A four-ligand hypercube model to quantify allosteric interactions within the GABAA receptor complex

The mechanism of action of retigabine (ezogabine), a first-in-class K+ channel opener for the treatment of epilepsy

The pharmacologic profile of retigabine [RTG (international nonproprietary name); ezogabine, EZG (U.S. adopted name)], is different from all currently approved antiepileptic drugs (AEDs). Its primary mechanism of action (MoA) as a positive allosteric modulator of KCNQ2-5 (K(v) 7.2-7.5) ion channels defines RTG/EZG as the first neuronal potassium (K(+)) channel opener for the treatment of epilepsy. KCNQ2-5 channels are predominantly expressed in neurons and are important determinants of cellular excitability, as indicated by the occurrence of human genetic mutations in KCNQ channels that underlie inheritable disorders including, in the case of KCNQ2/3, the syndrome of benign familial neonatal convulsions. In vitro pharmacologic studies demonstrate that the most potent action of RTG/EZG is at KCNQ2-5 channels, particularly heteromeric KCNQ2/3. Furthermore, mutagenesis and modeling studies have pinpointed the RTG/EZG binding site to a hydrophobic pocket near the channel gate, indicating how RTG/EZG can stabilize the open form of KCNQ2-5 channels; the absence of this site in KCNQ1 also provides a clear explanation for the inbuilt selectivity RTG/EZG has for potassium channels other than the KCNQ cardiac channel. KCNQ channels are active at the normal cell resting membrane potential (RMP) and contribute a continual hyperpolarizing influence that stabilizes cellular excitability. The MoA of RTG/EZG increases the number of KCNQ channels that are open at rest and also primes the cell to retort with a larger, more rapid, and more prolonged response to membrane depolarization or increased neuronal excitability. In this way, RTG/EZG amplifies this natural inhibitory force in the brain, acting like a brake to prevent the high levels of neuronal action potential burst firing (epileptiform activity) that may accompany sustained depolarizations associated with the initiation and propagation of seizures. This action to restore physiologic levels of neuronal activity is thought to underlie the efficacy of RTG/EZG as an anticonvulsant in a broad spectrum of preclinical seizure models and in placebo-controlled trials in patients with partial epilepsy. In this article, we consider the pharmacologic characteristics of RTG/EZG at the receptor, cellular, and network levels as a means of understanding the novel and efficacious MoA of this new AED as defined in both preclinical and clinical research.

Allosteric modulation of [3H]EBOB binding to GABAA receptors by diflunisal analogues

Allosteric modulatory effects of 12 biphenyl derivatives of diflunisal and two fenamates were studied on A-type receptors of GABA (GABAAR) via [3H]4'-ethynylbicycloorthobenzoate (EBOB) binding to synaptic membrane preparations of rat forebrain. A simplified ternary allosteric model was used to determine binding affinities of the compounds and the extents of cooperativity with GABA. Structure activity analysis revealed that 4-hydroxy substituents of the biphenyls contribute to their micromolar binding affinities more than 3-carboxyl groups. Electron-withdrawing fluorinated substituents, especially in ortho position, were also advantageous. These factors also strongly enhanced the cooperativity with GABA binding. The correlation between displacing potency of the allosteric agents and cooperativity with GABA suggests that these processes are associated with common mechanisms. The pharmacological relevance of these interactions is discussed. These data help to differentiate the structural requirements of these agents to act on GABAergic neurotransmission versus nonsteroidal anti-inflammatory effects.

Allosteric modulation of 5-HT3 serotonin receptors

[(3)H]Granisetron binding to 5-HT(3) type serotonin receptors was examined in homogenates of rat forebrain and NG 108-15 cells. We have applied an allosteric model to 5-HT(3) receptor binding for the first time. Slope factors of displacement improved the modelling. Serotonin displaced [(3)H]granisetron binding with micromolar potency in forebrain and with nanomolar potency in NG 108-15 cells. Racemic and (+)verapamil, ifenprodil and GYKI-46903 were used as representative allosteric inhibitors of 5-HT(3) receptors. They displaced [(3)H]granisetron binding with great negative cooperativity (alpha>10) and exerted great negative cooperativity with serotonin binding (beta>10). Great negative cooperativity of these agents with serotonin and [(3)H]granisetron binding cannot be distinguished from dual competitive displacement. Trichloroethanol (data from literature) had no cooperativity with [(3)H]granisetron binding (alpha~1) and exhibit positive cooperativity with serotonin (beta<1) in displacement. The allosteric model can lead to a more quantitative method in vitro to develop allosteric agents for 5-HT(3) receptors.