Chemical Methodology for Labelling and Bioconjugation - The Chemistry of Molecular Imaging - page 45

2.4 conclusIons

As this chapter summarises, a rich collection of chemical and biochemical reactions currently exists in the literature for
labelling a wide variety of functional groups on target molecules specifically or nonspecifically at a range of settings from
in vitro to live cells and even live animals. Selection of proper bioconjugation strategies is thus important to successful label-
ling of target molecules for imaging. While doing so, one should consider the compatibility of the reactions with the target
molecule and its reaction medium, as well as the efficiency of the conjugation reaction. Labelling target molecules with
imaging moieties of a short half-life requires high reaction yields and fast reaction kinetics. Site-specific labelling chemistry
gives uniform products but often requires introduction of a chemical tag to the target molecule through genetic encoding or
chemoenzymatic reactions. These chemical tags range from small functional groups (a few atoms) to large proteins (several
to tens of kda), the size of which should also be taken into consideration to avoid perturbations on the normal biological
functions of the target molecule. bioorthogonal reactions allow direct labelling in biological media with high specificity
and efficiency and thus are in high demand, but only a few are available. during the final compilation of the topic, significant
progress in conjugation chemistry has been made toward clinical applications. For example, strain-promoted cycloadditions,
such as TCo-tetrazine reaction and cyclooctyne-azide cycloaddition, have been applied in tumour pre-targeting using
nanoparticles [363, 364]. The Rao lab has applied the CbT condensation in in situ self-assembly of nanoparticles in living
mice, leading its way toward early cancer detection [365] and therapy response monitoring [366-368]. ongoing efforts of
many chemists and biologists will lead to the discovery of more efficient, site-specific, and biocompatible biochemical
methods and further enlarge the repertoire of labelling strategies for the development of molecular imaging probes.

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