1. Introduction
Cystic fibrosis (CF) is an incurable disease that affects the production of healthy mucus. This leads to a buildup of abnormal mucus that damages the lungs, digestive system, and other vital organs [1,2]. The global prevalence of diagnosed CF cases remains challenging to determine due to underdiagnosis limitations. However, the estimates suggest CF cases range from approximately 70,000 to over 160,000 [3,4]. The variations in CF prevalence across studies can be attributed to the heterogeneous approaches implemented by individual countries in their CF-screening programs [5]. This underscores the need for standardized methodologies to investigate CF prevalence on a global scale.
CF is caused by defects in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, following an autosomal recessive pattern of inheritance. An individual inherits two copies of the CFTR gene, one from each parent; however, only those who receive two mutated copies will develop the disease. [6]. Despite the existence of over 2000 different genetic variations in the CFTR gene, around 700 are identified as disease-causing variants [7,8,9]. Moreover, data suggest that the prevalence of CFTR mutation carriers ranges from 4% to 5% in the general population [10,11,12,13].
In a healthy state, the CFTR protein resides in the apical membrane of epithelial cells in organs like the lungs, liver, pancreas, intestines, and sweat glands. It functions as a chloride channel, regulating the movement of ions and water across these cells, maintaining a thin low-viscosity layer of mucus [14,15]. However, in patients with CF (pwCF), the absence or malfunction of the CFTR protein leads to the production of thick, sticky mucus that can damage organs over time, leading to serious health complications [15].
Lungs are the organs most severely affected in pwCF due to impaired mucociliary clearance caused by thickened mucus secretions. This dysfunction traps inhaled debris and pathogens, creating a breeding ground for chronic infections that are often accompanied by a chronic neutrophilic inflammatory response [16,17]. While the inflammatory response is crucial for fighting infection, prolonged neutrophil recruitment and inflammation can contribute to collateral damage in the lung tissues [18]. Neutrophils exacerbate lung damage through destructive degranulation and the release of neutrophil extracellular traps (NETs), which consist of DNA and antimicrobial proteins. Although NETs are intended to trap and eliminate infectious agents, their dysregulation contributes to excessive inflammation and tissue destruction in CF patients. The prolonged presence of neutrophils and their harmful by-products further damages lung tissues, leading to a progressive decline in lung function and worsening disease severity [19].
Pseudomonas aeruginosa (Pa) poses one of the greatest concerns for lung infections in pwCF. While other bacterial threats like Staphylococcus aureus and Haemophilus influenza can also establish infections, particularly in the early stages of life, Pa is particularly dangerous due to its ability to thrive in damaged airways, contribute to progressive lung disease, and resist antibiotic treatment [20,21]. By adulthood, it is estimated that Pa colonizes the airways of up to 80% of CF individuals, significantly impacting their health [22,23,24]. It has been shown that in the chronic state, Pa isolates are predominantly mucoid, often overexpressing alginate, which contributes to the formation of highly structured biofilms [25,26]. Furthermore, within biofilms, a subpopulation of bacterial cells (approximately 1%) enters a dormant state known as persistence, which exhibit neither active growth nor immediate death, even when exposed to high concentrations of antibiotics [27].
The early detection of Pseudomonas infection is crucial for improving the life expectancy of pwCF, as eradication therapy with intensive antibiotics is most successful in newly diagnosed cases [28,29,30]. However, once chronic infection is established, Pa becomes resistant to eradication, leading to more severe complications. Chronic Pa infections significantly accelerate the decline in lung function in pwCF, with estimates suggesting a 5–10% decrease in lung capacity per year. This decline can lead to serious complications, including early death [31,32,33]. To address this, a multi-pronged strategy is typically employed to manage the infection burden and increase life expectancy. This involves antimicrobial treatment to suppress bacterial growth, anti-inflammatory medications to manage chronic lung inflammation, and airway clearance therapies, including CFTR modulators, which enhance chloride transport, improve mucus viscosity, and reduce the frequency and severity of lung infections by preventing bacterial build-up [34,35,36]. A recent study in the US reported a significant reduction in mortality rates, with the median age of death increasing from 24 years in 1999 to 37 years in 2020, a change attributed to the implementation of aggressive multi-approach treatments in pwCF [37].
2. Overview of Phase IV Clinical Trials
Phase IV clinical trials are conducted after a drug or therapy has received regulatory approval and becomes available to the public. These studies, which include post-marketing surveillance (PMS), are crucial for evaluating the long-term safety, effectiveness, and potential adverse effects of treatments in a broader and more diverse patient population than those involved in earlier trial phases [38,39]. Unlike phases I–III, which are performed under controlled conditions with carefully selected participants, phase IV trials examine how therapies perform in real-world settings, offering a deeper insight into their practical applications, potential risks, and broader efficacy (Table 1) [40].
Modifying an approved treatment plan to suit a specific population, such as managing infections in pwCF, qualifies as a phase IV trial [38,39]. For CF patients, who often require prolonged therapy, PMS is particularly important for monitoring the long-term safety and efficacy of treatments [41]. The real-world data collected through phase IV trials enable clinicians to determine whether these therapies maintain their effectiveness over time, reduce the frequency of exacerbations, and ensure an acceptable safety profile for prolonged use. Additionally, these trials can identify emerging issues such as drug resistance and individual variations in response, and allow for the refinement of treatment regimens to improve the long-term outcomes in CF care [42].
4. Treatment Approaches for Management of Pa Infections in pwCF
Antibiotics are a key component of a multifaceted approach to manage infections in pwCF which includes anti-inflammatory therapies, mucolytics, bronchodilators, and CFTR modulators to improve lung function and reduce mucus build-up [36,123]. Around 80% of newly acquired Pseudomonas infections can be eradicated using a combinatorial therapeutic approach involving oral, inhaled, and intravenous antibiotics [124]. However, once the infection transitions to a chronic state, eradication becomes nearly impossible, and treatment focuses primarily on suppressing bacterial load and managing symptoms. The optimal antibiotic regimens, including the best combination of drugs, appropriate dosages, and treatment durations, are still areas of active investigation [125,126,127,128]. To date, no single antibiotic or established combination has been proven to achieve the definitive eradication of Pa in pwCF [129]. The diversity of these approaches reflects the ongoing search for the most effective treatment strategies for this complex and long-lasting condition.
4.1. Systemic Drugs
Systemic antibiotics are often chosen for their ability to achieve high systemic concentrations, making them effective in treating severe or widespread infections [130]. However, while systemic antibiotics can be highly effective in the short term, their long-term use raises concerns, including their potential toxicity and alterations in the pathogenicity of gut microbiota [131,132]. Limited data exist regarding the safety profiles of intravenous (IV) tobramycin and colistin for treating acute pulmonary exacerbations (PEx) in CF patients. Concerningly, IV tobramycin has demonstrated a significantly higher frequency of AEs compared with the two IV-dosing regimens of colistin (NCT02918409). Nephrotoxicity emerged as the most significant risk associated with IV tobramycin treatment, potentially leading to treatment failure and a worsening of the patient’s condition. This was evidenced by a higher incidence in patients receiving IV tobramycin (25%) compared with only 4% in those treated with both colistin regimens combined in this two-week duration study. In fact, nephrotoxicity is a well-recognized AE associated with IV administration of anti-Pseudomonas medications like tobramycin and colistin, as observed in several previous studies [133,134,135,136]. Notably, it has been demonstrated that renal function can recover to pre-treatment levels within 4 weeks after discontinuing tobramycin [137]. In contrast, colistin-induced nephrotoxicity exhibits a dose- and time-dependent relationship, with reported incidence varying between 10% and 70% across multiple studies [138,139,140,141,142]. This variation can also be attributed to the different diagnostic criteria and targeted groups of patients employed by studies [143]. Thus, the systemic administration of antibiotics presents a double-edged sword, necessitating judicious use, especially for chronic conditions.
4.2. Inhaled Drugs
Studies have shown that inhaled antibiotics are successful in reducing Pa density in the airways, improving lung function, and decreasing the frequency of PEx in pwCF with chronic infections [55,144]. Inhaled antibiotics offer a distinct advantage over systemic treatment because of their reduced systemic toxicity and the ability to achieve higher concentrations within the respiratory tract. Despite that, mild to serious AEs can arise in most patients with prolonged use [45,49,145]. In an extended phase IV study focused on the long-term safety of tobramycin inhalation powder (TIP) (NCT01519661 and NCT01775137), the drug demonstrated good tolerability with no unpredictable safety signals identified, and its efficacy was maintained throughout the observation period [146]. The reported AEs were comparable to those observed in other long-term studies from phase III [146,147,148]. Infective PEx was the most common adverse event and the primary reason for study discontinuation with a comparable severity to those in relevant studies [146,147,148]. Cough, the second most common adverse event, was found to decrease in frequency over time. This differentiates this phase IV investigation from previous trials that reported consistently higher rates of cough [146,147,148]. This decrease in cough frequency may be attributed to the improved inhalation techniques by patients, and the effectiveness of TIP in rapidly reducing bacterial load over time. This reduction in bacteria likely leads to a subsequent decrease in neutrophilic airway inflammation, which in turn, reduces sputum production and purulence [149,150,151]. While hemoptysis was also common, most cases were mild and likely linked to the underlying disease, as explained earlier by Thompson et al. (2015) [146,152]. Overall, TIP can be considered a safe and tolerable long-term treatment option for CF patients with established lung disease, although some AEs warrant monitoring. However, limitations inherent to the phase IV open-label, single-arm design necessitate consideration [146]. This is because the design is susceptible to reporting bias, especially for subjective AEs. Furthermore, the lack of a comparator group restricts the ability to definitively assess the treatment’s relative efficacy and safety profile when compared with alternative therapies or placebo.
Following the 1980s’ success of localized delivery of tobramycin, more effective treatments for chronic Pa infections in CF have been introduced via inhalation including colistin, aztreonam, and fluoroquinolones [153]. Nebulizers and dry powder inhalers (DPIs) represent two primary modes of direct drug delivery to the respiratory tract. In a comparative study between TIP and nebulized solutions of tobramycin (TIS) and colistimethate (NCT01844778), TIP administration via the T-326 inhaler demonstrated improved usability, reduced total delivery time, and significantly lower contamination rates compared with nebulizers [154]. Indeed, several studies have demonstrated that the reduced administration time and user-friendliness of inhaler devices positively impacts patient adherence to treatment regimens. This ease of use likely translates to minimized medication-handling errors and improved patient satisfaction with the therapy [155,156,157,158,159]. Moreover, DPIs have minimal risk of microbial contamination as frequent cleaning between uses is unnecessary [160]. In spite of the advantages, DPIs have a number of limitations compared with nebulizers. Certain antibiotics are currently unavailable in DPI formulations, restricting their use in this delivery method. Furthermore, the effective utilization of DPIs often requires a proper inhalation technique, potentially limiting their suitability for young children, older adults, or patients with compromised conditions [161].
4.3. Combination Therapies
While combination therapies offer the potential advantage of broader coverage against resistant bacteria, a careful evaluation of potential side effects and drug interactions is needed [162]. A common regimen involves TIP with oral azithromycin, a treatment prescribed to approximately 75% of patients with chronic lung infections [163]. However, the efficacy of this combination remains unclear. In a phase IV trial intended to investigate the impact of adding azithromycin to a standard TIP regimen (NCT02677701), the co-administration of azithromycin with TIP did not produce a synergistic effect in terms of improving lung function, nor did it result in significant changes in AEs or bacterial load when compared with a placebo [164]. Similarly, Mayer-Hamblett et al. (2018) reported no significant difference in the eradication rate of early Pa infection or clinical outcomes between CF patients receiving tobramycin solution for inhalation (TIS) with azithromycin and those receiving TIS alone [165]. On the other hand, several randomized controlled trials involving azithromycin have demonstrated improvements in lung function, a decrease in PEx and hospitalizations, and a reduced need for intravenous antibiotics in pwCF [166,167,168]. These benefits are likely attributable to the anti-inflammatory properties of azithromycin when used long-term. This can be explained by the shorter duration of a phase IV study (NCT02677701) compared with the other controlled trials and also that the Mayer-Hamblett study (2018) targeted early-stage infections before the establishment of chronic Pa colonization [165]. Therefore, the long-term use of azithromycin could be beneficial in improving lung function as a prophylactic measure, not through direct antimicrobial activity, but by mitigating inflammatory consequences and disease progression. However, more optimized studies are required to confirm these benefits in CF patients.
4.4. Optimal Dosing
Emerging evidence suggests that optimal dosing regimens may differ in CF population compared with non-CF patients [169]. Medication misuse, including overuse, can lead to undesirable side effects, compromising the intended therapeutic benefit and potentially culminating in treatment failure. To address this, two studies (NCT01429259 and NCT02421120) investigated the pharmacokinetics and tolerability of meropenem and ceftolozane/tazobactam (CZ/TAZ) in children and adult CF patients experiencing acute PEx, respectively. Interestingly, meropenem exhibited a higher clearance rate compared with previously reported values in healthy children [170,171,172,173]. In contrast, the findings of CZ/TAZ clearance revealed no significant difference between adult with and without CF condition [174,175,176]. However, the conclusive determination of optimal dosing regimens remains uncertain due to limitations in both phase IV studies. The relatively small sample sizes and the comparable baseline characteristics of the patient groups limit the generalizability of the findings. Larger and well-designed studies are warranted to definitively establish whether dosing strategies for these antibiotics differ between CF and non-CF populations.
4.5. CFTR Modulators
CFTR modulators have been shown to reduce Pa load in the sputum of CF patients [177,178]. These modulators include potentiators, such as Ivacaftor, which maintain the CFTR channels in an open state, allowing chloride ions to flow across cell membranes, and correctors, like Lumacaftor, which help ensure the protein folds correctly and reaches the cell surface [179,180,181]. Although they do not act as antibiotics, restoring CFTR function improves the lung environment, enhancing the effectiveness of antibiotic treatments by improving their penetration into biofilms and reducing the risk of reinfection by facilitating mucus clearance and decreasing bacterial colonization [177]. However, research in adults has shown that while Pa load decreases, chronic infections often persist and remain difficult to fully eradicate, particularly in individuals with advanced lung disease [178].
5. Challenges and Limitations in Treatments
Pa infections remain a major challenge in the management of CF patients, complicating efforts to improve the outcomes in this incurable condition. The genetic and phenotypic diversity of Pa populations in pwCF plays a critical role in treatment challenges and failures. This diversity allows bacterial subpopulations to transition between acute and chronic infection states, driven not only by mutations and the transfer of mobile genetic elements (MGEs) but also by the bacterium’s ability to modulate gene expression in response to environmental cues within the CF lung [182,183,184,185]. This on/off genetic regulation enables Pa to adapt dynamically to fluctuating conditions such as oxygen levels, nutrient availability, and immune pressures. Such adaptability promotes biofilm formation, antibiotic resistance, and immune evasion, allowing the pathogen to persist and thrive in the hostile lung environment. These adaptive strategies are particularly problematic in the context of the underlying pathophysiological conditions in CF patients, where thickened mucus and compromised immune responses create an ideal environment for Pa colonization [186,187,188]. The combined effects of genetic variability, biofilm development, and phenotypic flexibility further complicate efforts to eradicate the bacterium, often leading to persistent and recurrent infections despite aggressive therapy. This underscores the urgent need for continuous monitoring, personalized treatment strategies, and the reassessment of current therapeutic approaches to ensure effective long-term management of Pa infections in CF care [23,189,190,191,192].
Pa can readily develop resistance to most antibiotics. This situation is further complicated by the fact that prolong antibiotic exposure in pwCF selects for aggressive resistant strains, which makes it increasingly difficult to find effective antibiotics for treatment [193]. Pa is also notorious for its ability to form complex structures called biofilms. These complex structures can significantly impede therapeutic efforts by creating a formidable physical barrier around bacterial cell, effectively evading both immune defences and antibiotic action. Bacteria residing within biofilms also exhibit a downregulated metabolic state, leading to a restricted influx of external substrates [194,195]. It has been found that persister cells within the biofilm can tolerate up to 1000× minimum inhibitory concentration (MIC) of antibiotics under in vitro conditions [27,196].
The complex interplay between abnormal mucus production, impaired mucociliary clearance, and biofilm formation by Pa creates a fertile ground for chronic and recalcitrant infections in CF patients which can trigger the development of severe symptoms [197]. The presence of Pa infection is associated with more than 2.5-fold increase in the risk of mortality in CF patients over an 8-year period [198,199]. Acute PEx represents the worsening of symptoms and most common cause of death in CF patients. In fact, the increased frequency of PEx contributes to accelerated lung function decline and diminished quality of life [200,201]. As shown by two independent prognostic models, the higher frequency of annual cystic fibrosis PEx was directly linked to decreased 2-year and 5-year survival rates [202,203]. However, the prompt detection of bacterial cases accompanied by multifaceted management are crucial to minimize the risk, severity, and duration of exacerbations [204].
6. Emerging Treatments
Beyond conventional options, promising emerging therapies are demonstrating potential in the fight against P. aeruginosa infections in CF. Phage therapy is considered to be the most promising approach to treat CF infection [205]. Phages (or bacteriophages) are bacterial viruses with a remarkable degree of host specificity, exclusively targeting and lysing bacterial cells without affecting human cells. They were used as a therapeutic option in the early 20th century, particularly in Eastern Europe, before the widespread discovery and adoption of antibiotics. However, since the challenge of antimicrobial resistance intensifies, phage therapy re-emerges as a promising strategy for managing infections caused by MDR bacteria [206,207].
Gene-editing methods offer significant potential for addressing microbial infections in pwCF, either by fixing the CFTR gene mutation, which is the root cause of complications, or by directly targeting the causative pathogens [208,209,210,211]. CRISPR-Cas technology has emerged as a leading tool in this area. It can be customized to specifically target a CFTR mutation using a guide RNA (gRNA), which directs the Cas enzyme to the defective DNA sequence, enabling correction of the CFTR mutation. This restoration of normal CFTR function improves mucus clearance and reduces the likelihood of chronic infections [209,210]. Furthermore, CRISPR-Cas can be adapted to directly fight Pa infections by targeting bacterial DNA. This can be achieved by designing gRNAs that bind to specific genes responsible for bacterial virulence or antibiotic resistance, leading to the disruption of these genetic elements. As a result, the bacteria become less able to form biofilms or resist antibiotics, making it easier for the immune system to clear the infection [208,211].
Additionally, the development of biofilm-disrupting agents offers a potential solution against chronic bacterial infections [212]. These agents target the structural integrity of the biofilm matrix, which is composed of extracellular polymeric substances (EPS) that shield bacteria from antibiotics and the immune system. These agents can degrade the EPS, inhibit its formation, or disrupt quorum sensing that regulate biofilm development. By breaking down this protective barrier, the bacterial cells become more susceptible to antibiotic treatment and immune clearance [213]. One recent example is the human hormone hANP which has demonstrated promise in dismantling Pa biofilms and enhancing the efficacy of conventional antibiotics [214]. However, large-scale clinical trials are necessary to validate the efficacy and safety of these emerging therapies, especially in the context of CF. Additionally, addressing the potential for phage resistance and ensuring cost-effectiveness of these novel approaches requires further investigation.
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Mohammed Alqasmi www.mdpi.com
