Cidofovir ([(S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)
cytosine] [
HPMPC])-resistant forms of camelpox,
cowpox,
monkeypox, and vaccinia viruses were developed by prolonged passage in Vero 76 cells in the presence of
drug. Eight- to 27-fold-higher concentrations of
cidofovir were required to inhibit the resistant viruses than were needed to inhibit the wild-type (WT) viruses. Resistant viruses were characterized by determining their cross-resistance to other
antiviral compounds, examining their different replication abilities in two cell lines, studying the biochemical basis of their drug resistance, and assessing the degrees of their virulence in mice. These viruses were cross resistant to
cyclic HPMPC and, with the exception of vaccinia virus, to (S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)adenine. Three of the four resistant
cowpox and monkeypox viruses exhibited reduced abilities to infect and replicate in 3T3 cells compared to their abilities in Vero 76 cells. Compared to the WT virus
polymers the resistant cowpox virus
DNA polymerase was 8.5-fold less sensitive to inhibition by
cidofovir diphosphate, the active form of the
drug. Intracellular phosphorylation of [3H]
cidofovir was not stimulated or inhibited by
infection with resistant cowpox virus. In intranasally infected BALB/c mice, WT cowpox virus was 80-fold more virulent than the resistant virus.
Cidofovir treatment (100 mg/kg of
body weight, given one time only as early as 5 min after virus challenge) of a resistant cowpox virus
infection could not protect mice from mortality. However, the
drug prevented mortality in 80 to 100% of the mice treated with a single 100-mg/kg dose at 1, 2, 3, or 4 days after WT virus challenge. By application of these results to human
orthopoxvirus infections, it is anticipated that resistant viruses may be untreatable with
cidofovir but their virulence may be attenuated. Studies will need to be conducted with
cidofovir-resistant monkeypox virus in monkeys to further support these hypotheses.