Development and validation of a potent and specific inhibitor for the CLC-2 chloride channel

CLC-2 is a voltage-gated chloride channel that is widely expressed in many mammalian tissues. In the central nervous system (CNS), CLC-2 is expressed in neurons and glia. Studies to define how this channel contributes to normal and pathophysiological function in the CNS have been controversial, in part due to the absence of precise pharmacological tools for modulating CLC-2 activity. Herein, we describe the development and optimization of AK-42, a specific small-molecule inhibitor of CLC-2 with nanomolar potency (IC50 = 17 ± 1 nM). AK-42 displays unprecedented selectivity (>1000-fold) over CLC-1, the closest CLC-2 homolog, and exhibits no off-target engagement against a panel of 58 common channels, receptors, and transporters expressed in brain tissue. Computational docking, validated by mutagenesis and kinetic studies, indicates that AK-42 binds to an extracellular vestibule above the channel pore. In electrophysiological recordings of mouse CA1 hippocampal pyramidal neurons, AK-42 acutely and reversibly inhibits CLC-2 currents; no effect on current is observed on brain slices taken from CLC-2 knockout mice. These results establish AK-42 as a powerful new tool for investigating CLC-2 neurophysiology. Significance Statement The CLC-2 ion channel facilitates selective passage of Cl− ions across cell membranes. In the central nervous system (CNS), CLC-2 is expressed in both neurons and glia and is proposed to regulate electrical excitability and ion homeostasis. CLC-2 has been implicated in various CNS disorders, including certain types of epilepsy and leukodystrophy. Establishing a causative role for CLC-2 in neuropathologies, however, has been limited by the absence of selective reagents that enable acute and specific channel modulation. Our studies have resulted in the identification of a highly potent, small-molecule inhibitor that enables specific block of CLC-2 Cl− currents in hippocampal brain slices. This precise molecular tool should enable future efforts to identify and treat CLC-2-related disease. ### Competing Interest Statement The authors have declared no competing interest.