Small-conductance calcium activated potassium channels are encoded by the KCa2.1-2.3 (= SK1-3) genes and are best known for underlying the apamin-sensitive medium afterhyperpolarization current (mAHP) in neurons. Depending on the type of neuron, the function of KCa2 channels varies from determining instantaneous firing rates, over setting tonic firing frequencies, to regulating burst firing and potentially catecholamine release. Pharmacological modulation of KCa channels therefore offers the opportunity to significantly affect neuronal excitability. While KCa2 channel blockers like the bee venom apamin increase firing rates and induce seizures in rodents, KCa2 channel activators slow down neuronal firing and have therefore been proposed for the treatment of CNS disorders that are characterized by hyperexcitability such as epilepsy, ataxia, and neuropathic pain. However, this compelling therapeutic hypothesis currently remains largely untested because none of the existing KCa2 channel activators such as EBIO (EC50 300 μM) or NS309 are suitable for in vivo use. Using the neuroprotective drug riluzole as a synthetic template, our laboratory recently designed SKA-31 (EC50 2 uM), the first KCa2 channel activator, which is potent enough to be used in vivo, and demonstrated in collaboration with the NIH Anticonvulsant Screening Program (ASP) that the compound and several of its derivatives are effective anticonvulsants. Unfortunately, SKA-31 also activates KCa3.1 channels, which are expressed on vascular endothelium, and thus reduces blood pressure in mice. Using a combination of classical medicinal chemistry and automated and manual electrophysiology we intend to further explore the structure activity relationship around SKA 31 and EBIO in order to improve selectivity for KCa2 over KCa3.1 as well as potency and brain penetration. The best new KCa2 activators will then be evaluated for selectivity over a panel of cloned ion channels and characterized for activity on native KCa2 channels using hippocampal slices. Compounds selectively activating cloned and native KCa2 channels will further be evaluated for pharmacokinetic properties and brain penetration in rats using HPLC/MS. In parallel, we will submit selected compounds to the ASP, where the compounds will we tested in acute seizure models. Promising compounds will then be tested in amygdala kindled mice and rats with kainate-induced epilepsy, two models that are more representative of human refractory epilepsy. The design of brain penetrant and potentially subtype selective KCa2 channel activators would help to validate KCa2 channels as novel pharmacological targets for the treatment of epilepsy and would further provide the scientific community with tool compounds to study the role of KCa2 channels in ataxia, neuropathic pain and cognition.
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