Date of Award:

5-1995

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Animal, Dairy, and Veterinary Sciences

Department name when degree awarded

Toxicology

Committee Chair(s)

Reed P. Warren

Committee

Reed P. Warren

Committee

David Drown

Committee

Robert Sidwell

Abstract

Resistance of human viral pathogens to various antiviral drugs is a serious medical problem. Two modes of drug resistance in cytomegalovirus infections have been observed, the first being altered (decreased) drug metabolism by the infected cells, and the second reduced sensitivity of the viral deoxyribonucleic acid polymerase enzyme to the active form of the drug. Mice infected with the murine cytomegalovirus have been used extensively as an animal model for the human cytomegalovirus, and drug-resistant strains in this model have been identified. To better understand the mode of drug resistance of the virus, the metabolism of two antiviral drugs, 9-(1,3-dihydroxy-2-propoxymethyl)guanine (ganciclovir) and (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (cidofovir), was studied in cells infected with the virus.

The degree of resistance of the mutant virus strain to these two drugs and also to the drug phosphonoformic acid (foscarnet) was measured in viral plaque reduction assays. The resistant strain was 14-,4-, and 11-fold less sensitive to the drugs ganciclovir, foscarnet, and cidofovir, respectively, than a sensitive (wildtype) strain.

Metabolism of the antiviral drugs ganciclovir and cidofovir was studied in C127I mouse mammary tumor cells infected with the mutant strain. Uninfected C127I cells and C127I cells infected with the sensitive strain of murine cytomegalovirus were used as controls. The cells were treated with tritium-labeled ganciclovir or cidofovir and studied under a variety of parameters. Among these were duration of treatment, multiplicity of infection, and concentration of compound. After incubating, the cells were acid extracted and analyzed with high-pressure liquid chromatography. The radioactivity of each sample was measured on a scintillation counter and converted into picomoles of drug per million cells.

No significant difference was observed between the virus strains in terms of metabolism or catabolism of the two drugs. This effect remained constant, even when controlling for parameters such as the amount of virus infecting each cell, duration of treatment, or concentration of drug. Based on these results, it appears that the mode of resistance in this mutant strain of virus to ganciclovir and cidofovir is not due to an alteration in metabolism of these two compounds by infected cells. Thus, it is proposed that drug resistance in this mutant strain of virus is due to altered viral deoxyribonucleic acid polymerase function.

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