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Antisense Oligonucleotides: A Novel Therapeutic Approach

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naufan003
Antisense Oligonucleotides: A Novel Therapeutic Approach

Introduction to Antisense Technology

Antisense oligonucleotides are short, synthetic, modified nucleic acid sequences designed to bind to messenger RNA (mRNA) sequences. They work by specifically binding to their target mRNA through complementary base pairing, thereby modulating gene expression. The binding of antisense oligonucleotides to mRNA can result in several downstream effects such as degradation of the mRNA by RNase H, inhibition of translation and splicing of the pre-mRNA. By altering gene expression through modulation of mRNA functions, antisense oligonucleotides can block production of disease-causing proteins. This provides a unique therapeutic opportunity for treatment of various genetic disorders and other diseases that are caused due to abnormal protein expression or function.

Mechanism of Action

The core principle behind antisense technology is the base-pairing rules of nucleic acid hybridization. Antisense Oligonucleotides are designed to be complementary to a specific target mRNA sequence. When administered, the antisense oligonucleotide binds to its target mRNA through Watson-Crick base pairing rules. Following binding, the antisense-mRNA duplex recruits RNase H, an endogenous enzyme, which then degrades the target mRNA. This degradation prevents formation of the protein encoded by that mRNA. Alternatively, antisense oligonucleotides can also act by steric blockade of translation or splicing by binding to the mRNA. By any of these mechanisms, antisense oligonucleotides can specifically inhibit expression of a target gene at the post-transcriptional level.

Modifications for Stability and Potency

Naturally occurring DNA and RNA are rapidly degraded in living organisms by nucleases. To overcome this limitation, chemically modified antisense oligonucleotides have been developed with modifications introduced into the sugar-phosphate backbone or nucleotides. Common sugar modifications include phosphorothioate (PS) linkages and 2’-O-methyl/fluorine substitutions which increase nuclease resistance. Additional base modifications like 5-methylcytosine and pseudouridine help reduce immunogenicity. These chemical modifications play a key role in improving the stability, potency and pharmacokinetic properties of antisense oligonucleotides without compromising their core antisense activity. Second generation PS-modified antisense oligonucleotides with other sugar modifications are currently being evaluated in clinical trials.

Targets and Clinical Applications

Due to their high target specificity, antisense oligonucleotides are actively being explored as potential therapeutics for various genetic disorders. Antisense drugs that inhibit production of proteins involved in metabolic disorders are among the leading applications. Inotersen and Volanesorsen, which target production of transthyretin (TTR), have been approved for treatment of hereditary TTR amyloidosis and familial chylomicronemia syndrome, respectively. Other antisense drugs targeting apolipoprotein(a), huntingtin, androgen receptor and androgen-receptor splicing variants are in clinical trials for cardiovascular, neurodegenerative and cancer indications. Future opportunities lie in developing antisense therapies for diseases resulting from infections like COVID-19, neurological disorders like Alzheimer's and muscle diseases. Progress in modification chemistries, delivery and formulation holds promise for realization of the therapeutic potential of antisense oligonucleotides.

Delivery Challenges and Nanoparticle Formulations

Successful delivery of therapeutic oligonucleotides to target tissues or cells remains a major challenge. Naked oligonucleotides administered systemically are rapidly cleared by the kidneys or nucleases. They also face barriers such as low cellular uptake, endosomal entrapment and sequestration by reticuloendothelial system. To surmount these limitations, various nanocarrier systems like liposomes, polymers, inorganic nanoparticles and viral vectors are being explored. Among these, lipid nanoparticles and novel biodegradable polymer-based systems have shown promise for antisense delivery in clinical studies. For instance, patisiran, an antisense drug for treatment of hereditary transthyretin-mediated amyloidosis, employs a lipid nanoparticle formulation which provides efficient delivery to hepatocytes. Continuing advances in material design, surface modifications and targeting ligands are further improving the pharmacokinetic properties of nanoparticle delivery systems for various antisense applications.

Conclusion

With over two decades of research and development, antisense oligonucleotides have emerged as a promising new class of sequence-specific drugs. Significant advances have been made in understanding their mechanism of action, enhancing stability through chemical modifications and developing effective delivery technologies. The approval of several antisense drugs validates this approach for treatment of previously untreatable genetic disorders. Looking ahead, further optimization of antisense chemistry and delivery holds promise to realize their therapeutic potential for various other conditions. Moreover, combination with complementary nucleic acid approaches like RNAi or gene editing further expands the functional scope of nucleic acid-based medicines.


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