Antisense Oligonucleotides and RNA Interference - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 331

tumors [1] . However, the mechanism of their antitumor activity was never eluci-
dated, and their activity also varied with their size and structure. Compared to studies
done on DNA, more extensive research has been reported on using RNA and poly-
ribonucleotides as medicinal agent. In early stages of development, double-stranded
polyriboinosine-polyribonocytidine was the most extensively studied polynucleotide
[14] . Miller et al. were the first to try modifying the phosphate backbone of oligo-
nucleotides to improve their properties, and they synthesized the first chemically
modified oligonucleotide belonging to the class of methyl phosphotriester oligonu-
cleotides [15,16] . Thereafter, ribozymes, another class of catalytic oligonucleotides,
appeared as a new tool to investigate gene expression. Depending on the structure,
ribozymes can degrade or modify the target mRNA to produce correct sequence
[17,18] . AS ODNs and ribozymes were already in clinical practice as genetic thera-
peutics long before small interfering RNAs (siRNAs) were developed as potential
medicinal agents. In 1992, Fire et al. were the first to describe the RNAi as a mecha-
nism of action of AS ODNs for the destruction of target mRNA [19] . They are a nat-
ural cellular defense mechanism, by virtue of which the presence of double-stranded
viral DNA triggers the mRNA degradation. Introduction of 20- to 23- nucleotide-
long siRNA could exhibit antiviral activity by blocking the expression of viral pro-
teins, which led to the progress of siRNA in therapeutics to block the production of
disease-related proteins.
7.2.2 Present Scenario
Presently, three decades after the emergence of the antisense concept, the basics of
this technology and the key steps to challenges in therapeutics are well comprehen-
sible. The main attention of researchers and pharmaceutical industries today is to
make this technology available for its therapeutic applications. Thus, today's focus
is not only to design an antisense molecule with good affinity and specificity or
to predict its in vivo effect, but also to practically approach its formulation, taking
care of the ultimate pharmacological and toxicological aspects at the initial stage of
development to avoid rejection at the final clinical phases. Incredible progress has
been made in the rational design and appropriate selection of antisense agents, its
formulations, delivery carriers, doses, and dosage regimens, and most importantly in
the design of preclinical and clinical trials [20-22] . Since the discovery of antisense
technology, there have been numerous advances both in the structure and properties
of oligonucleotides used as therapeutic agents. Compared to that of whole DNA or
RNA, today RNAi is achieved using siRNA, dsRNA (double-stranded RNA), and
shRNA (short hairpin RNA). Several modifications such as novel bases, sugars, con-
jugates, and chimeric technology have been tried to improve the pharmacokinetic
and pharmacodynamic properties of these oligonucleotides [11,12,20-22] . Because
the biological activity of an AS ODN at the site of action is dependent on factors
such as its concentration, concentration of mRNA, rate of synthesis and degrada-
tion of mRNA, and the type of terminating mechanism, various strategies have been
employed to improve the properties of systemically administered oligonucleotides.
Further, a more potent gene therapy, ribozymes, has gained more attention than