Synthesis and Quantitative Structure−Activity Relationship of a Novel Series of Small Conductance Ca2+-Activated K+ Channel Blockers Related to Dequalinium

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Abstract
The synthesis, pharmacological testing, and quantitative structure−activity relationship studies of a novel series of bisquinolinium small conductance Ca2+-activated K+ channel blockers (23) related to dequalinium are described. In this series, two quinolinium rings are linked via the 4-position to an α,ω-diamino alkylene chain and the ring N atom is quaternized with a methyl or benzyl group. The exocyclic N atom can be replaced by O, S, or CH2 but with some loss of potency. The quinoline groups do not have to be quaternized for blocking activity, as long as they are basic enough to be protonated at the site of action. For the quaternary compounds, there is considerable steric tolerance for the group R attached to the ring N atom of the quinoline; a benzyl group gave the optimum potency in this series. Moreover, and in contrast to previously reported results for dequalinium analogues, there is no correlation of activity with N1 charge or EHOMO. On the other hand, a good correlation was obtained between the blocking potency of the compounds and ELUMO [pEMR = 1.16(±0.26)ELUMO + 5.33(±1.29) (n = 11, r = 0.83, s = 0.243)]. It has been possible to combine this equation with the previously reported ELUMO correlation for a series of dequalinium analogues to include all the compounds of both series [pEMR = 1.17(±0.15)ELUMO + 5.33(±0.76) (n = 24, r = 0.85, s = 0.249)]. A possible physical meaning for the ELUMO correlation based upon the principle of maximum hardness is discussed.