We evaluated the interaction of a biochemically active concentration of
cyclopentenyl cytosine (
CPE-C), an investigational
antimetabolite which inhibits
CTP synthetase, on the cytotoxicity of
arabinosyl-5-azacytosine (
Ara-AC) and 1-beta-D-arabinofuranosylcytosine (
Ara-C) in HCT 116 colon
carcinoma cells. A 3-h exposure to 0.5 microM
CPE-C depleted
CTP pools by over 90% and decreased
dCTP pools by 57%; the effect on
CTP pools persisted for up to 24 h following washout of
CPE-C. A 3-h pre-exposure to 0.5 microM
CPE-C augmented the growth inhibition resulting from a 24-h exposure to
Ara-AC. The combination of 1 microM
cytidine and
deoxycytidine fully reversed the enhancement associated with
CPE-C pretreatment, to a level of growth inhibition expected from either
CPE-C or
Ara-AC alone. A striking enhancement of toxicity was observed in clonogenic studies with pre-exposure to
CPE-C at a nonlethal dose followed by either
Ara-AC or
Ara-C.
CPE-C increased the formation of
Ara-AC and
Ara-C nucleotides by as much as 3-fold, and this was accompanied by increased incorporation of the arabinosyl
nucleotides into
methanol-precipitable material. Analysis of purified
RNase-treated
nucleic acids by
cesium sulfate density centrifugation confirmed that a 3-h pre-exposure to
CPE-C increased [3H]-
Ara-C incorporation into
DNA at 4 and 24 h by 2.4- and 2.7-fold, respectively. Thus, these studies indicate that
CPE-C can function as a biochemical modulator. Following a brief exposure to a nonlethal concentration,
CPE-C is capable of augmenting the cytotoxicity and intracellular metabolism of
Ara-AC and
Ara-C.