Enhanced tumor cell radiosensitivity and abrogation of G2 and S phase arrest by the Hsp90 inhibitor 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin.

PURPOSE: Because of the potential for affecting multiple signaling pathways, inhibition of Hsp90 may provide a strategy for enhancing tumor cell radiosensitivity. Therefore, we have investigated the effects of the orally bioavailable Hsp90 inhibitor 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17-DMAG) on the radiosensitivity of human tumor cells in vitro and grown as tumor xenografts. EXPERIMENTAL DESIGN: The effect of 17-DMAG on the levels of three proteins (Raf-1, ErbB2, and Akt) previously implicated in the regulation of radiosensitivity was determined in three human solid tumor cell lines. A clonogenic assay was then used to evaluate cell survival after exposure to 17-DMAG followed by irradiation. For mechanistic insight, the G(2)- and S-phase checkpoints were evaluated in 17-DMAG-treated cells. Finally, the effect of in vivo administration of 17-DMAG in combination with radiation on the growth rate of xenograft tumors was determined. RESULTS: 17-DMAG exposure reduced the levels of the three radiosensitivity-associated proteins in a cell line-specific manner with ErbB2 being the most susceptible. Corresponding concentrations of 17-DMAG enhanced the radiosensitivity of each of the tumor cell lines. This sensitization seemed to be the result of a 17-DMAG-mediated abrogation of the G(2)- and S-phase cell cycle checkpoints. The oral administration of 17-DMAG to mice bearing tumor xenografts followed by irradiation resulted in a greater than additive increase in tumor growth delay. CONCLUSIONS: These data indicate that 17-DMAG enhances the in vitro and in vivo radiosensitivity of human tumor cells. The mechanism responsible seems to involve the abrogation of radiation-induced G(2)- and S-phase arrest.

Flavopiridol enhances human tumor cell radiosensitivity and prolongs expression of gammaH2AX foci.

Flavopiridol is a cyclin-dependent kinase (CDK) inhibitor, which has recently entered clinical trials. However, when administered as a single agent against solid tumors, the antitumor actions of flavopiridol have been primarily cytostatic. Given its reported effects on cell cycle regulation, transcription, and apoptosis, flavopiridol may also influence cellular radioresponse. Thus, to evaluate the potential for combining this cyclin-dependent kinase inhibitor with radiation as a cancer treatment strategy, we have investigated the effects of flavopiridol on the radiation sensitivity of two human prostate cancer cell lines (DU145 and PC3). The data presented here indicate that exposure to flavopiridol (60-90 nM) after irradiation enhanced the radiosensitivity of both DU145 and PC3 cells. This sensitization occurred in the absence of significant reductions in cell proliferation, retinoblastoma protein phosphorylation, or P-TEFb activity. Moreover, the post-irradiation addition of flavopiridol had no effect on radiation-induced apoptosis or the activation of the G2 cell cycle checkpoint. However, flavopiridol did modify the time course of gammaH2AX expression in irradiated cells. Whereas there was no significant difference in radiation-induced gammaH2AX foci at 6 h, at 24 h after irradiation, the number of cells expressing gammaH2AX foci was significantly greater in the flavopiridol-treated cells. These results indicate that flavopiridol can enhance radiosensitivity of human tumor cells and suggest that this effect may involve an inhibition of DNA repair.