Synthesis and Antiviral Evaluation of Alkoxyalkyl Esters of Phosphonopropoxymethyl-Guanine and Phosphonopropoxymethyl-Diaminopurine

Advances in the treatment of varicella-zoster virus infections

Varicella-zoster virus (VZV) causes two distinct diseases, varicella (chickenpox) and shingles (herpes zoster). Chickenpox occurs subsequent to primary infection, while herpes zoster (usually associated with aging and immunosuppression) appears as a consequence of reactivation of latent virus. The major complication of shingles is postherpetic neuralgia. Vaccination strategies to prevent varicella or shingles and the current status of antivirals against VZV will be discussed in this chapter. Varivax®, a live-attenuated vaccine, is available for pediatric varicella. Zostavax® is used to boost VZV-specific cell-mediated immunity in adults older than 50 years, which results in a decrease in the burden of herpes zoster and pain related to postherpetic neuralgia. Regardless of the availability of a vaccine, new antiviral agents are necessary for treatment of VZV infections. Current drugs approved for therapy of VZV infections include nucleoside analogues that target the viral DNA polymerase and depend on the viral thymidine kinase for their activation. Novel anti-VZV drugs have recently been evaluated in clinical trials, including the bicyclic nucleoside analogue FV-100, the helicase-primase inhibitor ASP2151, and valomaciclovir (prodrug of the acyclic guanosine derivative H2G). Different candidate VZV drugs have been described in recent years. New anti-VZV drugs should be as safe as and more effective than current gold standards for the treatment of VZV, that is, acyclovir and its prodrug valacyclovir.

Emerging drugs for varicella-zoster virus infections

INTRODUCTION

Varicella-zoster virus (VZV) is the etiological agent of two distinct diseases, varicella (chickenpox) and shingles (herpes zoster). Chickenpox occurs following primary infection, while herpes zoster (usually associated with ageing and immunosuppression) is the consequence of reactivation of the latent virus. Post-herpetic neuralgia is the major complication of shingles.

AREAS COVERED

This review will discuss vaccination strategies and the current status of antivirals against VZV. A live attenuated vaccine, Varivax, is available for pediatric varicella while Zostavax was developed to boost VZV-specific cell-mediated immunity in adults older than 60 years and, via this mechanism, to decrease the burden of herpes zoster and pain associated with post-herpetic neuralgia. Despite the availability of a vaccine, there is a need for new antiviral agents. Current drugs approved for the treatment of VZV infections include nucleoside analogs that target the viral DNA polymerase and depend on the viral thymidine kinase. Novel anti-VZV drugs have recently been evaluated in clinical trials, including the bicyclic nucleoside analog FV-100, the helicase-primase inhibitor ASP2151 and valomaciclovir (prodrug of the acyclic guanosine derivative H2G).

EXPERT OPINION

New anti-VZV drugs should be as safe as and more effective than acyclovir and its prodrug valacyclovir (current gold standard for the treatment of VZV).

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.

Alkoxyalkyl prodrugs of acyclic nucleoside phosphonates enhance oral antiviral activity and reduce toxicity: current state of the art

Although the acyclic nucleoside phosphonates cidofovir, adefovir and tenofovir are approved for treating human cytomegalovirus, hepatitis B and HIV infections, respectively, their utility is limited by low oral bioavailability, renal toxicity and poor cell penetration. Research over the past decade has shown that these undesirable features can be eliminated by esterifying the compounds with an alkoxyalkyl group, in effect disguising them as lysophospholipids. In this modified form, the drugs are readily taken up in the gastrointestinal tract and have a prolonged circulation time in plasma. The active metabolite also has a long half life within cells, permitting infrequent dosing. Because these modified drugs are not recognized by the transport mechanisms that cause the accumulation of acyclic nucleoside phosphonates in renal tubular cells, they lack nephrotoxicity. Alkoxyalkyl esterification also markedly increases the in vitro antiviral activity of acyclic nucleoside phosphonates by improving their delivery into cells. For example, an alkoxyalkyl ester of cyclic-cidofovir, a less soluble compound, retains anti-CMV activity for 3 months following a single intravitreal injection. Two of these novel compounds, hexadecyloxypropyl-cidofovir (CMX001) and hexadecyloxypropyl-tenofovir (CMX157) are now in clinical development. This article focuses on the hexadecyloxypropyl and octadecyloxyethyl esters of cidofovir and (S)-HPMPA, describing their synthesis and the evaluation of their in vitro and in vivo activity against a range of orthopoxviruses, herpesviruses, adenoviruses and other double-stranded DNA viruses. The extension to other nucleoside phosphonate antivirals is highlighted, demonstrating that this novel approach can markedly improve the medicinal properties of these drugs.

Antiviral activities of novel 5-phosphono-pent-2-en-1-yl nucleosides and their alkoxyalkyl phosphonoesters

Three acyclic nucleoside phosphonates are currently approved for clinical use against infections caused by cytomegalovirus (Vistide), hepatitis B virus (Hepsera), and human immunodeficiency virus type 1 (Viread). This important antiviral class inhibits viral polymerases after cellular uptake and conversion to their diphosphates, bypassing the first phosphorylation, which is required for conventional nucleoside antivirals. Small chemical alterations in the acyclic side chain lead to marked differences in antiviral activity and the spectrum of activity of acyclic nucleoside phosphonates against various classes of viral agents. We synthesized a new class of acyclic nucleoside phosphonates based on a 5-phosphono-pent-2-en-1-yl base motif in which the oxygen heteroatom usually present in acyclic nucleoside phosphonates has been replaced with a double bond. Since the intrinsic phosphonate moiety leads to low oral bioavailability and impaired cellular penetration, we also prepared the hexadecyloxypropyl esters of the 5-phosphono-pent-2-en-1-yl nucleosides. Our earlier work showed that this markedly increases antiviral activity and oral bioavailability. Although the 5-phosphono-pent-2-en-1-yl nucleosides themselves were not active, the hexadecyloxypropyl esters were active against DNA viruses and hepatitis B virus, in vitro. Notably, the hexadecyloxypropyl ester of 9-(5-phosphono-pent-2-en-1-yl)-adenine was active against hepatitis B virus mutants resistant to lamivudine, emtricitabine, and adefovir.