Gimeracil, an inhibitor of dihydropyrimidine dehydrogenase, inhibits the early step in homologous recombination

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. (Cancer Sci 2011; 102: 1712–1716)