Nanocrystals: Production, Cellular Drug Delivery, Current and Future Products - Intracellular Delivery: Fundamentals and Applications - page 429

and in a modified version by Bäumler et al. (Staedtke et al. 2010 ). They loaded
Amphotericin B nanosuspension (AmB-NS) into human red blood cells (RBCs).
The AmB-NS-RBCs were then taken up by phagocytosis by leukocytes, which are
the main effector arms of antifungal defense. The leukocytes carry drugs also in
areas of inflammation. The loading of the RBCs with nanoparticles should be
preferentially performed ex vivo in purified cell populations, also the phagocytosis
by the leukocytes. This avoids competitive uptake (e.g. in vivo after injection of
nanosuspension by MPS organs, ex vivo by other cells present in the suspension).
In case the nanocrystals should be loaded into cells which have no phagocytic
capacity (e.g. RBC), the nanocrystals need to be small enough to be internalized by
pinocytosis, i.e. below 100 nm. This was performed for example with Amphotericin
B nanocrystals which were 65 nm in size.
Basically three targeting levels can be differentiated:

1. a specific organ
2. a certain cell population within the organ
3. and ideally a specific compartment within the cells,

whereas level three is the most challenging. To our knowledge very little work has
been done to study the intracellular fate of nanocrystals.
The reason for this is surely the problem of simultaneous distribution and
dissolution of the nanocrystals inside the cell. One approach to tackle this problem
is the use of fluorescent nanocrystals, and studying simultaneously dissolution
and resulting drug distribution inside the cell. With different nanocrystals different
drug distributions should be achievable. This is definitely one field of investigation
in the future.

8 Conclusions and Perspectives

Delivery to target cells and subsequent intracellular delivery by internalization of
particles is under investigation for more than half a century, dating back to the
1950s. This involves identification of mechanisms to localize nanocarriers in target
cells or cell compartments, and in parallel the development of suitable nanocarriers,
preferentially usable in patients at the end of the day.
At the beginning targeting mechanisms investigated were very simple, e.g. in
the 1960s effect of charge on i.v. injected particles (Wilkins and Myres 1966 ).
Meanwhile the targeting mechanisms got rather complex, e.g. via antibodies or via
modulation of the protein adsorption patterns in the blood to enrich the particle in
a target cell. In future it will get even more complex when considering the confor-
mation of these proteins and the role of the conformation in cellular uptake.
Parallel went the development of nanocarriers. Using the number of nanocarriers
for drug delivery on the market for treatment of patients as a performance measure,
the success was limited. The most important nanocarriers in the second half of
the last century were i.v. nanoemulsions and liposomes. In 2000 the nanocrystals