Voltage-gated sodium channels are responsible for the initial phase of the action potential, which is a wave of electrical depolarisation usually initiated at the soma of the neuron and propagated along the axon to the terminals. At the terminals, the action potential triggers the influx of calcium and the release of neurotransmitter. Drugs, such as lidocaine, that block voltage-gated sodium channels are used as local anaesthetics. Other sodium channel blockers, such as lamotrigine and carbamazepine are used to treat epilepsy. In the latter case, partial inhibition of voltage-gated sodium channels reduces neuronal excitability and reduces seizure propagation. In the case of local anaesthetics, regional block of sodium channels on sensory neurons prevents the conduction of painful stimuli. A key feature of these drugs is their state-dependent mechanism of action. The drugs are thought to stabilise an inactivated conformation of the channel that is adopted rapidly after the channel opens. This inactivated state provides a refractory period before the channel returns to its resting (closed) state ready to be reactivated. As a result, state-dependent sodium channel blockers inhibit the firing of neurons at high frequency, for example in response to painful stimuli, and will help to prevent repetitive firing during periods of prolonged neuronal depolarisation that might occur, for example, during a seizure. Action potentials triggered at lower frequencies, for example in the heart, will not be significantly affected by these drugs, although the safety margin differs in each case, since at high enough concentrations each of these drugs is capable of blocking the resting or open states of the channels.
The voltage-gated sodium channel family is made up of 9 subtypes, four of which are found in the brain, NaV1.1, 1.2, 1.3 and 1.6. Of the other subtypes, NaV1.4 is found only in skeletal muscle, NaV1.5 is specific to cardiac muscle, and NaV1.7, 1.8, and 1.9 are found predominantly in sensory neurons. The hypothesised binding site for state-dependent sodium channel blockers is the local anaesthetic (LA) binding site in the inner vestibule of the pore on transmembrane S6 of domain IV. Critical residues are located in a highly conserved region among the different subtypes, thus presenting a challenge for the design of new subtype selective drugs. Drugs such as lidocaine, lamotrigine and carbamazepine do not distinguish between the subtypes. However, selectivity can be achieved, and can be further enhanced functionally, as a result of the different frequencies at which the channels operate.
Drugs that block voltage-gated sodium channels in a state-dependent manner are also used in the treatment of bipolar disorder, either to reduce symptoms of mania or depression, or as mood stabilisers to prevent the emergence of mood episodes. Clinical and preclinical evidence also suggests that state-dependent sodium channel blockers may help to reduce the symptoms of schizophrenia. For example, lamotrigine has been shown to reduce symptoms of psychosis induced by ketamine in healthy human volunteers, and furthermore, studies in patients suggest that the drug can augment the antipsychotic efficacy of some atypical antipsychotic drugs, such as clozapine or olanzapine. It is hypothesised that efficacy in these psychiatric disorders may result in part from a reduction of excessive glutamate release. The reduction in glutamate release is thought to be a consequence of sodium channel inhibition in key brain areas, such as the frontal cortex. However, interaction with voltage-gated calcium channels may also contribute to the efficacy of these drugs.
WO 2007/042240 (Glaxo Group Limited) describes a series of quaternary alpha-aminocarboxamide derivatives as modulators of voltage-gated sodium channels.
The object of the invention is to identify alternative compounds which modulate voltage-gated sodium channels.
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