Comparison of the RNase H cleavage kinetics and blood serum stability of the north-conformationally constrained and 2'-alkoxy modified oligonucleotides.

The RNase H cleavage potential of the RNA strand basepaired with the complementary antisense oligonucleotides (AONs) containing North-East conformationally constrained 1',2'-methylene-bridged (azetidine-T and oxetane-T) nucleosides, North-constrained 2',4'-ethylene-bridged (aza-ENA-T) nucleoside, and 2'-alkoxy modified nucleosides (2'-O-Me-T and 2'-O-MOE-T modifications) have been evaluated and compared under identical conditions. When compared to the native AON, the aza-ENA-T modified AON/RNA hybrid duplexes showed an increase of melting temperature (DeltaTm = 2.5-4 degrees C per modification), depending on the positions of the modified residues. The azetidine-T modified AONs showed a drop of 4-5.5 degrees C per modification with respect to the native AON/RNA hybrid, whereas the isosequential oxetane-T modified counterpart, showed a drop of approximately 5-6 degrees C per modification. The 2'-O-Me-T and 2'-O-MOE-T modifications, on the other hand, showed an increased of Tm by 0.5 C per modification in their AON/RNA hybrids. All of the partially modified AON/RNA hybrid duplexes were found to be good substrates for the RNase H mediated cleavage. The Km and Vmax values obtained from the RNA concentration-dependent kinetics of RNase H promoted cleavage reaction for all AON/RNA duplexes with identical modification site were compared with those of the reference native AON/RNA hybrid duplex. The catalytic activities (Kcat) of RNase H were found to be greater (approximately 1.4-2.6-fold) for all modified AON/RNA hybrids compared to those for the native AON/RNA duplex. However, the RNase H binding affinity (1/Km) showed a decrease (approximately 1.7-8.3-fold) for all modified AON/RNA hybrids. This resulted in less effective (approximately 1.1-3.2-fold) enzyme activity (Kcat/Km) for all modified AON/RNA duplexes with respect to the native counterpart. A stretch of five to seven nucleotides in the RNA strand (from the site of modifications in the complementary modified AON strand) was found to be resistant to RNase H digestion (giving a footprint) in the modified AON/RNA duplex. Thus, (i) the AON modification with azetidine-T created a resistant region of five to six nucleotides, (ii) modification with 2'-O-Me-T created a resistant stretch of six nucleotides, (iii) modification with aza-ENA-T created a resistant region of five to seven nucleotide residues, whereas (iv) modification with 2'-O-MOE-T created a resistant stretch of seven nucleotide residues. This shows the variable effect of the microstructure perturbation in the modified AON/RNA heteroduplex depending upon the chemical nature as well as the site of modifications in the AON strand. On the other hand, the enhanced blood serum as well as the 3'-exonuclease stability (using snake venom phosphodiesterase, SVPDE) showed the effect of the tight conformational constraint in the AON with aza-ENA-T modifications in that the 3'-exonuclease preferentially hydrolyzed the 3'-phosphodiester bond one nucleotide away (n + 1) from the modification site (n) compared to all other modified AONs, which were 3'-exonuclease cleaved at the 3'-phosphodiester of the modification site (n). The aza-ENA-T modification in the AONs made the 5'-residual oligonucleotides (including the n + 1 nucleotide) highly resistant in the blood serum (remaining after 48 h) compared to the native AON (fully degraded in 2 h). On the other hand, the 5'-residual oligonucleotides (including the n nucleotide) in azetidine-T, 2'-O-Me-T, and 2'-O-MOE-T modified AONs were more stable compared to that of the native counterpart but more easily degradable than that of aza-ENA-T containing AONs.