DNAs, viral

Not all DNAs are double helices duplex DNA) Some types of viral DNA are single stranded and even a few DNA triple and quadruple helices are known... 

It is likely that ara-HxMP similarly exerts its antiviral activity in the form of the triphosphate, ara-HxTP, since ara-HxTP inhibits HSV-1 DNA polymerase (49). Another possible explanation of the antiviral activity of ara-HxTP is that it is metaboHcaHy converted to ara-AMP. In fact, it has been shown at Wellcome Research Laboratories that ara-HxMP is a substrate for adenylosuccinate synthetase, and that the resulting arabinofuranosyladenylosuccinate is cleaved to ara-AMP by adenylosuccinate lyase (1). The selective action of ara-A against HSV appears to be a consequence of the preferential inhibition of ara-ATP against HSV-1 and HSV-2 polymerases. Ara-ATP also inhibits normal cellular DNA polymerases, which may be the reason for its cellular toxicity. Also, it has been observed that ara-A is incorporated uniformly throughout the HSV-1 genome, which may result in defective viral DNA (50). 

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. 

The incorporation of acyclovir triphosphate into calf thymus DNA primer template has been shown to be much more rapid and extensive with HSV-1 DNA polymerase than with vero cell DNA polymerase a. This incorporation of acyclovir ceased after 15 min since the template is chain terminated by the acyclovir incorporation, as there is no 3 -hydroxyl group on which to continue elongation. The viral DNA polymerase is also inactivated by tight binding to the terminated template. 

Basically, AZT is anabohcaHy phosphorylated to AZT mono-, di-, and tri-phosphates by various enzymes (kinases) of a target ceU (159). AZT-triphosphate competes with other phosphorylated pyrimidine nucleosides for incorporation into HIV DNA by the viral reverse transcriptase. Incorporation of the AZT-triphosphate into reverse transcriptase results in viral DNA chain termination. Reverse transcriptase is essential in the repHcative cycle of HIV. 

HBV infection remains a major worldwide public health problem. The World Health Organization estimates that there are still 350 million chronic carriers of the vims, who are at risk of developing chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. The success of IFN-a treatment - mainly performed as combined treatment with adenine-arabinoside - has been measured by the normalization of liver enzymes, loss of HBe antigen and of detectable viral DNA in the serum of patients. It has been estimated from several clinical trials that as many as 40% of treated HBV patients would respond to therapy with IFN-a or combined treatment with nucleoside analogues and IFN-a. 

All acyclic and carbocyclic guanosine analogues depicted in Fig. 1 follow the same modus operandi as exemplified for acyclovir (ACV) in Fig. 5, in that they need three phosphorylations to be converted to their active metabolite, the triphosphate form, which then interacts with the target enzyme, the viral DNA polymerase, as a chain terminator (De Clercq 2002). In its DNA chain-terminating... 

After me ANPs (i.e., cidofovir, adefovir, and tenofovir) have been released (intra-or extraceUularly) from their prodrugs through the mtervention of infra- or extracellular esterases, they need only two phosphorylation steps to be converted to their active metabolites (i.e., HPMPCpp, PMEApp, and PMPApp), which will then compete with the natural substrates (dCTP for HPMPCpp, and dATP for PMEApp and PMPApp) for incorporation into me viral DNA (Fig. 6a). 

The integration of newly synthesized viral DNA into the host chromosome is a multi-step process (Anthony 2004 Van Made and Debyser 2005) that relies on the integrity of the last 10-20 base pairs at both ends of the viral cDNA besides a fully functional integrase (see Fig. 1). Initially, integrase recogiuzes the long-terminal repeat (LTR) of the retro-transcribed viral DNA and performs endonucleolytic... 

Second, the INSTl, but not an RTl, may conceivably inhibit the virus production from the pool of resting CD4 T cells that are in a state of pre-integration latency (Murray et al. 2007). Upon activation, the preformed pro-viral DNA that is already located in the nucleus integrates into the genome of these cells, allowing them to contribute to the viral load. 

To summarize, terminase inhibitors point the way toward a switch in strategy for developing HCMV inhibitors, with the aim of achieving a quality different from that of established DNA polymerase inhibitors. Intervention with viral DNA maturation arrests the replicative cycle at the DNA cleavage and packaging step, leading to an accumulation of empty procapsids and unprocessed concatemeric DNA. 

Terminase inhibition is an antiviral approach that may also be of consequence for other members of the herpesvirus group. In addition, since a similar DNA maturation process does not occur in higher cells, this principle offers the potential for high selectivity, in contrast to many of the viral DNA polymerase inhibitors, which also interact with cellular enzymes and hence can have severe side effects. 

Viruses are discussed more fully elsewhere (Chapter 3). However, there are certain groups of viruses, called bacteriophages (phages), which can attack bacteria. This attack involves the injechon of viral DNA into baeterial eells which then proceed to make new virus particles and destroy eells. Some viruses, known as temperate viruses, do not cause this catastrophic event when they infect their host, but can pass genetic material from one cell to another. 

Studies on the transformahon of tissue cultures with DNA-containing vimses have shown that, although complete vims particles cannot be found in the infected, transformed cells, viral DNA is present and is bound to the transformed cell DNA as provirus, analogous to the prophage of lysogenic bacteria. 

RNA oncogenic vimses have an unusual enzyme, reverse transcriptase, which is capable of making DNA copies from an RNA template. Cells transformed by these retrovimses have been shown to possess DNA transcripts of the viral RNA. It appears that the transformahon from normal to malignant is associated with the acquisition by the cell of viral DNA. 

Human immunodeficiency virus (HIV) is a retrovirus, i.e. its RNA is converted in human cells by the en me reverse transcriptase to DNA which is incorporated into the human genome and is responsible for producing new HIV particles. Zidovudine (azidothymidine, AZT Fig. 5.22F) is a stmctural analogue of thymidine (Fig. 5.22A) and is used to treat AIDS patients. Zidovudine is converted in both infected and uninfected cells to the mono-, di- and eventually triphosphate derivatives. Zidovudine triphosphate, the aetive form, is a potent inhibitor of HIV replication, being mistaken for thymidine by reverse transeriptase. Premature ehain termination of viral DNA ensues. However, AZT is relatively toxic because, as pointed out above, it is converted to the triphosphate by eellular enzymes and is thus also aetivated in uninfected cells. 

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... 

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