Medical Pharmacology Chapter 36:  Antiviral Drugs

• Antiviral Drugs

  • Acyclovir and Related Drugs continued:

    • Drug Resistance

      • Acyclovir and its derivatives represent first-line drugs both for HSV infection prophylaxis as well as for treatment.⁴ 

        • These agents are "prodrugs" and require several phosphorylation steps to form the active compound.

        • The first phosphorylation step is dependent on a virus-encoded thymidine kinase (TK).

        • Host cellular GMP kinase and nucleoside diphosphate kinase catalyze the two additional phosphorylation steps.

      • Long-term administration of acyclovir and related agents can lead to the development of resistance, particularly in immunocompromised patients. ⁴ 

        • The likelihood of the development of acyclovir-resistance in herpes simplex virus isolates (HSV isolates) is quite different between immunocompetent and immunocompromised individuals.

        • The most important factor may be that host responses are impaired and less pathogenic viral strains that normally would not survive in the context of a normal host response may continue to replicate.

          • The prevalence of acyclovir resistance in HSV in immunocompetent patients ranges from about 0.3% to 0.7%.

          • By contrast, the prevalence of HSV infections with diminished acyclovir susceptibility in immunocompromised patients is about 10 times higher, 3.5% to about 10%.

      • Taking into account the mechanism of acyclovir-antiviral action, viral mutations that confer resistance to acyclovir have been described in the UL23 gene which codes for the viral thymidine kinase (TK) which catalyzes the initial acyclovir phosphorylation step.⁴  

      • Another mutation sites that confers resistance is associated with the UL30 gene which codes for the DNA polymerase enzyme.⁴  

        • HSV depends on a functional DNA polymerase to replicate; whereas, an intact viral thymidine kinase may be dispensable in some mammalian tissues.

          • Accordingly, a viable acyclovir-resistant virus is more likely found as a result of a mutation in the UL23 gene compared to UL30 gene mutations.

          • Several phenotypes for acyclovir-resistance thymidine kinase (TK) mutants have been described.

            • These include TK-negative mutants which lack thymidine kinase catalytic activity, TK-low-producer mutants which results in decreased enzymatic activity nad TK-altered isolates show substrate-specific mutations in that they phosphorylate thymidine but not acyclovir.

            • Most (95%) of acyclovir-resistant HSV clinical isolates are either TK-negative or TK-low-producer mutants.⁴ 

      •  

        Mechanism of Action of Antiviral Agents with Activity Against Herpesviruses⁵

        "TK, thymidine kinase ANP, acyclic nucleotide phosphate P, phosphate DP, diphosphate TP, triphosphate dNTP, deoxynucleotide triphosphate". Attribution:  Fig. 1 (slightly modified) from reference 5.

   

  • Acyclic Nucleotide Phosphonates, e.g. Cidofovir (Vistide)

    • The drugs considered to this point are "classical" acyclic nucleoside analogues.

      • Examples of these drugs include acyclovir, ganciclovir and penciclovir.

        • Furthermore, other agents not yet discussed are similar in requiring an initial phosphorylation step.

          • Examples of these drugs include dideoxynucleoside analogues such as zidovudine (AZT) and lamivudine (3TC).

            • Therefore, in cells in which the initial nucleoside kinase exhibits reduced activity or is even not present the nucleoside analogues are inactive.

    • By contrast, another class of drugs are phosphonate derivatives, the acyclic nucleoside phosphonate's or ANPs.

      • ANPs are converted by nucleotide kinase (GMP kinase or AMP kinase) to a monophosphate form, an analog of diphosphate and then to the triphosphate analog by a nucleoside diphosphate kinase.

        • The active form is represented by the triphosphate (ANPpp) analogues.

        • Cidofovir (Vistide) is one example.

          • Cidofovir (Vistide)

        •  

          Acyclovir (left) and Acyclovir Monophosphate (right)⁶

          The structure on the left is acyclovir and the structure on the right is acyclovir monophosphate, the formation of which is likely dependent on a viral kinase.  The active drug is acyclovir triphosphate, formed by two subsequent phosphorylation steps catalyzed by host cell kinases.6 Attribution:  Figure slightly modified from structures presented in reference 6.

           

        • Acyclovir (left) and Cidofovir (right)⁶

          The structure on the left is acyclovir; three phosphorylation steps are needed to form the active drug, acyclovir triphosphate.6 The first and critical step is dependent on a viral kinase.  If that kinase is absent, as may occur in drug resistance, the active drug will not be effective. By contrast, the structure on the right is cidofovir already exhibiting a phosphonate group. Therefore, the "first step" is not needed.  To transition from the inactive prodrug cidofovir to the active diphosphylated form, only two steps are needed in total.  This process then does not depend on the presence of viral kinase activity.6 Attribution:  Figure slightly modified from structures presented in reference 6.
          Cidofovir (left) is a phosphonate and requires only two phosphorylation steps (1 and 2) for transformation active diphosphorylated species. Attribution:  Figure slightly modified from structures presented in reference 6.

       

      • Cidofovir (Vistide) inhibits viral DNA synthesis.

        • This inhibition occurs by terminating viral DNA chain elongation.⁶

          • In the case of cidofovir, two consecutive ("tandem") incorporations are required to terminate DNA in an efficient way in accord with the mechanism demonstrated for cytomegaloviral inhibition.⁶

            • Other acyclic phosphonate-type antiviral drugs exhibit slightly different mechanisms.⁶  

              • For example, adefovir (Preveon) acts by incorporating one molecule at the 3'-end of the growing DNA chain with this addition sufficient to terminate elongation.

              • This requirement is also noted for tenofovir.⁶  

        • As cidofovir is a phosphonate, phosphorylation proceeds only by cellular, not viral enzymes.¹ 

          • Accordingly, cidofovir can inhibit acyclovir-resistant thymidine kinase (TK) or thymidine kinase-modified herpes simplex virus (HSV) or varicella-zoster virus (VZV) strains.¹ 

          • Additionally, ganciclovir-resistant cytomegalovirus (CMV) strains exhibiting UL97 mutations (not strains with DNA polymerase mutations) would likely be sensitive to cidofovir.

          • Cidofovir is also effective in some foscarnet-resistant CMV isolates.

            • Cidofovir in combination with ganciclovir or foscarnet inhibits CMV replication in a synergistic manner.¹

          • The molecular target for cidofovir is the viral DNA-dependent DNA polymerase.⁶

            • For tenofovir and adefovir the molecular target in the human immunodeficiency virus (HIV) or hepatitis B is the virus RNA-dependent DNA polymerase (reverse transcriptase)⁶

      • Clinical Case:  Intralesion Cidofovir Treatment of Acyclovir-Resistant Herpes Simplex Virus⁷

        An example of treating acyclovir-resistant HSV with cidofovir is noted in the following clinical case.7  In this case a young HIV-positive male with a history of recurrent genital HSV infection presented with a left-sided nasal lesion. Biopsy specimens characterized extensive granulation tissue and noted that an "indurated, ulcerated plaque recurred and progressed despite surgical removal and corticosteroid injections." 15 months later the nasal lesion had extended to the other nares. Biopsy and pathologic examination revealed HSV changes with significant granulation but absent evidence of T-cell lymphoma or infection. On an outpatient basis the patient had been treated on multiple occasions with oral acyclovir for genital herpes simplex infections without nasal lesion improvement. The mass recurred again despite subsequent pharmacological treatment with IV acyclovir and a partial rhinectomy. The patient was then treated several times with IV acyclovir, vancomycin, along with further surgical debridement. Biopsies of the lesion both confirmed HSV infection and indicated clinical acyclovir resistance.7   This time the patient began receiving IV cidofovir and experienced total lesion clearance after about six weeks of treatment. Upon therapy discontinuation, however, the lesion recurred rapidly an extended towards the lower eyelid. IV cidofovir treatment was restarted. However, the notion of long-term use of IV cidofovir was considered problematic given the risk of renal toxicity. Accordingly, other treatment options were considered. After IV cidofovir had been initiated and only limited lesion improvement noted, the patient began receiving intralesional (IL) cidofovir. Such intralesional injections have been performed in other settings and described in the primary literature. Following intralesional cidofovir treatment, the lesion exhibited substantial improvement.7   "Progression of patient's herpes simplex virus lesion. A, Initial lesion after debridement. B, Recurrence of lesion after treatment with intravenous cidofovir. C. Progression of lesion after initiation of intralesional cidofir. Skin lesion after final treatment with intralesional cidofovir."7 The reader is directed to this case study for more detail.7  Castelo-Soccio L Bernardin R Stern J Goldstein SA Kovarik C Successful Treatment of Acyclovir-Resistant Herpes Simplex Virus with Intralesional Cidofovir.  Arch Dermatol 146(2):  124-126, February 2010. (complete reference) NIH Public Access Author Manuscript:  http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2874880/