Comparison of the bactericidal action of amikacin, netilmicin and tobramycin in free and liposomal formulation against Pseudomonas aeruginosa.

The rates at which free, cationic and anionic liposomal forms of amikacin, netilmicin and tobramycin kill Pseudomonas aeruginosa were studied in vitro. Control inocula with no antibiotic yielded 6.76, 9.53 and 9.74 log CFU/ml at 0, 6 and 24 h, respectively. Empty anionic or cationic liposomes had no effect on bacterial growth. The killing rates of free antibiotics against the bacterial strain were not enhanced by the addition of either empty anionic or cationic liposomes. After 6 and 24 h of exposure at 1, 2 and 4 times the minimum inhibitory concentrations, free amikacin, netilmicin and tobramycin demonstrated a more rapid bactericidal effect than encapsulated anionic or cationic liposomes. The killing rates of liposomal aminoglycosides were lower than those of free aminoglycosides at identical concentrations, suggesting that only fractions of the encapsulated drugs were released from liposomes.