Improvement of the absorption of oral (R,S)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine, an anti-varicella-zoster virus drug, in rats and monkeys

Antiviral Activity against VZV and HSV Type 1 and Type 2 of the (+) and (−) Enantiomers of (R,S)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine, in Comparison to other Closely Related Acyclic Nucleosides

The separate (+) and (−) enantiomers of the acyclic guanosine analogue 9-[4-hydroxy-2-(hydroxymethyl)-butyl]guanine (2HM-HBG) were tested for inhibition of varicella-zoster virus (VZV), herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2). In all cases the (−) enantiomer was the most active enantiomer. The (+) enantiomer was 10 times less active than the racemate against VZV and inactive against HSV-1 and -2.

The parent compound, 9-(4-hydroxybutyl)guanine, containing an unbranched side chain, was inactive against VZV, whereas substitution with a hydroxymethyl group at the 2 or 3 position of the side chain conferred anti-VZV activity. The effect of hydroxymethyl substitution may increase the recognition of the compound by the VZV thymidine kinase by increasing its similarity to the natural substrate thymidine. Further substitution of the side chain of the parent compound with oxygen, fluorine or hydroxyl groups did not confer antiviral activity against VZV.

Two VZV strains were isolated which could be grown in the presence of high concentrations of 2HM-HBG and which were also cross-resistant to other nucleoside analogues. These strains induced very little viral thymidine kinase activity in infected cells and thus were probably deficient for a functional thymidine kinase.

2HM-HBG exhibited a persistent antiviral effect even after the nucleoside was removed from the medium of VZV-infected cells, whereas acyclovir did not show this effect.

Prodrugs of nucleoside analogues for improved oral absorption and tissue targeting

Nucleoside analogues are widely used for the treatment of antiviral infections and anticancer chemotherapy. However, many nucleoside analogues suffer from poor oral bioavailability due to their high polarity and low intestinal permeability. In order to improve oral absorption of these polar drugs, prodrugs have been employed to increase lipophilicity by chemical modification of the parent. Alternatively, prodrugs targeting transporters present in the intestine have been exploited to facilitate the transport of the nucleoside analogues. Valacyclovir and valganciclovir are two successful valine ester prodrugs transported by the PepT1 transporter. Recently, research efforts have focused on design of prodrugs for tissue specific delivery to improve efficacy and safety. This review presents advances of prodrug approaches for improved oral absorption of nucleoside analogues and recent developments in tissue targeting.

Efficacy of (-)-9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine in African green monkeys infected with simian varicella virus

(-)-9-[4-Hydroxy-2-(hydroxymethyl)butyl]guanine was evaluated for its efficacy in African green monkeys infected with simian varicella virus. Treatment by intramuscular injection was initiated 48 h after virus inoculation and was continued for 10 days; the treatment showed therapeutic effects on rash and viremia at dosages down to 1 mg/kg of body weight per day.

Future prospects in antiviral therapy

Two important stumbling blocks to the development of effective and nontoxic antiviral drugs are the intracellular localization of the virus and the fact that a virus uses host cell functions to multiply. Therefore, new antiviral drugs must act on a virus-specific function. Most currently available useful antiviral drugs are the result of compound screening of large numbers of possible agents. Advances in our understanding of the molecular biology and biochemistry of the viral multiplication cycle and new laboratory techniques for determining the molecular sites of action have now made it possible to develop and screen new antiviral drugs in a more purposeful manner. Another possible option in antiviral therapy is combination therapy using drugs that enhance the therapeutic effect or diminish side-effects. The most promising new antiviral drugs are discussed according to the different steps they affect in the viral multiplication process. Combination therapy is also reviewed.

Improved fluorometric high-performance liquid chromatographic assay for (-)-carbovir in rat blood and urine

Carbovir is a carbocyclic guanosine analogue with potent in vitro activity against the human immunodeficiency virus. All of the activity resides in the (-)-enantiomer. An ion-paired liquid chromatographic assay for (-)-carbovir was developed on a Spherisorb C8 column with fluorescence detection (275 nm excitation, 345 nm emission). Guanosine nucleosides are fluorescent at a pH less than 2.5, and fluorescence detection resulted in a four-fold improvement in the limit of quantitation (0.039 microgram/ml) compared to the previously developed assay with ultraviolet detection. Standard curves were processed with an internal standard at (-)-carbovir concentrations of 0.039-40 micrograms/ml in whole rat blood with a solid-phase extraction technique. Total variability was less than 16% at all concentrations and less than 10% at concentrations greater than 0.3 microgram/ml. Within-day variability was less than 7.5% at concentrations greater than 0.3 microgram/ml. Urine was analyzed directly after dilution and an diethyl ether wash to remove impurities. The total coefficients of variation were less than 10% from 0.5-20 micrograms/ml in urine. The concentrations of (-)-carbovir in rat blood were detectable for as long as 8 h after intravenous and oral doses of 20 and 60 mg/kg, respectively.