Localization of sterically stabilized liposomes in experimental rat Klebsiella pneumoniae pneumonia: dependence on circulation kinetics and presence of poly(ethylene)glycol coating.

Preferential localization of liposomes at sites of infection or inflammation has been demonstrated in a variety of experimental models. Most studies report enhanced localization at the target site of poly(ethyelene) glycol (PEG)-coated liposomes as compared to conventional non-coated liposomes. It is generally accepted that the prolonged circulation time of PEG-coated liposomes increases target site exposure, which results in increased target localization. A quantitative relationship between circulation kinetics and localization at the pathological site has not been defined as yet. Besides, an effect of the PEG coating itself has been suggested, as theoretically the PEG coating may facilitate liposome extravasation. In the present study, in a rat model of an acute unilateral Klebsiella pneumoniae pneumonia, circulation kinetics of PEG-coated liposomes were manipulated by incorporation of different amounts of phosphatidylserine (PS) and variation of lipid dose, additionally allowing evaluation of the saturability of the localization process. In addition, this paper addresses the effect of the PEG coating, by comparing the circulation kinetics and target localization of long-circulating 'PEG-free' and PEG-coated liposomes. It is shown that the degree of liposome localization at the target site is positively linearly related to the area under the blood concentration time curve (AUC) of the liposome formulations, irrespective of PEG coating. This finding is discussed in relation to the equation of Kedem and Katchalsky, which describes protein influx into sites of infection or inflammation.