Inhibition of HIF-1α by the anticancer drug TAS106 enhances X-ray-induced apoptosis in vitro and in vivo

In a previous study, we showed that a novel anticancer drug, 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl) cytosine (TAS106, ECyd) increased the antitumour efficacy of X-irradiation. However, its effects on hypoxic cells in tumours remain unclarified. Here, we show that TAS106 enhances the induction of apoptosis in X-irradiated human gastric adenocarcinoma MKN45 and MKN28 cells under hypoxia in vitro. At the same time, the accumulation of HIF-1 alpha observed under hypoxia was shown to be decreased to the level of normoxia in the presence of 0.1 mu M TAS106. To study the function of HIF-1 alpha protein in apoptosis of hypoxic cells, we employed an HIF-1 alpha reductive approach using its specific antisense oligodeoxynucleotide. The reduction of HIF-1 alpha gene expression dramatically enhanced X-ray-induced apoptosis in hypoxic cells. In in vivo experiments in which MKN45 cells were transplanted into severe combined immunodeficient (SCID) mice, TAS106 (0.5 mg kg(-1)) suppressed HIF-1 alpha expression and subsequently reduced the area of the hypoxic region in the tumour and enhanced the induction of apoptosis in the hypoxic region when combined with 2Gy of X-irradiation. These results suggest the possibility that TAS106 acts as a potent radiosensitiser through the inhibition of HIF-1 alpha expression and can be a useful agent against radiotherapy-resistant hypoxic cells in solid tumours.