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MSCs should be differentiated to adipogenic, chondrogenic, and osteogenic
lineages. The potential for alterations in chondrogenic differentiation of
MSCs in vitro have been seen for various transfection agents and may be
dose dependent or incubation time dependent (Arbab et al . 2004b; Bulte et al .
2004; Henning et al . 2009). For HSCs, differentiation to colony-forming units
should be determined for labeled and unlabeled cells. Endothelial progenitor
cells (EPCs) can be differentiated to colony-forming units, endothelial cells,
and cord-like structures in a Matrigel system. NSCs should be differentiated
to neuron and glial cell lineages. The differential capacity of magnetically
labeled cells can also be determined by tracking the in vivo migration and
targeted incorporation or labeled stem cells in different organs (Watson et
al . 2006).
15.3 Important Note
The production and marketing of ferumoxides have been discontinued.
Although there are alternative SPIO agents that can be used for cell label-
ing, none of them are approved by the FDA or made in a cGMP facility and
available in the United States. Amag Pharmaceuticals ( http://www.amag-
pharma.com ) will be introducing ferumoxytol, a 30-nm carboxyl methyl-
dextran-coated SPION as a clinically approved iron supplement in patients
with renal failure, and presently ferumoxytol is being evaluated in clinical
trials ( http://clinicaltrials.gov/ct2/results?term=ferumoxytol ) as an MRI
contrast agent. It is also not clear whether ferumoxytol can be used to label
nonphagocytic adherent cells or cells grown in suspension. According to a
review by Neuwelt et al. (2008), who have been running clinical trials with
intravenous fermoxytol, combining the agent with protamine in vivo does
not result in leukocyte labeling as compared to ferumoxides and protamine.
Daldrup-Link et al. (2003) did label HSCs with P7228 (similar to ferumoxy-
tol) plus lipofectin, but Prussian blue images were not provided of the
labeled cells. Until such a time as a protocol can be developed for labeling
stem cells with ferumoxytol, the cell-labeling community will need to use
non-USP- (U.S. Pharmaceutical) grade material for cell labeling. For experi-
mental animal studies for SPIO labeling of stem cells or other nonphagocytic
cells, the ultimate removal of ferumoxides from the marketplace is probably
not an issue since other manufacturers synthesize SPIOs (e.g., Bangs, http://
www.bangslabs.com ; BioPAL Inc., http://www.biopal.com ) that can be used
to label nonphagocytic cells magnetically. For planned clinical trials in the
United States and possibly in other countries, investigators will need to seek
approval or postpone studies until it can be determined whether ferumoxy-
tol can be substituted for ferumoxides for cell labeling.
 
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