Radiosensitization by the Histone Deacetylase Inhibitor PCI-24781

Purpose: PCI-24781 is a novel broad spectrum histone deacetylase inhibitor that is currently in phase I clinical trials. The ability of PCI-24781 to act as a radiation sensitizer and the mechanisms of radiosensitization were examined. Experimental Design: Exponentially growing human SiHa cervical and WiDr colon carcinoma cells were exposed to 0.1 to 10 μmol/L PCI-24781 in vitro for 2 to 20 h before irradiation and 0 to 4 h after irradiation. Single cells and sorted populations were analyzed for histone acetylation, H2AX phosphorylation, cell cycle distribution, apoptotic fraction, and clonogenic survival. Results: PCI-24781 treatment for 4 h increased histone H3 acetylation and produced a modest increase in γH2AX but negligible cell killing or radiosensitization. Treatment for 24 h resulted in up to 80% cell kill and depletion of cells in S phase. Toxicity reached maximum levels at a drug concentration of ∼1 μmol/L, and cells in G 1 phase at the end of treatment were preferentially spared. A similar dose-modifying factor (DMF 0.1 = 1.5) was observed for SiHa cells exposed for 24 h at 0.1 to 3 μmol/L, and more radioresistant WiDr cells showed less sensitization (DMF 0.1 = 1.2). Limited radiosensitization and less killing were observed in noncycling human fibroblasts. Cell sorting experiments confirmed that depletion of S-phase cells was not a major mechanism of radiosensitization and that inner noncycling cells of SiHa spheroids could be sensitized by nontoxic doses. PCI-24781 pretreatment increased the fraction of cells with γH2AX foci 24 h after irradation but did not affect the initial rate of loss of radiation-induced γH2AX or the rate of rejoining of DNA double-strand breaks. Conclusions: PCI-24781 shows promise as a radiosensitizing agent that may compromise the accuracy of repair of radiation damage.