Tobramycin is an aminoglycoside antimicrobial produced by Streptomyces tenebrarius. It acts primarily by disrupting protein synthesis leading to altered cell membrane permeability, progressive disruption of the cell envelope, and eventual cell death.

Tobramycin has in vitro activity against Gram-negative bacteria including P. aeruginosa. It is bactericidal in vitro at peak concentrations equal to or slightly greater than the minimum inhibitory concentration.

Susceptibility Testing

Interpretive criteria for inhaled antibacterial products are not defined. The in vitro antimicrobial susceptibility test methods used to determine the susceptibility for parenteral tobramycin therapy can be used to monitor the susceptibility of P. aeruginosa isolated from cystic fibrosis patients (1, 2, 3). The relationship between in vitro susceptibility test results and clinical outcome with TOBI Podhaler therapy is not clear. A single sputum sample from a cystic fibrosis patient may contain multiple morphotypes of P. aeruginosa and each morphotype may require a different concentration of tobramycin to inhibit its growth in vitro. Patients should be monitored for changes in tobramycin susceptibility.

Development of Resistance

In clinical studies, some increases from baseline to the end of the treatment period were observed in the tobramycin MIC for P. aeruginosa morphotypes. In general, a higher percentage of patients treated with TOBI Podhaler had increases in tobramycin MIC compared with placebo or patients treated with TOBI inhalation solution.

The clinical significance of changes in MICs for P. aeruginosa has not been clearly established in the treatment of cystic fibrosis patients.

Cross-Resistance

Some emerging resistance to aztreonam, ceftazidime, ciprofloxacin, imipenem, or meropenem were observed in the TOBI Podhaler clinical trials. As other anti-pseudomonal antibiotics were concomitantly utilized in many patients in the clinical trials, the association with TOBI Podhaler is not clear.

Other
No trends were observed in the isolation of treatment-emergent bacterial respiratory pathogens (Burkholderia cepacia, Stenotrophomonas maltophilia, Staphylococcus aureus and Achromobacter xylosoxidans).
13 NONCLINICAL TOXICOLOGY
13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility
Carcinogenicity studies were not conducted with TOBI Podhaler. A two-year rat inhalation toxicology study to assess carcinogenic potential of TOBI (tobramycin inhalation solution, USP) has been completed. Rats were exposed to TOBI for up to 1.5 hours per day for 95 weeks. Serum levels of tobramycin of up to 35 µg/mL were measured in rats, in contrast to the maximum 1.99 ± 0.59 µg/mL level observed in cystic fibrosis patients in TOBI Podhaler clinical trials. There was no drug-related increase in the incidence of any variety of tumor.
Additionally, tobramycin has been eva luated for genotoxicity in a battery of in vitro and in vivo tests. The Ames bacterial reversion test, conducted with 5 tester strains, failed to show a significant increase in revertants with or without metabolic activation in all strains. Tobra