Inactivity of the bicyclic pyrimidine nucleoside analogues against simian varicella virus (SVV) does not correlate with their substrate activity for SVV-encoded thymidine kinase

Herpes simplex virus and varicella zoster virus: recent advances in therapy

PURPOSE OF REVIEW

The mainstay of antiviral therapy for the alpha-herpesviruses [herpes simplex virus (HSV)-1, HSV-2, and varicella zoster virus (VZV)] over the past 40 years has been the nucleoside analogues such as aciclovir. Although conventional antiviral therapy has reduced mortality in severe disease, novel agents are needed to address the emergence of resistance and toxicity associated with current second-line therapy. Treatment and prophylaxis of VZV and HSV reactivations remains a challenge.

RECENT FINDINGS

A number of compounds have recently been evaluated in human clinical trials, amongst them brincidofovir, an intracellularly acting derivative of cidofovir currently undergoing phase III trials. The helicase-primase inhibitors are a new class of antiviral agent and may circumvent resistance to existing agents. Amenamevir and pritelivir are two examples of these agents that have been evaluated clinically along with novel nucleoside analogues such as valomaciclovir and FV-100. Tenofovir, an agent used in HIV and hepatitis B therapy, may also have a role in the prevention of HSV-2 acquisition and reduce viral shedding.

SUMMARY

Although several novel antiviral agents have undergone clinical trials in recent years, all are yet to gain licensure. Brincidofovir appears to be the candidate with most promise for adoption into routine practice in the near future.

Evaluation of Novel Phosphoramidate ProTides of the 2′-Fluoro Derivatives of a Potent Anti-Varicella Zoster Virus Bicyclic Nucleoside Analogue

Background:

Recently, the synthesis and antiviral activity of a series of 2′-fluoro derivatives of the most potent anti-varicella zoster virus (VZV) agent reported to date, the bicyclic nucleoside analogue Cf1743, have been reported.

Methods:

Here, we present molecular modelling studies for the interaction of these compounds with VZV-encoded thymidine kinase (TK) and we report the synthesis of a series of phosphoramidate ProTides of these compounds designed to bypass the nucleoside kinase dependence of the parent nucleoside analogues.

Results:

The phosphoramidate prodrugs were equally effective as their parent compounds against VZV in cell culture, but lost antiviral potency against TK-deficient VZV strains.

Conclusions:

ProTide-based kinase bypass is not successful in this case.

FV-100: The Most Potent and Selective Anti-Varicella Zoster Virus Agent Reported to Date

Bicyclic aryl furano pyrimidines represent the most potent anti-varicella zoster virus (VZV) agents reported to date. Lead compounds have 50% effective concentration (EC50) values in vitro that are in the subnanomolar range and selectivity index values that exceed 1 million. They have an absolute requirement for VZV thymidine kinase and most likely act as their phosphate forms. Some structural modification (such as aryl substitution in the base moiety) is tolerated, whereas little sugar modification is acceptable.

The Cf1743 compound has proved to be significantly more potent than all reference anti-VZV compounds, as measured either by inhibition of infectious virus particles and/or viral DNA production; however, the high lipophilicity and very low water solubility of this compound gives poor oral bioavailability (<14%). Use of the modified cyclodextrin captisol and the synthesis of the 5′-monophosphate prodrug of Cf1743 has significantly improved water solubility, but does not give any enhancement in oral bioavailability. By contrast, the synthesis of the ether series does not give any further improvement in terms of solubility. The most promising prodrug to emerge to date is the hydrochloric salt of the 5′-valyl-ester, designated as FV-100. Its uptake into cells has been studied using fluorescent microscopy and biological assays, which have indicated that the compound is efficiently taken up by the cells after a short period of incubation.

2'-Fluorosugar analogues of the highly potent anti-varicella-zoster virus bicyclic nucleoside analogue (BCNA) Cf 1743

We report the preparation of 2'-alpha-F, 2'-beta-F and 2',2'-difluoro analogues of the leading anti-varicella zoster virus (VZV) pentylphenyl BCNA Cf 1743. VZV thymidine kinase showed the highest phosphorylating capacity for the beta-fluoro derivative, that retained equal antiviral potency as the parent compound. In contrast, the alpha-fluoro- and 2',2'-difluoro BCNA derivatives were markedly less (approximately 100-fold) antivirally active.

Aryl furano pyrimidines: the most potent and selective anti-VZV agents reported to date

Bicyclic aryl furano pyrimidines represent the most potent anti-VZV agents reported to date. Lead compounds have EC50 values in vitro as low as 0.1 nM and selectivity index values exceeding one million. They have an absolute requirement for VZV thymidine kinase (TK) and most likely act as their phosphate forms. Some structural modification, such as aryl substitution, is tolerated, while little sugar modification is acceptable. We herein summarise their biological profiles and structure activity relationships as discovered to date.

[New antivirals for herpesviruses]

The long-term treatment of herpesvirus infections with current antivirals leads to the development of drug-resistant viruses. Because currently available antivirals finally target the viral DNA polymerase, mutant resistant to one drug often shows cross-resistance to other drugs. This evidence highlights the need for the development of new antivirals that have the different viral targets. Recently, high-through-put screening of large compound collections for inhibiting specific viral enzymes, or in vitro cell culture assay, has identified several new antivirals. These include the inhibitors of helicase/primase complex, terminase complex, portal protein and UL97 protein kinase. This review will focus on these new compounds that directly inhibit viral replication.

Susceptibilities of several clinical varicella-zoster virus (VZV) isolates and drug-resistant VZV strains to bicyclic furano pyrimidine nucleosides

Varicella-zoster virus (VZV) is responsible for primary infections as well as reactivations after latency in the dorsal root ganglia. The treatment of such infections is mandatory for immunocompromised patients and highly recommended for elderly patients with herpes zoster infections (also called zona or shingles). The treatment of choice is presently based on four molecules, acyclovir (ACV), valaciclovir, famciclovir, and (in Europe) brivudine (BVDU). We present here our data on the antiviral activity of a new class of potent and selective anti-VZV compounds, bicylic pyrimidine nucleoside analogues (BCNAs), against a broad variety of clinical isolates and different drug-resistant virus strains. The results show that the BCNAs are far more potent inhibitors than ACV and BVDU against clinical VZV isolates as well as the VZV reference strains Oka and YS. The BCNAs were not active against ACV- and BVDU-resistant VZV strains bearing mutations in the viral thymidine kinase gene but kept their inhibitory potential against virus strains with mutations in the VZV DNA polymerase gene. Mutant virus strains selected in the presence of the BCNAs were solely cross-resistant to drugs, such as ACV and BVDU, that depend for their antiviral action on metabolic activation by the viral thymidine kinase.

Discovery and development of BVDU (brivudin) as a therapeutic for the treatment of herpes zoster

This Commentary is dedicated to the memory of Dr. Jacques Gielen, the late Editor of Biochemical Pharmacology, whom I have known as both an author and reviewer for the Journal for about 25 years. This is, quite incidentally, about the time it took for bringing brivudin (BVDU) [(E)-5-(2-bromovinyl)-2'-deoxyuridine] from its original description as an antiviral agent to the market place (in a number of European countries, including Germany and Italy) for the treatment of herpes zoster in immunocompetent persons. BVDU is exquisitely active and selective against varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV-1). BVDU owes this high selectivity and activity profile to a specific phosphorylation by the virus-encoded thymidine kinase, followed by a potent interaction with the viral DNA polymerase. The (E)-5-(2-bromovinyl)-substituent can be considered as the hallmark for the activity of BVDU against VZV and HSV-1. Extensive clinical studies have indicated that BVDU as a single (oral) daily dose of 125 mg (for no more than 7 days) is effective in the treatment of herpes zoster, as regards both short-term (suppression of new lesion formation) and long-term effects (prevention of post-herpetic neuralgia). In this sense, BVDU is as efficient and/or convenient, if not more so, than the other drugs (acyclovir, valaciclovir, famciclovir) that have been licensed for the treatment of herpes zoster. There is one caveat; however, BVDU should not be given to patients under 5-fluorouracil therapy, as the degradation product of BVDU, namely (E)-5-(2-bromovinyl)uracil (BVU), may potentiate the toxicity of 5-fluorouracil, due to inhibition of dihydropyrimidine dehydrogenase, the enzyme involved in the catabolism of 5-fluorouracil.