ENGINEERING OF CELL-PENETRATING PEPTIDE-CONJUGATED INTRACELLULAR DELIVERY SYSTEMS - Biologically-Responsive Hybrid Biomaterials - page 217

activity. This system can also be PEGylated. Helper lipids like
dioleoylphosphatidylethanolamine (DOPE) and cholesteryl hemisuccinate
(CHEMS) can be added to promote endosomal release and nuclear delivery of
the cargo [43].
CPPs have also been used to enhance the intracellular delivery of DNA by
PEI-based DNA complexes [130, 131] and by dendrimers [132-134].
RNAi technology has also been improved with CPP-mediated delivery of
SiRNA [59, 135]. The first report about CPP-mediated delivery of siRNA was
published in 2003 by Simeoni et al.[136]. SiRNA were non-covalently
complexed with the MPG peptide leading to strong down regulation of the target
protein. Davidson et al. has reported a string RNAi effect in hard to transfect
primary neuronal cells [137]. Here, siRNA directed against several endogenous
proteins were coupled to penetratin via a disulfide bond. We have described
recently a novel approach for siRNA cellular delivery using siRNA encapsulated
into liposomes additionally bearing arginine octamer (R8) molecules attached to
their surface (R8-liposomes). The R8-liposomal siRNA demonstrated a very high
stability against degradation by the blood serum and very high transfection
efficiency into SK-MES-1 lung tumor cells resistant towards the Lipofectamine
2000-mediated transfection. siRNA in R8-liposomes effectively inhibited the
targeted gene and significantly reduced the proliferation of cancer cells [138].
Arginine-9 (R9) was also investigated as a potential carrier for siRNAs [139].
Penetratin and transportan featuring terminal cysteines have been covalently
attached to a 5'-thiol modified siRNA and produced expression knockdown for
up to seven days [140]. Successful delivery of siRNA has also been reported
using MEND technology [141].
Even though there are many examples of successful CPP-mediated
internalization, several studies with CPP-siRNA revealed that TAT and
penetratin alone affected gene expression and that penetratin-siRNA elicited an
immunological response, challenging the use of these CPPs in delivery systems
[142, 143].
2.4. Stimuli-sensitive nanoparticulate drug delivery system
Ideal nanoparticular drug delivery system should be able to (1) specifically
accumulate in the required organ or tissue, and then (2) penetrate inside target
cells delivering its load (drug or DNA) intracellularly. Organ or tissue (tumor,
infarct) accumulation could be achieved by the passive targeting via the EPR
effect [113, 114] assisted by prolonged circulation of such nanocarrier (for
example, as a result of its coating with protecting polymer such as PEG); or by