TOBI: reducing the impact of pseudomonal infection

Antimicrobial molecules in the lung: formulation challenges and future directions for innovation

Inhaled antimicrobials have been extremely beneficial in treating respiratory infections, particularly chronic infections in a lung with cystic fibrosis. The pulmonary delivery of antibiotics has been demonstrated to improve treatment efficacy, reduce systemic side effects and, critically, reduce drug exposure to commensal bacteria compared with systemic administration, reducing selective pressure for antimicrobial resistance. This review will explore the specific challenges of pulmonary delivery of a number of differing antimicrobial molecules, and the formulation and technological approaches that have been used to overcome these difficulties. It will also explore the future challenges being faced in the development of inhaled products and respiratory infection treatment, and identify future directions of innovation, with a particular focus on respiratory infections caused by multiple drug-resistant pathogens.

Activity and interactions of liposomal antibiotics in presence of polyanions and sputum of patients with cystic fibrosis

Background

To compare the effectiveness of liposomal tobramycin or polymyxin B against Pseudomonas aeruginosa in the Cystic Fibrosis (CF) sputum and its inhibition by common polyanionic components such as DNA, F-actin, lipopolysaccharides (LPS), and lipoteichoic acid (LTA).

Methodology

Liposomal formulations were prepared from a mixture of 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine (DMPC) or 1,2-Dipalmitoyl-sn-Glycero-3-Phosphocholine (DPPC) and Cholesterol (Chol), respectively. Stability of the formulations in different biological milieus and antibacterial activities compared to conventional forms in the presence of the aforementioned inhibitory factors or CF sputum were evaluated.

Results

The formulations were stable in all conditions tested with no significant differences compared to the controls. Inhibition of antibiotic formulations by DNA/F-actin and LPS/LTA was concentration dependent. DNA/F-actin (125 to 1000 mg/L) and LPS/LTA (1 to 1000 mg/L) inhibited conventional tobramycin bioactivity, whereas, liposome-entrapped tobramycin was inhibited at higher concentrations - DNA/F-actin (500 to 1000 mg/L) and LPS/LTA (100 to 1000 mg/L). Neither polymyxin B formulation was inactivated by DNA/F-actin, but LPS/LTA (1 to 1000 mg/L) inhibited the drug in conventional form completely and higher concentrations of the inhibitors (100 to 1000 mg/L) was required to inhibit the liposome-entrapped polymyxin B. Co-incubation with inhibitory factors (1000 mg/L) increased conventional (16-fold) and liposomal (4-fold) tobramycin minimum bactericidal concentrations (MBCs), while both polymyxin B formulations were inhibited 64-fold.

Conclusions

Liposome-entrapment reduced antibiotic inhibition up to 100-fold and the CFU of endogenous P. aeruginosa in sputum by 4-fold compared to the conventional antibiotic, suggesting their potential applications in CF lung infections.

Optimum peripheral drug deposition in patients with cystic fibrosis

In order to identify the optimum particle size and breathing pattern for high peripheral deposition of inhaled drugs in patients with cystic fibrosis, regional deposition in these patients was studied systematically as a function of particle size, inhalation volume and flow rate. Regional deposition was assessed using the single-breath regional deposition technique in which the concentration profile of inhaled and exhaled non-radioactive, monodisperse test particles is analyzed. Using this technique particle deposition within the functional dead space volume and peripherally can be assessed. Regional deposition was measured in 12 patients with cystic fibrosis using 2, 3, 4, and 5.5 microm particles, inhalation volumes of 500, 1000, 1500, and 2000 cm(3), and inhalation flow rates of 100, 250, 500, and 750 cm(3)/sec. Peripheral deposition was highest when 2-3-microm particles were inhaled with air-flow rates of 250-500 cm(3)/sec. With these parameters peripheral deposition increased with increasing inhalation volume and reached values of about 60% of the total drug inhaled. It has been shown that high peripheral drug deposition can be achieved in patients with CF when inhalations are performed using an optimized combination of particle size and breathing pattern.

Inhaled Tobramycin (TOBI??)

Specifically formulated for nebulisation using the PARI LC PLUS reusable nebuliser, tobramycin solution for inhalation (TSI) [TOBI] provides a high dose of tobramycin (an aminoglycoside antibacterial with good activity against Pseudomonas aeruginosa) to the lungs of patients with cystic fibrosis, while maintaining low serum concentrations of the drug, thus reducing the risk of systemic toxicity. Intermittent (28-day on/28-day off) treatment with TSI 300 mg twice daily significantly (p < 0.001) improved lung function and sputum P. aeruginosa density compared with placebo (randomised double-blind trials), and was significantly (p = 0.008) more effective than colistin for improvement in forced expiratory volume in 1 second (small nonblind trial) in patients aged > or =6 years with cystic fibrosis and chronic P. aeruginosa infection. Improvements in lung function were most marked in adolescent patients (aged 13-17 years) in placebo-controlled trials. Improvements were maintained for up to 96 weeks in patients in an open-label extension study. Fewer TSI than placebo recipients required parenteral antipseudomonal agents or hospitalisation. In addition, TSI 300 mg twice daily for 28 days reduced P. aeruginosa density in the lower airways of patients aged <6 years with early colonisation and cystic fibrosis, although TSI is not currently indicated in this patient group. A decrease in tobramycin susceptibility of P. aeruginosa isolates and an increase in fungal organisms (Candida albicans and Aspergillus species) during prolonged intermittent treatment with TSI 300 mg twice daily was not associated with adverse clinical outcome. There was no evidence of selection for the most resistant isolates. TSI is generally well tolerated, with no renal toxicity or hearing loss in clinical trials, although transient mild or moderate tinnitus occurred more frequently in TSI than placebo recipients. Bronchospasm after administration of TSI was transient and occurred with a similar incidence to that with placebo; TSI is preservative free and specifically formulated for the lung in terms of osmolality and pH. In conclusion, TSI provides an effective means of delivering tobramycin to the lungs of patients with cystic fibrosis with chronic P. aeruginosa infection, improving lung function and sputum P. aeruginosa density in these patients without the nephrotoxicity or ototoxicity of parenteral aminoglycosides. Further data on the potential for and clinical significance of increased tobramycin resistance and fungal colonisation during TSI treatment would be beneficial, as would longer-term data. In the meantime, TSI represents a valuable option for suppressive antipseudomonal therapy in patients with cystic fibrosis.