Delivery to Intracellular Targets by Nanosized Particles - Intracellular Delivery: Fundamentals and Applications - page 78

(containing a panoply of hydrolytic enzymes), while receptors are recycled to the
cell surface. It follows that a drug carrier that is internalized in this way, and its
contents, will come into contact with an environment in which it can be degraded.
This is an important feature for polymeric delivery systems because it allows drug
release and prevents the accumulation of polymer within the cells but can inactivate
or sequester the active agent.
Factors which influence the uptake of drug carriers by macrophages include
size, surface charge, surface coating and the presence of specific ligands. In-vitro
studies have often observed more rapid uptake of larger particles, although this may
be a consequence of more rapid sedimentation of larger objects in unstirred culture
medium (Barratt et al. 1986 ; Tabata and Ikada 1988 ). However, more rapid clear-
ance of larger particles is also observed in vivo (Senior and Gregoriadis 1982 ) and
complement consumption also increases with particle size (Vonarbourg et al. 2006 ),
suggesting that the surface of larger particles is more susceptible to opsonization
than that of particles with a smaller radius of curvature. Nanoparticles and lipo-
somes with positively or negatively charged surfaces are taken up more rapidly than
neutral ones (Tabata and Ikada 1988 ; Heath et al. 1985 ) and those with hydrophobic
surfaces are phagocytosed more readily than those with a hydrophilic surface. In
particular, the presence of end-on hydrophilic chains such as PEG (Woodle 1998 ;
Gref et al. 2000 ) or dextran (Jaulin et al. 2000 ) on the surface reduces uptake by
phagocytes. Analysis of the kinetics of binding and internalization for various par-
ticle types suggests that the rate-limiting step is binding to the particle surface and
that once they are bound PEG-covered particles are internalized at the same rate as
non PEGylated ones (Mosqueira et al. 2001 ; Martina et al. 2007 ).
Some specific ligands can increase particle uptake by macrophages and this
phenomenon can be exploited to deliver biologically active material to these cells.
Liposomes containing phosphatidylserine are preferentially taken up by mac-
rophages (Schroit and Fidler 1982 ) by means of a receptor whose primary purpose
is to clear senescent erythrocytes and fragments of cells after apoptosis (Fadok
et al. 2000 ). Another receptor which can be utilized to promote capture of drug
carriers by macrophages is the mannose/fucose receptor, which allows these cells
to capture and destroy a number of microorganisms. One example is the delivery of
an immunomodulator in mannose-grafted liposomes, in order to stimulate the anti-
tumoral properties of macrophages (Barratt et al. 1987 ). More recently, this strategy
has been applied to the delivery of Amphotericin B (Vyas et al. 2000 ; Nahar et al.
2010 ) and another fungally derived antibiotic (Mitra et al. 2005 ) to macrophages
for treatment of leishmaniasis. Another targeting ligand which has been used in a
similar application is the tetrapeptide tuftsin (Thr-Lys-Pro-Arg, Agrawal et al.
2002 ). This peptide has the advantage of being both a targeting element and a mac-
rophage activator. The anti-leishmanial activity of the drug is thus reinforced by
macrophage-mediated effects.
Most other cell types are capable of internalizing carrier systems by endocytic
mechanisms. The best documented pathway is that of clathrin-dependent endocy-
tosis, involving so-called “coated pits”. These are invaginations of the plasma
membrane enriched in the protein clathrin, to which the intracellular portion of