Inactivators of herpes simplex virus ribonucleotide reductase: hematological profiles and in vivo potentiation of the antiviral activity of acyclovir

High-Throughput Screening of the ReFRAME Library Identifies Potential Drug Repurposing Candidates for Trypanosoma cruzi

Chagas disease, caused by the kinetoplastid parasite Trypanosoma cruzi, affects between 6 and 7 million people worldwide, with an estimated 300,000 to 1 million of these cases in the United States. In the chronic phase of infection, T. cruzi can cause severe gastrointestinal and cardiac disease, which can be fatal. Currently, only benznidazole is clinically approved by the FDA for pediatric use to treat this infection in the USA. Toxicity associated with this compound has driven the search for new anti-Chagas agents. Drug repurposing is a particularly attractive strategy for neglected diseases, as pharmacological parameters and toxicity are already known for these compounds, reducing costs and saving time in the drug development pipeline. Here, we screened 7680 compounds from the Repurposing, Focused Rescue, and Accelerated Medchem (ReFRAME) library, a collection of drugs or compounds with confirmed clinical safety, against T. cruzi. We identified seven compounds of interest with potent in vitro activity against the parasite with a therapeutic index of 10 or greater, including the previously unreported activity of the antiherpetic compound 348U87. These results provide the framework for further development of new T. cruzi leads that can potentially move quickly to the clinic.

Effective Prophylactic Therapy for Exposure to Monkey B Virus (Macacine alphaherpesvirus 1)

Zoonotic monkey B virus (Macacine alphaherpesvirus 1; BV) infections are extremely serious and usually fatal. Drugs currently used for treatment were developed for the treatment of herpes simplex virus but are less effective against BV. Effective suppression of viral replication in the skin could prevent the virus from invading the nervous system. To test this hypothesis, the efficacy of topical administration of several drugs against lethal BV infection was evaluated in female BALB/c mice that were infected by scarification. Drugs were then applied to the site of inoculation. As 3% preparations, most drugs were only minimally effective or ineffective. In contrast, ganciclovir and cidofovir were very effective. The ED₅₀ for cidofovir was 0.007%, compared with 1.1% for ganciclovir. At 0.5%, cidofovir protected against both death and neurologic signs, whereas 5% ganciclovir only protected against death but not neurologic involvement. All genotypes of BV were equally susceptible to cidofovir and ganciclovir. For maximal effectiveness, treatment with both cidofovir and ganciclovir had to be initiated within 8 h of infection. Cidofovir was completely protective when administered only on the day of infection, whereas a minimum of 5 d of treatment was required for maximal ganciclovir efficacy. These studies showed that topical cidofovir treatment started soon after BV exposure was very effective in preventing BV from invading the nervous system, whereas ganciclovir treatment was only partially effective. In addition, cidofovir was protective against a ganciclovir-resistant BV mutant, whereas ganciclovir was not. These studies showed that topical cidofovir treatment started soon after BV exposure is more effective than ganciclovir in preventing BV from invading the CNS.

Ribonucleotide reductase inhibitors hydroxyurea, didox, and trimidox inhibit human cytomegalovirus replication in vitro and synergize with ganciclovir

Ganciclovir (GCV) is a deoxyguanosine analog that is effective in inhibiting human cytomegalovirus (HCMV) replication. In infected cells GCV is converted to GCV-triphosphate which competes with dGTP for incorporation into the growing DNA strand by the viral DNA polymerase. Incorporated GCV promotes chain termination as it is an inefficient substrate for elongation. Because viral DNA synthesis also relies on cellular ribonucleotide reductase (RR) to synthesize deoxynucleotides, RR inhibitors are predicted to inhibit HCMV replication. Moreover, as dGTP competes with GCV-triphosphate for incorporation, RR inhibitors may also synergize with GCV by reducing intracellular dGTP levels and there by promoting increased GCV-triphosphate utilization by DNA polymerase. To investigate potential of RR inhibitors as anti-HCMV agents both alone and in combination with GCV, HCMV-inhibitory activities of three RR inhibitors, hydroxyurea, didox, and trimidox, were determined. In both spread inhibition and yield reduction assays RR inhibitors had modest anti-HCMV activity with 50% inhibitory concentrations ranging from 36±1.7 to 221±52μM. However, all three showed significant synergy with GCV at concentrations below their 50% inhibitory and 50% toxic concentrations. These results suggest that combining GCV with relatively low doses of RR inhibitors could significantly potentiate the anti-HCMV activity of GCV in vivo and could improve clinical response to therapy.

Dihydrofolate reductase from Kaposi's sarcoma-associated herpesvirus

Kaposi's sarcoma-associated herpesvirus (KSHV) is the first human virus known to encode dihydrofolate reductase (DHFR), an enzyme required for nucleotide and methionine biosynthesis. We have studied the purified KSHV-DHFR enzyme in vitro and analyzed its expression in cultured B-cell lines derived from primary effusion lymphoma (PEL), an AIDS-associated malignancy. The amino acid sequence of KSHV-DHFR is most similar to human DHFR (hDHFR), but the viral enzyme contains an additional 23 amino acids at the carboxyl-terminus. The viral DHFR, overexpressed and purified from E. coli, was catalytically active in vitro. The K(m) of KSHV-DHFR for dihydrofolate (FH(2)) was 2.4 microM, which is significantly higher than the K(m) of recombinant hDHFR (rhDHFR) for FH(2) (390 nM). K(m) values for NADPH were similar for the two enzymes, about 1 microM. KSHV-DHFR was inhibited by folate antagonists such as methotrexate (K(i): 200 pM), aminopterin (K(i): 610 pM), pyrimethamine (K(i): 29 nM), trimethoprim (K(i): 2.3 microM), and piritrexim (K(i): 3.9 nM). In all cases, K(i) values for these folate antagonists were higher for KSHV-DHFR than for rhDHFR. The viral enzyme was expressed at levels two- to tenfold higher than hDHFR in PEL cell lines as an early lytic cycle gene. KSHV-DHFR mRNA and protein appeared from 6 to 24 h after chemical induction of the KSHV lytic cycle. Epitope-tagged KSHV-DHFR and rhDHFR both localized to the nucleus of transfected cells, while other KSHV nucleotide metabolism genes localized to the cytoplasm. DHFR activity was not essential for viral replication in cultured PEL cells. Since hDHFR was not detectable in peripheral blood mononuclear cells (PBMCs), KSHV-DHFR may function to provide increased DHFR activity in vivo in infected cells that have little or none of their own enzyme.

Enhancement of antiherpetic activity of cytarabine by hydroxyurea in human embryonic skin-muscle fibroblasts

Pharmacological induction of low deoxyribonucleoside triphosphate (dNTP) levels is a novel approach to combine inhibition of virus replication. In accordance with this concept, the alteration of antiherpes activity of cytarabine (AraC) in combination with hydroxyurea (HU), an inhibitor of ribonucleotide reductase, was studied. The results were confirmed by virus yield assays in human embryonic skin-muscle fibroblasts (HESMF). It was demonstrated that HU significantly enhanced the antiviral activity of AraC against HSV-1 and HSV-2 in HESMF cultures and this inhibition was reversed by an excess of deoxycytidine (dCyt). The observations suggest that the reduction in dCTP level accounts for the potentiating effect of HU on the antiviral activity of AraC.

Antiviral activity of a selective ribonucleotide reductase inhibitor against acyclovir-resistant herpes simplex virus type 1 in vivo

The present study reports the activity of BILD 1633 SE against acyclovir (ACV)-resistant herpes simplex virus (HSV) infections in athymic nude (nu/nu) mice. BILD 1633 SE is a novel peptidomimetic inhibitor of HSV ribonucleotide reductase (RR). In vitro, it is more potent than ACV against several strains of wild-type as well as ACV-resistant HSV mutants. Its in vivo activity was tested against cutaneous viral infections in athymic nude mice infected with the ACV-resistant isolates HSV type 1 (HSV-1) dlsptk and PAAr5, which contain mutations in the viral thymidine kinase gene and the polymerase gene, respectively. Following cutaneous infection of athymic nude mice, both HSV-1 dlsptk and PAAr5 induced significant, reproducible, and persistent cutaneous lesions that lasted for more than 2 weeks. A 10-day treatment regimen with ACV given topically four times a day as a 5% cream or orally at up to 5 mg/ml in drinking water was partially effective against HSV-1 PAAr5 infection with a reduction of the area under the concentration-time curve (AUC) of 34 to 48%. The effects of ACV against HSV-1 dlsptk infection were not significant when it was administered topically and were only marginal when it was given in drinking water. Treatment under identical conditions with 5% topical BILD 1633 SE significantly reduced the cutaneous lesions caused by both HSV-1 dlsptk and PAAr5 infections. The effect of BILD 1633 SE against HSV-1 PAAr5 infections was more prominent and was inoculum and dose dependent, with AUC reductions of 96 and 67% against infections with 10(6) and 10(7) PFU per inoculation site, respectively. BILD 1633 SE also significantly decreased the lesions caused by HSV-1 dlsptk infection (28 to 51% AUC reduction). Combination therapy with topical BILD 1633 SE (5%) and ACV in drinking water (5 mg/ml) produced an antiviral effect against HSV-1 dlsptk and PAAr5 infections that was more than the sum of the effects of both drugs. This is the first report that a selective HSV RR subunit association inhibitor can be effective against ACV-resistant HSV infections in vivo.

The effects of interferon-alpha and acyclovir on herpes simplex virus type-1 ribonucleotide reductase

Herpes simplex virus-type 1 (HSV-1) encodes both the small (UL40) and large (UL39) subunits of the enzyme, ribonucleotide reductase. Treatment of HSV-1-infected cells with interferon-alpha (IFN-alpha) reduced the levels of both enzyme subunits. Reduced steady state levels of the large subunit were demonstrated by immunoblot using polyclonal antibody specific for the viral enzyme. Reduction in the amount of small subunit was shown by a reduction in the electron spin resonance signal derived from the iron-containing tyrosyl free-radical present in this subunit. Treatment of cells with 100 IU/ml of IFN-alpha decreased levels of both subunits resulting in a reduction in enzyme activity as measured by conversion of CDP to dCDP. The decrease in the amount of the large subunit was not due to a reduction in the level of its mRNA. The combination of IFN-alpha and ACV treatment of human cornea stromal cells did not result in a further reduction in amounts of ribonucleotide reductase relative to that detected with IFN-alpha alone. The IFN-alpha-induced reduction in ribonucleotide reductase activity is the likely cause of decreased levels of dGTP which we have previously demonstrated in IFN-alpha-treated, infected cells.

In vitro inhibition of human cytomegalovirus replication by desferrioxamine

Desferrioxamine (DFO) is commonly used in therapy as a chelator of ferric ion in disorders of iron overload. We found that DFO inhibits human cytomegalovirus (HCMV) replication in infected cultures of human foreskin fibroblasts (HFF) at concentrations that have been achieved in humans with no significant adverse effects. The concentrations of DFO required for 50 and 90% reduction in the production of a HCMV-late antigen ranged for several HCMV strains from 3.1 to 4.9 microM and from 14.2 to 17.3 microM, respectively. DFO concentration of 60 microM had no significant effect on the viability of HFF cells. Inhibitory effects of DFO on HCMV replication were completely prevented by co-incubation with stoichiometric amounts of Fe3+.

Deletion of the varicella-zoster virus large subunit of ribonucleotide reductase impairs growth of virus in vitro

Cells infected with varicella-zoster virus (VZV) express a viral ribonucleotide reductase which is distinct from that present in uninfected cells. VZV open reading frames 18 and 19 (ORF18 and ORF19) are homologous to the herpes simplex virus type 1 genes encoding the small and large subunits of ribonucleotide reductase, respectively. We generated recombinant VZV by transfecting cultured cells with four overlapping cosmid DNAs. To construct a virus lacking ribonucleotide reductase, we deleted 97% of VZV ORF19 from one of the cosmids. Transfection of this cosmid with the other parental cosmids yielded a VZV mutant with a 2.3-kbp deletion confirmed by Southern blot analysis. Virus-specific ribonucleotide reductase activity was not detected in cells infected with VZV lacking ORF19. Infection of melanoma cells with ORF19-deleted VZV resulted in plaques smaller than those produced by infection with the parental VZV. The mutant virus also exhibited a growth rate slightly slower than that of the parental virus. Chemical inhibition of the VZV ribonucleotide reductase has been shown to potentiate the anti-VZV activity of acyclovir. Similarly, the concentration of acyclovir required to inhibit plaque formation by 50% was threefold lower for the VZV ribonucleotide reductase deletion mutants than for parental virus. We conclude that the VZV ribonucleotide reductase large subunit is not essential for virus infection in vitro; however, deletion of the gene impairs the growth of VZV in cell culture and renders the virus more susceptible to inhibition by acyclovir.

Solar simulator-induced herpes simplex labialis: use in evaluating treatment with acyclovir plus 348U87

Models of UV radiation induced herpes labialis utilizing crude light sources have previously been used to examine the efficacy of antivirals. We sought to improve upon this model by using a solar simulator. Initial studies revealed that 13 of 34 (38%) subjects with a history of recurrent HSV labialis receiving three minimal erythema doses (MED's) of UV light developed herpes labialis. We next evaluated the effects of combined therapy with topical ACV and 348U87, a ribonucleotide reductase inhibitor, begun immediately after UV exposure for the prevention and treatment of herpes labialis. No significant reduction in the incidence or severity of herpes labialis was detected although the study was terminated after the interim analysis revealed no benefit, thus reducing the power to detect a difference. This lack of effect may be explained by the general poor efficacy of topical treatment for recurrent HSV infection. Further studies of ACV + 348U87 in vehicles that should increase the penetration and stability of the drugs are planned.

Aciclovir. A reappraisal of its antiviral activity, pharmacokinetic properties and therapeutic efficacy

Aciclovir (acyclovir) is a nucleoside analogue with antiviral activity in vitro against the herpes simplex viruses (HSV), varicella zoster virus (VZV), Epstein-Barr virus (EBV), cytomegalovirus (CMV) and human herpesvirus 6 (HHV-6). Topical, oral or intravenous aciclovir is well established in the treatment of ophthalmic, mucocutaneous and other HSV infections, with intravenous aciclovir the accepted treatment of choice in herpes simplex encephalitis. The efficacy of aciclovir is increased with early (preferably during the prodromal period) initiation of treatment but, despite significant clinical benefit, viral latency is not eradicated, and pretreatment frequencies of recurrence usually continue after episodic acute treatment is completed. Intravenous administration has also shown benefit in the treatment of severe complications of HSV infection in pregnancy, and neonatal HSV infections. Recurrence of HSV has been completely prevented or significantly reduced during suppressive therapy with oral aciclovir in immunocompetent patients. Use of oral aciclovir is effective but controversial in the treatment of otherwise healthy individuals with varicella (chickenpox), and in some countries it has been recommended for use only in cases which may be potentially severe. The development of rash and pain associated with herpes zoster (shingles) is attenuated with oral or intravenous aciclovir therapy, ocular involvement is prevented, and post-herpetic neuralgia appears to be decreased. Similarly, in a few patients with zoster ophthalmicus, oral aciclovir has reduced the frequency and severity of long term ocular complications and post-herpetic neuralgia, and herpes zoster oticus is improved with intravenous aciclovir. Oral aciclovir has prevented recurrence of HSV genital or orofacial infections during suppressive therapy in > 70% of immunocompetent patients in most clinical trials. Suppression of latent HSV, VZV and CMV infections has been achieved in many immunocompromised patients receiving the oral or intravenous formulations. Aciclovir also appears to offer partial protection from invasive CMV disease in CMV-seropositive bone marrow transplant recipients. The few comparative trials published have shown aciclovir to be at least as effective as other investigated antivirals in the treatment of HSV infections in immunocompetent patients, and more effective than inosine pranobex in the prophylaxis of genital herpes. Similarly, in isolated clinical trials, oral aciclovir appears as effective as topical idoxuridine and oral brivudine in some parameters in immunocompetent patients with VZV infections, and the intravenous formulation appears at least as effective as oral brivudine and intravenous vidarabine in treating these infections in immunocompromised patients.(ABSTRACT TRUNCATED AT 400 WORDS)

Topical treatment of infection with acyclovir-resistant mucocutaneous herpes simplex virus with the ribonucleotide reductase inhibitor 348U87 in combination with acyclovir

The thiocarbonohydrazone 348U87 inactivates herpes simplex virus ribonucleotide reductase and potentiates the activity of acyclovir against wild-type and acyclovir-resistant strains of herpes simplex virus. We treated 10 human immunodeficiency virus-infected patients with acyclovir-resistant anogenital herpes simplex virus infection with a topical preparation of 348U87 (3%) in combination with acyclovir (5%) in an open-labelled study. Transient improvement with combination therapy occurred frequently; however, target lesions reepithelialized completely in only 1 of 10 patients. Termination of study drug therapy was most often due to cessation of therapeutic effect before complete resolution of lesions. As currently formulated, topical 348U87 offers little therapeutic benefit for this indication.

2-Acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (1110U81) potently inhibits human cytomegalovirus replication and potentiates the antiviral effects of ganciclovir

We studied the effects of 2-acetylpyridine 5-[(dimethylamino)thiocarbonyl]-thiocarbonohydrazone (1110U81 or A1110U), a potent inhibitor of the ribonucleotide reductases encoded by herpes simplex virus types 1 and 2 and by varicella-zoster virus, against human cytomegalovirus (HCMV) replication in infected MRC-5 cells. We show that 1110U81 is a potent inhibitor of HCMV DNA replication (50% inhibitory concentration [IC50], 3.6 microM; IC90, 5.6 microM) and also potentiates the effects of ganciclovir (GCV) against HCMV. The IC90 of GCV is reduced from 65 microM when GCV alone is given to 2.8 microM when GCV is combined with 1110U81 at a molar ratio of 1:1.

Potential for combined therapy with 348U87, a ribonucleotide reductase inhibitor, and acyclovir as treatment for acyclovir-resistant herpes simplex virus infection

Inhibitors of the ribonucleotide reductase of herpes simplex viruses (HSV) potentiate the activity of acyclovir in vitro and in animal studies. In addition, the combination of the ribonucleotide reductase inhibitor 348U87 and acyclovir has synergistic therapeutic effects against infections in mice due to thymidine kinase-deficient, thymidine kinase-altered, and DNA polymerase mutants of HSV. We performed a pilot study of topical combination therapy with 348U87 (3%) and acyclovir (5%) cream for acyclovir-resistant, anogenital HSV infections in ten human immunodeficiency virus (HIV)-infected patients. Our results, with lack of complete reepitheliazation of lesions in all patients and poor virologic response, suggest that this therapy is unlikely to be useful for this indication.

Binding of 2-acetylpyridine-5-[(2-chloroanilino)thiocarbonyl] thiocarbonohydrazone (BW348U87) to human serum albumin

Dissociation constants and rate constants for the binding of 2-acetylpyridine-5-[(2-chloroanilino)thiocarbonyl]thiocar bonohydrazone (I, BW348U87), an agent that enhances the antiherpetic efficacy of acyclovir, to human serum albumin have been determined. I quenched the fluorescence of human serum albumin, whereas the absorbance of I at 370 nm increased upon binding to the protein. The absorbance change was attributed to preferential binding of anionic I (pKa 7.0). Titration data indicated multiple binding sites for I. The dissociation constant of the high-affinity site was 0.11 microM. The time course for binding of I to 100 nM human serum albumin was biphasic. The early and late phases were described by first-order rate constants that had maximal values of 100 and 11 sec-1, respectively. The rate constant for the dissociation of I from human serum albumin was estimated to be 6 sec-1. Dodecyl sulfate and octanoate displaced I from human serum albumin.I with rate constants of 4.5 and 7.3 sec-1, respectively. Since the fluorescence emission spectrum of human serum albumin and the absorption spectrum of I overlapped significantly (the spectral overlap integral, J, was 2.6 x 10(-14) M-1 cm3), the possibility of Förster dipole-dipole energy transfer was considered. However, a significant fraction of the fluorescence quenching by I resulted from a conformational change in the protein upon binding of I and was not the result of dipole-dipole energy transfer. Nonetheless, the distance between one of the binding regions for I on human serum albumin and a tryptophan residue in the protein was estimated to be 31 A by this method. The high affinity of I for albumin could be related to its low hematological toxicity.