N(6)-2-(4-aminophenyl)ethyl-adenosine enhances the anticonvulsive action of conventional antiepileptic drugs in the kindling model of epilepsy in rats

2-Phenethylamines in Medicinal Chemistry: A Review

A concise review covering updated presence and role of 2-phenethylamines in medicinal chemistry is presented. Open-chain, flexible alicyclic amine derivatives of this motif are enumerated in key therapeutic targets, listing medicinal chemistry hits and appealing screening compounds. Latest reports in discovering new bioactive 2-phenethylamines by research groups are covered too.

Adenosine receptor agonists differentially affect the anticonvulsant action of carbamazepine and valproate against maximal electroshock test-induced seizures in mice

Background.Adenosine is regarded as an endogenous anticonvulsant and its agonists have been proved to affect the anticonvulsant activity of a number of antiepileptic drugs (AEDs) in animal models of seizures.

Aim.To evaluate effects of adenosine agonists on carbamazepine (CBZ) and valproate (VPA) in mouse model of generalized tonic-clonic convulsions.

Methods.The following adenosine receptor agonists were used: A1– cyclohexyladenosine, A2A– CGS 21 680, A3– N6-benzyl-NECA and A1(preferentially) and A2– 2-chloroadenosine. Their possible anticonvulsant effects were studied in a threshold electroconvulsive test for maximal electroconvulsions. The protective activity of AEDs alone or in combinations with adenosine agonists was evaluated in the form of their respective ED50values necessary to protect 50% of mice against tonic extension of the hind limbs, following maximal electroshock, delivered through ear electrodes. The specificity of interactions between AEDs and adenosine agonists was challenged with an adenosine receptor A1and A2antagonist, aminophylline (5 mg/kg). The effects of AEDs alone or with adenosine agonists were tested for the occurrence of adverse effects (AE) (impairment of motor coordination) in a chimney test. All combinations with an enhancement the protective activity of CBZ or VPA were verified with the free plasma or brain concentration of these AED.

Results.Adenosine receptor agonists (cycloheksyladenosine up to 4 mg/kg; CGS 21 680 – 8 mg/kg; N6-benzyl-NECA – 1 mg/kg; 2-chloroadenosine – 2 mg/kg) did not significantly affect the threshold for maximal electroconvulsions. Cycloheksyladenosine (1 mg/kg), N6-benzyl-NECA (0.5 and 1 mg/kg) and 2-chloroadenosine (1 mg/kg) potentiated the anticonvulsant activity of CBZ. Valproate’s protective action was enhanced by one adenosine agonist – cycloheksyladenosine (1 mg/kg). Only the combination of CBZ + N6-benzyl-NECA (1 mg/kg) was resistant to aminophylline (5 mg/kg). Pharmacokinetic interactions were evident in case of the combination of CBZ + N6-benzyl-NECA (1 mg/kg) and resulted in an increased free plasma concentration of this CBZ. Interestingly, total brain concentration of CBZ confirmed the pharmacokinetic interaction as regards CBZ + N6-benzyl-NECA (1 mg/kg).

Conclusion.The best profile was shown by the combination of CBZ + 2-chloroadenosine which involved no AE or a pharmacokinetic interaction. The remaining positive combinations in terms of anticonvulsant activity were associated with general profound AE and pharmacokinetic interactions in some of them.

Role of the purinergic signaling in epilepsy

Adenine nucleotides and adenosine are signaling molecules that activate purinergic receptors P1 and P2. Activation of A1 adenosine receptors has an anticonvulsant action, whereas activation of A2A receptors might initiate seizures. Therefore, a significant limitation to the use of A1 receptor agonists as drugs in the CNS might be their peripheral side effects. The anti-epileptic activity of adenosine is related to its increased concentration outside the cell. This increase might result from the inhibition of the equilibrative nucleoside transporters (ENTs). Moreover, the implantation of implants or stem cells into the brain might cause slow and persistent increases in adenosine concentrations in the extracellular spaces of the brain. The role of adenosine in seizure inhibition has been confirmed by results demonstrating that in patients with epilepsy, the adenosine kinase (ADK) present in astrocytes is the only purine-metabolizing enzyme that exhibits increased expression. Increased ADK activity causes intensified phosphorylation of adenosine to 5'-AMP, which therefore lowers the adenosine level in the extracellular spaces. These changes might initiate astrogliosis and epileptogenesis, which are the manifestations of epilepsy. Seizures might induce inflammatory processes and vice versa. Activation of P2X7 receptors causes intensified release of pro-inflammatory cytokines (IL-1β and TNF-α) and activates metabolic pathways that induce inflammatory processes in the CNS. Therefore, antagonists of P2X7 and the interleukin 1β receptor might be efficient drugs for recurring seizures and prolonged status epilepticus. Inhibitors of ADK would simultaneously inhibit the seizures, prevent the astrogliosis and epileptogenesis processes and prevent the formation of new epileptogenic foci. Therefore, these drugs might become beneficial seizure-suppressing drugs.

The role of alpha-2 adrenoceptors in the anticonvulsant effects of adenosine on pentylenetetrazole-induced seizure threshold in mice

Adenosine has anticonvulsant effects in various models of seizures. Alpha-2 adrenoceptors have also demonstrated different effects in different models of epilepsy. In this study, the role of alpha-2 adrenoceptors in the anticonvulsant effects of adenosine in mice was determined according to the method of intravenous pentylenetetrazole-induced seizure. In this study, N(6)-cyclohexyladenosine (CHA) (a selective A1 receptor agonist), clonidine (an alpha-2 adrenoceptors agonist), yohimbine (an alpha-2 adrenoceptors antagonist) and 8-cyclopentyl-1,3-dimethylxanthine (8-CPT) (a selective A1 receptor antagonist) were used. CHA at doses of 0.5, 1 and 2mg/kg significantly increased seizure threshold with the maximum anticonvulsant effect at 2mg/kg. Yohimbine (0.1, 1 and 10mg/kg), clonidine (0.1, 0.5, 1 and 2mg/kg) and 8-CPT (0.5, 1, 2 and 4mg/kg) had no effect on seizure by itself. Combination of yohimbine (10mg/kg) and CHA (0.25mg/kg) increased clonic seizure latency showing that yohimbine and CHA have an additive effect. Increasing the seizure threshold created by combining ineffective doses of yohimbine (10mg/kg) and CHA (0.25mg/kg) was completely inhibited by 8-CPT (4mg/kg) or clonidine (1 and 2mg/kg). Clonidine (0.5, 1 and 2mg/kg) inhibited the anticonvulsant effects of CHA (2mg/kg). Combination of 8-CPT (1mg/kg) and clonidine (0.5mg/kg) which completely inhibited the anticonvulsant effect of CHA (2mg/kg) indicates that 8-CPT and clonidine have an additive effect. In conclusion, adenosine and yohimbine exhibit an additive effect on the enhancement of the pentylenetetrazole-induced seizure threshold in mice, indicating the interaction of alpha-2 adrenoceptors and A1 adenosine receptors.

The interaction of adenosine and morphine on pentylenetetrazole-induced seizure threshold in mice

Adenosine agonists or low doses of morphine exert anti-convulsant effects in different models of seizures. On the other hand, a tight interaction has been reported between morphine and adenosine in various paradigms. This study investigated the effect of the interaction of adenosine and morphine on seizure susceptibility in the intravenous mouse model of pentylenetetrazole (PTZ)-induced clonic seizures. The researchers used acute systemic administration of morphine, N(6)-cyclohexyladenosine (CHA) (a selective A1 receptor agonist), naltrexone (an opioid receptor antagonist) and 8-Cyclopentyl-1,3-dimethylxanthine (8-CPT) (a selective A1 receptor antagonist). Acute administration of morphine (0.25, 0.5 and 1 mg/kg) or CHA (0.25, 0.5, 1, 2 and 4 mg/kg) raised the threshold of seizures induced by PTZ. Non-effective dose of 8-CPT (2 mg/kg) inhibited the anticonvulsant effects of CHA (0.5 and 1 mg/kg). Combination of sub-effective doses of morphine (0.125 mg/kg) and CHA (0.125 mg/kg) increased clonic seizure latency showing the additive effect of morphine and CHA. The enhanced latency induced by combination of low doses of morphine and CHA completely reversed by 8-CPT (2 mg/kg) or naltrexone (1 mg/kg). Moreover, 8-CPT (2 mg/kg) inhibited anticonvulsant effects of morphine (0.25 and 0.5 mg/kg) and naltrexone (1 mg/kg) inhibited anticonvulsant effects of CHA (0.25, 0.5 and 1 mg/kg). Combination of low doses of 8-CPT (1 mg/kg) and naltrexone (0.5 mg/kg) inhibited the anticonvulsant effect of CHA (0.5 and 1 mg/kg). In conclusion, adenosine and morphine exhibit an additive effect on the enhancement of the pentylenetetrazole-induced seizure threshold in mice, probably through A1 or μ receptors.

Synergistic combinations of anticonvulsant agents: what is the evidence from animal experiments?

PURPOSE

Combination therapy is often used in the treatment of seizures refractory to monotherapy. At the same time, the pharmacodynamic mechanisms that determine the combined efficacy of antiepileptic drugs (AEDs) are unknown, and this prevents a rational use of these drug combinations. We critically evaluate the existing evidence for pharmacodynamic synergism between AEDs from preclinical studies in animal models of epilepsy to identify useful combinations of mechanisms and to determine whether study outcome depends on the various research methods that are in use.

METHODS

Published articles were included if the studies were placebo-controlled, in vivo, or ex vivo animal studies investigating marketed or experimental AEDs. The animal models that were used in these studies, the primary molecular targets of the tested drugs, and the methods of interpretation were recorded. The potential association of these factors with the study outcome (synergism: yes or no) was assessed through logistic regression analysis.

RESULTS

In total, 107 studies were identified, in which 536 interaction experiments were conducted. In 54% of these experiments, the possibility of a pharmacokinetic interaction was not investigated. The majority of studies were conducted in the maximal electroshock model, and other established models were the pentylenetetrazole model, amygdala kindling, and the DBA/2 model. By far the most widely used method for interpretation of the results was evaluation of the effect of a threshold dose of one agent on the median effective dose (ED50) of another agent. Experiments relying on this method found synergism significantly more often compared with experiments relying on other methods (p<0.001). Furthermore, experiments including antagonists of the AMPA receptor were more likely to find synergism in comparison with all other experiments (p<0.001).

CONCLUSIONS

Intensive preclinical research into the effects of AED combinations has not led to an understanding of the pharmacodynamic properties of AED combinations. Specifically, the majority of the preclinical studies are not adequately designed to distinguish between additive, synergistic, and antagonistic interactions. Quantitative pharmacokinetic-pharmacodynamic studies of selectively acting AEDs in a battery of animal models are necessary for the development of truly synergistic drug combinations.

Influences of caffeine to nitric oxide production and zonisamide concentration in the brain of seizure-susceptible EL mice

To investigate changes of nitric oxide (NO) productions and zonisamide (ZNS) concentrations in the brain of seizure-susceptible EL mice given caffeine orally, mice were given caffeine (600 microg/mL) solution ad libitum as a drinking fluid for 1-3 weeks. Nitric oxide production in the brain was determined by measuring levels of nitrite plus nitrate (NOx). The brain NOx levels of mice treated with caffeine for 3 weeks were significantly higher than the control. Seizures in mice treated with caffeine for 2 and 3 weeks were not suppressed by ZNS at a dose of 75 mg/kg. Serum ZNS concentrations of mice with caffeine intake for 1-3 weeks were higher than in untreated mice. Conversely, brain ZNS concentrations of mice with caffeine intake for the same periods were significantly lower than in untreated mice. These results suggested that caffeine influenced brain NO production and ZNS concentrations in the seizure susceptibility of EL mice.