Biomedical Engineering Reference
In-Depth Information
3.3
TATp-Modified Liposomes and Micelles: A Multifunctional
Approach
An ideal nanoparticular drug delivery system (DDS) should be able to (1) specifically
accumulate in the required organ or tissue, and then (2) penetrate target cells to
deliver its load (drug or DNA) intracellularly. Organ or tissue (tumor, infarct) accu-
mulation could be achieved by the passive targeting via the EPR effect (Maeda
et al. 2000 ) assisted by prolonged circulation of such a nanocarrier (for example, as
a result of its coating with protecting polymer such as PEG); or by antibody-
mediated active targeting (Torchilin 2004 ) and (Jaracz et al. 2005 ), while the intra-
cellular delivery could be mediated by certain internalizable ligands (folate,
transferrin) (Gabizon et al. 2004 ) and (Widera et al. 2003 ) or by CPPs (Gupta et al.
2005 ). Ideally, such a DDS should simultaneously carry on its surface various
active moieties, i.e. be multifunctional and possess the ability to “switch on” certain
functions (such as intracellular penetration) only when necessary, for example
under the action of local stimuli characteristic of the target pathological zone (first
of all, increased temperature or lowered pH values characteristic of inflamed, isch-
emic, and neoplastic tissues). These “smart” DDS should be built in such a way that
during the first phase of delivery, a non-specific cell-penetrating function is shielded
by the function providing organ/tissue-specific delivery (sterically protecting polymer
or antibody). Upon accumulating in the target, protecting polymer or antibody
attached to the surface of the DDS via the stimuli-sensitive bond should detach
under the action of local pathological conditions (abnormal pH or temperature) and
expose the previously hidden second function to allow for the subsequent delivery
of the carrier and its cargo inside cells (Fig. 8 ).
With this in mind, we prepared targeted long-circulating PEGylated liposomes
and PEG-PE-based micelles possessing several functionalities (Sawant et al. 2006 ;
Kale and Torchilin 2007a ). First, such systems targeted a specific cell or organ by
attaching the monoclonal antibody (infarct-specific antimyosin antibody 2G4 or
cancer-specific antinucleosome antibody 2C5) to their surface via reactive pNP-
PEG-PE moieties. Second, these liposomes and micelles were additionally modified
with TATp moieties attached to the surface of the nanocarrier by using TATp-short
PEG-PE derivatives. PEG-PE used for liposome surface modification or for micelle
preparation was made degradable by inserting a pH-sensitive hydrazone bond
between PEG and PE (PEG-Hz-PE). Under normal pH values, TATp functions on
the surface of nanocarriers were “shielded” by the long PEG chains (pH-degradable
PEG 2000 -PE or PEG 5000 -PE) or by long pNP-PEG-PE moieties used to attach anti-
bodies to the nanocarrier (non-pH-degradable PEG 3400 -PE or PEG 5000 -PE). At pH
7.5-8.0, both liposomes and micelles demonstrated high specific binding with
antibody substrates, but very limited internalization by NIH/3T3 or U-87 cells.
However, upon brief incubation (15-30 min) at lower pH values (pH 5.0-6.0) nano-
carriers lost their protective PEG shell because of acidic hydrolysis of the PEG-
Hz-PE and were effectively internalized by cells via TATp moieties (Fig. 9a ).
In vivo , TATp-modified pGFP-loaded liposomal preparations have been adminis-
tered intratumorally in tumor-bearing mice, and the efficacy of tumor cell transfection
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