Thymidine kinase acyclovir

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. 

FIGURE 49 2 Conversion of acyclovir to acyclovir triphosphate leading to DNA chain termination. Acyclovir is converted to the monophosphate (MP) derivative by a herpes virus thymidine kinase. Acyclovir-MP is dien phosphory-lated to acyclovir-DP and acyclovir-TP by cellular enzymes. Uninfected cells convert very little or no drug to the phos-phorylated derivatives. Thus, acyclovir is selectively activated in cells infected with herpesviruses that code for appropriate thymidine kinases. Incorporation of acyclovir-MP from acyclovir-TP into the primer strand during viral DNA replication leads to chain termination and formation of an inactive complex with the viral DNA polymerase. (Adapted from Elion, 1982, with permission.)... 

The antiviral mechanism of action of acyclovir has been reviewed (72). Acyclovir is converted to the monophosphate in herpes vims-infected cells (but only to a limited extent in uninfected cells) by viral-induced thymidine kinase. It is then further phosphorylated by host cell guanosine monophosphate (GMP) kinase to acyclovir diphosphate [66341 -17-1], which in turn is phosphorylated to the triphosphate by unidentified cellular en2ymes. Acyclovir triphosphate [66341 -18-2] inhibits HSV-1 viral DNA polymerase but not cellular DNA polymerase. As a result, acyclovir is 300 to 3000 times more toxic to herpes vimses in an HSV-infected cell than to the cell itself. Studies have shown that a once-daily dose of acyclovir is effective in prevention of recurrent HSV-2 genital herpes (1). HCMV, on the other hand, is relatively uninhibited by acyclovir. 

Moderate in vivo antiherpes vims activity was demonstrated by 9-P-Dxylofuranosylguanine [27462-39-1] (xylo-G, 38), C qH N O, and the 5 -mono-and 3, 5 -cycHc phosphates of (38), although none was as active as ara-A (89). Generally, guanine base-modified analogues of acyclovir are less active than acyclovir because they are not readily phosphorylated by herpes thymidine kinase. 

TACV Triacyclic analogue of acyclovir TGCV Triacyclic analogue of ganciclovir ANP Acyclic nucleoside phosphonate TK Thymidine kinase... 

Acyclovir (acycloguanosine. Fig. 5.221) is a novel type of nucleoside analogue which becomes achvated only in herpes-infected host cells by a herpes-specific enzyme, thymidine kinase. This enzyme inihates conversion of acyclovir initially to a monophosphate and then to the antiviral triphosphate which inhibits viral DNA polymerase. The host cell polymerase is not inhibited to the same extent, and the antiviral triphosphate is not produced in uninfected cells. Ganciclovir (Fig. 5.22J) is up to 100... 

Acyclovir is a particularly important alternate substrate for thymidine kinase ... 

In HSV and VZV, the most common mechanism of resistance to acyclovir involves mutations that result in decreased thymidine kinase activity. Therefore, these viral mutants exhibit cross-resistance to other antiviral agents that require thymidine kinase activation, such... 

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. 

Mutations in DNA polymerase or thymidine kinase may result in resistance. Acyclovir-resistant HSV strains that exhibit thymidine kinase deficiency are also resistant to famciclovir and penciciovir. 

Acyclovir requires three phosphorylation steps for activation. It is converted first to the monophosphate derivative by the virus-specified thymidine kinase and then to the di- and triphosphate compounds by host cell enzymes (Figure 49-3). Because it requires the viral kinase for initial phosphorylation, acyclovir is selectively activated—and the active metabolite accumulates—only in infected cells. Acyclovir triphosphate inhibits viral DNA synthesis by two mechanisms competition with deoxyGTP for the viral... 

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... 

Famciclovir is the diacetyl ester prodrug of 6-deoxypencidovir, an acyclic guanosine analog (Figure 49-2). After oral administration, famciclovir is rapidly deacetylated and oxidized by first-pass metabolism to penciclovir. It is active in vitro against HSV-1, HSV-2, VZV, EBV, and HBV. As with acyclovir, activation by phosphorylation is catalyzed by the virus-specified thymidine kinase in infected cells, followed by competitive inhibition of the viral DNA polymerase to block DNA synthesis. Unlike acyclovir, however, penciclovir does not cause chain termination. Penciclovir triphosphate has lower affinity for the viral DNA polymerase than acyclovir triphosphate, but it achieves higher intracellular concentrations. The most commonly encountered clinical mutants of HSV are thymidine kinase-deficient these are cross-resistant to acyclovir and famciclovir. 

Note Herpes simplex virus encodes a virus-specific thymidine. kinase, which phosphorylates the nucleoside analog acyclovir (acycloguanosine) to form acycloguanosine monophosphate. After further phosphorylation, the resulting acycloguanosine triphosphate is incorporated by the viral DNA polymerase into viral DNA, causing chain termination in virus-infected cells.]... 

Acyclovir (ACV) is not a true nucleoside, because the guanine residue is attached to an open-chain structure, but it mimics deoxyribose well enough for the compound to be accepted as a substrate by a thymidine kinase specified by certain herpes-type viruses. The normal thymidine kinase in mammalian cells does not recognize ACV as a substrate, however, so only virus-infected cells convert ACV to its monophosphate. Once the first phosphate has been added, the second phosphate is added by cellular guanylate kinase several other cellular kinases can add the third phosphate. The triphosphate is a more potent inhibitor of the viral DNA polymerases than of cellular DNA polymerases and also inactivates the former but not the latter. The net result is that ACV has been an effective treatment of, and prophylaxis for, genital herpes. Also it can result in dramatic relief of pain associated with shingles caused by reactivation of latent varicella-zoster virus, and has been successful in many patients with herpes encephalitis. 

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... 

Penciclovir triphosphate has lower affinity for the viral DNA polymerase than acyclovir triphosphate, but it achieves higher intracellular concentrations and has a more prolonged intracellular effect in experimental systems. The most commonly encountered clinical mutants of HSV are thymidine kinase-deficient and are cross-resistant to acyclovir and famciclovir. 

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. 

---