PARENT SESSION

Experimental and Clinical Therapeutics

(PP151) Radiation-induced activation of NQO1 potentiates the cytotoxicity of bio-reductive drugs.

Park, Heon Joo^*,1, Choi, Eun Kytung², Ji, In-Mi ¹, Kim, Eun Jung¹, Lee, So Ryung², WIlliams, Brent ³, Griffin, Robert ³, Ha, Sung Whan⁴, Lim, Byung Uk¹, Song, Chang Won³, ¹ Dept of Microbiology, Inchon, South Korea, South Korea² Dept of Radiation Oncology, Seoul, South Korea, South Korea³ Dept of Therapeutic Radiology-Radiation Oncology, Minneapolis, MN, USA⁴ Dept of Radiation Oncology, Seoul, South Korea, South Korea

ABSTRACT- NAD(P)H:Quinone Oxidoreductase(NQO1) has been known to play cardinal role in the activation of bio-reductive anti-cancer drugs such as mitomycin C, tirapazamine, EO9 and RHI. Beta-Lapachone (B-lap) is a potent bio-reductive drug, originally obtained from Lapacho tree. It has been reported that the cytotoxicity of B-lap is positively related to NQO1 activity and that B-Lap inhibits radiation-induced DNA damage repair. The purpose of the present study was to further reveal the mechanisms underlying the synergistic interaction between ionizing radiation (IR) and B-lap using human A549 lung epithelial cancer cells. A 4 h incubation of A549 cells with 5 M B-lap caused rapid apoptosis and reduced the clonogenic cell survival to about 10%. When cells were treated with B-lap for 4 h, rinsed and irradiated, the resultant cell death was merely additive. In contrast, synergistic cell death occurred when cell were irradiated first and then treated with B-lap. Interestingly, the synergistic interaction was evident even when the B-lap treatment was applied 12-24 h after irradiation, strongly demonstrating that more than inhibition of radiation-induced damage by B-lap was involved in the interaction between the two treatments. We found that IR with doses as low as 2.5 Gy causes a long lasting up-regulation of NQO1, as determined with enzymatic activity, western blot analysis for NQO1 and confocal microscopy of cells for immunoflurorescent staining of NQO1. Co-treatment of cells with B-lap and dicoumarole, an inhibitor of NQO1, effectively reduced the cell death caused by the combination of IR with B-lap. These results strongly indicated that the synergism between IR and B-lap was due to radiation-induced increase in NQO1 activity resulting in sensitization of cells to B-lap. The NQO1 level in many human tumors has been demonstrated to be significantly higher than that in original normal tissues. Preliminary studies indicated that radiation-induced up-regulation of NQO1 also sensitizes cells to other bio-reductive drugs. It is concluded that the NQO1 level can be further increased by IR selectively in tumors to potentiate the response of tumors to B-lap or other bio-reductive anticancer drugs.

Key words: Beta-Lapachone, NQO1, Bioreductvie drugs, Radiaiton