Page 59 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN002G-51 May 25, 2001 13:44

Bioconjugate Chemistry 97

activity, antibody binding, radioactivity, fluorescence, use of the polymer PEG is to modify the biological prop-
etc.). In addition, gel electrophoresis, high-performance erties of proteins. By attaching PEG groups to lysine side
liquid chromatography, and mass spectrometry are fre- chains, it is possible to reduce the likelihood that the im-
quently employed to confirm composition, molecular mune system will mount a response to the protein. This
weight, homogeneity, and even details regarding sites of permits repeated use of the protein over several cycles
attachment. Bioconjugates involving larger components of treatment. Another application of PEG conjugation is
such as particles or surfaces are characterized using mi- to modify the rate at which a protein leaves the circula-
croscopy and other surface-science techniques. tion, allowing therapeutic bioconjugates to circulate for
longer periods. The important properties of nucleic acids
B. Challenges and their analogs involve modification of gene expression
processes in target cells. This may occur by the insertion
The precise specification of the chemical linkages formed of new genes into cells, or by interference with the expres-
whenaproteinorotherbiomoleculeischemicallyattached sion of existing genes by “antisense” binding. Antisense
to another biomolecule, the product often having molecu- oligodeoxyribonucleotides (ODNs) can inhibit gene ex-
lar weight >100,000 daltons and multiple possible points pression by hybridization to complementary messenger
of attachment, is rarely attempted. In the past and still to- RNA sequences inside cells.
day, researchers often work with mixtures of molecular
conjugates having different points of attachment but sim-
ilar composition. In special cases, a particularly reactive B. Imaging
naturally occurring residue such as cysteine permits easy Bioconjugates of low molecular weight are used exten-
preparation of homogeneous products. sively in imaging sites of human disease by nuclear tech-
niques such as 99m Tc scintigraphy (making images from
V. NOMENCLATURE gamma photons). Here a coordination complex formed
between the metal technetium and an organic ligand with
The complexity of typical bioconjugates has so far targeting properties (e.g., a small peptide, a hormone ana-
defeated efforts to systematize their nomenclature. It is log, etc.) serves to localize the radiotracer in the tar-
customary to name products by referring to starting ma- get tissue. Positron Emission Tomography (PET) with
18
terials; thus antibody enzyme conjugates are named ac- F-labeled fluorodeoxyglucose or other small molecules
cording to the individual molecules involved. provides higher resolution images by detecting two anni-
hilation photons in coincidence.
Magnetic Resonance Imaging (MRI) sometimes makes
VI. EXAMPLES
use of bioconjugates containing paramagnetic metals such
as gadolinium or manganese conjugated to polymers to
A. Targeted Therapy
modify their relaxivity properties.
An important application of bioconjugates involves the Optical imaging probes that fluoresce in the near-
treatment of disease by specifically targeting disease sites. infrared wavelength region can be conjugated with tar-
Combining the properties of an antibody, synthetic poly- geting molecules and used for in vivo imaging.
mer, or liposome with those of a drug, nucleic acid, or
radionuclide is currently an important research activity.
1. Separation and Analysis in vitro
Some therapeutic products are now commercially avail-
able, and many clinical trials are under way. Immobilization of biomolecules by linking them to sur-
The important properties of antibodies are their spe- faces, such as the wells of microtiter plates, magnetic
cific binding to molecular targets on diseased cells and beads, or chromatography supports for subsequent, spe-
the physiological consequences of their large size, which cific binding of target molecules is often the first step to in
influence where they localize and how long they remain vitro analysis. The nature of the linkage may be as simple
there. Artificial polymers such as PEG share some of the as adsorption of a protein to a hydrophobic surface, or it
latter properties, as do liposomes. Smaller molecules can may involve covalent attachment of the biomolecule to re-
be used in place of these targeting moieties, including active groups on the surface using the chemical principles
smaller fragments of antibodies that retain binding speci- discussed above. With modern techniques, it is usually
ficity (Fab fragments, single-chain antigen binding pro- possible to accomplish this surface attachment without
teins, targeting peptides, etc.) and non-protein molecules significant loss of biological properties.
basedonnucleicacids(aptamers)orotherstructuresfound Conjugates containing antibodies that specifically bind
to bind to or accumulate in biological targets. A typical to a molecule of interest are widely used in bioanalytical