Pharmaceutical Manufacturing Handbook: Production and

464 LIPOSOMES AND DRUG DELIVERY
c
a a a a a a a a
b b
Targeting by target specific antibody
and/or long circulation
Incubation at lowered pH
Removal of PEG chains
aa
a
a a
However, the labile nature of the hydrazone bond is used to formulate “ smart ”
drug delivery systems where the basic idea is to introduce in parallel a pH - responsive
ligand spacer in the lipid bilayer (which is PEG 5000 – Hz – PE), a temporarily
shielded biotin or TATp and mAb (monoclonal antibody) attached to the surface
of the drug delivery system via a noncleavable bond (TATp – PEG 2000 – PE) [183] .
Such a system will be able to respond to environment stimuli such as pH changes,
where, for example, at acidic pH (5.0 – 6.0) PEG5000 molecules will be detached from
the carrier surface and biotin or TATp will be available to either bind to avidin or
be internalized by the cells, respectively (Figure 8 ). The monoclonal antibody
and biotin or TAT has been attached on pNP – PEG – PE. The produced mAb DDS
(drug delivery system) demonstrated clear immunoreactivity toward the antigen.
However, some affi nity decrease was observed for the antibodies modifi ed with the
pNP – PEG – PE anchor and incorporated onto the immunoliposomes. Biotin binding
to avidin was pH dependent with higher retention (75%) at pH 5.0, where the
shielding PEG molecules were cleaved away. Signifi cant increase of DDS uptake
by the cells was achieved when TAT was incorporated on the liposome surface at
pH 5.0.
An avidin – biotin system has been used to attach antibodies in the bilayer of
DDSs. Xiao et al. developed a three - step strategy to improve the tumor - to - tissue
ratio of anticancer agents [184] . Two antibodies specifi c for the CA - 125 antigen that
is highly expressed on NIH:OVCAR - 3 cells were used. These cells were prelabeled
with biotinylated anti - CA - 125 antibody and fl uoroscein isothiocyanate (FITC) –
labeled streptavidin (SAv) prior to administration of biotinylated liposomes. Both
antibodies were specifi cally bound to the cell surface of OVCAR - 3 cells but not to
SK - OV - 3 cells, which do not express the specifi c antibody. Antibody biotinylation
did not affect its immunoreactivity.
Schnyder et al. explored the targetability of biotinylated immunoliposomes to
skeletal muscle cell line in vitro [185] . OX26 mAb binds to transferrin receptor and
is covalently attached to streptavidin by introducing thiol groups using 2 - iminothiolane
(Traut ’ s reagent). Immunoliposomes consisted of DSPC (5.2 μ mol), cholesterol
c
b b
De-shielding of the “hidden” function
FIGURE 8 Schematic for design of multifunctional drug delivery system (DDS) that
includes pH - cleavable PEG - Hz - PE (a), temporarily “ shielded ” biotin or TATp (b), and
monoclonal antibody (c) attached to surface of DDS via pH - noncleavable spacer. ( Reprinted
with permission from ref. 183 . Copyright 2006 by the Americam Chemical Society .)
a
a a