Treatments for

1. A method for treating an infection by a virus which is a member of the Flaviviridae family of viruses, in a mammalian host, comprising administering to said host a therapeutically effective amount of a compound represented by structural formula IV

or a pharmaceutically acceptable salt thereof or a pharmaceutical composition thereof which composition comprises a pharmaceutically acceptable carrier; wherein R 3 and R 4 are independently —OH or a pharmaceutically acceptable leaving group, wherein R 5 is —OH, a straight or branched chain C 9 to C 24 alkylphosphate or a straight or branched chain C 9 to C 24 alkenylphosphate group or a pharmaceutically acceptable leaving group and wherein R 1 and R 2 are independently C 1 to C 10 alkyl or wherein R 1 and R 2 taken together with N form a C 3 to C 7 ring represented by the following structural formula:

wherein n and m are independently 0, 1, 2 or 3 and Q is CH 2 , NR, O, S, SO or SO 2 ; and R is independently H, C 1 to C 6 alkyl or C 1 to C 6 acyl or wherein R 1 and R 2 , taken together with the N, are represented by the structural formula:

and wherein said pharmaceutically acceptable leaving groups groups are capable of being converted to —OH, -phosphate, —F or —CH 3 when the compound of structural formula IV is administered in vivo and are independently represented by structural formula

wherein Y═H, CH 3 , CH 3 CH 2 —, CH 3 CH 2 CH 2 —, Me 2 CH—, Me 2 CH 2 CH 2 —, CH 3 CH 2 CH(Me)—, PhCH 2 —, HOOCCH 2 CH 2 —, HSCH 2 —, HOOCCH 2 —, MeSCH 2 CH 2 —, HOCH 2 —,

H 2 N(CH 2 ) 4 —, or CH 3 CH(OH)—, or a pharmaceutically acceptable salt thereof,

or Y, taken together with the alpha-carbon and N, form

wherein the pharmaceutically acceptable leaving groups are capable of being converted to —OH, -phosphate, —F or —CH 3 when the compound of structural formula IV is administered in vivo and are independently represented by a structural formula selected from the group consisting of:

2. The method claim 1 wherein the virus is hepatitis C virus.

3. The method of claim 1 , wherein the compound represented by formula IV is administered in association with interferon-alfa, pegylated interferon-alfa or albumin-interferon-alpha.

4. The method of claim 1 wherein the compound represented by formula IV is administered in association with an interferon-alfa selected from the group consisting of interferon alfa-2a, interferon alfa-2b, interferon alfa-2c, interferon alfa n-1, interferon alfa n-3 and consensus interferon.

5. The method of claim 1 wherein the compound represented by formula IV is administered in association with a pegylated interferon-alfa selected from the group consisting of pegylated interferon alfa-2a, pegylated interferon alfa-2b, pegylated interferon alfa-2c, pegylated interferon alfa n-1, pegylated interferon alfa n-3, and pegylated consensus interferon.

6. The method of claim 1 , wherein the compound represented by formula IV is administered in association with ribavirin.

7. The method of claim 1 wherein R 5 ═

or a corresponding ion thereof.

8. The method of claim 1 wherein R 3 ═R 4 ═R 5 ═—OH.

9. The method of claim 1 , wherein R 1 and R 2 are C 1 -C 5 alkyl.

10. The method of claim 9 , wherein R 1 and R 2 are isopropyl.

11. The method of claim 1 , wherein R 1 and R 2 taken together with N are represented by the structural formula:

wherein n and m are independently 0, 1, 2 or 3 and Q is CH 2 , NR, O, S, SO or SO 2 ; and R is independently H, C 1 to C 6 alkyl or C 1 to C 6 acyl.

12. The method of claim 1 , wherein R 1 and R 2 taken together with N are represented by the following structural formula:

13. The method of claim 1 wherein the host is administered the compound represented by structural formula IV following transplantation of a liver into said host or transfusion of blood into said host.

14. A method for treating an infection by a virus which is a member of the Flaviviridae family of viruses, in a mammalian host, comprising administering to said host a therapeutically effective amount of a compound represented by a structural formula selected from the group consisting of:

15. A composition represented by structural formula IV

or a pharmaceutically acceptable salt thereof or a pharmaceutical composition thereof which composition comprises a pharmaceutically acceptable carrier; wherein R 3 and R 4 are independently —OH or a pharmaceutically acceptable leaving group, wherein R 5 is a straight or branched chain C 9 to C 24 alkylphosphate or a straight or branched chain C 9 to C 24 alkenylphosphate group and wherein R 1 and R 2 are independently C 1 to C 10 alkyl or wherein R 1 and R 2 taken together with N form a C 3 to C 7 ring represented by the following structural formula:

wherein n and m are independently 0, 1, 2 or 3 and Q is CH 2 , NR, O, S, SO or SO 2 ; and R is independently H, C 1 to C 6 alkyl or C 1 to C 6 acyl or wherein R 1 and R 2 , taken together with the N, are represented by the structural formula:

and wherein said pharmaceutically acceptable leaving groups are capable of being converted to —OH,-phosphate, —F or —CH 3 when the compound of structural formula IV is administered in vivo and are independently represented by the structural formula

wherein Y═H, CH 3 , CH 3 CH 2 —, CH 3 CH 2 CH 2 —, Me 2 CH—, Me 2 CH 2 CH 2 —, CH 3 CH 2 CH(Me)—, PhCH 2 —, HOOCCH 2 CH 2 —, HSCH 2 —, HOOCCH 2 —, MeSCH 2 CH 2 —, HOCH 2 —

H 2 N(CH 2 ) 4 —, or CH 3 CH(OH)—, or a pharmaceutically acceptable salt thereof,

or Y, taken together with the alpha-carbon and N, form

or wherein said pharmaceutically acceptable leaving groups are capable of being converted to —OH, -phosphate, —F or —CH 3 when the compound of structural formula IV is administered in vivo and are independently represented by a structural formula selected from the group consisting of:

16. A composition which is represented by a structural formula selected from the group consisting of:

17. The method of claim 1 wherein the subject is human.

18. The method of claim 14 wherein the subject is human.

19. The method of claim 14 wherein the compound is administered in association with interferon alfa-2b.

20. The method of claim 14 wherein the compound is administered in association with interferon alfa-2a.

21. The method of claim 14 wherein the compound is administered in association with pegylated interferon alfa-2b.

22. The method of claim 14 wherein the compound is administered in association with pegylated interferon alfa-2a.

23. The method of claim 14 wherein the compound is administered in association with ribavirin.

24. The method of claim 14 wherein the compound is administered in association with ribavirin and interferon alfa-2b.

25. The method of claim 17 wherein the virus is hepatitis C virus.

26. The method of claim 18 wherein the virus is hepatitis C virus.