Radiosensitization by the histone deacetylase inhibitor PCI-24781.
OBJECTIVE
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.


METHODS
Exponentially growing human SiHa cervical and WiDr colon carcinoma cells were exposed to 0.1 to 10 micromol/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 gammaH2AX 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 approximately 1 micromol/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 micromol/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 gammaH2AX foci 24 h after irradiation but did not affect the initial rate of loss of radiation-induced gammaH2AX 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.
Banuelos
Carmen A
CA
Medical Biophysics Department, British Columbia Cancer Research Center, 675 W. 10th Avenue, Vancouver, BC, Canada.
Banáth
Judit P
JP
MacPhail
Susan H
SH
Zhao
Jin
J
Reitsema
Tarren
T
Olive
Peggy L
PL
D001572:Benzofurans / Q000494:pharmacology*; D045744:Cell Line, Tumor; D053903:DNA Breaks, Double-Stranded / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D056572:Histone Deacetylase Inhibitors; D006657:Histones / Q000032:analysis / Q000378:metabolism; D006801:Humans; D006877:Hydroxamic Acids / Q000494:pharmacology*; D011836:Radiation Tolerance / Q000187:drug effects*; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*
D001572:Benzofurans; D004791:Enzyme Inhibitors; C468783:H2AX protein, human; D056572:Histone Deacetylase Inhibitors; D006657:Histones; D006877:Hydroxamic Acids; D011838:Radiation-Sensitizing Agents; C512352:abexinostat