Mass Transport via Cellular Barriers and Endocytosis - Intracellular Delivery: Fundamentals and Applications - page 17

3.2 Pathways Exploited by Nanoparticles

Many nanoparticles, especially those used in therapeutic drug delivery, are intended
to deposit a drug at a specific intracellular location, such as the cytosol. The transit
time for the cell to internalize the nanoparticle and deliver it to the cytosol, as well
as the steady-state level of the nanoparticle and its cargo, influenced by degradation
and dose, will ultimately determine the overall efficacy of the drug delivery system.
This section of the chapter will explore which endocytic pathways are candidates
for nanoparticle internalization.
In many instances, nanoparticles may enter cells via multiple endocytic path-
ways. Park et al.'s (Hyung Park et al. 2006 ) investigation of hydrophobically
modified glycol chitosan (HGC) is one such example. HGC is a promising
nanoparticle-based drug carrier used for transport of anti-cancer drugs, genes,
and peptides. Hence, understanding the endocytic pathways used in its uptake and
subsequently its intracellular localization and bioavailability, is of clinical inter-
est. The advantages of using HGC are high circulation time, rapid cellular uptake,
low toxicity and small size of the particle. The inhibition studies conducted on
this type of particle employed chlorpromazine, filipin III, and amiloride to inhibit
clathrin-mediated endocytosis, caveolae-mediated endocytosis, and macropino-
cytosis, respectively (Schnitzer et al. 1994 ). These studies demonstrated that
while HGC used all three endocytic pathways, their primary method of entry into
the cell is macropinocytosis.
Nanoparticles also enter cells using non-endocytic pathways. For example,
Taylor et al. ( 2010 ) demonstrated that incubation of gold nanoparticles with cells at
4°C failed to inhibit nanoparticle uptake, suggesting an energy-independent path-
way for entry into the cell, such as diffusion. The internalized gold nanoparticles
also failed to co-localize with Rab5a or LAMP-1 (Lysosomal-Associated Membrane
Protein 1), markers for early endosomes and lysosomal membranes, respectively.
Although targeting to lysosomal pathways is generally undesirable and actively
avoided in drug delivery due to degradation of the drug, some nanoparticle drug
delivery systems take advantage of this pathway in order to increase delivery. For
instance, Sahay et al. ( 2009 ) have described the delivery of the cytotoxic drug,
doxorubicin, via cross-linked-micelles, or polymeric micelles with cross-linked
ionic cores made of poly(methacrylic acid) and nonionic shells made of poly(ethylene
oxide). The cross-linked-micelles are not internalized in normal epithelial cells, but
are instead taken up via a caveolae-mediated endocytic pathway in cancer cells.
Subsequently, the cross-linked-micelles bypass the early endosomes and arrive at
the lysosomes, where the pH-sensitive hydrazone bonds of doxorubicin are cleaved.
This event leads to the accumulation of doxorubicin in the nucleus of the cancer
cell 5 h after administration. Hence, the method of uptake and intracellular destina-
tion in this instance confers the selective toxicity of this nanoparticle drug delivery
method. Many other investigators also have documented the use of pH sensitive
carriers for lysosomal release of therapeutics.