Bolus doses of
5-chlorodeoxycytidine (CldC) administered with modulators of
pyrimidine metabolism, followed by X-irradiation, resulted in a 2-fold dose increase effect against RIF-1
tumors in C3H mice. Pool size studies of the fate of [14C]-CldC in BDF1 mice bearing Sarcoma-180
tumors, which demonstrated the rapid formation of 5-chlorodeoxycytidylate (CldCMP), and incorporation of CldC as such in RIF-1
tumor DNA, indicate that CldC is a substrate for
deoxycytidine kinase, as our past Km studies have shown. Our data indicate that
5-chlorodeoxyuridine triphosphate (CldUTP) accumulates from both the
cytidine deaminase-
thymidine kinase pathway, as well as from the
deoxycytidine kinase-
dCMP deaminase pathway, in
tumor tissue. As shown in a previous study,
tetrahydrouridine (H4U), a potent inhibitor of
cytidine deaminase, can effectively inhibit the
enzyme in the normal tissues of BDF1 mice. When H4U was administered with the modulators N-(phosphonacetyl)-
L-aspartic acid (
PALA) and 5-fluorodeoxycytidine (FdC), the levels of CldC-derived
RNA and
DNA directed metabolites increased in
tumor and decreased in normal tissues compared to when CldC was administered alone. These modulators inhibit the de novo pathway of
thymidine biosynthesis, lowering
thymidine triphosphate (
TTP) levels, which compete with CldUTP for incorporation into
DNA.
5-Benzylacyclouridine (BAU), an inhibitor of
uridine phosphorylase, was also utilized.
DNA incorporation studies using C3H mice bearing RIF-1
tumors showed that the extent of incorporation of
5-chlorodeoxyuridine (CldU) into
DNA correlates with the levels of
cytidine and
dCMP deaminases; this is encouraging in view of their high activity in many human
malignancies and the low activities in normal tissues, including those undergoing active replication. Up to 3.9% replacement of
thymidine by CldU took place in RIF-1
tumors, whereas incorporation into bone marrow was below our limit of detection. CldC did not result in
photosensitization under conditions in cell culture in which radiosensitization to X rays was obtained. Thus, the combination of CldC with modulators of its metabolism has potential as a modality of selective radiosensitization for ultimate clinical use in a wider range of
tumors than those of the brain.