Lessons learned from marketed and investigational prodrugs

Acyclovir, penciclovir, and their prodrugs have been widely used during the past two decades for the treatment of herpesvirus infections. In spite of the distribution of over 2.3 × 10 6 kg of these nucleoside analogues, the prevalence of acyclovir resistance in herpes simplex virus isolates from immunocompetent hosts has remained stable at approximately 0.3%. In immuncompromised patients, in whom the risk for developing resistance is much greater, the prevalence of resistant virus has also remained stable but at a higher level, typically 4 to 7%. These observations are examined in the light of characteristics of the virus, the drugs, and host factors.

Herpes Simplex Virus Resistance to Acyclovir and Penciclovir after Two Decades of Antiviral Therapy

Improved oral drug delivery: solubility limitations overcome by the use of prodrugs

Herpes simplex viruses (HSV) type 1 and type 2 are responsible for recurrent orolabial and genital infections. The standard therapy for the management of HSV infections includes acyclovir (ACV) and penciclovir (PCV) with their respective prodrugs valacyclovir and famciclovir. These compounds are phosphorylated by the viral thymidine kinase (TK) and then by cellular kinases. The triphosphate forms selectively inhibit the viral DNA polymerase (DNA pol) activity. Drug-resistant HSV isolates are frequently recovered from immunocompromised patients but rarely found in immunocompetent subjects. The gold standard phenotypic method for evaluating the susceptibility of HSV isolates to antiviral drugs is the plaque reduction assay. Plaque autoradiography allows the associated phenotype to be distinguished (TK-wild-type, TK-negative, TK-lowproducer, or TK-altered viruses or mixtures of wild-type and mutant viruses). Genotypic characterization of drug-resistant isolates can reveal mutations located in the viral TK and/or in the DNA pol genes. Recombinant HSV mutants can be generated to analyze the contribution of each specific mutation with regard to the drug resistance phenotype. Most ACV-resistant mutants exhibit some reduction in their capacity to establish latency and to reactivate, as well as in their degree of neurovirulence in animal models of HSV infection. For instance, TK-negative HSV mutants establish latency with a lower efficiency than wild-type strains and reactivate poorly. DNA pol HSV mutants exhibit different degrees of attenuation of neurovirulence. The management of ACV- or PCV-resistant HSV infections includes the use of the pyrophosphate analogue foscarnet and the nucleotide analogue cidofovir. There is a need to develop new antiherpetic compounds with different mechanisms of action. Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) are responsible for recurrent orolabial and genital infections. In the United States, the seroprevalence of HSV-1 and HSV-2 infections has been estimated to be approximately 50% and 20%, respectively (243). Transmission occurs by contact with secretions from an infected person with either overt infection or asymptomatic excretion of virus. After primary infection, HSV establishes long-term latency in the ganglia of sensory nerves, from which it can reactivate episodically. These reactivations may be accompanied by symptoms or may be clinically silent. The most common clinical manifestations of HSV infections are vesicular lesions affecting the mucous membranes principally of the mouth, nose, or eyes for HSV-1 and the anogenital region for HSV-2. However, an increasing proportion of genital infections are caused by HSV-1 (243). The symptoms associated with both viruses are usually self-limiting, and the goals of treatment are to accelerate lesion healing and to prevent transmission in immunocompetent individuals (61). Both HSV-1 and HSV-2 can also cause infrequent but serious diseases, such as encephalitis and disseminated neonatal infections. HSV infections may be severe in immunocompromised patients, particularly those with defects in cell-mediated immunity. In patients with HIV infection and in recipients of solid organ or bone marrow transplants, herpetic lesions can be extensive, tend to persist for longer periods, and have the potential to disseminate. Recurrences tend to be more frequent and may be atypical in appearance (140). In addition to persistent mucocutaneous lesions, HSV can also lead to disseminated visceral infections, such as esophagitis, hepatitis, or pneumonia, as well as meningoencephalitis, in immunocompromised hosts (94, 99, 174).

Resistance of Herpes Simplex Viruses to Nucleoside Analogues: Mechanisms, Prevalence, and Management

Purpose. General use of nucleoside analogues in the treatment of viral infections and cancer is often limited by poor oral absorption. Valacyclovir, a water soluble amino acid ester prodrug of acyclovir has been reported to increase the oral bioavailability of acyclovir but its absorption mechanism is unknown. This study characterized the intestinal absorption mechanism of 5′-amino acid ester prodrugs of the antiviral drugs and examined the potential of amino acid esters as an effective strategy for improving oral drug absorption.

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5′-Amino Acid Esters of Antiviral Nucleosides, Acyclovir, and AZT Are Absorbed by the Intestinal PEPT1 Peptide Transporter

Summary Eighteen amino acid esters of the antiherpetic drug, acyclovir, were synthesized as potential prodrugs for oral administration. The esters were examined for in vitro antiviral activity against herpes simplex virus Type 1 (HSV-1). They were found to have less potency than the parent compound. Their efficiencies as pro­ drugs were evaluated in rats by measuring the urinary recovery of acyclovir. Ten prodrugs produced greater amounts of the parent drug in the urine. The L-amino acid esters were better prodrugsthan the correspond­ ing D- or D,L-isomers, suggesting the involvement of a stereoselectivetransporter. The L-valyl ester, 256U87, was the best prodrug. Sixty three per cent of its administered dose was excreted as acyclovir in the urine, a considerable improvement over acyclovir itself, for which this value was 19%. Since 256U87 was stable in aqueoussolutions, its conversion to acyclovir in vivo was probably enzyme catalyzed. This L-valyl ester prodrug of acyclovir is now undergoing clinical evaluation.

T. Bumette

Papers

Amino acid ester prodrugs of acyclovir