Pharmacological Modulation of Ion Channels for the Treatment of Cystic Fibrosis

Abstract

Cystic fibrosis (CF) is a life-shortening monogenic disease caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR) protein, an anion channel that transports chloride and bicarbonate across epithelia. Despite clinical progress in delaying disease progression with symptomatic therapies, these individuals still develop various chronic complications in lungs and other organs, which significantly restricts their life expectancy and quality of life. The development of high-throughput assays to screen drug-like compound libraries have enabled the discovery of highly effective CFTR modulator therapies. These novel therapies target the primary defect underlying CF and are now approved for clinical use for individuals with specific CF genotypes. However, the clinically approved modulators only partially reverse CFTR dysfunction and there is still a considerable number of individuals with CF carrying rare CFTR mutations who remain without any effective CFTR modulator therapy. Accordingly, additional efforts have been pursued to identify novel and more potent CFTR modulators that may benefit a larger CF population. The use of ex vivo individual-derived specimens has also become a powerful tool to evaluate novel drugs and predict their effectiveness in a personalized medicine approach. In addition to CFTR modulators, pro-drugs aiming at modulating alternative ion channels/transporters are under development to compensate for the lack of CFTR function. These therapies may restore normal mucociliary clearance through a mutation-agnostic approach (ie, independent of CFTR mutation) and include inhibitors of the epithelial sodium channel (ENaC), modulators of the calcium-activated channel transmembrane 16A (TMEM16, or anoctamin 1) or of the solute carrier family 26A member 9 (SLC26A9), and anionophores. The present review focuses on recent progress and challenges for the development of ion channel/transporter-modulating drugs for the treatment of CF.

Keywords: CFTR modulators; ENaC; SLC26A9; TMEM16A; anionophores; drug development; precision medicine.

Figure 1

Overall structure of CFTR protein. CFTR structure is composed of five functional domains: two transmembrane domains (TMD1 and TMD2), two nucleotide-binding domains (NBD1 and NBD2) and an intrinsically disordered regulatory domain (RD). Ribbon diagram of two conformations of human CFTR: dephosphorylation, ATP-free conformation (left, PDB: 5UAK) (data from Liu et al) and phosphorylated, ATP-bound conformation (right, PDB: 6MSM) (data from Zhang et al). Notably, only a small portion of RD is depicted as most of its structure remains undetermined due to being intrinsically unstructured.

Figure 2

Site of action of the different CFTR modulator drugs. CFTR modulator drugs may be grouped into five main types according to their actions on CFTR mutations: read-through agents (for class I mutants), correctors (for class II mutants), potentiators (for classes III and IV mutants), amplifiers (for class V mutants, and possibly all others, except VII) and stabilizers (for class VI mutants). These molecules have a different putative site of action in order to correct specific defects in CFTR mutations. Some examples of promising CFTR modulators that are under experimental and clinical investigation have been provided (see text for further details). Notes: Adapted from Lopes-Pacheco M. CFTR modulators: the changing face of cystic fibrosis in the era of precision medicine. Front Pharmacol. 2020;10:1662. Copyright © 2020 Lopes-Pacheco. Creative Commons Attribution License (CC BY).

Figure 3

Modulation of ion channels/transporters as alternative therapies for CF. In healthy airways, CFTR, ENaC, TMEM16A and SLC26A9 are expressed at the plasma membrane (PM) of epithelial cells where they contribute to ion and water homeostasis. In CF airways, due to the absence of functional CFTR, Cl^– secretion is compromised and Na⁺ absorption is upregulated, leading to a dehydrated air surface liquid (ASL) and impaired mucociliary clearance (MCC). Expression of TMEM16A and SLC26A9 at the PM are also diminished in CF ciliated cells, although the role of TMEM16A overexpression in secretory cells and its role in mucus secretion are still controversial. Alternative therapies for CF thus include blocking ENaC, enhancing SLC26A9 expression at the PM, and modulating TMEM16A. Although for the latter is still not clear whether activators or inhibitors are beneficial, a TMEM16A potentiator in currently under clinical investigation.

Figure 4

Summary of different strategies to inhibit ENaC. (A) Indirect inhibition: SPX-101 is a peptide analogue mimicking the inhibitory actions of SPLUNC1; (B) Direct inhibition: both amiloride and BI 1265162 promote direct inhibition of ENaC by binding to channel and decreasing its open probability; (C) Genetic inhibition: IONIS-ENAC-2.5Rx is an antisense oligonucleotide that recruits RNase H to degrade ENaC mRNA and consequently decrease ENaC protein synthesis and channel function; ARO-ENaC is a small interfering RNA (siRNA) that also promotes degradation of ENaC mRNA transcripts by the RNA-induced silencing complex (RISC) mechanism.