Retigabine

Absorption

Rapidly absorbed and distributed, with an absolute oral bioavailability of 60%. Pharmacokinetics of ezogabine suggest first-order kinetics. Tmax, single oral dose = 30-120 minutes; Time to steady state = 3 days

Toxicity

Lethal Dose, acute, oral, rat = 100 mg/kg; Lethal Dose, chronic, oral, rat = 5.1 mg/kg/day, 90-day; Most common adverse effects that lead to discontinuation of therapy include dizziness and somnolence.

Description

Retigabine was approved in March 2011 by the European Commission for the adjunctive treatment of partial-onset seizures in adults who have epilepsy; in June 2011, the U.S. FDA approved the same drug, known in the United States as ezogabine. Retigabine differs from all currently approved antiepileptic drugs in that it acts as a selective positive allosteric modulator (opener) of KCNQ2-5 potassium channels, which are key regulators of neuronal excitability. The discovery of retigabine was based on modification of an analgesic agent flupirtine that had serendipitously shown potent anticonvulsant activity in animal models of epilepsy. Changing a central 2,3,6-triaminopyridine to a 1,2,4-triaminobenzene decreased analgesic activity while enhancing antiepileptic activity. Retigabine (D-23129) was shown to be a broad spectrum anticonvulsant with oral activity in a variety of animal models. The mechanism of action of retigabine was discovered well after its in vivo activity was recognized. Retigabine can be prepared by reductive amination of 2-ethoxycarbonylamino-5-(4-fluorobenzylamino)- nitrobenzene with 4-fluorobenzaldehyde, followed by hydrogenation in the presence of Raney nickel.

Chemical properties

Purple Solid

Originator

ASTA Medica group (Germany)

The Uses of Retigabine

Retigabine may be used as a reference standard in the determination of retigabine in biological matrices using high-performance liquid chromatography coupled with tandem mass spectrometry with positive ion atmospheric pressure chemical ionization (HPLC-APCI-MS/MS).

Background

Ezogabine (D23129) is a close structural analog of the centrally acting analgesic flupitrine. It is a neuronal potassium channel opener being developed as a first-in-class antiepileptic drug (AED) and is currently being studied in Phase 3 trials as an adjunctive treatment for partial-onset seizures in adult patients with refractory epilepsy. FDA approved in June 10, 2011 under the name of ezogabine.

Indications

Adjuvant treatment of partial-onset seizures.

Definition

ChEBI: A substituted aniline that is benzene-1,2,4-triamine bearing ethoxycarbonyl and 4-fluorobenzyl substituents at positions N-1 and N-4 respectively. An anticonvulsant used to treat seizures associated with epilepsy in adults.

brand name

Trobalt (European Union), Potiga

General Description

Retigabine is an antiepileptic drug, which activates neuronal KCNQ-type K+ channels by inducing a large hyperpolarizing shift of steady-state activation. Retigabine can be prepared by reductive amination of 2-ethoxycarbonylamino-5-(4-fluorobenzylamino)- nitrobenzene with 4-fluorobenzaldehyde, followed by hydrogenation in the presence of Raney nickel.

Biochem/physiol Actions

Retigabine (Ezogabine) is a Kv7.2-7.5 (KCNQ2-5) neuronal potassium channel opener witrh anticonvulsant activity.

Mechanism of action

Retigabine has a novel mechanism of action for an antiseizure drug, acting as positive allosteric modulator of the neuronal potassium channels KNCQ (Kv2 to 5).  Under normal physiologic conditions, KNCQ channels help establish the  neuronal resting membrane potential, by providing a continual  hyperpolarizing influence.

Pharmacokinetics

As compared to other antiepileptic agents, ezogabine is unique in that it selectively activates potassium ion channels Kv 7.2-Kv7.5 and not cardiac Kv 7.1, thereby avoiding cardiac side effects. The antiepileptics, as a drug class, are routinely used in the treatment of a number of disease states in addition to epilepsy. Ezogabine is highly efficacious in a broad-spectrum of in vivo epilepsy and seizure models. A comparison of antiepileptic form activity of ezogabine with that of conventional anticonvulsants in in vitro models suggests that retigabine is especially likely to be useful in the treatment of pharmacoresistant epilepsy. Retigabine clearly attenuates pain-like behaviors in various animal models of neuropathic pain; it may also prove to be useful in treatment of clinical anxiety disorders. Clinical data obtained thus far indicate that retigabine is well tolerated in humans when titrated up to its therapeutic dose range. No tolerance, drug dependence, or withdrawal liability has been reported. Thus, retigabine may prove to be useful in the treatment of a diverse range of disease states in which neuronal hyperexcitability is a common underlying factor.

Pharmacology

Retigabine is quickly absorbed, and reaches maximum plasma concentrations between half an hour and 2 hours after a single oral dose. It has a moderately high oral bioavailability (50–60%), a high volume of distribution (6.2 L/kg), and a terminal half-life of 8 to 11 hours. Retigabine requires thrice-daily dosing due to its short half-life.
Retigabine is metabolized in the liver, by N-glucuronidation and acetylation. The cytochrome P450 system is not involved. Retigabine and its metabolites are excreted almost completely (84%) by the kidneys.

Clinical Use

Retigabine is an investigational antiepileptic drug with a novel mechanism of action that involves opening of neuronal voltageactivated K+ channels that serves to stabilize the membrane potential and to control neuronal excitability. Thus, retigabine also may prove to be useful in the treatment of other diseases associated with neuronal hyperexcitability. GSK (GlaxoSmithKline) intends to permanently stop producing this product, trobalt? (retigabine) tablets (50 mg, 100 mg, 200 mg, 300 mg and 400 mg) was no longer be available after June 2017. This is due to the very limited use of the drug, not for reasons of efficacy or safety.

Side Effects

Dizziness, somnolence, fatigue, confusional state, aphasia, abnormal coordination, tremor, balance disorder, memory impairment, gait disturbance, blurred vision, constipation, anxiety, psychotic disorders, paraesthesia, wt gain, nausea, dyspepsia, urinary retention and hesitation, dysuria; discolouration of ocular tissues (including the retina), skin, lips, and nails; prolongation of QT interval.

Synthesis

Commercially available 4-fluorobenzaldehyde (188) was condensed with 4-amino-2-nitroaniline (189) and the resulting imine was reduced with sodium borohydride to give aniline 190 in 79¨C85% yield. Aniline 190 was acylated with diethylcarbonate (191) to give nitrobenzene 192 in 80¨C88% yield. Reduction of the nitro group via catalytic hydrogenation provided retigabine/ezogabine (XVIII) in 70¨C90% yield. An alternative method (not shown) involved initial reduction of nitrobenzene 190 with Zn/NH4Cl followed by acylation with diethylcarbonate (191) or ethyl chloroformate/Hunig?ˉs base, providing retigabine/ezogabine (XVIII) in 46¨C81% overall yield.

Metabolism

Ezogabine is metabolized exclusively via phase II hepatic N-glucurodination and acetylation. N-glucurodination is the major metabolic pathway of the two and form two major N-glucuronide metabolites. The enzymes involved are UGT1A1, 1A9, 1A4, and 1A3. However, the product of the N-acetyl pathway is a weak, active metabolite referred to as NAMR. The enzyme that is involved in the N-acetyl pathway is called N-acetyltransferase 2. The pharmacokinetics of NAMR and ezogabine are similar. The cytochrome P450 enzyme system is not involved with the metabolism of ezogabine.

storage

Store at +4°C