Biology Reference
In-Depth Information
entrapped in negative or positive charged liposomes when incubated with LPS were
not signifi cantly leaked from the liposomes which were not lysed. It is possible then
that the higher concentrations of LPS/LTA may contribute to the stabilization of the
liposomes, reduce antibiotic release, and thus prevent the leakage of the antibiotics
leading to reduction of their interaction with bacteria.
If the lipid bilayers of liposomes can decrease antibiotic interactions with the poly-
anionic components found in CF lungs and reduce bacterial growth within a 3 hr pe-
riod much more strongly than free antibiotics, its long term advantage and presence
in an 18 hr period would be advantageous (Table 2). Unfortunately, prolonged contact
between polyanions and the formulations greatly increased the free and liposomal
polymyxin B bactericidal concentrations, with liposomal tobramycin exhibiting bet-
ter activity than free tobramycin. The dissimilar inhibitory effects on tobramycin and
polymyxin B may be attributed to differences in their mechanisms of action, as tobra-
mycin, a polar drug can enter the cell while polymyxin B a lipophilic agent interacts
with LPS on the cellular surface. The interaction of polymyxin B with cell surface
LPS, in addition to the interaction with the polyanions might leads to competition at
the LPS binding site of bacteria, ultimately reducing antibiotic binding.
In light of the higher bactericidal activities and lower inactivation of liposomal
antibiotics in the presence of polyanionic components
in vitro
, we sought to compare
the bactericidal activity of these formulations against endogenous
P. aeruginosa
in
CF sputa to that of the free drug. As shown in Figure 4, the antibacterial activity of
liposomal antibiotics was more effective than the free antibiotics by 4-fold, although
due to a large microbial population in the CF sputum, neither of the formulations fully
eradicated bacterial growth. While liposomal tobramycin (128 mg/l) reduced growth,
liposomal polymyxin B (8 mg/l) fell into clinically acceptable levels. The high con-
centrations of antibiotics, tobramycin in particular, required to lower growth, may be
primarily due to samples containing antibiotic resistant strains, or the sputum and its
contents impeding antibiotic effects by acting as a physical barrier or inhibitor. Several
studies have dealt with the inhibitory properties of sputum on antibiotics [34, 42, 69]
while there have been a limited number of studies focused on liposomal penetration
and interaction with sputum [53-55, 70]. The majority of these studies have focused on
gene therapy and their transport across the sputum, but a recent work by Meers et al.
[54] showed the ability of labeled neutral liposomes to penetrate sputum, and further-
more, aminoglycosidic amikacin-entrapped liposomes were more effi cacious than free
amikacin in reducing bacterial growth in a rat
P. aeruginosa
infection model. In our
study, due to issues of confi dentiality, we did not have access as to the clinical status of
the patients or their pathology laboratory reports. Nevertheless, delivery of antibiotics
via a liposomal system enhanced their antibacterial activity in sputum.
Although liposome entrapment of antibiotics and their increased effi cacy is not a
novel fi nding, neutral liposome-entrapped antibiotics tended to be more bactericidal in
sputum and in the presence of sputum components when compared to free antibiotics,
but with reduced effi cacies over a longer period of time
in vitro
(18 hr exposure). This
decrease in effi cacy appears to be the result of pro-longed interactions of the liposomes
with the polyanionic factors found in sputum. As prophylactic and anti-infl ammatory
