The immunosuppressive agent mycophenolate mofetil markedly potentiates the activity of lobucavir [1R(1alpha,2beta,3alpha)]-9-[2,3-bis(hydroxymethyl)cyclobutyl]guanine against different herpes viruses

Toward inhibition of human cytomegalovirus replication with compounds targeting cellular proteins

Antiviral therapy for human cytomegalovirus (HCMV) currently relies upon direct-acting antiviral drugs. However, it is now well known that these drugs have shortcomings, which limit their use. Here I review the identification and investigation of compounds targeting cellular proteins that have anti-HCMV activity and could supersede those anti-HCMV drugs currently in use. This includes discussion of drug repurposing, for example the use of artemisinin compounds, and discussion of new directions to identify compounds that target cellular factors in HCMV-infected cells, for example screening of kinase inhibitors. In addition, I highlight developing areas such as the use of machine learning and emphasize how interaction with fields outside virology will be critical for development of anti-HCMV compounds.

BK Virus in Kidney Transplant Recipients: The Influence of Immunosuppression

The incidence of BK virus infection in kidney transplant recipients has increased over recent decades, coincident with the use of more potent immunosuppression. More importantly, posttransplant BK virus replication has emerged as an important cause of graft damage and subsequent graft loss. Immunosuppression has been accepted as a major risk for BK virus replication. However, the specific contribution of individual immunosuppressive medications to this risk has not been well established. The purpose of this paper is to provide an overview of the recent literature on the influence of the various immunosuppressant drugs and drug combinations on posttransplant BK virus replication. Evidence supporting the various immunosuppression reduction strategies utilised in the management of BK virus will also be briefly discussed.

Herpes simplex pneumonia: Combination therapy with oral acyclovir and aerosolized ribavirin in an immunocompetent patient

BACKGROUND

Herpes simplex viruses (HSVs) are known to cause respiratory tract infections in immunocompromised hosts and, in rare instances, in immunocompetent hosts. Numerous in vitro and in vivo studies have shown that aerosolized administration of ribavirin can potently and selectively inhibit viral replication in pulmonary disease, thereby increasing the effectiveness of acyclovir in HSV.

OBJECTIVE

In this case study, we reported on a 46-year-old immunocompetent woman with HSV type 1 pneumonia with bilateral pulmonary infiltrates but without mucocutaneous lesions.

METHODS

The diagnosis was confirmed using cytology, viral culture, and serology. Because of the persistence of fever and dyspnea, we chose an antiviral therapy. The patient received oral acyclovir and aerosolized ribavirin to improve the antiviral effectiveness of the acyclovir and to reduce the symptoms and the time to resolution of the pulmonary disease.

RESULTS

After 3 days of therapy, dyspnea and fever decreased and hypoxemia improved. After 2 weeks, computed tomography showed complete resolution of pulmonary abnormalities. The patient did not report any adverse effects.

CONCLUSIONS

In our case study, we demonstrated that therapy with a combination of aerosolized ribavirin and oral acyclovir may be useful to reduce the severity of viral infection, the adverse effects, and the days of hospitalization. To our knowledge, this is the first report in the literature of the synergistic effects of the combination of aerosolized ribavirin and oral acyclovir in the treatment of an immunocompetent patient with HSV pneumonia.

Immunosuppression in hepatitis B virus and hepatitis C virus transplants: special considerations

The management of the immunosuppression treatment must take account its consequences on viral replication. Such treatment operates on the emerging balance between the recurrence of the virus on the graft and the immune response of the host. Randomized and prospective trials are currently ongoing with the purpose of determining the opportunity and relevance of each immunosuppressive agent in the treatment. In HBV patients, good control of HBV reinfection by prophylactic strategies using HBIG, lamivudine, or both have decreased the impact of immunosuppression on HBV recurrence. In contrast, HCV recurrence is now a major problem. The mechanisms of viral recurrence need to be deepened thus requiring new studies. The absence of in vitro and in vivo systems to study HCV reinfection is a lack in the comprehension of the relation between HCV and immunosuppression. It will allow adapting the effectiveness of the immunosuppression treatment. The treatment's primary target is to avoid graft rejection, and its secondary objective is to limit the risk of viral recurrence.

Erosive enterocolitis in mycophenolate mofetil-treated renal-transplant recipients with persistent afebrile diarrhea

BACKGROUND

Diarrhea is the most frequently reported adverse event in mycophenolate mofetil (MMF)-treated transplant patients. The aim of this study was to explore the gastrointestinal tract in MMF-treated renal transplant recipients with persistent afebrile diarrhea to characterize its nature and etiology.

METHODS

Renal transplant recipients with persistent afebrile diarrhea (daily fecal output >200 g) were prospectively investigated for infections, morphologic, and functional (gastrointestinal motility and intestinal absorptive capacity) integrity of the gastrointestinal tract; 26 patients met the inclusion criteria.

RESULTS

All but one patient had an erosive enterocolitis. Seventy percent of the patients had malabsorption of nutrients, contributing to the diarrhea. In +/-60%, an infectious origin was demonstrated and successfully treated with antimicrobial agents without changes in immunosuppressive regimen. In +/-40%, no infection occurred, but a Crohn's disease-like pattern of inflammation was noted. These patients also had a less pronounced bile-acid malabsorption but a significant faster colonic transit time, correlating with the trough level of mycophenolic acid (MPA). Cessation of MMF, however, was associated with allograft rejection in one third of these patients.

CONCLUSIONS

Persistent afebrile diarrhea in renal transplant recipients is characterized by erosive enterocolitis, which is of infectious origin in +/-60%. In +/-40%, a Crohn's disease-like (entero-)colitis was present. Because reduction or cessation of MMF was the only effective therapy, MPA or one of its metabolites may be suggested as a possible cause. However, reduction or cessation of MMF was associated with an increased risk for rejection.

Antiviral agents active against human herpesviruses HHV-6, HHV-7 and HHV-8

A series of antiviral compounds were examined for their activity against human herpesvirus type 6 (HHV-6), type 7 (HHV-7) and type 8 (HHV-8). They were selected either because they are already approved for clinical use in the treatment of herpesvirus infections (acyclovir, valaciclovir, penciclovir, famciclovir, ganciclovir, brivudin, foscarnet and cidofovir) or have demonstrated marked activity against herpesviruses (lobucavir, H2G, A-5021, D/L-cyclohexenyl G and S2242). In view of their host cell specificity, different cells and assays had to be used for determining antiviral activity against these three viruses. The most potent compounds with the highest antiviral selectivity index were: (i) for HHV-6; foscarnet, S2242, A-5021 and cidofovir; (ii) for HHV-7; S2242, cidofovir and foscarnet; and (iii) for HHV-8; S2242, cidofovir and ganciclovir. As mycophenolic acid has been shown to enhance significantly the activity of acyclic guanosine analogues (such as acyclovir, penciclovir and ganciclovir) in vitro against HSV-1, HSV-2, VZV and HCMV, it would seem worth evaluating whether mycophenolic acid also potentiates the activity of these acyclic guanosine analogues against HHV-6, -7 and -8.

VX-497: a novel, selective IMPDH inhibitor and immunosuppressive agent

Inosine monophosphate dehydrogenase (IMPDH) is an essential rate-limiting enzyme in the purine metabolic pathway, catalyzing the de novo synthesis of guanine nucleotides required for lymphocyte proliferation. IMPDH has therefore been an attractive target for developing immunosuppressive drugs (e.g., CellCept and mizoribine). Here we describe the immunosuppressive activity of VX-497, a novel noncompetitive inhibitor of IMPDH. VX-497 (MW 452.5) is orally bioavailable and inhibits the proliferation of primary human, mouse, rat, and dog lymphocytes at concentrations of approximately 100 nM. The inhibitory effect of VX-497 on lymphocytes is reversed in the presence of exogenous guanosine, but not in the presence of adenosine or uridine, confirming that the antilymphocytic activity of VX-497 is specifically due to inhibition of IMPDH. The antiproliferative effect of VX-497 in cells is also reversed within 48 h of its removal. Based on evaluation of VX-497 in several lymphoid and nonlymphoid cells, the antiproliferative effect of VX-497 is observed to be most pronounced on lymphoid and keratinocyte cells as compared with fibroblasts. In vivo, oral administration of VX-497 inhibits the primary IgM antibody response in a dose-dependent manner, with an ED(50) value of approximately 30-35 mg/kg in mice. Single daily dosing of VX-497 is observed to be as effective as twice-daily dosing in this model of immune activation. These studies demonstrate that VX-497 is a potent, specific, and reversible IMPDH inhibitor that selectively inhibits lymphocyte proliferation.

Acyclic/carbocyclic guanosine analogues as anti-herpesvirus agents

Several guanosine analogues, i.e. acyclovir (and its oral prodrug valaciclovir), penciclovir (in its oral prodrug form, famciclovir) and ganciclovir, are widely used for the treatment of herpesvirus [i.e. herpes simplex virus type 1 (HSV-1), and type 2 (HSV-2), varicella-zoster virus (VZV) and/or human cytomegalovirus (HCMV)] infections. In recent years, several new guanosine analogues have been developed, including the 3-membered cyclopropylmethyl and -methenyl derivatives (A-5021 and synguanol) and the 6-membered D- and L-cyclohexenyl derivatives. The activity of the acyclic/carbocyclic guanosine analogues has been determined against a wide spectrum of viruses, including the HSV-1, HSV-2, VZV, HCMV, and also human herpesviruses type 6 (HHV-6), type 7 (HHV-7) and type 8 (HHV-8), and hepatitis B virus (HBV). The new guanosine analogues (i.e. A-5021 and D- and L-cyclohexenyl G) were found to be particularly active against those viruses (HSV-1, HSV-2, VZV) that encode for a specific thymidine kinase (TK), suggesting that their antiviral activity (at least partially) depends on phosphorylation by the virus-induced TK. Marked antiviral activity was also noted with A-5021 against HHV-6 and with D- and L-cyclohexenyl G against HCMV and HBV. The antiviral activity of the acyclic/carbocyclic nucleoside analogues could be markedly potentiated by mycophenolic acid, a potent inhibitor of inosine 5'-monophosphate (IMP) dehydrogenase. The new carbocyclic guanosine analogues (i.e. A-5021 and D- and L-cyclohexenyl G) hold great promise, not only as antiviral agents for the treatment of herpesvirus infections, but also an antitumor agents for the combined gene therapy/chemotherapy of cancer, provided that (part of) the tumor cells have been transduced by the viral (HSV-1, VZV) TK gene.

The anti-herpesvirus activity of (1'S,2'R)-9-[[1',2'-bis(hydroxymethyl)-cycloprop-1'-yl]methyl]guanine is markedly potentiated by the immunosuppressive agent mycophenolate mofetil

Mycophenolic acid (MPA), the active form of the immunosuppressive agent mycophenolate mofetil (MMF), was found to markedly potentiate the anti-herpesvirus activity of the novel anti-herpesvirus agent A-5021, (1'S,2'R)-9-[[1',2'-bis(hydroxymethyl)cycloprop-1'-yl]methyl]guanine. For example, at a concentration of 1 microg/ml MPA, the activity of A-5021 against HSV-1, HSV-2 and TK(-) HSV-1 increased by a factor of 130, 14 and > or = 189, respectively. Exogenously added guanosine reversed this potentiating effect, suggesting that a depletion of the endogenous dGTP pools enhanced the inhibitory effect of the 5'-triphosphate metabolite of A-5021 on the viral DNA polymerase. The combined effect of A-5021 and MPA on the growth of uninfected Vero cells was additive rather than synergistic. The combination of topically applied MMF (5%) with 0.05% A-5021 (a subactive concentration) completely protected against HSV-1-induced cutaneous lesions in hairless mice, whereas therapy with either compound used alone had no protective effect. These findings may have implications for those transplant recipients that receive MMF as (part of) their immunosuppressive therapy and that develop intercurrent herpesvirus infections for which they need treatment.

Ribavirin and mycophenolic acid potentiate the activity of guanine- and diaminopurine-based nucleoside analogues against hepatitis B virus

Mycophenolic acid [the active metabolite of the immunosuppressive agent mycophenolate mofetil (MMF)] and ribavirin were found to potentiate the anti-HBV activity of the guanine-based nucleoside analogues penciclovir (PCV), lobucavir (LBV) and 3'-fluorodideoxyguanosine (FLG) and diaminopurine dioxolane (DAPD). Ribavirin and mycophenolic acid are both inhibitors of inosine 5'-monophosphate dehydrogenase and cause a depletion of intracellular dGTP levels. It may be assumed that the 5'-triphosphorylated derivatives of the guanine-based nucleoside analogues, in the presence of reduced levels of dGTP, inhibit more efficiently the priming reaction as well as the reverse transcription and DNA-dependent DNA polymerase activity of the HBV polymerase. This assumption is corroborated by the observation that exogenously added guanosine reversed the potentiating effect of ribavirin and mycophenolic acid on the anti-HBV activity of the guanosine analogues. Our observations may have implications for those (liver) transplant recipients that receive MMF as (part of their) immunosuppressive regimen and that, because of de novo or persistent infection with HBV, need specific anti-HBV therapy.

Guanosine analogues as anti-herpesvirus agents

Several guanosine analogues, i.e. acyclovir (and its oral prodrug valaciclovir), penciclovir (in its oral prodrug form, famciclovir) and ganciclovir, are widely used for the treatment of herpesvirus (i.e. HSV-1, HSV-2, VZV and HCMV) infections. In recent years, several new guanosine analogues have been developed, including the 3-membered (cyclopropyl) sugar derivative A-5021 and the 6-membered D- and L-cyclohexenyl derivatives. Prominent features shared by all guanosine analogues are the following. They depend for their phosphorylation on the virus-encoded thymidine kinase (TK), which makes them particularly effective against those viruses (HSV-1, HSV-2 and VZV) that encoded for such TK. They are also active against HCMV, whether or not they are subject of phosphorylation by the HCMV-induced UL97 protein kinase. Their antiviral activity can be markedly potentiated by mycophenolic acid, an IMP dehydrogenase inhibitor, and they hold great promise, not only as antiviral agents for the treatment of herpesvirus infections, but also as antitumor agents for the combined gene therapy/chemotherapy of cancer, provided that (part of) the tumor cells have been transfected by the viral TK gene.

Effect of antimetabolite drugs of nucleotide metabolism on the anti-human immunodeficiency virus activity of nucleoside reverse transcriptase inhibitors

A number of attempts are currently underway to combine antimetabolite drugs of nucleotide metabolism with a nucleoside reverse transcriptase inhibitor (NRTI) targeting human immunodeficiency virus (HIV) to improve the antiviral efficacy of the NRTIs and to better control HIV drug resistance. Hydroxyurea, a ribonucleotide reductase inhibitor, is currently combined with the NRTI didanosine (2',3'-dideoxyinosine) in clinical trials. However, other cellular target enzymes, including thymidylate synthase, inosinate dehydrogenase, cytidine-5'-triphosphate synthetase, and other enzymes from the de novo nucleotide biosynthesis pathway, can also be considered to potentiate the antiviral action of NRTIs. The underlying reasons for the potentiation of the antiviral activity of the NRTIs by antimetabolite drugs of nucleotide metabolism can be multiple. Decreased endogenous 2'-deoxynucleoside-5'-triphosphate (dNTP) pools result in a better competition of the NRTI (as its triphosphate derivative), with the dNTPs for the virus-encoded reverse transcriptase to be recognized as a substrate for the DNA polymerization reaction and subsequently to be incorporated into the growing viral DNA chain. Also, an increased metabolism (phosphorylation) of the NRTI by stimulatory enzyme feedback mechanisms may result in the production of higher levels of NRTI triphosphate. Thus, higher intracellular ratios of NRTI-triphosphate/dNTP created by well-defined combinations of NRTIs and antimetabolite drugs enable a more profound inhibitory effect of the NRTI against the reverse transcriptase (and thus, against the virus) and a better suppression of resistant (mutant) virus strains. A profound evaluation of this relatively new concept in the clinical setting will reveal whether this approach will establish a place in future treatment modalities of HIV infections.

Hydroxyurea potentiates the antiherpesvirus activities of purine and pyrimidine nucleoside and nucleoside phosphonate analogs

Hydroxyurea has been shown to potentiate the anti-human immunodeficiency virus activities of 2',3'-dideoxynucleoside analogs such as didanosine. We have now evaluated in vitro the effect of hydroxyurea on the antiherpesvirus activities of several nucleoside analogs (acyclovir [ACV], ganciclovir [GCV], penciclovir [PCV], lobucavir [LBV], (R)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine [H2G], and brivudin and nucleoside phosphonate analogs (cidofovir [CDV] and adefovir [ADV]). When evaluated in cytopathic effect (CPE) reduction assays, hydroxyurea by itself had little effect on CPE progression and potentiated in a subsynergistic (herpes simplex virus type 1 [HSV-1]) to synergistic (HSV-2) fashion the antiviral activities of ACV, GCV, PCV, LBV, H2G, ADV, and CDV. Hydroxyurea also caused marked increases in the activities of ACV, GCV, PCV, LBV, and H2G (compounds that depend for their activation on a virus-encoded thymidine kinase [TK]) against TK-deficient (TK(-)) HSV-1. In fact, in combination with hydroxyurea the 50% effective concentrations of these compounds for inhibition of TK(-) HSV-1-induced CPE decreased from values of 20 to > or = 100 microg/ml (in the absence of hydroxyurea) to values of 1 to 5 microg/ml (in the presence of hydroxyurea at 25 to 100 microg/ml). When evaluated in a single-cycle virus yield reduction assay, hydroxyurea at a concentration of 100 microg/ml inhibited progeny virus production by 60 to 90% but had little effect on virus yield at a concentration of 25 microg/ml. Under these assay conditions hydroxyurea still elicited a marked potentiating effect on the antiherpesvirus activities of GCV and CDV, but this effect was less pronounced than that in the CPE reduction assay. It is conceivable that the potentiating effect of hydroxyurea stems from a depletion of the intracellular deoxynucleoside triphosphate pools, thus favoring the triphosphates of the nucleoside analogues (or the diphosphates of the nucleoside phosphonate analogues) in their competition with the natural nucleotides at the viral DNA polymerase level. The possible clinical implications of these findings are discussed.