Modified oligonucleotides with increased stability

1. A branched compound comprising two or more oligonucleotides, wherein:

(a) the oligonucleotides are connected to one another by one or more moieties selected from the group consisting of a linker, a spacer and a branching point, and

(b) at least one oligonucleotide is a modified oligonucleotide comprising at least one modified intersubunit linkage of Formula (I):

wherein:

each B is, independently, a base pairing moiety;

W is selected from the group consisting of O, OCH 2 , OCH, CH 2 , and CH;

each X is, independently, selected from the group consisting of halo, hydroxy, and C 1-6 alkoxy;

Y is selected from the group consisting of O − , OH, OR, NH − , NH 2 , S − , and SH;

Z is selected from the group consisting of O and CH 2 ;

when Y is O − or S − , either Z or W is not O;

R is a protecting group; and

is an optional double bond.

2. The branched compound of claim 1 , wherein each oligonucleotide is double-stranded and comprises a sense strand and an antisense strand, wherein the sense strand and the antisense strand each have a 5′ end and a 3′ end.

3. The branched compound of claim 1 , wherein:

Z is CH 2 and W is CH 2 .

4. The branched compound of claim 1 , wherein the base pairing moiety B is selected from the group consisting of adenine, guanine, cytosine, and uracil.

5. The branched compound of claim 1 , wherein the modified oligonucleotide is incorporated into a modified siRNA, said modified siRNA having a 5′ end, a 3′ end and complementarity to a target, wherein the siRNA comprises a sense and antisense strand, and at least one modified intersubunit linkage of Formula (I).

6. The branched compound of claim 1 , wherein R is a protecting group selected from the group consisting of dimethoxytrityl (DMTr), succinate, tert-butyl dimethylsilyl (TBDMS), benzoyl (Bz), benzyl (Bn), methoxyethoxymethyl ether (MOM), methoxybenzyl ether (PMB), methylthiomethyl ether, pivaloyl (Piv), tetrahydropyranyl (THP), tetrahydrofuranyl (THF), trityl (Trt), triisopropylsilyl (TIPS), tert-butyldiphenylsilyl (TBDPS), and acetate; and

wherein is an optional double bond.

7. A branched compound comprising two or more oligonucleotides, wherein:

(a) the oligonucleotides are connected to one another by one or more moieties selected from a linker, a spacer and a branching point, and

(b) at least one oligonucleotide is a modified oligonucleotide comprising a modified intersubunit linkage represented by Formula (I):

wherein:

each B is, independently, a base pairing moiety;

W is selected from the group consisting of —O—CH 2 — and —O—CH═;

each X is, independently, selected from the group consisting of halo, hydroxy, and C 1-6 alkoxy;

Y is selected from the group consisting of O − , OH, OR, NH − , NH 2 , S − , and SH;

Z is selected from the group consisting of —O— and —CH 2 —;

R is a protecting group; and

is an optional double bond.

8. The branched compound of claim 7 , wherein the modified intersubunit linkage is represented by Formula VI:

wherein:

each B is, independently, a base pairing moiety;

each X is, independently, selected from the group consisting of fluoro, hydroxy, and C 1-6 alkoxy;

Y is selected from the group consisting of O − , OH, OR, S − , and SH;

Z is selected from the group consisting of —O— and —CH 2 —;

R is a protecting group; and

is an optional double bond.

9. The branched compound of claim 7 , comprising two oligonucleotides connected to one another by way of a linker.

10. The branched compound of claim 7 , wherein each oligonucleotide is double-stranded and comprises an antisense strand having complementarity to a target mRNA molecule and a sense strand having complementarity to the antisense strand, and wherein each of the antisense strand and the sense strand is, independently, from 10 to 50 nucleotides in length.

11. The branched compound of claim 1 wherein is a single bond.

12. The branched compound of claim 1 wherein is a double bond.

13. The branched compound of claim 1 , wherein the branched compound comprises 2 oligonucleotides.

14. The branched compound of claim 1 wherein the branched compound comprises 4 oligonucleotides.

15. The branched compound of claim 1 wherein the branched compound comprises 6 oligonucleotides.

16. The branched compound of claim 1 wherein the branched compound comprises 8 oligonucleotides.

17. The branched compound of claim 2 , wherein each double-stranded oligonucleotide is independently connected to a linker, spacer or branching point at the 3′ end or at the 5′ end of the sense strand or the antisense strand.

18. The branched compound of claim 17 , wherein each linker is independently selected from the group consisting of an ethylene glycol chain, an alkyl chain, a peptide, an RNA, a DNA, a phosphate, a phosphonate, a phosphoramidate, an ester, an amide, a triazole, and combinations thereof.

19. The branched compound of claim 17 , wherein any carbon or oxygen atom of the linker is optionally replaced with a nitrogen atom.

20. The branched compound of claim 17 , wherein any carbon or oxygen atom of the linker bears a hydroxyl substituent.

21. The branched compound of claim 17 , wherein any carbon or oxygen atom of the linker bears an oxo substituent.

22. The branched compound of claim 2 , wherein each antisense strand independently comprises at least 16 contiguous nucleotide and has complementarity to a target.

23. The branched compound of claim 2 , wherein each antisense strand independently comprises at least 17 contiguous nucleotides and has complementarity to a target.

24. The branched compound of claim 2 , wherein each antisense strand independently comprises at least 18 contiguous nucleotides and has complementarity to a target.

25. The branched compound of claim 2 , wherein each antisense strand independently comprises at at least 19 contiguous nucleotides and has complementarity to a target.

26. The branched compound of claim 2 , wherein each antisense strand independently comprises at least 20 contiguous nucleotides and has complementarity to a target.

27. The branched compound of claim 3 , wherein the modified intersubunit linkage of Formula I is a modified intersubunit linkage of Formula II:

28. The branched compound of claim 1 , wherein Z is CH 2 and W is O.

29. The branched compound of claim 28 , wherein the modified intersubunit linkage of Formula I is a modified intersubunit linkage of Formula III:

30. The branched compound of claim 1 , wherein Z is O and W is CH 2 .

31. The branched compound of claim 30 , wherein the modified intersubunit linkage of Formula I is a modified intersubunit linkage of Formula IV:

32. The branched compound of claim 1 , wherein Z is O and W is CH.

33. The branched compound of claim 32 , wherein the modified intersubunit linkage of Formula I is a modified intersubunit linkage of Formula V:

34. The branched compound of claim 1 , wherein Z is O and W is OCH 2 .

35. The branched compound of claim 1 , wherein the modified intersubunit linkage of Formula I is a modified intersubunit linkage of Formula VI:

wherein:

each X is independently, selected from the group consisting of fluoro, hydroxy, and Ci-6 alkoxy;

Y is selected from the group consisting of O—, OH, and OR;

Z is selected from the group consisting of O and CH 2 and W is OCH 2 , and

is an optional double bond.

36. The branched compound of claim 35 , wherein the modified intersubunit linkage of Formula I is a modified intersubunit linkage of Formula VIa:

37. The branched compound of claim 1 , wherein Z is CH 2 and W is CH.

38. The branched compound of claim 37 , wherein the modified intersubunit linkage of Formula I is a modified intersubunit linkage of Formula VII:

39. The branched compound of claim 6 , wherein is a single bond.

40. The branched compound of claim 6 , wherein is a double bond.

41. The branched compound of claim 7 , wherein is a single bond.

42. The branched compound of claim 7 , wherein is a double bond.

43. The branched compound of claim 8 , wherein is a single bond.

44. The branched compound of claim 8 , wherein is a double bond.