PARENT SESSION
1:30 PM to 3:30 PM
Monday, April 22, 2002
Poster Session 15 DNA Damage and Repair II
Room: Nevada Exhibition Center

(P20-206) Motexafin gadolinium (MGd) Inhibits potentially lethal damage repair in cancer cells following exposure to ionizing radiation.

Magda, Darren^*,1, Lee, Intae¹, Lecane, Phillip¹, Miller, Richard¹, Biaglow, John², ¹ Pharmacyclics, Inc., Sunnyvale, CA² Department of Radiation Oncology, Philadelphia, PA

ABSTRACT-
To evaluate the effect of motexafin gadolinium(Xcytrin™, MGd) on potentially lethal damage repair (PLDR) and sub-lethal damage repair (SLDR) in cancer cells following exposure to ionizing radiation. For PLDR measurements, A549 human lung adenocarcinoma cells (10E5 cells per flask) were seeded 10 days prior to overnight treatment of plateau phase cultures with 50 M MGd. Cells were subcultured immediately following irradiation with 10 Gy or after holding for four hours at ambient temperature. For SLDR measurements, exponential phase cultures of human glioma U87 were exposed to continuous or split doses of ionizing radiation, with cultures held at room temperature for various time intervals between doses. Exponential phase A549 cells were subjected to FACS analysis following treatment with MGd and 0.25 mg/mL dichlorofluorescin acetate (DCFA) or 0.5 mg/mL propidium iodide. In the absence of MGd, there was up to a 38-fold increase in clonogenic survival when cultures were allowed to recover for 4 hr prior to subculture, relative to cells treated with trypsin immediately after irradiation (recovery factor, RF = 38). However, this recovery was abrogated in cultures incubated with MGd (RF = 1.2). Similarly, exponential phase cultures of U87 glioma cells treated with a split dose of ionizing radiation (5 Gy, 7 Gy) displayed a RF of ca. 14, relative to cultures treated with a continuous (12 Gy) dose. This recovery was inhibited by MGd treatment (RF = 0.9). Fluorescence at 530 nm, corresponding to DCFA oxidation, was enhanced in A549 cells exposed to MGd. Fluorescence at >650 nm, corresponding to intrinsic fluorescence of MGd, was also noted. No changes in cell cycle distribution were observed as a consequence of MGd treatment. In conclusion, redox cycling by MGd leads to the formation of reactive oxygen species (e.g., superoxide anion, hydrogen peroxide) in vitro. MGd enhances radiation response in A549 and U87 by inhibition of potentially lethal and sublethal damage repair.

KEYWORDS: Motexafin Gadolinium (MGd), DNA repair, PLDR, SLDR