Ataluren in cystic fibrosis: development, clinical studies and where are we now?

 

Abstract

Introduction

Cystic fibrosis, often referred to as CF, stands as one of the most prevalent genetically acquired conditions globally, significantly impacting an individual’s lifespan. This inherited disorder is fundamentally characterized by a malfunction of the cystic fibrosis transmembrane conductance regulator, or CFTR protein. This dysfunction invariably leads to the development of progressive lung disease, alongside a wide array of other systemic effects that can affect multiple organ systems throughout the body. A notable subset of individuals afflicted with cystic fibrosis, approximately 10% worldwide, possess a specific type of genetic alteration known as a class I nonsense mutation. This particular mutation results in the premature presence of a stop codon during protein synthesis, leading to the production of a truncated, and thus non-functional or severely impaired, CFTR protein.

 

Areas Covered

 

This discussion delves into the remarkable discovery of ataluren, a small-molecule drug. In laboratory settings, ataluren has demonstrated the ability to enable a process termed “read-through” of these premature termination codons, thereby facilitating the synthesis of a complete, full-length CFTR protein. Furthermore, a comprehensive review of the various clinical studies that have been conducted to evaluate ataluren’s effectiveness in individuals with cystic fibrosis will be presented. Initial, early-phase, short-term crossover studies provided encouraging results, showing an observable improvement in nasal potential difference measurements, a key indicator of CFTR function. However, a subsequent phase III randomized controlled trial, which aimed to establish definitive efficacy, did not reveal a statistically significant difference for its primary outcome measure, which was lung function. Nevertheless, a post-hoc analysis of the data from this trial suggested a potential benefit in a specific subgroup of patients, specifically those who were not concurrently receiving tobramycin, an antibiotic. Another randomized controlled trial, specifically designed to investigate the effects of ataluren in patients who were not receiving tobramycin, has reportedly concluded and indicated no significant benefit. It is important to note that the full, peer-reviewed publication of these latter findings is still pending.

 

Expert Opinion

 

The fundamental concept of employing a small-molecule therapeutic agent to facilitate the read-through of premature termination codons in nonsense mutations carries considerable intuitive appeal from a scientific and medical perspective. This approach directly targets the underlying genetic defect. Despite this promising theoretical basis, it is crucial to acknowledge that, at the current juncture, there remains a lack of robust, high-quality evidence definitively demonstrating the clinical efficacy of ataluren as a treatment for individuals living with cystic fibrosis. Further rigorous investigation is warranted to conclusively establish its therapeutic value.

 

Introduction

 

Cystic fibrosis, commonly abbreviated as CF, represents the most frequently encountered life-limiting autosomal recessive genetic disorder across both Europe and North America. In the United Kingdom, for instance, this condition impacts over 10,000 individuals. The hallmark of cystic fibrosis is its progressive lung disease, which inexorably advances to end-stage bronchiectasis and ultimately culminates in respiratory failure. Beyond the primary pulmonary manifestations, a range of other significant clinical features can present, including pancreatic insufficiency, the development of CF-related diabetes, compromise of nutritional status, liver dysfunction, and male infertility. The fundamental genetic cause of cystic fibrosis lies in pathogenic mutations within the CF transmembrane-conductance regulator, or CFTR, gene. This critical gene provides the instructions for producing the CFTR protein, a transmembrane chloride and bicarbonate channel that is predominantly located on the apical membrane of epithelial cells throughout the body. While cystic fibrosis is most prevalent among populations of Northern European ancestry, it is also observed, albeit at a lower frequency, across a wide spectrum of ethnically diverse populations worldwide.

 

Approximately 2000 distinct CFTR mutations have been identified to date. Among this substantial number, a comparatively small subset of these mutations is responsible for the vast majority of disease cases. CFTR mutations are systematically categorized based on the specific underlying mechanisms that lead to dysfunction of the CFTR protein, including issues with its synthesis, processing, and trafficking within the cell. Mutations that result in the defective expression of the CFTR protein are classified as class I mutations. Class II mutations encompass those that describe dysfunctional CFTR processing and a subsequent failure of the CFTR protein to traffic correctly to the cell surface. In instances of class III mutations, the CFTR protein is indeed expressed at the cell surface, but a defect in nucleotide binding leads to impaired channel gating, which in turn results in abnormal epithelial chloride transport. For class IV mutations, the CFTR channel pore itself is defective, leading to impaired channel conductance. Splicing defects, categorized as class V mutations, reduce the overall quantity of correctly transcribed CFTR protein available at the cell surface. Lastly, class VI mutations are characterized by reduced stability of the functional CFTR protein, leading to its rapid turnover at the cell surface. Despite this systematic classification, it is important to recognize that some mutations are not exclusively confined to a single class. A prominent example of this overlap is the most common CFTR mutation, Phe508del, which exhibits characteristics belonging to both class II and class III. Furthermore, while classes I through III are generally associated with a severe disease presentation, the observable variation in phenotype, even within the same classes and genotypes, strongly suggests the influential involvement of multifactorial environmental and non-CFTR genetic factors in determining disease severity and progression.

 

Class I mutations constitute a diverse collection of mutation types. These include nonsense mutations, frameshift mutations, and large deletion or insertion mutations. All of these distinct types of class I mutations ultimately lead to the total or partial absence of CFTR protein expression. Consequently, class I mutations are functionally defined by a complete absence of chloride conductance in affected epithelial cells, which then results in a severe disease phenotype.

 

The frequency of individual CFTR mutations exhibits considerable global variation among different ethnic groups. For instance, the Phe508del CFTR variant accounts for approximately 70% of mutations observed in Northern Europe. In contrast, among individuals of Ashkenazi Jewish ancestry, class I CFTR mutations are the most commonly encountered genetic alterations. The evolution of coordinated multidisciplinary care, diligently delivered by specialized centers and rigorously guided by evidence-based clinical guidelines, has yielded significant incremental improvements in the survival rates of individuals with cystic fibrosis. This advancement parallels a steady enhancement in the understanding of the condition since its initial recognition in 1938. When cystic fibrosis was first medically described, it was tragically associated with a very poor survival prognosis, often limited to early childhood. However, due to continuous advancements in treatment and care, the median survival rate has now extended to over 40 years in nations possessing well-funded and robust healthcare systems. Historically, the primary approaches to treatment have focused on managing symptoms and attempting to delay the progression of the disease. These mainstays of treatment have included intensive chest physiotherapy, the lifelong administration of antibiotics, pancreatic enzyme replacement therapy, and the use of inhaled mucolytics. While these strategies have achieved considerable clinical success in ameliorating symptoms, none of them have addressed the fundamental underlying CFTR defect. Instead, they have primarily sought to mitigate the downstream effects of the CFTR dysfunction.

 

Ivacaftor represents a significant advancement in cystic fibrosis therapeutics. It is a small molecule CFTR potentiator, a type of drug that works by improving the function of CFTR proteins, particularly in the context of class III mutations. Most notably, it has shown remarkable efficacy in individuals carrying the Gly551Asp mutation, which accounts for approximately 3% to 5% of mutant CFTR alleles worldwide. In large-scale multicenter randomized controlled trials, ivacaftor demonstrated a substantial improvement in lung function, specifically by about 10 percentage points of predicted forced expiratory volume in 1 second, or FEV1. Beyond pulmonary benefits, the trials also reported significant positive impacts on sweat chloride levels, a key diagnostic marker for CF, as well as improvements in respiratory symptoms, body mass index, and overall quality of life. As a direct consequence of these compelling results, ivacaftor is now widely prescribed as a mutation-specific treatment in numerous countries. The success of ivacaftor, which has provided a substantial breakthrough over the last decade for the relatively small cohort of cystic fibrosis patients with eligible mutations, stands as one of the best examples to date of precision medicine. Crucially, it provides compelling “proof of concept” that directly targeting CFTR dysfunction is a highly promising and potentially transformative therapeutic strategy for cystic fibrosis. Subsequent research and development efforts have intensively focused on discovering and developing other small-molecule approaches aimed at restoring function in other common CFTR mutations. A particularly notable outcome of these efforts is the development of the combination treatment, known as Orkambi, which comprises lumacaftor, a CFTR corrector, and ivacaftor, for use in patients who are homozygous for the Phe508del mutation. However, it is important to note that the clinical benefits observed in randomized controlled trials with this combination treatment have been relatively modest when compared to the profound improvements seen with ivacaftor in Gly551Asp patients. The intricate biology of Phe508del CFTR continues to present complex challenges in therapeutic development.

 

Ataluren, also identified by its designation PTC-124, is a pharmaceutical agent that was specifically discovered for its ability to target class I nonsense CFTR mutations. Its mechanism of action involves enabling “read through” of messenger RNA, or mRNA, which subsequently leads to the expression of the full-length CFTR protein. This article will thoroughly review and discuss the historical context and background surrounding the development of ataluren, elaborate on its precise mechanisms of action, and present the evidence derived from clinical trials regarding ataluren’s potential as a therapeutic intervention for individuals living with cystic fibrosis.

 

Overview of the Market

 

Globally, it is estimated that approximately 10% of all individuals who have been diagnosed with cystic fibrosis carry at least one class I nonsense mutation within their CFTR gene. However, it is imperative to acknowledge that the prevalence of these specific mutations is not uniform across all geographical regions and ethnic groups; instead, it demonstrates significant variability. For instance, a striking statistical observation from Israel reveals that more than 45% of patients suffering from cystic fibrosis in that country possess a nonsense mutation. This figure stands in stark contrast to the situation in the United Kingdom, where the proportion of patients carrying such a mutation is comparatively much lower, less than 15%. Among the diverse array of class I mutations that have been identified, the Gly542X mutation is particularly noteworthy as it is recognized as the most commonly occurring type.

 

Introduction to the Compound – What is Ataluren?

 

Nonsense mutations are a specific type of genetic alteration that lead to the presence of in-frame premature termination codons, often referred to as PTCs. These PTCs have a profound effect on the process of ribosomal translation, causing it to prematurely cease, which consequently results in the production of a truncated and ultimately non-functional CFTR protein. Furthermore, the presence of these PTCs triggers a sophisticated cellular regulatory mechanism known as nonsense-mediated mRNA decay, or NMD. This NMD pathway serves to actively degrade messenger RNA molecules that contain PTCs, thereby reducing any further synthesis of these undesirable shortened proteins. Nonsense mutations are fundamentally defined by the existence of PTCs and their intricate regulation via the NMD pathway, both of which are inherently linked processes. Given the pervasive absence of functional protein that characterizes this class of mutation, it is generally observed that patients afflicted with such mutations typically exhibit a severe clinical phenotype.

 

Therapeutic strategies aimed at addressing cystic fibrosis that is caused by nonsense mutations encompass approaches such as promoting PTC read-through and inhibiting NMD, both of which are designed to encourage the production of full-length CFTR protein. This particular discussion, however, will primarily concentrate on the advancements in our understanding of PTC read-through, specifically concerning the compound ataluren.

 

Historically, aminoglycosides were the first class of drugs to demonstrate the remarkable ability to facilitate ribosomal read-through of PTCs. The prevailing hypothesis suggests that their selective binding to specific sites on ribosomes induces the insertion of a random amino acid at the PTC position on the messenger RNA. This action effectively “masks” the premature termination codon, allowing the ribosome to continue translation and produce a complete, full-length, and functionally active protein. This groundbreaking observation was initially made in the 1960s, following the compelling demonstration of phenotypic repair in defective bacterial genotypes when treated with streptomycin. Further corroborating evidence emerged from the 1970s onwards, as this read-through activity of aminoglycosides was successfully replicated in mammalian cells. A pivotal study in 1996, utilizing cystic fibrosis as a disease model, rigorously tested PTC read-through as a novel therapeutic approach. This study provided compelling evidence by demonstrating the production of full-length and functional CFTR protein in cell lines expressing PTCs after being treated with aminoglycosides. Indeed, a number of subsequent studies have consistently shown an improvement in CFTR function at a cellular level following treatment with gentamicin. Nevertheless, the clinical utility of aminoglycosides for the purpose of restoring CFTR function as a chronic treatment is significantly limited by several factors, including the necessity of parenteral administration and the substantial risks of otic and renal toxicities associated with this class of drugs.

 

Ataluren was identified through a high-throughput screening process as a compound that similarly promoted the read-through of PTCs. Structurally, it is distinctly different from aminoglycosides and represents the first drug of its kind. The initial discovery of ataluren was based on the demonstration of its read-through activity in firefly luciferase reporter systems. Ataluren has also undergone investigation for its potential use in nonsense mutation Duchenne muscular dystrophy, or nmDMD. In recognition of some clinical benefit observed in slowing the rate of decline in patients with advanced disease, ataluren was granted conditional approval for marketing in 2014 by the European Medicines Agency for this indication. While it was not approved for this indication by the United States Food and Drug Administration, as of late 2024, the FDA has accepted a resubmission of a New Drug Application for review for this indication, providing renewed hope. The initial refusal in February 2016 was based on a perceived lack of convincing evidence regarding its effectiveness. Although further data are still required, ataluren’s development offers hope and marks a significant advancement in drug research for this group of patients who otherwise possess no known disease-modifying treatment options.

 

Chemistry

 

Ataluren is a small molecule with a molecular weight of 284 Daltons, and it does not belong to the aminoglycoside class of compounds. Its specific chemical structure is formally known as 3-[5-(2-fluorophenyl)-[1,2,4]oxa-diazol-3-yl]-benzoic acid, with the chemical formula C15H9FN2O3. When prepared as an aqueous suspension, its anhydrous, free carboxylic acid form exhibits oral bioavailability, meaning it can be effectively absorbed into the bloodstream after being taken by mouth.

 

Pharmacodynamics

 

Over the past decade, numerous studies have meticulously investigated ataluren’s pharmacodynamic properties and its precise mechanism of action, yielding a variety of findings. In one particular study, which utilized cell lines genetically engineered to express a firefly luciferase gene containing a PTC at codon 190, ataluren consistently demonstrated dose-dependent read-through activity across all three known types of PTCs: UGA, UAG, and UAA. This activity was quantified by measuring the observed amount of luciferase activity. Notably, in this study, ataluren exhibited potency at considerably lower concentrations compared to gentamicin. It was further demonstrated that ataluren’s nonsense suppression capabilities in this study were primarily attributable to its promotion of PTC read-through activity, with only a minimal effect observed on the NMD regulatory mechanism. Ataluren displayed a remarkable selectivity for premature termination codons, importantly not affecting normal termination codons. Unlike gentamicin, it did not exhibit any antibacterial activity.

 

A subsequent study, published in 2007 shortly after ataluren’s initial discovery, further affirmed its role as a read-through agent. In a cystic fibrosis mouse model specifically engineered to express the Gly542X mutation, treatment with ataluren resulted in the successful expression of CFTR protein at the apical surface of the mice’s intestinal glands. This observation was consistent with effective in vivo nonsense mutation suppression activity.

 

Despite these findings, there have been conflicting reports within the scientific literature concerning the methodology employed in ataluren’s discovery and the intricacies of its mechanisms of action. Two separate studies, utilizing alternative reporter assays, were unable to reproduce evidence of ataluren’s purported read-through activity. The authors of these studies suggested that the initial observation of ataluren’s read-through ability might, in fact, have been due to an off-target activity, specifically its action as a firefly luciferase inhibitor. However, these counter-reports have recently faced rebuttal from Roy and colleagues, who subsequently demonstrated read-through activity in several non-luciferase reporter assays. Further evidence provided by Roy and colleagues elucidated ataluren’s selectivity for the ribosomal A site and confirmed that it promotes the insertion of near-cognate transfer RNA molecules at the PTC during the process of protein synthesis. Moreover, Roy and colleagues also demonstrated that tobramycin, an aminoglycoside known for its similar ribosomal selectivity, acts as a strong inhibitor of ataluren. This finding is consistent with a previous hypothesis proposed by Kerem and colleagues in a post-hoc in vitro study conducted as part of a phase III trial.

 

The ex vivo human intestinal organoid model has also been employed to further investigate the functional effects of ataluren. In this sophisticated model, intestinal epithelial cells, carefully derived from rectal biopsy tissue, are cultured to generate organoids. These organoids are microscopic three-dimensional structures that intricately mimic the physiological architecture of the intestine, containing crypt-like structures and a central lumen that is lined by a differentiated apical epithelium capable of expressing CFTR. The addition of forskolin, a potent activator of CFTR, can be utilized to assess the effects of CFTR activity on fluid secretion by measuring the degree of organoid swelling. In a study employing this model, intestinal organoids were derived from patients who were compound heterozygous for five different nonsense mutations, in combination with either a frameshift mutation or the common Phe508del mutation. Interestingly, pretreatment with ataluren, either in isolation or in combination with ivacaftor and lumacaftor, did not result in either total organoid swelling or luminal swelling across all donor organoids. This particular finding suggested a failure of ataluren to induce detectable read-through activity in this specific experimental model.

 

Pharmacokinetics and Metabolism

 

Early-phase clinical investigations, specifically Phase I and Phase II studies, have consistently demonstrated that ataluren is rapidly absorbed when administered orally. These studies also revealed a dose-proportional increase in pharmacokinetic parameters, indicating that as the dose of ataluren increases, its presence in the body follows a predictable pattern. Importantly, no significant effects of gender or age on these pharmacokinetic parameters were observed, suggesting that the drug behaves similarly across diverse patient demographics. The maximum concentration of ataluren in the bloodstream, or peak plasma levels, is typically attained approximately two hours after a dose is administered. The drug’s elimination half-life, which is the time it takes for half of the drug to be cleared from the body, ranges between three and six hours. Furthermore, repeated dosing with ataluren showed no evidence of drug accumulation within the body, nor did it induce any significant metabolic auto-induction, processes that could complicate long-term treatment. Phase I studies also indicated that a relatively small percentage of the administered ataluren is excreted in the urine as the unchanged parent drug.

 

Clinical Efficacy

 

In all clinical studies discussed in this section, the participants enrolled were individuals diagnosed with cystic fibrosis who possessed two disease-causing CFTR mutations, with at least one of these mutations being a class I nonsense mutation. This specific genetic criterion ensured that the study population was relevant to ataluren’s targeted mechanism of action.

 

Phase II Studies

 

An initial Phase II study was conducted involving 23 adult participants. This investigation was designed as a prospective, open-label crossover trial, where participants received two distinct cycles of treatment. The first cycle included 23 participants, while the second cycle involved 21 individuals. Each cycle consisted of a 14-day treatment period, with a daily dose of 16 mg/kg in the first cycle and 40 mg/kg in the second, followed by a 14-day washout period during which no placebo was administered.

 

The primary objective of this study was to assess treatment response as measured by nasal potential difference, commonly abbreviated as nasal PD. Nasal PD serves as a reliable surrogate measure for evaluating the presence and functional activity of the CFTR protein on the surface of respiratory epithelial cells. The measurement process of nasal PD involves the sequential perfusion of specific compounds across the nasal epithelial surface. This sequence initially blocks the absorption of sodium ions and subsequently aims to enhance CFTR-mediated chloride transport. This intricate sequence of events results in dynamic changes in potential difference, ultimately leading to hyperpolarization. In individuals with cystic fibrosis, the most consistent and defining abnormality observed is the absence of this expected hyperpolarization.

 

A favorable treatment response was prospectively defined as an increase in total chloride transport of negative five millivolts or more. Normal chloride transport was also predefined as a nasal PD value that was at least as electrically negative as negative five millivolts. In this study, each patient served as their own control. This design was based on the premise that all participants initially presented with an abnormal baseline nasal PD, and that less than 5% of cystic fibrosis patients with nonsense mutations have ever been observed to exhibit nasal PD values indicative of “hyperpolarizers,” or individuals with normal chloride transport.

 

Improvements in total chloride transport were indeed observed in the majority of patients across both treatment cycles. Specifically, in the first cycle, 13 out of 23 patients, representing 57% of the cohort, showed improvement that reached the normal range of total chloride transport, a statistically significant finding. In the second cycle, 9 out of 21 patients, or 43%, similarly demonstrated this improvement, also achieving statistical significance. Notably, these observed improvements in nasal potential difference reverted to baseline values during the 14-day off-treatment washout period, strongly suggesting that the effects were directly mediated by ataluren. A small, though statistically significant, increase in forced expiratory volume in 1 second, FEV1, was also noted in the first phase of treatment, although the precise numerical values were not included in the original publication. Additionally, a small but statistically significant increase in body weight, with a mean change of +0.6 kilograms, was observed exclusively in the first treatment phase.

 

Following this initial trial, an open-label extension study was conducted with 19 of the original participants. This follow-on study aimed to evaluate the effects of 12 weeks of continuous ataluren treatment. Patients in this extension were allocated to the dosing regimen that had previously elicited their best response in the preceding study. The results showed statistically significant and ongoing improvement in total chloride transport. This improvement appeared to be progressive with increasing duration of treatment, but it was not found to be dose-dependent. A subsequent Phase II prospective crossover trial was the first to investigate the same primary outcome in a pediatric population, enrolling 30 participants aged 6 to 18 years. These children were randomized into two cohorts: one receiving a high-to-low dosing regimen and the other a low-to-high regimen. They were assessed in a similar manner, with two cycles consisting of 14 days on treatment followed by 14 days off. Using nasal PD as the metric, a total chloride response was observed in 50% of the patients, which amounted to 15 individuals. Furthermore, 47% of the patients, or 14 individuals, were observed to enter the normal range of total chloride transport, with higher response rates seen at higher dose levels in both cohorts. This study also marked a significant milestone as it was the first to use immunohistochemistry to observe an increased apical CFTR protein expression on nasal epithelial cells after treatment with ataluren, although it was not possible to correlate this directly with changes in nasal PD due to the small sample size and inherent variability in measurements. Consistent with previous findings, no statistically significant changes were observed in other clinical outcomes such as FEV1 and body weight in this pediatric cohort. Encouragingly, this study also provided valuable data demonstrating the safety profile of ataluren for use in children.

 

Across all of these Phase II studies, no consistent association could be established between the observed treatment response and any particular genotype or specific combination of genotypes. It is important to acknowledge that all these Phase II investigations were characterized by relatively small sample sizes and short trial durations. Furthermore, their crossover design, while useful for initial proof-of-concept, is not considered ideal for comprehensively studying the long-term clinical outcomes of a potentially disease-modifying therapeutic agent.

 

Phase III Studies

 

The promising results from the earlier phase II studies provided the impetus for the initiation of a pivotal phase III multicenter randomized controlled trial. This larger investigation enrolled a cohort of 238 patients, all of whom were six years of age or older. Participants in this extensive trial were randomly assigned to receive either ataluren, administered at a dose of 40 mg/kg per day in three divided doses, or a placebo, for a duration of 48 weeks. The primary outcome measure for this study was the change in percentage predicted forced expiratory volume in 1 second, or FEV1, a standard assessment of lung function. The rate of pulmonary exacerbations served as a key secondary outcome.

 

Upon analysis of the complete study population, no statistically significant difference in the mean relative change in FEV1 at 48 weeks was observed between the ataluren and placebo treatment groups. The mean difference favored ataluren by 1.76%, but this result fell within a wide 95% confidence interval ranging from -0.43% to 3.95%, indicating a lack of statistical significance. While the mean rate of pulmonary exacerbations was numerically lower in the ataluren-treated group compared to the placebo group over the 48-week period, with a rate ratio of 0.77, this difference did not reach statistical significance. Similar to the observations made in the phase II studies, there were no notable changes in sweat chloride concentrations, which was an exploratory outcome in this particular study. Furthermore, no significant difference in total chloride transport, as assessed by nasal potential difference, was detected following treatment.

 

Despite the overall lack of statistical significance in the primary analysis, a post-hoc analysis was subsequently performed. This analysis focused on a subgroup of patients, specifically differentiating between those receiving chronic inhaled tobramycin and those who were not. It is important to note that a subsequent Cochrane review identified this post-hoc analysis as carrying a high risk of bias due to selective reporting. Nevertheless, this analysis suggested that in the subgroup of patients who were not receiving tobramycin, the mean relative change in percentage predicted FEV1 at week 48 favored ataluren by 5.7%, with a 95% confidence interval of 1.5% to 10.1%, which was statistically significant. This intriguing observation was further supported by in vitro findings from a luciferase reporter assay, which demonstrated that ataluren-induced read-through activity was diminished when cells were co-incubated with a combination of ataluren and aminoglycoside antibiotics such as gentamicin or tobramycin. This suggested a potential inhibitory interaction between these classes of drugs.

 

To rigorously investigate these compelling observations regarding tobramycin, a further phase III study was specifically designed and undertaken. This trial enrolled 279 patients who were explicitly not receiving chronic inhaled tobramycin. However, in March 2017, a press release from PTC Therapeutics announced that the results of this confirmatory trial similarly failed to achieve its primary and secondary endpoints. While the full, peer-reviewed data from this study remained unpublished at the time of the original text’s writing, the brief announcement reported that the change in percentage predicted FEV1 favored ataluren by a mere 0.6%, a result that was not statistically significant. Similarly, the rate of pulmonary exacerbations in the ataluren group was 14% lower than in the placebo group, but this difference also did not reach statistical significance. In light of these consistently discouraging results, PTC Therapeutics made the decision to discontinue its clinical development program for ataluren in cystic fibrosis.

 

Safety and Tolerability

 

Preclinical pharmacology investigations conducted in animal models, specifically rats and dogs, indicated that oral administration of ataluren, even at high doses reaching up to 1500 mg/kg, did not induce any discernible neurological, pulmonary, or cardiovascular toxicities. These findings suggested a favorable safety profile in initial assessments. Across all human phase II studies, no clinically significant adverse effects that could be definitively attributed to ataluren were observed. However, it is important to note that none of these early-phase trials incorporated a placebo control group, which limits the ability to isolate drug-specific adverse events definitively. The majority of reported adverse events were classified as mild to moderate in severity, and crucially, their frequency or intensity did not demonstrate a dose-dependent increase. In the first phase II trial involving adult participants, the most commonly reported adverse event was dysuria, experienced by four individuals. Nevertheless, this symptom was not accompanied by any other urinary abnormalities, suggesting it was an isolated occurrence. This particular observation of dysuria was not replicated in the subsequent trial conducted in pediatric patients. Throughout the entire series of phase II studies, blood markers indicative of renal and liver function, such as creatinine levels and aminotransferase levels, remained consistently stable, providing reassurance regarding the drug’s impact on these vital organ systems.

 

A significant observation emerged from the first phase III trial, where a statistically significant increase in episodes of acute kidney injury was reported in the ataluren treatment group. These events were broadly described as renal failure, acute renal failure, renal impairment, or hypercreatininemia, and the rate ratio was notably high at 17.70, with a wide confidence interval of 1.28 to 244.40. The investigators attributed these instances of acute kidney injury in the ataluren arm to the concomitant use of systemic nephrotoxic antibiotics, specifically aminoglycosides and vancomycin, as well as instances of dehydration among the patients. In response to these findings, the study protocol was subsequently amended after seven months. This amendment specifically prohibited the concurrent use of ataluren with these systemic antibiotics and actively encouraged increased patient hydration to mitigate the risk of renal complications. Overall, ataluren was generally well tolerated by participants across all studies, and patient compliance with the treatment regimen did not present a significant issue.

 

Regulatory Affairs

 

The proprietary rights to ataluren are held by PTC Therapeutics, and the compound is commercially marketed under the trade name Translarna. Within the European Union Member States, ataluren has received approval for the treatment of nonsense mutation Duchenne muscular dystrophy in ambulatory patients aged over five years. Furthermore, it has been designated as an orphan medicinal product by the European Medicines Agency, recognizing its therapeutic potential for a rare disease. The United States Food and Drug Administration has also granted orphan drug designation to ataluren for the treatment of nonsense mutation Duchenne muscular dystrophy.

 

Conclusion

 

Class I nonsense CFTR mutations are identified in approximately 10% of individuals diagnosed with cystic fibrosis worldwide. These mutations are strongly associated with a severe clinical phenotype due to the fundamental absence of full-length, functional CFTR protein. Ataluren is an orally administered small-molecule drug specifically designed to facilitate the read-through of messenger RNA beyond a premature termination codon, thereby theoretically enabling the expression of a complete, functional protein. Laboratory investigations have provided in vitro evidence that ataluren does indeed facilitate this read-through process in cellular models that express nonsense CFTR mutations. Early-phase crossover studies conducted in patients with nonsense CFTR mutations also demonstrated favorable electrophysiological changes, specifically in nasal potential difference, following short-term ataluren treatment. However, despite these early promising signals, subsequent phase III randomized, blinded, and placebo-controlled clinical trials have, on two separate occasions, failed to demonstrate a significant clinical benefit from ataluren treatment. Consequently, at the present time, there is a lack of high-quality evidence to conclusively establish the clinical efficacy of ataluren in patients with cystic fibrosis. It is crucial to acknowledge that the most recent randomized controlled trial has not yet been formally published in a peer-reviewed format; thus, comments regarding its outcomes are based on a press release issued by PTC Therapeutics.

 

Expert Opinion

 

The narrative surrounding the identification of ataluren and its subsequent journey through development and evaluation as a potential therapeutic intervention for individuals with cystic fibrosis caused by class I nonsense CFTR mutations is, from both an academic and clinical perspective, a truly fascinating one. The story commenced with exciting foundational discovery science, leading to the identification of a small molecule that appeared capable of facilitating the translation of full-length protein, thereby addressing the core problem introduced by pathogenic premature termination codons. Within the realm of laboratory and basic science, there was a period of debate concerning its precise mechanism of action and the most appropriate methodologies for studying it, with the most recent publications confirming its in vitro efficacy.

 

Utilizing changes in nasal potential difference as a measurable indicator of CFTR function, short-term crossover studies conducted in both adults and children afflicted with cystic fibrosis due to class I nonsense CFTR mutations demonstrated some positive effects. Appropriately, large-scale multicenter randomized controlled trials were subsequently undertaken. The first of these extensive studies yielded essentially negative results, with the exception of a post-hoc analysis that focused specifically on participants who were not receiving tobramycin. The scientific rationale suggesting a relevant interaction with tobramycin offered renewed hope, leading to the execution of a further randomized controlled trial specifically in patients who were not concurrently receiving tobramycin. While the complete findings of this latter trial have yet to be formally published in a peer-reviewed journal, current understanding, derived from a press release by PTC Therapeutics, indicates that the results are negative, and the company has consequently decided to cease its pursuit of ataluren as a treatment for cystic fibrosis. Therefore, it is our considered opinion that there is currently no compelling, good-quality evidence to substantiate the clinical efficacy of ataluren in cystic fibrosis. With the benefit of hindsight, the existing evidence now suggests that the data associated with the post-hoc analysis from the subgroup not receiving tobramycin were ultimately not robust enough to serve as the sole foundation for an additional phase III study.

 

From a patient’s viewpoint, we suspect that this entire development story is particularly frustrating and deeply disappointing. Nonetheless, the overarching therapeutic approach of employing a small molecule to facilitate the read-through and expression of full-length, functional CFTR protein remains scientifically sound and, in our professional judgment, continues to warrant further dedicated investigation. Upon reflection, the comprehensive experience with ataluren to date profoundly illustrates several critical points pertinent to cystic fibrosis research. Firstly, it underscores the paramount importance of continuous and rigorous basic science research, employing the most valid and predictive experimental models. Secondly, it highlights the crucial significance of meticulous study design within the challenging field of translational cystic fibrosis research. There remain ongoing complexities and challenges surrounding which endpoints are most relevant and appropriate in early-phase versus later-phase studies within cystic fibrosis. These challenges inherently contribute to the difficulties encountered in designing and conducting adequately powered phase III studies, particularly within subsets of a patient population that is already considered to have an orphan disease.