SKA-111, a Positive KCa Channel Gating Modulator with Selectivity for KCa3.1

Abstract

Intermediate and small-conductance Ca2+ activated potassium (KCa) channels play an important role in regulating membrane excitability and Ca2+ signaling. Pharmacological activation of these voltage-independent channels has therefore been suggested for the treatment of various diseases. While KCa2 activators can reduce neuronal excitability in CNS disorders, KCa3.1 activators are being discussed as endothelial targeted antihypertensives. KCa channels are gated by Ca2+ binding to a constitutively associated calmodulin at the C-terminus. Recent work (Zheng M. 2012 Nat Commun 3:1021) demonstrated that the benzimidazolone 1-EBIO binds at the interface of the CAM N-lobe and the calmodulin binding domain (CAMBD) of KCa2.2. Mutations of A477V/L480M and A477I in KCa2.2 were shown to increase and decrease potency of 1-EBIO, respectively. Here we report that the same mutations (A625V/L628M and A625I) in KCa2.3 also alter the potency of KCa2/3 activators of the benzothiazole class such as our recently developed SKA-31, a pharmacological tool compound that has been widely used to study the role of KCa2 and KCa3.1 channels. However, like 1-EBIO and NS309, SKA-31 suffers from a lack of selectivity between KCa3.1 and KCa2 channels making it difficult to interpret in vivo observations made with this compound. Via a structure activity relationship (SAR) study we have optimized the pharmacophore of SKA-31 and have now identified SKA-111 (5-methylnaphtho[1,2-d]thiazol-2-amine), which displays 160-fold selectivity for KCa3.1 (EC50 = 118 ± 29 nM) over KCa2.3 (EC50 = 11.2 ± 4 μM). SKA-111 constitutes a novel pharmacological tool to study the role of KCa3.1 in blood pressure regulation. SKA-111 selectively potentiates native KCa3.1 currents in porcine carotid endothelium and lowers blood pressure in mice. SKA-111 should further be useful to help understand what structural elements confer selectivity between KCa3.1 and KCa2 activation to the benzothiazole pharmacophore.