Viral thymidine kinase

FIAC also strongly inhibits HCMV and Epstein-Barr vims (EBV) in vitro the two vimses known not to induce a specific viral thymidine kinase for their repHcation. However, HCMV may stimulate cellular kinases that can anabolize FIAC to its 5 -triphosphate, which specifically inhibits the HCMV-encoded DNA polymerase. This selective activity suggests that FIAC should be evaluated against HCMV infections. FIAC-ttiphosphate incorporated into DNA has shown strong in vitro activity against the DNA polymerases of human hepatitis B vims (HBV) and of woodchuck hepatitis vims (WHV) (37). 

Acyclovir Zovirax) is a guanine nucleoside analogue most effective against HSV-1 and HSV-2, but it has some activity against VCV, CMV, and EBV. Valacyclovir (Valtrex) is the L-valine ester prodrug of acyclovir. Acyclovir is converted to its active metabolite via three phosphorylation steps. First, viral thymidine kinase converts acyclovir to acyclovir monophosphate. Next, host cell enzymes convert the monophosphate to the diphosphate and then to the active compound, acyclovir triphosphate. Because viral thymidine kinase has a much greater affinity for acyclovir triphosphate than does mammalian thymidine kinase, acyclovir triphosphate accumulates only in virus-infected cells. 

Cidofovir is approved for the treatment and prophylaxis of CMV retinitis in AIDS patients. It has also been used in the treatment of acyclovir-resistant (viral thymidine kinase-dehcient) HSV infections, polyomavirus-associated progressive multifocal leukoencephalopathy, condylomata acuminata (anogenital warts), and mollus-cum contagiosum. 

Famciclovir (Famvir) is the diacetyl ester prodrug of the acyclic guanosine analogue 6-deoxypenciclovir Dena-vir). Penciciovir has activity against HSV-1, HSV-2, VZV, and HB V. After oral administration, famciclovir is converted to penciciovir by first-pass metabolism. Penciciovir has a mechanism of action similar to that of acyclovir. It is first monophosphorylated by viral thymidine kinase then it is converted to a triphosphate by cellular kinases. Penciciovir triphosphate acts as a competitive inhibitor of viral DNA polymerase, but unlike acyclovir, it does not cause chain termination. 

L D. The conversion of penciclovir to its active form requires initial monophosphorylation by viral thymidine kinases, then conversion to its active triphosphate form by cellular enzymes. Thus, the concentration of penciclovir triphosphate is particularly high in cells infected with its target viruses (e.g., HSV, VZV, HBV). Foscarnet is a pyrophosphate analogue that does not require activation. Oseltamivir is a neuraminidase inhibitor that is con-... 

Resistance to acyclovir can develop in HSV or VZV through alteration in either the viral thymidine kinase or the DNA polymerase, and clinically resistant infections have been reported in immunocompromised hosts. Most clinical isolates are resistant on the basis of deficient thymidine kinase activity and thus are cross-resistant to valacyclovir, famciclovir, and ganciclovir. Agents such as foscarnet, cidofovir, and trifluridine do not require activation by viral thymidine kinase and thus have preserved activity against... 

Mechanism of Action. Acyclovir inhibits viral DNA replication by inhibiting the function of the DNA polymerase enzyme.42 This drug is taken into virus-infected cells and converted to acyclovir triphosphate by an enzyme known as viral thymidine kinase 42 The phosphorylated drug directly inhibits the function of the viral DNA polymerase, thus impairing the replication of viral genetic material. The virus also incorporates the drug into viral DNA strands, which halts further production of DNA because of the presence of a false nucleic acid.42... 

Mechanism of Action. Foscarnet works somewhat like acyclovir and ganciclovir that is, foscarnet inhibits the DNA polymerase enzyme necessary for viral DNA replication. Foscarnet differs from these other antiviral drugs, however, in that it does not require phosphorylation (activation) by enzymes such as viral thymidine kinase. Certain strains of viruses are thymidine-kinase deficient, meaning that these viruses lack the enzyme needed to activate antiviral agents... 

Wagner, E.F., Stewart, TA., and Mintz, B. (1981) The human fi-globin gene and a functional viral thymidine kinase gene in developing mice. Proc. Natl. Acad. Sci. USA 78, 5016-5020. 

In the past, various serendipitous discoveries have capitalized on the differential expression of enzymes by host and viral infected cells. For example, the prodrug Acyclovir, used widely for the treatment of herpes simplex and herpes zoster infections, is selectively activated through phosphorylation by viral thymidine kinase to acyclovir monophosphate which is then converted to the triphosphate, which inhibits DNA polymerase, by host cellular enzymes. Similarly several 2, 3 -dideoxynucleoside analogs such as Zidovudine (azidothymidine, AZT) and 2, 3 -didehydro-3 -deoxythymidine (D4T) have potent antiviral activity against human immunodeficiency vims (HIV). These compounds are selectively phosphoiylated intracellularly to the 5 -triphosphate derivatives which inhibit the viral reverse transcriptase. 

Resistance Altered or deficient thymidine kinase and DNA polymerases have been found in some resistant viral strains. [Note Cytomegalovirus (CMV) is resistant because it lacks a specific viral thymidine kinase.]... 

Penciclovir [pen SIK lo veer] is an acyclic guanosine nucleoside derivative that is active against herpes simplex virus Types I and II, and against varicella-zoster virus. Penciclovir is only administered topically (Figure 37.8). Penciclovir is monophosphorylated by viral thymidine kinase, and cellular enzymes form the nucleoside triphosphate, which inhibits herpes DNA polymerase. Penciclovir triphosphate has an intracellular half-life 20 to 30 times longer than does acyclovir triphosphate (see p. 365). Penciclovir is negligibly absorbed from topical application, and is well tolerated. Both healing and pain are shortened approximately one-half day in duration, compared to placebo-treated subjects. 

Aciclovir is an acyclic purine nucleoside. Its antiviral activity depends upon intracellular phosphorylation to its triphosphate derivative. Because of its higher affinity for viral thymidine kinase, aciclovir is phosphorylated at a much higher rate by the viral enzyme. Thus, it is almost exclusively active in infected cells, fulfilling one of the selectivity principles of antiviral drugs. In addition, aciclovir triphosphate serves as a better substrate for viral than for host cell DNA polymerase and thereby causes preferential termination of viral DNA sjmthesis (1). 

Acyclovir is an acyclic guanosine derivative that requires three phosphorylation steps for activation. Conversion to the monophosphate is carried out by the virus thymidine kinase, and conversion to the di- and triphosphate is carried out by host kineases. The active nucleotide triphosphate inhibits viral replication by competing with the viral deoxy-GTP for the viral DNA polymerase. Thus, acyclovir triphosphate is inserted into the growing viral DNA, leading to irreversible chain termination. Reduced susceptibility to acyclovir is due to altered viral thymidine kinase. Because the MOA of ganciclovir is similar to that of acyclovir, HSV that is resistant to acyclovir is also resistant to ganciclovir. 

Acyclovir-resistant HSV has been isolated from patients with AIDS. The primary mechanism of resistance appears to be a deficiency in viral thymidine kinase. Strategies that have been employed for management of severe acyclovir-resistant HSV infections include increasing the dose of acyclovir, discontinuing acyclovir, and use of an alternative antiviral agent. Vidarabine and foscarnet, because they do not require phosphorylation by thymidine kinase, are examples of potential alternative agents. A randomized comparison of foscarnet and vidarabine indicated that foscarnet is more effective and associated with fewer adverse reactions than vidarabine. ... 

Treatment of HSV typically involves the nse of nucleoside analogs such as acyclovir (acycloguanosine) that inhibit the viral DNA polymerase. Acyclovir is a prodrng that is activated by viral thymidine kinase. The activated drug, acyclo-GTP, inhibits the viral DNA polymerase by acting as a chain terminator when it is incorporated into viral DNA. 

Monophosphorylated by viral thymidine kinase (TK), then further bioactivated by host-cell kinases to the triphosphate. Acyclovir-triphosphate is both a substrate for and inhibitor of viral DNA polymerase when incorporated into the DNA molecule, it acts as a chain terminator because it lacks the ribosyl 3 hydroxyl group. 

B. mutation in the gene that encodes DNA polymerase G loss of ability to produce viral thymidine kinase... 

Fluoroidoaracystosine (FIAC, Fig. 7-16) is a potent and selective inhibitor of herpes viruses. Like acyclovir and BVDU, it, too, must be phosphorylated by viral thymidine kinase to act. Its selectivity toward the viral enzyme is several thousand times higher so that it is active against HSV-1 and -2 and herpes zoster virus (HZV) with a good therapeutic index. Clinical studies seem promising. 

Acycloguanosine — P — P — P Inhibits viral thymidine kinase Inhibition of viral DNA synthesis... 

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