Enhanced effect of liposome-encapsulated amikacin on Mycobacterium avium-M. intracellulare complex infection in beige mice

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Abstract
We examined the therapeutic effects of free and liposome-encapsulated amikacin on Mycobacterium avium-M. intracellulare complex infection by using the beige-mouse model of the disease. In the first series of studies, intravenous administration of four weekly doses of 5 mg of amikacin per kg encapsulated in large (approximately 0.4-.mu.m diameter), unilamellar liposomes arrested the growth of M. avium-M. intracellulare complex organisms in the liver, as measured by CFU counts. M. avium-M. intracellulare complex levels in untreated animals and in those treated with the same dose of free amikacin increased by several orders of magnitude over 8 weeks. Liposome-encapsulated amikacin was also effective against M. avium-M. intracellulare complex organisms in the spleen and kidneys, reducing the CFU counts by about 1,000-fold compared with those of both untreated controls and free-drug-treated mice. In the lungs, a slight reduction in CFU was observed in the liposome-encapsulated-amikacin-treated group, but only at the 8-week point. Neither free nor liposome-encapsulated amikacin reduced the colony counts in the lymph nodes compared with those of control animals. Reductions in CFU in all organs greater than those caused by the liposome preparation could be achieved by intramuscular administration of free amikacin, but only at a 10-fold-higher dose given 6 days a week for 8 weeks. In the second series of studies, we investigated the effects of (i) doubling the dose of liposome-encapsulated amikacin and (ii) increasing the size of the liposomes and prolonging the treatment to five injections. Administration of 10 mg of amikacin per kg in liposomes 2 to 3 .mu.m in diameter was more effective in the liver than 5 or 10 mg of amikacin per kg in liposomes 0.2 .mu.m in diameter. A slight reduction in the CFU levels in the lungs was observed with the higher dose, irrespective of liposome size. Our results indicate that liposome-based delivery of amikacin enhances its anti-M. avium-M. intracellulare complex activity, particularly in the liver, spleen, and kidney, and may therefore improve the therapy of this disease.