Although early diagnosis for colorectal cancer has been significantly improved, the occurrence of locally advanced colorectal cancer is still a major medical problem since these patients have extremely poor prognosis. Clearly, new approaches for treatment of colorectal cancer are needed. Two major obstacles which exist relative to standard radiation and chemotherapy protocols are: (i) resistance of colorectal tumors to treatment and (ii) dose limitations of treatment due to fibrosis of normal intestinal tissue. Radio- and chemoresistance are characterized either as pre-existing resistance, based on constitutive expression of certain proteins such as MDR1 or anti-apoptotic proteins such as Bcl-2 and on loss of pro-apoptotic factors such as p53, or as inducible resistance. Inducible resistance is a relatively poorly characterized phenomenon in which tumors transiently induce resistance following exposure to treatment. In several studies, tumor response correlates with induction of apoptosis. Thus, it is likely that one major component of tumor chemo/radioresistance is suppression of apoptosis. We have reported that the transcription factor NF-KB provides a powerful anti-apoptotic mechanism through its ability to transcriptionally regulate genes encoding proteins which suppress apoptosis. Importantly, NF-KB activation is basally detected in colorectal tumor tissue (although expression may predominantly be localized in tumor-associated macrophages) and is strongly activated in tumor cells following radiation or chemotherapy exposure. Based on these findings, we have shown that inhibition of NF-KB through a gene delivery approach strongly potentiates chemotherapy-induced experimental colorectal tumor cytotoxicity through the induction of apoptosis. Furthermore, we have used an FDA-approved drug (PS-341, a proteasome inhibitor) which strongly blocks NF-KB activation to enhance colorectal tumor xenograft responses to both radiation and to CPT-11 based chemotherapy. We hypothesize that radiation-induced NF-KB activation and associated downstream transcriptional responses will occur in colorectal tumors and that this will correlate with a decreased therapeutic response by providing an anti-apoptotic signal. To test our hypothesis, we propose to: (i) determine whether radiation-induced activation of NF-KB and associated induced genes occurs in pre-operative radiochemotherapy and determine whether this responses correlates with clinical response and (ii) determine the ability of PS-341 to modulate NF-KB-dependent responses and to measure the toxicity of PS-341 delivered with pre-operative radiochemotherapy. Additionally, these studies will measure NF-KB activation and induction of transcription factor-dependent transcription responses in normal intestinal tissue following radiochemotherapy as a potential molecular correlate for cancer therapy-induced intestinal fibrosis. We also hypothesize that adjuvant radiochemotherapy responses will be improved and overall toxicity reduced through the use of more specific inhibitors of the NF-KB pathway. Thus, we will measure the efficacy of NF-KB inhibitors as an adjuvant approach with radiation or with the newest combination of chemotherapy regimens on different experimental tumors including liver metastases. These phase I clinical studies and basic translational experiments have the potential to significantly improve therapeutic approaches for advanced colorectal cancer.
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