Synthesis and Structure−Activity Relationship Studies of 2-(N-Substituted)-aminobenzimidazoles as Potent Negative Gating Modulators of Small Conductance Ca2+-Activated K+ Channels

Small conductance Ca2+-activated K+ channels (SK channels) participate in the control of neuronal excitability, in the shaping of action potential firing patterns, and in the regulation of synaptic transmission. SK channel inhibitors have the potential of becoming new drugs for treatment of various psychiatric and neurological diseases such as depression, cognition impairment, and Parkinson's disease. In the present study we describe the structure−activity relationship (SAR) of a class of 2-(N-substituted)-2-aminobenzimidazoles that constitute a novel class of selective SK channel inhibitors that, in contrast to classical SK inhibitors, do not block the pore of the channel. The pore blocker apamin is not displaced by these compounds in binding studies, and they still inhibit SK channels in which the apamin binding site has been abolished by point mutations. These novel SK inhibitors shift the concentration−response curve for Ca2+ toward higher values and represent the first example of negative gating modulation as a mode-of-action for inhibition of SK channels. The first described compound in this class is NS8593 (14), and the most potent analogue identified in this study is the racemic compound 39 (NS11757), which reversibly inhibits SK3-mediated currents with a Kd value of 9 nM.