In vitro radiosensitization by pentoxifylline does not depend on p53 status.

The mode by which the xanthine derivative, pentoxifylline, induces a radiosensitizing effect in cell cultures is a key and controversial radiobiological issue and requires further elucidation. Six human glioblastoma cell lines were tested for the effect of pentoxifylline treatment at maximum G2/M block on the basis of cell survival, mitotic activity, and micronucleus formation after exposure to gamma radiation. Cell survival was measured by the colony-forming assay. Micronucleus formation (an indicator of DNA damage) and the proportion of binucleated cells (a representation of mitotic activity) were determined using the cytokinesis-block assay. Remarkably, exposure to a single dose of 4 Gy produced strong G2/M blocks in both p53 mutant and wild-type cells. Addition of pentoxifylline at the peak of radiation-induced G2/M blocks resulted in a p53-independent reduction in cell survival in all cell lines. This radiosensitization was strongly correlated with the magnitude of the radiation-induced G2/M block. The changes observed in mitotic activity and micronucleus yield were also p53-independent. These results are at variance with the view that pentoxifylline preferentially sensitizes p53 mutant cells, and that sensitization occurs only when cells are irradiated in the presence of the drug. The data suggest that the effectiveness of pentoxifylline as radiosensitizer depends on the proportion of cells that are arrested in the G2/M phase transition following exposure to ionizing radiation. These findings can assist in the identification of cancers that may benefit from therapies using G2/M checkpoint abrogators.