1. A G551D missense mutation in the cystic fibrosis transmembrane conductance receptor (CFTR) is a genetic cause of cystic fibrosis (CF).
2. In utero administration of the CFTR-potentiating drug, VX-770 (ivacaftor) in a ferret genetic model of cystic fibrosis provided protection from CF pathologies in the pancreas, intestine, and male reproductive tract.
Evidence Rating Level: 2 (Good)
Study Rundown: Cystic fibrosis (CF) is a disease that results from recessive genetic mutations in the cystic fibrosis transmembrane receptor (CFTR). CFTR is an anion channel that helps move chloride and bicarbonate across epithelia. Dysfunctional CFTR impairs the proper development of gastrointestinal tract, male reproductive tract, and the exocrine pancreas, amongst other abnormalities. These consequences often manifest early in life and affect the health of infants with CF. Genetic large animal models of CF suffer severe manifestations of disease, including a high incidence of meconium ileus at birth, which results in death of the animal. In this study, the authors deliver ivacaftor, a pharmacologic therapy that potentiates the function of the CFTRC551D mutant receptor, in utero in a ferret CF model to determine whether the pathologies associated with CF could be averted. To study these effects, the authors generated a knock-in ferret harboring the CFTRG551D allele, a CFTR-causing mutation found in 2-5% of patients with CF. They demonstrated that VX-770 could activate the CFTRG551D in vitro using intestinal organoids, intestine, and pancreatic ductal epithelium from CFTR mutant ferrets. Additionally, pathologies associated with CF, including meconium ileus, abnormalities of the male reproductive structures, and the growth and exocrine function of the pancreas, could be partially ameliorated with in utero administration of VX-770. VX-770 administration was initiated at a time roughly approximate to the start of the third trimester in humans. Withdrawal of VX-770 resulted in reestablishment of the disease, suggesting that VX-770 helps ameliorate developmental abnormalities, and is also necessary for sustained organ function. These studies provide a proof-of-concept for future clinical trials in humans to assess the impact of in utero administration of VX-770 for preventative treatment of the developmental abnormalities associated with CF.
Relevant Reading: Therapies for cystic fibrosis
In-Depth [pre-clinical study]: The objective of this study was to model cystic fibrosis using a genetic large animal model, and test the efficacy of early, in utero administration CFTR-potentiating agent, VX-770 (ivacaftor) in averting CF-associated developmental pathologies. VX-770 is the best-characterized CFTR-potentiator, helping to rescue gating defects in CFTR, and is currently approved for patients with CF, 1 year or older, with particular genetic mutations. Using a genetic ferret model of CF, the authors show that in utero administration of ivacaftor partially averted developmental pathologies associated with CF.
A knock-in ferret model of CF was created using recombinant adeno-associated virus-mediated homologous recombination in fetal ferret fibroblasts to create the CFTRG551D mutation followed by somatic cell nuclear transfer. Responsiveness to VX-770 was assessed using forskolin-induced swelling assays on intestinal organoids derived from CFTRWT/WT, CFTRKO/KO, CFTRG551D/KO, and CFTRG551D/G551D ferrets. Swelling of CFTRG551D/G551D organoids at 0.8uM forskolin was reduced by >50% compared to wild-type organoids, while knockout organoids failed to swell at any forskolin concentration. Further studies on freshly excited intestine and pancreas from mutant and wild-type animals showed that treatment of CFTRG551D/G551D intestine with VX-770 could produce a cyclic adenosine monophosphate (cAMP)-inducible chloride current.
After establishing the efficacy of VX-770 in vitro, the authors proceeded to ask whether VX-770 could rescue the severe meconium ileus phenotype in CF ferrets. VX-770 was administered to pregnant ferrets at embryonic day 28, during a gestational period of 42 days (corresponding to roughly the third trimester in humans). Pharmacokinetic experiments established the VX-770 dose required to obtain therapeutic concentrations of VX-770 in fetal tissues. Administration of VX-770 increased the percentage of CFTRG551D/G551D kits passing meconium following birth (96%), compared to CFTRKO/KO kits (26%). Additionally, VX-770 rescued vas deferens and epididymis development only in CFTRG551D/G551D ferrets, but not in CFTRG551D/KO ferrets. Other effects of VX-770 administration in CFTRG551D/G551D included preservation of glycemic stability and pancreatic sufficiency and improved overall growth and survival. Furthermore, cessation of VX-770 in adult CFTR mutant ferrets led to increased incidence of lung and pancreatic disease, suggesting the need for continued VX-770 administration beyond the developmental period.
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