Radiosensitization by pemetrexed of human colon carcinoma cells in different cell cycle phases.
OBJECTIVE
The novel folate antimetabolite Alimta (pemetrexed disodium, LY231514) exhibits antitumor activity in a broad array of human malignancies and was recently found to enhance radiation-induced cell killing in vitro. In the present study, a possible cell cycle phase-specific radiosensitization by pemetrexed was assessed.


METHODS
Widr human colon carcinoma cells were synchronized by serum withdrawal/stimulation that yielded about 80% cells with G1 DNA content 6 h after replating and more than 60% S-phase cells after 22 h, as assessed by flow cytometry. The respective cultures were irradiated with doses up to 12 Gy in combination with a subtoxic pemetrexed exposure (1.06 microM for 2 h: about 80% survival), or after mock treatment. Survival curves were generated by the clonogenic assay; apoptosis was measured by sub-G1 DNA flow cytometry.


RESULTS
The combination treatment of the G1 cells and of the more radioresistant S-phase cell preparations yielded survival rates that were lower than expected for independent cell killing. Radiosensitization, calculated as the ratio of the mean inactivation doses without or with drug exposure (enhancement ratio), was not significantly different for the two cell preparations (enhancement ratio of 2.1 and 2.3, respectively) and was similar to the previously reported value for log-phase cells. Pemetrexed exposure was unable to stimulate an apoptotic response of these cells to radiation.


CONCLUSIONS
Radiosensitization by pemetrexed is not cell cycle phase-specific, and the relative radioresistance of S-phase cells is retained. Apoptosis seems to have no influence on radiosensitization in this cell line.
Bischof
Marc
M
Department of Radiotherapy, Laboratory for Radiobiology, University of Heidelberg, Heidelberg, Germany. marc_bischof@med.uni-heidelberg.de
Huber
Peter
P
Stoffregen
Clemens
C
Wannenmacher
Michael
M
Weber
Klaus-Josef
KJ
D017209:Apoptosis / Q000187:drug effects / Q000528:radiation effects; D002453:Cell Cycle / Q000187:drug effects* / Q000528:radiation effects*; D003110:Colonic Neoplasms / Q000378:metabolism*; D004307:Dose-Response Relationship, Radiation; D005971:Glutamates / Q000494:pharmacology*; D006147:Guanine / Q000031:analogs & derivatives* / Q000494:pharmacology*; D006801:Humans; D000068437:Pemetrexed; D011836:Radiation Tolerance / Q000187:drug effects*; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D005971:Glutamates; D011838:Radiation-Sensitizing Agents; D000068437:Pemetrexed; D006147:Guanine