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Arthropod-borne viruses represent a significant public health threat worldwide, yet there are few antiviral therapies or prophylaxes targeting these pathogens. In particular, the development of novel antivirals for high-risk populations such as pregnant women is essential to prevent devastating disease such as that which was experienced with the recent outbreak of Zika virus (ZIKV) in the Americas. One potential avenue to identify new and pregnancy-acceptable antiviral compounds is to repurpose well-known and widely used FDA-approved drugs. In this study, we addressed the antiviral role of atovaquone, an FDA Pregnancy Category C drug and pyrimidine biosynthesis inhibitor used for the prevention and treatment of parasitic infections. We found that atovaquone was able to inhibit ZIKV and chikungunya virus virion production in human cells and that this antiviral effect occurred early during infection at the initial steps of viral RNA replication. Moreover, we were able to complement viral replication and virion production with the addition of exogenous pyrimidine nucleosides, indicating that atovaquone functions through the inhibition of the pyrimidine biosynthesis pathway to inhibit viral replication. Finally, using an ex vivo human placental tissue model, we found that atovaquone could limit ZIKV infection in a dose-dependent manner, providing evidence that atovaquone may function as an antiviral in humans. Taken together, these studies suggest that atovaquone could be a broad-spectrum antiviral drug and a potential attractive candidate for the prophylaxis or treatment of arbovirus infection in vulnerable populations, such as pregnant women and children. IMPORTANCE The ability to protect vulnerable populations such as pregnant women and children from Zika virus and other arbovirus infections is essential to preventing the devastating complications induced by these viruses. One class of antiviral therapies may lie in known pregnancy-acceptable drugs that have the potential to mitigate arbovirus infections and disease, yet this has not been explored in detail. In this study, we show that the common antiparasitic drug atovaquone inhibits arbovirus replication through intracellular nucleotide depletion and can impair ZIKV infection in an ex vivo human placental explant model. Our study provides a novel function for atovaquone and highlights that the rediscovery of pregnancy-acceptable drugs with potential antiviral effects can be the key to better addressing the immediate need for treating viral infections and preventing potential birth complications and future disease., (Copyright © 2019 American Society for Microbiology.)

The pyrimidine de novo nucleotide synthesis consists of 6 sequential steps. Various inhibitors against these enzymes have been developed and evaluated in the clinic for their potential anticancer activity: acivicin inhibits carbamoyl-phosphate-synthase-II, N-(phosphonacetyl)-L- aspartate (PALA) inhibits aspartate-transcarbamylase, Brequinar sodium and dichloroallyl-lawsone (DCL) inhibit dihydroorotate-dehydrogenase, and pyrazofurin (PF) inhibits orotate-phosphoribosyltransferase. We compared their growth inhibition against 3 cell lines from head-and-neck-cancer (HEP-2, UMSCC-14B and UMSCC-14C) and related the sensitivity to their effects on nucleotide pools. In all cell lines Brequinar and PF were the most active compounds with IC50 (50% growth inhibition) values between 0.06-0.37 µM, Acivicin was as potent (IC50s 0.26-1 µM), but DCL was 20-31-fold less active. PALA was most inactive (24-128 µM). At equitoxic concentrations, all pure antipyrimidine de novo inhibitors depleted UTP and CTP after 24 hr exposure, which was most pronounced for Brequinar (between 6-10% of UTP left, and 12-36% CTP), followed by DCL and PF, which were almost similar (6-16% UTP and 12-27% CTP), while PALA was the least active compound (10-70% UTP and 13-68% CTP). Acivicin is a multi-target inhibitor of more glutamine requiring enzymes (including GMP synthetase) and no decrease of UTP was found, but a pronounced decrease in GTP (31-72% left). In conclusion, these 5 inhibitors of the pyrimidine de novo nucleotide synthesis varied considerably in their efficacy and effect on pyrimidine nucleotide pools. Inhibitors of DHO-DH were most effective suggesting a primary role of this enzyme in controlling pyrimidine nucleotide pools.

Peptide nucleic acids (PNAs) are a powerful tool for recognition of double-stranded DNA. Strand invasion is most efficient when pyrimidine PNAs are linked to form a bisPNA in which one strand binds by Watson-Crick base pairing while the other binds by Hoogsteen base pairing to the newly formed PNA-DNA duplex. Within many genes, however, polypyrimidine target sequences may not be located in optimal positions relative to transcription factor binding sites, and this deficiency may complicate attempts to identify potent antigene PNAs. To increase the versatility of strand invasion by PNAs, we have synthesized bisPNAs and bisPNA-peptide conjugates containing a mixed base extension of the Watson-Crick polypyrimidine strand. We find that these tail-clamp PNAs (TC-PNAs) bind duplex DNA and inhibit transcription. DNA recognition occurs with single-stranded or TC-bisPNAs and requires attachment of positively charged amino acids. Association rate constants, k(a), for binding to DNA by TC-PNAs are as high as 35000 M(-1) s(-1) and are usually only a fewfold lower than for analogous PNAs that lack mixed base extensions. The ability to bind duplex DNA is not always necessary for inhibition of transcription, possibly because PNAs can bind to accessible DNA within the transcription bubble created by RNA polymerase. These results, together with similar findings independently obtained by Nielsen and colleagues [Bentin, T., Larsen, H. J., and Nielsen, P. E. (2003) Biochemistry 42, 13987-13995], expand the range of sequences within duplex DNA that are accessible to PNAs and suggest that TC-PNA-peptide conjugates are good candidates for further testing as antigene agents.

Purine and pyrimidine nucleotides play critical roles in DNA and RNA synthesis as well as in membrane lipid biosynthesis and protein glycosylation. They are necessary for the development and survival of mature T lymphocytes. Activation of T lymphocytes is associated with an increase of purine and pyrimidine pools. However, the question of how purine vs pyrimidine nucleotides regulate proliferation, cell cycle, and survival of primary T lymphocytes following activation has not yet been specifically addressed. This was investigated in the present study by using well-known purine (mycophenolic acid, 6-mercaptopurine) and pyrimidine (methotrexate, 5-fluorouracil) inhibitors, which are used in neoplastic diseases or as immunosuppressive agents. The effect of these inhibitors was analyzed according to their time of addition with respect to the initiation of mitogenic activation. We showed that synthesis of both purine and pyrimidine nucleotides is required for T cell proliferation. However, purine and pyrimidine nucleotides differentially regulate the cell cycle since purines control both G(1) to S phase transition and progression through the S phase, whereas pyrimidines only control progression from early to intermediate S phase. Furthermore, inhibition of pyrimidine synthesis induces apoptosis whatever the time of inhibitor addition whereas inhibition of purine nucleotides induces apoptosis only when applied to already cycling T cells, suggesting that both purine and pyrimidine nucleotides are required for survival of cells committed into S phase. These findings reveal a hitherto unknown role of purine and pyrimidine de novo synthesis in regulating cell cycle progression and maintaining survival of activated T lymphocytes.

Potent inhibitors of enzymes catalyzing reactions in the de novo pathways for biosynthesis of purine and pyrimidine nucleotides are synthetic or natural-product analogues of pathway intermediates or, more recently, inhibitors rationally designed from a knowledge of the catalytic mechanism. Such inhibitors may be effective drugs against cancer, inflammatory disorders, or various infections. For human cancer, the purine pathway may be a better target for inhibition than the pyrimidine pathway, where toxic side effects are more apparent. Drugs such as methotrexate and 6-mercaptopurine have multiple sites of action, making it difficult to quantitatively predict their effects upon cells. Rational design of inhibitors based upon the X-ray structure of the target enzyme has the prospect of yielding drugs with only one site of action in human cells. Such a drug is VX-497, a potent inhibitor of the purine enzyme, IMP dehydrogenase.

Rat corticoencephalic cell cultures were investigated by high performance liquid chromatography for changes in the levels of adenosine 5'-triphosphate (ATP), guanosine 5'-triphosphate (GTP), uridine 5'-triphosphate (UTP), cytidine 5'-triphosphate (CTP), and the respective nucleoside diphosphates. Hypoxia was induced by gassing the incubation medium for 30 min with 100% argon. Removal of glucose was caused by washing the cultures in glucose-free medium at the beginning of the 30 min incubation period. Whereas hypoxia or glucose-deficiency alone failed to alter the nucleotide levels, the combination of these two manipulations was clearly inhibitory. Diazoxide (300 microM) an opener of ATP-dependent potassium channels (K(ATP)) did not alter the nucleotide contents either in a normoxic and glucose-containing medium, or a hypoxic and glucose-free medium. By contrast, the K(ATP) channel antagonist tolbutamide (300 microM) aggravated the hypoxic decrease of nucleotide levels in a glucose-free medium, although it was ineffective in a normoxic and glucose-containing medium. Hypoxia and glucose-deficiency decreased the ATP/ADP and UTP/UDP ratios, but failed to change the GTP/GDP ratio. Diazoxide and tolbutamide (300 microM each) had no effect on the nucleoside triphosphate/diphosphate ratios either during normoxic or during hypoxic conditions. In conclusion, corticoencephalic cultures are rather resistant to in vitro ischemia. Although they clearly respond to the blockade of plasmalemmal K(ATP) channels (plasmaK(ATP)) by tolbutamide, these channels appear to be maximally open as a consequence of the fall in intracellular nucleotides and, therefore, diazoxide has no further effect.

Transformed cells are characterized by imbalances in metabolic routes. In particular, different key enzymes of nucleotide metabolism and DNA biosynthesis, such as CTP synthetase, thymidylate synthase, dihydrofolate reductase, IMP dehydrogenase, ribonucleotide reductase, DNA polymerase, and DNA methyltransferase, are markedly up-regulated in certain tumor cells. Together with the concomitant down-modulation of the purine and pyrimidine degradation enzymes, the increased anabolic propensity supports the excessive proliferation of transformed cells. However, many types of cancer cells have maintained the ability to differentiate terminally into mature, non-proliferating cells not only in response to physiological receptor ligands, such as retinoic acid, vitamin D metabolites, and cytokines, but also following exposure to a wide variety of non-physiological agents such as antimetabolites. Interestingly, induction of tumor cell differentiation is often associated with reversal of the transformation-related enzyme deregulations. An important class of differentiating compounds comprises the antimetabolites of purine and pyrimidine nucleotide metabolism and nucleic acid synthesis, the majority being structural analogs of natural nucleosides. The CTP synthetase inhibitors cyclopentenylcytosine and 3-deazauridine, the thymidylate synthase inhibitor 5-fluoro-2'-deoxyuridine, the dihydrofolate reductase inhibitor methotrexate, the IMP dehydrogenase inhibitors tiazofurin, ribavirin, 5-ethynyl-1-beta-D-ribofuranosylimidazole-4-carboxamide (EICAR) and mycophenolic acid, the ribonucleotide reductase inhibitors hydroxyurea and deferoxamine, and the DNA polymerase inhibitors ara-C, 9-(2-phosphonylmethoxyethyl)adenine (PMEA), and aphidicolin, as well as several nucleoside analogs perturbing the DNA methylation pattern, have been found to induce tumor cell differentiation through impairment of DNA synthesis and/or function. Thus, by selectively targeting those anabolic enzymes that contribute to the neoplastic behavior of cancer cells, the normal cellular differentiation program may be reactivated and the malignant phenotype suppressed.

Nucleotides are phosphate esters of nucleosides that contain a sugar linked through a glycosidic linkage with purine and pyrimidine bases. Purine and pyrimidine nucleotides are major components of the cells that make up the monomeric units of DNA and RNA, and they function in all cellular processes. Biosynthesis, interconversion, catabolism and other aspects of nucleotide metabolism, along with various cellular roles of nucleotides, will be discussed, and the possible use of dietary sources of preformed purines and pyrimidines will be considered.

1. An improvement of the chemotherapy of hepatocellular carcinoma with adriamycin or 5-fluorouracil and a reduction of side effects has been achieved by intra-arterial administration of the drugs. This treatment provides a somewhat extended survival but no cure. 2. The treatment of hepatocellular carcinoma in patients by reduction of an inactive precursor of a cytocidal alkylating agent by azoreductase of the tumor showed no therapeutic effect. 3. A selective hepatocellular uptake of drugs coupled to asialoglycoproteins has been described. An application of this concept for the chemotherapy of hepatocellular carcinoma seems doubtful since a loss of binding proteins for desialylated glycoproteins during experimental hepatocarcinogenesis has been demonstrated. 4. The increased uptake of 5-fluorouridine in hepatomas after induction of a tissue-specific depletion of uridine 5'-triphosphate and cytidine 5'-triphosphate provides an effective experimental chemotherapy with limited side effects. A clinical use of this new concept for the chemotherapy of hepatocellular carcinoma may serve as a useful approach.

The parasite Trypanosoma cruzi metabolizes allopurinol by a sequential conversion to allopurinol mononucleotide and aminopurinol mononucleotide. The latter is incorporated into RNA. This transformation of a widely used innocuous agent, allopurinol, into a toxic adenine analog appears to account for the antiprotozoan effect of allopurinol. These unique enzymatic activities appear to occur only in T. cruzi and the pathogenic lesihaminae. Allopurinol may serve as a model for the synthesis of similar antiprotozoan agents.