The anti-protozoal drug pentamidine blocks K _IR2.x-mediated inward rectifier current by entering the cytoplasmic pore region of the channel: Research paper

Background and purpose: Pentamidine is a drug used in treatment of protozoal infections. Pentamidine treatment may cause sudden cardiac death by provoking cardiac arrhythmias associated with QTc prolongation and U-wave alterations. This proarrhythmic effect was linked to inhibition of hERG trafficking, but not to acute block of ion channels contributing to the action potential. Because the U-wave has been linked to the cardiac inward rectifier current (I _K1), we examined the action and mechanism of pentamidine-mediated I _K1 block. Experimental approach: Patch clamp measurements of I _K1 were made on cultured adult canine ventricular cardiomyocytes, K _IR2.1-HEK293 cells and K _IR2.x inside-out patches. Pentamidine binding to cytoplasmic amino acid residues of K _IR2.1 channels was studied by molecular modelling. Key results: Pentamidine application (24 h) decreased I _K1 in cultured canine cardiomyocytes and K _IR2.1-HEK293 cells under whole cell clamp conditions. Pentamidine inhibited I _K1 in K _IR2.1-HEK293 cells 10 min after application. When applied to the cytoplasmic side under inside-out patch clamp conditions, pentamidine block of I _K1 was acute (IC ₅₀ = 0.17 μM). Molecular modelling predicted pentamidine-channel interactions in the cytoplasmic pore region of K _IR2.1 at amino acids E224, D259 and E299. Mutation of these conserved residues to alanine reduced pentamidine block of I _K1. Block was independent of the presence of spermine. K _IR2.2, and K _IR2.3 based I _K1 was also sensitive to pentamidine blockade. Conclusions and implications: Pentamidine inhibits cardiac I _K1 by interacting with three negatively charged amino acids in the cytoplasmic pore region. Our findings may provide new insights for development of specific I _K1 blocking compounds.