In vitro and in vivo evidence that the antiviral activity of 2',3'-dideoxycytidine is target cell dependent in a feline retrovirus animal model

Efficacy and Toxicity of Long-Term Administration of 2′,3′-dideoxycytidine in the LP-BM5 Murine-Induced Immunodeficiency Model

The LP-BM5 murine retrovirus-induced immunodeficiency model was used to evaluate the efficacy and toxicity of long-term 2′,3′-dideoxycytidine (DDC) therapy. A mean plasma drug concentration of 0.2 + 0.02 μm of DDC for 3 months was found to reduce splenomegaly, lymphoadenopathy and hypergammaglobulinemia in infected mice. However, DDC also reduced spleen weight in control mice and spleen haemopoiesis in both infected and uninfected animals. In the bone marrow the most prominent feature of DDC treatment was a marked reduction of megakariocytes, while in the liver an hepatocellular vacuolation was evident in uninfected animals. DDC reduced, but did not prevent, LP-BM5 integration in lymph node DNA and Pr 60gag expression in spleen lymphocytes and bone marrow cells. Furthermore, DDC reduced the mitochondrial DNA content and restored the mitogen proliferation of T cells but not that of B cells in infected mice. Thus, DDC appears to be of some, but limited, efficacy in murine AIDS, with a toxicity profile involving more cell types than previously thought.

Prophylactic Efficacy and Bone Toxicity Associated with Phosphonoformate Therapy against Retrovirus Infection

Phosphonoformate (PFA) is a simple pyrophosphate analogue which is a topical and parenteral treatment for human herpes virus infections and is currently undergoing evaluation for treatment of human immunodeficiency virus (HIV) and cytomegalovirus infections associated with (AIDS). In this study, antiretroviral activity of PFA was demonstrated by two separate treatment regimens. In the first, an inoculum of feline leukaemia virus (FeLV) in plasma from viraemic cats was treated with 1024 μM PFA prior to intravenous inoculation into susceptible animals. Three of four cats given the PFA treated inoculum were protected from viraemia by the PFA treatment, while 2 of 2 challenge controls receiving sham treated inoculum and 6 of 6 untreated challenge controls became viraemic. In the second regimen, a long-term continuous intravenous infusion of PFA (1000 mg kg−1 day−1) was administered to 6 young cats beginning 1–2 days prior to and extending 4 weeks following intravenous inoculation with FeLV. Five of the six PFA-treated cats also received heparin intravenously and acetyl salicylic acid (aspirin) orally to reduce risk of thrombosis. Six cats (heparin controls) received only heparin and aspirin and were inoculated with FeLV in an identical manner. Six cats served as untreated challenge controls. Four of 6 PFA-treated cats were protected from FeLV antigenaemia. In contrast, all 6 heparin-control animals and all 6 challenge-control animals became persistently viraemic as evidenced by continuous expression of FeLV p27 antigen. All challenged cats including the 4 protected by PFA treatment developed antibody to FeLV, indicating that PFA did not prevent primary virus infection. Significant toxic effects of PFA treatment were reduced weight-gain and rickets-like bone lesions in the cats receiving the 4 week treatment. Additionally, decreased serum alkaline phosphatase, phosphorus, and calcitriol concentrations, presumably related to the bone lesions, were observed. Results of this study suggest that the antiviral effect of PFA involves an immediate and direct mechanism targeted at cell-free virus and that long-term continuous intravenous infusion of PFA has significant anti-retroviral activity in vivo.

Efficacy of antiviral chemotherapy for retrovirus-infected cats: What does the current literature tell us?

GLOBAL IMPORTANCE

The two feline retroviruses, feline immunodeficiency virus (FIV) and feline leukaemia virus (FeLV), are global and widespread, but differ in their potential to cause disease.

VIRAL INFECTION - FIV

FIV, a lentivirus that shares many properties with human immunodeficiency virus (HIV), can cause an acquired immune deficiency syndrome, which predisposes cats to other infections, stomatitis, neurological disorders and tumours. Although secondary infections are common, specific opportunistic infections or acquired immunodeficiency virus-defining infections, such as those that occur with HIV, are not commonly reported in FIV-infected cats. In most naturally infected cats, FIV does not cause a severe clinical syndrome; with appropriate care, FIV-infected cats can live many years before succumbing to conditions unrelated to their FIV infection. Thus, overall survival time is not necessarily shorter than in uninfected cats, and quality of life is usually high over many years or lifelong.

VIRAL INFECTION - FELV

FeLV, an oncornavirus, is more pathogenic than FIV. Historically, it was considered to account for more disease-related deaths and clinical syndromes in cats than any other infectious agent. Recently, the prevalence and importance of FeLV have been decreasing, mainly because of testing and eradication programmes and the use of FeLV vaccines. Progressive FeLV infection can cause tumours, bone marrow suppression and immunosuppression, as well as neurological and other disorders, and leads to a decrease in life expectancy. However, with appropriate care, many FeLV-infected cats can also live several years with a good quality of life.

PRACTICAL RELEVANCE

A decision regarding treatment or euthanasia should never be based solely on the presence or absence of a retrovirus infection. Antiviral chemotherapy is of increasing interest in veterinary medicine, but is still not used commonly.

EVIDENCE BASE

This article reviews the current literature on antiviral chemotherapy in retrovirus-infected cats, focusing on drugs that are currently available on the market and, thus, could potentially be used in cats.

Feline leukemia virus outbreak in the critically endangered Iberian lynx (Lynx pardinus): high-throughput sequencing of envelope variable region A and experimental transmission

The Iberian lynx is the most endangered felid species. During winter/spring 2006/7, a feline leukemia virus (FeLV) outbreak of unexpected virulence killed about 2/3 of the infected Iberian lynxes. All FeLV-positive animals were co-infected with feline hemoplasmas. To further characterize the Iberian lynx FeLV strain and evaluate its potential virulence, the FeLV envelope gene variable region A (VRA) mutant spectrum was analyzed using the Roche 454 sequencing technology, and an in vivo transmission study of lynx blood to specified-pathogen-free cats was performed. VRA mutations indicated weak apolipoprotein B mRNA editing enzyme and catalytic polypeptide-like cytidine deaminase (APOBEC) restriction of FeLV replication, and variants characteristic of aggressive FeLV strains, such as FeLV-C or FeLV-A/61C, were not detected. Cats exposed to FeLV/Candidatus Mycoplasma haemominutum-positive lynx blood did not show a particularly severe outcome of infection. The results underscore the special susceptibility of Iberian lynxes to infectious diseases.

Antiretroviral efficacy of a 98% solution of glycerol or ethylene oxide for inactivation of feline leukemia virus in bone

OBJECTIVE

To determine whether infectious retrovirus was inactivated in bones from FeLV-infected cats after ethylene oxide (ETO) sterilization or preservation in a 98% solution of glycerol in an in vitro cell culture system.

SAMPLE POPULATION

Metatarsal bones obtained from 5 FeLV-infected cats and cultured with feline fibroblast cells.

PROCEDURE

Metatarsal bones were treated with 100% ETO, a 98% solution of glycerol, or left untreated. Twenty-five flasks of feline fibroblast cells were assigned to 5 groups: negative control, positive control, ETO-treated bone, glycerol-treated bone, and untreated bone with 5 replicates/group for 4 passages. Media and cell samples were harvested from every flask at each passage to measure FeLV p27 antigen and the number of copies of provirus per 100 ng of DNA, respectively.

RESULTS

All negative control and ETO-treated group replicates were negative for FeLV p27 antigen and provirus throughout the study. All positive control group replicates were positive for FeLV p27 antigen and provirus at passages 1 to 4. Untreated bone group replicates were positive for FeLV p27 antigen at passages 3 and 4 and provirus beginning at passage 2. Glycerol-treated group replicates had delayed cell replication and were negative for FeLV p27 antigen and provirus at passages 1 to 4 and 2 to 4, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Ethylene oxide sterilization of bone from FeLV-infected cats appeared to abrogate transmission of infectious retrovirus and effectively sterilized bone allografts. Impact for Human Medicine-Additional studies to confirm effectiveness of ETO treatment of allograft tissues for prevention of pathogen transmission via transplantation are warranted.

Demineralization for inactivation of infectious retrovirus in systemically infected cortical bone: in vitro and in vivo experimental studies

BACKGROUND

Clinical and experimental studies have demonstrated viral transmission through the transplantation of fresh-frozen infected bone. While sterilization methods sufficient to inactivate the human immunodeficiency virus (HIV) have been shown to markedly alter osteoconductive and osteoinductive properties of bone allografts, the ability of a process for creating demineralized bone matrix to abrogate transmission of a retrovirus has not been investigated, to our knowledge. We hypothesized that a clinically accepted demineralization procedure would alter the nucleic acids of the feline leukemia virus (FeLV, a retrovirus with a structure and replication cycle similar to those of HIV), inactivating the virus in infected bone and rendering it noninfectious.

METHODS

Bone infected with FeLV was demineralized with a method employed for creating demineralized bone matrix powder. The effects of demineralization on cellular and (pro)viral nucleic acids were examined with use of gel electrophoresis and quantitative polymerase chain reaction, respectively. To compare the infectivity of the demineralized bone matrix with that of mineralized bone particles in cell cultures and in animals in which they had been implanted, we measured FeLV p27 antigen and (pro)viral nucleic acids as well as antiviral antibodies.

RESULTS

Demineralization of FeLV-infected bone appeared to inactivate the virus by degradation and fragmentation of the DNA, rendering it noninfectious in both in vitro and in vivo test systems. In contrast, untreated mineralized FeLV-infected bone contained intact nucleic acids and readily transmitted the virus in both test systems.

CONCLUSIONS

The demineralization process inactivated infectious retrovirus in infected cortical bone, thereby preventing disease transmission.

Effects of 2',3'-dideoxycytidine and 2',3'-dideoxycytidine 5'-triphosphate on phospholipid metabolism in permeabilized rat hepatocytes

Both 2',3'-dideoxycytidine (ddC) and 2',3'-dideoxycytidine 5'-triphosphate (ddCTP) inhibit the synthesis of the major phospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE) in permeabilized rat hepatocytes. For PC, this appears to be based on competitive inhibition of cholinephosphotransferase (CDPcholine:1,2-diacylglycerol cholinephosphotransferase; EC 2.7.8.2). The study was based on short-term incubations (6-12 min) of the nucleoside/nucleotide analogs with alpha-toxin permeabilized rat hepatocytes. At a concentration of 1 mM, ddC and ddCTP decreased the incorporation of radiolabelled glycerol-3-phosphate into PC by approximately 50% as compared with control. This was accompanied by a significant increase in diacylglycerol labelling. In the presence of 1 mM CDP-ethanolamine and increasing concentrations of ddC(TP) (0.01-1 mM), the incorporation of radiolabelled glycerol-3-phosphate into PE was decreased to approximately 60% of the control value. When both PC and PE synthesis were operative, the inhibition by ddC(TP) was restricted to PC synthesis. ddC and ddCTP were found to have inhibition constants (K(i)) of 496 microM and 452 microM, respectively, for the inhibition of PC synthesis from CDP-choline. Although the inhibitory concentrations of the nucleoside analog and its triphosphate ester are much higher than the in vivo plasma concentrations, the possibility is raised that the peripheral neuropathy, seen as a dose-dependent adverse effect of ddC treatment in acquired immunodeficiency syndrome therapy is, at least partly, caused by a perturbation of the phospholipid constitution of neuronal membranes.

Evidence that high-dosage zidovudine at time of retrovirus exposure reduces antiviral efficacy

The antiviral efficacy of prophylactic 3'-azido-3'-deoxythymidine (AZT) therapy administered by continuous infusion or intermittent injection was compared in pediatric cats infected with feline leukemia virus. A 4-week treatment regimen of AZT was initiated at -48, 8, or 96 h postinfection (p.i.). For AZT therapy begun at -48 h p.i., significant efficacy was attained when therapy was given by continuous infusion but not by intermittent injection. However, when AZT therapy was delayed until 96 h p.i., both continuous infusion and intermittent injection gave complete protection. The results suggest that intermittent AZT administration is less efficacious than continuous infusion. Higher peak AZT concentrations in plasma associated with intermittent injection compared with those associated with continuous infusion may be immunotoxic, thus reducing the drug-induced vaccine effect. Furthermore, AZT toxicity seemed to be restricted to a window of sensitivity close to the time of virus challenge because delaying the start of AZT therapy until 96 h p.i. was highly efficacious, regardless of the method of administration.

Differential antiviral activities and intracellular metabolism of 3'-azido-3'-deoxythymidine and 2',3'-dideoxyinosine in human cells

Dideoxynucleosides such as 3'-azido-3'-deoxythymidine (AZT) and 2',3'-dideoxyinosine (ddI) can effectively inhibit the replication of human immunodeficiency virus (HIV) in T lymphoid cells. There is evidence that HIV can infect and replicate in other cells including monocytoid cells and macrophages. The present study compared the antiretroviral activities of ddI and AZT in three lineages of human cells, i.e., MOLT4 (T lymphocytoid, CD4+), U937 (monocytoid, CD4+), and HT1080 (fibroblastoid, CD4-) cells. Feline leukemia virus, a retrovirus that causes immunodeficiency in cats, was used to infect the cells. The drug concentrations needed to reduce the viral p27 antigen titers in cell lysates by 50% (IC50s) were determined. The data show that AZT and ddI inhibited viral replication in all three cell lines. The IC50s of AZT were 0.02, 1.75, and 2.31 microM in MOLT4, HT1080, and U937 cells, respectively. For ddI, the IC50s were 4.31, 9.52, and 43.5 microM, respectively. These data indicate differential antiviral activities of ddI and AZT in the different cells with the following rank order of drug sensitivity: MOLT4 > HT1080 > U937. A study of the intracellular metabolism of [3H]AZT and [3H]ddI shows that the antiretroviral activities of AZT and ddI in the three cell lines correlated with the levels of their intracellular triphosphate metabolites.

Antiviral agents: characteristic activity spectrum depending on the molecular target with which they interact

The target protein (enzyme) with which antiviral agents interact determines their antiviral activity spectrum. Based on their activity spectrum, antiviral compounds could be divided into the following classes: (1) sulfated polysaccharides (i.e., dextran sulfate), which interact with the viral envelope glycoproteins and are inhibitory to a broad variety of enveloped viruses (i.e., retro-, herpes-, rhabdo-, and arenaviruses): (2) SAH hydrolase inhibitors (i.e., neplanocin A derivatives), which are particularly effective against poxvirus, (-)RNA viruses (paramyxovirus, rhabdovirus), and (+/-)RNA virus (reovirus); (3) OMP decarboxylase inhibitors (i.e., pyrazofurin) and CTP synthetase inhibitors (i.e., cyclopentenylcytosine), which are active against a broad range of DNA, (+)RNA, (-)RNA, and (+/-)RNA viruses; (4) IMP dehydrogenase inhibitors (i.e., ribavirin), which are also active against various (+)RNA and (-)RNA viruses and, in particular, ortho- and paramyxoviruses; (5) acyclic guanosine analogs (i.e., ganciclovir) and carbocyclic guanosine analogs (i.e., cyclobut-G), which are particularly active against herpesviruses (i.e., HSV-1, HSV-2, VZV, CMV); (6) thymidine analogs (i.e., BVDU, BVaraU), which are specifically active against HSV-1 and VZV because of their preferential phosphorylation by the virus-encoded thymidine kinase; (7) acyclic nucleoside phosphonates (i.e., HPMPA, HPMPC, PMEA, FPMPA), which, depending on the structure of the acyclic side chain, span an activity spectrum from DNA viruses (papova-, adeno-, herpes-, hepadna-, and poxvirus) to retroviruses (HIV); (8) dideoxynucleoside analogs (i.e., AZT, DDC), which act as chain terminators in the reverse transcriptase reaction and thus block the replication of retroviruses as well as hepadnaviruses; and (9) the TIBO, HEPT, and other TIBO-like compounds, which interact specifically with the reverse transcriptase of HIV-1 and thus block the replication of HIV-1, but not of HIV-2 or any other retrovirus.

Pre- and postexposure chemoprophylaxis: evidence that 3'-azido-3'-dideoxythymidine inhibits feline leukemia virus disease by a drug-induced vaccine response

The benefits of postexposure 3'-azido-3'-dideoxythymidine (AZT) prophylaxis following human immunodeficiency virus exposure are unknown. We describe a comprehensive assessment of pre- and postexposure AZT therapy in the feline leukemia virus (FeLV)-cat model for AIDS which included in vitro testing, an in vivo dose-response titration, a postexposure treatment study, plasma drug concentration determinations, and evaluation of the immune response to FeLV. In in vitro studies, AZT prevented FeLV infection of a feline T-lymphoid cell line, giving 50 and 90% inhibition concentrations of 4.6 and 11.1 mM, respectively. In all of the in vivo efficacy studies, AZT was administered by continuous subcutaneous infusion for 28 days. AZT toxicity was excessive at a dosage of 120 mg/kg of body weight per day, causing acute anemia, but AZT was tolerable at 60 mg/kg/day. In preexposure studies, AZT was efficacious in preventing chronic antigenemia at a dosage of > or = 15 mg/kg/day, at which plasma AZT concentrations averaged between 0.51 and 0.81 micrograms/ml (2.13 and 3.03 microM). As a postexposure treatment, at 60 mg/kg/day, AZT prevented chronic FeLV antigenemia when treatment was started up to 96 h post-virus inoculation (p.i.), but not when treatment was started at 192 h p.i. The 4-day period between 96 and 192 h p.i. appears to be critical for establishing chronic viremia. It is presumed that the increase in virus load between 4 and 8 days p.i. was able to overwhelm the immunologic functions responsible for containment of FeLV infection, even though AZT therapy effectively controlled viremia during the treatment period. The antibody response to FeLV varied depending on the time of AZT treatment initiation relative to virus challenge. When AZT treatment was started 48 h before or 8 h after FeLV challenge, antibodies to FeLV were not detected until after AZT treatment was discontinued at 28 days p.i. Following AZT treatment, however, antibody titers rapidly increased at a rate suggestive of a secondary immune response. When AZT treatment was initiate at later time points relative to virus challenge (24, 48, and 96 h p.i.), antibodies to FeLV became detectable during the treatment period. These results indicate that AZT treatment does not completely prevent FeLV infection, even when treatment begins before virus challenge, and that immune sensitization to FeLV proceeds during the prophylactic drug treatment period.

Prospects for Antiviral Chemotherapy in Veterinary Medicine: 1. Feline Virus Diseases

This paper, which is published in two parts, reviews the literature pertaining to antiviral chemotherapy of viruses of veterinary importance. While early reports in the 1970s referred to the chemotherapy of a number of different RNA and DNA viruses, there was considerable focus in the 1980s, initially on herpesviruses and latterly on retroviruses, particularly in cats. Details are given of the successful treatments of FeLV and FIV, which have been used as animal models for HIV therapy. The high costs of developing and registering a new chemical entity, especially for food species, in which extensive toxicity/residue data are required, is the main reason why specific antiviral compounds are not currently available for veterinary use, although some non-specific immune modulators are now emerging. Concurrent availability of appropriate diagnostic tools is a prerequisite for successful veterinary antiviral chemotherapy, as is a greater understanding of the pathogenesis of virus infections in animals and the development of more sophisticated means of drug delivery, appropriate to both food animal species and companion animals. Additionally, antiviral agents are valuable as research tools per se, as opposed to solely as chemotherapeutic agents.

Evaluation of antiviral activity and toxicity of dextran sulfate in feline leukemia virus-infected cats

The feline leukemia virus (FeLV) disease model was used to conduct a toxicity and antiretrovirus efficacy trial of dextran sulfate (DS; molecular mass, 7,000 to 8,000 Da). In vitro, FeLV infection of feline lymphoid cells was inhibited by 10 micrograms of DS per ml. DS was administered to cats by continuous intravenous infusion at doses of 600, 120, 24, or 4.8 mg/kg of body weight per day, beginning 24 h before FeLV challenge. Doses of 24 mg/kg/day and more were excessively toxic, causing intestinal lesions and death. Similar changes were observed in unchallenged animals receiving 24 mg/kg/day, indicating that toxicity was DS mediated. The dosage of 4.8 mg/kg/day was subtoxic but did not prevent the induction and persistence of FeLV viremia. The results demonstrate that DS by continuous intravenous infusion is excessively toxic at high doses and ineffective at preventing FeLV infection at a subtoxic dose in the FeLV cat model.