Biomedical Engineering Reference
In-Depth Information
AS ODNs [23-25] . Ribozymes are enzymes that cleave single-stranded regions
of RNA by transesterification or hydrolysis of a phosphodiester bond. To achieve
RNAi, either ribozyme-encoding sequences are incorporated into plasmids, or chem-
ically modified minimum ribozyme structure is administered [26] . Chemical modi-
fications have also been tried to synthesize nuclease-resistant ribozyme drugs [27] .
Moreover, earlier used AS ODNs exhibit gene silencing after entering the nucleus,
but some newer ones and siRNA need not enter the nucleus; this leads to posttran-
scriptional gene silencing by degrading target mRNA in the cytoplasm itself, making
the delivery challenge a little simpler than it was before with AS ODNs [18] . The
evolution of different antisense technologies has not closed the path of oglionucel-
otides; rather the progress and improvements in the oligonucleotides have hastened
the pace of newer antisense agents to reach the therapeutic platform. Currently, there
is swift progress in systematic research and development, while obviating the down-
side faced previously by AS ODNs at the earlier growth and clinical steps.
7.3 Strategies of Transcriptional Arrest
Downregulation of mRNA expression is made possible either via transcriptional
arrest of the RNA complementary to the disease-related protein or through the post-
transcriptional gene silencing (PTGS) phenomenon. Transcriptional arrest is an
alternative to inhibiting mRNA expression by AS ODNs posttranscription. The tran-
scriptional arrest of double-stranded DNA can be achieved by two distinct strategies,
namely, strand invasion and triple-strand formation [28] . Until recently, triple-strand
formation has been the most commonly used strategy to induce transcriptional arrest.
Triple strands are formed by involving Watson-Crick hydrogen bonds between the
third strand and the complementary strand of DNA duplex [28] . Homopyrimidine
oligonucleotides are capable of inhibiting transcription via triple-strand formation
[29,30] . Several modifications in oligonucleotides have been tried to improve bind-
ing to duplex DNA via triplex formation with high affinity and specificity [31-33] .
Strand invasion, though not a widely studied strategy to induce transcriptional arrest,
is being used by certain oligonucleotides, such as peptide nucleic acids (PNAs), to
inhibit transcription [34] . PTGS is the phenomenon in which antisense agents act
through degrading the transcribed target mRNA to prevent translation into the com-
plementary protein ( Fig. 7.1 ).
Antisense molecules lead to the manipulation and/or modification of DNA or
RNA through a number of different mechanisms to partially or completely eliminate
the normal cellular processing of the genetic message of a gene. Accomplishment
of this knockdown or knockout is the major challenge presented by the antisense
technique. To achieve clinically approved status, it is essential to have a better
understanding of the various pharmaceutical and pharmacological considerations,
availability of the active moiety to act at the desired site, therapeutically effective
concentration, its formulation into an appropriate dosage form, and different barriers
to reaching the target of interest [21,22] .
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