Organic Synthesis and Current Understanding of the Mechanisms of CFTR Modulator Drugs Ivacaftor, Tezacaftor, and Elexacaftor

The monogenic rare disease Cystic Fibrosis (CF) is caused by mutations in the gene encoding the CF transmembrane conductance (CFTR) protein, an anion channel expressed at the apical plasma membrane of epithelial cells. The discovery and subsequent development of CFTR modulators-small molecules acting on the basic molecular defect in CF-have revolutionized the standard of care for people with CF (PwCF), thus drastically improving their clinical features, prognosis, and quality of life. Currently, four of these drugs are approved for clinical use: potentiator ivacaftor (VX-770) alone or in combination with correctors lumacaftor, (VX-809), tezacaftor (VX-661), and elexacaftor (VX-445). Noteworthily, the triple combinatorial therapy composed of ivacaftor, tezacaftor, and elexacaftor constitutes the most effective modulator therapy nowadays for the majority of PwCF. In this review, we exploit the organic synthesis of ivacaftor, tezacaftor, and elexacaftor by providing a retrosynthetic drug analysis for these CFTR modulators. Furthermore, we describe the current understanding of the mechanisms of action (MoA's) of these compounds by discussing several studies that report the key findings on the molecular mechanisms underlying their action on the CFTR protein.

Elexacaftor/VX-445-mediated CFTR interactome remodeling reveals differential correction driven by mutation-specific translational dynamics

Cystic fibrosis (CF) is one of the most prevalent lethal genetic diseases with over 2000 identified mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Pharmacological chaperones such as lumacaftor (VX-809), tezacaftor (VX-661), and elexacaftor (VX-445) treat mutation-induced defects by stabilizing CFTR and are called correctors. These correctors improve proper folding and thus facilitate processing and trafficking to increase the amount of functional CFTR on the cell surface. Yet, CFTR variants display differential responses to each corrector. Here, we report that variants P67L and L206W respond similarly to VX-809 but divergently to VX-445 with P67L exhibiting little rescue when treated with VX-445. We investigate the underlying cellular mechanisms of how CFTR biogenesis is altered by correctors in these variants. Affinity purification-mass spectrometry multiplexed with isobaric tandem mass tags was used to quantify CFTR protein-protein interaction changes between variants P67L and L206W. VX-445 facilitates unique proteostasis factor interactions especially in translation, folding, and degradation pathways in a CFTR variant-dependent manner. A number of these interacting proteins knocked down by siRNA, such as ribosomal subunit proteins, moderately rescued fully glycosylated P67L. Importantly, these knockdowns sensitize P67L to VX-445 and further enhance the trafficking correction of this variant. Partial inhibition of protein translation also mildly sensitizes P67L CFTR to VX-445 correction, supporting a role for translational dynamics in the rescue mechanism of VX-445. Our results provide a better understanding of VX-445 biological mechanism of action and reveal cellular targets that may sensitize nonresponsive CFTR variants to known and available correctors.

KCa3.1 potentiation stimulates Cl- secretion in F508del and G551D CFTR corrected primary human bronchial epithelial cells

We previously identified potentiators of KCa3.1 (DCEBIO) that stimulate Cl^- secretion across HBEs expressing wild type (WT) CFTR. However, these compounds failed to stimulate Cl^- secretion in F508del CFTR HBEs. Drug discovery efforts identified CFTR potentiators (VX-770) and correctors (VX-445, VX-661) for CF disease causing mutations, including F508del and G551D. Herein, we evaluated the effect of KCa3.1 potentiation on Cl^- equivalent current (I_Cl) across primary HBEs expressing WT, F508del and G551D CFTR. Transepithelial impedance analysis was used to obtain estimates of apical (R_a) and basolateral membrane (BLM; R_b) resistances. In WT CFTR HBEs, DCEBIO stimulated I_Cl, which was increased by forskolin. Similarly, forskolin stimulated I_Cl, and this was increased by DCEBIO. The KCa3.1 blocker, TRAM-34 inhibited I_Cl. DCEBIO decreased R_b, whereas TRAM-34 increased R_b, consistent with BLM localization of KCa3.1. Following correction of F508del CFTR with VX-445+VX-661, DCEBIO failed to stimulate I_Cl, although the subsequent addition of forskolin+VX-770 increased I_Cl. Importantly, following stimulation of I_Cl with forskolin+VX-770, DCEBIO induced a further significant increase in I_Cl. As above, DCEBIO reduced R_b, whereas TRAM-34 increased R_b, consistent with BLM localized KCa3.1. Finally, we assessed KCa3.1 potentiation on I_Cl in G551D/F508del CFTR HBEs in the absence or presence of VX-445+VX-661. In both cases, DCEBIO failed to stimulate I_Cl. However, following stimulation with forskolin+VX-770, DCEBIO nearly doubled I_Cl. Our results demonstrate, following correction/potentiation of F508del and G551D CFTR, potentiation of KCa3.1 increases the Cl^- secretory response, suggesting this class of compounds may represent a novel means of further increasing Cl^- secretion across CF airway.

Partial Rescue of F508del-CFTR Stability and Trafficking Defects by Double Corrector Treatment

Deletion of phenylalanine at position 508 (F508del) in the CFTR chloride channel is the most frequent mutation in cystic fibrosis (CF) patients. F508del impairs the stability and folding of the CFTR protein, thus resulting in mistrafficking and premature degradation. F508del-CFTR defects can be overcome with small molecules termed correctors. We investigated the efficacy and properties of VX-445, a newly developed corrector, which is one of the three active principles present in a drug (Trikafta^®/Kaftrio^®) recently approved for the treatment of CF patients with F508del mutation. We found that VX-445, particularly in combination with type I (VX-809, VX-661) and type II (corr-4a) correctors, elicits a large rescue of F508del-CFTR function. In particular, in primary bronchial epithelial cells of CF patients, the maximal rescue obtained with corrector combinations including VX-445 was close to 60–70% of CFTR function in non-CF cells. Despite this high efficacy, analysis of ubiquitylation, resistance to thermoaggregation, protein half-life, and subcellular localization revealed that corrector combinations did not fully normalize F508del-CFTR behavior. Our study indicates that it is still possible to further improve mutant CFTR rescue with the development of corrector combinations having maximal effects on mutant CFTR structural and functional properties.