding the emergence of status epilepticus, 7 was
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Molecules 2015, 20, 19660–19673
the most protective. The decrease in the tremors (as seen with 9 and 10, the increase in time to onset
of seizures (as seen with all isomers), and protection against death (observed in the groups treated
with 7, 9 and 10 are of great interest for developing new antiepileptic drugs. Significant differences
in latency to death were observed (for the first time) between isomers, where 9 was more protective
than 8. The results suggest in this methodology, that isomer 9 was found to be the most promising,
since it was significantly effective in increasing the latency to onset of seizures. Protection in latency
to death of the animals was also observed. Substance 7, as well as 8, showed significant results only
in increased latency to onset of convulsions, and not for latency to death. The animals treated with 7
and 8, though increasing time to the first seizures, showed no protective effects for latency to death.
As a comparative study of test substances, the results presented stereoisomer 9 as the most promising.
These initial findings with this methodology are not enough to suggest muscarinic antagonism as
the source of their anticonvulsant action, but do reinforce the potential of stereoisomers 7, and
especially 9, and 10 for future research for the treatment of epilepsy. It was seen that all of the isomers
are effective in relation to anticonvulsant activity, showing only slight differences between them in
certain parameters. Stereochemical differences between the isomers did not significantly affect their
pharmacodynamics in the tested methodologies. A constitutional isomer of valprimide has been
shown effective in the pilocarpine-induced seizures test [47]. Another study of chiral derivatives
of pyrrolo[1,2-α] pyrazine (in the pilocarpine-induced seizures test) noted one derivative as the more
active among the series in preventing status epilepticus [48], differing from the previous findings where
all tested substances with structural similarity, also had similar pharmacological effects.
In the strychnine-induced seizures test, protection in animals treated with isomers regarding
the latency to onset of convulsions was not observed, presenting the possibility that the compound
does not act on spinal cord glycine receptors [49]. Strychnine, an important post-synaptic inhibitory
transmitter, acts as a convulsing agent by competitive antagonism with glycine receptors, [50].
In contrast, it was found that the isomers 7 and 9 (at 300 mg/kg), although possibly not affecting
glycine receptors, showed an increase in latency to death by still unknown mechanisms. The
structural isomers and enantiomers of pinene, a monoterpene with phytotoxic action, manifested
differences in physiological parameters as related to germination and corn growth, probably due to
differing mechanisms of action [51]. Studies like this, involving isomeric comparisons, are important
since the characteristics of the substances are evaluated, allowing the future, the development of
new drugs.
Electrophysiological experiments don’t conducted to corroborate the effectiveness of EC isomers,
although in a study conducted by Almeida and collaborators [25], only (+)-cis-EC was tested,
which reduced neural excitability. The use of other isomers for this purpose may confirm (or not)
involvement of the same in voltage-gated Na+ channels blockade. What we can safely say about these
stereoisomers is that behavioral seizures are affected, as seen in the pharmacological experiments.
These data are preliminary and require further study of the mechanism of action to indicate how
such neuroprotection occurs.
Despite the fact the dose tested in these methodologies (300 mg/kg) is relatively high, previous
studies conducted by Sousa [23] showed a high LD50 for one of the isomers of the epoxycarvone used
in this work. Due to structural similarity between four substances, this data suggests a possible low
toxicity and safety at the chosen dose for all stereoisomers. Research shows that the use of high doses
are not necessarily related to high toxicity, as other studies that also have reports of substances with
low toxicity, high doses with pharmacological activity and similarity to the isomers [52–57].
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Molecules 2015, 20, 19660–19673
3. Experimental Section
3.1. Reagents
The epoxycarvone isomers were obtained as described [21,27–30], and dissolved in a
Tween 80 5% emulsion. Pentylenetetrazole, pilocarpine, strychnine, and Tween 80 were purchased
from Sigma (St. Louis, MO, USA).
3.2. Preparation of Epoxycarvone Stereoisomers
The oily compounds were prepared in our laboratory according to the literature [27–30].
The 1H- and 13C-NMR measurements were obtained with a Mercury-Varian spectrometer (Palo Alto,
CA, USA) operating at 200 MHz (for 1H), and 50 MHz (for 13C). The infrared spectra were recorded on
a Bomen Michelson model 102 FTIR (Bomen, Chicago, IL, USA) and the most intense or representative
bands reported (in cm´1). Optical rotations were measured on an Optical Activity AA-10 automatic
polarimeter (Optical Activity Limited, Ramsey, UK) at ambient temperature.
3.2.1. Reduction of (´)-Carvone (1) and (+)-Carvone (2)
˝
Sodium borohydride (2.5 g, 66.1 mmol) was added at 20 C to a solution of 1 or 2 (10 g,
67 mmol), and CeCl3¨ 7H2O (25 g, 148.5 mmol) in methanol (500 mL). The mixture was stirred
for 5 min. Then, diethyl ether (100 mL) and water (100 mL) were added. The organic layer was
separated, and the aqueous layer was extracted with diethyl ether (3 ˆ 100 mL). The organic layers
were combined and dried over Na2SO4, and filtered. The filtrate was concentrated under reduced
pressure, and the residue was subjected to column chromatography on silica gel using a mixture
of ethyl hexane and ethyl acetate (8:2) as eleuent. (´)-cis-Carveol (3) and (+)-cis-carveol (4) were
obtained with 80% (53.46 mmol) and 78% (52.0 mmol) yields, respectively [27,28].
(3): rαs2D9 = ´33.7 (CHCl3, c 0.03); IR (KBr) νmax: 3461, 2945, 2900, 1650, 1500, 1050, 900 cm–1; 1H-NMR
(CDCl3): δ 5.44–5.39 (1H, m), 4.93 (2H, s), 4.25–4.21 (1H, dd, J = 8 Hz), 2.42–2.26 (5H, m), 2.22–2.12
(3H, m), 1.65 (3H, s), 1.56 (1H, s); 13C-NMR (CDCl3) δ: 146.3, 134.2, 125.3, 106.5, 68.7, 38.3, 36.7, 28.2,
27.2, 26.5. CAS 7632-16-8.
(4): rαs2D9 = +31.9 (CHCl3, c 0.03); IR (KBr) νmax: 3460, 2945, 2900, 1640, 1500, 1050, 880 cm–1; 1H-NMR
(CDCl3): δ 5.45–5.33 (1H, m), 4.87 (2H, s), 4.22–4.19 (1H, dd, J = 8 Hz), 2.46–2.35 (5H, m), 2.17–2.08
(3H, m), 1.62 (3H, s), 1.58 (1H, s); 13C-NMR (CDCl3) δ: 147.8, 136.3, 123.1, 104.7, 69.5, 39.6, 36.5, 28.5,
27.2, 26.4. CAS 1197-06-4.
3.2.2. Epoxidation of (´)-cis-Carveol (3) and (+)-cis-Carveol (4)
A solution of 3 or 4 (6 g, 39.5 mmol) in CH2Cl2 (20 mL) was added to a solution of
m-chloroperbenzoic acid (9.51 g, 80%, 55.11 mmol) in CH2Cl2 (100 mL), and the resultant mixture
was left at rest for 6 h. The filtered solution was washed with 10% aqueous NaHSO3, (2 ˆ 10 mL),
with 10% aqueous NaHCO3, (2 ˆ 10 mL) and concentrated on a rotary evaporator. The product was
purified by column chromatography on silica gel, using a mixture of hexane and ethyl acetate (9:1)
as eluent. (´)-Carveol epoxide (5) and (+)-carveol epoxide (6) were obtained with 67% (26.42 mmol),
and 65% (25.6 mmol) yields, respectively [28,29].
29
1
(5): rαsD = ´17.6 (CHCl3, c 0.05); IR (KBr) νmax: 3485, 2990, 2350, 1600, 1375, 900 cm–1; H-NMR
(CDCl3): δ 5.44–5.39 (1H, m), 4.93 (2H, s), 4.25–4.21 (1H, dd, J = 8 Hz), 2.42–2.26 (5H, m), 2.11–1.98
(3H, m), 1.65 (3H, s), 1.56 (1H, s); 13C-NMR (CDCl3): δ 146.3, 134.2, 125.3, 106.5, 68.7, 38.3, 36.7, 28.2,
27.2, 26.5. CAS 24120-79-4.
29
1
(6): rαsD = +16.8 (CHCl3, c 0.05); IR (KBr) νmax: 3500, 2980, 2350, 1600, 1350, 900 cm–1; H-NMR
(CDCl3): δ 5.48–5.31 (1H, m), 4.87 (2H, s), 4.33–4.14 (1H, dd, J = 8 Hz), 2.53–2.35 (5H, m), 2.17–1.90
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Molecules 2015, 20, 19660–19673
(3H, m), 2.15–2.01 (3H, m), 1.62 (1H, s), 1.58 (s); 13C-NMR (CDCl3) δ: 147.8, 136.3, 123.1, 104.7, 69.5,
39.6, 36.5, 28.5, 27.2, 26.4. CAS 39903-74-7.
3.2.3. Oxidation of (´)-Carveol Epoxide (5) and (+)-Carveol Epoxide (6)
A CrO3-pyridine complex (7.5 g, 42 mmol) was added to a solution of compound 5 or 6 (2 g,
11.90 mmol) in pyridine (125 mL), and the resultant mixture was stirred for 30 h. The solvent was
then concentrated on a rotary evaporator. The product was purified by column chromatography on
silica gel, a mixture of hexane and ethyl acetate (9:1) being used as eluent. (´)-trans-Carvone oxide
(9) and (+)-trans-carvone oxide (10) were obtained in 40% (5.35 mmol) and 45% (4.76 mmol) yield,
respectively [28].
29
1
(9): rαsD = ´33.2 (CHCl3, c 0.01); IR (KBr) νmax: 3020, 2985, 1700, 880 cm–1; H-NMR (CDCl3z): δ
4.69–4.65 (2H, m), 3.38 (1H, d, J = 4.5), 2.74 (1H, dd, J = 15.6, J = 10.2), 2.48–2.60 (1H, m), 2.04–2.17 (1H,
m), 2.02–2.16 (1H, m), 1.95 (1H, dd, J = 11.6, J = 15.2), 1.63 (3H, s), 1.33 (3H, s); 13C-NMR (CDCl3) δ:
206.5, 146.0, 110.3, 64.3, 58.3, 42.6, 41.4, 27.9, 18.8, 15.4. CAS 39903-98-5.
29
1
(10): rαsD = +32.1 (CHCl3, c 0.01); IR (KBr) νmax: 3025, 2975, 1720, 880 cm–1; H-NMR (CDCl3): δ
4.70–4.64 (2H, m), 3.36 (1H, d, J = 4.5), 2.75 (1H, dd, J = 15.8, J = 10.1), 2.49–2.61 (1H, m), 2.03–2.15 (1H,
m), 2.02–2.14 (1H, m), 1.96 (1H, dd, J = 11.3, J = 15.4), 1.63 (3H, s), 1.35 (3H, s); 13C-NMR (CDCl3) δ:
206.7, 145.0, 110.6, 62.0, 59.4, 42.4, 41.5, 28.4, 20.2, 15.7. CAS 18383-49-8.
3.2.4. Epoxidation of (´)-Carvone (1) and (+)-Carvone (2)
A solution of 1 or 2 g (5 g; 33.33 mmol) was dissolved in MeOH (35 mL), 30% H2O2 (1021 mL;
˝
9999 mmol) and 6 N KOH (16.64 mL; 99.99 mmol), and the mixture was stirred for 4 h at 0 C.
The two-phase system was vigorously stirred at room temperature, and the phases were separated;
the aqueous layer was extracted with Et2O (3 ˆ 20 mL). The organic phase was washed with water
(3 ˆ 20 mL), and dried with Na2SO3. The solvent was evaporated and the residue chromatographed
on silica gel, using a mixture of hexane and ethyl acetate (9:1) as eluent. (+)-cis-epoxycarvone
(7) and (´)-cis-epoxycarvone (8) were obtained in 76% (25.33 mmol) and 69% (23.0 mmol) yield,
respectively [29,30].
29
(7): rαsD = +30.5 (CHCl3, c 0.03); IR (KBr) νmax: 2990, 2900, 1720, 1675, 880 cm–1; 1H-NMR
(CDCl3): δ 4.69 (2H, m), 3.38 (1H, d, J = 4.5), 2.48–2.60 (1H, m), 2.04–2.17 (1H, m), 1.95 (1H, dd,
J = 11.6), 1.70 (3H, s), 1.63 (3H, s); 13C-NMR (CDCl3) δ: 206.2, 146.4, 111.2, 63.2, 58.5, 41.8, 35.2, 28.8,
20.9, 15.7. CAS 36616-60-1.
26
(8): rαsD = ´33.4 (CHCl3, c 0.03); IR (KBr) νmax: 2986, 2911, 1720, 1675, 890 cm–1; 1H-NMR
(CDCl3): δ 4.71 (2H, m), 3.39 (1H, d, J = 4.5), 2.49–2.59 (1H, m), 2.03–2.17 (1H, m), 1.97 (1H, dd,
J = 11.5), 1.70 (3H, s), 1.63 (3H, s); 13C-NMR (CDCl3) δ: 205.7, 146.8, 111.6, 63.2, 57.4, 41.4, 36.5, 28.8,
20.9, 15.5. CAS 39903-97-4.
3.3. Experimental Section
3.3.1. Animals
Swiss albino male mice (Mus musculus), 2–3 months old, weighing from 25 to 35 g were used.
The animals were housed in polyethylene cages and maintained under controlled temperature
˝
(21 ˘ 1 C), with a 12 h light/dark cycle (lights on at 6:00 a.m.), with food (Purinar commercial pellet
feed, Paulínia, São Paulo, Brazil), and water available ad libitum until 1 h prior to tests. The tests
were performed in the period between 12:00 and 17:00; mice were placed in polyethylene cages with
4 animals each and moved to the place of testing at least 1 h before tests to minimize eventual
behavioral alterations. Each animal was used only once and euthanized at the end of the test.
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Molecules 2015, 20, 19660–19673
All experimental procedures were analyzed and previously approved by the Ethics Committee on
Animal Use (CEUA) UFPB, under Certificate No. 0109/11.
3.3.2. Behavioral Experiments
Pentylenetetrazole-Induced Seizure Test
Briefly, the test animals were divided into six groups (n = 8): The experimental groups
were treated with (+)-cis-epoxycarvone (7), (´)-cis-epoxycarvone (8), (+)-trans-carvone oxide (9),
(´)-trans-carvone oxide (10) (300 mg/kg, i.p.), the control group received Tween 80 5% (10 mL/kg)
and the standard group was treated with diazepam (4 mg/kg, i.p.). After 30 min of treatment, the
mice were injected with pentylenetetrazole (75 mg/kg, i.p.), and each animal was observed for onset
of first convulsion (first generalized clonic seizure with loss of righting reflex) and mortality over
15 min [31,58].
Maximal Electroshock-Induced Seizure (MES)
Similarly to the previous tests, in a single trial, animals were divided into six groups (n = 8):
The experimental groups were treated with (+)-cis-epoxycarvone (7), (´)-cis-epoxycarvone (8),
(+)-trans-carvone oxide (9), (´)-trans-carvone oxide (10) (300 mg/kg, i.p.), the control group received
a Tween 80 5% emulsion and the standard group was treated with phenytoin (25 mg/kg, i.p.). After
30 min, the seizures were induced by a pair of auricular clip electrodes by means of an electroshock
of 150 pulses/s and 0.5 s duration (ECT UNIT 7801). The main parameters evaluated were: the
number of animals with tonic seizures (complete hind limb extensions), death scores, and seizure
duration [32,59].
Pilocarpine-Induced Seizure Test
In this test, animals were divided into six groups (n = 8): The experimental groups were treated
with (+)-cis-epoxycarvone (7), (´)-cis-epoxycarvone (8), (+)-trans-carvone oxide (9), (´)-trans-carvone
oxide (10) (300 mg/kg, i.p.), the control group received Tween 80 5% (10 mL/kg), and the standard
group was treated with diazepam (4 mg/kg, i.p.). After 30 min of treatments, the animals received
pilocarpine (400 mg/kg, i.p.). To compare seizure severity after pilocarpine administration, in a
period of 1 h, mice were observed with appearance of peripheral cholinergic signs (miosis, diarrhea,
and urination), stereotyped movements (continuous sniffing, paw licking, and rearing), tremors,
seizures, status epilepticus, mortality, latency to onset seizures and death [33,34,60,61].
Strychnine-Induced Seizure Test
The animals were divided into six groups (n = 8): The experimental groups were treated
with (+)-cis-epoxycarvone (7), (´)-cis-epoxycarvone (8), (+)-trans-carvone oxide (9), (´)-trans-carvone
oxide (10) (300 mg/kg, i.p.), the control group received Tween 80 5% (10 mL/kg) and the standard
group was treated with diazepam (4 mg/kg, i.p.). After 30 min of the above treatment, animals of
all groups were treated with strychnine (20 mg/kg, i.p.), the latency for the onset of first seizure and
death as well as the seizure and mortality rate were evaluated [35,36].
3.3.3. Statistical Analysis
The results presented were obtained using ANOVA, followed by Tukey’s post-hoc test. Data
were analyzed using the GraphPad Prism program version 5.00 (GraphPad Sotware Incorporated,
San Diego, CA, USA), and the values obtained, were expressed as the mean ˘ standard error of
mean. Fisher’s exact test was used to analyze the percentages of animals with peripheral cholinergic
signs, stereotyped movements, tremors, seizures, status epilepticus and the mortality rate. Differences
were considered statistically significant when p < 0.05.
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Molecules 2015, 20, 19660–19673
4. Conclusions
The results indicate that the four epoxycarvone stereoisomers tested are active in several
seizure models, effectively decreasing tonic seizures, and also in increasing the animal survival
rate after induced seizures. The differences in isomer stereochemistry exerted little influence on
pharmacological activity in some of the animal models used. Further studies should be conducted
to characterize the mechanisms of action of these four epoxycarvone isomers, and to clarify the
structure-activity relationships of the tested compounds.
Acknowledgments: This research was supported by the Conselho Nacional de Desenvolvimento Científico
e Tecnológico (CNPq), the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and
the Fundação de Apoio à Pesquisa e Inovação Tecnológica do Estado de Sergipe (FAPITEC/SE)—EDITAL
FAPITEC/SE/FUNTEC/CNPq N˝ 10/2011.
Author Contributions: Paula Regina Rodrigues Salgado performed the research (pharmacological experiments),
analyzed the data and wrote the paper; Renan Marinho Braga and Cynthia Germoglio Farias de Melo performed
research (pharmacological experiments); Diogo Vilar da Fonsêca performed research (pharmacological
experiments), and wrote the paper; Luciana Nalone Andrade analyzed the data and helped in performing the
chemistry experimental section; Reinaldo Nóbrega de Almeida and Damião Pergentino de Sousa designed the
research and appraisal of the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the epoxycarvone stereoisomers are not available from the authors.
© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open
access article distributed under the terms and conditions of the Creative Commons by
Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).
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