Biomedical Engineering Reference
In-Depth Information
of concept and are still far from being the perfect delivery system. In order to design
similar carriers for other sub-cellular compartments it is necessary to find suitable
self-assembling molecules with an affinity for the intended sub-cellular compart-
ment. To this end, recent work on the sub-cellular distribution of micelle forming
agents offers some interesting insights (Bae et al.
2005
; Maysinger et al.
2007
;
Savic et al.
2006, 2009
; Xiong et al.
2008
).
Imaging studies based on the use of a variety of organelle-specific dyes, gold and
fluorescent polymers have provided detailed insight into the sub-cellular distribution
of block copolymer micelles (Maysinger et al.
2007
; Savic et al.
2003
). Both imaging
techniques, i.e. confocal fluorescence microscopy (to detect the fluorophore-labeled
copolymers) and transmission electron microscopy (to detect the gold-labeled copo-
lymers) demonstrate that poly(caprolactone)-b-poly(ethylene oxide) micelles (PCL-
b-PEO micelles) do not enter the nucleus. With respect to the cytosolic distribution
of PCL-b-PEO micelles, however, the two different imaging techniques used in these
studies suggest quite a different sub-cellular disposition. TEM images show most of
the gold labeled micelles to be localized in endosomes/lysosomes and a few of them
were seen at or in mitochondria (Maysinger et al.
2007
). Confocal fluorescence
microscopic images, on the other hand, show fluorescent PCL-b-PEO micelles
almost evenly distributed throughout the cytosol (Savic et al.
2003
). Therefore, not
surprisingly, cell staining with organelle-specific dyes and overlaying the correspond-
ing confocal fluorescence images reveal partial colocalization of PCL-b-PEO
micelles with lysosomes, with the Golgi apparatus and the Endoplasmic Reticulum,
with the mitochondria and the Endoplasmic Reticulum and with mitochondria alone.
Considering the nature of the micelle corona, which is entirely made up of non-
functionalized polyethylene oxide, a highly hydrophilic polymer, any specific interac-
tion with or any specific affinity for any of the cell organelles could not be expected
per se. It would be very interesting to see to what extent modifying the micelle corona
with organelle-specific ligands would alter the intracellular distribution of such
micelles, which then potentially could become nanocontainers that distribute cargo to
defined cytoplasmic organelles. However, the distinctive distribution of nonfunction-
alized PCL-b-PEO micelles throughout the cytosol makes them highly suitable for
multiple cytoplasmic targeting (Savic et al.
2003
), which has most recently been
proven to be relevant for the delivery of effector molecules of the cell signaling path-
ways that are activated in the cytosol (Savic et al.
2009
). This study suggests that
micelle-based intracellular delivery of potent, poorly water-soluble, cell-death-pathway
inhibitors may represent a useful addition to established delivery of cytocidal block-
copolymer micelle-incorporated bioactives (Savic et al.
2009
).
4
Conclusion and Perspectives
From the examples discussed so far it would seem that nanocarrier systems could
be designed to achieve true molecular level targeting inside cells. However to say
that these systems will be available soon is perhaps premature given what little is
currently known about the sub-cellular dynamics associated with nanocarrier
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