The inhibition of
thymidylate synthase (TS) by
5-fluorouracil (5-FU) was known to increase the incorporation of radiolabelled
iododeoxyuridine (IdUrd) into
DNA. The relatively non-toxic compounds such as
thiol-containing
antioxidant pyrrolidinodithiocarbamte (
PDTC) or aromatic
fatty acid phenylbutyrate (PB) had been reported to enhance the cytotoxic efficacy of
5-FU. We designed a novel strategy through triplet combination of PB,
PDTC and
5-FU to increase the radiolabelled IdUrd uptake and investigated the underlying mechanisms. The growth inhibition and [(125)I]IdUrd-
DNA incorporation by PB,
PDTC,
5-FU in different combinations were tested on parent or p21(Waf1) transfected Hep3B cells. The combination of PB and
PDTC was more effective in enhancing
5-FU cytotoxicity than either
drug alone. The combination of PB/
PDTC and
5-FU blocked cells in S-phase and resulted in 8.5-fold increase of radiolabelled IdUrd-
DNA incorporation. The transfection of p21(Waf1) did not change the general pattern of enhancement. Intriguingly, the combination of PB and
PDTC effectively down-regulated
NF-kappaB and TS and prevented their up-regulation from
5-FU treatment than either
drug alone through a p21(Waf1)-independent mechanism. Based on this strategy, the 3-drug combination offered potential for improved radiolabelled IdUrd molecular
radiotherapy for
hepatoma treatment.