Influence of adenosine agonists and antiepileptic drugs on theophylline-induced seizures in rats

Seizures is a major toxicity of theophylline. The mechanism of theophylline-induced seizures is not known, but antagonism at adenosine receptors may be a possibility. The effect of pretreatment with different doses of adenosine (100, 500 and 1000 mg/kg, i.p.), and the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA), 1, 5 and 10 mg/kg, i.p., was studied against seizures induced by theophylline in rats. Both these drugs, at all dose levels tested, failed to protect theophylline seizures. Thus adenosinergic system is unlikely to be involved in mediating the convulsant action of theophylline. On the other hand, the conventional antiepileptic drugs, i.e. diazepam (4 mg/kg), sodium valproate (300 mg/kg) and phenobarbitone (50 mg/kg), but not carbamazepine, afforded some protection. The modification of course of seizures, by the antiepileptic drugs suggests the involvement of some other alternate mechanism in theophylline-induced seizures.     

Gamma-aminobutyric acid and glutamic acid receptors may mediate theophylline-induced seizures in mice

The effects of drugs affecting GABA and glutamic acid receptors on theophylline-induced seizures were investigated in mice. Theophylline elicited tonic seizures in mice in a dose dependent manner. Muscimol, DABA and AOAA significantly prolonged the onset and significantly decreased the incidence of theophylline-induced seizures. Baclofen significantly delayed the onset of the tonic seizures induced by theophylline. Bicuculline and picrotoxin significantly shortened the onset and significantly increased the incidence of seizures induced by a low dose of theophylline and also significantly antagonized muscimol-attenuating effect against theophylline seizures. N-methyl-DL-aspartic acid significantly shortened the onset and significantly increased the incidence of seizures elicited by a low dose of theophylline. D-(-)-2-amino-phosphonopentanoic acid effectively delayed the onset and significantly decreased the incidence of seizures elicited by theophylline and also significantly antagonized the potentiating effect of N-methyl-DL-aspartic acid on seizures induced by a low dose of theophylline. Dextromethorphan and ketamine profoundly shortened the onset of theophylline-induced seizures. Clonidine effectively prolonged the onset and significantly decreased the incidence of theophylline-induced seizures. These data indicate that GABA(A) and N-methyl-D-aspartic acid receptors may mediate theophylline-elicited tonic seizures in mice.     

Influence of endothelium in dose-dependent inhibition and potentiation by isoniazid of isosorbide dinitrate relaxation of rat aorta

The influence of in vivo administration of isoniazid on the relaxant effect of isosorbide dinitrate was determined by pretreating rats with various doses of isoniazid and obtaining concentration-response curves to isosorbide dinitrate in aortic rings from these animals. In rings with endothelium, isoniazid potentiated responses to isosorbide dinitrate at doses of 10, 30, and 100 mg/kg; 3 and 300 mg/kg were without effect. In endothelium-denuded preparations, potentiation was present only at 10 mg/kg; 3 and 300 mg/kg inhibited relaxation. Other experiments indicated that isoniazid potentiation was prevented by pyridoxine, was reproduced with theophylline, and did not occur with 3-morpholinosydnonimine or papaverine. These results were deemed compatible with the hypothesis that isoniazid inhibits transsulfuration of homocysteine and causes its accumulation in vascular smooth muscle and endothelial cells, where it functions as a thiol intermediate and leads to enhanced bioactivation of isosorbide dinitrate. Potentiation appeared to occur only with moderate increases of homocysteine.     

Effect of histamine receptor antagonists on aminophylline-induced seizures and lethality in mice

The aim of this study was to evaluate the effects of H(1) (antazoline and astemizole) or H(2) (cimetidine and famotidine) histamine receptor antagonists on the clonic phase, tonic seizures and morality of mice challenged with aminophylline to induce convulsions in mice. Moreover, the total plasma and brain concentrations of theophylline were evaluated. Astemizole (1 mg/kg) did not affect the threshold for aminophylline-induced seizures, but when administered at a dose of 2 mg/kg, it significantly reduced the CD(50) value of aminophylline from 249 mg/kg to 211 mg/kg (p < 0.01). The remaining histamine receptor antagonists studied i.e., antazoline (up to 1 mg/kg), cimetidine (up to 40 mg/kg) and famotidine (up to 10 mg/kg) had no impact on seizure susceptibility in aminophylline-induced convulsions. Furthermore, astemizole (2 mg/kg) decreased latency to the clonic phase of aminophylline-induced convulsions from 51.1 +/- 4.5 to 32.1 +/- 4.3 min (p < 0.01). It is noteworthy that astemizole, a novel H(1) receptor antagonist, did not alter the brain and plasma levels of theophylline, so the existence of pharmacokinetic interactions was excluded. Our results indicate that some interactions between methylxanthines and histamine receptor antagonists may be clinically important since these drugs are usually combined during the treatment of status asthmaticus.     

Possible role of free radicals in theophylline-induced seizures in mice

Theophylline is a methylxanthine bronchodilator with a narrow therapeutic index and is prone to induce seizures, the mechanisms for which are not clearly defined. Free radicals have considerable neurotoxic potential and the present study evaluated the possible involvement of these bioactive moieties in aminophylline-induced seizures in mice. Aminophylline (50-250 mg/kg) induced convulsions and mortality in mice in a dose-dependent manner. The anti-oxidants, melatonin (25-100 mg/kg) and N-actylcysteine (100 and 200 mg/kg) attenuated aminophylline seizures and mortality. Similar antagonism of aminophylline seizures was also observed after pretreatments with nitric oxide (NO) synthase inhibitors, L-NAME (3 and 10 mg/kg) and 7-nitroindazole (10 and 30 mg/kg). Further, combined treatment with otherwise sub-effective doses of melatonin and L-NAME or 7-nitroindazole produced marked protective effects against these seizures. Aminophylline-induced seizures enhanced malondialdehyde (MDA) concentrations and NO metabolite (NOx) levels in the brain homogenates of mice, and these were attenuated by melatonin and L-NAME pretreatments. The results are suggestive of the possible involvement of free radicals (reactive oxygen and reactive nitrogen species) in the convulsiogenic effects of aminophylline.     

Lack of protection by pyridoxine or hydrazine pretreatment against monosodium L-glutamate-induced seizures.

Pretreatment of rats with hydrazine (100 mg/kg), a compound which raises brain gamma-aminobutyric acid (GAGA) 175 percent in 12 hr was not able to prevent the occurrence of seizures induced by monosodium L-glutamate (MSG). Pyridoxine (50 mg/kg) the cofactor essential in the conversion of glutamate to GABA, also failed to prevent convulsions induced by parenteral MSG administration. It is concluded that the mechanism of action of MSG-induced seizures is neither by decreasing brain GABA levels or interfering with the pyridoxine cofactor.     

PROCONVULSANT EFFECT OF PAPAVERINE ON THEOPHYLLINE-INDUCED SEIZURES IN RATS

1. The effects of papaverine, an adenosine uptake inhibitor, were tested against theophylline-induced seizures. 2. Rats were infused with a constant intravenous infusion of theophylline at 10.3 mg/min immediately following intraperitoneal pretreatment with either papaverine 35 mg/kg or saline vehicle. 3. Papaverine produced a potent proconvulsant effect towards theophylline seizures, reducing the onset time and cumulative dose to seizure and theophylline concentrations in arterial serum, cerebrospinal fluid and brain, respectively, at onset of maximal seizures.     

Effects of Saiko-ka-ryukotsu-borei-to, a Japanese Kampo medicine, on tachycardia and central nervous system stimulation induced by theophylline in rats and mice

Effects of Saiko-ka-ryukotsu-borei-to (SRBT) on theophylline-induced tachycardia in anesthetized rats and theophylline-induced locomotion and convulsions in mice were examined. An intraduodenal administration of SRBT (1 g/kg) prevented theophylline (5 mg/kg, i.v.)-induced tachycardia in rats. SRBT also attenuated an increase in arterial blood pressure with a slow reduction in heart rate of rats treated with theophylline, with no influence on the plasma level of theophylline. However, SRBT did not change the beating rate of right atrium isolated from rats in the absence or presence of theophylline or isoproterenol. The locomotor activity of theophylline in mice was reduced by the treatment with SRBT. Furthermore, the latency of convulsions in mice induced by administration of theophylline at a higher dose (240 mg/kg, i.p.) was prolonged by treatment with SRBT (1 g/kg, p.o.) and seven out of fifteen mice were saved from death due to convulsions. These results suggest that theophylline-induced tachycardia and central nervous stimulation are suppressed by SRBT and that SRBT may reduce the undesirable actions of theophylline on the cardiovascular and central nervous systems.     

Reversal of acute theophylline toxicity by calcium channel blockers in dogs and rats

Theophylline, widely used in the treatment of pulmonary diseases, has a narrow therapeutic index; the recommended plasma levels being 10–20 μg/ml in humans. The misuse or abuse of theophylline can cause life-threatening central nervous system and cardiovascular effects. Increased intracellular Ca²⁺ levels are thought to play an important role in theophylline toxicity and death. The objective of this study was to determine whether Ca²⁺ channel blockers, e.g. verapamil, nifedipine, or diltiazem, prevent sudden death caused by theophylline treatment in rats and dogs. Groups of Sprague-Dawley rats were treated with theophylline alone (150 mg/kg i.p.) or with theophylline pretreatment followed by administration of verapamil (0.25 to 0.5 mg/kg i.p.), nifedipine (0.25 to 1.0 mg/kg i.p.), or diltiazem (0.5 to 1.0 mg/kg i.p.), 2.5 to 15 min later. The rats were observed for toxic signs and survival over a period of 15 days. All three calcium channel blockers significantly reduced the theophylline-induced sudden death in rats. In a separate study, neither verapamil (0.5 mg/kg i.p.) nor nifedipine (1.0 mg/kg i.p.) prevented the theophylline-induced myocardial necrosis in the rat. In beagle dogs, verapamil (0.5 mg/kg i.v.) prevented theophylline (15 mg/kg/min i.v. for 10 min)-induced hypotension, arrhythmias, and sudden death. Our results support previously reported findings that calcium plays a major role in theophylline-induced toxicity and death.     

Evaluation of diazepam and pyridoxine as antidotes to isoniazid intoxication in rats and dogs

Because information regarding efficacious treatment of acute isoniazid (INH) toxicity is incomplete and controversial, diazepam and pyridoxine were investigated as iv antidotes in rats and dogs following administration of po lethal doses of INH. There is a marked species variation in the lethality of INH; the lowest consistently lethal dose is 1500 mg/kg for rats and 75 mg/kg for dogs. Of the two species, the dog more closely approximates man's sensitivity to the lethal effect of INH overdose (80–150 mg/kg). Species variation was also observed in the effects of the antidotes. Diazepam exerted dose-related protection against convulsions in rats; paradoxically, survival was increased by the lowest dose (1 mg/kg) but not by higher doses. In dogs, however, diazepam failed to prevent convulsions but provided dose-related protection against death. Pyridoxine, in rats, did not protect against INH toxicity, but in dogs it showed dose-related effectiveness against convulsions, and all doses (75–300 mg/kg) prevented lethality. Significantly, the highest dose of pyridoxine tested in rats (750 mg/kg) was substantially below the optimal pyridoxine-to-INH antidotal ratio recommended for man (a dose that at least equals the amount of INH ingested), but that dose of pyridoxine would be larger than its LD50 for rats. Combined administration of diazepam with pyridoxine protected against convulsions and death in rats and dogs. Used concurrently, the two antidotes are clearly synergistic for controlling the manifestations of experimental INH overdose. These results have important implications for the management of acute INH intoxication in man.     

Potentiation of pyridoxine by depressants and anticonvulsants in the treatment of acute isoniazid intoxication in dogs

Treatments for acute isoniazid (INH) intoxication have included, singly and in various combinations, a great variety of drugs. As a consequence it is difficult to evaluate the efficacy of these antidotes, except for pyridoxine, the most commonly recommended one. In some cases of INH poisoning evaluation is further complicated because of concurrent alcohol ingestion. The objectives of this investigation were to determine whether ethanol enhances the toxic effects of acute INH overdose, as suggested by some clinical reports, and to evaluate the antidotal efficacy of phenobarbital, pentobarbital, phenytoin, ethanol, or diazepam when each is administered in combination with pyridoxine. Male dogs were either pretreated with iv ethanol and challenged 1 hr later with po INH, 50 or 75 mg/kg, or they were given INH, 75 mg/kg, and injected iv 30 min later with the test drugs, alone or in combination with pyridoxine. Ethanol pretreatment not only did not enhance the toxicity of INH but, in fact, it reduced the severity of convulsions, although it did not change the mortality rate. In the antidotal study, none of the five CNS depressants or anticonvulsants protected against clonic-tonic seizures or death. Pyridoxine, however, reduced the severity of the seizures and prevented death, although it did not completely block convulsions. The combination antidotal treatments (pyridoxine plus each of the CNS drugs) were the most effective; they prevented both convulsions and lethality. It is suggested that pyridoxine is the basic antidote for treatment of acute INH poisoning, and that the addition of an anticonvulsant or a CNS depressant to the therapy enhances effectiveness.     

Effect of orally administered dextromethorphan on theophylline- and pentylenetetrazol-induced seizures in rats

Dextromethorphan, widely used as an antitussive, has recently been shown to protect animals against maximal electroshock and excitatory amino acid (N-methyl-D-aspartate)-induced convulsions. Its protective efficacy against theophylline-induced seizures was determined in this investigation in view of the limited effectiveness of presently available anticonvulsants against this manifestation of serious theophylline intoxication. Rats were pretreated with an oral dose of dextromethorphan (50 mg/kg) or saline solution. Fifteen minutes later, the rats were infused intravenously with theophylline [approximately 11 mg/(kg.min)] until the onset of maximal seizures. Pretreatment with dextromethorphan was associated with a significant decrease in the concentrations of theophylline in the cerebrospinal fluid and serum at the pharmacologic endpoint. To further explore this unanticipated effect, a similar experiment was performed with the convulsant pentylenetetrazol (PTZ), which was infused at a rate of approximately 3.4 mg/(kg.min) until the onset of maximal seizures. Dextromethorphan-pretreated animals required a significantly larger dose of PTZ than did controls to produce the first myoclonic jerk, but a significantly smaller dose of the convulsant to produce maximal seizures. Serum and cerebrospinal fluid concentrations of PTZ at onset of maximal seizures were significantly lower in dextromethorphan-treated than in control animals. The proconvulsant activity of dextromethorphan with respect to theophylline-induced maximal seizures is similar to that of phenytoin, and is consistent with other pharmacologic evidence of such similarity.     

A serious interaction of dantrolene and theophylline

Dantrolene and theophylline have in common pronounced actions on several muscular systems. Therefore we investigated the interaction of these 2 drugs in rats. Normal cases of dantrolene (4 or 2 mg/kg) caused an increase in the lethality produced by theophylline, but without showing the expected seizures of theophylline toxicity. This may be caused by a synergistic action on the heart or blood vessels. On the other hand, a small dose of dantrolene decreased theophylline-induced seizures and death. This may be due to the effect of dantrolene on calcium release in skeletal muscles. The dosage of dantrolene should be decreased when used with theophylline.     

Comparative effect of theophylline and aminophylline on theophylline blood concentrations and peripheral blood eosinophils in patients with asthma

The comparative effects of a new theophylline preparation (Theodrip) and aminophylline on blood concentrations of theophylline were examined in 74 patients with asthma. Subjects were intravenously administered 200 mg of Theodrip or 250 mg of aminophylline for 1 h. The mean increases in blood theophylline concentration after Theodrip or aminophylline administration were 8.80 +/- 1.80 mg/l and 8.81 +/- 2.15 mg/l, respectively. In addition, these patients were divided into four groups based on baseline theophylline concentrations before infusion of Theodrip or aminophylline: i) na?ve patients (not administered theophylline); ii) those with a baseline theophylline concentration of 0-5 mg/l; iii) those with a baseline theophylline concentration of 5-10 mg/l; iv) those with a baseline theophylline concentration of 10-15 mg/l. Mean increases in blood theophylline concentration after administration of Theodrip in each group were similar to those after aminophylline administration. We found no significant differences between Theodrip and aminophylline. However, when the comparative effects of Theodrip and aminophylline on peripheral blood eosinophil counts were examined, Theodrip, but not aminophylline, reduced blood eosinophil counts. With acute exacerbations of bronchial asthma, it is expected that Theodrip, but not aminophylline, may have an antiinflammatory effect. In conclusion, it is suggested that Theodrip is a more useful drug than aminophylline in patients with acute exacerbations of bronchial asthma.     

Pharmacokinetic and pharmacodynamic interactions of aminophylline and topiramate in the mouse maximal electroshock-induced seizure model

The aim of this study was to determine the influence of acute (single) and chronic (twice daily for 14 consecutive days) treatments with aminophylline (theophylline(2).ethylenediamine) on the anticonvulsant potential of topiramate (a broad-spectrum antiepileptic drug) in the mouse maximal electroshock-induced seizure model. Additionally, the effects of acute and chronic administration of aminophylline on the adverse effect potential of topiramate were assessed in the chimney test (motor performance). To evaluate pharmacokinetic characteristics of interaction between topiramate and aminophylline, total brain concentrations of topiramate and theophylline were estimated with fluorescence polarization immunoassay technique. Results indicate that aminophylline in non-convulsive doses of 50 and 100 mg/kg (i.p.), both in acute and chronic experiments, markedly attenuated the anticonvulsant potential of topiramate by raising its ED(50) value against maximal electroconvulsions. Aminophylline at a lower dose of 25 mg/kg did not affect significantly the ED(50) value of topiramate in the acute experiment, but the drug markedly increased the ED(50) value of topiramate during the chronic treatment in mice. Only, aminophylline at 12.5 mg/kg, in both acute and chronic experiments, did not affect the antielectroshock action of topiramate in mice. Moreover, aminophylline at a dose of 100 mg/kg had no impact on the adverse effect potential of topiramate in the chimney test. Pharmacokinetic evaluation of total brain concentrations of topiramate and theophylline revealed that topiramate significantly increased total brain theophylline concentrations following both acute and chronic applications of aminophylline. Conversely, aminophylline did not alter total brain concentrations of topiramate in mice. Based on this preclinical study, one can conclude that aminophylline attenuated the antiseizure action of topiramate in the mouse maximal electroshock-induced seizure model and the observed interaction between drugs was both pharmacokinetic and pharmacodynamic in nature.     

Comparative pharmacokinetics of theophylline and aminophylline in man.

1 The pharmacokinetics of theophylline and aminophylline was compared after oral administration and intravenous infusion. 2 Theophylline (250 mg) and aminophylline (390 mg) were taken orally by eight healthy volunteers in a randomized cross-over study. 3 In another cross-over study theophylline and aminophylline were administered intravenously to six healthy volunteers at a dose corresponding to 5 mg/kg pure theophylline. 4 The protein binding of the theophylline in serum collected during the intravenous study was studied by ultrafiltration. The serum concentration of theophylline was measured by high pressure liquid chromatography. 5 Almost identical concentration-time curves were found for theophylline and aminophylline in both of the studies. No significant difference was found in the pharmacokinetic parameters or protein binding with the two preparations.     

Anticonvulsant effect of denzimol in DBA/2 mice

The anticonvulsant activity of N-[beta-[4-beta-phenylethyl)phenyl]-beta-hydroxyethyl]-imidazole hydrochloride, denzimol, was studied following intraperitoneal administration in DBA/2 mice (seizures induced by sound). Protection against sound-induced seizures was observed after intraperitoneal administration of denzimol (3-15 mg/kg). The ED50 values for the suppression of tonic, clonic and wild running phases of sound-induced seizures were 1.24, 2.61 and 6.03 mg/kg, respectively. This protective action was significantly reduced by pretreatment with aminophylline (25 mg/kg i.p.), CGS 8216 (1 or 5 mg/kg i.p.) and Ro 15-1788 (2.5 mg/kg i.p). The present experiments suggest an involvement of purinergic and benzodiazepine mechanisms in the anticonvulsant action of denzimol.     

Antagonists of adenosine and alpha-2-adrenoceptors reverse the anticonvulsant effects of tizanidine in DBA/2 mice

The anticonvulsant activity of 5-chloro-4-(2-imidazolin-2yl-amino)-2,1,3-benzothiazole, tizanidine, was studied following intraperitoneal (i.p.) administration in DBA/2 mice (which show sound-induced seizures). Protection against sound-induced seizures was observed after tizanidine, (0.5-4 mg/kg i.p.). The ED50 values for suppression of the tonic, clonic and wild running phases of sound-induced seizures were 0.54, 0.76 and 1.43 mg/kg, respectively. This protective action was significantly reduced by pretreatment with aminophylline (25 mg/kg i.p.), yohimbine (1 mg/kg i.p.) or piperoxan (20 mg/kg i.p.). Methysergide, a serotonin antagonists, did not significantly reduce the anticonvulsant effects of tizanidine. The present experiments suggest an involvement of purinergic and adrenergic mechanisms in the anticonvulsant action of tizanidine.     

A Comparative Study of Aminophylline- and Acepifylline-induced Seizures and Death in the Chemoconvulsion Model in Rats

The convulsive, pro-convulsive and lethal effects of two theophylline-containing bronchodilating agents, aminophylline and acepifylline, have been evaluated in rats. Aminophylline (theophylline ethylenediamine) caused seizures and death in a dose-dependent manner; an intraperitoneal dose of 250 mg kg^? caused seizures and death in all rats. Intraperitoneal doses of acepifylline (theophylline ethanoate of piperazine) up to 1000 mg kg^?, however, did not cause seizure or death. Further, pre-treatment of the rats by intraperitoneal administration of a subconvulsive dose (100 mg kg^?) of aminophylline caused a significant decrease in CD50 and LD50 values for pentylenetetrazole and a significant increase in the number of positive responders (i.e. rats with a pentylenetetrazole-induced seizure score of 3 or more on a seizure scale ranging from 0 to 6) and death rate compared with those obtained for rats pre-treated with an equivalent intraperitoneal dose (140 mg kg^?) of acepifylline (‘equivalent dose’ referred to here denotes the theophylline content of the two preparations). The study has established the neurosafety profile of acepifylline and documents a safer alternative to aminophylline for use in asthmatics suffering from concomitant epilepsy or other seizure-prone neurological defects.     

Role of nitric oxide in systemic effect of theophylline on mouse body temperature

In the present study, the interaction of nitric oxide synthase (NOS) inhibitors, L-NAME (N(G)-nitro-L-arginine methyl ester HCl) and L-NA (N(omega)-nitro-L-arginine), and its precursor, L-arginine (2-(S)-2-amino-5-[(aminoiminomethyl)amino] pentatonic acid), with theophylline on mouse body temperature was studied. Intraperitoneal (i.p.) injection of different doses of theophylline altered body temperature. Lower doses of theophylline (12.5 and 25 mg/kg) increased, but a higher dose (100 mg/kg) reduced, the animals' body temperature. The combination of L-arginine (20 and 40 mg/kg) with the highest dose of theophylline potentiated the hypothermic effect induced by the latter drug, while L-arginine by itself did not alter body temperature. L-NAME (10-80 mg/kg) or L-NA (10 mg/kg) plus a lower dose of theophylline (12.5 mg/kg) reduced the theophylline-induced hyperthermic response. L-NA (1, 5, and 10 mg/kg) in combination with the high dose of theophylline (100 mg/kg) also induced greater hypothermia. Both L-NAME and L-NA by themselves reduced body temperature. It is concluded that nitric oxide (NO) may be involved in the effects of theophylline on body temperature in mice.