5-Chloro-2-Guanidinobenzimidazole (ClGBI) Is a Non-Selective Inhibitor of the Human H(V)1 Channel

5-chloro-2-guanidinobenzimidazole (ClGBI), a small-molecule guanidine derivative, is a known effective inhibitor of the voltage-gated proton (H(+)) channel (H(V)1, K(d) ≈ 26 μM) and is widely used both in ion channel research and functional biological assays. However, a comprehensive study of its ion channel selectivity determined by electrophysiological methods has not been published yet. The lack of selectivity may lead to incorrect conclusions regarding the role of hHv1 in physiological or pathophysiological responses in vitro and in vivo. We have found that ClGBI inhibits the proliferation of lymphocytes, which absolutely requires the functioning of the K(V)1.3 channel. We, therefore, tested ClGBI directly on hK(V)1.3 using a whole-cell patch clamp and found an inhibitory effect similar in magnitude to that seen on hH(V)1 (K(d) ≈ 72 μM). We then further investigated ClGBI selectivity on the hK(V)1.1, hK(V)1.4-IR, hK(V)1.5, hK(V)10.1, hK(V)11.1, hK(Ca)3.1, hNa(V)1.4, and hNa(V)1.5 channels. Our results show that, besides H(V)1 and K(V)1.3, all other off-target channels were inhibited by ClGBI, with K(d) values ranging from 12 to 894 μM. Based on our comprehensive data, ClGBI has to be considered a non-selective hH(V)1 inhibitor; thus, experiments aiming at elucidating the significance of these channels in physiological responses have to be carefully evaluated.