PR-000350, a novel hypoxic radiosensitizer, is a
2-nitroimidazole nucleoside analog and has begun to be used for clinical
cancer therapy. In this study, using U937 monoblastoid cells we investigated the mechanisms of enhanced cell killing by
PR-000350. When cells were irradiated under an extremely hypoxic condition, the apoptotic rate was strongly suppressed. However, a remarkable increase in the DNA fragmentation rate as well as in the ladder formation was observed when hypoxic cells were irradiated in the presence of 5 mM
PR-000350.
DNA histograms of the
PR-000350 treated group showed enhancement of the sub-G1 fraction and simultaneous suppression of the progression of the cell cycle from the S to G2/M phase at 4-8 h after X-irradiation, suggesting the importance of the S phase in the induction of apoptotic cell death. Flow cytometric and immunohistochemical analyses after
BrdU labelling revealed that apoptotic cell death is induced mainly in the
BrdU-positive cells. In addition, by using cell synchronization technique it was proved that the S phase is the most sensitive fraction to the
radiosensitizing effect of
PR-000350. These results suggest that
PR-000350 strongly enhances
tumor cell killing by promoting X-ray induced-apoptosis preferentially in the S-phase fraction.
PR-000350 is a new type radiosensitizer and promise to provide an effective anti-
cancer activity against hypoxic
tumor cells that are resistant to the usual
radiotherapy.