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Magnetic drug targeting 175

Figure 6.2 MNP (magnetic nanoparticle) possessing various ligands to enable multifunctionality
from a single nanoparticle platform.

(Sharma et al., 2015), and their magnetic susceptibility is 20 (Aslibeiki et al., 2012).
Considering their hydrodynamic diameter, SPIONs are classified into three categories:
oral (300 nm to 3.5 μm), standard (SSPIO, 50 mm to 150 nm), and ultrasmall (USPIO,
less than 50 nm), which place them in a range of sizes comparable with those of cells
in the human body (10 100 μm), viruses (20 450 nm), proteins (5 50 nm), or genes
(2 nm wide and 10 100 nm long). This shows that they are able to come into direct
contact with a targeted biological entity. SPIONs with hydrodynamic diameters
between 10 and 100 nm in size are considered optimal for intravenous administration,
while particles between 200 and 10 nm are either cloistered in the spleen or removed
by the kidneys.
SPIONs are suitable for biological applications because they do not attract with
each other, so that the peril of clustering in a medical application is minimized. Iron-
cored MNPs are biocompatible since the organism is adapted to metabolize ferritin,
and to use it in subsequent metabolic processes. SPIONs have so far been used as con-
trast agents in MRI, with products on the market approved by the Food and Drug
Agency (FDA), so they could be accepted for other types of applications too (Schütt
et al., 1997; Markides et al., 2012).
However, the route of administering and the surface properties of SPIONs are what
determine the efficiency of cell absorption, the biodistribution they have, their metaboli-
zation and their toxic potential. Magnetite and maghemite are both iron oxides thus
they appear naturally as nanometric crystals in the crust of the earth through volcanoes
or fires. Therefore it would seem that there are no intrinsic risks associated with them.