2',3'-Dideoxyuridine (
ddU) is ineffective at controlling human immunodeficiency virus type 1 (HIV-1)
infection in human T cells, because it is not biotransformed to the active 5'-triphosphate. The metabolic block resides in the poor substrate affinity of
ddU for cellular
nucleoside kinases. This problem cannot be overcome by supplying the preformed
nucleotides, because such compounds are unable to penetrate cells. To circumvent the requirement of
ddU for enzymic phosphorylation, we have prepared bis(pivaloyloxymethyl)
2',3'-dideoxyuridine 5'-monophosphate (
piv2 ddUMP), as a potential membrane-permeable
prodrug of
ddUMP, and investigated its metabolism and anti-HIV activity in two human T cell lines, one with wild-type
thymidine kinase activity (MT-4) and the other deficient in
thymidine kinase activity (CEM-tk-). The 5'-mono-, di-, and triphosphates of
ddU were formed in both cell lines after exposure to piv2-ddUMP. In contrast, phosphorylated metabolites were not observed in cells treated with
ddU or
ddUMP alone. piv2-ddUMP also reduced the cytopathic effects of HIV-1 in MT-4 cells (ED50, 4.75 microM) and inhibited virus production in culture fluid (ED50, 20 microM). In addition, piv2-ddUMP protected CEM-tk- cells from HIV-1
infection, as demonstrated by inhibition of intracellular p24
antigen levels (ED50, 3 microM) and
reverse transcriptase activity in culture medium (Ed50, 2.5 microM). Based on these findings, we propose that the "masked
nucleotide" strategy may make available for development
nucleoside analogues hitherto considered inactive because of failure to undergo biotransformation to the corresponding 5'-monophosphates. Moreover, by circumventing metabolic dependency on
nucleoside kinases, the strategy may overcome acquired resistance to
nucleoside analogues caused by the loss or depletion of
nucleoside kinases.