In search of new inhibitors of HIV-1 replication: synthesis and study of 1-(2'-Deoxy-beta-D-Ribofuranosyl)-1,2,4-triazole-3-carboxamide as a selective viral mutagenic agent.

With the emergence of HIV strains resistant or cross-resistant to nearly all antiretroviral regimen, novel therapy approaches have to be considered. As a part of our current work on viral mutagenic compounds, we prepared 1-(2' -deoxy-beta-D-ribofuranosyl)-1,2,4-triazole-3-carboxamide (2' -deoxy-ribavirin) and its 5' -triphosphate derivative. The nucleoside mutagenic activity was evaluated on HIV-1 NL4-3 in CEMx174 cell culture. After 2.5 months, no reduction on HIV-1 viability was observed. On the other hand, in vitro experiments with purified HIV-1 RT demonstrated that the triphosphate analog can be incorporated opposite to several natural nucleosides.

Nucleoside and nucleotide inhibitors of HIV-1 replication.

HIV-1 reverse transcriptase (RT) is one of the main targets for antiviral therapy. Two classes of RT inhibitors can be distinguished: those that are nucleoside or nucleotide analogues (sharing the common NRTIs abbreviation) and those that are not. This review focuses on the NRTIs, which are highly efficient in slowing down viral replication and are used in combination regimens. Unfortunately, the current inhibitors do not completely suppress viral replication and due to the high capacity of adaptation of HIV, allow the selection of drug-resistant viruses. Resistance mechanisms to NRTIs have been extensively investigated and can be divided into two types: improved discrimination of a nucleotide analogue relative to the natural substrate or increased phosphorolytic cleavage of an analogue-blocked primer. This knowledge is important both for the development of new drugs designed to target resistant strains and for the development of new antiviral strategies. The NRTIs currently in clinical trials and new developments in this area are also reviewed.

Low enantioselectivities of human deoxycytidine kinase and human deoxyguanosine kinase with respect to 2'-deoxyadenosine, 2'-deoxyguanosine and their analogs.

The antiviral activity of L-nucleoside analogs depends in part on the enantioselectivity of nucleoside kinases which catalyse their monophosphorylation. The substrate properties of human recombinant deoxycytidine kinase (dCK) and human recombinant deoxyguanosine kinase (dGK) with respect to L-adenosine and L-guanosine analogs, in the presence of saturating amounts of ATP and relatively high concentrations of substrates, demonstrated a marked lack of enantioselectivity of both these enzymes. Human dCK catalysed the phosphorylation of D- and L-enantiomers of beta-dA, beta-araA, and beta-dG with enantioselectivities favoring the unnatural enantiomer for the adenosine derivatives and the natural enantiomer for 2'-deoxyguanosine. No other tested L-adenosine or L-guanosine analog was a substrate of dCK. Similarly, D- and L-enantiomers of beta-dA, beta-araA, and beta-dG were substrates of human dGK but with different enantioselectivities compared to dCK, especially concerning beta-dA. The present results indicate that human dCK and dGK have similar properties including substrate properties, relaxed enantioselectivities, and possibly catalytic cycles.

Enzymatic synthesis of unlabeled and beta-(32)P-labeled beta-L-2', 3'-dideoxyadenosine-5'-triphosphate as a potent inhibitor of adenylyl cyclases and its use as reversible binding ligand.

beta-L-2',3'-Dideoxyadenosine-5'-triphosphate (beta-L-2', 3'-dd-5'-ATP) was prepared enzymatically from the corresponding monophosphate by the use of adenylate kinase, creatine phosphate, and creatine kinase in a single step. The beta-(32)P-labeled analog was prepared similarly, but in a two step reaction. beta-L-2', 3'-dd-5'-ATP inhibited adenylyl cyclase from rat brain competitively with respect to substrate (5'-ATP.Mn(2+)) and exhibited an IC(50) approximately 24 nM. The labeled ligand was used in the development of a reversible binding assay for adenylyl cyclases. Binding of beta-L-2',3'-dd-[beta-(32)P]5'-ATP was saturable with increasing concentrations of ligand and increased in proportion to membrane protein, and was enhanced by Mn(2+) to a greater extent than by Mg(2+). Binding was displaced with adenine nucleotides known to be either competitive or noncompetitive inhibitors but not by agents known not to act on the cyclase, or by 3-isobutyl-1-methylxanthine, creatine phosphate, or creatine kinase. Binding was rapid, with a half-time for the on-rate <1.8 min and for the off-rate <0.8 min. The potency and mechanism of the inhibition of this ligand and the pattern of agents that displace binding suggest an interaction with adenylyl cyclase per se and to a configuration of the enzyme consistent with an interaction at the catalytic active site. The data suggest that this is a pretransition state inhibitor and contrasts with the equipotent 2',5'-dd-3'ATP, a post-transition state noncompetitive inhibitor.

Interaction of beta-L-adenosine-5'-triphosphate (L-ATP) with human deoxycytidine kinase, human DNA primase and T4 DNA ligase: does the chance direct enzymatic enantioselectivity?

We demonstrate that L-ATP: 1) as well as its natural D-enantiomer, acts as a phosphate donor in the reaction catalysed by human deoxycytidine kinase; 2) inhibits human DNA-primase and the ATP-dependent T4 DNA ligase. Thus, the lack of enantioselectivity of the enzymes is more frequent than it was believed a few years ago and we suggest that it would depend on chance more than on an evolutionary strategy.

Base pairing of anhydrohexitol nucleosides with 2,6-diaminopurine, 5-methylcytosine and uracil asbase moiety.

Hexitol nucleic acids (HNAs) with modified bases (5-methylcytosine, 2,6-diaminopurine or uracil) were synthesized. The introduction of the 5-methylcytosine base demonstrates that N -benzoylated 5-methylcytosyl-hexitol occurs as the imino tautomer. The base pairing systems (G:CMe, U:D, T:D and U:A) obey Watson-Crick rules. Substituting hT for hU, hCMefor hC and hD for hA generally leads to increased duplex stability. In a single case, replacement of hC by hCMedid not result in duplex stabilization. This sequence-specific effect could be explained by the geometry of the model duplex used for carrying out the thermal stability study. Generally, polypurine HNA sequences give more stable duplexes with their RNA complement than polypyrimidine HNA sequences. This observation supports the hypothesis that, besides changes in stacking pattern, the difference in conformational stress between purine and pyrimidine nucleosides may contribute to duplex stability. Introduction of hCMeand hD in HNA sequences further increases the potential of HNA to function as a steric blocking agent.

L-ATP is recognized by some cellular and viral enzymes: does chance drive enzymic enantioselectivity?

We demonstrate that l-ATP is recognized by some enzymes that are involved in the synthesis of nucleotides and nucleic acids. l-ATP, as well as its natural d-enantiomer, acts as a phosphate donor in the reaction catalysed by human deoxycytidine kinase, whereas it is not recognized by either enantioselective human thymidine kinase or non-enantioselective herpes virus thymidine kinase. l-ATP strongly inhibits (Ki 80 microM) the synthesis of RNA primers catalysed by DNA primase associated with human DNA polymerase alpha, whereas RNA synthesis catalysed by Escherichia coli RNA polymerase is completely unaffected. Moreover, l-ATP competitively inhibits ATP-dependent T4 DNA ligase (Ki 25 microM), suggesting that it interacts with the ATP-binding site of the enzyme. Kinetic studies demonstrated that l-ATP cannot be used as a cofactor in the ligase-catalysed joining reaction. On the other hand, l-AMP is used by T4 DNA ligase to catalyse the reverse reaction, even though a high level of intermediate circular nicked DNA molecules accumulates. Our results suggest that a lack of enantioselectivity of enzymes is more common than was believed a few years ago, and, given the absence of selective constraints against l-nucleosides in Nature, this may depend on chance more than on evolutionary strategy.

Synthesis and biological evaluation of 9-(beta-L-arabinofuranosyl)adenine.

For the first time, the stereospecific synthesis of 9-(beta-L-arabinofuranosyl) adenine was carried out. Unfortunately, and unlike its "natural" D-counterpart Vidarabine, this L-enantiomer did not show significant activity when evaluated against a broad range of viruses.