Stereoselective phosphorylation of cyclopropavir by pUL97 and competitive inhibition by maribavir

Anti-CMV therapy, what next? A systematic review

Human cytomegalovirus (HCMV) is one of the main causes of serious complications in immunocompromised patients and after congenital infection. There are currently drugs available to treat HCMV infection, targeting viral polymerase, whose use is complicated by toxicity and the emergence of resistance. Maribavir and letermovir are the latest antivirals to have been developed with other targets. The approval of letermovir represents an important innovation for CMV prevention in hematopoietic stem cell transplant recipients, whereas maribavir allowed improving the management of refractory or resistant infections in transplant recipients. However, in case of multidrug resistance or for the prevention and treatment of congenital CMV infection, finding new antivirals or molecules able to inhibit CMV replication with the lowest toxicity remains a critical need. This review presents a range of molecules known to be effective against HCMV. Molecules with a direct action against HCMV include brincidofovir, cyclopropavir and anti-terminase benzimidazole analogs. Artemisinin derivatives, quercetin and baicalein, and anti-cyclooxygenase-2 are derived from natural molecules and are generally used for different indications. Although they have demonstrated indirect anti-CMV activity, few clinical studies were performed with these compounds. Immunomodulating molecules such as leflunomide and everolimus have also demonstrated indirect antiviral activity against HCMV and could be an interesting complement to antiviral therapy. The efficacy of anti-CMV immunoglobulins are discussed in CMV congenital infection and in association with direct antiviral therapy in heart transplanted patients. All molecules are described, with their mode of action against HCMV, preclinical tests, clinical studies and possible resistance. All these molecules have shown anti-HCMV potential as monotherapy or in combination with others. These new approaches could be interesting to validate in clinical trials.

DNA polymerases of herpesviruses and their inhibitors

Human herpesviruses are large double-stranded DNA viruses belonging to the Herpesviridae family. The main characteristics of these viruses are their ability to establish a lifelong latency into the host with a potential to reactivate periodically. Primary infections and reactivations with herpesviruses are responsible for a large spectrum of diseases and may result in severe complications in immunocompromised patients. The viral DNA polymerase is a key enzyme in the replicative cycle of herpesviruses, and the target of most antiviral agents (i.e., nucleoside, nucleotide and pyrophosphate analogs). However, long-term prophylaxis and treatment with these antivirals may lead to the emergence of drug-resistant isolates harboring mutations in genes encoding viral enzymes that phosphorylate drugs (nucleoside analogs) and/or DNA polymerases, with potential cross-resistance between the different analogs. Drug resistance mutations mainly arise in conserved regions of the polymerase and exonuclease functional domains of these enzymes. In the polymerase domain, mutations associated with resistance to nucleoside/nucleotide analogs may directly or indirectly affect drug binding or incorporation into the primer strand, or increase the rate of extension of DNA to overcome chain termination. In the exonuclease domain, mutations conferring resistance to nucleoside/nucleotide analogs may reduce the rate of excision of incorporated drug, or continue DNA elongation after drug incorporation without excision. Mutations associated with resistance to pyrophosphate analogs may alter drug binding or the conformational changes of the polymerase domain required for an efficient activity of the enzyme. Novel herpesvirus inhibitors with a potent antiviral activity against drug-resistant isolates are thus needed urgently.

Early diagnosis and successful treatment of cytomegalovirus peritonitis in children with primary nephrotic syndrome: case series and literature review

Cytomegalovirus (CMV) is a major pathogen in immunocompromised population and CMV infections in immunocompromised patients cause substantial morbidity and mortality. The common clinical manifestations of CMV infection are pneumonia, hepatitis, colitis and so on, while CMV peritonitis without gut perforation is rare. Reviewing the literature, CMV peritonitis in patients with nephrotic syndrome (NS) had not been reported. Only four cases of CMV peritonitis without gut perforation were reported in adults with other diseases. Two cases were diagnosed by reverse-transcription polymerase chain reaction (RT-PCR) of ascites while the other two cases by histopathological examination of peritoneal tissue. We report four cases of primary nephrotic syndrome complicated with CMV peritonitis. Four cases all diagnosed by RT-PCR of ascites (659–455 000 copies/mL). We mainly discusses the diagnosis and treatment of CMV peritonitis without gut perforation.

Advances in the Development of Therapeutics for Cytomegalovirus Infections

The development of therapeutics for cytomegalovirus (CMV) infections, while progressing, has not matched the pace of new treatments of human immunodeficiency virus (HIV) infections; nevertheless, recent developments in the treatment of CMV infections have resulted in improved human health and perhaps will encourage the development of new therapeutic approaches. First, the deployment of ganciclovir and valganciclovir for both the prevention and treatment of CMV infections and disease in transplant recipients has been further improved with the licensure of the efficacious and less toxic letermovir. Regardless, late-onset CMV disease, specifically pneumonia, remains problematic. Second, the treatment of congenital CMV infections with valganciclovir has beneficially improved both hearing and neurologic outcomes, both fundamental advances for these children. In these pediatric studies, viral load was decreased but not eliminated. Thus, an important lesson learned from studies in both populations is the need for new antiviral agents and the necessity for combination therapies as has been shown to be beneficial in the treatment of HIV infections, among others. The development of monoclonal antibodies, sirtuins, and cyclopropovir may provide new treatment options.

Pentostatin antagonizes the antiviral activity of MBX-2168 by inhibiting the biosynthesis of the active compound

MBX-2168 has a mechanism of action similar to that of acyclovir (ACV) and ganciclovir (GCV), but two unique steps differentiate this drug from ACV/GCV. First, MBX-2168 is, at least partially, phosphorylated by the endogenous cellular kinase TAOK3 to a monophosphate. The second involves the removal of a moiety at the 6-position of MBX-2168-MP by adenosine deaminase like protein-1 (ADAL-1). It has been previously demonstrated that co-incubation with pentostatin (dCF), an ADAL-1 inhibitor, antagonizes the anti-viral activity of MBX-2168. We therefore hypothesize that inhibiting ADAL-1 results in a reduction of active compound produced in virus-infected cells. To test this, we examined the effect dCF has on the conversion of MBX-2168 to a triphosphate in HSV-1 and HCMV-infected cells. Our results demonstrate incubation of MBX-2168 alone or with dCF in HCMV-infected cells resulted in 53.1 ± 0.7 and 39.4 ± 1.5 pmol triphosphate/10⁶ cells at 120 h, respectively. Incubation of MBX-2168 alone or with dCF in Vero cells resulted in 12.8 ± 0.1 and 6.7 ± 0.7 pmol triphosphate/10⁶ cells at 24 h, respectively. HSV-1-infected Vero cells demonstrated no statistical difference in triphosphate accumulation at 24 h (13.1 ± 0.3 pmol triphosphate/10⁶ cells). As expected, incubation with dCF resulted in the accumulation of MBX-2168-MP in both HFF (9.8 ± 0.9 pmol MBX-2168-MP/10⁶ cells at 120 h) and Vero cells (4.7 ± 0.3 pmol MBX-2168-MP/10⁶ cells at 24 h) while no detectable levels of monophosphate were observed in cultures not incubated with dCF. We conclude that dCF antagonizes the anti-viral effect of MBX-2168 by inhibiting the production of triphosphate, the active compound.

Antiviral Drugs Against Herpesviruses

The discovery of the nucleoside analogue, acyclovir, represented a milestone in the management of infections caused by herpes simplex virus and varicella-zoster virus. Ganciclovir, another nucleoside analogue, was then used for the management of systemic and organ-specific human cytomegalovirus diseases. The pyrophosphate analogue, foscarnet, and the nucleotide analogue, cidofovir, have been approved subsequently and constitute the second-line antiviral drugs. However, the viral DNA polymerase is the ultimate target of all these antiviral agents with a possible emergence of cross-resistance between these drugs. Recently, letermovir that targets the viral terminase complex was approved for the prophylaxis of human cytomegalovirus infections in hematopoietic stem cell transplant recipients. Other viral targets such as the protein kinase and the helicase-primase complex are also evaluated for the development of novel potent inhibitors against herpesviruses.

Filociclovir Is a Potent In Vitro and In Vivo Inhibitor of Human Adenoviruses

Human adenovirus (HAdV) infection is common in the general population and can cause a range of clinical manifestations, among which pneumonia and keratoconjunctivitis are the most common. Although HAdV infections are mostly self-limiting, infections in immunocompromised individuals can be severe. No antiviral drug has been approved for treating adenoviruses. Filociclovir (FCV) is a nucleoside analogue which has successfully completed phase I human clinical safety studies and is now being developed for treatment of human cytomegalovirus (HCMV)-related disease in immunocompromised patients. In this report, we show that FCV is a potent broad-spectrum inhibitor of HAdV types 4 to 8, with 50% effective concentrations (EC50s) ranging between 1.24 and 3.6 μM and a 50% cytotoxic concentration (CC50) of 100 to 150 μM in human foreskin fibroblasts (HFFs). We also show that the prophylactic oral administration of FCV (10 mg/kg of body weight) 1 day prior to virus challenge and then daily for 14 days to immunosuppressed Syrian hamsters infected intravenously with HAdV6 was sufficient to prevent morbidity and mortality. FCV also mitigated tissue damage and inhibited virus replication in the liver. The 10-mg/kg dose had similar effects even when the treatment was started on day 4 after virus challenge. Furthermore, FCV administered at the same dose after intranasal challenge with HAdV6 partially mitigated body weight loss but significantly reduced pathology and virus replication in the lung. These findings suggest that FCV could potentially be developed as a pan-adenoviral inhibitor.

Moving Past Ganciclovir and Foscarnet: Advances in CMV Therapy

PURPOSE OF REVIEW

CMV DNA polymerase inhibitors such as ganciclovir and foscarnet have dramatically reduced the burden of CMV infection in the HCT recipient. However, their use is often limited by toxicities and resistance. Agents with novel mechanisms and favorable toxicity profiles are critically needed. We review recent developments in CMV antivirals and immune-based approaches to mitigating CMV infection.

RECENT FINDINGS

Letermovir, an inhibitor of the CMV terminase complex, was approved in 2017 for primary CMV prophylaxis in adult seropositive allogeneic HCT recipients. Maribavir, an inhibitor of the CMV UL97 kinase, is currently in two phase 3 treatment studies. Adoptive immunotherapy using third-party T cells has proven safe and effective in preliminary studies. Vaccine development continues, with several promising candidates currently under study. No longer limited to DNA polymerase inhibitors, the prevention and treatment of CMV infections in the HCT recipient is a rapidly evolving field which should translate into improvements in CMV-related outcomes.

Biosynthesis and half-life of MBX-2168-triphosphate in herpes virus-infected cells

The third generation of methylenecyclopropane nucleoside analogs (MCPNAs) elicit an anti-viral effect against all three sub-classes of herpes viruses without inducing cytotoxicity in vitro. It has been previously established that the mechanism of action of MCPNAs is similar to that of ganciclovir (GCV) or acyclovir (ACV). However, the activation of MBX-2168, a third generation MCPNA, involves additional and unique enzymatic steps and this process has not been examined in virus-infected cells. To that end, herpes virus-infected cells were incubated with MBX-2168, synguanol, GCV, or ACV. Incubation of HCMV-infected cells with five times the EC₅₀ of MBX-2168 (4.0 μM), synguanol (10.5 μM), or GCV (25 μM) resulted in a time-dependent increase in triphosphate accumulation reaching a maximum of 48.1 ± 5.5, 45.5 ± 2.5, and 42.6 ± 3.7 pmol/10⁶ cells at 120 h, respectively. Additionally, half-lives of these compounds were similar in HCMV-infected cells (GCV-TP = 25.5 ± 2.7 h; MBX-2168-TP/synguanol-TP = 23.0 ± 1.4 h). HSV-1-infected cells incubated with five times the EC₅₀ of MBX-2168 (33.5 μM) or ACV (5.0 μM) demonstrated a time-dependent increase in triphosphate levels reaching a maximum of 12.3 ± 1.5 and 11.6 ± 0.7 pmol/10⁶ cells at 24 h, respectively. ACV-TP and MBX-2168-TP also had similar half-lives under these conditions (27.3 ± 4.8 h and 22.2 ± 2.2 h, respectively). We therefore conclude that although MBX-2168 does not follow the classical route of nucleoside analog activation, the metabolic profile of MBX-2168 is similar to other nucleoside analogs such as GCV and ACV that do.

The discovery and development of filociclovir for the prevention and treatment of human cytomegalovirus-related disease

Human cytomegalovirus (HCMV) infections are widespread among the human population. Infection is persistent and mostly asymptomatic, except in immunocompromised individuals, particularly transplant patients, where significant morbidity and mortality can occur. Currently approved drugs for treating HCMV-related disease [including ganciclovir (GCV), valganciclovir (VGCV), cidofovir (CDV) and foscarnet (FOS)] all target the viral DNA polymerase and suffer from dose-limiting toxicity and resistance issues. The most recently approved drug, letermovir (LMV), was approved only for prophylaxis in adult HCMV-seropositive stem cell transplant recipients. Although LMV is highly potent, high-grade resistance mutations in the terminase gene were shown to readily emerge in vitro and in treated patients. Therefore, there is a need for new drugs that can be used for combinatorial therapeutic and/or prophylactic regimens to counteract the emergence of resistant mutants. Filociclovir (FCV), also known as cyclopropavir or MBX-400, is a methylenecyclopropane nucleoside analog, which has successfully completed Phase I safety studies, and is now entering Phase II clinical efficacy studies for the treatment of HCMV-related disease in transplant patients. FCV is 10-fold more active than GCV against HCMV in vitro, and has activity against all human herpesviruses except HSV-1 and HSV-2. Recently, FCV was also shown to be highly potent against human adenoviruses. This activity spectrum suggests that FCV could be used to treat/prevent infection with several viruses that pose significant risk to transplant patients. The active triphosphate form of FCV (FCV-TP) reaches higher peak levels than GCV-TP in HCMV-infected cells, and exhibits about 10-fold higher affinity to HCMV DNA polymerase UL54. Furthermore, FCV was shown to retain activity against a panel of GCV-resistant HCMV isolates, suggesting that it could be a useful alternative therapy for treating patients infected with some GCV-resistant HCMV strains. This review summarizes the early discovery work of FCV and highlights the recent advances in the continued development of this clinical candidate.

Activation of 6-Alkoxy-Substituted Methylenecyclopropane Nucleoside Analogs Requires Enzymatic Modification by Adenosine Deaminase-Like Protein 1

To determine the mechanism of action of third-generation methylenecyclopropane nucleoside analogs (MCPNAs), DNA sequencing of herpes simplex virus 1 (HSV-1) isolates resistant to third-generation MCPNAs resulted in the discovery of G841S and N815S mutations in HSV-1 UL30. Purified HSV-1 UL30 or human cytomegalovirus (HCMV) UL54 was then subjected to increasing concentrations of MBX-2168-triphosphate (TP), with results demonstrating a 50% inhibitory concentration (IC₅₀) of ∼200 μM, indicating that MBX-2168-TP does not inhibit the viral DNA polymerase. Further metabolic studies showed the removal of a moiety on the guanine ring of MBX-2168. Therefore, we hypothesized that enzymatic removal of a moiety at the 6-position of the guanine ring of third-generation MCPNAs is an essential step in activation. To test this hypothesis, pentostatin (deoxycoformycin [dCF]), an adenosine deaminase-like protein 1 (ADAL-1) inhibitor, was coincubated with MBX-2168. The results showed that dCF antagonized the effect of MBX-2168, with a >40-fold increase in the 50% effective concentration (EC₅₀) at 50 μM dCF (EC₅₀ of 63.1 ± 8.7 μM), compared with MBX-2168 alone (EC₅₀ of 0.2 ± 0.1 μM). Purified ADAL-1 demonstrated time-dependent removal of the moiety on the guanine ring of MBX-2168-monophosphate (MP), with a K_m of 17.5 ± 2.4 μM and a V _max of 0.12 ± 0.04 nmol min^-1 Finally, synguanol-TP demonstrated concentration-dependent inhibition of HSV-1 UL30 and HCMV UL54, with IC₅₀s of 0.33 ± 0.16 and 0.38 ± 0.11 μM, respectively. We conclude that ADAL-1 is the enzyme responsible for removing the moiety from the guanine ring of MBX-2168-MP prior to conversion to a TP, the active compound that inhibits the viral DNA polymerase.

Phase Ib Trial To Evaluate the Safety and Pharmacokinetics of Multiple Ascending Doses of Filociclovir (MBX-400, Cyclopropavir) in Healthy Volunteers

Filociclovir (MBX-400, cyclopropavir) is an antiviral agent with activity against cytomegalovirus (CMV). A phase 1, double-blind, randomized, placebo-controlled (3:1 ratio), single-center, multiple-ascending-dose trial was conducted to assess the safety, tolerability, and pharmacokinetics of filociclovir. Filociclovir (n = 18) or placebo (n = 6) was administered as a daily oral dose (100 mg, 350 mg, or 750 mg) for 7 days to normal healthy adults (ages, 25 to 65 years) who were monitored for 22 days. Safety assessments included clinical, laboratory, and electrocardiogram monitoring. Plasma and urine samplings were used to determine pharmacokinetic parameters. All study product-related adverse events were mild, most commonly gastrointestinal (17%), nervous system (11%), and skin and subcutaneous tissue (11%) disorders. One subject had reversible grade 3 elevation in serum creatinine and bilirubin, which was associated with an ∼1-log increase in plasma filociclovir exposure compared to levels for other subjects in the same (750-mg) cohort. No other serious adverse events were observed. Plasma exposures (area under the concentration-time curve from 0 to 24 h [AUC_0-24]) on days 1 and 7 were similar, suggesting negligible dose accumulation. There was a sublinear increase in plasma exposure with dose, which plateaued at the daily dose of 350 mg. The amount of filociclovir recovered in the urine remained proportional to plasma exposure (AUC). Doses as low as 100 mg achieved plasma concentrations sufficient to inhibit CMV in vitro (This study has been registered at ClinicalTrials.gov under identifier NCT02454699.).

Treatment and Prevention of CMV Disease in Transplant Recipients: Current Knowledge and Future Perspectives

This review summarizes the significant impact of cytomegalovirus (CMV) infection on solid organ and hematopoietic stem cell transplant recipients. A discussion of the various CMV prevention and treatment strategies is provided, including a detailed description of each of the available CMV antiviral drugs.

New therapies for human cytomegalovirus infections

The recent approval of letermovir marks a new era of therapy for human cytomegalovirus (HCMV) infections, particularly for the prevention of HCMV disease in hematopoietic stem cell transplant recipients. For almost 30 years ganciclovir has been the therapy of choice for these infections and by today's standards this drug exhibits only modest antiviral activity that is often insufficient to completely suppress viral replication, and drives the selection of drug-resistant variants that continue to replicate and contribute to disease. While ganciclovir remains the therapy of choice, additional drugs that inhibit novel molecular targets, such as letermovir, will be required as highly effective combination therapies are developed not only for the treatment of immunocompromised hosts, but also for congenitally infected infants. Sustained efforts, largely in the biotech industry and academia, have identified additional highly active lead compounds that have progressed into clinical studies with varying levels of success and at least two have the potential to be approved in the near future. Some of the new drugs in the pipeline inhibit new molecular targets, remain effective against isolates that have developed resistance to existing therapies, and promise to augment existing therapeutic regimens. Here, we will describe some of the unique features of HCMV biology and discuss their effect on therapeutic needs. Existing drugs will also be discussed and some of the more promising candidates will be reviewed with an emphasis on those progressing through clinical studies. The in vitro and in vivo antiviral activity, spectrum of antiviral activity, and mechanism of action of new compounds will be reviewed to provide an update on potential new therapies for HCMV infections that have progressed significantly in recent years.

In vitro evaluation of current and novel antivirals in combination against human cytomegalovirus

Human cytomegalovirus (HCMV) can cause severe disease in patients with compromised or immature immune systems. Currently approved pharmacotherapies for the treatment of systemic HCMV infections [ganciclovir (GCV), cidofovir (CDV), foscarnet] are limited by a high incidence of adverse effects and/or the development of drug resistance. Given that many of these drugs have the same viral target (HCMV-encoded DNA polymerase), cross-resistance is relatively common. The primary means to combat drug resistance is combination pharmacotherapy using therapeutics with different molecular mechanisms of action with the expectation that those combinations result in an additive or synergistic enhancement of effect; combinations that result in antagonism can, in many cases, be detrimental to the outcome of the patient. We therefore tested select combinations of approved (GCV, CDV, letermovir (LMV)) and experimental (brincidofovir (BCV), cyclopropavir (CPV), maribavir (MBV), BDCRB) drugs with the hypothesis that combinations of drugs with different and distinct molecular mechanisms of action will produce an additive and/or synergistic enhancement of antiviral effect against HCMV in vitro. Using MacSynergy II (a statistical package that measures enhancement or lessening of effect relative to zero/additive), select drug combination studies demonstrated combination indices ranging from 160 to 372 with 95% confidence intervals greater than zero indicating that these combinations elicit a synergistic enhancement of effect against HCMV in vitro. These data suggest that administration of a viral DNA polymerase inhibitor, MBV, and/or a viral terminase inhibitor in combination has the potential to address the resistance/cross-resistance problems associated with currently available therapeutics.

Development of a phosphorylated Momordica charantia protein system for inhibiting susceptible dose-dependent C. albicans to available antimycotics: An allosteric regulation of protein

A regulatory Momordica charantia protein system was constructed allosterically by in vitro protein phosphorylation, in an attempt to evaluate antimycological pluripotency against dose-dependent susceptibilities in C. albicans. Fungal strain lineages susceptible to ketoconazole, econazole, miconazole, 5-flucytosine, nystatin and amphotericin B were prepared in laboratory, followed by identification via antifungal susceptibility testing. Protein phosphorylation was carried out in reactions with 5'-adenylic, guanidylic, cytidylic and uridylic acids and cyclic adenosine triphosphate, through catalysis of cyclin-dependent kinase 1, protein kinase A and protein kinase C respectively. Biochemical analysis of enzymatic reactions indicated the apparent Michaelis-Menten constants and maximal velocity values of 16.57-91.97mM and 55.56-208.33μM·min^-1, together with an approximate 1:1 reactant stoichiometric ratio. Three major protein phosphorylation sites were theoretically predicted at Thr255, Thr102 and Thr24 by a KinasePhos tool. Additionally, circular dichroism spectroscopy demonstrated that upon phosphorylation, protein folding structures were decreased in random coil, β6-sheet and α1-helix partial regions. McFarland equivalence standard testing yielded the concentration-dependent inhibition patterns, while fungus was grown in Sabouraud's dextrose agar. The minimal inhibitory concentrations of 0.16-0.51μM (at 50% response) were obtained for free protein and phosphorylated counterparts. With respect to the 3-cycling susceptibility testing regimen, individuals of total protein forms were administrated in-turn at 0.14μM/cycle. Relative inhibition ratios were retained to 66.13-81.04% of initial ones regarding the ketoconazole-susceptible C. albicans growth. An inhibitory protein system, with an advantage of decreasing antifungal susceptibilities to diverse antimycotics, was proposed because of regulatory pluripotency whereas little contribution to susceptibility in itself.

Distribution and effects of amino acid changes in drug-resistant α and β herpesviruses DNA polymerase

Emergence of drug-resistance to all FDA-approved antiherpesvirus agents is an increasing concern in immunocompromised patients. Herpesvirus DNA polymerase (DNApol) is currently the target of nucleos(t)ide analogue-based therapy. Mutations in DNApol that confer resistance arose in immunocompromised patients infected with herpes simplex virus 1 (HSV-1) and human cytomegalovirus (HCMV), and to lesser extent in herpes simplex virus 2 (HSV-2), varicella zoster virus (VZV) and human herpesvirus 6 (HHV-6). In this review, we present distinct drug-resistant mutational profiles of herpesvirus DNApol. The impact of specific DNApol amino acid changes on drug-resistance is discussed. The pattern of genetic variability related to drug-resistance differs among the herpesviruses. Two mutational profiles appeared: one favoring amino acid changes in the Palm and Finger domains of DNApol (in α-herpesviruses HSV-1, HSV-2 and VZV), and another with mutations preferentially in the 3′-5′ exonuclease domain (in β-herpesvirus HCMV and HHV-6). The mutational profile was also related to the class of compound to which drug-resistance emerged.

Potency and Stereoselectivity of Cyclopropavir Triphosphate Action on Human Cytomegalovirus DNA Polymerase

Cyclopropavir (CPV) is a promising antiviral drug against human cytomegalovirus (HCMV). As with ganciclovir (GCV), the current standard for HCMV treatment, activation of CPV requires multiple steps of phosphorylation and is enantioselective. We hypothesized that the resulting CPV triphosphate (CPV-TP) would stereoselectively target HCMV DNA polymerase and terminate DNA synthesis. To test this hypothesis, we synthesized both enantiomers of CPV-TP [(+) and (-)] and investigated their action on HCMV polymerase. Both enantiomers inhibited HCMV polymerase competitively with dGTP, with (+)-CPV-TP exhibiting a more than 20-fold lower apparent Ki than (-)-CPV-TP. Moreover, (+)-CPV-TP was a more potent inhibitor than GCV-TP. (+)-CPV-TP also exhibited substantially lower apparent Km and somewhat higher apparent kcat values than (-)-CPV-TP and GCV-TP for incorporation into DNA by the viral polymerase. As is the case for GCV-TP, both CPV-TP enantiomers behaved as nonobligate chain terminators, with the polymerase terminating DNA synthesis after incorporation of one additional nucleotide. These results elucidate how CPV-TP acts on HCMV DNA polymerase and help explain why CPV is more potent against HCMV replication than GCV.

Antagonistic Relationship between Human Cytomegalovirus pUL27 and pUL97 Activities during Infection

Human cytomegalovirus (HCMV) is a member of the betaherpesvirus family. During infection, an array of viral proteins manipulates the host cell cycle. We have previously shown that expression of HCMV pUL27 results in increased levels of the cyclin-dependent kinase (CDK) inhibitor p21(Cip1). In addition, pUL27 is necessary for the full antiviral activity of the pUL97 kinase inhibitor maribavir (MBV). The purpose of this study was to define the relationship between pUL27 and pUL97 and its role in MBV antiviral activity. We observed that expression of wild-type but not kinase-inactive pUL97 disrupted pUL27-dependent induction of p21(Cip1). Furthermore, pUL97 associated with and promoted the phosphorylation of pUL27. During infection, inhibition of the kinase resulted in elevated levels of p21(Cip1) in wild-type virus but not a pUL27-deficient virus. We manipulated the p21(Cip1) levels to evaluate the functional consequence to MBV. Overexpression of p21(Cip1) restored MBV activity against a pUL27-deficient virus, while disruption reduced activity against wild-type virus. We provide evidence that the functional target of p21(Cip1) in the context of MBV activity is CDK1. One CDK-like activity of pUL97 is to phosphorylate nuclear lamin A/C, resulting in altered nuclear morphology and increased viral egress. In the presence of MBV, we observed that infection using a pUL27-deficient virus still altered the nuclear morphology. This was prevented by the addition of a CDK inhibitor. Overall, our results demonstrate an antagonistic relationship between pUL27 and pUL97 activities centering on p21(Cip1) and support the idea that CDKs can complement some activities of pUL97.

IMPORTANCE

HCMV infection results in severe disease upon immunosuppression and is a leading cause of congenital birth defects. Effective antiviral compounds exist, yet they exhibit high levels of toxicity, are not approved for use during pregnancy, and can result in antiviral resistance. Our studies have uncovered new information regarding the antiviral efficacy of the HCMV pUL97 kinase inhibitor MBV as it relates to the complex interplay between pUL97 and a second HCMV protein, pUL27. We demonstrate that pUL97 functions antagonistically against pUL27 by phosphorylation-dependent inactivation of pUL27-mediated induction of p21(Cip1). In contrast, we provide evidence that p21(Cip1) functions to antagonize overlapping activities between pUL97 and cellular CDKs. In addition, these studies further support the notion that CDK inhibitors or p21(Cip1) activators might be useful in combination with MBV to effectively inhibit HCMV infections.

TAOK3 phosphorylates the methylenecyclopropane nucleoside MBX 2168 to its monophosphate

Monohydroxymethyl methylenecyclopropane nucleosides (MCPNs) with ether or thioether substituents at the 6-position show promise as broad-spectrum herpes virus inhibitors. Their proposed mechanism of action involves sequential phosphorylation to a triphosphate, which can then inhibit viral DNA polymerase. The inhibition of herpes simplex virus (HSV) by these compounds is not dependent on the viral thymidine kinase (TK), which is known to phosphorylate acyclovir (ACV), a standard treatment for HSV infections. Previous studies on the mechanism of action of these compounds against human cytomegalovirus (HCMV) implicated a host kinase in addition to HCMV UL97 kinase in performing the initial phosphorylation. After first eliminating other candidate HSV-1 encoded kinases (UL13 and US3) as well as potential host nucleoside kinases, using activity-based fractionation, we have now identified the host serine-threonine protein kinase TAOK3 as the kinase responsible for transforming the representative monohydroxymethyl MCPN analog MBX 2168 to its monophosphate.

KSHV targeted therapy: an update on inhibitors of viral lytic replication

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi’s sarcoma, primary effusion lymphoma and multicentric Castleman’s disease. Since the discovery of KSHV 20 years ago, there is still no standard treatment and the management of virus-associated malignancies remains toxic and incompletely efficacious. As the majority of tumor cells are latently infected with KSHV, currently marketed antivirals that target the virus lytic cycle have shown inconsistent results in clinic. Nevertheless, lytic replication plays a major role in disease progression and virus dissemination. Case reports and retrospective studies have pointed out the benefit of antiviral therapy in the treatment and prevention of KSHV-associated diseases. As a consequence, potent and selective antivirals are needed. This review focuses on the anti-KSHV activity, mode of action and current status of antiviral drugs targeting KSHV lytic cycle. Among these drugs, different subclasses of viral DNA polymerase inhibitors and compounds that do not target the viral DNA polymerase are being discussed. We also cover molecules that target cellular kinases, as well as the potential of new drug targets and animal models for antiviral testing.

Metabolism of cyclopropavir and ganciclovir in human cytomegalovirus-infected cells

Human cytomegalovirus (HCMV) is a widespread pathogen that can cause severe disease in immunologically immature and immunocompromised patients. The current standard of therapy for the treatment of HCMV infections is ganciclovir (GCV). However, high incidence rates of adverse effects are prevalent and limit the use of this drug. Cyclopropavir (CPV) is 10-fold more effective against HCMV in vitro than GCV (50% effective concentrations [EC50s]=0.46 and 4.1 μM, respectively) without any observed increase in cytotoxicity (S. Zhou, J. M. Breitenbach, K. Z. Borysko, J. C. Drach, E. R. Kern, E. Gullen, Y. C. Cheng, and J. Zemlicka, J. Med. Chem. 47:566-575, 2004, doi:10.1021/jm030316s). We have previously determined that the viral protein kinase pUL97 and endogenous cellular kinases are responsible for the conversion of CPV to a triphosphate (TP), the active compound responsible for inhibiting viral DNA synthesis and viral replication. However, this conversion has not been observed in HCMV-infected cells. To that end, we subjected HCMV-infected cells to equivalently effective concentrations (∼5 times the EC50) of either CPV or GCV and observed a time-dependent increase in triphosphate levels for both compounds (CPV-TP=121±11 pmol/10(6) cells; GCV-TP=43.7±0.4 pmol/10(6) cells). A longer half-life was observed for GCV-TP (48.2±5.7 h) than for CPV-TP (23.8±5.1 h). The area under the curve for CPV-TP produced from incubation with 2.5 μM CPV was 8,680±930 pmol·h/10(6) cells, approximately 2-fold greater than the area under the curve for GCV-TP of 4,520±420 pmol·h/10(6) cells produced from incubation with 25 μM GCV. We therefore conclude that the exposure of HCMV-infected cells to CPV-TP is greater than that of GCV-TP under these experimental conditions.

Highlights in antiviral drug research: antivirals at the horizon

This review highlights ten "hot topics" in current antiviral research: (i) new nucleoside derivatives (i.e., PSI-352938) showing high potential as a direct antiviral against hepatitis C virus (HCV); (ii) cyclopropavir, which should be further pursued for treatment of human cytomegalovirus (HCMV) infections; (iii) North-methanocarbathymidine (N-MCT), with a N-locked conformation, showing promising activity against both α- and γ-herpesviruses; (iv) CMX001, an orally bioavailable prodrug of cidofovir with broad-spectrum activity against DNA viruses, including polyoma, adeno, herpes, and pox; (v) favipiravir, which is primarily pursued for the treatment of influenza virus infections, but also inhibits the replication of other RNA viruses, particularly (-)RNA viruses such as arena, bunya, and hanta; (vi) newly emerging antiarenaviral compounds which should be more effective (and less toxic) than the ubiquitously used ribavirin; (vii) antipicornavirus agents in clinical development (pleconaril, BTA-798, and V-073); (viii) natural products receiving increased attention as potential antiviral drugs; (ix) antivirals such as U0126 targeted at specific cellular kinase pathways [i.e., mitogen extracellular kinase (MEK)], showing activity against influenza and other viruses; and (x) two structurally unrelated compounds (i.e., LJ-001 and dUY11) with broad-spectrum activity against virtually all enveloped RNA and DNA viruses.

Human cytomegalovirus UL97 kinase is involved in the mechanism of action of methylenecyclopropane analogs with 6-ether and -thioether substitutions

Methylenecyclopropane nucleoside (MCPN) analogs are being investigated for treatment of human cytomegalovirus (HCMV) infection because of favorable preclinical data and limited ganciclovir cross-resistance. Monohydroxymethyl MCPNs bearing ether and thioether functionalities at the purine 6 position have antiviral activity against herpes simplex virus (HSV) and varicella-zoster virus (VZV) in addition to HCMV. The role of the HCMV UL97 kinase in the mechanism of action of these derivatives was examined. When tested against a kinase-inactive UL97 K355M virus, a moderate 5- to 7-fold increase in 50% effective concentration (EC50) was observed, in comparison to a 13- to 25-fold increase for either cyclopropavir or ganciclovir. Serial propagation of HCMV under two of these compounds selected for three novel UL97 mutations encoding amino acid substitutions D456N, C480R,and Y617del. When transferred to baseline laboratory HCMV strains, these mutations individually conferred resistance to all of the tested MCPNs, ganciclovir, and maribavir. However, the engineered strains also demonstrated severe growth defects and abnormal cytopathic effects similar to the kinase-inactive mutant. Expressed and purified UL97 kinase showed in vitro phosphorylation of the newly tested MCPNs. Thus, HCMV UL97 kinase is involved in the antiviral action of these MCPNs, but the in vitro selection of UL97-defective viruses suggests that their activity against more typical ganciclovir-resistant growth-competent UL97 mutants may be relatively preserved.

Human herpesvirus 6 U69 kinase phosphorylates the methylenecyclopropane nucleosides cyclopropavir, MBX 2168, and MBX 1616 to their monophosphates

Dihydroxymethyl and monohydroxymethyl methylenecyclopropane nucleosides are effective inhibitors of both variants of human herpesvirus 6 (HHV-6). We investigated involvement of HHV-6 U69 protein kinase in their mechanism of action. Phosphorylation of the dihydroxymethyl analogue cyclopropavir and monohydroxymethyl nucleosides with either a 6-ether moiety (MBX 2168) or a 6-thioether moiety (MBX 1616) with purified U69 was examined. All three compounds were substrates of this viral kinase and had similar Michaelis-Menten kinetic parameters.

Resistance of human cytomegalovirus to cyclopropavir maps to a base pair deletion in the open reading frame of UL97

Human cytomegalovirus (HCMV) is a widespread pathogen in the human population, affecting many immunologically immature and immunocompromised patients, and can result in severe complications, such as interstitial pneumonia and mental retardation. Current chemotherapies for the treatment of HCMV infections include ganciclovir (GCV), foscarnet, and cidofovir. However, the high incidences of adverse effects (neutropenia and nephrotoxicity) limit the use of these drugs. Cyclopropavir (CPV), a guanosine nucleoside analog, is 10-fold more active against HCMV than GCV (50% effective concentrations [EC50s] = 0.46 and 4.1 μM, respectively). We hypothesize that the mechanism of action of CPV is similar to that of GCV: phosphorylation to a monophosphate by viral pUL97 protein kinase with further phosphorylation to a triphosphate by endogenous kinases, resulting in inhibition of viral DNA synthesis. To test this hypothesis, we isolated a CPV-resistant virus, sequenced its genome, and discovered that bp 498 of UL97 was deleted. This mutation caused a frameshift in UL97 resulting in a truncated protein that lacks a kinase domain. To determine if this base pair deletion was responsible for drug resistance, the mutation was engineered into the wild-type viral genome, which was then exposed to increasing concentrations of CPV. The results demonstrate that the engineered virus was approximately 72-fold more resistant to CPV (EC50 = 25.8 ± 3.1 μM) than the wild-type virus (EC50 = 0.36 ± 0.11 μM). We conclude, therefore, that this mutation is sufficient for drug resistance and that pUL97 is involved in the mechanism of action of CPV.

Human herpesvirus vaccines and future directions

Over the last few years there has been an impressive increase in the virological and immunological tools available to detect both human herpesvirus (HHV) and immune control of replication post-solid organ transplantation. This has allowed a greater appreciation of pathogenesis, studies to be designed to evaluate potential vaccines, new approaches adopted for antiviral deployment and the success of interventions to be judged. This chapter aims to summarize the state-of-the-art in vaccine development and look forward to the role that vaccines, immune monitoring, viral kinetics and new antiherpesvirus agents may play in the future management of HHV infections after transplantation.

The biology of cytomegalovirus drug resistance

PURPOSE OF REVIEW

This review assesses recently published data on cytomegalovirus (CMV) antiviral drug resistance.

RECENT FINDINGS

Resistance is typically encountered after prolonged ganciclovir treatment for posttransplant primary CMV infection and is diagnosed by the detection of characteristic mutations in the viral UL97 kinase and UL54 DNA polymerase genes in clinical specimens. One of seven canonical UL97 mutations is detected in most cases of ganciclovir resistance, but many viral sequence variants of unknown relevance are being reported after drug exposure in vitro and in vivo. Rapid technical advances in recombinant phenotyping have shown that many of these variants confer no detectable drug resistance, whereas some unusual resistance mutations are newly confirmed. All currently marketed CMV antiviral drugs, including foscarnet and cidofovir, target the viral DNA polymerase, and cross-resistance may result from some UL54 mutations. To decrease cross-resistance and toxicity, there is an ongoing effort to develop anti-CMV drugs with different resistance pathways and alternative targets, such as the UL97 kinase or UL56-UL89 terminase enzymes.

SUMMARY

An increasing volume of information correlating CMV genotypes and drug susceptibility phenotypes is becoming available. This will improve the interpretation of sequence-based assays currently used for clinical diagnosis and guide the development of new antiviral drugs.

Progress in the development of new therapies for herpesvirus infections

Resurgent interest in antiviral drugs for the treatment of herpesvirus has led to the development of new compounds that are progressing through clinical trials. This is important because there are few therapeutic options for resistant infections and some viruses such as human cytomegalovirus remain underserved. New compounds include conventional DNA polymerase inhibitors such as valomaciclovir and cyclopropavir, as well as CMX001 that has a broad spectrum of antiviral activity that includes all the herpesviruses. It also includes compounds with new molecular targets such as maribavir (MBV), FV-100, AIC361, and AIC246. Recent advances with each of these compounds will be reviewed including their virus specificity, mechanism of action, and stage of development. The potential of these new compounds to improve clinical outcome will also be discussed.

Cyclopropavir susceptibility of cytomegalovirus DNA polymerase mutants selected after antiviral drug exposure

Human cytomegalovirus (CMV) UL54 DNA polymerase (pol) mutants with known patterns of resistance to current antivirals ganciclovir (GCV), foscarnet (FOS), and cidofovir (CDV) were tested for cyclopropavir (CPV) susceptibility by a standardized reporter-based yield reduction assay. Exonuclease and A987G (region V) mutations at codons commonly associated with dual GCV-CDV resistance in clinical isolates paradoxically conferred increased CPV susceptibility. Various polymerase catalytic region mutations conferring FOS resistance with variable low-grade GCV and CDV cross-resistance also conferred CPV resistance, with 50% effective concentration (EC(50)) increases of 3- to 13-fold. CPV EC(50) values against several pol mutants were increased about 2-fold by adding UL97 mutation C592G. Propagation of a CMV exonuclease mutant under CPV selected for pol mutations less often than UL97 mutations. In 21 experiments, one instance each of mutations E756D and M844V, which were shown individually to confer 3- to 4-fold increases in CPV EC(50), was detected. Unlike GCV and CDV, exonuclease mutations are not a preferred mechanism of CPV resistance, but mutations in and near pol region III may confer CPV resistance by affecting its recognition as an incoming base for DNA polymerization.

Cyclopropavir inhibits the normal function of the human cytomegalovirus UL97 kinase

Cyclopropavir (CPV) is active against human cytomegalovirus (CMV), as well as both variants of human herpesvirus 6 and human herpesvirus 8. The mechanism of action of CPV against CMV is similar to that of ganciclovir (GCV) in that it is phosphorylated initially by the CMV UL97 kinase, resulting in inhibition of viral DNA synthesis. Resistance to CPV maps to the UL97 kinase but is associated primarily with H520Q mutations and thus retains good antiviral activity against most GCV-resistant isolates. An examination of CMV-infected cultures treated with CPV revealed unusual cell morphology typically associated with the absence of UL97 kinase activity. A surrogate assay for UL97 kinase activity confirmed that CPV inhibited the activity of this enzyme and that its action was similar to the inhibition seen with maribavir (MBV) in this assay. Combination studies using real-time PCR indicated that, like MBV, CPV also antagonized the efficacy of GCV and were consistent with the observed inhibition of the UL97 kinase. Deep sequencing of CPV-resistant laboratory isolates identified a frameshift mutation in UL27, presumably to compensate for a loss of UL97 enzymatic activity. We conclude that the mechanism of action of CPV against CMV is complex and involves both the inhibition of DNA synthesis and the inhibition of the normal activity of the UL97 kinase.

Recent developments in antiviral drugs for cytomegalovirus

Human cytomegalovirus continues to impact adversely on the outcome of solid organ and stem cell transplantation and remains a major cause of congenital abnormalities. In the absence of a vaccine, antiviral drugs have been the mainstay of therapy. Although very few anticytomegalovirus drugs are currently licensed, there are multiple opportunities within the viral life cycle for drug development. In this article we summarize some of the key new antiviral agents undergoing preclinical and clinical development against a range of targets in the viral life cycle, highlighting those where further development is warranted or being undertaken.

The search for new therapies for human cytomegalovirus infections

Ganciclovir (GCV), the therapy of choice for human cytomegalovirus (CMV) infections and foscarnet, a drug used to treat GCV-resistant CMV infections was approved more than twenty years ago. Although cidofovir and a prodrug of GCV have since been added to the armamentarium, a highly effective drug without significant toxicities has yet to be approved. Such a therapeutic agent is required for treatment of immunocompromised hosts and infants, which bear the greatest burden of disease. The modest antiviral activity of existing drugs is insufficient to completely suppress viral replication, which results in the selection of drug-resistant variants that remain pathogenic, continue to replicate, and contribute to disease. Sustained efforts, largely in the biotech industry and academia, have identified highly active lead compounds that have progressed into clinical studies with varying levels of success. A few of these compounds inhibit new molecular targets, remain effective against isolates that have developed resistance to existing therapies, and promise to augment existing therapies. Some of the more promising drugs will be discussed with an emphasis on those progressing to clinical studies. Their antiviral activity both in vitro and in vivo, spectrum of antiviral activity, and mechanism of action will be reviewed to provide an update on the progress of potential new therapies for CMV infections.

Cytomegalovirus UL97 mutations affecting cyclopropavir and ganciclovir susceptibility

Among the 7 most common UL97 mutations encountered in ganciclovir-resistant clinical cytomegalovirus isolates, the associated cyclopropavir cross-resistance varies from insignificant (L595S) to substantial (M460I and H520Q) as determined by recombinant phenotyping. Mutations M460I and H520Q were preferentially selected in vitro under cyclopropavir and conferred 12- to 20-fold increases in 50% effective concentration (EC(50)) values, while M460V, C592G, A594V, and C603W conferred 3- to 5-fold increases. Uncommon mutations M460T and C603R increased cyclopropavir EC(50)s by 8- to 10-fold.

Phosphorylation of antiviral and endogenous nucleotides to di- and triphosphates by guanosine monophosphate kinase

Many fraudulent nucleosides including the antivirals acyclovir (ACV) and ganciclovir (GCV) must be metabolized to triphosphates to be active. Cyclopropavir (CPV) is a newer, related guanosine nucleoside analog that is active against human cytomegalovirus (HCMV) in vitro and in vivo. We have previously demonstrated that CPV is phosphorylated to its monophosphate (CPV-MP) by the HCMV pUL97 kinase. Consequently, like other nucleoside analogs phosphorylated by viral kinases, CPV most likely must be converted to a triphosphate (CPV-TP) in order to elicit antiviral activity. Once formed by pUL97, we hypothesized that guanosine monophosphate kinase (GMPK) is the enzyme responsible for the conversion of CPV-MP to CPV-DP. Incubation of CPV-MP with GMPK resulted in the formation of CPV-DP and, surprisingly, CPV-TP. When CPV-DP was incubated with GMPK, a time-dependent increase in CPV-TP occurred corresponding to a decrease in CPV-DP thereby demonstrating that CPV-DP is a substrate for GMPK. Substrate specificity experiments revealed that GMP, dGMP, GDP, and dGDP are substrates for GMPK. In contrast, GMPK recognized only acyclovir and ganciclovir monophosphates as substrates, not their diphosphates. Kinetic studies demonstrated that CPV-DP has a K(M) value of 45±15μM. We were, however, unable to determine the K(M) value for CPV-MP directly, but a mathematical model of experimental data gave a theoretical K(M) value for CPV-MP of 332±60μM. We conclude that unlike many other antivirals, cyclopropavir can be converted to its active triphosphate by a single cellular enzyme once the monophosphate is formed by a virally encoded kinase.