Biomedical Engineering Reference
In-Depth Information
7.6 Types of Antisense Agents
To date, a broad array of disease targets have been explored utilizing various antisense
agents. AS ODNs, ribozymes, aptamers, and siRNA are the available techniques to
achieve suppression or elimination of a genetic message related to a particular disease.
Oligonucleotide-based antisense techniques represent the first clinically successful
approach to target an ocular disease. None of the antisense agent has become available
for systemic applications. AS ODNs and siRNA, being large, ionic, and structurally
similar to natural nucleic acids, cannot be used
per se
as genetic medicines. Hence, to
serve as effective drugs, these must possess some desirable pharmacokinetic and phar-
macodynamic properties, which vary with their oligonucleotide length, sequence, and
chemical class
[98,99]
. Various modifications in the basic structure of AS ODNs have
been tried to improve their properties and reduce toxicities while maintaining their tar-
get specificity and imparting resistance to nucleases. To elicit a biological response, an
AS ODN must be absorbed from the site of administration and distributed to various
tissues with maximum uptake by the target cells while being resistant to any chemical
or enzymatic degradation.
Amendments that can be protective to nucleases while maintaining the desired
characteristics of antisense effect can be introduced in DNA as well as RNA nucleo-
tides (
Fig. 7.5
) by alteration in the base and modifications in the phosphate back-
bone
[100]
. Further, the 2-OH group can also be tailored in RNA nucleotides.
Synthetically modified AS ODNs can be grouped into three broad categories
viz
.
first-, second-, and third-generation AS ODNs.
7.6.1 First-Generation AS ODNs
These include phosphorothioate oligonucleotides (
Fig. 7.6A
) synthesized by replac-
ing one of the nonbridging oxygen atoms in the phosphate backbone with a sulfur
atom
[101]
and methylphosphonates prepared by replacing a nonbridging oxygen
atom with a methyl group at each phosphorus in the oligonucleotide chain.
Methylphosphonates are neutral with excellent stability in biological milieu
[101]
,
but cannot activate RNase H activity
[102]
. Their cellular uptake occurs by adsorp-
tive endocytosis
[103]
and not by membrane diffusion
[104]
. Phosphorothioate oli-
gonucleotides are the most widely studied oligonucleotides and were the first to be
synthesized chemically
[105]
. This modification was primarily tried to improve the
stability of AS ODNs toward nucleases, but phosphorothioate oligonucleotides were
Figure 7.5
Possible sites for chemical
modification of AS ODNs to improve
their properties. Note that the 2-OH
site is only available in RNA.
Base
B
O
H
H
H
Ribose moiety (2'-OH group)
Phosphate backbone
O
P
OH
O
-
O
O
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