Pharmacological Rescue of Mutant CFTR Function for the Treatment of Cystic Fibrosis. In: Fermini B, Priest BT, editors. Ion Channels. Berlin: Springer Berlin Heidelberg; 2008. pp. 91-120. [DOI: 10.1007/7355_2008_022]

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.

Tezacaftor-Ivacaftor in Residual-Function Heterozygotes with Cystic Fibrosis

BACKGROUND

Cystic fibrosis is an autosomal recessive disease caused by mutations in the CFTR gene that lead to progressive respiratory decline. Some mutant CFTR proteins show residual function and respond to the CFTR potentiator ivacaftor in vitro, whereas ivacaftor alone does not restore activity to Phe508del mutant CFTR.

METHODS

We conducted a randomized, double-blind, placebo-controlled, phase 3, crossover trial to evaluate the efficacy and safety of ivacaftor alone or in combination with tezacaftor, a CFTR corrector, in 248 patients 12 years of age or older who had cystic fibrosis and were heterozygous for the Phe508del mutation and a CFTR mutation associated with residual CFTR function. Patients were randomly assigned to one of six sequences, each involving two 8-week intervention periods separated by an 8-week washout period. They received tezacaftor-ivacaftor, ivacaftor monotherapy, or placebo. The primary end point was the absolute change in the percentage of predicted forced expiratory volume in 1 second (FEV1) from the baseline value to the average of the week 4 and week 8 measurements in each intervention period.

RESULTS

The number of analyzed intervention periods was 162 for tezacaftor-ivacaftor, 157 for ivacaftor alone, and 162 for placebo. The least-squares mean difference versus placebo with respect to the absolute change in the percentage of predicted FEV1 was 6.8 percentage points for tezacaftor-ivacaftor and 4.7 percentage points for ivacaftor alone (P<0.001 for both comparisons). Scores on the respiratory domain of the Cystic Fibrosis Questionnaire-Revised, a quality-of-life measure, also significantly favored the active-treatment groups. The incidence of adverse events was similar across intervention groups; most events were mild or moderate in severity, with no discontinuations of the trial regimen due to adverse events for tezacaftor-ivacaftor and few for ivacaftor alone (1% of patients) and placebo (<1%).

CONCLUSIONS

CFTR modulator therapy with tezacaftor-ivacaftor or ivacaftor alone was efficacious in patients with cystic fibrosis who were heterozygous for the Phe508del deletion and a CFTR residual-function mutation. (Funded by Vertex Pharmaceuticals and others; EXPAND ClinicalTrials.gov number, NCT02392234 .).

Small-molecule CFTR activators increase tear secretion and prevent experimental dry eye disease

Dry eye disorders, including Sjögren's syndrome, constitute a common problem in the aging population, with limited effective therapeutic options available. The cAMP-activated Cl(-) channel cystic fibrosis transmembrane conductance regulator (CFTR) is a major prosecretory channel at the ocular surface. We investigated whether compounds that target CFTR can correct the abnormal tear film in dry eye. Small-molecule activators of human wild-type CFTR identified by high-throughput screening were evaluated in cell culture and in vivo assays, to select compounds that stimulate Cl(-)-driven fluid secretion across the ocular surface in mice. An aminophenyl-1,3,5-triazine, CFTRact-K089, fully activated CFTR in cell cultures with EC50 ∼250 nM and produced an ∼8.5 mV hyperpolarization in ocular surface potential difference. When delivered topically, CFTRact-K089 doubled basal tear volume for 4 h and had no effect in CF mice. CFTRact-K089 showed sustained tear film bioavailability without detectable systemic absorption. In a mouse model of aqueous-deficient dry eye produced by lacrimal ablation, topical administration of 0.1 nmol CFTRact-K089 3 times daily restored tear volume to basal levels, preventing corneal epithelial disruption when initiated at the time of surgery and reversing it when started after development of dry eye. Our results support the potential utility of CFTR-targeted activators as a novel prosecretory treatment for dry eye.-Flores, A. M., Casey, S. D., Felix, C. M., Phuan, P. W., Verkman, A. S., Levin, M. H. Small-molecule CFTR activators increase tear secretion and prevent experimental dry eye disease.

Discovery of N-(2,4-di-tert-butyl-5-hydroxyphenyl)-4-oxo-1,4-dihydroquinoline-3-carboxamide (VX-770, ivacaftor), a potent and orally bioavailable CFTR potentiator

Quinolinone-3-carboxamide 1, a novel CFTR potentiator, was discovered using high-throughput screening in NIH-3T3 cells expressing the F508del-CFTR mutation. Extensive medicinal chemistry and iterative structure-activity relationship (SAR) studies to evaluate potency, selectivity, and pharmacokinetic properties resulted in the identification of N-(2,4-di-tert-butyl-5-hydroxyphenyl)-4-oxo-1,4-dihydroquinoline-3-carboxamide (VX-770, 48, ivacaftor), an investigational drug candidate approved by the FDA for the treatment of CF patients 6 years of age and older carrying the G551D mutation.

On the mechanism of CFTR inhibition by a thiazolidinone derivative

The effects of a thiazolidinone derivative, 3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl)methylene]-2-thioxo-4-thiazolidinone (or CFTRinh-172), on cystic fibrosis transmembrane conductance regulator (CFTR) gating were studied in excised inside-out membrane patches from Chinese hamster ovary cells transiently expressing wild-type and mutant CFTR. We found that the application of CFTRinh-172 results in an increase of the mean closed time and a decrease of the mean open time of the channel. A hyperbolic relationship between the closing rate and [CFTRinh-172] suggests that CFTRinh-172 does not act as a simple pore blocker. Interestingly, the potency of inhibition increases as the open time of the channel is increased with an IC50 in the low nanomolar range for CFTR channels locked in an open state for tens of seconds. Our studies also provide evidence that CFTRinh-172 can bind to both the open state and the closed state. However, at least one additional step, presumably reflecting inhibitor-induced conformational changes, is required to shut down the conductance after the binding of the inhibitor to the channel. Using the hydrolysis-deficient mutant E1371S as a tool as the closing rate of this mutant is dramatically decreased, we found that CFTRinh-172-dependent inhibition of CFTR channel gating, in two aspects, mimics the inactivation of voltage-dependent cation channels. First, similar to the recovery from inactivation in voltage-gated channels, once CFTR is inhibited by CFTRinh-172, reopening of the channel can be seen upon removal of the inhibitor in the absence of adenosine triphosphate (ATP). Second, ATP induced a biphasic current response on inhibitor-bound closed channels as if the ATP-opened channels "inactivate" despite a continuous presence of ATP. A simplified six-state kinetic scheme can well describe our data, at least qualitatively. Several possible structural mechanisms for the effects of CFTRinh-172 will be discussed.