Alternative chloride transport pathways as pharmacological targets for the treatment of cystic fibrosis

Cystic fibrosis is a hereditary disease that originates from mutations in the epithelial chloride channel CFTR. Whereas established therapies for the treatment of cystic fibrosis target CFTR to repair its function, alternative therapeutic strategies aim for the restoration of chloride transport by the activation of other chloride transport proteins such as TMEM16A or SLC26A9 or by the application of synthetic anionophores. TMEM16A is an anion-selective channel that is activated by the binding of Ca$^{2+}$ from the cytoplasm. Pharmacological efforts aim for the increase of its open probability at resting Ca$^{2+}$ concentrations. SLC26 is an uncoupled chloride transporter, which shuttles chloride across the membrane by an alternate-access mechanism. Its activation requires its mobilization from intracellular stores. Finally, anionophores are small synthetic molecules that bind chloride to form lipid-soluble complexes, which shuttle the anion across the membrane. All three approaches are currently pursued and have provided promising initial results