**2. Premature termination codons**

In approximately 10% of patients with CF, the responsible mutation results in a nonsense mutation that terminates the CFTR protein production due to a premature stop codon in the CFTR messenger ribonucleic acid (mRNA). The resultant truncated protein cannot properly transport chloride ion across the membrane. PTC Therapeutics, Inc. had discovered a small molecule drug ataluren (PTC124®) which enables the mRNA containing the premature stop codon to be read through the ribosome. The molecule is a 1,2,4-oxadiazole with a molecular weight of 284 Daltons. Using cell culture as well as a mouse model of CF, investigators determined that oral administration of ataluren was effective as long plasma concentrations in the range of 2 to 10 mcg/mL will result in functional CFTR activity. [1, 2] There was no evidence of nonspecific read through of normal stop codons. It did not appear to be teratogenic in rats and rabbits. However, there did appear to be inhibition of cytochrome P450 (CYP2C9) at therapeutic concentrations of ataluren. Therefore, monitoring blood levels of medications which are primarily metabolized by this enzyme (such as warfarin or phenytoin) may be needed clinically.

**Figure 1.** Structural formula of atalaren

Phase 1 studies indicate that serum ataluren levels of 2 to 10 mcg/mL can be achieved with a three times per day (TID) dosing schedule. Since administration with meals can result in prolonged levels in the blood, dosing after a meal appears to be desired.

Phase 2a studies have been conducted in 30 children and 47 adult patients in the United States, Israel, and Europe. [3, 4, 5] Using transepithelial potential difference (TEPD), which assesses transepithelial chloride conductance, as the primary outcome measure, these studies found statistically significant improvements. Pulmonary function testing also showed positive trends with decreased sputum volume and thickness, easier elimination of sputum, decreased coughing, and increased quality of life. There were no serious adverse drug effects. Phase 2a extension study looked at a 3-month administration of ataluren and confirmed its effectiveness and safety. [6] Optimal responses appear to be with doses of 10-, 10-, 20-mg/kg on the TID schedule and were seen with numerous different missense mutations.

The encouraging results of these preliminary studies prompted PTC Therapeutics to conduct a Phase 3, randomized, double-blind, placebo-controlled study to look at the effects of 48 weeks of therapy on pulmonary function and clinical symptoms in CF patients with the appropriate mutation. Over 400 patients were enrolled and showed positive trends favoring ataluren over placebo with an increase in percent of predicted for forced expiratory volume in 1 second (FEV1) and decreased pulmonary exacerbation rate. [7] A subgroup of patients who were not receiving inhaled antibiotics (primarily tobramycin) showed an even greater improvement on ataluren. The study continued to show the drug was well tolerated and had a good safety profile. There were some cases of creatinine elevation which were associated with the combi‐ nation of potential nephrotoxic antibiotics with ataluren. These results resulted in conducting another international, multicenter study in which patients will not be allowed to receive treatment with chronic inhaled aminoglycosides (such as tobramycin or TOBI) in early 2015. The aim is to randomize approximately 208 patients who will receive either ataluren or placebo for 48 weeks.
