ENGINEERING OF CELL-PENETRATING PEPTIDE-CONJUGATED INTRACELLULAR DELIVERY SYSTEMS - Biologically-Responsive Hybrid Biomaterials - page 213

translocation of liposomes by TATp or penetratin was proportional to the number
of peptide molecules attached to the liposomal surface and as few as five peptide
number was already sufficient to enhance the intracellular delivery of liposomes.
The kinetics of the uptake was peptide-and cell-type dependent [119]. Coupling
of TATp to the outer surface of liposomes was also described that resulted in an
enhanced binding and endocytosis of the liposomes in ovarian carcinoma cells
[120]. Here, TATp was covalently coupled to the maleimide-derivatized PEG-
DSPE present in the liposomal bilayer via a sulfhydryl-maleimide coupling
reaction.
Antp (43-58) and TATp coupled to small unilamellar liposomes were
accumulated in higher proportions within tumor cells and dendritic cells than
unmodified control liposomes [121]. It was found that the uptake was time- and
concentration dependent and at least 100 PTD molecules per small unilamellar
liposomes were required for efficient uptake inside cells. The uptake of the
modified liposomes was inhibited by the preincubation of liposomes with
heparin, suggesting the role of heparin sulfate proteoglycans in CPP-mediated
uptake.
Conventional liposomes have limited intracellular targeting capacity and are
cleared rapidly by the lungs. Therefore, the modification of liposome aerosols for
lung drug delivery was investigated. The effect of three CPPs - TATp, Antp, and
octaarginine conjugated to neutral liposomes was studied. It was found that the
modified liposomes were efficiently internalized by airway epithelial cells in
culture and were able to deliver cargo (e.g. dextran) to the cytoplasm of the cells
[91].
The requirement of direct unhindered contact of CPP with cells was also
shown in case of TATp-modified micelles [122]. These micelles when loaded
with anticancer drug paclitaxel demonstrated increased cytotoxicity with various
cancer cells in vitro . This was considered as a result of increased interaction with
cells of TATp-modified micelles compared to non-modified-micelles. Another
application of CPPs is the labeling of cells with semiconductor nanocrystals or
quantum dots (QDs). QDs are more popular than standard fluorophores for
studying tumor pathophysiology since they are photostable, more robust and
stable light emitters and relatively insensitive to the wavelength of the excitation.
They are also capable of distinguishing tumor vessels form both the perivascular
cells and the matrix, with concurrent imaging. QDs trapped within PEG-PE
micelles bearing TATp-PEG-PE linker were successfully applied for labeling
mouse endothelial cells in vitro . For in vivo tracking, bone marrow-derived
progenitor cells were labeled with TATp-bearing QD-containing micelle ex vivo ,
and then injected in the mouse bearing tumor in a cranial window model. It was