5-ETHOXY-2'-DEOXYURIDINE, A NOVEL SUBSTRATE FOR THYMIDINE PHOSPHORYLASE, POTENTIATES THE ANTITUMOR-ACTIVITY OF 5-FLUOROURACIL WHEN USED IN COMBINATION WITH INTERFERON, AN INDUCER OF THYMIDINE PHOSPHORYLASE EXPRESSION
El. Schwartz et al., 5-ETHOXY-2'-DEOXYURIDINE, A NOVEL SUBSTRATE FOR THYMIDINE PHOSPHORYLASE, POTENTIATES THE ANTITUMOR-ACTIVITY OF 5-FLUOROURACIL WHEN USED IN COMBINATION WITH INTERFERON, AN INDUCER OF THYMIDINE PHOSPHORYLASE EXPRESSION, Cancer research, 55(16), 1995, pp. 3543-3550
Clinical studies have demonstrated that the combination of 5-fluoroura
cil (FUra) and IFN-alpha has activity in the treatment of advanced col
orectal cancer. Treatment of human colon carcinoma cells with IFN caus
ed a 5-fold increase in the level of thymidine phosphorylase (TP) mRNA
and an 8-fold increase in TP enzyme activity, Since TP catalyzes the
first step in the direct conversion of FUra to deoxyribonucleotides, i
ts induction by IFN is a potential biochemical mechanism for the modul
ation of the antitumor activity of FUra. In contrast to the activity m
easured in cell extracts, however, thymine utilization by intact cells
was increased less than 2-fold by IFN, suggesting that the metabolic
activation of PUra by TP in the IFN-treated cells was similarly subopt
imal. This was likely due to a rate-limiting amount of cosubstrate for
TP, and in this study, a series of 5-substituted 2'-deoxyuridine anal
ogues were synthesized and tested as potential deoxyribose donors for
TP. One of the compounds, the novel pyrimidine analogue 5-ethoxy-2'-de
oxyuridine (EOdU), was found to be a substrate for the transferase rea
ction of TP, to have little or no direct cytotoxicity, to selectively
increase the cellular levels of 5-fluoro-dUMP, to enhance the inhibito
ry effect of FUra on thymidylate synthase activity, and to potentiate
the cytotoxicity of FUra and IFN in human colon carcinoma cells, EOdU
was tested in vivo against HT-29 cells grown as xenografts in nude mic
e. The combination of EOdU + FUra + IFN-alpha 2a produced tumor regres
sions and a significantly greater delay in tumor growth when compared
to FUra + IFN-alpha 2a, FUra + EOdU, or FUra or IFN used alone; tumors
were 72% smaller in the EOdU + FUra + IFN-alpha 2a-treated animals co
mpared to the saline control group. A comparable antitumor effect was
also found when a related nucleoside analogue, 5-propynyloxy-2'-deoxyu
ridine, was used with FUra + IFN, and it also showed modulating activi
ty when used with only FUra. The antitumor activity of the three agent
combination (nucleoside + IFN + FUra) was comparable to that of a hig
her dose of PUra used alone, but it was substantially less toxic to th
e animals than the higher dose of FUra, indicating that the modulating
agents improved the therapeutic index of FUra. The substitution of a
hybrid recombinant IFN-alpha A/D, active in both human and murine tiss
ues, in place of the species-specific human IFN-alpha 2a did not furth
er increase the efficacy of the combination, suggesting that a direct
effect on the tumor cells rather than host-mediated actions of IM was
the predominant mechanism for antitumor effects observed in vivo. Thes
e studies demonstrate a potential novel approach to increase the effic
acy and selectivity of FUra, which incorporates two complementary bioc
hemical actions: the selective induction by TPN of expression of a gen
e regulating pyrimidine synthesis, coupled with the rational design of
a cosubstrate for the induced enzyme.