2'-fluoro-5-iodo-aracytosine, a potent and selective anti-herpesvirus agent

Activity of 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodouracil against simian varicella virus infections in African green monkeys

The fluorinated pyrimidines 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodouracil (FIAU) and 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-methyluracil (FMAU) are highly effective inhibitors of herpesvirus infections in vitro and in vivo. This report is concerned with an evaluation of their activities in African green monkeys (Cercopithecus aethiops) infected with simian varicella virus, a herpesvirus closely related to human varicella-zoster virus. Oral or intravenous administration of FIAU at 50 mg/kg per day as divided doses beginning 48 h after virus inoculation prevented the development of evidences of clinical infection. Oral treatment with FIAU at 30 mg/kg per day deferred as late as 7 days after virus inoculation modified the course of the disease. When treatment was started 48 h after virus inoculation, daily doses of FIAU as small as 1 mg/kg inhibited development of infections; daily doses of 0.2 mg/kg were ineffective. At the latter dose FMAU prevented development of clinical disease, suggesting that it was more active than FIAU. No signs of FIAU toxicity were observed, with the single exception of an early but transitory elevation in aspartate aminotransferase activity in serum.

Activities of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine and its metabolites against herpes simplex virus types 1 and 2 in cell culture and in mice infected intracerebrally with herpes simplex virus type 2

As measured by plaque and yield reduction assays, several metabolites of 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine (FIAC) were highly active against herpes simplex virus types 1 and 2. These metabolites included the 2'-deoxy-2'-fluoroarabinosyl derivatives of 5-iodouracil (FIAU), cytosine (FAC), uracil (FAU), and thymine (FMAU). In mice inoculated intracerebrally with herpes simplex virus type 2, the relative order of potency of these compounds and licensed antiviral drugs was as follows: FMAU much greater than FIAC approximately equal to FIAU greater than acyclovir approximately equal to vidarabine much greater than FAC approximately equal to FAU. One of the main metabolites of FMAU, 2'-fluoro-5-hydroxymethyl-arabinosyluracil, was essentially inactive in vivo. FIAC-, FIAU-, FMAU-, FAC-, and FAU-resistant herpes simplex virus variants prepared in cell culture were found to be (i) devoid of viral thymidine kinase, (ii) cross-resistant to one another and resistant to drugs requiring viral thymidine kinase for activation, and (iii) sensitive to vidarabine or phosphonoformate. These results indicate that FIAC, FIAU, and FMAU require the virally encoded thymidine kinase for activation and suggest that the antiviral activity of FAU and FAC in cell cultures is also mediated by this enzyme. The interaction of the fluoroarabinosyl pyrimidine nucleosides with herpes simplex virus thymidine kinase in a cell-free system is also described.

Simple quantitative radioiodination of (E)-5-(2-iodovinyl)-2'-deoxyuridine (IVDU) by exchange labelling

The synthesis of [125I]-(E)-5-(2-iodovinyl)-2'-deoxyuridine [( 125I]IVDU) by a halogen isotope exchange reaction has been monitored in various reaction conditions. Quantitative incorporation of radioiodine into IVDU was obtained within 20 min by a simple and safe method that is suitable for application in any nuclear medicine laboratory. The radiolabelled antiviral agent can be used without further purification.

Human cytomegalovirus-induced DNA polymerase and its interaction with the triphosphates of 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-methyluracil, -5-iodocytosine, and -5-methylcytosine

Human cytomegalovirus-induced DNA polymerase and cellular DNA polymerase alpha were purified by successive chromatography on DEAE-cellulose, phosphocellulose, heparin agarose, and single-stranded DNA agarose columns. The purified virus-induced DNA polymerase was resolved to consist of two polypeptides corresponding to molecular weights of 140,000 and 58,000, as analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Virus-induced DNA polymerase and cellular alpha polymerase were examined for their sensitivities to the triphosphates of 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-methyluracil (FMAUTP), -5-iodocytosine (FIACTP), and -5-methylcytosine (FMACTP). The inhibitive effects of these triphosphates on the DNA polymerases were competitive with regard to the natural substrates; thus FMAUTP competes with dTTP, and FIACTP and FMACTP compete with dCTP. The inhibition constants (Ki) for FMAUTP, FIACTP, and FMACTP of virus-induced DNA polymerase are 0.06, 0.30, and 0.47 microM, respectively. Cellular DNA polymerase alpha is much less sensitive to these inhibitors, and its Ki values for FMAUTP, FIACTP, and FMACTP are 0.45, 3.10, and 2.90 microM, respectively. In addition, human cytomegalovirus-induced DNA polymerase, but not cellular DNA polymerase alpha, can utilize these analog triphosphates as alternate substrates for their corresponding natural deoxyribonucleoside triphosphates in in vitro DNA synthesis.

Murine mammary FM3A carcinoma cells transformed with the herpes simplex virus type 1 thymidine kinase gene are highly sensitive to the growth-inhibitory properties of (E)-5-(2-bromovinyl)-2'-deoxyuridine and related compounds

Murine mammary carcinoma (FM3A TK-/HSV-1 TK+) cells, which are thymidine kinase (TK)-deficient but have been transformed with the herpes simplex virus type 1 (HSV-1) TK gene are inhibited in their growth by (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), (E)-5-(2-iodovinyl)-2'-deoxyuridine (IVDU) and (E)-5-(2-bromovinyl)-2'-deoxycytidine (BVDC) at 0.5, 0.5 and 0.8 ng/ml, respectively; i.e., a concentration 5000 to 20 000-fold lower than that required to inhibit the growth of the corresponding wild-type FM3A/0 cells. Hence, transformation of tumor cells with the HSV-1 TK gene makes them particularly sensitive to the cytostatic action of BVDU and related compounds.

Biotransformation and elimination of [2-14C]-1-(2-deoxy-2'-fluoro-beta-D -arabinofuranosyl)-5-iodocytosine in immunosuppressed patients with herpesvirus infections

The metabolism of the drug [2-14C]-1-(2'-deoxy-2'-fluoro-beta-D -arabinofuranosyl)-5-iodocytosine (FIAC), a potent inhibitor of herpesvirus replication, was studied in immunosuppressed patients with herpesvirus infections. FIAC was administered intravenously by 15-min infusion and by mouth 24 h later to four patients at doses of 50 or 100 mg/m2. FIAC was cleared from the plasma primarily by biotransformation in liver, kidney, and peripheral blood, with a terminal-phase half-life of 0.92 to 1.80 h (mean, 1.36 h) after intravenous administration. The area under the concentration-time curve from zero to infinity (AUC0-infinity) for FIAC was 1.6 to 4.7% (mean, 3.4%) of the AUC0-infinity for total radioactivity. 1-(2'-Deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodouracil (FIAU) was the major metabolite; the AUC0-infinity for FIAU was 54.3 to 72.5% (mean, 63.4%) of the AUC0-infinity for total radioactivity. The terminal-phase half-life for FIAU was 3.32 to 4.49 h (mean, 3.91 h); FIAU was cleared from plasma by renal elimination and further biotransformation. lesser amounts of 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)uracil, 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)cytosine, the glucuronide conjugates of these metabolites, and the glucuronide conjugates of FIAC and FIAU were also formed. A comparison of the AUC0-infinity for total radioactivity after intravenous and oral administration suggested that nearly all of the oral dose was absorbed. Plasma levels of FIAU, also a potent inhibitor of herpesvirus replication in vitro, exceeded the 50% effective dose for herpes simplex virus and varicella-zoster virus as late as 12 h after administration of FIAC.

Metabolic activation of the nucleoside analog 9-[( 2-hydroxy-1-(hydroxymethyl)ethoxy]methyl)guanine in human diploid fibroblasts infected with human cytomegalovirus

9-[( 2-Hydroxy-1-(hydroxymethyl)ethoxy]-methyl)guanine (BW B759U) is a more potent inhibitor of human cytomegalovirus (HCMV) in vitro than is the related nucleoside analog acyclovir (ACV). BW B759U was selectively activated to the 5'-triphosphate (BW B759U-triphosphate) in cells infected with HCMV to levels at least 10-fold higher than those measured for ACV-triphosphate and up to as much as 100-fold higher than the levels found in uninfected cells. BW B759U-triphosphate accumulated in HCMV-infected cells with time; the rate of this increase was dependent upon the drug dose and virus multiplicity of infection. Enzyme activities that catalyzed the phosphorylation of thymidine and 2'-deoxycytidine increased 3- to 7-fold in extracts of cells early after HCMV infection but thereafter declined. No concomitant increase in the rate of BW B759U phosphorylation was detected under these assay conditions. Maximal rate of accumulation of both BW B759U-triphosphate and ACV-triphosphate after a short exposure to drug occurred in the late phase of the infective cycle, as the titer of extracellular virus reached a peak in untreated cultures, but after the decline of stimulated host deoxypyrimidine kinase activities. Once formed, the BW B759U-triphosphate pool decreased very slowly and thus it persisted for several days in both HCMV-infected and uninfected cells.

Combination chemotherapy: interaction of 5-methoxymethyldeoxyuridine with trifluorothymidine, phosphonoformate and acycloguanosine against herpes simplex viruses

Methoxymethyldeoxyuridine (MMUdR) when used in combination with either trifluorothymidine (F3TdR) or phosphonoformate (PFA) showed synergistic activity against herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2) in vitro, whereas MMUdR and acycloguanosine (ACG) combination was antagonistic against herpes viruses. HSV-1 mutants resistant to ACG, arabinofuranosyladenine (Ara-A), MMUdR or PFA were isolated. Drug-resistant HSV-1 virus mutants were analyzed for cross sensitivity to ACG, Ara-A, F3TdR, MMUdR, MMUdR-5'-monophosphate (MMUdR-MP) and PFA. The Ara-A-resistant (Ara-AR) virus exhibited 3-fold resistance to MMUdR-MP (ID50 = 105 microM). The ACG-resistant (ACGR) mutant was 160-fold less sensitive to MMUdR (ID50 greater than 1138 microM). The MMUdR-resistant (MMUdRR) mutant remained sensitive to all other antiviral drugs in vitro. Ara-A provided protection against HSV-1 encephalitis in immunosuppressed mice inoculated with a low dose (200 PFU/mouse) of MMUdRR virus or wild-type HSV-1. F3TdR decreased incorporation of tritiated deoxyuridine [( 3H]UdR) in RK-13 cells by 50% at 0.068 microM. Under similar conditions, MMUdR (up to 600 microM) and PFA (up to 208 microM) were without effect on incorporation of [3H]UdR into DNA. In combination chemotherapy experiments, MMUdR (up to 300 microM) used along with F3TdR (up to 1.08 microM) neither decreased nor enhanced cytotoxicity of F3TdR as measured by incorporation of [3H]UdR into cellular DNA. Similarly, MMUdR (up to 300 microM) in combination with PFA (up to 166 microM) was nontoxic to host cells.

Ab initio studies of the antiviral drug 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl) thymine

The antiviral drug 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl) thymine (FMAU) exhibits a high therapeutic index against a number of herpesviruses. As with other drugs which are nucleoside analogs, such as 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl)-5-iodocytosine and others, the activity seems to be strongly related to the presence of a fluorine atom in the sugar moiety, in the arabino position. Theoretical calculations, using quantum-chemical methods, are used to elucidate the role of the fluorine in the arabino position and to provide information about the sugar puckering. The results seem to indicate that the fluorine atom prevents the rotation of the base around the sugar-base bond, locking it into an anti structure (Fig. 2,A3) which might be related to its exposure to enzymatic attack. The sugar study confirms the endo position (above the plane) of the C'2 carbon as opposed to the endo position of the C'3 carbon.

New antiviral compounds

Several potent and selective antiviral agents against herpes virus infections have been developed. However, the majority of compounds against other viral diseases has not yet reached such high standard. Based on progress in molecular virology it can, however, be anticipated that similar concepts of selective inhibition will also be developed for other virus groups. In addition to virus-induced enzymes, viral proteins other than enzymes with specific activities will be identified. The identification of active sites will lead to the design of new and specific inhibitors. Moreover, studies on the mode of action of the huge number of known antiviral compounds may provide the basis for new and potent approaches to specific virus chemotherapy. New inhibitors of viral replication may also be derived from 2'-5'A and other mediators of the interferon induced antiviral state. However, since 2'-5'A does not enter cells, is rapidly degraded by phosphodiesterases, and affects viral and cellular protein synthesis, only analogs which do not have these disadvantages may qualify as antiviral drugs. In addition to refinements at the molecular level quantitative assays for a better evaluation of antiviral agents for clinical use are required. For clinical trials, rapid diagnosis, early initiation of treatment, and quantitative evaluation of the antiviral effects of a drug need to be developed. Moreover, new methods of drug delivery and/or drug targeting will improve potency and selectivity of antiviral compounds. Drug carriers have already successfully been used in cancer therapy (Poste and Fidler, 1981) they should be also applicable to virus chemotherapy. Finally, a better understanding of the pathogenesis and the natural course of viral diseases will contribute to the development of more effective and safe antiviral agents.

Inhibition of human and woodchuck hepatitis virus DNA polymerase by the triphosphates of acyclovir, 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine and E-5-(2-bromovinyl)-2'-deoxyuridine

The triphosphates of acyclovir (ACV), 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine (FIAC) and E-5-(2-bromovinyl)-2'-deoxyuridine (BVdU) have been examined for their inhibitory effects on the endogenous DNA polymerase reactions of human hepatitis B virus (HBV) and woodchuck hepatitis virus (WHV). All three triphosphates (ACVTP, FIACTP and BVdUTP) inhibited the HBV and WHV DNA polymerases by competing with the corresponding natural substrates. FIACTP was the most potent inhibitor of HBV and WHV DNA polymerase while ACVTP was the least effective inhibitor. The inhibitory properties of these compounds were compared with those of the 5'-triphosphates of 1-beta-arabinofuranosyl-cytosine (ara-CTP) and 1-beta-arabinofuranosylthymine (ara-TTP). The 50% inhibitory doses for HBV and WHV DNA polymerases were in the following order: FIACTP less than BVdUTP less than ara-TTP less than ACVTP less than ara-CTP. BVdUTP appeared to be an efficient alternate substrate to dTTP for HBV DNA polymerase while FIACTP was much less efficient when substituted for dCTP. ACVTP did not act as an alternate substrate to dGTP and appeared to prevent DNA chain elongation.

Effect of eight antiviral drugs on the reactivation of herpes simplex virus in explant cultures of latently infected mouse trigeminal ganglia

The effect of several antiviral drugs on the reactivation of herpes simplex virus type 1 in explant cultures of latently infected mouse trigeminal ganglia was investigated. Phosphonoacetate and phosphonoformate, which act directly on the virus-induced DNA polymerase, require a drug concentration of 400 micrograms/ml for the inhibition of virus reactivation in latently infected ganglia. Arabinosyladenine and arabinosyladenine monophosphate, which are phosphorylated to triphosphates by cellular enzymes and inhibit virus synthesis either by blocking the DNA polymerase or by incorporation into viral DNA, require a concentration of only 100 micrograms/ml for the inhibition of the reactivation process. Drugs that are phosphorylated by the virus-induced thymidine kinase, such as acyclovir, arabinosylthymine, bromovinyldeoxyuridine, and three fluorinated pyrimidine nucleosides require the lowest drug concentrations for complete inhibition of virus reactivation in latently infected ganglia explant cultures. Our data suggest that the inhibition of virus reactivation is dependent not only on drug concentration, but also on the number of latently infected neurons in the ganglia.

2'Fluoro-5-iodo-arabinosyl-cytosine: a new agent for herpes infections in the immunosuppressed patient

Concurrent with our increased understanding of mechanisms of viral replication, new antiviral agents were developed with greater selectivity and sensitivity. Acyclovir was the first of these. We now present the initial compound of a series of 2'-Fluoro pyrimidine nucleosides with potent antiviral activity. This agent demonstrates both potency and sensitivity in vitro and in vivo against herpes simplex virus types I and II, and varicella zoster virus and is selective in vitro against cytomegalovirus. Initial clinical trials show the drug to be well tolerated and to be more effective than adenine arabinoside against varicella zoster in the immunosuppressed patient. Future developmental plans with the drug are outlined. FIAC (2'-Fluoro-5-iodo-aracytosine) is still at an early stage of clinical development and any comparison with acyclovir is premature; rather, therapy of severe viral infections in the future should consider combinations of such agents that show differential selectivity at multiple sites of action.

Kinetics of the interaction of monophosphates of the antiviral nucleosides 2'-fluoro-1-beta-D-arabinofuranosylpyrimidine and (E)-5-(2-bromovinyl)-2'-deoxyuridine with thymidylate kinases from Vero cells and herpes simplex virus types 1 and 2

The affinities of the monophosphates of 2'-fluoro-5-iodo-1-beta-D-arabinofuranosyluracil and its 5-methyl analog for cellular thymidylate kinase were two or more orders of magnitude greater than for the thymidine-thymidylate kinases from herpes simplex virus types 1 and 2. In contrast, the monophosphate of (E)-5-(2-bromovinyl)-2'-deoxyuridine was found to have a higher affinity for the viral enzymes than for the cellular enzyme.

Cell-specific antiviral activity of 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl)-5-iodocytosine (FIAC) against Marek's disease herpesvirus and turkey herpesvirus

Three new fluoroarabinosylpyrimidine nucleosides (FIAC, FIAU and FMAU) were tested for in vitro activity against oncogenic and nononcogenic strains of Marek's disease virus (MDV) and herpesvirus of turkeys (HVT). Marek's disease is a herpesvirus-induced lymphoma in chickens. Nononcogenic strains of MDV and HVT can protect against this disease. All viruses were inhibited by 1 microM of these drugs in chick kidney cell (CKC) cultures, but only FMAU and FIAU were active in chicken embryo fibroblast (CEF) and spleen cell cultures. It was determined that whereas CKC produced the enzyme 2'-deoxycytidine-deaminase which is needed to deaminate FIAC to FIAU, CEF were devoid of this enzyme activity. In addition, the deaminase inhibitor 3,4,5,6-tetrahydrouridine prevented the antiviral activity of FIAC in CKC. FMAU was not active against two Marek's disease-derived lymphoblastoid tumor cell lines.

Susceptibility of vaccine strains of varicella-zoster virus to antiviral compounds

Using a plaque reduction assay, we determined the 50% effective doses of six antiviral compounds against low- and high-passage viruses of the KMcC and Oka strains of varicella-zoster virus vaccine. The potency, as indicated by the ranges of 50% effective doses (micrograms per milliliter) of the antiviral compounds, in decreasing order was as follows: (E)-5-(2-bromovinyl)-2'-deoxyuridine, 0.0007 to 0.0035; 1-(2'-flouro-2-deoxy-beta-D-arabinofuranosyl)-5-iodocytosine, 0.0063 to 0.0091; aphidicolin, 0.092 to 0.180; acyclovir, 0.79 to 1.81; vidarabine, 0.62 to 2.10; and phosphonoformic acid, 8.18 to 16.4. Susceptibility to the various antiviral compounds was independent of passage level or strain. These data, along with the available in vivo data, indicate that varicella-zoster virus vaccine infections requiring antiviral therapy most probably would be treated as effectively as would natural varicella infections.

Herpes virus infections in patients with neoplastic disease. Diagnosis and therapy

Herpes viruses are among the most common and troublesome opportunistic pathogens infecting patients with neoplastic diseases. The recent development of partially effective and relatively nontoxic antiviral agents offers promise for the prophylaxis or therapy of these infections in high-risk groups. Vidarabine and acyclovir have shown efficacy in several herpes virus infections and are now licensed in the United States. Alpha interferon may also be useful in the prophylaxis or early therapy of certain herpes virus infections. Newer antiviral agents and combination therapies are under study. Early and rapid diagnosis of such infections is critical to the development of effective therapy.

Efficacy and selectivity of some nucleoside analogs as anti-human cytomegalovirus agents

1-(2'-Deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine (FIAC), 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-methyluridine (FMAU), 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-iodouridine (FIAU), and 1-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-5-ethyluridine (FEAU) were evaluated for antiviral activities against human cytomegalovirus (HCMV) and compared with 9-[(2-hydroxyethoxy)methyl]guanine (acyclovir) and E-5-(2'-bromovinyl)-2'-deoxyuridine (BVDU). The relative anti-HCMV potencies of these compounds, as determined by calculating the dose of drug which inhibited 50% plaque formation, were in order of decreasing potency: FIAC greater than FIAU greater than FMAU greater than acyclovir greater than FEAU greater than BVDU. The antiviral activity of FIAC occurred at levels much lower than those that caused cytotoxic or cytostatic effects in uninfected fibroblasts. Neither thymidine nor deoxycytidine reversed the anti-HCMV activity of FIAC, indicating that this drug was not acting as an analog of the natural nucleosides. FIAC was not phosphorylated by cytosols of HCMV-infected cells to a greater extent that by those of uninfected cells, indicating that, unlike the antiviral activity against herpes simplex virus type 1, the selectivity of this drug is probably not based on a virus-specified pyrimidine kinase.

Antiherpes activity of (E)-5-(2-bromovinyl)- and 5-vinyl-1-beta-D-arabinofuranosyluracil and some other 5-substituted uracil arabinosyl nucleosides in two different cell lines

Of a series of 5-substituted 1-beta-D-arabinofuranosyluracil (5-X-araU) analogues, (E)-5-(2-bromovinyl)-araU(BrVaraU) and 5-vinyl-araU (VaraU) were the most potent inhibitors of plaque formation by two herpes simplex virus type 1 (HSV-1) strains in human embryonic lung fibroblast (HELF) cell cultures. They were not only more active than 5-methyl-araU (MaraU, araT) and 5-ethyl-araU (EaraU), but even more than 1000 times more potent than the 5-fluoro, 5-iodo, 5-formyl and 5-trifluoromethyl (FaraU, IaraU, faraU, CF3araU) analogues. BrVaraU and VaraU were superior to 9-(2-hydroxyethoxymethyl)guanine (Acyclovir, ACV) and comparable in potency with 2'-fluoro-5-iodoaracytosine (FIAC) and 2'-fluoro-5-methylarauracil (FMAU). Their anti-HSV-1 potency was surpassed only by (E)-5-(2-bromovinyl)-2'-deoxyuridine (BrVUdR). Surprisingly, in a HSV-1 plaque inhibition assay in African green monkey kidney (Vero) cells, BrVaraU and VaraU were nearly 100 times active or even inactive. In contrast, the antiherpes activity of ACV, FIAC, FMAU and BrVUdR differed only marginally in the two cell lines. The following order of (decreasing) activity against HSV-2 in HELF cells was found: FIAC = FMAU greater than MaraU (araT) greater than ACV greater than VaraU greater than BrVUdR greater than CF3araU greater than IaraU greater than FaraU = Eara U greater than BrVaraU greater than araU greater than faraU. When deoxyribose is replaced by arabinose in 5-X-UdR analogues, a slight increase in anti-HSV-1-77 activity was observed for the 5-vinyl or 5-ethyl substituent, whereas the other 5-X-araU nucleosides were two to more than 100 times less active than their deoxyribosyl counterparts. However, the sugar exchange led to a strong reduction in anti-HSV-2 activity regardless of the 5-substituent.

Epstein-Barr virus: inhibition of replication by three new drugs

Epstein-Barr virus (EBV) is the cause of infectious mononucleosis and is associated with three human malignancies. Acyclovir [9-(2-hydroxyethoxymethyl)guanine], the first clinically useful drug effective against replication of EBV, is without effect against latent or persistent EBV infection. Three nucleoside analogs, E-5-(2-bromovinyl)-2'-deoxyuridine, 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-iodocytosine, and 1-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-5-methyluracil are potent inhibitors of EBV replication in vitro. Moreover, in contrast to the reversibility of viral inhibition by Acyclovir, these three drugs have prolonged effects in suppressing viral replication even after the drugs are removed from persistently infected cell cultures.

Therapeutic activities of 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl)-5-iodocytosine and -thymine alone and in combination with acyclovir and vidarabine in mice infected intracerebrally with herpes simplex virus

The therapeutic effectiveness of two new antiviral agents, 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl)-5-iodocytosine and 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl)thymine, was compared with that of acyclovir and vidarabine. In mice inoculated intracerebrally with high 50% lethal doses of herpes simplex virus type 2, nontoxic intraperitoneal or oral treatments with the two new fluorinated antiviral agents were highly effective in reducing mortality. The two drugs were also effective when treatment was begun as late as 48 h after virus inoculation. The relative order of potencies of the drugs when compared on a molar basis or in terms of therapeutic index was 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl)thymine much greater than 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl)-5-iodocytosine greater than vidarabine approximately to acyclovir. The new pyrimidine analogs were also found to lack immunosuppressive activity in mice. The combination of 1-(2-fluoro-2-deoxy-beta-=D-arabinofuranosyl)-5-iodocytosine and vidarabine was the most effective; significantly greater reduction in mortality was achieved with this combination than with either drug alone. Thirty minutes after intraperitoneal treatment with the fluorinated analogs, the drugs (or their metabolites) were transported to the brains of virus-inoculated and normal mice at levels about one-third to two thirds those in the blood. The levels of 1-(2-fluoro-2-deoxy-beta-D-arabinofuranosyl)thymine in the blood or brain were consistently higher than those found with equivalent intraperitoneal doses of the 5-iodocytosine analog.

Activity of 1-(2'-fluoro-2'-deoxy-beta-D-arabinofuranosyl)thymine against herpes simplex virus in cell cultures and rabbit eyes

A new antiviral compound 1-(2'-fluoro-2'-deoxy-beta-D-arabinofuranosyl)thymine (2'-fluoro-5-methyl-ara-uracil [FMAU]), was compared with acyclovir and idoxuridine in vitro against two strains of both herpes simplex virus type 1 (HSV-1) and HSV-2. Determinations of the 50% effective dose varied slightly with each strain and with the host cells employed. The 50% effective dose for FMAU and acyclovir against HSV-1 ranged from 0.1 microM to 0.5 to 0.6 microM in rabbit kidney cells and from 0.5 microM to 0.6 to 0.78 microM in Vero cells. Beginning 4 days post-inoculation, topical FMAU therapy given five times per day to rabbits with acute herpetic keratitis either suppressed or delayed the severity of corneal epithelial involvement, conjunctivitis, iritis, and corneal clouding. Responses to treatment with FMAU were similar to those obtained with acyclovir and significantly better than those attained with idoxuridine and vidarabine. At 30 to 40 days after the end of treatment, rabbit eyes were subjected to iontophoresis with epinephrine in an attempt to induce reactivation and enhance detection of previously latent HSV-1. Latent HSV-1 was detected in 67 to 92% of trigeminal ganglia in FMAU-treated animals and in 90% of placebo-treated animals.

Comparison of 2'-fluoro-arabinosyl pyrimidine nucleosides and 1-beta-D-arabinofuranosylcytosine on immunological parameters in vitro

1-beta-D-arabinofuranosylcytosine (ara C)3, 2'-fluoro-5-iodo-arabinofuranosyl-cytosine (FIAC) and 2'-fluoro-5-methyl-arabinofuranosyluracil (FMAU) were analyzed for immunosuppressive activity in vitro. In assay systems quantifying both humoral and cellular immune reactivity, FMAU and FIAC were less immunosuppressive than ara C by several orders of magnitude.

Virus-induced thymidine kinases as markers for typing herpes simplex viruses and for drug sensitivity assays

A rapid, reproducible and objective new method for typing herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) based on the effects of virus-induced thymidine kinases on various antiviral drugs has been developed. When several laboratory strains and clinical isolates were typed by this method and compared to the results obtained by the immunofluorescence antibody typing method, agreement was found for all the viruses. The new technique has the added advantage of determining the sensitivity of HSV strains to antiviral drugs.

Biochemical and biophysical approaches to improving the anticancer effectiveness of Ara-adenine

Ara-C at very low dosage has been reported to decrease the host toxicity of ara-AMP or ara-A in combination with 2'-deoxycoformycin, a potent adenosine deaminase inhibitor, while increasing the toxicity to intracerebral L1210 leukemia. The possibility of increasing the selectivity of ara-A by prior administration of ara-C is explored. The importance of deoxynucleoside kinases, some of which may be cancer-induced, in obtaining selective anticancer effects is discussed. The possibility of a conformational basis for the differing degrees of selectivity and activity of various novel arabinosyl nucleosides is evaluated. The levels of cyclic nucleotides, which have opposing effects on leukemia, may possibly be manipulated to interfere with the growth of cancer cells. Approaches to minimizing major metabolic distortions, such as the progressive accumulation of dATP associated with the use of potent adenosine deaminase inhibitors and which limit the therapeutic effects of ara-A, are proposed.

Efficacy of (E)-5-(2-bromovinyl)-2'-deoxyuridine against different herpes simplex virus strains in cell culture and against experimental herpes encephalitis in mice

(E)-5-(2-Bromovinyl-2'-deoxyuridine (BrVUdR) showed strong antiviral activity against different laboratory strains and clinical isolates of herpes simplex virus type 1 (HSV-1) on primary rabbit testes (PRT) cells with a 50% inhibition of plaque formation (ID50) at 0.01-0.02 microM. One laboratory strain (HSV-1-S), however, was completely refractory even at concentrations as high as 100 microM. In contrast, the ID50S for all herpes simplex virus type 2 (HSV-2) strains were about 10(2) - 10(3) times higher (8-25 microM) than for the HSV-1 strains. No toxicity in mice treated with 140 mg BrVUdR/kg/day for 14 days was observed, and successful treatments of herpes encephalitis in mice induced experimentally by intracerebral infection with one laboratory strain (HSV-1-Kupka) and one clinical isolate (HSV-1-64) were achieved. Treatment of encephalitis in mice induced by the strain HSV-1-S insensitive to BrVUdR in cell culture failed to be effective. Similar antibody titers against HSV-1 were found in surviving mice of the control and of the BrVUdR-treated groups.

Quantitative autoradiographic mapping of herpes simplex virus encephalitis with a radiolabeled antiviral drug

2'-Fluoro-5-methyl-l-beta-D-arabinosyluracil (FMAU) labeled with carbon-14 was used to image herpes simplex virus type 1-infected regions of rat brain by quantitative autoradiography. FMAU is a potent antiviral pyrimidine nucleoside which is selectively phosphorylated by virus-coded thymidine kinase. When the labeled FMAU was administered 6 hours before the rats were killed, the selective uptake and concentration of the drug and its metabolites by infected cells (defined by immunoperoxidase staining of viral antigens) allowed quantitative definition and mapping of HSV-1-infected structures in autoradiograms of brain sections. These results show that quantitative autoradiography can be used to characterize the local metabolism of antiviral drugs by infected cells in vivo. They also suggest that the selective uptake of drugs that exploit viral thymidine kinase for their antiviral effect can, by appropriate labeling, be used in conjunction with clinical neuroimaging techniques to define infected regions of human brain, thereby providing a new approach to the diagnosis of herpes encephalitis in man.

Capacity of deoxycytidine to selectively antagonize cytotoxicity of 5-halogenated analogs of deoxycytidine without loss of antiherpetic activity

Enzyme kinetic studies from this laboratory (M. Dobersen and S. Greer, Biochemistry 17:920-928, 1978) suggested that deoxycytidine could antagonize the toxicity of 5-halogenated analogs of deoxycytidine without interfering with their antiviral activity. Antagonism by deoxycytidine of the toxicity of 5-chlorodeoxycytidine without impairing its anti-herpes simplex virus type 2 activity is demonstrated in the present studies. Tetrahydrouridine, an inhibitor of cytidine deaminase, was utilized. The high Km for deoxycytidine (0.6 mM) with respect to the herpes pyrimidine nucleoside kinase as compared with the low Km for 5-chlorodeoxycytidine (1.1 microM) accounts for the absence of antagonism of the antiviral activity. The high Km for 5-chlorodeoxycytidine (56 microM) as compared with the low Km of deoxycytidine (2 microM) with respect to mammalian deoxycytidine kinase accounts, in great part, for the antagonism of toxicity. In addition, antagonism of toxicity by deoxycytidine is the result of factors other than the kinetic parameters of nucleoside kinases, as indicated by its antagonism of the cytotoxicity of 5-chlorodeoxyuridine. This may be attributed to replenishment of low dCTP pools, diminished because of effector inhibition of ribonucleoside diphosphate reductase by Cl-dUTP. Resistance of the herpes-encoded enzymes to effector control may also play a role in the selective antagonism. Cell culture studies with high concentrations of tetrahydrouridine and 2'-deoxytetrahydrouridine suggest that competition by deoxycytidine for deaminases may not play a major role. The fact that deoxycytidine antagonizes the toxicity of chlorodeoxyuridine also argues against competition for the deaminases as a major reason for its effect. Limited studies with a topical herpes simplex virus type 2 infection system indicate heightened efficacy of 5-chlorodeoxycytidine (and tetrahydrouridine) when deoxycytidine is coadministered. The concepts of selective antagonism of a chemotherapeutic agent derived from these studies may be applied to other approaches that extent beyond viral chemotherapy.

Effect of 9-(2-hydroxyethoxymethyl)guanine on viral-specific polypeptide synthesis in human cytomegalovirus-infected cells

Plaque-forming assay resulted in a 50 percent inhibitory dose by 9-(2-hydroxyethoxymethyl)guanine (acyclovir) against Towne strain human cytomegalovirus (HCMV) of approximately 98 mumol. At the drug concentration of 200 mumol, we did not detect any significant inhibition of viral DNA synthesis by cRNA-DNA hybridization. However, at this drug concentration, the synthesis of at least two viral-specific late polypeptides (150K and 67K) was significantly retarded up to 48 hours after infection, but resumed at 72 hours. These data suggest that acyclovir or its in vivo transformed derivative had a transient effect on viral-specific polypeptide synthesis in HCMV-infected human fibroblasts at a high drug concentration.

C(2')-substituted purine nucleoside analogs. Interactions with adenosine deaminase and purine nucleoside phosphorylase and formation of analog nucleotides

Four C(2')-substituted 2'-deoxyadenosines were examined as substrates for human erythrocytic adenosine deaminase and for formation of intracellular nucleotide analogs in human erythrocytes, lymphocytes and murine Sarcoma 180 cells: 9-(2'-deoxy-2'-fluoro-beta-D-ribofuranosyl)adenine, 9-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)adenine, 9-(2'-azido-2'-deoxy-beta-D-ribofuranosyl)adenine (2'-N3-riboA) and 9-(2-azido-2'-deoxy-beta-D-arabinofuranosyl)adenine. All four adenosine analogs were substrates of human erythrocytic adenosine deaminase, but the corresponding inosine analogs (synthesized by the adenosine deaminase reaction) were highly resistant to cleavage by human erythrocytic purine nucleoside phosphorylase. Only 9-(2'-deoxy-2'-fluoro-beta-D-ribofuranosyl)hypoxanthine underwent very slow phosphorolysis, and no inhibition of inosine phosphorolysis was detected when a 30 microM concentration of any studied inosine analog was added to a reaction mixture containing 30 microM inosine (the Km concentration). Kinetic parameters were determined for the deamination of the adenosine analogs. The greatest affinity for adenosine deaminase was found with 2'-N3-ribo A (Ki = 2 microM), but the reaction velocity was highest with the F-substituted analogs. All four adenosine analogs formed triphosphate nucleotides after incubation with human erythrocytes, murine Sarcoma 180 cells, or human lymphocytes (tested only with the F analogs) in the presence of deoxycoformycin.

Comparative efficacy of antiherpes drugs in different cell lines

A selection of antiherpes compounds including bromovinyldeoxyuridine (BVDU), acyclovir (ACV), and fluoroiodoaracytosine (FIAC) were compared for their inhibitory effects on herpes simplex virus type 1 (strain KOS) replication in a wide variety of human, simian, feline, and murine cell lines. The 50% inhibitory doses of the compounds varied considerably from one cell line to another, i.e., from 0.01 to 2 micrograms/ml for ACV and from 0.004 to 0.2 micrograms/ml for BVDU and FIAC. However, the relative order of antiviral potency remained constant, thus (in order of decreasing potency) BVDU greater than FIAC greater than ACV, for all cell lines studied except for the murine embryo cells, in which the order of decreasing potency was BVDU greater than ACV greater than FIAC, and for the feline lung cells, in which the order of decreasing potency was FIAC greater than ACV greater than BVDU.

Incorporation of metabolites of 2'-fluoro-5-iodo-1-beta-D-arabinofuranosylcytosine into deoxyribonucleic acid of neoplastic and normal mammalian tissues

The radioactivity of 14C-labeled 2'-fluoro-5-iodo-1-beta-D-arabinofuranosylcytosine ([2-14C]-FIAC), a new and potent antiherpetic agent, was shown previously to be incorporated into the DNA fractions of mammalian and neoplastic tissues. The present work was undertaken to learn the nature of the incorporated moieties. Enzyme degradation of highly purified DNA from the small intestine of mice treated with [2-14C]FIAC and analysis of the resulting nucleosides failed to reveal the presence of unchanged FIAC. Rather, three metabolites were found, namely, the 2'-fluoro-1-beta-D-arabinofuranosyl nucleosides of cytosine (FAC), thymine (FMAU) and 5-iodouracil (FIAU). Labeled metabolites of FIAC were also found in the DNA isolated from P815 leukemic cells in mice given [2-14C]FIAC. It is of interest to note that FMAU, FIAU and FAC are, like FIAC, potent antiherpetic agents.