Intracellular Bacteria and Protozoa - Intracellular Delivery: Fundamentals and Applications - page 770

GEN as high as 80 mg/kg showed only a transient increase in blood urea nitrogen
and no rise in serum creatinine. These results confirm that liposomal GEN is less
toxic in mice than is free GEN and is extremely effective-therapy for disseminated
Salmonella infections in mice (Fierer et al. 1990 ).
In a simmilar approach, GEN was loaded in 1.2-10.0 mm MLV (eggPC) and i.v.
administered to rats and mice at 20 mg/kg. The half-life of liposomal GEN in serum
was substantially prolonged (mean half-lives in serum of free GEN in mice and rats
were 1.0 and 0.6 h, respectively, whereas liposomal GEN had apparent mean half-
lives of 3.8 h in mice and 4.0 h in rats) and the apparent rate of elimination and
clearance of active drug from the blood was dose dependent. As the total dose
increased, the rate of elimination and systemic clearance decreased (the apparent
half-life in serum of liposomal GEN increased as the dose Increased). This was
consistent with a saturable process (such as phagocytosis by fixed or circulating
cells) as a primary mechanism in the removal of drug from the blood. Liposome
encapsulation resulted in higher and more prolonged activity in organs rich in RES
cells (especially spleen and liver). In a model of murine salmonellosis where infec-
tion is by the i.v. route with S. typhimurium , the initial septicemia is followed by
localization of the organism within the major RES organs, liposomal GEN greatly
enhanced survival when given as a single dose (10 mg/kg) at 1 or 2 days after infec-
tion as well as up to 7 days before infection. The mechanism of the enhanced thera-
peutic effect of liposomal GEN in this model was probably related to targeting of
the antibiotic to the organ, cellular, and possibly subcellular site of bacterial resi-
dence (Swenson et al. 1990 ).
Two further works determined the biodistribution and therapeutic efficacy of
liposomal fluoroquinolone CIP. For instance, CIP was loaded in MLV (DPPC:
DPPG: chol, 4.1:0.9:4 M ratio) and i.v. injected in BALB/c mice infected per os
with the fatal S. dublin at 20 mg/kg. Untreated, the infection begins in Peyer's
patches, spreads to the regional lymph nodes, and then disseminates to the liver and
spleen, leading to bacteremia and death 10-14 days after infection. A single admin-
istration of liposomal CIP was ten times more effective than a single sc injection of
free CIP at preventing mortality and the result was nearly identical to the one
achieved with comparable doses of free CIP given twice daily for 5 days. Treatment
with liposomal CIP produced dose-dependent decreases in bacterial counts in
spleen, stool, and Peyer's patches, indicating that the drug had distributed to all
areas of inflammation, not just to the major reticuloendothelial system organs.
Although liposomal CIP was cleared rapidly from the blood, drug persisted in the
liver and spleen for at least 48 h after administration of a dose of liposomal CIP.
The improved survival may have been due to the persistence of CIP in the liver and
spleen for at least 2 days after a single injection of 20 mg/kg. This had the effect of
prolonging therapy well beyond the time when the drug had disappeared from the
circulation. In contrast, at 8 h after administration of the same dose of free CIP,
there was no measurable drug in the spleen (Magallanes et al. 1993 ). In general, a
drug that is actively retained inside a liposome by a transmembrane ion gradient
leaks more slowly than a drug that has been passively encapsulated (Hope and
Wong 1995 ; Cullis et al. 1997 ). CIP pharmacokinetics, accumulation at infection