Cytisine inhibits the anticonvulsant activityof phenytoin and lamotrigine in mice

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Short communication

Cytisine inhibits the anticonvulsant activity of phenytoin and lamotrigine in mice

Piotr Tutka^1,2, Tomasz Mróz², Jerzy Bednarski², Andrzej Styk², Jaromir Ognik², Jerzy Mosiewicz³, Jarogniew £uszczki^4,5

1Department of Pharmacology, Institute of Nursing and Health Sciences, University of Rzeszów, Warzywna 1, PL 35-959 Rzeszów, Poland

2Department of Experimental and Clinical Pharmacology, Medical University of Lublin, Jaczewskiego 8, PL 20-090 Lublin, Poland

3Department of Internal Medicine, Medical University of Lublin, Staszica 16, PL 20-081 Lublin, Poland

4Department of Pathophysiology, Medical University of Lublin, Jaczewskiego 8, PL 20-090 Lublin, Poland

5Department of Physiopathology, Institute of Agricultural Medicine, Jaczewskiego 2, PL 20-090 Lublin, Poland Correspondence: Piotr Tutka, e-mail: tutka@umlub.pl

Abstract:

Background: Cytisine (CYT), the most commonly used drug for smoking cessation in Poland, was experimentally found to induce convulsions. There is a lack of studies on the influence of CYT on the anticonvulsant activity of antiepileptic drugs (AEDs).

Methods: The effects of CYT on the anticonvulsant activity of six AEDs were examined in maximal electroshock (MES)-induced seizures in mice.

Results: Single intraperitoneal (ip) administration of CYT in a subthreshold dose of 2 mg/kg antagonized the protective activity of ip phenytoin and lamotrigine against MES-induced seizures in mice. A dose of 1 mg/kg did not reverse the protective activity of phenytoin and lamotrigine. CYT in a dose of 2 mg/kg had no effect on the anticonvulsive activity of carbamazepine, oxcarbazepine, phenobarbital, and valproate magnesium.

Conclusion: CYT ability to antagonize the anticonvulsive activity of phenytoin and lamotrigine can be of serious concern for epileptic smokers, who might demonstrate therapeutic failure to these drugs resulting in possible breakthrough seizure attacks.

Key words:

cytisine, epilepsy, lamotrigine, maximal electroshock, nicotinic receptors, phenytoin, smoking cessation

Introduction

Cytisine (CYT) is a plant alkaloid used in medicine for hundreds years, including smoking cessation [4, 14, 16, 19]. Recent clinical trials conducted in Poland demonstrated that CYT was at least equally, if not

more, effective for smoking cessation as the available pharmacological therapies [18, 21]. The evidence suggests that the antismoking effects of CYT are de- termined by its partial agonism at brain a4b2 nico- tinic acetylcholine receptors (nAChRs) [9]. CYT, binding to these receptors and causing a moderate and sustained release of mesolimbic dopamine, attenuates

Pharmacological Reports 2013, 65, 195200 ISSN 1734-1140

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[17, 18, 21]; however, despite its 40+ years on the mar- ket, the number of preclinical and clinical studies on its safety is very limited and existing data are not suffi- cient to support registration of the drug in most Euro- pean countries and the US [15]. Given the life-saving potential of the drug across the globe there is an urgent need to undertake a rigorous evaluation of its safety.

The activation of brain nAChRs is involved in clonic-tonic convulsions induced by nicotine [3]. It was experimentally found that CYT could also induce convulsions [1, 12], but its effect on the course and clinical outcome of epilepsy in patients remains un- known. In addition, there is a lack of studies on the effects of CYT on the anticonvulsant activity of antiepileptic drugs in epileptic patients.

Therefore, we examined the influence of CYT on the anticonvulsive action of six antiepileptic drugs on maximal electroshock (MES)-induced seizures, which are thought to be an experimental model of human generalized tonic-clonic seizures in mice.

Materials and Methods

Animals

The experiments were performed on adult male Swiss mice weighing 20–25 g. The animals were kept in standard laboratory conditions on a natural light-dark cycle, with ambient temperature of 18–22°C, relative humidity of 52–58%, and unlimited access to chow pellets and water. All animals were acclimatized to their home cages for 1 week before testing. The experimental groups, consisting of 8 mice, were chosen by means of a randomized schedule. Each mouse was used only once. The tests were performed between 8:00 and 14:00 h. The control groups were always tested on the same day as the corresponding experimental groups. The experimental protocol and procedures were followed according to “Principles of Laboratory Animal Care” (NIH publication No. 86–23, revised 1985), approved by the Medical University of Lublin Ethics Committee for the use of experimental animals and confirmed with the European Communi- ties Council Directive (86/609/EEC).

mazepine (CBZ, Sigma-Aldrich, St. Louis, MO, USA), cytisine (CYT, Tabex, Sopharma-Poland Sp. z o.o.

Warszawa, Poland), lamotrigine (LTG, Lamitrin, GlaxoSmithKline Export Ltd., Brentford, Great Brit- ain), oxcarbazepine (OXC, Trileptal; Novartis Pharma GmbH, Nürnberg, Germany), phenobarbital (PHB, Pharmaceutical Company “Unia”, Warszawa, Poland), phenytoin (DPH, Warsaw Pharmaceutical Works Polfa S.A., Warszawa, Poland), valproate magnesium (VPA, ICN Polfa Rzeszów S.A., Rzeszów, Poland).

All the drugs were suspended in a 1% solution of Tween 80 (Sigma-Aldrich, St. Louis, MO, USA) in sterile saline (NaCl, 0.9%, Baxter Terpol, Sieradz, Poland) immediately before intraperitoneal (ip) administration at a volume of 10 ml/kg of body weight. Fresh drug solu- tions or suspensions were prepared ex tempore on each experimental day. Control animals were injected with equivalent amounts of sterile saline or 1% solution of Tween 80 in sterile saline using the same route.

Electroconvulsions

All the procedures were conducted after at least 30 min of acclimatization to the experimental conditions. Electroconvulsions were produced with a cur- rent delivered via ear-clip electrodes by a Rodent Shocker generator (constant-current stimulator Type 221, Hugo Sachs Elektronik, Freiburg, Germany).

The criterion for convulsant activity was tonic hindlimb extension (i.e., the hind limbs of animals be- came extended at 180° to the plane of the body axis).

MES seizure threshold test

Animals were subjected to the current stimulation with constant duration (0.2 s) and different intensities (5–10 mA). Each time the number of convulsing out of total animals in an experimental group was regis- tered and CS₅₀value (current strength₅₀), i.e., median current strength (in mA) necessary to induce tonic convulsions in 50% of animals, was calculated.

MES seizure test

Mice were challenged with set current intensity and stimulus duration (25 mA and 0.2 s, respectively) [7].

All animals in control groups produced seizures.

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Anticonvulsant activity of the antiepileptic drugs was determined in mice pretreated with these drugs before MES as ED₅₀values (i.e., doses of the drugs in mg/kg, protecting 50% of animals against MES).

Experimental design

Three to five experimental groups were used each time to construct the dose-response curve for each desired value to be calculated. The duration of CYT and antiepileptic drugs pretreatment was based on information concerning its biological activity obtained from the literature and confirmed in our pilot experiments.

Two doses of CYT (2 and 4 mg/kg) were screened to determine subthreshold dose of CYT, i.e., the high- est dose that did not significantly change electroconvulsive threshold in mice.

In order to determine the effect of CYT on the protective activity of the antiepileptic drugs, they were injected ipin the different doses. The study groups received progressive doses of the antiepileptic drugs in combination with CYT in a subthreshold dose and control groups received progressive doses of the antiepileptics and vehicle. CYT was administered 15 min, CBZ, OXC, and VPA 30 min, LTG and PHB 60 min, and DPH 120 min before tests. The antiepileptic drugs were administered in the following doses: CBZ – 10–14, DPH – 4–14, LTG – 2–7, OXC – 4–10, PHB – 15–35, and VPA – 200–350 mg/kg. If a significant effect of CYT on ED₅₀ of the antiepileptics was found, then lower dose of CYT (1 mg/kg) was given to mice following the same experimental procedure as for a subthreshold dose of CYT.

Statistics

The CS₅₀ and ED₅₀ values with their respective 95%

confidence limits, which were transformed into standard errors (SE) as needed, were calculated and statis-

tically analyzed by the computer-assisted log-probit method, according to Litchfield and Wilcoxon [5] fol- lowed by one-way ANOVA with the post-hoc Dun- nett’s test for multiple comparisons [6]. The CS50val- ues for CYT were compared with the respective CS₅₀ values in vehicle-treated control group. In MES, the ED₅₀ values of the antiepileptic drugs for mice pretreated with CYT were compared with the respective ED₅₀ values of the antiepileptic drugs administered with vehicle. The index of probability of less than 0.05 (p < 0.05) was considered significant in com- parative analysis.

Results

Effects of CYT on electroconvulsive threshold CYT administered ip in a dose of 4 mg/kg signifi- cantly reduced the threshold for electrically-induced seizures, decreasing the CS₅₀from 7.5 (6.8–8.3) to 6.1

Cytisine and the anticonvulsant activity of antiepileptics

Piotr Tutka et al.

Fig. 1. Effect of cytisine (CYT) on seizure threshold in electrically- induced convulsions in mice. CYT was administered ip in doses of 2 and 4 mg/kg for 15 min before MEST test. Each bar reflects the CS₅₀value (with SE), i.e., median current strength in mA necessary to induce tonic convulsions in 50% of animals

Tab. 1. Effect of cytisine (CYT) in a subthreshold dose (in mg/kg) on the anticonvulsant activity of carbamazepine (CBZ), oxcarbazepine (OXC), phenobarbital (PHB), and valproate magnesium (VPA) expressed as their ED₅₀doses in mg/kg (with 95% confidence limits in parenthe- ses) in the MES test in mice

Drug Drug + CYT 0

ED50(mg/kg)

Drug + CYT 2 ED50(mg/kg)

Statistics

CBZ 11.0 (10.2–11.9) 11.3 (10.3–12.4) p > 0.05

OXC 7.7 (6.5–9.2) 7.1 (6.3–8.0) p > 0.05

PHB 20.8 (18.0–24.1) 24.5 (21.2–28.3) p > 0.05

VPA 268 (226–317.9) 262.7 (220.2–313.4) p > 0.05

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trol). The dose of CYT 2 mg/kg was determined to be the subthreshold dose (Fig. 1).

Effects of CYT on the anticonvulsant activity of the antiepileptic drugs in MES test

CYT administered ip in a dose of 2 mg/kg did not influ- ence the ED₅₀s of CBZ, OXC, PHB, and VPA (Tab. 1).

CYT administered ip in a dose of 2 mg/kg signifi- cantly increased the ED₅₀of DPH from 6.5 (5.3–7.9) to 9.5 (7.8–11.6) mg/kg (p < 0.01). The dose of 1 mg/kg of CYT did not change significantly the ED₅₀ of DPH [9.7 (8.1–11.8) vs. 9.7 (8.1–11.6); non signifi- cant] (Fig. 2).

CYT did not change significantly the ED50of LTG [4.2 (3.5–5.1) vs. 4.3 (3.6–5.1); non significant] (Fig. 3).

Discussion

In this study, we demonstrated that systemic administration of CYT caused a significant reduction of the seizure threshold for electroconvulsions in mice. CYT in a single dose of 4 mg/kg decreased the electroconvulsive threshold whereas in a dose of 2 mg/kg did not change the threshold. The dose of 2 mg/kg was applied in subsequent follow up experiments.

CYT ip pretreatment in the single subthreshold dose (2 mg/kg) increased the ED50of ip DPH by 46% and ip LTG by 72% against MES-induced seizures, which are thought to be a model of human generalized tonic- clonic seizures. It was further observed that lower dose of CYT (1 mg/kg) had failed to antagonize the antiseizure efficacy of DPH and LTG. CYT administered in the subthreshold dose of 2 mg/kg did not change the antiseizure activity of CBZ, OXC, PHB, and VPA.

It was reported that CYT induced clonic-tonic convulsions in animals [1, 12]. The drug is also well known to produce seizures after an accidental or intentional massive overdose in humans. The case of a 48-year-old woman who took 40–50 tablets of CYT (1.5 mg/tablet) for suicidal purpose and experienced a loss of con- sciousness and clonic seizures was reported [13]. She was hospitalized and treated with infusions of sodium chloride and glucose as well as with analeptics, vitamins and phenobarbital. After 5 days she was discharged from the hospital without any symptoms.

Although large doses of CYT could cause seizures, there are no reports describing safety of the drug in the doses considered to be therapeutic in smokers with epilepsy. Post-marketing data on varenicline, another nAChRs partial agonist approved for smoking cessation, has linked this drug to an increase in neu- ropsychiatric symptoms such as seizures [20]. It is well known that nicotine in large doses can induce clonic-tonic seizures. Nicotine-induced seizures are centrally mediated and involve the activation of a4 and a7 nAChRs subtypes [3, 12]. The safety concerns associated with the use of low dosages of nicotine in seizure condition among smokers are still discussed.

Fig. 2. Effect of cytisine (CYT) on the antiepileptic activity of pheny- toin (DPH) in MES test in mice. CYT was administered ip in doses of 2 (subthreshold dose) and 1 mg/kg for 15 min before MES test. Each bar reflects the ED₅₀value (with 95% confidence limits), i.e., dose of DPH in mg/kg, protecting 50% of animals against MES-induced convulsions

Fig. 3. Effect of cytisine (CYT) on the antiepileptic activity of lamotrig- ine (LTG) in MES test in mice. CYT was administered ip in doses of 2 (subthreshold dose) and 1 mg/kg for 15 min before MES test. Each bar reflects the ED₅₀value (with 95% confidence limits), i.e., dose of LTG in mg/kg, protecting 50% of animals against MES-induced convulsions

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Both CYT and nicotine are agonists at nAChRs and through their agonism might, at least theoretically, predispose epileptic patients to lower the seizure threshold. It is reasonable to pay attention to epileptic patients who continue to smoke while taking CYT for smoking cessation. It is of serious concern that in these patients either CYT and nicotine alone or both agents taken together may affect the anticonvulsive activity of antiepileptic drugs.

Indeed, the results of this study showed that CYT significantly and dose-dependently antagonized the protective activity of two among six tested antiepileptic drugs. CYT antagonized the protective activity of DPH and LTG, drugs acting via Na⁺ channel block- ade. This is the first report because virtually nothing is known about the effectiveness of the antiepileptic drugs in animals treated with this drug. CYT is not the only nAChRs agonist inhibiting the anticonvulsive activity of antiepileptics. Recently, it was found that low-dose nicotine inhibited the protective efficacy of topiramate against kainic acid-induced seizures that have been widely recognized as a model for human temporal lobe epilepsy [11]. The action of nicotine was reversed by mecamylamine, a noncompetitive blocker of nAChRs, and this observation led to the conclusion that the inhibitory effect of nicotine against the anticonvulsive activity of topiramate is mediated through nAChRs. It was reported earlier that nicotine caused excitation of hippocampal neu- rons through nAChRs, leading to synaptic release of glutamate [8] and seizure activity. According to Damaj et al. [3], a7 nAChRs contribute to nicotine- induced excitation and its proconvulsant effect. CYT was found to be a full agonist at a7 nAChRs [16].

Thus, as observed in this study, it might be possible that CYT inhibits the protective activity of DPH and LTG through facilitation of the excitatory glutamater- gic system via a7 nAChRs. However, an identifica- tion of pharmacological mechanism(s) associated with CYT effect is not possible on the basis of our results and detailed animal studies investigating this issue are needed.

In summary, the present study provides the first information about the effect of CYT on the beneficial action of the antiepileptic drugs against MES-induced seizures. CYT ability to antagonize the anticonvulsive action of DPH and LTG can be of serious concern for epileptic smokers, who might demonstrate therapeutic failure to DPH and LTG resulting in possible breakthrough seizure attacks.

Acknowledgments:

This study was supported by grant PW 528/07 and DS 454/09 from Medical University of Lublin, Lublin, Poland.

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Received: May 9, 2012; in the revised form: September 26, 2012;

accepted: October 8, 2012.