Evidence for the efficacy of aztreonam for inhalation solution in the management of Pseudomonas aeruginosa in patients with cystic fibrosis

Pulmonary infections in cystic fibrosis

In cystic fibrosis (CF), the genetic defect of the cystic fibrosis transmembrane conductance regulator (CFTR) gene causes dehydration of the airways, leading to the accumulation of thick, viscous mucus in the airways of affected individuals. Recurrent and ultimately chronic airway infection is a major complication in CF and leads to the increased morbidity and mortality found in this disease. A diversity of pathogens can be found in these patients: at a younger age, common agents are Staphylococcus aureus, or Haemophilus influenzae, but other pathogens such as Pseudomonas aeruginosa can cause recurrent and ultimately chronic, pulmonary infection during a patient’s life [Burns et al. 2001; Li et al. 2005]. Other pathogens have become increasingly common in chronically infected CF patients, including Burkholderia cepacia complex, Achromobacter xylosoxidans, Stenotrophomonas maltophilia and nontuberculous mycobacteria. Many of these microorganisms are resistant to multiple antibiotics, making treatment very difficult.

Endobronchial infection leads to a local inflammatory response which is ineffective in clearing the infection. Inflammation leads to airway damage, which in turn increases the risk of new infections, creating a vicious circle ultimately leading to end-stage lung disease and the need of lung transplantation.

Over the last 50 years, CF treatment regimens have improved immensely, and survival of these patients is increasing. Improvements in lung infection treatment are closely related to the treatment of P. aeruginosa. All CF patients will become intermittently colonized with P. aeruginosa during childhood. Without specific, anti-P. aeruginosa treatment, the development of chronic infection is inevitable. Development of infection is characterized by increased, P. aeruginosa-specific antibodies and acquisition of the mucoid phenotype of the bacteria [Li et al. 2005]. P. aeruginosa is still the most common cause of chronic, pulmonary infection in adult CF patients [Cystic Fibrosis Foundation, 2013; ECFS Patient Registry, 2012].

With the introduction of aggressive, antibiotic treatment for P. aeruginosa infection, using courses of intravenous antibiotics, survival increased. At the same time, lung function decline was slowed or prevented in patients treated this way [Frederiksen et al. 1996]. The problem with intravenous antibiotics, however, is the need for patients to be hospitalized. In spite of introduction of home intravenous treatment, the use of inhaled antibiotics has changed treatment of chronic P. aeruginosa infection into a more flexible treatment, available through out-patient clinics.

Inhaled antibiotics

The rationale for use of inhaled antibiotics is based on the endobronchial location of infections in CF patients. Endobronchially located sputum is not easily reached through systemic administration of antibiotics. The use of inhaled antibiotics favours high concentrations at the localization of the microorganisms without systemic exposure and the risk of side effects, and has been a part of CF treatment for many years. In Europe, nebulized colomycine was the first antibiotic used this way during the 1980s [Littlewood et al. 1985; Frederiksen et al. 1999]. For many years, the intravenous preparation was used for inhalation, until a specific preparation for inhalation was made. In the US, nebulized tobramycin using a formulation made specifically for inhalation, has been approved by the US Food and Drug Administration (FDA) and used in standard CF treatment since the 1990s [Ramsey et al. 1999].

Initially, inhaled antibiotics were used in CF patients chronically infected with P. aeruginosa, and led to clinical stabilization and a decrease in the need for courses of intravenous antibiotics. Over the years, however, inhaled antibiotics are being used in an increasing number of patients, and most CF centres now use inhaled colomycine or tobramycin for treatment of early, intermittent P. aeruginosa infection even in young CF patients [Döring and Høiby, 2004; Saiman and Siegel, 2003].

Microbiologically, microorganisms naïve to antibiotics are often quite sensitive, and when a new treatment is introduced, the effect can be extraordinary. Using the antibiotic over longer periods of time increases the risk of development of resistance [Burns et al. 1998] or a diminished effect in the patients if repeated courses of treatment are used [Assael et al. 2012]. Although both colomycine and tobramycin have been proven efficient in treating P. aeruginosa airway infections in CF, the possibility of even more antibiotics available for inhalation, would further improve the treatment results. Furthermore, change of type of antibiotic with intervals would increase the chance of keeping the patient clinically stable for longer periods of time.

Prior to the original studies on inhaled tobramycin and inhaled colomycine, patients never received any kind of inhaled antibiotics. Chronic P. aeruginosa infection was treated with courses of intravenous anti-P. aeruginosa antibiotics which were given a few times a year, depending on the treatment policy of the individual CF centres and on the clinical stability of the patient.

When introducing the first inhaled antibiotics, the results were overwhelming. The double-blind, placebo-controlled study of inhaled tobramycin performed by Ramsey and colleagues showed an increase in forced expiratory volume in 1 second (FEV₁) of 10% of predicted in the tobramycin group compared with the placebo group [Ramsey et al. 1999]. Inhaled antibiotics were not used as part of standard treatment when the study was performed, and included patients had a mean FEV₁ of approximately 50% of predicted. Dornase alfa, which is now part of standard CF treatment in many CF centres, was approved in 1994, and since the tobramycin study was performed in 1995 and 1996, it is safe to assume that participants did not receive dornase alfa and that most of the microorganisms tested were naïve to tobramycin. These reasons may explain the amazing effect of tobramycin found in this study.

Since CF treatment has improved in many ways over recent years, with the introduction of dornase alfa and the approval of tobramycin for inhalation, CF patients have become more stable, with a marked increase in lung function since the 1990s [Cystic Fibrosis Foundation, 2013]. The initiation of treatment using inhaled antibiotics meant a huge difference to patients in terms of improved clinical status and stability. When new treatments are introduced in CF patients now, these treatments need to be compared with well-known treatments to prove efficacy. Furthermore, the clinical stability of these patients is very different from the first studies, and the magnitude of the treatment effect of new drugs may not be as pronounced as in the earlier studies.

Investigating new treatments

When investigating the effect of new treatments, comparison of a new treatment with placebo can be ethically impossible if it means some participants are deprived of a treatment known to improve their condition, which means new treatments need to be compared head-on against old treatments. Investigators often aim at noninferiority compared with older treatments known to be efficient. In order to prove superiority to older treatments, a very large number of participants and extended study periods are needed, making investigation more difficult.

Study endpoints have varied in different studies. Since many CF patients are now respiratory stable with normal levels of lung function, a significant improvement of lung function can be difficult to achieve. Other endpoints may be time to pulmonary exacerbation (and need for extra antibiotics and hospitalization), improvement of nutritional status, quality of life or stabilization of a patient’s clinical status. These new endpoints may be as meaningful in the eyes of the patient and his/her care-taker as a significant improvement in lung function of a few per cent of predicted.

Inhaled aztreonam

Aztreonam lysine for inhalation was developed in the US and approved by FDA as the second antibiotic for inhalation [US Food and Drug Administration, 2010]. The approval is for ‘improvement of respiratory symptoms and pulmonary function in patients with cystic fibrosis and P. aeruginosa’. The reasons for approval were that not all CF patients could use tobramycin, that aztreonam does not bind to sputum, leaving all inhaled drug active, and that aztreonam has a favourable antibiotic profile with activity against most aerobic Gram-negative microorganisms.

Aztreonam is a monobactam with wide-spectrum activity against aerobic Gram-negative bacteria and has been used for parenteral treatment for a variety of serious infections. The intravenous formulation is an arginine formulation, and since arginine salt is a substrate for the production of nitric oxide in the lung, the use of this for inhalation could lead to increased airway inflammation. In order to prevent this potential adverse effect, a lysine salt formulation of aztreonam was developed.

The aztreonam lysine for inhalation has been tested in CF patients in a number of studies. A pilot study by Gibson and colleagues in 12 adult CF subjects on microbiology, safety and pharmacokinetics revealed high sputum concentrations but low systemic exposure and no change in antibiotic resistance in microorganisms found in this group of patients [Gibson et al. 2006]. Three different doses of aztreonam, from 75 to 225 mg, were tested in adult and adolescent CF patients. Sputum concentrations of aztreonam lysine remained above the minimum inhibitory concentration to inhibit 50% of the bacterial strains (MIC₅₀) for up to 4 hours after inhalation for all different doses. Adverse events were mostly from the respiratory system such as coughing, crackles and increased sputum. All adverse events were consistent with those reported in CF patients with mild to moderate disease [Caron et al. 2003].

Following this, a phase II study was initiated [Retsch-Bogart et al. 2008], including a total of 105 patients receiving two different doses of aztreonam (75 or 225 mg) or two different volumes of placebo, matching the dose of the active agent when compared to volume. This study was conducted in the USA. Participants had CF, were colonized with P. aeruginosa at the time of screening, were able to produce sputum, were above the age of 13 years and had FEV₁ of above 40% of predicted. The duration of the study was up to 37 days. The treatment was given twice daily for 14 days. In both aztreonam-treated groups, sputum density of P. aeruginosa decreased significantly compared with the placebo groups. FEV₁ increased by day 7 in both aztreonam-treated groups, but the increase was most significant in patients with FEV₁ below 75% of predicted prior to study start. Use of inhaled short-acting bronchodilators prior to inhalation of aztreonam was associated with a greater reduction in P. aeruginosa bacterial density, greater improvement in FEV₁ from baseline to day 14 and higher plasma aztreonam concentrations in the group treated with 225 mg doses. These results led to some conclusions for subsequent inhaled aztreonam studies: the duration of therapy should be increased to 28 days; refrain from using the 225 mg dose in future studies, because there was a trend toward increased respiratory symptoms in patients receiving higher doses; bronchodilator use should be used for pretreatment in all participants; and finally, a new dosing regimen of three daily doses should be compared with the two daily doses used so far.

In the next study of inhaled aztreonam (AIR-CF2), the treatment was prolonged to 28 days and was given immediately after completion of 28 days’ treatment using inhaled tobramycin. Participants were randomized to aztreonam BID, aztreonam TID or placebo [McCoy et al. 2008]. The aztreonam dose was 75 mg. Primary endpoints were change in FEV₁, in bacterial density of sputum, change in quality of life scores and time to need for additional intravenous or inhaled anti-P. aeruginosa antibiotics as a marker for symptoms of exacerbations. This setup ensured that participants initiated aztreonam-treatment at a peak of clinical stability and suppression of the chronic lung infection. Participants were followed for up to 87 days without further planned antibiotic treatment. Only 90 of 246 patients completed the study, but more than half of discontinued participants met the primary endpoint, since the occurrence of adverse events (mostly respiratory in nature) led to initiation of extra anti-P. aeruginosa treatment, thus meeting the primary endpoint of the study. Patients receiving aztreonam (both groups were pooled) had a significantly longer period of time without need of extra antibiotics compared with placebo. The CFQ-R score specific for respiratory symptoms significantly improved in the aztreonam-treated group [McCoy et al. 2008].

Another study conducted in parallel (AIR-CF1) with this study was comparing inhaled aztreonam TID (dose 75 mg), but including patients who had not previously been receiving maintenance therapy as recommended [Flume et al. 2007]. Participants received 28 days of treatment with aztreonam or placebo [Retsch-Bogart et al. 2009] and were followed up for another 14 days. Although these patients were slightly older (mean age in AIR-CF1 29.6 years, in AIR-CF2 26.2 years) and received less maintenance therapy than recommended, lung function levels were comparable between the two studies (mean FEV₁ in AIR-CF1 54.6% predicted, in AIR-CF2 55.1% predicted). In this study, however, the group of aztreonam lysine-treated patients had a significant increase in FEV₁ after only 28 days and improvement in the quality of life score compared with the placebo group.

Participants of both studies were offered the opportunity of continuing in a prolonged study of repeated cycles of treatments over the next 18 months (AIR CF3). Participants from the AIR-CF2 study who had received BID treatment with either aztreonam or placebo were continued on aztreonam BID, while all other participants continued on aztreonam TID. Inhaled aztreonam was given in 28 days on/28 days off cycles for up to nine treatments. This treatment was well tolerated, and compliance was good. During this open-label study, all participants received routine CF therapy as prescribed by their primary CF-care provider, and interestingly, half of the participants received a mean of 3–4 courses of inhaled tobramycin during the study. It is not stated whether tobramycin was used as add-on treatment or during the off-periods. The group treated with aztreonam TID had a somewhat more pronounced improvement in FEV₁ compared with the BID treated group. All participants had improved quality of life scores. During AIR-CF3, no increase in aztreonam resistance was found throughout the study [Oermann et al. 2011]. Susceptibility to other, commonly used anti-P. aeruginosa antibiotics was stable as well.

The latest study is a European–USA multicentre study comparing aztreonam to tobramycin inhalation solution in three cycles of treatment [Assael et al. 2012] of either aztreonam 75 mg TID or tobramycin 300 mg BID. Enrolled patients were not tobramycin-naïve, but received respiratory treatment according to recommendations, including dornase alfa, azithromycin and hypertonic saline. All participants were considered clinically stable. Results showed superiority of aztreonam compared to tobramycin, with a significant increase in FEV₁ compared with baseline, not only after 28 days, but even after three cycles of treatment. Furthermore, aztreonam-treated patients had significantly longer exacerbation-free intervals compared with the tobramycin-treated group. The study was extended after 3 months as a crossover study, with eligible participants continuing for 6 months with another three cycles of aztreonam on/off. The effect of inhaled aztreonam during the extended study in the group of patients receiving inhaled tobramycin during the first 6 months, was similar to the effect found during the primary study in the group receiving inhaled aztreonam during the first 6 months.

Further studies are ongoing. Studies in children with chronic P. aeruginosa lung infection [ClinicalTrials.gov identifier: NCT01404234] and of aztreonam for P. aeruginosa infection eradication [ClinicalTrials.gov identifier: NCT01375049] have been completed, but results are not yet available. A study of possible reduction in airway inflammation during inhaled aztreonam treatment in CF patients with chronic P. aeruginosa infection is currently recruiting [ClinicalTrials.gov identifier: NCT01736839]. Finally, an ongoing study of continuous therapy using inhaled aztreonam or placebo alternating with inhaled tobramycin as a treatment regimen for chronic P. aeruginosa infection is currently recruiting [ClinicalTrials.gov identifier: NCT01641822].

The future of treatment

The use of inhaled antibiotics in the USA has been based on a 1 month on/one month off cycle. Several studies have shown how patients became clinically stable when receiving inhaled antibiotics, and how lung function declined and growth of P. aeruginosa increased during the off-periods.

The introduction of a new antibiotic for inhalation offers the opportunity of improving anti-P. aeruginosa treatment by changing this practice to cycles alternating between different inhaled antibiotics. The perspective of this means greater stability for the patients, and help suppressing the development of resistance, since alternating antibiotics would prevent resistant strains from out-growing the sensitive strains. Other uses may be as add-on therapy to other inhaled antibiotics, since combination therapy may be beneficial, especially in patients with resistant bacterial strains [McCaughey et al. 2013; Yu et al. 2012].

Studies conducted so far have focused on the use of inhaled aztreonam in CF patients with chronic P. aeruginosa infection. The next generation of CF patients, having received early, aggressive treatment for intermittent P. aeruginosa infection may be different, and the possible use of inhaled aztreonam in intermittently colonized patients need to be investigated, as well as there is a need for agreement on how to treat intermittent infections.

The use of aztreonam in other Gram-negative bacterial infections apart from P. aeruginosa may be possible. Oermann and colleagues found Burkholderia species strains to be sensitive to aztreonam [Oermann et al. 2010]. A recent study, however, found no effect when comparing aztreonam inhalations with placebo in a large group of CF patients chronically infected with Burkholderia species [Tullis et al. 2013]. A. xylosoxidans shows resistance to aztreonam [Gibson et al. 2006]. Other causes of infection, such as Stenotrophomonas maltophilia or Bordetella bronchiseptica, may also be possible targets for the use of inhaled aztreonam.