Gimeracil (5-chloro-2, 4-dihydroxypyridine) is an inhibitor of
dihydropyrimidine dehydrogenase (DPYD), which degrades
pyrimidine including
5-fluorouracil in the blood.
Gimeracil was originally added to an oral fluoropyrimidine derivative S-1 to yield prolonged
5-fluorouracil concentrations in serum and
tumor tissues. We have already reported that
gimeracil had
radiosensitizing effects by partially inhibiting homologous recombination (HR) in the repair of
DNA double strand breaks. We investigated the mechanisms of
gimeracil radiosensitization. Comet assay and radiation-induced focus formation of various kinds of
proteins involved in HR was carried out.
siRNA for DPYD were transfected to HeLa cells to investigate the target
protein for radiosensitization with
gimeracil. SCneo assay was carried out to examine whether DPYD depletion by
siRNA inhibited HR repair of
DNA double strand breaks. Tail moments in neutral comet assay increased in
gimeracil-treated cells.
Gimeracil restrained the formation of foci of Rad51 and
replication protein A (RPA), whereas it increased the number of foci of Nbs1, Mre11, Rad50, and FancD2. When HeLa cells were transfected with the DPYD
siRNA before irradiation, the cells became more radiosensitive. The degree of radiosensitization by transfection of DPYD
siRNA was similar to that of
gimeracil.
Gimeracil did not sensitize DPYD-depleted cells. Depletion of DPYD by
siRNA significantly reduced the frequency of neopositive clones in SCneo assay.
Gimeracil partially inhibits the early step in HR. It was found that DPYD is the target
protein for radiosensitization by
gimeracil. The inhibitors of DPYD, such as
gimeracil, could enhance the efficacy of
radiotherapy through partial suppression of HR-mediated DNA repair.