Pharmacokinetics of intravitreally injected liposome-encapsulated tobramycin in normal rabbits

Bacterial endophthalmitis, which is a devastating complication of intraocular surgery or eye trauma, has a poor prognosis. Intravitreal injection of antimicrobial agents has become a part of the standard treatment of endophthalmitis. The authors investigate the pharmacokinetics of intravitreal liposome-encapsulated tobramycin as a possible method of prolonging the duration of therapeutic concentrations. Tobramycin was encapsulated into liposomes of phosphatidylcholine, phosphatidic acid, and alpha-tocopherol by the reverse phase evaporation method. The final liposomal suspension contained tobramycin, 7.0 mg/ml, 60.5% encapsulated. One eye received an intravitreal injection of either liposome-encapsulated tobramycin (LET), tobramycin phosphated-buffered saline (TS) or a mixture of tobramycin and liposome-encapsulated saline (TEL), and the results were as follows: 1. Concentrations of free tobramycin were significantly lower with LET than with TS or TEL at 1 hour after intravitreal injection. 2. Concentrations of free and total tobramycin were significantly higher with LET than with TS or TEL at 5 and 8 days after intravitreal injection. Concentrations of free tobramycin with TS were lower than the minimal inhibitory concentration(MIC) of tobramycin for Pseudomonas aeruginosa at 8 days after intravitreal injection, while those with LET were higher than the MIC of tobramycin for Pseudomonas aeruginosa 18 days after injection.