Retigabine for Seizures

Retigabine for SeizuresDefinitionEzogabine ( get together States) or retigabine (Europe) has the chemical name N-2-amino-4-(4-fluorobenzylamino) phenyl carbamic dot ethyl ester.It was shown that the anticonvulsive action of ezogabine is a consequence of its ability to open the KCNQ (Kv7)- sheath green channels that are quiet of four subunits and are found in the membranes of neurons 2. The importance of this type of potassium channels in the ripening of epilepsy was understood only after its dysfunction was linked to benign familial neonatal convulsions 3,4. Ezogabine binds to all subtypes of the KCNQ potassium channels, including the KCNQ2-5- subtypes that are expressed in brain tissue. The especially heteromeric KCNQ2/3and KCNQ3/5 potassium channels are extremely expressed in the central nervous system and more important in the control of neuronic excitability than other types because they mediate M currents 5,6. Ezogabine is the only anticonvulsant that has a machine of act ion that involves disruption potassium channels. This fact was the initial rationale for the drugs development, as researchers sought to produce a new drug that would be effective in patients with drug therapy-resistant epilepsy. Ezogabine is the first antiepileptic drug with this mechanism of action and was developed to fill this gap and help with the treatment of drug-resistant patients. Finally, clinical trials documented the effectiveness of ezogabine as adjuvant therapy for the reduction of partial-onset seizure frequency, and it received marketing authorization in 2011 in both the United States and Europe 17The propertiesEzogabine is a poorly water soluble small molecule (molecular weight 303.3). . The standard system for identification of ezogabine is reverse phase high performance liquid chromatography (RP-HPLC) with or without mass spectrometry employ electro spray ionization and quadrupole time-of-flight mass analyzer 19. The results of ezogabine identification could b e confirmed by nuclear magnetized resonance spectroscopy and infrared spectroscopy 19.Mechanism of actionThe mechanism of action of ezogabine was investigated using the electrophysiological measurements of loft currents in Chinese hamster ovary (CHO) cells (transfected with cDNA encoding humanM1 muscarinic receptors,KCNQand CD8cDNAs) and rat ganglion cells, using the amphotericin-B perforatedpatch technique 22. The effectuate of ezogabine were tested on ionic currents from both heteromeric KCNQ2/3 and homomeric KCNQ1, KCNQ2, KCNQ3 and KCNQ4 potassium channels. The potassium currents were enhanced by ezogabine through all types of channels, merely KCNQ1. The effects of ezogabine were in any case tested in a nonher study on HEK293 cells stably transfected with KCNQ5 channels 23 it was shown that ezogabine concentration-dependently also enhances the potassium current through this type of channel. After binding for the KCNQ channel, ezogabine decreases the membranepotential thresho ld for activating of the channel (makes it more negative, i.e., shifts it toward the hyperpolarizing end) and increases the probability of maximum opening of the KCNQ channel 22,23. All of these effects make neurons slight prone to depolarization and activation.Pharmacokinetics and metabolismThe absolute bioavailability of orally-administered RTG is 60% 34. Absorption is rapid and Cmax is typically reached 30 cxx min after a single oral dose 35-38. Cmax is moderately increased when RTG is taken with a high-fat meal, while the area under the plasma concentration-time curve (AUC) remains unchanged 39. RTG is rapidly eliminated following first-order kinetics, with a terminal half-life of approximately 7.5 h the head is 0.58 0.76 lh-1kg-1 35-37. Young women may reach 30% higher Cmax values than men after accounting for body weight, but weight adjusted clearance is similar 36. RTG has a large volume of distribution (8.7 lkg-1) and a plasma protein binding of some 80% 35,39. In he althy volunteers, RTG pharmacokinetics is linearly dose-proportional for doses of 50 350 g BID and not affected by multiple administrations indicating that RTG does not influence its own metabolism 35. In epilepsy patients, RTG dose linearity was demonstrated everywhere the therapeutic dose range of 200 400 mg t.i.d. 35,40. RTG is not metabolized by cytochrome P450 (CYP) enzymes 41,42. Rather, it is extensively metabolized by N-glucuronidation, leading to organization of two major N-glucuronides, and to a smaller extent by N-acetylation, leading to formation of the N-acetyl metabolite of retigabine (NAMR, formerly AWD21-360), which also undergoes glucuronidation 41,42. In vitro assays using recombinant human UDPglucuronosyltransferases (UGTs) identified UGT1A1, 1A9, 1A4, and 1A3 as the principal enzymes catalyzing RTG glucuronidation 43,44. In subjects with reduce UGT1A1 metabolic capacity (Gilberts syndrome), RTG pharmacokinetics is unchanged and N-acetylation partially compens ates for reduced glucuronidation, in addition to compensation by other UGT1A isoforms 45. Other UGT1 and UGT2 family members also appear to be involved, because complete loss of UGT1A metabolic capacity (Crigler-Najjar type II) did not completely abolish RTG glucuronidation in vitro 44. Individuals with genetic polymorphisms in the N-acetyltransferase 2 (NAT2) gene leading to a slow acetylation phenotype had a lower total exposure to NAMR than rapid acetylators 45. gum elastic and tolerability of RTG treatment is apparently not altered in subjects with Gilberts syndrome or slow acetylation polymorphisms 45. RTG and its metabolites are primarily excreted by the kidney 42,45. A 30% slower RTG clearance in elderly subjects vs. young subjects is consistent with the known decline of renal function with age 36. Systemic RTG clearance can be reduced by 50% in patients with moderate/severe impairment of liver function (Child-Pugh score 7) or kidney function (creatinine clearance Safety an d tolerabilityThree Phase III clinical trials were pivotal to the eventual approval of ezogabine for use to treat seizures. The first was a study by Porter et al. during which placebo or ezogabineat daily doses of 600, 900, or 1200 mg was administered 9. gear up 1 compared placebo to ezogabine 1200 mg/day, and RESTORE 2 compared subjects taking 600 or 900 mgof ezogabine daily with those taking placebo 4,10. As would be expected and consistent with other antiepileptic agents, the most commonly observed adverse effects seen with ezogabine use are CNS-related. At least 10% of study participants in Phase III clinical trials experienced somnolence, fatigue, sloppiness, dizziness, tremor, abnormal thinking, vertigo,speech disorders, or amnesia 4,9,10. Most of these adverse effects appear to be related to dose, and are typically reported to be mild or moderate in nature. As in the Phase III trials, the majority (62%) of adverse events were CNS-related. In addition, several ezogabine user s experienced neuropsychiatric events including hallucinations (2%), confusion (9%), and psychosis (1%) 12. The majority of these episodes occurred within the first two months of drug exposure in individuals with no documented tale of a psychiatric disorder.