Compositions and methods of treating muscle atrophy and myotonic dystrophy

1. A method of treating muscle atrophy or myotonic dystrophy in a subject in need thereof, comprising administering to the subject an siRNA molecule conjugate comprising an anti-transferrin receptor binding moiety conjugated to an siRNA molecule that hybridizes to a target sequence of a human DMPK mRNA and mediates RNA interference against the human DMPK mRNA preferentially in muscle cells in the subject, wherein the target sequence of the human DMPK mRNA comprises exons 1-13 of the human DMPK mRNA excluding CUG repeats, thereby treating muscle atrophy or myotonic dystrophy in the subject.

2. The method of claim 1 , wherein the siRNA molecule comprises at least one 2′ modified nucleotide, at least one modified internucleotide linkage, or at least one inverted abasic moiety.

3. The method of claim 1 , wherein the anti-transferrin receptor binding moiety binds to a transferrin receptor on a cell surface of a muscle cell.

4. The method of claim 1 , wherein the siRNA molecule comprises a sense strand and an antisense strand, and wherein the sense strand and the antisense strand each independently comprises at least one 2′ modified nucleotide, at least one modified internucleotide linkage, or at least one inverted abasic moiety.

5. The method of claim 1 , wherein the siRNA molecule hybridizes to at least 8 contiguous bases of the target sequence of the human DMPK mRNA.

6. The method of claim 1 , wherein the siRNA molecule is from about 8 to about 50 nucleotides in length or from about 10 to about 30 nucleotides in length.

7. The method of claim 1 , wherein the siRNA molecule conjugate comprises a linker connecting the anti-transferrin receptor binding moiety to the siRNA molecule.

8. The method of claim 2 , wherein the at least one 2′ modified nucleotide: comprises 2′-O-methyl, 2′-O-methoxyethyl (2′-O-MOE), 2′-O-aminopropyl, 2′-deoxy, 2′-deoxy-2′-fluoro, 2′-O-aminopropyl (2′-O-AP), 2′-O-dimethylaminoethyl (2′-O-DMAOE), 2′-O-dimethylaminopropyl (2′-O-DMAP), 2′-O-dimethylaminoethyloxyethyl (2′-O-DMAEOE), or 2′-O—N-methylacetamido (2′-O-NMA) modified nucleotide; comprises locked nucleic acid (LNA) or ethylene nucleic acid (ENA); or comprises a combination thereof.

9. The method of claim 2 , wherein the at least one modified internucleotide linkage comprises a phosphorothioate linkage or a phosphorodithioate linkage.

10. The method of claim 2 , wherein the siRNA molecule comprises three or more 2′ modified nucleotides selected from 2′-O-methyl modified nucleotide and 2′-deoxy-2′-fluoro modified nucleotide.

11. The method of claim 1 , wherein the siRNA molecule conjugate has a drug to anti-transferrin receptor binding moiety ratio of from about 1 to about 4.

12. The method of claim 1 , wherein the siRNA molecule comprises a 5′-terminal vinylphosphonate modified nucleotide.

13. The method of claim 1 , wherein the muscle atrophy is associated with myotonic dystrophy type 1 (DM1).

14. The method of claim 1 , wherein the myotonic dystrophy is DM1.

15. The method of claim 1 , wherein the siRNA molecule conjugate is formulated for parenteral administration.

16. The method of claim 1 , wherein the anti-transferrin binding moiety is an anti-transferrin receptor antibody or antigen binding fragment thereof.

17. The method of claim 16 , wherein the anti-transferrin receptor antibody comprises two light chain variable domains and two heavy chain variable domains.