Resistance of herpes simplex viruses to nucleoside analogues: mechanisms, prevalence, and management

Abstract

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-low-producer, 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.

FIG. 1.

Mutations identified in the UL23 gene of HSV-1 (A) and HSV-2 (B) isolates resistant to ACV. The ATP-binding site (ATP), the nucleoside-binding site (NBS), and the six regions of the UL23 gene that are conserved among Herpesviridae are shown by the black boxes. The six highly conserved regions are located at amino acids (AA) 56 to 62 (site 1), 83 to 88 (site 2), 162 to 164 (site 3), 171 to 173 (site 4), 216 to 222 (site 5), and 284 to 289 (site 6) for HSV-1 and 56 to 62, 83 to 88, 163 to 165, 172 to 174, 217 to 223, and 285 to 290 for HSV-2. The additions (a), deletions (d), or both additions and deletions (a/d) reported in homopolymer runs, as well as the nucleotides (Nt) involved, are indicated below vertical bars. Substitutions of amino acids reported in the UL23 gene that are included in the boxes correspond to those identified in conserved regions, and those outside the boxes are located in nonconserved regions. Underlined mutations correspond to the HSV-2 mutations.

FIG. 2.

Mutations identified in the UL30 gene of HSV-1 (A) and HSV-2 (B) isolates resistant to ACV. Regions conserved among Herpesviridae genes are shown by the black boxes. The roman numbers (I to VII and δ-region C) corresponding to each of these regions are indicated above the boxes. Amino acid (AA) locations are noted below each of these regions for HSV-1 and HSV-2. Substitutions reported in the UL30 gene that are included in the boxes correspond to those identified in conserved regions, and those outside the boxes are located in nonconserved regions. Underlined mutations correspond to the HSV-2 mutations. Mutations E460D, G464V, K522E, and P561S in and outside Exo II are lethal to the virus; mutations Y577H and D581A in the Exo III motif in δ-region C are associated with hypersusceptibility to ACV; and none of the mutations in region I are spontaneously induced.

FIG. 3.

Management of HSV infections that are unresponsive to nucleoside analogues. IV, intravenous.