It was previously demonstrated that fluid liposomal-encapsulated tobramycin, named Fluidosomes, was successful in eradicating mucoid Pseudomonas aeruginosa in an animal model of chronic pulmonary infection, whereas free antibiotic did not reduce colony-forming unit (CFU) counts (C. Beaulac et al., Antimicrob. Agents Chemother. 40 (1996) 665-669; C. Beaulac et al., J. Antimicrob. Chemother. 41 (1998) 35-41). These liposomes were also shown to be bactericidal in in vitro tests against strong resistant P. aeruginosa 64 microg/ml). The time needed to reach the maximal fusion rate was about 5 h for the resistant strain comparatively to much shorter time for the sensitive strain. The specific characteristics of Fluidosomes could help overcome bacterial resistance related to permeability barrier and even enzymatic hydrolysis considering the importance of synergy in the whole process of antibiotic resistance.

It has been shown previously that tobramycin encapsulated in fluid liposomes (composed of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylglycerol (DMPG)) eradicated mucoid Pseudomonas aeruginosa in an animal model of chronic pulmonary infection. Exponential cultures of P. aeruginosa, Stenotrophomonas maltophila, Burkholderia cepacia, Escherichia coli and Staphylococcus aureus were treated with (i) free tobramycin, (ii) sub-MIC tobramycin encapsulated in DPPC/DMPG liposomes, (iii) control liposomes without antibiotic or (iv) control liposomes combined with free tobramycin. Bacterial colonies were counted 0, 1, 3, 6 and 16 h after addition of antibiotic. After 3 h, the growth of B. cepacia, E. coli and S. aureus was reduced 129, 84 and 566 times respectively in cultures treated with encapsulated antibiotic compared with those treated with free antibiotic. Six hours and 16 h after treatment, the maximal reduction of growth between strains treated with liposome-encapsulated tobramycin and free tobramycin was 84, 129, 166, 10(5) and 10(4) times respectively for P. aeruginosa, B. cepacia, E. coli, S. maltophilia and S. aureus. The liposomes were stable at 4 degrees C and at room temperature for the whole period studied. At 37 degrees C, equivalent stability was observed for the first 16 h of the study. Administration of antibiotic encapsulated in DPPC/DMPG liposomes may thus greatly improve the management of resistant infections caused by a large range of microorganisms. The strong bactericidal activity of the encapsulated antibiotic at sub-MIC doses of the strains tested cannot be explained only as a result of prolonged residence time of liposome-encapsulated tobramycin and the resulting release of entrapped antibiotic at the bacterial site; rather, direct interaction of chemoliposomes and bacteria, probably by a fusion process, may explain the bactericidal effect of the sub-MIC antibiotic doses used.

The pulmonary residence time of free and liposome-encapsulated tobramycin was studied with uninfected rats and rats infected with Pseudomonas aeruginosa. Chronic infection in lungs was established by intratracheal administration of 108 CFU of P. aeruginosa PA 508 prepared in agar beads. After 3 days, a single dose (300 ,ug) of free or liposome-encapsulated tobramycin was given intratracheally to both infected and uninfected rats. At various time intervals (0.25 to 16 h) after drug instillations, the remaining tobramycin was evaluated in blood, lungs, and kidneys by a microbiological assay. Intratracheal instillation of liposome-encapsulated tobramycin resulted in high and sustained levels of tobramycin in lungs of uninfected and infected rats over the 16-h period studied; however, the tobramycin levels were two times higher in uninfected rats. There was no tobramycin detected in the blood or kidneys from these animals. In contrast, the intratracheally instilled free tobramycin was cleared within 3 and 1 h from the lungs of uninfected and infected animals, respectively. These data suggest that the encapsulation of tobramycin in liposomes can result in a significant increase of its residence time within lungs. This study also shows that pulmonary infection was associated with a lowering of tobramycin levels in lungs.

Despite controversies associated with forms and value of antibiotic therapy for cystic fibrosis patients, antibiotherapy remains a cornerstone in the management of those patients. Locally administered liposome-encapsulated antibiotics may offer advantages over free antibiotics, including sustained concentration of the antibiotic, minimal systemic absorption, reduced toxicity, and increased efficacy. We evaluated the efficacy of free and encapsulated tobramycin in fluid and rigid liposomal formulations administered to rats chronically infected with Pseudomonas aeruginosa. Chronic infection in lungs was established by intratracheal administration of 10(5) CFU of a mucoid variant of P. aeruginosa PA 508 prepared in agar beads. Antibiotic treatments were given intratracheally at time intervals of 16 h. After the last treatment, lung bacterial counts were determined and tobramycin levels in the lungs and kidneys were evaluated by high-performance liquid chromatographic analysis and microbiological assay. Two independent experiments showed that animals treated with encapsulated tobramycin in fluid liposomes had a number of CFU less than the minimal CFU number required to be statistically acceptable compared with > or = 10(6) CFU per pair of lungs for animals treated with encapsulated tobramycin in rigid liposomes, free antibiotic, or liposomes without tobramycin. Tobramycin measured in the lungs at 16 h after the last treatment following the administration of encapsulated antibiotic was still active, and its concentration was > or = 27 micrograms/mg of tissue. Low levels of tobramycin were detected in the kidneys (0.59 to 0.87 micrograms/mg of tissue) after the administration of encapsulated antibiotic, while 5.31 micrograms/mg of tissue was detected in the kidneys following the administration of free antibiotic. These results suggest that the local administration of fluid liposomes with encapsulated tobramycin could greatly improve the management of chronic pulmonary infection in cystic fibrosis patients.