An improved synthesis of 2'-O,4'-C-ethylene nucleic acid (ENA) and thermodynamic studies of duplex formation containing the guanosine ENA unit.

Oligonucleotides containing 2'-O,4'-C-ethylene nucleic acids (ENA) have been proven highly effective for antisense therapeutics. 2'-O,4'-C-Ethyleneguanosine and its phosphoramidite were previously obtained from 3,5-di-O-benzy1-4-C-(p-tolulenesulfonyloxyethyl)-1,2-di-O-acetyl-α-D-erythropentofuranose by glycosylation, but with limited efficiency. Using 3,5-di-O-benzy1-4-C-(2-t-butyldiphenylsilyloxyethyl)-1,2-di-O-acetyl-α-D-erythropentofuranose as an alternative substrate, we developed several methods to obtain 2'-O,4'-C-ethyleneguanosine derivatives with much higher yields than previously reported. These methods were also applicable for the synthesis of 2'-O,4'-C-ethyleneadenosine and 2'-O,4'-C-ethylene-5-methyluridine derivatives. Moreover, we investigated the thermodynamic benefit of DNA strands containing 2'-O,4'-C-ethyleneguanosines during duplex formation with complementary RNA. Only a single modification by the nucleoside resulted in a 10-fold greater binding constant of the DNA/RNA duplex.

In vivo antisense activity of ENA oligonucleotides targeting PTP1B mRNA in comparison of that of 2'-MOE-modified oligonucleotides.

The 2'-0-(2-methoxy)ethyl (2'-MOE)-modified gapmer antisense oligonucleotide ISIS113715, which targets protein-tyrosine phosphatase IB (PTP1B) mRNA, increases insulin sensitivity and normalizes plasma glucose levels in diabetic ob/ob and db/db mice. In the present study, the efficacy of the isosequential 2'-O,4'-C-ethylene-bridged nucleic acid (ENA)-modified oligonucleotide ENA-1 was compared with that of ISIS113715 in order to further improve the down-regulation of PTP1B in db/db mice. Intraperitoneal administration of ENA-1 more effectively decreased the plasma glucose levels in db/db mice than ISIS113715. Moreover, ENA-1 decreased the expression of PTP1B in the liver and fat of db/db mice more effectively than ISIS113715. These data indicate that ENA modifications enhance the ability of antisense oligonucleotides and make them superior to second-generation 2'-MOE modifications. We would like to thank to Drs. Shinya Tsutsumi and Kenji Kawai for the T(m) measurement and autoradiography experiments. ENA is a registered trademark of Mitsubishi-Kagaku Foods Corporation.

Fine-tuning of ENA gapmers as antisense oligonucleotides for sequence-specific inhibition.

For gene validation and the development of oligonucleotide agents, 2'-O,4'-C-ethylene-bridged nucleic acid (ENA) antisense gapmers are widely available. An in vitro Escherichia coli RNase H reaction analysis using ENA gapmers and an RNA oligonucleotide with mouse peptidylarginine deiminase 4 (PADI4) gene sequences revealed that the RNA oligonucleotide was specifically cleaved in the only reported case of the use of an ENA gapmer with an antisense sequence. On the other hand, duplexes of the full-length transcripts of PADI4 mRNA and ENA gapmers with a wide DNA window were cleaved not only at the target site, but also at nontarget sites by RNase H derived from partial base-pairing between the transcript and the ENA gapmer. When the DNA window region of the ENA gapmer was shortened to 5 or 6 nucleotides, the nontarget cleavage was effectively diminished. Moreover, the specific inhibition of PADI4 mRNA expression was observed in the cotransfection of PADI4 cDNA and ENA gapmers containing a short DNA region into NIH3T3 cells. These results demonstrated that ENA gapmers with a short DNA region improved the sequence-specificity of mRNA downregulation. These optimized ENA gapmers could reduce the "off-target" effect and be applicable to gene validation and oligonucleotide therapeutics.

Design of ENA gapmers as fine-tuning antisense oligonucleotides with sequence-specific inhibitory activity on mouse PADI4 mRNA expression.

In order to evaluate the function of the mouse peptidylarginine deiminase 4 (PADI4) gene, gapmer-designed ENA antisense oligonucleotides were utilized. Antisense ENA gapmers were found to inhibit the mRNA expression of the PADI4 gene, but ENA gapmers with sense sequences as controls only partially inhibited this mRNA expression. An in vitro E. coli RNase H reaction analysis using transcripts of the PADI4 gene and their ENA gapmers revealed that the mixtures of the transcripts and ENA gapmers with sense sequences were cleaved at non-target sites by RNase H derived from partial base-pairing between the transcript and the ENA gapmer. In an in vitro E. coli RNase H reaction analysis, when the DNA region of the ENA gapmer was shortened to 5 or 6 nucleotides, the non-target cleavage disappeared and the specific inhibition of PADI4 mRNA expression was observed. These results demonstrated that ENA gapmers with a short DNA region improved the sequence-specificity of mRNA down-regulation. As well, they suggest that we should be alert to the utility of antisense oligonucleotides with a wide DNA region in terms of sequence specificity, and additionally, that optimized ENA gapmers could be useful for application to gene validation.

Direct comparison of in vivo antisense activity of ENA oligonucleotides targeting PTP1B mRNA with that of 2'-O-(2-methoxy)ethyl-modified oligonucleotides.

Protein-tyrosine phosphatase 1B (PTP1B) inhibitory activity of the 2'-O-(2-methoxy)ethyl (2'- MOE)-modified gapmer antisense oligonucleotide, ISIS113715, was previously reported. This antisense oligonucleotide increases insulin sensitivity and normalizes plasma glucose levels in diabetic ob/ob and db/db mice. In the present study, the isosequential 2'-O,4'-C-ethylene-bridged nucleic acid (ENA)-modified oligonucleotide, ENA-1, was synthesized, and its ability to further improve the downregulation of PTP1B in db/db mice was examined. We demonstrated that, compared with ISIS113715, intraperitoneal and subcutaneous administration of ENA-1 more effectively decreased the plasma glucose levels in db/db mice. Moreover, ENA-1 decreased expression of PTP1B in the liver and fat of db/db mice more effectively than ISIS113715. We describe for the first time the functional comparison of 2'-MOE- and ENA-modified antisense oligonucleotides. Our data indicate that the enhancement of the efficacy of antisense oligonucleotides by ENA modifications is superior to that of second-generation 2'-MOE modifications in certain aspects.