Antioxidant Inhibitors Potentiate the Cytotoxicity of Photodynamic Therapy

Abstract

Photodynamic therapy (PDT) is an increasingly popular anticancer treatment that uses photosensitizer, light, and tissue oxygen to generate cytotoxic reactive oxygen species (ROS) within illuminated cells. Acting to counteract ROS-mediated damage are various cellular antioxidant pathways. In this study, we combined PDT with specific antioxidant inhibitors to potentiate PDT cytotoxicity in MCF-7 cancer cells. We used disulphonated aluminium phthalocyanine photosensitizer plus various combinations of the antioxidant inhibitors: diethyl-dithiocarbamate (DDC, a Cu/Zn-SOD inhibitor), 2-Methoxyestradiol (2-ME, a Mn-SOD inhibitor), L-buthionine sulfoximine (BSO, a glutathione synthesis inhibitor) and 3-amino-1,2,4-Triazole (3-AT, a catalase inhibitor). BSO, singly or in combination with other antioxidant inhibitors, significantly potentiated PDT cytotoxicity, corresponding with increased ROS levels and apoptosis. The greatest potentiation of cell death over PDT alone was seen when cells were pre-incubated for 24 hours with 300 μM BSO plus 10 mM 3-AT (1.62-fold potentiation) or 300 μM BSO plus 1 μM 2-ME (1.52-fold), or with a combination of all four inhibitors (300 μM BSO, 10 mM 3-AT, 1 μM 2-ME, 10 μM DDC: 1.4-fold).Because many of these inhibitors have already been clinically tested, this work facilitates future in vivo studies.

Amplification of oxidative stress by a dual stimuli-responsive hybrid drug enhances cancer cell death

“…However, cancer cells also generate a large amount of intrinsic antioxidants such as GSH, superoxide dismutase and catalase to counteract ROS and tolerate ROS-induced oxidative stress, leading to altered redox balance 1,17 . In particular, a large amount of GSH is known to play a critical role in cellular redox maintenance and antioxidative systems 8,10 . We also found that DU145 cells have the highest level of GSH and H 2 O 2 and the least level of GSH and H 2 O 2 were observed with normal NIH3T3 cells † † † ** ** *** *** *** *** *** ( Supplementary Fig.…”

“…We developed QCA that exerts dual and synergistic therapeutic actions in cancer environments associated with a high concentration of H 2 O 2 and acidic pH 8,27 . As shown in Fig.…”

“…Simple treatment of cinnamaldehyde may not always lead to a preferential and sufficient killing of cancer cells. Depletion of cellular antioxidant GSH by QM would seem to be a logical approach in enhancing the anticancer potential of ROSgenerating cinnamaldehyde 8 . Therefore, appealing features of rationally designed hybrid anticancer drug QCA are the dual stimuli responsiveness and sequential therapeutic actions, which results in synergistic anticancer effects in a cancer cell-specific manner.…”

“…Accumulating evidence has suggested that abnormal cancer cells are under oxidative stress associated with an increased production of ROS due to disrupted ROS homeostasis and cancer cells have also adapted to oxidative stress by activating antioxidant systems often through the upregulation of glutathione (GSH) 2,[7][8][9][10] . Ironically, cancer cells are also known to utilize ROS to drive proliferation and other events required for tumour development 11,12 .…”

“…Hulsman et al 7 reported that the potent anticancer activity of nitric oxide-donating aspirin is attributed to QM, but not nitric oxide. Combined use of an ROSgenerating agent with an inhibitor of ROS scavenging systems has been also reported to amplify oxidative stress and be effective in the treatment of advanced solid tumours in terms of both efficacy and selectivity 8,19 . Therefore, manipulating the ROS levels by redox modulation is a logical approach to selectively kill cancer cells without causing significant toxicity to normal cells and would have broad therapeutic applications for cancer treatment.…”