Acyclovir: Mechanism of Action
The drug acyclovir (Zovirax) does not have activity against viral pathogens until it is converted to the active form acyclovir triphosphate. This process is initiated by the viral enzyme thymidine kinase; subsequently, human cellular kinases perform the second and third phosphorylation steps to complete the process (Figure 1).[,] Acyclovir triphosphate, the active form of acyclovir, is present in 40- to 100-fold higher concentrations in herpes simplex virus (HSV)-infected cells than in uninfected cells. Acyclovir triphosphate has a two-pronged mechanism of action: (1) it competes with 2-deoxyguanosine triphosphate (dGTP) as a substrate for viral DNA polymerase and (2) once it becomes incorporated into the replicating viral DNA, it acts as a chain terminator because it does not have a terminal 3' hydroxyl group. In contrast, penciclovir, the active component of the prodrug famciclovir (Famvir), does not act as a chain terminator because it has a terminal 3' hydroxyl group that permits viral primer-template extension. Foscarnet (Foscavir) is a pyrophosphate analog that directly inhibits viral DNA polymerase by reversibly blocking the pyrophosphate binding site of the viral polymerase (in addition to inhibiting the cleavage of pyrophosphate from deoxynucleotide triphosphates).
Mechanism of Acyclovir Resistance in HSV
Acyclovir resistance can occur from one of four types of mechanisms: (1) absent production of viral thymidine kinase (TK-negative mutants); (2) a partial decrease in the production of viral thymidine kinase (TK-partial mutants); (3) altered viral thymidine kinase substrate specificity that results in phosphorylation of thymidine but not acyclovir (TK-altered mutants); and (4) altered viral DNA polymerase (DNA polymerase mutants).[,,] The herpes thymidine kinase is a 376-amino-acid protein encoded by the UL23 gene and the herpes DNA polymerase is a larger protein (approximately 1,200-amino-acids long) and it is encoded by the UL30 gene. The DNA polymerase mutants result from a mutation in the HSV pol gene that causes a decreased binding of acyclovir-triphosphate to viral DNA polymerase. Among the different types of resistance mutations, the most common are the absent or decreased production of viral thymidine kinase (TK-negative and TK-partial mutants (Figure 2).[,] Given that most assays can not easily differentiate low TK-partial mutants from TK-negative mutants, some experts have referred to both of these mutants as TK-deficient mutants.
Epidemiology of Acyclovir-Resistant HSV in HIV-infected Patients
The first reported case of acyclovir-resistant HSV infection in a HIV-infected patient appeared in 1982. Subsequent studies established that HIV-infected persons have a markedly higher incidence of acyclovir-resistant HSV infection when compared with HIV-negative persons. Most cases of acyclovir-resistant HSV have involved HSV type 2.[,] Among HIV-infected individuals with acyclovir-resistant HSV infection, they typically have advanced AIDS, a history of recurrent HSV infection, and significant prior treatment with acyclovir, famciclovir, or valacyclovir (Valtrex).[,] Rare reports have documented acyclovir-resistant HSV infection in persons without prior treatment with acyclovir, famciclovir, or valacyclovir. Person-to-person transmission of acyclovir-resistant HSV has not been documented.
Although prior research in animal models suggested that thymidine kinase-negative HSV isolates have decreased virulence, several reports have shown that acyclovir-resistant HSV can cause significant clinical disease in humans.[,,,] Most HIV-infected persons who develop acyclovir-resistant HSV present with gradually expanding, extensive, ulcerated lesions that fail to respond to conventional therapy.[,,] These lesions can develop anywhere on the body, but most often occur in the perianal (Figure 3) or oral area (Figure 4). Less frequently, patients can develop raised, hypertrophic lesions (Figure 5). Mucocutaneous dissemination rarely occurs. One report described a patient with fatal acyclovir-resistant HSV meningoencephalitis who developed acyclovir resistance during therapy of a persistent perirectal ulcer caused by HSV.
The first goal is to establish the diagnosis of HSV. If acyclovir resistance is suspected, testing for HSV should include a viral culture of the sample, since resistance testing will require isolation of virus. Laboratory testing for acyclovir resistance includes a variety of phenotypic assays, such as the plaque-reduction assay, dye-uptake assays, and viral DNA-inhibition assays. Among these assays, the plaque reduction assay is the most frequently used and resistance is characteristically defined as an IC50 greater than 2 ug/ml, with non-resistant isolates typically having an IC50 of approximately 0.1 ug/ml.[,] Within a single lesion, heterogeneous virus populations may exist composed of susceptible and resistant strains.
All of the drugs that require initial activation by HSV thymidine kinase (acyclovir, famciclovir, and valacyclovir) generally produce ineffective results against HSV strains that have absent or decreased thymidine kinase production. Drugs that requiree activation via a similar viral kinase, such as ganciclovir (Cytovene) and valganciclovir (Valcyte), are also ineffective. Rarely, acyclovir-resistant HSV infections result from altered viral thymidine kinase and these strains of HSV could theoretically respond to famciclovir. Because foscarnet does not require activation by viral thymidine kinase, it retains activity against HSV strains with absent, partially reduced, or altered production of thymidine kinase. Several clinical studies have shown foscarnet is the most effective drug in treating acyclovir-resistant herpes simplex lesions and it was typically given for 10 to 50 days.[,,] The 2013 guidelines for the prevention and treatment of opportunistic infections recommends using foscarnet as first-line therapy given at a dose of 80 to 120 mg/kg/day IV in 2 to 3 divided doses until a clinical response has occurred (Figure 6). The duration of therapy is usually at least 21 to 28 days. Foscarnet can cause significant adverse effects and administration of foscarnet therapy for acyclovir-resistant HSV should be done by an expert or with expert consultation.
Cidofovir (Vistide), formerly HPMPC, is an antiviral compound that reaches the cell in a monophosphorylated form whereby it is activated by cellular kinases. Because cidofovir does not require activation by a viral kinase, it should theoretically have activity against acyclovir-resistant HSV infections. Several reports have suggested that use of either topical cidofovir 0.3% or 1.0% gel applied once daily[,] or intravenous cidofovir were effective in the treatment of acyclovir-resistant HSV infections, but there are insufficient data to make significant conclusions regarding the use of cidofovir for acyclovir-resistant HSV infections. One AIDS Clinical Trial Group study reported fair responses to topical 1% ophthalmic trifluridine solution (Viroptic) among patients with acyclovir-resistant HSV infections. More recently, two reports have described excellent clinical response with topical imiquimod (Aldara) 5% cream.[,] The 2013 opportunistic infections guidelines list topical therapy with trifluridine, cidofovir, and imiquimod, along with intravenous cidofovir as alternative therapies for the treatment of acyclovir-resistant mucocutaneous HSV, with a duration of 21 to 28 days or longer, based on the patient's clinical response (Figure 6).