Mechanism of Action
Tobramycin is an aminoglycoside antibacterial 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. 3
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.
Treatment for 6 months with tobramycin inhalation solution in two clinical studies did not affect the susceptibility of the majority of P. aeruginosa isolates tested; however, increased minimum inhibitory concentrations (MICs) were noted in some patients. The clinical significance of this information has not been clearly established in the treatment of P. aeruginosa in cystic fibrosis patients [see Clinical Studies (14)].
Interpretive criteria for inhaled antibacterial products are not defined. The in vitro antimicrobial susceptibility test methods used for parenteral tobramycin therapy can be used to monitor the susceptibility of P. aeruginosa isolated from cystic fibrosis patients. 4,5,6 If decreased susceptibility is noted, the results should be reported to the clinician. Susceptibility breakpoints established for parenteral administration of tobramycin do not apply to aerosolized administration of tobramycin inhalation solution. The relationship between in vitro susceptibility test results and clinical outcome with tobramycin inhalation solution therapy is not clear. A single sputum sample from a cystic fibrosis patient may contain multiple morphotypes of Pseudomonas aeruginosa and each morphotype may have a different level of in vitro susceptibility to tobramycin.
For specific information regarding susceptibility test interpretive criteria and associated test methods and quality control standards recognized by FDA for this drug, please see: https://www.fda.gov/STIC.
A two-year rat inhalation toxicology study to assess carcinogenic potential of tobramycin inhalation solution has been completed. Rats were exposed to tobramycin inhalation solution for up to 1.5 hours per day for 95 weeks. The clinical formulation of the drug was used for this carcinogenicity study. Serum levels of tobramycin of up to 35 mcg/mL were measured in rats, in contrast to the average 1 mcg/mL levels observed in cystic fibrosis patients in clinical trials. There was no drug-related increase in the incidence of any variety of tumor.
Additionally, tobramycin has been evaluated 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. Tobramycin was negative in the mouse lymphoma forward mutation assay, did not induce chromosomal aberrations in Chinese hamster ovary cells, and was negative in the mouse micronucleus test.
Subcutaneous administration of up to 100 mg/kg of tobramycin did not affect mating behavior or cause impairment of fertility in male or female rats.
Two identically designed, double-blind, randomized, placebo-controlled, parallel group, 24-week clinical studies (Study 1 and Study 2) at a total of 69 cystic fibrosis centers in the United States were conducted in cystic fibrosis patients with P. aeruginosa with tobramycin inhalation solution. Subjects who were less than 6 years of age, had a baseline creatinine of > 2 mg/dL, or had B. cepacia isolated from sputum were excluded. All subjects had baseline FEV1 % predicted between 25% and 75%. In these clinical studies, 258 patients received tobramycin inhalation solution therapy on an outpatient basis (see Table 2) using a PARI LC PLUS nebulizer along with a DeVilbiss Pulmo-Aide compressor.
|Cycle 1||Cycle 2||Cycle 3|
|28 days||28 days||28 days||28 days||28 days||28 days|
|TOBRAMYCINInhalation Solution regimenn=258||TOBRAMYCIN InhalationSolution 300 mg twice daily||No drug||TOBRAMYCIN InhalationSolution 300 mg twice daily||No drug||TOBRAMYCIN InhalationSolution 300 mg twice daily||No drug|
|Placebo regimenn=262||Placebo twice daily||Placebo twice daily||Placebo twice daily|
All patients received either tobramycin inhalation solution or placebo (saline with 1.25 mg quinine for flavoring) in addition to standard treatment recommended for cystic fibrosis patients, which included oral and parenteral anti-pseudomonal therapy, Beta 2-agonists, cromolyn, inhaled steroids, and airway clearance techniques. In addition, approximately 77% of patients were concurrently treated with dornase alfa.
In each study, tobramycin inhalation solution-treated patients experienced significant improvement in pulmonary function. Improvement was demonstrated in the tobramycin inhalation solution group in Study 1 by an average increase in FEV1 % predicted of about 11% relative to baseline (Week 0) during 24 weeks compared to no average change in placebo patients. In Study 2, tobramycin inhalation solution-treated patients had an average increase of about 7% compared to an average decrease of about 1% in placebo patients. Figure 1 shows the average relative change in FEV1 % predicted over 24 weeks for both studies.
In each study, tobramycin inhalation solution therapy resulted in a significant reduction in the number of P. aeruginosa colony forming units (CFUs) in sputum during the on-drug periods. Sputum bacterial density returned to baseline during the off-drug periods. Reductions in sputum bacterial density were smaller in each successive cycle. (see Figure 2).
Patients treated with tobramycin inhalation solution were hospitalized for an average of 5.1 days compared to 8.1 days for placebo patients. Patients treated with tobramycin inhalation solution required an average of 9.6 days of parenteral anti-pseudomonal antibiotic treatment compared to 14.1 days for placebo patients. During the 6 months of treatment, 40% of tobramycin inhalation solution patients and 53% of placebo patients were treated with parenteral anti-pseudomonal antibiotics.
The relationship between in vitro susceptibility test results and clinical outcome with tobramycin inhalation solution therapy is not clear. However, 4 tobramycin inhalation solution patients who began the clinical trial with P. aeruginosa isolates having MIC values ≥ 128 μg/mL did not experience an improvement in FEV1 or a decrease in sputum bacterial density.
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