Antisense Oligonucleotides as Potential Drugs
405
between the mRNA and the antisense oligonucleotide. Different modes of action
are to be considered. The ODNs can either simply function as a steric block
to interfere with the ribosomal translation or induce the action of particular
RNases to destroy the mRNA in the hybrid formed with the ODN. In addition
to this pharmacodynamic consideration, pharmacokinetic processes are important
for optimal drug activity. Key points are the mode of delivery of the ODNs to the
patient, to assure that the target is reached in the cells or organs of concern. The
synthesis of the oligonucleotides has to be straightforward, economic, and reliable.
The ODNs should be fairly metabolically stable, possess high afFnity to the target,
be nontoxic, and preferentially orally available. The Frst generation of ODNs are
the phosphorothioate analogues that have been extensively tested. However, further
developments of analogues for improvement are actively pursued.
This article will feature the design of oligos, their synthesis, modiFcations, and
a discussion of the mechanisms of action. The ODN application will be illustrated
with some selected examples, with the emphasis on those that are in clinical trials.
1
Introduction
The classical treatment of a disease is the
inhibition of function of the protein that
is presumed to be the causative agent of
the disease, mostly with a small molecular
weight drug. Even though this concept is
very successful, problems arise when the
drug is not very speciFc for the protein
in question, for example, when the wild
type and the mutated protein are not
sufFciently different in their afFnity for
the drug. Thus, the idea to interfere earlier
in the process of disease development by
preventing the synthesis of the unwanted
protein at the Frst place is attractive. It
is based on the idea that the wild type
and the mutated protein are sufFciently
different in their nucleotide sequence
that it should be possible to sequence-
speciFcally inhibit either transcription or
translation for the latter but not the former.
The antisense methodology follows the
route of translation interference. This
approach has been applied beyond gene
functional analysis to application in the
clinic where a considerable number of
trials are underway.
In the original concept, the antisense
method for the sequence-speciFc inhibi-
tion of gene expression was quite simply
steric blockage of translation by binding of
the oligonucleotide (ODN) to the mRNA
target. However, it is now generally ac-
cepted that the mode of action is more
complicated. Even though some of the
chemically modiFed antisense oligonu-
cleotides (AS-ODN) seem to act by this
mechanism, the most commonly used AS-
ODN act by degradation of the RNA target
by RNase H in the hybrid formed upon
annealing of the AS-ODN. As it was recog-
nized early that the unmodiFed ODNs are
unsuitable for most
in vitro
and certainly
for
in vivo
applications, chemical modiFca-
tion has played a major part in developing
this methodology. The phosphorothioate
analogues have played a crucial role in this
development.
More recently, RNA interference has
been described for the inhibition of gene
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