**3. Potentiators**

variable levels of function (or Class III, IV, or VI effects). Class III mutations result in amino acid substitutions which affect how the CFTR protein is regulated on the cell surface, usually decreasing the channel opening. Class IV mutations result in amino acid substitutions which affect how the CFTR protein functions in its conduction of chloride ions. Class V mutations result in decreased production of normal functioning CFTR protein. Therefore, there is a reduced amount of normal CFTR protein on the cell surface. Finally, class VI mutations are

Awareness of the types of genetic mutations affecting the normal functioning of the CFTR protein has resulted in searches for treatments directed at various CFTR dysfunctions. This chapter will review recent attempts to develop treatments specific for the various classes of

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

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.

those which affect the stability of CFTR.

132 Cystic Fibrosis in the Light of New Research

**2. Premature termination codons**

mutations.

needed clinically.

**Figure 1.** Structural formula of atalaren

The presence of some CFTR on the cell surface in mutation classes III, IV, and VI suggests another approach may be to increase the "gating" function of these protein molecules. The most common gating mutation is G551D, a missense mutation that results in the replacement of a glycine for an aspartic acid at position 551 of the CFTR protein. The resulting CFTR protein is present at the cell surface but does not open and close properly, which is called defective channel gating. The effort to find an effective "potentiator" of the channel opening has resulted in the Federal Drug Administration (FDA) approval of the first medication which modulates the function of CFTR, ivacaftor (Kalydeco), in January 2012.

Ivacaftor (or VX-770) was developed by Vertex Pharmaceuticals, Inc., to potentiate the action of the mutated CFTR on the cell surface. It has a molecular weight of 392 and a molecular formula of C24H28N2O3. Studies with recombinant cell lines with G551D-CFTR indicated that VX-770 did result in increased total chloride transport by greater than 10%. [8] There was evidence to indicate that it acted directly on the mutated CFTR protein to keep the channel open. It apparently has similar activity on other mutant CFTR forms resulting from other CFTR gating mutations. However, since the G551D mutation affects about 5% of all CF patients and

**Figure 2.** Structural formula of ivacaftor

is the most common gating mutation, initial studies focused on patients with this particular mutation.

Phase 1 studies indicated that ivacaftor is primarily metabolized in the liver and moderate hepatic impairment may reduce its elimination. Since *in vitro* studies indicated that ivacaftor is a substrate of CYP3A4/5 and therefore, strong (e.g., ketoconazole) and moderate (e.g., fluconazole) CYP3A inhibitors as well as strong CYP3A inducers (e.g., rifampin) will affect ivacaftor's metabolism. A detailed list of the numerous drug interactions is found in the package insert of ivacaftor (Kalydeco®) (see http://www.accessdata.fda.gov/drugsatfda\_docs/ label/2012/203188lbl.pdf).

An initial phase 2 study by Accurso found that the safety profile of ivacaftor in adult patients with at least one G551D mutation was excellent for a wide variety of daily doses (from 50 mg to 500 mg for 2 to 4 weeks). An improvement in FEV1 and drop in sweat chloride were also noted (although these were not primary end points of the study). [9] This prompted two randomized, placebo-controlled, double blind studies eventually resulting in FDA approval. Ivacaftor 150 mg twice daily for 48 weeks was studied in 167 patients 12 years or older with at least one G551D mutation. There was a 10.6% improvement in predicted FEV1 from baseline versus placebo. Additionally, a 55% reduction in pulmonary exacerbation was also observed. Sweat chloride levels dropped to a mean of 47.8 mmol/L compared with 100.0 mmol/L in the placebo group. There was improvement in quality of life measurements as well as a more significant increase in weight gain (4.1 kg increase) in those receiving ivacaftor. [10] An additional pediatric study of 52 patients 6 to 12 years of age showed a 12.5% improvement in FEV1 and a 2.8 kg increase in weight. A decrease in sweat chloride of 54 mmol/L was also seen. [11] In both studies, the incidence of adverse events was similar in both treated and placebo groups.

Similar results were observed in studies examining non-G551D gating mutations. [12] These 36 patients also showed significant improvements in FEV1, sweat chloride measurements, changes in BMI, and quality of life indices. Therefore, the FDA also approved ivacaftor for G178R, S549N, S549R, G551S, G970R, G1244E, S1251N, G1349D, and S1255P mutations in February 2014. Finally, in December 2014, one additional mutation, R117H, was added by the FDA to the list of approved CF mutations for ivacaftor administration after 69 patients greater than 6 years of age were studied showing similar efficacy and safety data. [13] However, their decrease in sweat chloride was only 24.0 mmol/L and BMI treatment differences were not significant.

Since its initial FDA approval in 2012, follow-up studies in 144 patients with the G551D mutation have confirmed the persistence of lung function improvement (9.4% increase in adults and 10.3% in children) for up to 144 weeks on ivacaftor. [14] Additionally, treated patients continued to have reduced hospitalizations and decreased Pseudomonas aeruginosa culture rates, which may be related to increased mucociliary clearance. [15] There was a reduced rate of decline in FEV1. The absolute increase in weight was 5.1 kg in adults and 14.8 kg in children after almost 3 years, indicating that there was an increased rate of weight gain and improved BMI. [16] Not all the nutritional improvement may be attributed to improved lung function since treated patients also showed a normalizing in small bowel pH which may have improved pancreatic enzyme function.[14]

Although the FDA approval was for patients greater than 6 years of age, Davies et al. found ivacaftor to be well-tolerated in 34 children between 6 months and 5 years of age at doses 50 to 75 mg twice a day. [17] They also showed a significant decrease in sweat chloride with a mean drop of 44 mmol/L, and many patients showed improvement in pancreatic function with increases in stool elastase. However, 14.7% showed a higher rate of elevated liver function test results indicating the need to especially monitor these younger patients.
