Antisense molecules

Author: Prof. Dr. med. Peter Altmeyer

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Last updated on: 29.10.2020

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Antisense drugs; Antisense oligonucleotides; Antisense oligonucleotide therapeutics; ASOs; ASO Therapeutics; gene modulators

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Zamecnik P 1978

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Antisense oligonucleotides are chemically modified and thus synthetic, short-chain single-stranded nucleic acids that contain the complementary (and thus "contrary to the original sense" = anti-sense opposite) genetic information of a gene. They bind firmly to the messenger RNA of the gene, which means that translation thus influences or even prevents the conversion of the mRNA into a protein in the ribosomes and thus the biosynthesis of proteins (Benimetskaya L et al. 1997; Bennett CF et al. 2017).

Antisense oligonucleotides are spontaneously taken up by cells in small amounts. By using certain carriers (e.g. certain carrier lipids), an improvement in their uptake into the cell (transfection) and a better intracellular distribution can be achieved. The therapeutic effect of antisense oligonucleotides (ASO) has now been well documented in a wide range of clinical pictures (Askari FK et al. 1996). Perhaps their most significant advantage over other therapeutics is that knowledge of the gene target sequence directly provides knowledge of possible complementary oligonucleotide therapeutics. Thus, the concept of a targeted modulation of disease-causing proteins with antisense oligonucleotides has shown positive results in various clinical studies in viral, inflammatory and oncological diseases. In the last 5 years, more than 100 therapeutic approaches based on antisense oligonucleotides have been tested in Phase I clinical studies, of which about 25% have reached Phase II / III.

General information
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A good example of a positive therapeutic effect of this substance group is the antisense agent milasen, which is tailored to the gene mutation in Batten's disease, a rare genetic defect that leads to neurological deficits.

Furthermore, Exondys51® is an antisense oligonucleotide for the treatment of Duchenne muscular atrophy.

Since July 2017, Nusinersen (Spinraza®) is the first causal, disease-modifying therapy for the treatment of 5q-associated spinal muscular atrophy (SMA) available on the German market. This gene modulator increases the production of the complete and functional SMN protein, binds to the SMN2 pre-mRNA and thus promotes the inclusion of exon 7 in the mRNA. Thus, functional SMN protein can be produced. The half-life in CSF is 135 to 177 days (Finkel RS et al. 2017).

Of particular interest are fomiviruses and mipomers which have been approved for the treatment of cytomegalovirus retinitis and familial hypercholsterinemia (Adams BD et al. 2017). Thus, in the registration studies, treatment with mipomers (Kynamro®), an antisense oligonucleotide (blocks the messenger RNA for the synthesis of apolipoprotein B in the liver) as an adjunct therapy, led to statins at the primary efficacy time point, PET) resulted in a statistically significant reduction of LDL-C levels by 24.7% in patients with homozygous familial hypercholesterolemia (HoFH) and by 35.9% in patients with severe heterozygous familial hypercholesterolemia (HeFH) compared to the respective baseline values, which can be considered clinically relevant. In 2012, the FDA voted against EMA 2013 for approval of this antisense oligonucleotide.

Fomivirsen(Vitravene®) is a 21mer antisense DNA phosphorothioate oligonucleotide that has been used since 1996 as an antiviral for the treatment of cytomegalovirus (CMV) infections in immunodeficiency (e.g. AIDS). Fomiviruses have a sequence complementary to the mRNA, the major immediate-early (MIE) transcriptional unit of the human cytomegalovirus (CMV). By binding the aRNA to the complementary mRNA, the translation of this viral mRNA is blocked (Anderson KP et al. 1996). No protein synthesis takes place. The cell's own enzyme ribonuclease H is activated by the double-strand formation and cleaves the mRNA in this area. The oligonucleotide is thereby released again and can bind to another mRNA.

The oral antisense oligonucleotide (Mongersen) has been used in several clinical studies in patients with Crohn's disease. Mongersen inhibits SMAD7, a molecule that blocks the release ofTGF-ß1 (Monteleone G et al. 2015). SMAD7 is overexpressed in Crohn's disease, among other things. This overexpression leads to disturbances in the "TGF-beta" signaling pathway, whose undisturbed function has an anti-inflammatory effect. Blocking of the TGF-beta signaling pathway by overexpressed SMAD7 (or CYLD) suppresses the function of regulatory T cells and leads to inflammatory reactions, a mechanism which is considered to be of pathogenetic importance in Crohn's disease. The approval of Mongersen is expected.

Antisense oligonucleotides in tumor diseases: In tumor diseases, several proliferation-promoting proteins (oncogenes) usually contribute to growth at the same time. Therefore, the inhibition of individual oncogenic proteins is not very promising. However, certain protein kinases play a superordinate role in the regulation of cell proliferation and differentiation. In leukemia cell lines, for example, inhibition of protein kinase A1 (PKA1) with an antisense oligonucleotide leads to an equilibrium shift in favour of the expression of protein kinase A2 (PKA2).

Inhibition of the Bcl-2 protein: Oblimersen (Genasense®; also known as Augmeroses or bcl-2 antisense oligodeoxynucleotide G3139) is a synthetic 18-base single-stranded phosphorothioate DNA oligonucleotide designed to down-regulate bcl-2 mRNA expression (Klasa et al 2002). Oblimersen selectively hybridizes to the first 6 codons of the open reading frame encoding the Bcl-2 protein and ultimately prevents translation of the Bcl-2 protein. Bcl-2- is a central molecule that is overexpressed by many tumor cells. This shifts the balance between survival and apoptosis towards the survival of the tumor cells. Thus, in many tumor entities neoplastic cells can survive by protecting themselves from programmed cell death (apoptosis). Oblimersen is able to enhance the effect of cytostatic therapy against a number of tumour entities by completely blocking Bcl-2, for example in chronic lymphatic leukaemia, B-cell lymphomas, breast carcinoma in bronchial carcinoma and malignant melanoma (Ott PA et al. 2013; Herbst RS et al. 2004). In later studies, however, the efficiency of this therapeutic principle in metastatic malignant melanoma could not be proven (Bedikian AY et al. 2014).

Non-antisense-mediated effects" of oligonucleotides: In addition to the specific antisense-mediated inhibition of a target protein, further effects of antisense oligonucleotides can be demonstrated that occur independently of the specific antisense effect. These so-called "non-antisense-mediated effects" of oligonucleotides are based on their direct binding to proteins (Hartmann G et al. 1997). Such effects can induce antiviral, anti-adhesive or even immunostimulatory effects.

Thus, from a therapeutic point of view, the non-antisense-mediated immune stimulation of the oligonucleotides is of interest. This effect is due to general differences in the base composition of bacterial DNA and the DNA of vertebrates. In bacterial DNA, the double sequence cytidine base with subsequent guanidine base (CpG dinucleotide) is frequently found. In vertebrate DNA, this two-part sequence is less common and is altered by the addition of a methyl group to the cytidine base (in contrast to unmethylated bacterial DNA). Vertebrates recognize this difference in the base composition of DNA via a mechanism that has not yet been clarified. In vertebrates, the recognition of bacterial DNA leads to the physiological activation of an unspecific immune response. The immunostimulatory property of bacterial DNA can be imitated by synthetic oligonucleotides. A phosphorothioate modification of these oligonucleotides additionally enhances their immunostimulatory effect. Monocytes, macrophages and dendritic cells are also directly activated. In these cells there is an increased synthesis of the cytokines tumor necrosis factor-alpha and interleukin-12. However, bacterial DNA or oligonucleotides with CpG dinucleotides do not exert a direct influence on T lymphocytes.

In inflammatory (including oncological and viral) processes, proteins are known to contribute significantly to the pathogenesis of the respective disease. In inflammatory diseases, for example, the inhibition of the formation of pro-inflammatory cytokines and leukocyte/endothelial adhesion molecules leads to a reduction of undesired inflammatory reactions. In acute and chronic inflammatory diseases, the proinflammatory tumor necrosis factor-alpha (TNFa) plays a central mediator function. The synthesis of TNF-alpha can be specifically inhibited in cell cultures with antisense oligonucleotides.

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The continuous improvement of innovative RNA modifications as well as the improvement of release mechanisms (e.g. as nanoparticles) will open up the development of future RNA-based therapeutics for a broader spectrum of chronic diseases (Adams BD et al. 2017).

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  3. Askari FK et al (1996) Antisense oligonucleotide therapy. New Engl J Med 334: 316-320.Bedikian AY et al. (2014) Dacarbazine with or without oblimersen (a Bcl-2 antisense oligonucleotides) in chemotherapy-naive patients with advanced melanoma and low-normal serum lactate dehydrogenase: 'The AGENDA trial'. Melanoma research 24:237-243.
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  11. Mercuri E et al (2018) Nusinersen versus sham control in later-onset spinal muscular atrophy. N Engl J Med 378:625-635
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Last updated on: 29.10.2020