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Inhibition of axenically grown Entamoeba histolytica by purine nucleoside analogs and actions of natural nucleosides.

The effects of the nucleoside analogs tubercidin, nebularin, formycin B and 3'-deoxyinosine on axenically grown Entamoeba histolytica were tested. Both tubercidin and nebularin showed pronounced inhibitory action, 50% of growth inhibition (IC50) being obtained at 0.14 and 0.82 microM, respectively. Formycin B and 3'-deoxyinosine were essentially inactive or weakly active at concentrations above 10 microM. Natural nucleosides, including adenosine, inosine, guanosine, thymidine, uridine and cytidine caused no significant effects at concentrations of 0.01-1 microM, however, significant inhibitory action was observed at or above 10 microM with cytidine and thymidine. The exploration of cytotoxic nucleosides as antiamebal drugs is of continued interest.

Animals↗

Fluctuations in nucleoside uptake and binding of the inhibitor of nucleoside transport, nitrobenzylthioinosine, during the replication cycle of HeLa cells.

Binding of the potent nucleoside transport inhibitor 6-[(4-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine (NBMPR) and rates of uptake of several nucleosides were examined at 4-hr intervals during the replication cycle of HeLa S3 cells. Monolayer cultures of synchronous cells, obtained by mitotic detachment, were assayed for high-affinity binding of NBMPR and for rates of uptake of thymidine, uridine, cytidine, adenosine, inosine, and guanosine. The number of NBMPR binding sites per cell doubled between 4 and 16 hr after detachment (late G1 and S phase); during this interval, V max 'S for uptake of cytidine and adenosine doubled, and for uridine and thymidine uptake increased about 4- and 8-fold, respectively. Rates of inosine and guanosine uptake at extracellular concentrations below saturation increased 2-fold between G1 and S phase of the cell cycle. Km 'S for cellular uptake of thymidine, uridine, cytidine, and adenosine did not change with progress through the cycle. The results presented suggest that changes in nucleoside uptake during the HeLa cell cycle were due, in part, to changes in the activity of NBMPR-sensitive transport elements in the membrane.

Binding Sites↗

Initial rate kinetics and evidence for duality of mediated transport of adenosine, related purine nucleosides, and nucleoside analogues in L1210 cells.

In studies using a rapid kinetic technique, evidence was derived for multiplicity of systems mediating [3H]adenosine transport in L1210 cells. A variety of approaches were used in discriminating between transport and kinase-mediated phosphorylation. Under these conditions, two systems mediating influx were delineated which exhibited high-affinity [Km = 13.9 +/- 2 (S.E.) microM] or low-affinity [Km = 199 +/- 27 microM] for [3H]-adenosine. Both systems exhibited high capacities, but that associated with the low-affinity system (V 37 degrees max = 263 +/- 43 nmol = 99.6 +/- 12 nmol sec/g, dry weight). The relative difference in affinity of these two systems during influx was also reflected in the values for influx Ki obtained with other nucleosides and nucleoside analogues. Influx of [3H]-adenosine by each mediated system was inhibited by 6-(2-hydroxy-5-nitrobenzyl)thioguanosine, a specific transport inhibitor, and by 9-beta-D-arabinofuranosylpurine-6(1H)thione which is not phosphorylated in L1210 cells. Influx kinetics were the same in L1210 cells, in adenosine triphosphate-depleted L1210 cells (L1210/ara-C/MMPR) which have substantially reduced ability for [3H]adenosine phosphorylation, and in the presence of 2'-deoxycoformycin, a potent inhibitor of adenosine deaminase. The same multiplicity in mediated influx of [3H]adenosine was shown at 0 degrees when transport became rate limiting to total uptake. The high-affinity system mediating [3H]adenosine influx was also elucidated in L1210 cell plasma membrane vesicles in the presence or absence of 2'-deoxycoformycin. Almost all of the natural nucleosides examined competed less effectively with [3H]adenosine for influx by the high-affinity system than by the low-affinity system. These results are discussed with respect to possible pharmacological implications.

Adenosine↗

Functional expression of Na(+)-dependent nucleoside transport systems of rat intestine in isolated oocytes of Xenopus laevis. Demonstration that rat jejunum expresses the purine-selective system N1 (cif) and a second, novel system N3 having broad specificity for purine and pyrimidine nucleosides.

Isolated stage VI oocytes from Xenopus laevis expressed uridine transport activity after microinjection of mRNA from rat jejunum. Uridine uptake during 30 min (10 microM, 20 degrees C) by mRNA-injected oocytes reached 2.5 pmol/oocyte, compared with endogenous uptake by water-injected oocytes of about 0.05 pmol/oocyte. The expressed transport activity was 96% Na(+)-dependent, saturable (apparent Km = 15 microM) and inhibited by phloridzin (IC50 = 100 microM). Nucleoside inhibition studies resolved the expressed transport activity into two components: 1) a novel Na(+)-dependent system of broad purine and pyrimidine specificity that was inhibited by low concentrations of guanosine, inosine, adenosine, uridine, thymidine, and cytidine and 2) a Na(+)-dependent system of narrower specificity that was inhibited by low concentrations of guanosine, inosine, adenosine, and uridine and by high concentrations of thymidine and cytidine. The characteristics of the latter system are consistent with those of the Na(+)-dependent nucleoside transport system N1 (cif), previously identified in a number of cell types and tissues, including intestinal epithelia and cultured cells of intestinal origin. The broad specificity system, which was also detected in mRNA-injected oocytes using thymidine as permeant, has been given the provisional designation N3 to distinguish it from the previously described N1 (purine-selective) and N2 (pyrimidine-selective) Na(+)-linked nucleoside transporters. Rat jejunal transporters N1 and N3 were both expressed maximally by the same mRNA size fraction (1.6-3.0 kb, peak 2.3 kb).

Animals↗

Structure-activity relationships for phosphorylation of nucleoside analogs to monophosphates by nucleoside kinases.

The mammalian deoxyribonucleoside kinases thymidine kinase 1 and 2, deoxycytidine kinase and deoxyguanosine kinase phosphorylate deoxyribonucleosides and provide an alternative to de novo synthesis of DNA precursors. Their activities are essential for activation of several chemotherapeutically important nucleoside analogs. These four salvage kinase enzymes exhibit distinct substrate specificities for nucleoside analogs modified in the base and glycon moieties. In this review their. structure-activity relationships are discussed. Alternative routes for phosphorylation of nucleoside analogs are also reviewed, such as the phosphotransfer capacity of 5'-nucleotidase and protein kinases.

Adenosine Kinase↗

Comparison of the interaction of uridine, cytidine, and other pyrimidine nucleoside analogues with recombinant human equilibrative nucleoside transporter 2 (hENT2) produced in Saccharomyces cerevisiae.

The human equilibrative nucleoside transporters I and 2 (hENT1, hENT2) share 50% amino acid identity and exhibit broad selectivities, accepting purine and pyrimidine nucleosides as permeants. The permeant selectivity of hENT2 is less well understood because of the low abundance of the native transporter in cells amenable to functional analysis. Recent studies of hENT2 produced in recombinant form in functional expression systems have shown that it differs from hENT1 in that it transports nucleobases. To further understand the structural requirements for permeant interaction with hENT2, we compared the relative abilities of uridine, cytidine, and their analogues to inhibit transport of [3H]uridine by recombinant hENT1 and hENT2 produced in yeast. hENT1 and hENT2 tolerated halogen modification at the 5 position of the base and the 2' and 5' positions of the ribose moieties of uridine whereas removal of the hydroxyl group at the 3' position of the ribose moiety of uridine eliminated interaction with both transporters. hENT2 displayed a lower ability, compared with hENT1, to interact with cytidine and cytidine analogues, suggesting a low tolerance for the presence of the amino group at the 4 position of the base.

Biological Transport↗

Up-regulation of the high-affinity pyrimidine-preferring nucleoside transporter concentrative nucleoside transporter 1 by tumor necrosis factor-alpha and interleukin-6 in liver parenchymal cells.

BACKGROUND/AIMS: Concentrative nucleoside transporter 1 (CNT1), a high affinity transporter for pyrimidine nucleosides, is responsible for their Na+-dependent concentrative uptake into hepatocytes. Though CNT1 protein amounts increase in rat liver soon after partial hepatectomy, the physiological regulators of CNT1 expression have not yet been identified. METHODS: Rat hepatoma cell lines and hepatocytes isolated from fetuses and adult rats were used to identify single agents able to up-regulate CNT1 expression and activity in liver. TNF-alpha receptor-I (TNFRI) and IL-6 knock-out mice were also used to study CNT1 regulation in vivo. RESULTS: TNF-alpha and IL-6 independently induced CNT1 protein expression in cultured liver parenchymal and FAO hepatoma cells by PI-3 kinase- and ERK-dependent mechanisms, respectively. In vivo data showed that transporter protein levels were low in livers from TNFRI knock-out mice, but not in those from IL-6 deficient animals. However, IL-6 administration only partially restored CNT1 expression in the former model. CONCLUSIONS: This study identifies TNF-alpha as a major in vivo modulator of the nucleoside transporter CNT1 and suggests a secondary role for IL-6 in mediating CNT1 up-regulation by TNF-alpha in vivo. Evidence is provided that two independent pathways are involved in the up-regulation of CNT1 by TNF-alpha and IL-6.

Animals↗

A comparison of three highly active antiretroviral treatment strategies consisting of non-nucleoside reverse transcriptase inhibitors, protease inhibitors, or both in the presence of nucleoside reverse transcriptase inhibitors as initial therapy (CPCRA 058 FIRST Study): a long-term randomised trial.

BACKGROUND: Long-term data from randomised trials on the consequences of treatment with a protease inhibitor (PI), non-nucleoside reverse transcriptase inhibitor (NNRTI), or both are lacking. Here, we report results from the FIRST trial, which compared initial treatment strategies for clinical, immunological, and virological outcomes. METHODS: Between 1999 and 2002, 1397 antiretroviral-treatment-naive patients, presenting at 18 clinical trial units with 80 research sites in the USA, were randomly assigned in a ratio of 1:1:1 to a protease inhibitor (PI) strategy (PI plus nucleoside reverse transcriptase inhibitor [NRTI]; n=470), a non-nucleoside reverse transcriptase inhibitor (NNRTI) strategy (NNRTI plus NRTI; n=463), or a three-class strategy (PI plus NNRTI plus NRTI; n=464). Primary endpoints were a composite of an AIDS-defining event, death, or CD4 cell count decline to less than 200 cells per mm3 for the PI versus NNRTI comparison, and average change in CD4 cell count at or after 32 months for the three-class versus combined two-class comparison. Analyses were by intention-to-treat. This study is registered ClinicalTrials.gov, number NCT00000922. FINDINGS: 1397 patients were assessed for the composite endpoint. A total of 388 participants developed the composite endpoint, 302 developed AIDS or died, and 188 died. NNRTI versus PI hazard ratios (HRs) for the composite endpoint, for AIDS or death, for death, and for virological failure were 1.02 (95% CI 0.79-1.31), 1.07 (0.80-1.41), 0.95 (0.66-1.37), and 0.66 (0.56-0.78), respectively. 1196 patients were assessed for the three-class versus combined two-class primary endpoint. Mean change in CD4 cell count at or after 32 months was +234 cells per mm3 and +227 cells per mm3 for the three-class and the combined two-class strategies (p=0.62), respectively. HRs (three-class vs combined two-class) for AIDS or death and virological failure were 1.15 (0.91-1.45) and 0.87 (0.75-1.00), respectively. HRs (three-class vs combined two-class) for AIDS or death were similar for participants with baseline CD4 cell counts of 200 cells per mm3 or less and of more than 200 cells per mm3 (p=0.38 for interaction), and for participants with baseline HIV RNA concentrations less than 100 000 copies per mL and 100,000 copies per mL or more (p=0.26 for interaction). Participants assigned the three-class strategy were significantly more likely to discontinue treatment because of toxic effects than were those assigned to the two-class strategies (HR 1.58; p<0.0001). INTERPRETATION: Initial treatment with either an NNRTI-based regimen or a PI-based regimen, but not both together, is a good strategy for long-term antiretroviral management in treatment-naive patients with HIV.

Adult↗

Mutation patterns of the reverse transcriptase genes in HIV-1 infected patients receiving combinations of nucleoside and non nucleoside inhibitors.

A genotyping assay was used to define human immunodeficiency virus type 1 (HIV-1) reverse transcriptase codons in plasma samples from 80 HIV-1 patients extensively treated with two nucleoside reverse transcriptase (zidovudine and lamivudine) and one non nucleoside reverse transcriptase (nevirapine) inhibitor. The frequencies of T215S/Y/F, M41L, D67N, L210W K70R, K219Q mutations, detectable in plasma samples, conferring resistance to zidovudine were 61.2, 56.2, 36.2, 31.5, 27.5 and 17.5%, respectively. Mutations (M184V or M184I) conferring resistance to lamivudine were detected in an extremely high percentage of patients (61%). Among mutations correlated to high (K103N, V106A, Y181C/I, Y188C/H/L, G190A/C/E/Q/S/T) or moderate (V108I, V118I) levels of nevirapine resistance, the predominant amino acid change was a substitution at 103 codon, present in 24 of 80 samples tested. Finally Q151M, the marker mutation able to confer resistance to all nucleoside analogues, was detected in seven patients with a viral load of between 1 x 10(4) and 9 x 10(4) HIV-1 RNA copies/ml. The relationship between the genotype and the viral load showed that the incidence of some specific mutations [M41L, T215Y (correlated to zidovudine resistance) and K103N (correlated to all NNRTIs drugs)] significantly (P=0.001) increased with higher viral load. Our results, albeit limited to a small cohort, showed a high frequency of mutations correlated to drugs in use, suggesting a need for therapeutic change in the near future and demonstrating that the development of genotyping tests helps to guide the therapeutic management of HIV-1 infected people. Our data highlight the dangers of selecting antiretroviral therapy without previous antiretroviral drug testing. Although the cost of these assays is a concern, prescribing inefficacious drugs could create serious problems for HIV-1 patients.

Acquired Immunodeficiency Syndrome↗

Nucleosides and nucleotides. 122. 2'-C-cyano-2'-deoxy-1-beta-D-arabinofuranosylcytosine and its derivatives. A new class of nucleoside with a broad antitumor spectrum.

Design, synthesis, and tumor cell growth inhibitory effects of 2'-C-cyano-2'-deoxy-1-beta-D-arabinofuranosyl derivatives of cytosine (1i, CNDAC), thymine (6a), uracil (6c), and adenine (6d) have been described. The synthesis of the target compounds was achieved from the corresponding 2'-keto nucleosides 2a-d. Cyanohydrins of 2a-d were converted to thionocarbonates, which were deoxygenated to give the desired 2'-beta-cyano-2'-deoxy derivatives 5a-d, followed by deprotection to furnish the target nucleosides. Of these nucleosides, CNDAC was the most potent inhibitor of cell growth with an IC50 value of 0.53 microM against L1210 cells. In vitro cytotoxicity of CNDAC against human tumor cell lines was also examined; compared with that of 1-beta-D-arabinofuranosylcytosine (ara-C) and 5-fluorouracil (5-FU), CNDAC was more cytotoxic to several cell lines refractory to ara-C. The in vivo effect of CNDAC on M5076 mouse reticulum cell sarcoma was very strong; 99% tumor volume inhibition on day 20 was achieved when it was administrated orally on days 1, 4, 7, 10, 13, and 16 at a dose of 400 mg/kg/day, while 5'-deoxy-5-fluorouridine (5'-DFUR) and 5-FU caused only 50% inhibition at a dose of 500 mg/kg/day and 28% inhibition at a dose of 50 mg/kg/day, respectively, on the same schedule. These results indicated that CNDAC may have potential as a new antineoplastic agent with a broad antitumor spectrum.

Adenine↗

Nucleosides and nucleotides. 97. Synthesis of new broad spectrum antineoplastic nucleosides, 2'-deoxy-2'-methylidenecytidine (DMDC) and its derivatives.

A new type of antineoplastic nucleoside, 2'-deoxy-2'-methylidenecytidine (DMDC) has been synthesized from the corresponding 2'-keto pyrimidine nucleosides 3 and 8 by the Wittig reaction. During the course of the reaction, we found that an intermediate betaine could pick a proton from the excess triphenylphosphonium bromide to form the 2'-phosphonium salts 5 and 10, which could be further converted into the 2'-deoxy-2'-methylidene nucleosides 4 and 9 by treatment with sodium hydride. Various 5-substituted DMDC derivatives 19a-e,h and their uracil congeners 16a-h were also synthesized from the corresponding 5-substituted uridines 12a-f,h. Among them, DMDC as well as 2'-deoxy-2'-methylidene-5-fluorocytidine (19a) showed potent antileukemic activity against murine L1210 cells in culture. The activity of DMDC and 19a toward various human tumor cells in culture compared with 1-beta-D-arabinofuranosylcytosine and 5-fluorouracil was also examined. In vivo antitumor activity of DMDC against L1210 was also described.

Animals↗

Nucleosides. 116. 1-(beta-D-Xylofuranosyl)-5-fluorocytosines with a leaving group on the 3' position. Potential double-barreled masked precursors of anticancer nucleosides.

Syntheses of five pairs of cytosine and 5-fluorocytosinexylofuranosyl nucleosides in which the 3'-hydroxyl group is replaced by Cl, Br, OMs, or OTs are described. Those xylosyl nucleosides with a good leaving group at the 3' position exhibit good inhibitory activity against L5178Y and P815 mouse leukemic cells in vitro at rather low concentrations, and like that of ara-C this cytotoxicity is reversed by 2'-deoxycytidine but not by thymidine. Xylosylcytosines are not active against ara-C resistant lines of L5178Y and P815 cells; however, the corresponding 5-fluorocytosine analogues exhibit significant cytotoxicity against these ara-C resistant leukemic cell lines, and this activity is reversed by thmidine but not by deoxycytidine. These data support the "double-barreled" masked precursor hypothesis in that xylosyl-5-fluorocytosines substituted at the 3' position by a good leaving group exhibit activity akin to that of ara-C in the ara-C sensitive lines, while these nucleosides act as 5-fluoropyrimidines in the ara-C resistant lines.

Animals↗

Nucleosides. 139. Synthesis and anticytomegalovirus and antiherpes simplex virus activity of 5'-modified analogues of 2'-fluoroarabinosylpyrimidine nucleosides.

In order to determine if modification of the 5'-position reduces or abolishes the antiviral activity of 2'-fluoro-5-iodo-ara-C (FIAC), 2'-fluoro-5-iodo-ara-U (FIAU), or 2'-fluoro-5-methyl-ara-U (FMAU) against human cytomegalovirus (HCMV) and herpes simplex virus (HSV), the 5'-deoxy, 5'-mercapto, and 5'-amino analogues of these nucleosides were prepared. 5'-Deoxy-FIAC and 5'-deoxy-FIAU were prepared by catalytic hydrogenation of 5'-iodo-FIAC and 5'-iodo-FIAU to 5'-deoxy-FAC and 5'-deoxy-FAU, respectively, followed by reiodination at C-5. Reduction of 5'-iodo-FMAU afforded 5'-deoxy-FMAU. These 5'-deoxy nucleosides were found to be inactive against HCMV, indicating that the conversion to 5'-phosphate by the cellular enzyme(s) is a requirement for antiviral activity against this virus. Other 5'-modified (NH2 and SH) analogues were also prepared from 5'-O-tosyl-FIAC and 5'-O-tosyl-FMAU. Treatment of these tosylates with LiN3 in DMF afforded the corresponding 5'-N3 products. Catalytic hydrogenation of 5'-N3-FMAU afforded 5'-NH2-FMAU, whereas 5'-NH2-FIAC was obtained by treatment of 5'-N3-FIAC with Ph3P in pyridine. 5'-Mercapto analogues were prepared by treatment of 5'-O-tosyl-3'-O-acetyl nucleosides with KSAc followed by deacetylation. 5'-NH2-FMAU was the only compound that showed good activity against HSV-1 and HSV-2 in vitro. However, this compound was less potent and had a lower therapeutic index than FMAU.

Animals↗

Nucleosides and nucleotides. 175. Structural requirements of the sugar moiety for the antitumor activities of new nucleoside antimetabolites, 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)cytosine and -uracil1.

We previously designed 1-(3-C-ethynyl-beta-d-ribo-pentofuranosyl)uracil (EUrd) and its cytosine congener (ECyd) as potential multifunctional antitumor nucleoside antimetabolites. They showed potent and broad-spectrum antitumor activity against various human and mouse tumor cells in vitro and in vivo. To clarify the structure-activity relationship of the sugar moiety, various 3'-C-carbon-substituted analogues, such as 1-propynyl, 1-butynyl, ethenyl, ethyl, and cyclopropyl derivatives, of ECyd and EUrd were synthesized. We also prepared 3'-deoxy analogues and 3'-homologues of ECyd and EUrd with different configurations to determine the role of the 3'-hydroxyl group and the length between the 3'-carbon atom and the ethynyl group and a 2'-ethynyl derivative of ECyd to determine the spatial requirements of the ethynyl group. The in vitro tumor cell growth inhibitory activities of these nucleosides against mouse leukemic L1210 and human KB cells showed that ECyd and EUrd were the most potent inhibitors in the series, with IC50 values of 0.016 and 0.13 microM for L1210 cells and 0.028 and 0.029 microM for KB cells, respectively. Only 3'-C-1-propynyl and -ethenyl derivatives of ECyd showed greatly reduced cytotoxicity. We found that the cytotoxic activity of these nucleosides predominantly depended on their first phosphorylation by uridine/cytidine kinase.

Animals↗

Aryl H-phosphonates. 12. Synthetic and (31)P NMR studies on the preparation of nucleoside H-phosphonothioate and nucleoside H-phosphonodithioate monoesters.

Transformation of nucleoside H-phosphonate monoesters into the corresponding H-phosphonothioate and H-phosphonodithioate derivatives and possible side-reactions that may accompany this process were studied using (31)P NMR spectroscopy. These provided new insight into a possible mechanism involved in this transformation and constituted the basis for development of efficient methods for the preparation of nucleoside H-phosphonothioate and nucleoside H-phosphonodithioate monoesters using readily available H-phosphonate monoesters as starting materials.

Esters↗

Synthesis and Antiviral Activity of the alpha-Analogues of 1,5-Anhydrohexitol Nucleosides (1,5-Anhydro-2,3-dideoxy-D-ribohexitol Nucleosides).

1,5-Anhydro-2,3-dideoxy-D-ribohexitol nucleosides were synthesized starting from 4,6-di-O-benzyl-1,5-di-O-pivaloyl-3-deoxy-D-glucitol using a ring closure procedure. The target nucleoside adopts a (4)C(1) conformation as also demonstrated for the corresponding 1,5-anhydrohexitol nucleosides with beta-configuration of the base-substituted carbon atom. The cytosine congener demonstrated a moderate but selective activity against Herpes simplex virus types 1 and 2.

Journal Article↗

Conformation of nucleosides: circular dichroism study on the syn-anti conformational equilibrium of 2-substituted benzimidazole nucleosides.

The solution conformations of 2-substituted derivatives of 1-(beta-D-ribofuranosyl)benzimidazole have been determined by circular dichroism spectroscopy in aqueous solutions. It is shown that analogs with methyl, amino, or methylamino substituents at position 2 of the benzimidazole ring (position 8 of the purine ring) have predominantly anti conformations, whereas analogs with chloro, aza, methoxy, or methylmercapto substituents have predominantly syn conformations. The preferred solution conformations of the benzimidazole nucleosides and analogous purine nucleosides are compared. The results demonstrate that the replacement of nitrogen by carbon at position 3 of the purine ring of purine (beta) nucleosides leads to important conformational consequences, which are strengthened or neutralized by substituents at position 8 of the purine ring.

Benzimidazoles↗

Anti-cowpox virus activities of certain adenosine analogs, arabinofuranosyl nucleosides, and 2'-fluoro-arabinofuranosyl nucleosides.

Nucleoside analogs were investigated for their potential to inhibit cowpox virus (a surrogate for variola and monkeypox viruses) in cell culture and in lethal respiratory infections in mice. Cell culture antiviral activity was determined by plaque reduction assays, with cytotoxicity determined by cell proliferation assays. Selectivity indices (SI's, 50% cytotoxic concentration divided by 50% virus-inhibitory concentration) were determined for 15 compounds. Three arabinofuranosyl (Ara) nucleosides showed activity in mouse mammary tumor (C127I) cells: guanine (Ara-G), thymine (Ara-T), and adenine (Ara-A) with SI's of 113, 61, and 95, respectively. The 2'-fluoro-Ara nucleosides of 5-F-cytosine (FIAC), 5-methyluracil (FMAU), and 5-iodouracil (FIAU) exhibited SI's of 148, 77, and 29, respectively. Other potent compounds included cidofovir (a positive control) and 3'-O-methyladenosine, with SI values of 164 and 56, respectively. In general, assays performed in African green monkey kidney (Vero) cells produced lower SI's than in C127I cells, except for 5-iodo-2'-deoxyuridine (IDU) which had an SI of > 71 in Vero cells and 3.1 in C127I cells. Intranasal infection of mice with cowpox virus was followed a day later by twice daily intraperitoneal treatment with compounds for 5 days. Ara-A was active at 300 mg/kg/day (40% survival), FMAU at 100 mg/kg/day (70% survival), and cidofovir (given for 1 day only) at 100 mg/kg (80-100% survival). None of the other compounds, including IDU, prevented death nor delayed the time to death. Cidofovir had the best potential for treating orthopoxvirus infections of those tested.

Adenosine↗