5-Bromo-2'-deoxyuridine (BrdUrd) was found to increase the cytotoxicity induced by
1,3-bis(2-chloroethyl)-1-nitrosourea (
BCNU) and
cisplatin in human
glioma cells. At a fixed concentration of BrdUrd and
BCNU, the greatest cell loss was observed in exponentially growing cells. As cells approached plateau growth, cytotoxicity was reduced as indicated by greater cell viability. Under varying growth conditions the percentage of
thymine replacement by
bromouracil in
DNA, as determined by gas chromatography/mass spectrometry analysis, declined as cultures approached maximum density. These data indicate BrdUrd must be incorporated into
DNA for the enhanced effect to be observed. In exponentially growing cells, sensitization was dependent upon both the concentration of BrdUrd and
alkylating agent. Using regression analysis (at 95% CL), a relationship between the level of
bromouracil in
DNA and the extent of enhanced cytotoxicity was observed at two concentrations of
BCNU (r2 = 0.99, 0.96). Although it is known that bifunctional
alkylating agents exert cytotoxicity by forming cross-links between
cDNA strands, increased cross-link formation was not observed in BrdUrd substituted
DNA as determined by alkaline elution. The data suggest that DNA damage induced by halogenated
pyrimidines may not involve interstrand cross-links and that these agents may be useful in the treatment of
glioma in combination with
alkylating agents.