|Year : 2018 | Volume
| Issue : 2 | Page : 73-76
Oligonucleotides: A new approach against undruggable cardiovascular disorders
Department of Cardiology, AIIMS, New Delhi, India
|Date of Web Publication||10-Sep-2018|
Dr. Arvind Dambalkar
Department of Cardiology, AIIMS, New Delhi
Source of Support: None, Conflict of Interest: None
Oligonucleotides are short DNA or RNA molecules used for a variety of indications in medicine. They can be used for artificial gene synthesis, polymerase chain reactions, and DNA sequencing and can be used as molecular probes. They are usually found as small RNA molecules that function in the regulation of gene expression (e.g., microRNA). These antisense oligonucleotides, based on DNA or RNA chemistries, regulate gene expression by binding to an RNA target.
Keywords: Antisense oligonucleotide, oligonucleotides, siRNA
|How to cite this article:|
Dambalkar A. Oligonucleotides: A new approach against undruggable cardiovascular disorders. J Pract Cardiovasc Sci 2018;4:73-6
|How to cite this URL:|
Dambalkar A. Oligonucleotides: A new approach against undruggable cardiovascular disorders. J Pract Cardiovasc Sci [serial online] 2018 [cited 2018 Sep 21];4:73-6. Available from: http://www.j-pcs.org/text.asp?2018/4/2/73/240967
| Introduction|| |
Oligonucleotides have recently emerged as an effective therapy against a variety of diseases. These molecules are rapidly gaining importance in therapy for various genetic disorders, viral infections, cancers, and recently cardiovascular disorders. The molecules have overcome a lot of hurdles during their development over decades and are now being used in various undruggable diseases.
Oligonucleotides are small messenger RNA (mRNA) which can target any gene by posttranslational modification of the mRNA [Figure 1].
They are mainly of two types – antisense oligonucleotide (ASO) and small-interfering RNA (siRNA). Both these molecules are different in structure but share fundamental principle; an oligonucleotide binds a target RNA through Watson–Crick base pairing, resulting duplex directs degradation/modulation of the target mRNA.
In 1978, Zamecnik first reported a synthetic oligonucleotide complementary to Rous sarcoma virus 35 S RNA inhibiting its protein expression. Another oligonucleotide called short-interfering RNA (siRNA) was discovered by Craig Mello and Andrew Fire, for which they were awarded the Nobel Prize in 2006 for its discovery.
Further developments were slow initially primarily due to instability and nonselective distribution of the drug. The last decade witnessed milestones in developing the molecule as a targeted therapy for a wide range of diseases.
Oligonucleotides were conjugated to specific molecules via triantennary N-acetylgalactosamine (GalNAc) which facilitates targeted delivery to hepatocytes. The oligonucleotides have also been linked to synthetic antigen binding (Fab) fragment of antibody against CD markers which target them to specific organs. CD71 is a specific marker for heart and muscle cells.
| Mechanism of Action|| |
The ASO consists of complementary base pairs of the targeted mRNA. The ASO is modified further by GalNAc conjugated with a therapeutic ASO, which are internalized and then released inside the cell, where they can hybridize to their cognate pre-mRNA and induce cleavage of the RNA–DNA heteroduplex mainly. However, multiple mechanisms have been observed which lead to inhibition of gene expression.
ASO, the single-stranded DNA oligonucleotide, passes through the cell membrane and enters into the nucleus from the cytoplasm. In the cell nucleus, the ASO binds or hybridizes to the target mRNA, resulting in the formation of a sense–antisense duplex. If ASO is completely complementary to the mRNA, the formation of the duplex initiates the recruitment of the RNase H and enzyme, an endogenous nuclease. RNase H degrades the target mRNA, which results in inhibition of target mRNA expression. The oligonucleotide moves on and then hybridizes to another target mRNA. If ASO is not completely complementary to target mRNA, it causes steric hindrance to attachment of ribosomes or leads to abnormal splicing of target mRNA.
The mechanism of action of siRNA is analogous to ASO; however, there are subtle differences. The siRNA molecule consists of two strands of complementary oligonucleotides. The guide strand is complementary to mRNA, and the passenger strand acts as prodrug and stabilizes the molecule. The loaded RNA-induced silencing complex (RISC) has a long half-life, and as few as 100–200 loaded RISC complexes per cell are sufficient to eliminate expression of the targeted gene. Once siRNA enters the cell a helicase separates the sense and antisense strands of the oligonucleotide. RISC, an endogenous conglomerate of functional components, associates with the ASO. The antisense strand of the oligonucleotide hybridizes to the target mRNA, resulting in the formation of a sense–antisense duplex. The nuclease component of RISC is an endogenous nuclease that degrades the target mRNA. This results in the inhibition of target mRNA expression
Oligonucleotides in cardiology
Oligonucleotide therapies are being increasingly investigated in various diseases of cardiology, and the success achieved by these drugs has been phenomenal. Recently, the phase II and III trials were completed siRNA (inclisiran) for dyslipidemia in high cardiovascular risk patients which showed favorable response rates and minimal side effects profile. Proprotein convertase subtilisin/kexin type 9 (PCSK9), an enzyme (serine protease) encoded by the PCSK9 gene, is predominantly produced in the liver. PCSK9 binds to the low-density lipoprotein receptor (LDL-R) on the surface of hepatocytes, leading to the degradation of the LDL-R and higher plasma LDL-cholesterol (LDL-C) levels. Antibodies to PCSK9 interfere with its binding of the LDL-R, leading to higher hepatic LDL-R expression and lower plasma LDL-C levels [Figure 2]. There are several strategies to lower free plasma PCSK9, and inclisiran is one such attempt.
|Figure 2: Inclisiran effective in reducing low-density lipoprotein cholesterol.|
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These trials conclusively demonstrated successful improvement of lipid profile with dosing as less as twice per year and have thus been recently approved by the United States Food and Drug Administration.
Familial homozygous hypercholesterolemia, a genetic disorder leading to dyslipidemia and accelerated atherosclerosis, was deemed undruggable till now as statin was ineffective in these patients. Mipomersen, an ASO, has been studied in these patients in a randomized trial and found significant improvement in dyslipidemia. Mipomersen consists of a 20-mer 2′-O-methoxyethyl-modified nucleotide complementary and specific to human apolipoprotein B (apoB)-100 mRNA. It distributes mainly to the liver, where it forms a duplex with the target mRNA, causing the mRNA to be cleaved by RNase H and therefore unable to be translated to apoB-100. Hepatic apoB mRNA silencing causes reduction in hepatic apoB and plasma total cholesterol, LDL-C, and apoB concentrations [Figure 3].
The molecule being synthetic could prove to be an economical way of lipid reduction in future patients with dyslipidemia, in addition to improved compliance to the drug.
Volanesorsen targeting APO3 is in phase III trial for hypertriglyceridemia, familial chylomicronemia syndrome, and familial partial lipodystrophy.
Eteplirsen has been approved for Duchene's muscular dystrophy. It induces skipping of exon 51 of the dystrophin gene by binding to a sequence within the dystrophin pre-mRNA and restored the translational reading frame to dystrophin transcripts in patients with Duchene's muscular dystrophy.
These molecules are also being investigated in hypertension, targeting the RAS pathway. Promising results have been shown in preclinical studies.
Idiopathic pulmonary hypertension is also being investigated as a target for oligonucleotide therapy and has met with success in diagnosing and treating idiopathic pulmonary arterial hypertension in animal studies.
In a randomized controlled trial, factor XI (FXI)-ASO, an ASO that specifically reduces FXI levels, was found to be superior to enoxaparin to prevent deep vein thrombosis in patients undergoing total knee arthroplasty. There was decreased risk of bleeding with these agents 2%–3% compared to 8% seen in enoxaparin. FXI-ASO could be used in triple therapy (dual antiplatelet therapy (DAPT) + anticoagulation) in view of better safety profile.
Diseases with disorders of protein accumulation such as amyloidosis can also be potentially treated with oligonucleotides. Animal studies have shown positive response in transthyretin and senile systemic amyloidosis with this therapy, which need to be further confirmed in human trials.
Toxicity study of phosphorothioate oligodeoxynucleotide and its analogs done on animals has shown to cause thrombocytopenia, dose-dependent elevation of liver transaminases, reduction of the levels of alkaline phosphatase, albumin, and total protein. Splenomegaly, lymphoid hyperplasia, diffused multiorgan mixed mononuclear cell infiltrates, lymph nodes necrosis, cytological alterations and necrosis in hepatocytes, and renal tubule regeneration were also observed.
However, the recent studies,,,,,,,,,,,,,,,,, have failed to replicate the concerns generated by preclinical trials. Injection site reactions and flu-like syndrome were the most commonly reported adverse effects in these trials. Creatine kinase elevation (5%) and myalgia (8%) were observed in patients receiving inclisiran. Liver enzymes were not significantly different compared to placebo. The study provides only 210-day follow-up of these patients and long-term safety profile data will be available in due course of time.
| Conclusions|| |
Diseases ranging from viral infections such as HIV, to degenerative disorders such as amyotrophic lateral sclerosis to various cancers and inflammatory diseases such as Crohn's disease and rheumatoid arthritis are being investigated, and approximately 40 drugs have been tested till now of which 20 have been tested in phase II and III trials. The success of oligonucleotide therapy might be the dawn of new era in pharmacotherapy and could be a hope for patients suffering from undruggable diseases.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]