Drug Targeting Organ-Specific Strategies

48 2 Brain-Specific Drug Targeting Strategies
An antibody used for brain targeting of immunoliposomes has to meet several requirements.
First, the antibody should recognize a structure which is present exclusively at the
blood–brain barrier. Second, the antibody should be able to cross the blood–brain barrier by
an active transport mechanism such as receptor-mediated transcytosis. Third, the epitope
against which the antibody is targeted should preferably not be species specific. Fourth, high
quantities of the antibody should be available. The OX26 mAb [38] meets several (but not
all) of the above requirements. In vitro experiments have demonstrated that OX26-immunoliposomes
can be taken up specifically by living RG2 rat glioma cells overexpressing the rat
transferrin receptor despite their particulate size of approximately 90 nm [113]. The fluorescent-labelled
OX26-immunoliposomes accumulated within an intracellular (endosomal)
compartment [114]. Similar results were obtained by incubation of fluorescent OX26-immunoliposomes
with freshly isolated rat brain capillaries [115] which revealed binding to the
luminal and basolateral membranes of the brain endothelium.
2.4.3.3 Drugs of Interest for Targeting to the Brain
Brain delivery of the anticancer drug daunomycin provides an example of the in vivo application
of OX26-immunoliposomes [111]. Different formulations of [ 3 H]-daunomycin were
i.v. administered to rats either as the free drug or encapsulated in conventional liposomes,
sterically-stabilized liposomes, or PEG-conjugated immunoliposomes (Table 2.3). Plasma
samples were taken at defined time points and after 1 h the animal was killed and drug concentrations
in brain tissue were determined.
Free daunomycin and not PEG-conjugated liposomes containing daunomycin, disappear
rapidly from the circulation. Plasma clearance of the liposome was reduced 66-fold by PEGconjugation.
Coupling 29 OX26 monoclonal antibodies per PEG-liposome partially reversed
the effect of PEG-conjugation on plasma clearance.
Analysis of the blood–brain barrier permeability surface area (PS) product indicated that
daunomycin, and to a lesser degree conventional liposomes, have the potential to penetrate
the blood–brain barrier. However, brain tissue accumulation of free daunomycin or conventional
liposomes was poor, being the result of their high systemic plasma clearance. The use
of PEG-conjugated liposomes reduced the blood–brain barrier PS product value to zero. No
brain uptake of the PEG-liposomes was observed, despite their marked increase in plasma
circulation time. Conversely, the use of PEG-conjugated OX26 immunoliposomes increased
the blood–brain barrier PS product, relative to PEG-liposomes, resulting in increased brain
uptake. Thus, optimal brain delivery of daunomycin was achieved using OX26 immunoliposomes
(see Table 2.3). Titration of the amount of OX26 conjugated per liposome (n between
3 and 197) revealed an increase in plasma clearance and a decrease in the systemic volume of
distribution of immunoliposomes at higher OX26 concentrations. Highest PS product values
and brain tissue accumulation was observed for immunoliposomes with 29 OX26 mAb. At
higher OX26 densities on the liposome, a saturation effect was observed resulting in a reduction
in volume of distribution, PS product and brain tissue accumulation of OX26 immunoliposomes.
Recently the OX26 immunoliposomes were used in a gene delivery approach to transport
expression vectors for luciferase or β-galactosidase through the BBB [116]. The plasmids