Very high affinity ClC-2 blocker (apparent KD ~ 50 pM). Slows ClC-2 activation and inhibits slow-gating but does not inhibit open ClC-2 channels. Selective for ClC-2 over other ClC family members (ClC-0, ClC-1, ClC-3 and ClC-4), CFTR, GABAC, CaCC and KV1.