Approaches to Achieving Sub-cellular Targeting of Bioactives Using Pharmaceutical Nanocarriers - Intracellular Delivery: Fundamentals and Applications - page 60

access, transport through the cytosol and organelle entry thus become the major
barriers that need to be overcome to mediate the efficient delivery of the bioactive
to its intended site of action.

2.1 Cytosolic Access

In the case of nanocarrier based intracellular delivery strategies, access to the
cytosol is not a trivial matter. Based on current understanding it is accepted that
all nanocarrier systems are subject to some form of endocytosis either receptor
mediated or more commonly non specific endocytosis. Nanocarriers may be sur-
face functionalized with endocytic targeting moieties such as transferrin, folic
acid, low-density lipoprotein, cholera toxin, riboflavin, nicotinic acid and the trip-
eptide RGD, which lead to internalization by either clathrin-dependent receptor
mediated endocytosis, caveolin-assisted endocytosis, lipid-raft assisted endocyto-
sis or macropinocytosis (Bareford and Swaan 2007 ; Rajendran 2010 ). Strictly
speaking a nanocarrier and the associated drug that is inside an endosome is not in
the cytosol and therefore much research has been devoted to mediating the release
of the bioactive cargo from the endosome into the cytosol ( Rajendran 2010 ).
Endosomal release may be mediated by a fusogenic mechanism where lipid-con-
taining nanocarriers fuse with the endosomal membrane increasing the fluidity of
the membrane and therefore the release of the cargo (Martin and Rice 2007 ).
Alternatively endosomolytic agents that can disrupt the endosomal membrane by
the so called proton sponge effect leading to the release of endosomal contents
(Yessine and Leroux 2004 ) or photochemical internalization (PCI) using photo-
sensitizer molecules that can rupture the endosomal membrane can also be utilized
(Shiraishi and Nielsen 2006 ). Such approaches are arguably necessary in the case
of agents such as siRNA since the target of the drug molecule is in the cytoplasm
itself (Patil and Panyam 2009 ; Yuan et al. 2006 ; Tahara 2010 ; Song 2010 ). On the
other hand retention in the endosome followed by subsequent lysosomal fusion is
a desirable outcome in the case of bioactives requiring delivery into the lysosome.
Recombinant human acid sphingomyelinase for example has been delivered intra-
cellularly by nanocarriers surface functionalized with the anti-ICAM antibody that
mediates endosomal uptake and subsequent lysosomal association of the enzyme.
This approach has shown promise in the treatment of Nieman Pick's disease type
A and B (Muro et al. 2006 ). Most viruses utilize a cell penetrating peptide (CPP)
that aids in the internalization of the virus in the cell. Some of these peptides can
be anchored with the therapeutic drug molecule to facilitate its internalization into
the cell. HIV's Tat peptide is one such CPP, which has aided in the delivery of
large proteins and DNA into the cell either when it is directly conjugated to the
molecule (Fittipaldi and Giacca 2005 ; Schwarze et al. 1999 ; Fawell et al. 1994 ) or
conjugated to a nanocarrier such as a liposome (Torchilin et al. 2001 ). Other CPPs,
D-penetratin and Syn-B, have been used to enhance the intracellular delivery of
doxorubicin in the brain (Rousselle et al. 2000 ).