Cyclosaligenyl-2',3'-didehydro-2',3'-dideoxythymidine-5'-monophosphate (cyc
loSal-d4TMP) is a potent and selective inhibitor of human immunodeficiency
virus replication in cell culture and differs from other nucleotide prodrug
approaches in that it is designed to selectively deliver the nucleotide 5'
-monophosphate by a controlled, chemically induced hydrolysis. Its antivira
l efficacy in cell culture is at least as good as, if not superior to, that
of d4T. CycloSal-d4TMP was found to lead to the efficient intracellular re
lease of d4TMP in a variety of cell lines, including both wild-type CEM and
thymidine kinase-deficient CEM/TK- cells. Under similar experimental condi
tions, exposure of CEM/TK- cells to d4T failed to result in significant d4T
TP levels. The intracellular conversion of cycloSal- d4TMP proved to be bot
h time and dose dependent. The half-life of d4TTP generated intracellularly
from d4T- or cycloSal-d4TMP-treated CEM cells was similar to 3.5 h, and th
e intracellular ratios of d4TTP/d4TMP in cells exposed to cycloSal-d4TMP gr
adually increased from 1 to 3.4 upon prolonged incubation. Radiolabeled cyc
loSal- d4TMP could be separated as its two R-p and S-p diastereomers on hig
h-performance liquid chromatography. The R-p diastereomer of cycloSal- d4TM
P was 3- to 7-fold more efficient in releasing d4TMP and generating d4TTP t
han the S-p cycloSal- d4TMP diastereomer. This correlated well with the 5-f
old more pronounced antiviral activity of the R-p diastereomer versus the S
-p diastereomer. d4TMP is a poor substrate for the cytosolic 5'(3')-deoxyri
bonucleotidase (V-max/K-m for d4TMP: 0.08 of V-max/K-m for dTMP) and is onl
y slowly hydrolyzed to d4T. This contributes to the efficient conversion of
the prodrug of d4TTP.