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Case 2: Discussion

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)[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)[1].

Acyclovir: Mechanisms for Resistance

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)[3,4,5]. 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[6]. 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)[5,7]. 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[5].

Background and Epidemiology

The first reported case of acyclovir-resistant HSV infection in a HIV-infected patient appeared in 1982[8]. Subsequent studies established that HIV-infected persons have a markedly higher incidence of acyclovir-resistant HSV infection when compared with HIV-negative persons[9]. Most cases of acyclovir-resistant HSV have involved HSV type 2[5,10]. 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)[9,11]. Rare reports have documented acyclovir-resistant HSV infection in persons without prior treatment with acyclovir, famciclovir, or valacyclovir[5]. Person-to-person transmission of acyclovir-resistant HSV has not been documented.

Clinical Manifestations

Although prior research in animal models suggested that thymidine kinase-negative HSV isolates have decreased virulence[12], several reports have shown that acyclovir-resistant HSV can cause significant clinical disease in humans[10,11,12,13]. Most HIV-infected persons who develop acyclovir-resistant HSV present with gradually expanding, extensive, ulcerated lesions that fail to respond to conventional therapy[7,10,14]. 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[13]. 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[11].


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[5]. 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[5], with non-resistant isolates typically having an IC50 of approximately 0.1 ug/ml[3,12]. Within a single lesion, heterogeneous virus populations may exist composed of susceptible and resistant strains.

Recommended Therapy

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 such as ganciclovir (Cytovene) and valganciclovir (Valcyte), that use a similar viral kinase 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 in these studies foscarnet was used at a dose at 40-60 mg/kg every 8 hours, with reduced dosage for renal insufficiency) for to 10 to 50 days[14,15,16]. The 2009 guidelines for the prevention and treatment of opportunistic infections recommend 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[17]. The duration of therapy is usually at least 21 to 28 days (Figure 6).

Alternative Therapy

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[18,19] or intravenous cidofovir[20]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[21]. More recently, two reports have described excellent clinical response with topical imiquimod (Aldara) 5% cream[22,23]. The 2009 opportunistic infections guidelines note topical therapy with trifluridine, cidofovir, and imiquimod has been successful as shown in case reports and these topical agents are listed as alternative therapies to foscarnet[17]. Topical therapy may require prolonged application (at least 21 to 28 days)[17].

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    Figure 1. Mechanism of Action of Acyclovir

    In this HSV-infected human cell, the acyclovir molecules enter the cell and are converted to acyclovir monophosphate by the HSV enzyme thymidine kinase (TK). Enzymes in the human cell add two more phosphates to eventually form the active drug acyclovir triphosphate. The acyclovir triphosphate competes with 2-deoxyguanosine triphosphate (dGTP) as a substrate for viral DNA polymerase, as well as acting as a chain terminator. In actual infection, the HSV releases its naked capsid that delivers DNA to the human nucleus; the active drug acyclovir triphosphate exerts its action on the viral DNA located in the nucleus.

    Figure 1
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    Figure 2. Most Common Mechanism of Acyclovir Resistance in Herpes Simplex Virus Infection

    The most common types of acyclovir-resistant HSV infection involve either absent production of viral thymidine kinase (TK-negative mutants), or partially reduced production of viral thymidine kinase (TK-partial mutants). In this illustration, the notation TK- refers to deficient production of viral thymidine kinase, either through TK-negative mutants or TK-partial mutants. As a result of the deficient production of TK, the HSV strains fail to effectively generate acyclovir triphosphate and thus acyclovir does not effectively inhibit viral DNA polymerase.

    Figure 2
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    Figure 3. Perianal Lesion Caused by Acyclovir-Resistant Herpes Simplex Virus
    Figure 3
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    Figure 4. Perioral Lesion with Extension to Face Caused by Acyclovir-Resistant Herpes Simplex Virus
    Figure 4
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    Figure 5. Hypertrophic Inguinal Lesion Caused by Acyclovir-Resistant Herpes Simplex Virus
    Figure 5
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    Figure 1. Therapy for Acyclovir-Resistant Mucocutaneous HSV

    This table is based on recommendations from Kaplan JE, Benson C, Holmes KK, et al. Guidelines for prevention and treatment of opportunistic infections in HIV-infected adults and adolescents: recommendations from CDC, the National Institutes of Health, and the HIV Medicine Association of the Infectious Diseases Society of America. MMWR Recomm Rep. 2009;58(RR-4):1-207.

    Figure 6