Purine 8-substitution modulates the ribonuclease L binding and activation abilities of 2',5'-oligoadenylates

8-Oxo-7,8-dihydroadenine and 8-Oxo-7,8-dihydroadenosine-Chemistry, Structure, and Function in RNA and Their Presence in Natural Products and Potential Drug Derivatives

A description and history of the role that 8-oxo-7,8-dihydroadenine (8-oxoAde) and 8-oxo-7,8-dihydroadenosine (8-oxoA) have in various fields has been compiled. This Review focusses on 1) the formation of this oxidatively generated modification in RNA, its interactions with other biopolymers, and its potential role in the development/progression of disease; 2) the independent synthesis and incorporation of this modified nucleoside into oligonucleotides of RNA to display the progress that has been made in establishing its behavior in biologically relevant systems; 3) reported synthetic routes, which date back to 1890, along with the progress that has been made in the total synthesis of the nucleobase, nucleoside, and their corresponding derivatives; and 4) the isolation, total synthesis, and biological activity of natural products containing these moieties as the backbone. The current state of research regarding this oxidatively generated lesion as well as its importance in the context of RNA, natural products, and potential as drug derivatives is illustrated using all available examples reported to date.

Inhibition of HIV-1 replication and activation of RNase L by phosphorothioate/phosphodiester 2',5'-oligoadenylate derivatives

2',5'-Oligoadenylate (2-5A) derivatives have been designed to act distal to the human immunodeficiency virus-1 (HIV-1)-induced blockade in the 2-5A synthetase/RNase L antiviral pathway. Stereochemical modification of individual internucleotide linkages of the 2-5A molecule was accomplished by phosphoramidite and phosphotriester chemical syntheses. Phosphorothioate/phosphodiester trimer and tetramer 2-5A derivatives revealed differences in the stereodynamics of activation of RNase L and inhibition of HIV-1 replication. The first and second internucleotide linkages are critical for activation of recombinant, human RNase L; A(Rp)ApA, A(Sp)ApA and ApA(Rp)A are agonists (IC50 = 2 x 10(-7), 2 x 10(-6) and 8 x 10(-6) M); ApA(Sp)A is an antagonist. The second and third internucleotide linkages are crucial for activation of murine RNase L; ApA(Rp)A, ApA(Rp)ApA, and ApApA(Rp)A are agonists (IC50 = 5 x 10(-7) M); ApA(Sp)A, ApA(Sp)ApA, and ApApA(Sp)A are antagonists. Inhibition of HIV-1-induced syncytia formation by the phosphorothioate/phosphodiester derivatives is specific for derivatives with substitution at the 2',3'-terminus. ApA(Rp)A, ApA(Sp)A, ApApA(Rp)A, and ApApA(Sp)A are potent inhibitors of HIV-1-induced syncytia formation (80-, 10-, 40-, and 15-fold more inhibitory, respectively, than solvent control). HIV-1 infection results in enhanced uptake and accumulation of ApA(Rp)A and ApA(Sp)A (7- and 10-fold, respectively). These stereochemically modified 2-5A derivatives are taken up preferentially by HIV-1-infected cells and show promise in anti-HIV-1 chemotherapy.

The Relationship between Conformation and Biological Activity of 8-substituted Analogues of 2′,5′-Oligoadenylates

Analogues of the 2′,5′-linked adenylate trimer 5′-monophosphates, p5′A2′p5′A2′p5′A (pA3) (1a), containing 8-hydroxyadenosine and 8-mercaptoadenosine in the first, second, and third nucleotide positions were tested for their ability to bind to and activate RNase L of mouse L cells. The oligomer, p5′ASH2′p5′ASH2′p5′ASH(pASH3) (1c) had little capacity to bind to RNase L. On the other hand, an analogue of the p5′AOH2′p5′AOH2′p5′AOH(pAOH3) (1b) bound almost as well as the parent 2-5A [pppA(2′p5′A)2] (P3A3) (1d) to RNase L. The 8-substituted analogues of 2-5A were more resistant to degradation by (2′,5′) phosphodiesterase. Finally, the monophosphate, pASH3(1c) which possessed higher anti-HIV activity than pAg (1a) or pAOH3(1b).

Characterization and biological activity of 8-substituted analogues of 2',5'-oligoadenylates

Analogues of the 2',5'-linked adenylate trimer 5'-monophosphates (p5'A2'p5'A2'p5'A) containing 8-hydroxyadenosine and 8-mercaptoadenosine in the first, second, and third nucleotide positions were tested for their ability to bind to and activate RNase L of mouse L cells. The ability of p5'ASH2'p5'ASH2'p5'ASH (pASH3) (1c) to bind 2-5A dependent endonuclease was markedly decreased. On the other hand, an analogue of the 2',5'-linked adenylate trimer monophosphate substituted by 8-hydroxyadenosine in the first, second, and third nucleotide positions bound almost as well as parent 2-5A [pppA(2'p5'A)2] (p3A3) (1d) to RNase L. The 8-substituted analogues of 2-5A were more resistant to the degradation by the (2',5') phosphodiesterase. Of particular interest is monophosphate, pASH3 (1c) which possessed higher anti-HIV activity than pA3 (1a) or pAOH3 (1b).

A new and potent 2-5A analogue which does not require a 5'-polyphosphate to activate mouse L-cell RNase L

In order to explore the possibility of supplanting the requirement of a 5'-triphosphate moiety for the activation of the 2-5A-dependent endonuclease (RNase L) of mouse L-cells, two new tetrameric analogues of 2-5A were synthesized. The first tetramer, obtained by both a modified prebiotic synthetic approach as well as a phosphite triester solid phase oligonucleotide synthesis method, was p5'A2'p5'A2'p5'(br8A)2'p5'(br8A). The second oligonucleotide was derived from the former by a sequence involving periodate oxidation, reaction with n-hexylamine, and cyanoborohydride reduction, resulting in conversion of the 2'-terminal adenosine residue to 9-(3'-aza-4'-hexyl-1',2',3',4'-tetradeoxyhexopyranos-1(1)-yl)-8-++ +bromoadenine. Both of these oligomers, bearing only 5'-monophosphate groups, were found to be as potent as 2-5A itself as activators of the RNase L of mouse L-cells.