Targeting of antimicrobial agents by means of liposomes is under investigation and may be of importance in the treatment of infections that prove refractory to conventional forms of antimicrobial treatment. The ability to achieve a significantly longer residence time of liposomes in plasma and limited uptake of liposomes by the mononuclear phagocyte system opens up new areas of investigation and potential therapeutic application. By manipulating the liposomal composition, rates of uptake and intracellular degradation can be influenced and thereby the rates at which liposome-encapsulated agents are released and become available to exert their therapeutic action. With respect to the targeting of macrophage modulators at the mononuclear phagocyte system by means of liposomes for maximal stimulation of the nonspecific antimicrobial resistance, experimental evidence is now available of the potential usefulness of liposomes as carriers of these agents. This approach may also be of importance for the potentiation of treatment of severe infections.

The effect of free and liposome-encapsulated muramyl tripeptide phosphatidylethanolamide (MTPPE) and interferon-y (IFN-y) on the resistance against Listeria monocytogenes infection in mice was investigated. It was shown that administration of MTPPE or IFN-y at 24 h before bacterial inoculation led to increased resistance against L. monocytogenes infection in terms of a decrease in bacterial numbers in liver and spleen. Encapsulation of MTPPE and IFN-y in liposomes increased their efficacy 33- or 66-fold, respectively. In addition, liposomal encapsulation led to a more rapid decrease in bacterial numbers. The immunomodulator to lipid ratio appeared to be very important in the antibacterial effect of LE-MTPPE and LE-IFN-y. When nontherapeutic doses of liposome-encapsulated MTPPE or IFN-y were administered in a larger amount of lipid (so at higher lipid: immunomodulator ratio), these doses became effective. Exposure of macrophages in monolayer infected with L. monocytogenes in vitro to MTPPE had no effect, whereas exposure to IFN-y only led to growth inhibition of the intracellular bacteria. However, incubation of macrophages with a combination of MTPPE and IFN-y resulted in killing of the intracellular bacteria. Exposure of macrophages in vivo to both immunomodulators in combination can be effected by using liposomes as carriers. It was observed that administration of MTPPE and IFN-y co-encapsulated in liposomes resulted in a synergistic enhanced antibacterial resistance against L. monocytogenes. Both reactive oxygen and nitrogen intermediates seemed to play a role in the killing of L. monocytogenes by macrophages activated with a combination of MTPPE and IFN-y.

The effect of free and liposome-encapsulated muramyl tripeptide phosphatidylethanolamide (MTPPE) on resistance to Klebsiella pneumoniae infection in mice was investigated. It was shown that administration of MTPPE, at 24 h before bacterial inoculation, led to a dose-dependent antibacterial resistance in terms of increased clearance of bacteria from the blood and bacterial killing in various organs. The lowest effective dose of MTPPE was 50 ,ug per mouse. Administration of liposome-encapsulated MTPPE was also effective at a dose of 25 ,ug per mouse. The time of administration of both free and liposome-encapsulated MTPPE, with respect to the appearance of bacteria in the blood, was very important and indicated that repeated administration is necessary to obtain protection for a prolonged period. In view of the toxicity of MTPPE, it was an important observation that repeated administration of MTPPE in the liposome-encapsulated form also produced antibacterial resistance. Administration of free and liposome-encapsulated MTPPE resulted in increased numbers of granulocytes, monocytes, and lymphocytes in the blood of uninfected mice and prevented leukopenia in infected mice.

Administration of free muramyl tripeptide phosphatidylethanolamide (MIPPE) or liposome-encapsulated MTPPE (LE-MTPPE) in a twofold-lower dose at 24 h before bacterial inoculation resulted in clearance of intravenously inoculated KlebsieUla pneumoniae by tissue macrophages, whereas in control mice, bacteria were not effectively cleared from the blood. In addition, MTPPE and LE-MTPPE led to increased numbers of leukocytes in the blood, which could compensate for the leukopenia in mice resulting from infection with K. pneumoniae. In an attempt to elucidate the relative contributions of the activation of tissue macrophages and the recruitment of leukocytes to the antibacterial resistance induced by MTPPE and LE-MTPPE, mice were infected intraperitonealy with K. pneumoniae. In these MTPPE-and LE-MTPPE treated mice, intraperitoneal influx of leukocytes and the phagocytic capacity of leukocytes were not higher than in untreated control mice. However, MTPPEand LE-MTPPE-treated mice survived much longer, eventually 33% of the LE-MTPPEtreated mice survived, whereas all untreated control mice died as a result of bacterial septicemia. This prevention of early death appeared to be the result of an increased clearance of bacteria from the blood by activated tissue macrophages. It was observed that depletion of these tissue macrophages in liver and spleen abrogates the effect of LE-MTPPE treatment, indicating that tissue macrophages are of major importance in the LE-MTPPE-induced resistance against K. pneumoniae infection.

The effect of free muramyl tripeptide phosphatidylethanolamide (MTPPE) and liposome-encapsulated MTPPE (LE-MTPPE) on Klebsiella pneumoniae septicemia resulting from intraperitoneal bacterial inoculation was investigated in mice. When administering a single prophylactic dose at 24 h before bacterial inoculation, the percentage survival was 55% (MTPPE) or 40% (LE-MTPPE), whereas untreated control mice died. Only repeated prophylactic treatment with LE-MTPPE could further increase survival up to 85%.