Physics And Chemistry Basis Of Biotechnology - De Cuyper - tiera.ru

J.W.M. Bulte and L.H. Bryant Jr.
be detected without refocusing of 180° pulses (T2* effect). Of paramount importance
here is that water protons at distant sites can be affected, leading to a “blooming effect”,
i.e. an amplification of signal changes.
Magnetite particles need to be stabilised in order to prevent aggregation. Most
commonly this is accomplished by a coating of dextran. Immunoglobulins can then be
covalently linked to the polysaccharide coat using a periodate-oxidation/borohydridereduction
method, which, through the formation of Schiff bases as intermediates,
covalently links the amine (lysine) groups of the mab to the alcohol groups of the
dextran [36,37]. MION-(46L) iron oxide nanoparticles have been conjugated this way
to polyclonal IgG for detection of induced inflammation [38), to mab fragments for the
specific visualisation of cardiac infarct [39], and to intact mabs for immunospecific
detection of intracranial small cell lung carcinoma [40], ICAM- 1 gene expression [4 1],
and oligodendrocyte progenitors [42]. Alternative ways of attaching mabs to magnetic
nanoparticles include glutaraldehyde crosslinking [43], complexing through ultrasonic
sonication [44,45], using the biotin-streptavidin system [46] and amine-sulfhydryl
group linkage [47,48]. For in vivo applications, limited success (e.g. true specific
immunodetection) has been achieved thus far but this is likely to improve with the
development of smaller nanoparticles that facilitate endothelial penetration and exhibit
longer blood half-lives.
3. Other magnetically labelled ligands
Either paramagnetic chelates or magnetic nanoparticles can be linked to molecules
other than mabs in order to confer specificity for a targetable receptor. For the group of
paramagnetic agents, it has been demonstrated that “folated” gadolinium-dendrimers
can be targeted in vitro to folate-receptor bearing leukaemic cells [32], and induce
significant specific changes in relaxation times that is inhibitable by free, nonconjugated
folate. This may be used for in vivo imaging of folate-receptor
overexpressing tumours [49], but further work including the use of non-targeted
polymer controls is needed. Another approach of conferring specificity to a
paramagnetic label is to link it to an antisense oligonucleotide; a specific proton
relaxation enhancement has been achieved for 5S rRNA as a macromolecular target and
its labelled complimentary 6mer antisense sequence [50].
For magnetic iron oxide particles, the first use of a targetable ligand employed the
use of arabinogalactan [51,52] in lieu of bacterial dextran as the polysaccharide
coating: in this way, specific uptake in hepatocytes is achieved through uptake of the
asialoglycoprotein receptor. Similar results were obtained when asialofetuin was used
as a coating [53], and may be useful for improved detection of hepatocellular
carcinoma. (Synthetic) peptides can also be linked to MION-46 or other very small iron
oxide particles. For instance, cholecystokinin- [54] and secretin-[55] linked particles
have been employed for MR visualisation of their respective pancreatic receptor and
may aid in the diagnosis of pancreatic cancer. Transferrin is another example of a
targetable protein, since certain tumours are known to overexpress transferrin receptors.
Transferrin-iron oxide particles have been used for specific detection of gliosarcoma
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