Nanoparticles to Cross Biological Barriers - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 103

the substrates are internalized. These vesicles then fuse with the caveo-
somes or multivesicular bodies, thereby possibly bypassing lysosomal
incorporation and degradation [186]. Clathrin- and calveolae-independent
endocytosis may occur where ~90 nm vesicles are internalized without
the presence of these proteins. A special form of clathrin- and calveolae-
independent endocytosis is macropinocytosis [186]. In macropinocytosis,
the binding of receptor tyrosine kinases initiates the formation of ruffles
in the plasma membrane, which then engulf fluids [189]. These ruffles may
close off and form vesicles termed macropinosomes, which are directed
towards lysosomes.
Size can be a factor in the endocytosis of nanocarriers. While particles >500
nm are phagocytosed by macrophages and those <5 nm are rapidly cleared
by renal filtration and urinary excretion [190, 191], NPs <100 nm are able to
exit circulation via extravasation and are internalized by endocytosis [185,
186, 191]. Chithrani et al. [26] have shown size- and shape-dependent uptake
in HeLa cells, where 50-nm spherical gold nanoparticles were favorably endo-
cytosed. DeSimone's group have developed nanofabricated nanoparticles and
microparticles to study the effect of the size and shape on the cellular internal-
ization pathway [28] and NPs' in vivo biodistribution [192]. Nanoparticles were
more rapidly internalized through a caveolae-mediated endocytosis mecha-
nism by Hela cells than were microparticles, and the fraction of nanoparticles
taken up was substantially higher. The fastest uptake of nanoparticles (~150
nm) was found to be due to simultaneous internalization through multiple
pathways. Interestingly, rod-like nanoparticles were internalized much more
efficiently than their spherical counterparts. The influence of nanoparticle
shape on cellular uptake is hypothesized to be dependent on the region of
contact with cellular membranes. Various polystyrene ellipsoid and spherical
microparticles were investigated for uptake in alveolar macrophages, and it
was shown that for ellipsoid particles, internalization was favored when con-
tacting the cell in a perpendicular manner [193]. Furthermore, cationic nano-
carriers with a high aspect ratio will be internalized more easily [161]. Discher
et al. [194] have shown that filomicelle (20-60 nm diameter and 2-18 μm
length) circulation time was substantially longer than their spherical coun-
terparts, with a strong dependence on their length, with a plateau at length
>8 μm. In vivo results were correlated with in vitro fluid-flow experiments on
macrophages, showing a slower cell-uptake kinetic caused by shear-inducing
flow alignment and reduced cell-particle contact. In addition, filomicelles
were found to accumulate in the lung and be rapidly taken up by epithelial
cells. Paclitaxel-loaded filomicelles also increased apoptosis in human lung
tumor xenograft mice models and reduced tumor size 7 days post injection.
However, the data shows tumor reduction only at one time point, and com-
parative results with spherical particles were not published.
Because nanoparticles will first interact with the endothelial lining, Peetla
et al. [195] used Langmuir films to provide a biomimetic endothelial cell
model membrane (EMM) to understand the varied interactions between