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

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.

40 Years after the Registration of Acyclovir: Do We Need New Anti-Herpetic Drugs?

Herpes simplex virus types 1 and 2 HSV1 and 2, namely varicella-zoster VZV and cytomegalovirus CMV, are among the most common pathogens worldwide. They remain in the host body for life. The course of infection with these viruses is often asymptomatic or mild and self-limiting, but in immunocompromised patients, such as solid organ or bone marrow transplant recipients, the course can be very severe or even life-threatening. Unfortunately, in the latter group, the highest percentage of infections with strains resistant to routinely used drugs is observed. On the other hand, frequent recurrences of genital herpes can be a problem even in people with normal immunity. Genital herpes also increases the risk of acquiring sexually transmitted diseases, including HIV infection and, if present in pregnant women, poses a risk to the fetus and newborn. Even more frequently than herpes simplex, congenital infections can be caused by cytomegalovirus. We present the most important anti-herpesviral agents, the mechanisms of resistance to these drugs, and the associated mutations in the viral genome. Special emphasis was placed on newly introduced drugs such as maribavir and brincidofovir. We also briefly discuss the most promising substances in preclinical testing as well as immunotherapy options and vaccines currently in use and under investigation.

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.

Resistance to a Nucleoside Analog Antiviral Drug from More Rapid Extension of Drug-Containing Primers

Nucleoside analogs are mainstays of antiviral therapy. Although resistance to these drugs hinders their use, understanding resistance can illuminate mechanisms of the drugs and their targets. Certain nucleoside analogs, such as ganciclovir (GCV), a leading therapy for human cytomegalovirus (HCMV), contain the equivalent of a 3'-hydoxyl moiety, yet their triphosphates can terminate genome synthesis (nonobligate chain termination). For ganciclovir, chain termination is delayed until incorporation of the subsequent nucleotide, after which viral polymerase idling (repeated addition and removal of incorporated nucleotides) prevents extension. Here, we investigated how an alanine-to-glycine substitution at residue 987 (A987G), in conserved motif V in the thumb subdomain of the catalytic subunit (Pol) of HCMV DNA polymerase, affects polymerase function to overcome delayed chain termination and confer ganciclovir resistance. Steady-state enzyme kinetic studies revealed no effects of this substitution on incorporation of ganciclovir-triphosphate into DNA that could explain resistance. We also found no effects of the substitution on Pol's exonuclease activity, and the mutant enzyme still exhibited idling after incorporation of GCV and the subsequent nucleotide. However, despite extending normal DNA primers similarly to wild-type enzyme, A987G Pol more rapidly extended ganciclovir-containing DNA primers, thereby overcoming chain termination. The mutant Pol also more rapidly extended RNA primers, a previously unreported activity for HCMV Pol. Structural analysis of related Pols bound to primer-templates provides a rationale for these results. These studies uncover a new drug resistance mechanism, potentially applicable to other nonobligate chain-terminating nucleoside analogs, and shed light on polymerase functions.IMPORTANCE While resistance to antiviral drugs can hinder their clinical use, understanding resistance mechanisms can illuminate how these drugs and their targets act. We studied a substitution in the human cytomegalovirus (HCMV) DNA polymerase that confers resistance to a leading anti-HCMV drug, ganciclovir. Ganciclovir is a nucleoside analog that terminates DNA replication after its triphosphate and the subsequent nucleotide are incorporated. We found that the substitution studied here results in an increased rate of extension of drug-containing DNA primers, thereby overcoming termination, which is a new mechanism of drug resistance. The substitution also induces more rapid extension of RNA primers, a function that had not previously been reported for HCMV polymerase. Thus, these results provide a novel resistance mechanism with potential implications for related nucleoside analogs that act against established and emerging viruses, and shed light on DNA polymerase functions.

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.

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.

Summary

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.

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.

Synthesis of New Imidazopyridine Nucleoside Derivatives Designed as Maribavir Analogues

The strong inhibition of Human Cytomegalovirus (HCMV) replication by benzimidazole nucleosides, like Triciribine and Maribavir, has prompted us to expand the structure-activity relationships of the benzimidazole series, using as a central core the imidazo[4,5-b]pyridine scaffold. We have thus synthesized a number of novel amino substituted imidazopyridine nucleoside derivatives, which can be considered as 4-(or 7)-aza-d-isosters of Maribavir and have evaluated their potential antiviral activity. The target compounds were synthesized upon glycosylation of suitably substituted 2-aminoimidazopyridines, which were prepared in six steps starting from 2-amino-6-chloropyridine. Even if the new compounds possessed only a slight structural modification when compared to the original drug, they were not endowed with interesting antiviral activity. Even so, three derivatives showed promising cytotoxic potential.

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.).

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.

Ganciclovir-Resistant Cytomegalovirus Infection in Abdominal Solid Organ Transplant Recipients: Case Series and Review of the Literature

Ganciclovir-resistant cytomegalovirus (GR-CMV) is emerging as a significant infection in the abdominal transplant population. GR-CMV is difficult to manage, and treatment options are limited. We report a descriptive case series of 15 patients who had documented GR-CMV at our center and review the literature on treatment of GR-CMV. The first case in this series was detected in 2012; the majority of cases occurred after January 1, 2014, with approximately 50% occurring in 2015. UL97 and UL54 viral genome mutations were present in 100% and 40% of CMV-infected patients, respectively. GR-CMV infection occurred ≤ 1 year posttransplantation in 11 patients (73%). All patients experienced dose reduction of valganciclovir (the oral prodrug of ganciclovir) before the development of GR-CMV. Initial treatment for GR-CMV included a variety of regimens, all including reduction in maintenance immunosuppression. Of the 6 patients with detectable GR-CMV by polymerase chain reaction (PCR) who were discharged without GR-CMV treatment and had a length of stay (LOS) less than 14 days, 83% were subsequently readmitted for treatment of GR-CMV within 2 months (60% in < 20 days); none received leflunomide. Of six patients with a LOS ≥ 14 days, 80% had CMV PCR below quantification on hospital discharge, and only one patient was readmitted in less than 20 days; 83% received leflunomide. Following GR-CMV, there was a 50% rejection incidence, 27% graft loss, and 20% mortality. For patients with more than three admissions for GR-CMV treatment, 100% had a major complication: 60% rejection, 20% graft loss, and 40% mortality. Common clinical characteristics of patients with GR-CMV included high-risk serostatus, lymphocyte depletion, and history of valganciclovir dose reduction. Overall, outcomes were poor. It appears that hospital readmission rate was reduced when CMV was treated to negativity with an initial treatment regimen of reduced immunosuppression, foscarnet, intravenous immunoglobulins, and leflunomide.