ArticlesEffectiveness of PTC124 treatment of cystic fibrosis caused by nonsense mutations: a prospective phase II trial
Introduction
Cystic fibrosis results from mutations in the gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR), an apical cell-surface epithelial chloride channel that promotes chloride efflux and secondarily inhibits constitutive sodium influx via the epithelial sodium channel (ENaC).1 Dysfunction of this regulator leads to epithelial mucous dehydration and viscous secretions, which often cause chronic neutrophilic inflammation and occlusion of respiratory airways. Obstruction of pancreatic ducts, the biliary tract, and the vas deferens can occur. Patients typically develop progressive respiratory dysfunction and persistent pulmonary infections and often have pancreatic insufficiency, diminished bodyweight, chronic hepatobiliary inflammation, and male infertility. Respiratory failure is the most common cause of death. No means for correcting the genetic defect has been available; current medical treatments are palliative.
A nonsense mutation is a single point alteration in DNA that results in the inappropriate presence of a UAA, UAG, or UGA stop codon in the protein-coding region of the corresponding messenger RNA (mRNA) transcript. Such a stop codon causes premature cessation of translation, with protein truncation leading to loss of function and consequent disease. Nonsense mutations are responsible for about 10% of cystic fibrosis cases worldwide.2 However, in Israel, nonsense mutations are the cause of cystic fibrosis in most patients.3 Because people with such mutations produce little functional CFTR, these patients usually have a phenotype of severe cystic fibrosis.4
Certain aminoglycoside antibiotics (eg, gentamicin) can induce ribosomes to read through a premature stop codon in mRNA, resulting in incorporation of an amino acid and continuation of translation to produce a complete protein.5 We have previously shown that topical application of gentamicin drops to the nasal mucosa can cause a local increase in CFTR-mediated chloride transport as assessed by nasal transepithelial potential difference (PD) in patients who have sufficient CFTR transcripts that contain a nonsense mutation.6, 7 However, the inconvenience of parenteral administration and the potential for serious toxic effects preclude long-term systemic use of gentamicin for supression of nonsense mutations.
PTC124 (3-[5-(2-fluorophenyl)-[1,2,4]oxadiazol-3-yl]-benzoic acid) is a 284-Dalton, orally bioavailable, non-aminoglycoside compound that was specifically developed to induce ribosomes to read through premature stop codons, but not normal stop codons.8 When tested in a mouse model of stop-mutation-mediated cystic fibrosis, PTC124 generated production of full-length, functional CFTR.9 Phase I studies in healthy volunteers established the initial safety profile for PTC124,10 and defined dosing regimens to achieve target trough plasma concentrations (of 2 to 10 μg/mL) that are known to be active in preclinical models.8, 9
We aimed to use nasal PD to assess whether PTC124 could overcome the effects of a nonsense mutation by restoring the functional activity of CFTR and increasing total chloride transport. We also aimed to assess other nasal PD measures of ion-channel activity, the cellular levels of CFTR mRNA with a nonsense mutation, disease-related clinical parameters, safety of this treatment, compliance with treatment, and PTC124 pharmacokinetics.
Section snippets
Participants
Patients were referred by four participating cystic fibrosis clinics in Israel for treatment at a single centre. All patients were aged 18 years or older, and had cystic fibrosis as established by a typical clinical presentation, an abnormal sweat test (sweat chloride >40 mEq/L by pilocarpine iontophoresis), abnormal chloride transport (nasal transepithelial PD more electrically positive than −5 mV during nasal perfusion with chloride-free amiloride and isoproterenol),11, 12, 13, 14 and the
Results
23 patients who had never been given PTC124 were enrolled, and completed the treatment phase of the first cycle. One patient had an exacerbation of a pre-existing Mycobacterium abscessus infection in the first cycle, and thus did not participate in the second cycle. Another patient had rhinitis, which precluded baseline testing of nasal PD before the second cycle; therefore we had paired assessments of nasal PD in cycle 2 for 21 patients. For FEV1 and FVC, 22 patients had paired assessments in
Discussion
The trial exemplifies the concept of personalised medicine:18 integrating selection of patients with a specific genetic defect, use of a treatment designed to overcome that defect in gene expression, and direct assessment of protein function within disease-affected tissues. We used genotyping to identify patients in whom cystic fibrosis was caused by a CFTR nonsense mutation. We administered PTC124 to induce ribosomes to selectively bypass premature stop codon mutations in mRNA. We used nasal
References (35)
- et al.
CFTR genotype as a predictor of prognosis in cystic fibrosis
Chest
(2006) - et al.
Basic protocol for transepithelial nasal potential difference measurements
J Cyst Fibros
(2004) - et al.
Reproducibility of nasal potential difference measurements in cystic fibrosis 1
Chest
(2007) - et al.
Evidence of CFTR function in cystic fibrosis after systemic administration of 4-phenylbutyrate
Mol Ther
(2002) - et al.
A multicenter study of the effect of solution temperature on nasal potential difference measurements
Chest
(2003) - et al.
Cystic fibrosis
- et al.
Cystic fibrosis: a worldwide analysis of CFTR mutations–correlation with incidence data and application to screening
Hum Mutat
(2002) - et al.
Cystic fibrosis in Jews: frequency and mutation distribution
Genet Test
(1997) - et al.
Aminoglycoside suppression of a premature stop mutation in a Cftr-/- mouse carrying a human CFTR-G542X transgene
J Mol Med
(2002) - et al.
Gentamicin-induced correction of CFTR function in patients with cystic fibrosis and CFTR stop mutations
N Engl J Med
(2003)