B. Burton

Correction of the F508del-CFTR protein processing defect in vitro by the investigational drug VX-809

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene that impair the function of CFTR, an epithelial chloride channel required for proper function of the lung, pancreas, and other organs. Most patients with CF carry the F508del CFTR mutation, which causes defective CFTR protein folding and processing in the endoplasmic reticulum, resulting in minimal amounts of CFTR at the cell surface. One strategy to treat these patients is to correct the processing of F508del-CFTR with small molecules. Here we describe the in vitro pharmacology of VX-809, a CFTR corrector that was advanced into clinical development for the treatment of CF. In cultured human bronchial epithelial cells isolated from patients with CF homozygous for F508del, VX-809 improved F508del-CFTR processing in the endoplasmic reticulum and enhanced chloride secretion to approximately 14% of non-CF human bronchial epithelial cells (EC50, 81 ± 19 nM), a level associated with mild CF in patients with less disruptive CFTR mutations. F508del-CFTR corrected by VX-809 exhibited biochemical and functional characteristics similar to normal CFTR, including biochemical susceptibility to proteolysis, residence time in the plasma membrane, and single-channel open probability. VX-809 was more efficacious and selective for CFTR than previously reported CFTR correctors. VX-809 represents a class of CFTR corrector that specifically addresses the underlying processing defect in F508del-CFTR.

Ivacaftor potentiation of multiple CFTR channels with gating mutations

BACKGROUND The investigational CFTR potentiator ivacaftor (VX-770) increased CFTR channel activity and improved lung function in subjects with CF who have the G551D CFTR gating mutation. The aim of this in vitro study was to determine whether ivacaftor potentiates mutant CFTR with gating defects caused by other CFTR gating mutations. METHODS The effects of ivacaftor on CFTR channel open probability and chloride transport were tested in electrophysiological studies using Fischer rat thyroid (FRT) cells expressing different CFTR gating mutations. RESULTS Ivacaftor potentiated multiple mutant CFTR forms with defects in CFTR channel gating. These included the G551D, G178R, S549N, S549R, G551S, G970R, G1244E, S1251N, S1255P and G1349D CFTR gating mutations. CONCLUSION These in vitro data suggest that ivacaftor has a similar effect on all CFTR forms with gating defects and support investigation of the potential clinical benefit of ivacaftor in CF patients who have CFTR gating mutations beyond G551D.

Effect of ivacaftor on CFTR forms with missense mutations associated with defects in protein processing or function

BACKGROUND Ivacaftor (KALYDECO™, VX-770) is a CFTR potentiator that increased CFTR channel activity and improved lung function in patients age 6 years and older with CF who have the G551D-CFTR gating mutation. The aim of this in vitro study was to evaluate the effect of ivacaftor on mutant CFTR protein forms with defects in protein processing and/or channel function. METHODS The effect of ivacaftor on CFTR function was tested in electrophysiological studies using a panel of Fischer rat thyroid (FRT) cells expressing 54 missense CFTR mutations that cause defects in the amount or function of CFTR at the cell surface. RESULTS Ivacaftor potentiated multiple mutant CFTR protein forms that produce functional CFTR at the cell surface. These included mutant CFTR forms with mild defects in CFTR processing or mild defects in CFTR channel conductance. CONCLUSIONS These in vitro data indicated that ivacaftor is a broad acting CFTR potentiator and could be used to help stratify patients with CF who have different CFTR genotypes for studies investigating the potential clinical benefit of ivacaftor.

Use of Primary Cultures of Human Bronchial Epithelial Cells Isolated from Cystic Fibrosis Patients for the Pre-clinical Testing of CFTR Modulators

The use of human bronchial epithelial (HBE) cell cultures derived from the bronchi of CF patients offers the opportunity to study the effects of CFTR correctors and potentiators on CFTR function and epithelial cell biology in the native pathological environment. Cultured HBE cells derived from CF patients exhibit many of the morphological and functional characteristics believed to be associated with CF airway disease in vivo, including abnormal ion and fluid transport leading to dehydration of the airway surface and the loss of cilia beating. In addition, they can be generated in sufficient quantities to support routine lab testing of compound potency and efficacy and retain reproducible levels of CFTR function over time. Here we describe the development and validation of the CF HBE pharmacology model and its use to characterize, optimize, and select clinical candidates. It is expected that the pre-clinical testing of CFTR potentiators and correctors using epithelial cell cultures derived from CF patients will help to increase their likelihood of clinical efficacy.

WS14.1 Ivacaftor potentiates mutant CFTR forms associated with residual CFTR function

Objectives: Clinical studies have shown that ivacaftor, an oral CFTR potentiator, increased CFTR channel activity and improved lung function in patients with CF who have the G551D-CFTR gating mutation. The aim of this in vitro study was to evaluate whether ivacaftor potentiates mutant CFTR with non-gating CFTR channel defects. Methods: The pharmacological action of ivacaftor on over 50 mutant CFTR forms was evaluated in electrophysiological studies using a panel of Fischer rat thyroid cells or human bronchial epithelia cells isolated from patients with CF. Results: Ivacaftor potentiated multiple mutant CFTR forms that deliver sufficient amounts of functional CFTR to the cell surface to result in residual baseline chloride transport. These included CFTR mutations that result in mild defects in CFTR processing and delivery to the cell surface, reduced channel conductance, or reduced CFTR synthesis. In addition, ivacaftor also potentiated mutant CFTR forms that result in residual chloride transport due to defects in both channel gating activity and channel conductance, such as R117H-CFTR. Single-channel studies indicated that ivacaftor increased the channel open probability of R117H-CFTR to enhance chloride transport. In contrast, a minimal ivacaftor response was observed in cells expressing mutant CFTR forms associated with minimal baseline chloride transport. Conclusions: This in vitro study indicated that ivacaftor acts on multiple mutant CFTR forms with defects beyond channel gating and supports investigation of the potential clinical benefit of ivacaftor in patients with CF who have cell surface CFTR that results in residual chloride transport. Sponsored by Vertex.

WS06.2 R117H-CFTR has a defect in channel gating activity that can be potentiated by ivacaftor

Background The R117H mutation is associated with residual chloride transport and a delayed onset of CF symptoms. Although generally called a conductance mutation, R117H-CFTR exhibits defective channel gating. This study aimed to more fully explore the mechanistic defect of R117H-CFTR and to evaluate the effect of ivacaftor, a CFTR potentiator, on R117H-CFTR. Methods and Results In single-channel, patch-clamp studies, the channel open probability (gating activity) of R117H-CFTR was 22% of normal, whereas the channel conductance was 88% of normal. The channel gating activity of R117H-CFTR was characterized by frequent but brief openings of the channel compared with normal CFTR. The brief openings are consistent with a low channel open probability due in part to destabilization of the channel open state by R117H. The addition of ivacaftor increased the channel open probability of R117H-CFTR from 22% to 42% of normal. The increase in open probability was due to an increase in the frequency of channel opening and a decreased closed duration. Channel open duration and conductance were not increased. In Ussing chamber studies using R117H-5T/F508del-human bronchial epithelial cells from two donor bronchi, chloride transport was 23% of normal, which is consistent with residual chloride transport. Ivacaftor treatment for 18–24 hours increased chloride transport to 36% of normal. Conclusion These data confirm that R117H is a residual function CFTR mutation that causes a predominant defect in channel gating activity that is potentiated by ivacaftor. Based on these in vitro data, ivacaftor would be expected to enhance CFTR activity in people with the R117H mutation.