Blockade Of Mgmt Expression By O\u3csup\u3e6\u3c/sup\u3e Benzyl Guanine Leads To Inhibition Of Pancreatic Cancer Growth And Induction Of Apoptosis

Purpose: We sought to determine whether administration of a MGMT blocker, O6-benzyl guanine (O6BG), at an optimal biological dose alone or in combination with gemcitabine inhibits human pancreatic cancer cell growth. Experimental Design: Human pancreatic cancer L3.6pl and PANC1 cells were treated with O6BG, either alone or in combination with gemcitabine, and the therapeutic efficacy and biological activity of these drug combinations were investigated. Results: O6BG sensitized pancreatic cancer cells to gemcitabine. Protein and mRNA expression of MGMT, cyclin B1, cyclin B2, cyclin A, and ki-67 were significantly decreased in the presence of O 6BG. In sharp contrast, protein expression and mRNA message of p21cip1 were significantly increased. Interestingly, O6BG increases p53-mediated p21cip1 transcriptional activity and suppresses cyclin B1. In addition, our results indicate that p53 is recruited to p21 promoter. Furthermore, an increase in p21cip1 and a decrease in cyclin transcription are p53 dependent. The volume of pancreatic tumors was reduced by 27% in mice treated with gemcitabine alone, by 47% in those treated with O6BG alone, and by 65% in those mice given combination. Immunohistochemical analysis showed that O6BG inhibited expression of MGMT and cyclins, and increased expression of p21cip1. Furthermore, there was a significant decrease in tumor cell proliferation and an increase in tumor cell apoptosis. Conclusions: Collectively, our results show that decreased MGMT expression is correlated with p53 activation, and significantly reduced primary pancreatic tumor growth. These findings suggest that O6BG either alone or in combination with gemcitabine may provide a novel and effective approach for the treatment of human pancreatic cancer. © 2009 American Association for Cancer Research