The artificial cell design: liposomes - Artificial Cells, Cell Engineering and Therapy - page 88

later, small liposomes bearing PEG chains and biotin were administered; these
could circulate, extravasate and bind to the antibody present on the surface of the
tumour cells. This approach can concentrate the carrier within the tumour, but
will not necessarily promote internalization, since in order for the procedure to
function, the bispecific antibody must remain at the cell surface. However, this
may be sufficient in many cases; for example, if the carrier is loaded with a
radioisotope to provide local irradiation. An example of this is the use of a
bispecific antibody to the tumour antigen CA 125 and biotin to deliver biotin-
decorated immunoliposomes loaded with the radionuclide

90

Yo to human
ovarian cancer cells growing in nude mice (McQuarrie et al., 2004).
One strategy for avoiding the necessity of an internalizing antigen would be
to deliver a lipophilic prodrug which could diffuse passively into the target cell
after being released in the vicinity. Thus, Koning et al. (2002) synthesized a
dipalmitoylated derivative of 5-fluorodeoxyuridine and encapsulated it in
liposomes bearing an antibody specific for a rat colon carcinoma. Although only
a small proportion of the liposomes were internalized, the majority of the
prodrug was metabolized to the active drug, probably at the level of the cell
membrane. This approach allows a bystander effect whereby neighbouring cells
which do not express the surface antigen are also exposed to the drug.
Another means of achieving site-specific local high concentrations of drug is
the ADEPT (antibody-directed enzyme prodrug therapy) strategy. This requires
that an inactive prodrug is cleaved to the active agent by an enzyme not
normally present in the organism which is directed to the target cell. In the
original concept (Bagshawe et al., 1988), the enzyme was coupled directly to an
antibody either chemically or as a fusion protein. However, the amount of
enzyme delivered can be improved by using immunoliposomes which have
several enzyme molecules coupled to the surface as well as the targeting
antibody (Fonseca et al., 1999). In a variant of this approach, Huysmans et al.
(2005) have encapsulated a nucleoside hydrolase within liposomes containing
porins in their membrane, thus allowing specific transport of the substrate to the
aqueous core, while avoiding a host immune response to the exogenous enzyme.
A second step would be to direct these `nanoreactors' to the target cells by
means of specific ligands.

Targeting to tumour cells: transferrin receptors

Many proteins and peptides other than antibodies have been used to target
carriers to specific cell types. The transferrin receptor system has been widely
studied because it is over-expressed on many tumours. Transferrin, an iron-
transporting protein of 80 kDa, can be coupled to the surface of liposomes by a
number of different methods (Anabousi et al., 2005). Long-circulating lipo-
somes bearing transferrin have been shown to be taken up by receptor-mediated
endocytosis in Colon 26 cells (Ishida et al., 2001) and to deliver doxorubicin to