Lethal Mutagenesis of Picornaviruses with N-6-Modified Purine Nucleoside Analogues

RNA viruses exhibit extraordinarily high mutation rates during genome replication. Nonnatural ribonucleosides that can increase the mutation rate of RNA viruses by acting as ambiguous substrates during replication have been explored as antiviral agents acting through lethal mutagenesis. We have synthesized novel N-6-substituted purine analogues with ambiguous incorporation characteristics due to tautomerization of the nucleobase. The most potent of these analogues reduced the titer of poliovirus (PV) and coxsackievirus (CVB3) over 1,000-fold during a single passage in HeLa cell culture, with an increase in transition mutation frequency up to 65-fold. Kinetic analysis of incorporation by the PV polymerase indicated that these analogues were templated ambiguously with increased efficiency compared to the known mutagenic nucleoside ribavirin. Notably, these nucleosides were not efficient substrates for cellular ribonucleotide reductase in vitro, suggesting that conversion to the deoxyriboucleoside may be hindered, potentially limiting genetic damage to the host cell. Furthermore, a high-fidelity PV variant (G64S) displayed resistance to the antiviral effect and mutagenic potential of these analogues. These purine nucleoside analogues represent promising lead compounds in the development of clinically useful antiviral therapies based on the strategy of lethal mutagenesis

Synthesis of a 6-Methyl-7-Deaza Analogue of Adenosine that Potently Inhibits Replication of Polio and Dengue Viruses

Bioisosteric deaza analogues of 6-methyl-9-β-D-ribofuranosylpurine, a hydrophobic analogue of adenosine, were synthesized and evaluated for antiviral activity. Whereas the 1-deaza and 3-deaza analogues were essentially inactive in plaque assays of infectivity, a novel 7-deaza-6-methyl-9-β-D-ribofuranosylpurine analogue, structurally related to the natural product tubercidin, potently inhibited replication of poliovirus (PV) in HeLa cells (IC50 = 11 nM) and dengue virus (DENV) in Vero cells (IC50 = 62 nM). Selectivity against PV over cytotoxic effects to HeLa cells was >100-fold after incubation for 7 h. Mechanistic studies of the 5'-triphosphate of 7-deaza-6-methyl-9-β-D-ribofuranosylpurine revealed that this compound is an efficient substrate of PV RNA-dependent RNA polymerase (RdRP) and is incorporated into RNA mimicking both ATP and GTP

Lethal Mutagenesis of Poliovirus Mediated by a Mutagenic Pyrimidine Analogue

Lethal mutagenesis is the mechanism of action of ribavirin against poliovirus (PV) and numerous other RNA viruses. However, there is still considerable debate regarding the mechanism of action of ribavirin against a variety of RNA viruses. Here we show by using T7 RNA polymerase mediated production of PV genomic RNA, PV polymerase-catalyzed primer extension and cell-free PV synthesis that a pyrimidine ribonucleoside triphosphate analogue (rPTP) with ambiguous basepairing capacity is an efficient mutagen of the PV genome. The in vitro incorporation properties of rPTP are superior to ribavirin triphosphate. We observed a log-linear relationship between virus titer reduction and the number of rPMP molecules incorporated. A PV genome encoding a high-fidelity polymerase was more sensitive to rPMP incorporation, consistent with diminished mutational robustness of high-fidelity PV. The nucleoside (rP) did not exhibit antiviral activity in cell culture owing to the inability of rP to be converted to rPMP by cellular nucleotide kinases. rP was also a poor substrate for herpes simplex virus thymidine kinase. The block to nucleoside phosphorylation could be bypassed by treatment with the P nucleobase, which exhibited both antiviral activity and mutagenesis, presumably a reflection of rP nucleotide formation by a nucleotide salvage pathway. These studies provide additional support for lethal mutagenesis as an antiviral strategy, suggest that rPMP prodrugs may be highly efficacious antiviral agents, and provide a new tool to determine the sensitivity of RNA virus genomes to mutagenesis as well as interrogation of the impact of mutational load on the population dynamics of these viruses

Sensitivity of Mitochondrial Transcription and Resistance of RNA Polymerase II Dependent Nuclear Transcription to Antiviral Ribonucleosides

Ribonucleoside analogues have potential utility as anti-viral, -parasitic, -bacterial and -cancer agents. However, their clinical applications have been limited by off target effects. Development of antiviral ribonucleosides for treatment of hepatitis C virus (HCV) infection has been hampered by appearance of toxicity during clinical trials that evaded detection during preclinical studies. It is well established that the human mitochondrial DNA polymerase is an off target for deoxyribonucleoside reverse transcriptase inhibitors. Here we test the hypothesis that triphosphorylated metabolites of therapeutic ribonucleoside analogues are substrates for cellular RNA polymerases. We have used ribonucleoside analogues with activity against HCV as model compounds for therapeutic ribonucleosides. We have included ribonucleoside analogues containing 2′-C-methyl, 4′-methyl and 4′-azido substituents that are non-obligate chain terminators of the HCV RNA polymerase. We show that all of the anti-HCV ribonucleoside analogues are substrates for human mitochondrial RNA polymerase (POLRMT) and eukaryotic core RNA polymerase II (Pol II) in vitro. Unexpectedly, analogues containing 2′-C-methyl, 4′-methyl and 4′-azido substituents were inhibitors of POLRMT and Pol II. Importantly, the proofreading activity of TFIIS was capable of excising these analogues from Pol II transcripts. Evaluation of transcription in cells confirmed sensitivity of POLRMT to antiviral ribonucleosides, while Pol II remained predominantly refractory. We introduce a parameter termed the mitovir (mitochondrial dysfunction caused by antiviral ribonucleoside) score that can be readily obtained during preclinical studies that quantifies the mitochondrial toxicity potential of compounds. We suggest the possibility that patients exhibiting adverse effects during clinical trials may be more susceptible to damage by nucleoside analogs because of defects in mitochondrial or nuclear transcription. The paradigm reported here should facilitate development of ribonucleosides with a lower potential for toxicity

Sensitivity of Mitochondrial Transcription and Resistance of RNA Polymerase II Dependent Nuclear Transcription to Antiviral Ribonucleosides

<div><p>Ribonucleoside analogues have potential utility as anti-viral, -parasitic, -bacterial and -cancer agents. However, their clinical applications have been limited by off target effects. Development of antiviral ribonucleosides for treatment of hepatitis C virus (HCV) infection has been hampered by appearance of toxicity during clinical trials that evaded detection during preclinical studies. It is well established that the human mitochondrial DNA polymerase is an off target for deoxyribonucleoside reverse transcriptase inhibitors. Here we test the hypothesis that triphosphorylated metabolites of therapeutic ribonucleoside analogues are substrates for cellular RNA polymerases. We have used ribonucleoside analogues with activity against HCV as model compounds for therapeutic ribonucleosides. We have included ribonucleoside analogues containing 2′-C-methyl, 4′-methyl and 4′-azido substituents that are non-obligate chain terminators of the HCV RNA polymerase. We show that all of the anti-HCV ribonucleoside analogues are substrates for human mitochondrial RNA polymerase (POLRMT) and eukaryotic core RNA polymerase II (Pol II) in vitro. Unexpectedly, analogues containing 2′-C-methyl, 4′-methyl and 4′-azido substituents were inhibitors of POLRMT and Pol II. Importantly, the proofreading activity of TFIIS was capable of excising these analogues from Pol II transcripts. Evaluation of transcription in cells confirmed sensitivity of POLRMT to antiviral ribonucleosides, while Pol II remained predominantly refractory. We introduce a parameter termed the mitovir (<em><u>mito</u></em>chondrial dysfunction caused by anti<em><u>vir</u></em>al ribonucleoside) score that can be readily obtained during preclinical studies that quantifies the mitochondrial toxicity potential of compounds. We suggest the possibility that patients exhibiting adverse effects during clinical trials may be more susceptible to damage by nucleoside analogs because of defects in mitochondrial or nuclear transcription. The paradigm reported here should facilitate development of ribonucleosides with a lower potential for toxicity.</p> </div

Predicting adverse effects of antiviral ribonucleosides during preclinical development: The mitovir score.

<p>Correlations between (<b>A</b>) cytotoxicity in Huh-7 cells and MT4 cells (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1003030#ppat-1003030-t001" target="_blank"><b>Table 1</b></a><b>, CC₅₀</b>), (<b>B</b>) cytotoxicity in MT4 cells (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1003030#ppat-1003030-t001" target="_blank"><b>Table 1</b></a><b>, CC₅₀</b>) and the efficiency of nucleotide incorporation (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1003030#ppat-1003030-t002" target="_blank"><b>Table 2</b></a><b>, </b><b><i>k</i></b><b>_pol/</b><b><i>K</i></b><b>_d,app</b>), (<b>C</b>) cytotoxicity in MT4 cells (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1003030#ppat-1003030-t001" target="_blank"><b>Table 1</b></a><b>, CC₅₀</b>) and <i>mitovir score</i> for MT4 cells (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1003030#ppat-1003030-t002" target="_blank"><b>Table 2</b></a>), (<b>D</b>) cytotoxicity in MT4 cells (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1003030#ppat-1003030-t001" target="_blank"><b>Table 1</b></a><b>, CC₅₀</b>) and the <i>mitovir score</i> for each analogue corrected to account for the presence of the nucleotide with which the analogue competes, ATP or CTP, and (<b>E</b>) cytotoxicity in Huh-7 cells (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1003030#ppat-1003030-t001" target="_blank"><b>Table 1</b></a><b>, CC₅₀</b>) and <i>mitovir score</i> for Huh-7 cells (<a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1003030#ppat-1003030-t002" target="_blank"><b>Table 2</b></a>). Error bars represent s.d. Nonparametric (Spearman) correlations with r values shown. In parentheses are one-tailed P-values calculated from Spearman coefficients to provide a measure of statistical significance of correlation.</p