Nanoparticles to Cross Biological Barriers - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 102

multiple compartments such as the targeting ligand molecular properties
(size, charge, blood circulation half-life, and toxicity), the antigen proper-
ties (expression level, expression distribution, and recycling) and the inter-
actions between the molecule and its antigen (specificity and affinity). It
is suggested that a small molecule should be engineered with high blood-
circulation half-life, low toxicity, and high antigen-binding specificity to
bind to a homogeneously distributed antigen with a high expression level
and affinity in the 1-10 nanomolar range. Lower-affinity characteristics
have been shown to help diffusion farther into tissues owing to the relative
ease with which they dissociate from antigens compared to higher-affinity
binding. Also, lower-affinity antibodies, because of this dissociation, do
not irreversibly bind to antigens and undergo less endocytic degradation.
Conversely, retention time in the tumor decreases, leading to lower thera-
peutic efficacy [179, 180]. Low binding affinity entails a higher percent of
unbound antibodies that are free to diffuse out of the tumor via convec-
tive clearance. Moreover, the vascular architecture heterogeneity of tissues
and the reduced contact of cancer cells with blood flow [181] will affect the
therapy and imaging efficacy.
Cellular Interactions and Uptake: Size, Charge, and Kinetics
Nanoparticles must first cross the plasma membrane to deliver drugs or
agents to the cytosol. The cellular mechanisms that mediate nanoparticle
internalization will be controlled primarily by the adsorption of proteins
on the nanoparticle surface. Thus, the physical and chemical properties
of the nanoparticle—size, shape, surface charge, hydrophobicity, surface
functional groups, and targeting ligands—will determine nanoparticle-
protein interaction and ultimately cellular response [183, 184]. Moreover,
the mode by which nanoparticles enter cells is relevant because it will
dictate the initial cellular microenvironment to which the nanoparticle
will be exposed [185]. NPs will enter by a variety of methods, including
clathrin-mediated endocytosis, caveolae-mediated endocytosis, clathrin-
and caveolae-independent endocytosis, and macropinocytosis [185-187].
In clathrin-mediated endocytosis, receptor binding initiates the forma-
tion of a vesicle ~120 nm via the invagination of the cellular membrane
[186, 187]. The cytoplasmic face of the membrane is coated with clathrin
molecules, which aid in forming the budding vesicle. The clathrin coat is
shed intracellularly, and these vesicles are further directed towards early
endosomes, and then lysosomes or the trans-Golgi network. Caveolae are
flask-shaped cavities in the plasma membrane that are ~60 nm in diameter
and formed by membrane proteins identified as calveolins [186, 188]. When
substrates bind to the surface of the calveolae, vesicle budding occurs and