Key featuresShow Hide
- Modification to enhance cell delivery of an oligonucleotide
- Non-animal based cholesterol
- All carbon spacer
- Not susceptible to 1,2-diol elimination
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Despite advances in oligonucleotide therapeutics, the main issues remain cell delivery and cellular uptake. A number of strategies have been developed to combat this, the most widely used being the conjugation of a ‘delivery’ reagent to the oligonucleotide. In general the reagent is hydrophobic in nature, e.g. cholesterol, and is often attached via a cleavable linker. This is typically incorporated at the 5'-end of the oligo, and for siRNA is incorporated on the sense (passenger) strand.
Cholesteryl-conjugated oligonucleotides have been the subject of substantial interest in antisense and other studies due to the lipophilicity and good availability of cholesterol. One such study (1) has shown the use of cholesteryl-modified siRNA in therapeutic gene silencing.
For 5'-attachment, we have found 5'-Cholesterol-CE Phosphoramidite (LK2170) to offer specific advantages in oligo synthesis. Since the cholesterol is attached directly to aminohexanol, it is not susceptible to 1,2-diol elimination as observed in some other products. Lack of a trityl group simplifies purification; some cholesterol products must be used in trityl-on mode (to prevent 1,2-diol elimination during deprotection), then detritylated, and can subsequently be very difficult to purify. Similarly, 5'-Cholesterol-TEG-CE Phosphoramidite (LK2189) is a simple 5'-modifier without the complications of a 1,2-diol and trityl protection, but with the added benefit of solubility in acetonitrile conferred by the TEG spacer.
For 3'-modification we offer 3'-Cholesterol CPG 1000/110 (LK2394) and a 3'-Cholesterol TEG CPG, the latter in various pore sizes and functional loadings. LK2394 has an additional benefit in certain circumstances, since the modification is based on the natural sugar-phosphate backbone, hence there are no adverse structural effects on the oligo.
The strict guidelines imposed by regulatory authorities now make it essential to use non-animal based products in pharmaceutical drug development for humans.
With increasing frequency, therefore, our customers are requesting that we supply products with BSE/TSE statements. We have now developed an alternative route to these products that uses entirely plant-derived cholesterol, making them even better choices for modification of oligos.
- Therapeutic silencing of an endogenous gene by systemic administration of modified siRNAs, J. Soutschek, A. Akinc, B. Bramlage, K. Charisse, R. Constien, M. Donoghue, S. Elbashir, A. Geick, P. Hadwiger, J. Harborth, M. John, V. Kesavan, G. Lavine, R.K. Pandey, T. Racie, K.G. Rajeev, I. Röhl, I. Toudjarska, G. Wang, S. Wuschko, D. Bumcrot, V. Koteliansky, S. Limmer, M. Manoharan and H.-P. Vornlocher, Nature, 432, 173-178, 2004.
|LK2393||3'-Palmitate SynBase™ CPG 1000/110|
|LK2394||3'-Cholesterol SynBase™ CPG 1000/110|
Physical & Dilution Data
Dilution volumes (in ml) are for 0.1M solutions in dry, alcohol-free DCM, except for LK2189 which is dissolved in dry acetonitrile (LK4050). Adjust accordingly for other concentrations. For µmol pack sizes, products should be diluted as 100µmol/ml to achieve 0.1M, regardless of molecular weight.
LK2163, LK2170, LK2194 and LK2199 - Dilute in anhydrous, alcohol-free DCM to a concentration of 0.1M.
LK2189 - Use anhydrous acetonitrile to a concentration of 0.1M.
Prepare the amidite solution 5-10min before placing on the synthesiser to ensure complete dissolution.
LK2163, LK2170 and LK2194 - An increased coupling time of 15min is recommended for the phosphoramidites. Contrary to MacKellar et al1, we have found that when using LK2170, column washes with DCM before and after coupling are unnecessary. In our hands, omitting the DCM washes gave the highest final coupling results and there was no evidence of reagent precipitation in the lines.
LK2199 - 3-5min (3min up to 1umol).
LK2393 and LK2394 - Both CPG supports are used as any standard nucleoside support as per instrument instructions. However, non-nucleosidic modifications are slow to detritylate and require an initial detritylation prior to use in synthesis. In this case it is important not to use a cycle with an initial capping step.
It is recommmended that the oligonucleotide is synthesised DMT OFF when using the 3’ modifications (CPGs), otherwise the presence of the DMTr and hydrophobic group can result in difficult purification and solubility issues. None of the 5’ modifiers have DMTr blocking therefore this is not an issue with them.
Cleavage & Deprotection
For the amidites no changes are required from your standard method, however the optimum conditions are AMA for 2h at RT. The amidites - except for LK2189 - are stable to most common deprotection methods e.g. AMA, 10mins, 65oC (cholesterol-TEG has a tendency to cleave through the carbamate at elevated temperatures).
The CPG supports use the succinyl linker which will cleave under most ammonium hydroxide solution and AMA deprotection conditions (typically 1-2h at room temperature with ammonium hydroxide solution, and a few minutes at 65°C with AMA). The linker will also cleave with potassium carbonate at room temperature (>90% after 4h). Therefore cleavage and deprotection of the oligo is carried out according to the deprotection protocols required by the nucleobases and other modifiers (if present). When synthesising short oligos (<15 bases) it is, however, advantageous to add 20% EtOH to the cleavage and deprotection solution to ensure complete removal of the oligo from the resin.
Purification by RP-HPLC is recommended for oligonucleotides modified with hydrophobic labels. Where 3’-modifiers are used DMT ON purification is possible but makes the oligonucleotide extremely hydrophobic. As a result short oligos often have solubility issues. Also, removing the DMTr group in aqueous acetic acid can cleave the cholesterol label from the oligo through the carbamate bond.
Where modification is incorporated at the 5’ end, there is a significant difference in the retention time between the labelled and unlabelled oligo, making purification simple. In general the HPLC gradient must reach at least 95% MeCN to elute the product.
Whilst this removes the unlabelled failures from the labelled oligo very efficiently, if there is a requirement to remove any labelled deletion sequences IE-HPLC or PAGE is the preferred choice. Similarly, where the modifier is incorporated at the 3’ end, the latter is the preferred choice of purification. RP-HPLC gives limited separation in this case since the full-length and failure sequences are all labelled with the hydrophobic group.
Storage & Stability
All products are stored dry in a freezer at –10 to –30°C and are stable under these conditions for over 12 months. Diluted samples must be used within 24h.
- Synthesis and physical properties of anti-HIV antisense oligonucleotides bearing terminal lipophilic groups, C. MacKellar, D. Graham, D.W. Will, S. Burgess and T. Brown, Nucleic Acids Research, 20, 3411-3417, 1992.