Magnetic Resonance Tracking of Stem Cells with Iron Oxide Particles - Intracellular Delivery: Fundamentals and Applications - page 469

from 50 to 500 nm, and was absent when 1 mm particles were used (Rejman et al.
2004 ). In a study labelling human T-cells with particles of 33 nm or larger diameters,
uptake increased with diameter peaking at 107 nm before decreasing with diameter
from 207 nm to 1.4 mm (Thorek and Tsourkas 2008 ). However, it is difficult to infer
the pattern of T-cell uptake with particle diameters as Thorek et al 's study used
a variety of particle coatings at different sizes: dextran (33-107 nm), styrene
(207-289 nm) and silica (1.4 mm) coated particles.
It is possible to use transfection agents during labelling to generate particle
aggregates of varying sizes. However, results relating aggregate size to? uptake
have been contradictory as aggregate surface composition and charge among
particles greatly differed in these experiments (Matuszewski et al. 2005 ; Song
et al. 2007 ). Eliciting the relationship between particle size and cellular uptake
requires a set of particles of similar surface composition. Size-dependent uptake
of microgel iron oxide particles (MGIO) was demonstrated between the diame-
ters of 87 and 766 nm with 600 nm MGIO providing the maximum loading of
33 pg/MSC, which is three- to sixfold greater than with ferucarbotran (Lee et al.
2009 ). Interestingly, endothelial progenitor cells (EPC) labelled with either
600 nm MGIO or ferucarbotran resulted in similar quantity of cellular iron
(26-28 pg/EPC) (Lee et al. 2010b ). Taken together, these results suggest that uptake
is size-dependent in some cell types, including monocytes, lung carcinoma cells
and melanoma cells. In addition, macrophages and MSC have shown maximum
uptake efficiency at specific diameters. However, it remains to be determined if
certain cells such as EPC are insensitive particle diameters while controlling for
particle coating or surface charge.

4.4 Labelling Duration and Concentration

Current evidence shows that the number of particles internalised increases and
saturates with increasing labelling duration and concentration, independent of
particle type, size or cell type. The uptake of sub-100 nm dextran-, 200-300 nm
styrene- and 1.4 mm silica-coated particles increased with incubation duration but
was saturated by 4 h with non-phagocytic T-cells (Thorek and Tsourkas 2008 ).
The same study also demonstrated that the saturating concentration varied for dif-
ferent particle sizes. Labelling concentration is commonly expressed as mass of
iron per unit volume of labelling medium, for example mg Fe / ml or simply mg/
ml. The uptake for sub-100 nm dextran particle saturates at 0.025 mg/ml while
200-300 nm styrene particles and 1.4 mm silica particles are saturated at 0.1 mg/
ml. When 30 nm AMNP was used to label Hela cells and macrophages, the satu-
rating durations were 5 and 10 h respectively while the saturating concentration
was 0.5 mg/ml for both cell types, further demonstrating the variability of label-
ling efficiency between cell types (Wilhelm et al. 2003 ). The longest reported
labelling duration was 72 h where ferumoxide was incorporated into bone marrow
cells (Jendelova et al. 2003 ). Most studies are kept to a maximum labelling duration