Carba-LNA-5MeC/A/G/T Modified Oligos Show Nucleobase-Specific Modulation of 3′-Exonuclease Activity, Thermodynamic Stability, RNA Selectivity, and RNase H Elicitation : Synthesis and Biochemistry

Using the intramolecular 5-exo-5-hexenyl radical as a key cyclization step, we previously reported an unambiguous synthesis of carba-LNA thymine (cLNA-T), which we subsequently incorporated in antisense oligonudeotides (AON) and investigated their biochemical properties [J. Am. Chem. Soc. 2007, 129 (26), 8362-8379]. These cLNA-T incorporated oligos showed specific RNA affinity of +3.5-5 degrees C/modification for AON:RNA heteroduplexes, which is comparable to what is found for those of LNAs (Locked Nucleic Acids). These modified oligos however showed significantly enhanced nuclease stability (ca. 100 times more) in the blood serum compared to those of the LNA modified counterparts without compromising any RNase H recruitment capability. We herein report the synthesis of 5-methylcytosine-1-yl (C-Me), 9-adeninyl (A), and 9-guaninyl (G) derivatives of cLNA and their oligonucleotides and report their biochemical properties as potential RNA-directed inhibitors. In a series of isosequential carba-LNA modified AONs, we herein show that all the cLNA modified AONs are found to be RNA-selective, but the magnitude of RNA-selectivity of 7'-R-Me-cLNA-G (cLNA-G) (Delta T-m = 2.9 degrees C/modification) and intractable isomeric mixtures of 7'-(S/R)-Me-cLNA-T (cLNA-T, Delta T-m = 2.2 degrees C/modification) was found to be better than diastereomeric mixtures of 7'-(S/R)-Me-cLNA-C-Me with trace of cENA-C-Me (cLNA-C-Me, Delta T-m = 1.8 degrees C/modification) and 7'-R-Me-cLNA-A (cLNA-A, Delta T-m = 0.9 degrees C/modification). cLNA-C-Me modified AONs however exhibited the best nuclease stability, which is 4-, 7-, and 20-fold better, respectively, than cLNA-T, cLNA-A, and cLNA-G modified counterparts, which in turn was more than 100 times stable than that of the native. When the modification sites are appropriately chosen in the AONs, the cLNA-A, -G, and -C-Me modified sites in the AON:RNA hybrids can be easily recognized by RNase H, and the RNA strand of the hybrid is degraded in a s