Cyclooxygenase-2 (COX-2) enzyme inhibitors as potential enhancers of tumor radioresponse

Targeted therapies for stage III non-small cell lung cancer: integration in the combined modality setting

Combined modality therapy represents current standard therapy for locoregionally advanced non-small cell lung cancer. In particular, concomitant chemoradiotherapy has emerged as the preferred approach. At the same time, efforts to increase locoregional and systemic antitumor activity are necessary to further improve long-term survival rates for these patients. In recent years, multiple cellular targets have emerged in the development of novel antitumor therapies. Several of these are of high relevance in the carcinogenesis of lung cancer including the epidermal growth factor receptor (EGFR), the ras signaling pathway, tumor angiogenesis, and cyclooxygenase-2 (COX-2) expression. Novel agents directed against these targets are currently under development with promising early results in non-small cell lung cancer when administered as single agents or in combination with chemotherapy in stage IV or recurrent disease. Similarly their use with concurrent radiation therapy is supported by preclinical models. Selected early clinical trials utilizing these agents in combination with radiotherapy or chemoradiotherapy are discussed.

Initial experience combining cyclooxygenase-2 inhibition with chemoradiation for locally advanced pancreatic cancer

Pancreatic cancer is a lethal disease that is resistant to chemotherapy and radiotherapy. Gemcitabine has recently been shown to be an improvement over 5-fluorouracil in patients with advanced disease. It is also a potent radiosensitizer, which has led to the investigation of gemcitabine with concurrent radiotherapy. However, preliminary results indicate that there are significant limitations to this approach in this challenging disease. Pancreatic cancer cells have alterations in many molecular signaling pathways that may be responsible for their resistance to cytotoxic therapy and aggressive behavior. Cyclooxygenase-2 (COX-2) is commonly overexpressed in pancreatic tumors, and preclinical evidence indicates that selective COX-2 inhibition enhances both chemotherapy and radiotherapy response, without affecting normal tissue damage. We have initiated preclinical studies as well as a phase I clinical protocol evaluating the combination of gemcitabine and celecoxib (Celebrex) with radiotherapy. In preclinical studies, celecelecoxib strongly enhanced the antitumor efficacy of chemoradiation. However, preliminary observations from both the preclinical experiments as well as the clinical protocol have revealed more toxicity with this combination than with gemcitabine and radiotherapy alone. These observations require further study, but are cause for concern when combining gemcitabine, radiotherapy, and celecoxib.

Combination of radiation and celebrex (celecoxib) reduce mammary and lung tumor growth

The selective cyclooxygenase (COX)-2 inhibitor, celecoxib, alone and in combination with radiation was investigated in vitro and in vivo. Murine mammary tumor line (MCa-35) and human lung carcinoma line (A549) have high and low basal levels of COX-2 protein, respectively. Treatment of both tumor cells with celecoxib alone resulted in a dose- and time-dependent reduction of cell number (clonogenic cell death) and tumor cell growth rate in vitro; however, inhibition of tumor cell growth by celecoxib was not correlated with the reduction of COX-2 protein in tumor cells. Although both tumor cell types had similar DNA damage after celecoxib treatment, significant induction of tumor cell apoptosis was only observed in MCa-35. Celecoxib-mediated radiation sensitization also occurred in MCa-35 cells determined by clonogenic assay, in part due to a G2/M arrest at 8 to 24 hours after treatment. The tumor growth inhibitory effects of celecoxib were also studied in vivo. It was found that celecoxib inhibited both tumor growth after intragastric administration of celecoxib (5 daily doses of 50 mg/kg). Combined with a single 30-Gy dose of radiation, celecoxib resulted in additive effects on A549 tumors. Celecoxib-treated A549 tumors had marginal reduction of total and perfused blood vessels compared with untreated controls. Reduction of tumor angiogenic cytokine and growth factor mRNA was associated with decreased perfused vessels. Finally, reduction of vascular endothelial growth factor protein after celecoxib was also observed in both tumor lines by Western blot. Our results indicate that the selective inhibition of COX-2 combined with radiation has potential application in radiotherapy, and celecoxib-mediated antitumor effects may act through different mechanisms including direct inhibition of tumor cell proliferation, alteration of tumor cell cycle, and antiangiogenesis.

Cyclooxygenase-2 (COX-2) enzyme inhibitors and radiotherapy: preclinical basis

Cyclooxygenase-2 (COX-2), an enzyme induced by proinflammatory cytokines, mitogenic substances, oncogenes, growth factors, and hypoxia, among others, is involved in the metabolic conversion of arachidonic acid to prostaglandins in inflamed tissues and neoplasia. COX-2 is often overexpressed in malignant tumors and premalignant lesions and is linked to carcinogenesis, maintenance of progressive tumor growth, and facilitation of metastatic spread. Because COX-2 may also be a determinant of tumor radioresistance, its inhibition or inhibition of its products (prostaglandins) may improve tumor response to radiotherapy. Preclinical studies have shown that treatment with selective COX-2 inhibitors significantly enhances tumor response to radiation without appreciably affecting normal tissue radioresponse. The underlying mechanisms of the COX-2 inhibitor-radiation interactions seem to be multiple, with the enzyme inhibitor directly or indirectly augmenting tumor cell destruction by radiation. Thus, use of selective COX-2 inhibitors is a potential approach for improving cancer radiotherapy.

Combined-modality treatment of solid tumors using radiotherapy and molecular targeted agents

PURPOSE

Molecular targeted agents have been combined with radiotherapy (RT) in recent clinical trials in an effort to optimize the therapeutic index of RT. The appeal of this strategy lies in their potential target specificity and clinically acceptable toxicity.

DESIGN

This article integrates the salient, published research findings into the underlying molecular mechanisms, preclinical efficacy, and clinical applicability of combining RT with molecular targeted agents. These agents include inhibitors of intracellular signal transduction molecules, modulators of apoptosis, inhibitors of cell cycle checkpoints control, antiangiogenic agents, and cyclo-oxygenase-2 inhibitors.

RESULTS

Molecular targeted agents can have direct effects on the cytoprotective and cytotoxic pathways implicated in the cellular response to ionizing radiation (IR). These pathways involve cellular proliferation, DNA repair, cell cycle progression, nuclear transcription, tumor angiogenesis, and prostanoid-associated inflammation. These pathways can also converge to alter RT-induced apoptosis, terminal growth arrest, and reproductive cell death. Pharmacologic modulation of these pathways may potentially enhance tumor response to RT though inhibition of tumor repopulation, improvement of tumor oxygenation, redistribution during the cell cycle, and alteration of intrinsic tumor radiosensitivity.

CONCLUSION

Combining RT and molecular targeted agents is a rational approach in the treatment of solid tumors. Translation of this approach from promising preclinical data to clinical trials is actively underway.

Linking radiation oncology and imaging through molecular biology (or now that therapy and diagnosis have separated, it's time to get together again!)

Among the areas defined by the National Cancer Institute as "Extraordinary Opportunities for Research Investment" that are highly relevant to the technology-oriented disciplines within the broad field of radiology are cancer imaging, defining the signatures (ie, underlying molecular features) of cancer cells, and molecular targets of prevention and treatment. In molecular target credentialing, a specific molecular target is imaged, the molecular signature is defined, a treatment is given, and the effect of the intervention on the image findings and the signature is then evaluated. Such an approach is used to validate the proposed target as a legitimate one for cancer therapy or prevention and to provide the opportunity to ultimately individualize therapy on the basis of both the initial characteristics of the tumor and the tumor's response to an intervention. Therapeutic radiation is focused biology (ie, radiation produces molecular events in the irradiated tissue). Radiation can (a) kill cancer cells by itself, (b) be combined with cytotoxic or cytostatic drugs, and (c) serve to initiate radiation-inducible molecular targets that are amenable to treatment with drugs and/or biologic therapies. Focused biology can be anatomically confined with various types of external beams and with brachytherapy, and it can be used systemically with targeted radioisotopes. These new paradigms link diagnostic imaging, radiation therapy, and nuclear medicine in unique ways by way of basic biology. It is timely to develop new collaborative research, training, and education agendas by building on one another's expertise and adopting new fields of microtechnology, nanotechnology, and mathematical analysis and optimization.

Role of Cyclooxygenase-2 Inhibitors in Combination with Radiation Therapy in Lung Cancer

Cyclooxygenase-2 (COX-2) is an enzyme involved in prostaglandin production in pathologic states such as inflammatory disorders and cancer. The enzyme is often overexpressed in premalignant lesions and cancer of the lung. Overexpression of COX-2 in lung cancer is associated with more aggressive biological tumor behavior and adverse patient outcome. In preclinical studies, inhibition of this enzyme with selective COX-2 inhibitors enhances tumor response to radiation and chemotherapeutic agents. These findings have been rapidly advanced to clinical oncology. Clinical trials of the combination of selective COX-2 inhibitors with radiation therapy, chemotherapy, or both in patients with lung cancer have been initiated and some preliminary results are available. In this review, we describe the relationship between overexpression of COX-2 and lung cancer, the antitumor effect of selective COX-2 inhibitors, discuss the rationale for using selective COX-2 inhibitors combined with radiation therapy and chemotherapy, and summarize current clinical protocols and initial findings.

Chemoradiotherapy: emerging treatment improvement strategies

BACKGROUND

The use of chemotherapeutic drugs in combination with radiotherapy has become a common strategy for the treatment of advanced cancer. Solid evidence exists showing that chemotherapy administered during the course of radiotherapy (concurrent chemoradiotherapy) increases both local tumor control and patient survival in a number of cancer sites, including head and neck cancer. These therapy improvements, however, have been achieved at the expense of considerable toxicity, which underscores the need for further improvements.

METHODS

The current status of chemoradiotherapy clinical trials for head and neck cancer and research on the emerging treatment improvements were reviewed. A review of potential treatment improvement strategies focused on preclinical investigations on newer chemotherapeutic agents, notably taxanes and nucleoside analogues, as well as on molecular targets such as epidermal growth factor receptor (EGFR) or cyclooxygenase-2 (COX-2) enzyme.

RESULTS

Concurrent, but not induction (drugs given before radiotherapy), chemoradiotherapy improves locoregional tumor control and survival benefit in head and neck carcinoma relative to radiotherapy alone. In comparison, both concurrent and induction chemoradiotherapy showed therapeutic advantage over radiotherapy alone in the treatment of lung cancer. These therapeutic improvements were achieved with standard chemotherapeutic drugs, most commonly cisplatin-based chemotherapy. Biologically, chemotherapy interacts with radiation through a number of mechanisms, including inhibition of cellular repair, cell cycle effects, and inhibition of tumor cell regeneration. Potential avenues emerged to further improve chemoradiotherapy. One of these involves the newer chemotherapeutic agents, taxanes and nucleoside analogues, which in preclinical studies exhibited strong tumor radiosensitization and therapeutic gain. The clinical benefit of these agents is currently under testing. Another approach for improvement of chemoradiotherapy consists of inhibiting molecules selectively or preferentially expressed on tumor cells, such as EGFR and COX-2, both shown to render cellular resistance to drugs or radiation. Agents that selectively inhibit these molecules are becoming available at a rapid rate, and many of them have been shown in preclinical testing to be highly effective in improving tumor radioresponse or chemoresponse without affecting normal tissues.

CONCLUSIONS

Concurrent chemoradiotherapy, using standard chemotherapeutic agents, has emerged as an effective treatment for advanced cancer, but unfortunately at the expense of considerable increase in normal tissue toxicity. There are a number of potential emerging treatment strategies to further improve chemoradiotherapy. One consists of using newer chemotherapeutic drugs, which in preclinical studies are potent enhancers of tumor radioresponse. Another approach consists of targeting EGFR or COX-2 with selective inhibitors of these molecules.

Non steroidal anti-inflammatory drugs and COX-2 inhibitors as anti-cancer therapeutics: hypes, hopes and reality

Non-steroidal anti-inflammatory drugs (NSAIDs) and specific inhibitors of cyclooxygenase (COX)-2, are therapeutic groups widely used for the treatment of pain, inflammation and fever. There is growing experimental and clinical evidence indicating NSAIDs and COX-2 inhibitors also have anti-cancer activity. Epidemiological studies have shown that regular use of Aspirin and other NSAIDs reduces the risk of developing cancer, in particular of the colon. Molecular pathology studies have revealed that COX-2 is expressed by cancer cells and cells of the tumor stroma during tumor progression and in response to chemotherapy or radiotherapy. Experimental studies have demonstrated that COX-2 over expression promotes tumorigenesis, and that NSAIDs and COX-2 inhibitors suppress tumorigenesis and tumor progression. Clinical trials have shown that NSAIDs and COX-2 inhibitors suppress colon polyp formation and malignant progression in patients with familial adenomatous polyposis (FAP) syndrome. Recent advances in the understanding of the cellular and molecular mechanisms of the anti-cancer effects of NSAIDs and COX-2 inhibitors have demonstrated that these drugs target both tumor cells and the tumor vasculature. The therapeutic benefits of COX-2 inhibitors in the treatment of human cancer in combination with chemotherapy or radiotherapy are currently being tested in clinical trials. In this article we will review recent advances in the understanding of the anti-tumor mechanisms of these drugs and discuss their potential application in clinical oncology.

Coupled mRNA stabilization and translational silencing of cyclooxygenase-2 by a novel RNA binding protein, CUGBP2

Cyclooxygenase-2 (COX-2) expression is translationally silenced in epithelial cells undergoing radiation-induced apoptosis. CUGBP2, a predominantly nuclear protein, is also rapidly induced in response to radiation and translocates to the cytoplasm. Antisense-mediated suppression of CUGBP2 renders radioprotection through a COX-2-dependent prostaglandin pathway, providing an in vivo demonstration of translation inhibition activity for CUGBP2. CUGBP2 binds to two sets of AU-rich sequences (AREs) located within the first sixty nucleotides of the COX-2 3' untranslated region (3'UTR). Upon binding, CUGBP2 stabilizes a chimeric luciferase-COX-2 3'UTR mRNA but inhibits its translation. These findings identify a novel paradigm for RNA binding proteins in facilitating opposing functions of mRNA stability and translation inhibition and reveal a mechanism for inhibiting COX-2 expression in cancer cells.

Selective cyclooxygenase-2 inhibition: a target in cancer prevention and treatment

A major goal in the area of cancer prevention and treatment is to make rational use of defined molecular targets in order to block carcinogenesis. Studies conducted in experimental animal models for many human cancers, including those of lung, skin, mammary gland, urinary bladder, colon, and pancreas, have demonstrated that carcinogenesis often may be inhibited by the administration of a highly diverse group of biologic and chemical agents. One very promising and well-studied target is cyclooxygenase (COX)-2. Interestingly, a number of cancers appear to overexpress the COX-2 enzyme, which may play several roles in carcinogenesis. Recent clinical studies have demonstrated the effect of COX-2 inhibitors in the treatment of familial adenomatous polyposis, a genetic disorder that increases the risk for developing colorectal cancer. Ongoing clinical trials with COX-2 inhibitors will increase our understanding and may give us profound insights into the general applicability of this new targeted approach for cancer control.

COX-2 inhibitors for the prevention of breast cancer

The inducible prostaglandin synthase cyclooxygenase-2 (COX-2) is normally expressed predominantly in kidney and brain, and also has important roles in reproduction and inflammation. COX-2 misexpression has been observed in numerous human cancers, including the majority of colorectal cancers. Recently, COX-2 overexpression has been described in human breast cancer. COX-2 is present in about 40% of invasive breast carcinomas, particularly those that overexpress HER2/neu, and COX-2 expression correlates with poor patient prognosis. Manipulation of Cox-2 gene dosage by using transgenic overexpression and knockout approaches has revealed an important role for Cox-2 in tumorigenesis. Furthermore, translational experiments using rodent breast cancer models suggest COX-2 inhibition to be an effective strategy for both prevention and treatment of experimental breast cancers. Since COX-2 can contribute to multiple facets of tumorigenesis, including angiogenesis, several mechanisms are likely to underlie the anticancer action of COX inhibitors. Thus, selective COX-2 inhibitors offer considerable promise for the prevention and treatment of human breast cancer.

In vitro enhancement of tumor cell radiosensitivity by a selective inhibitor of cyclooxygenase-2 enzyme: mechanistic considerations

PURPOSE

Selective cyclooxygenase-2 inhibitors have been reported to enhance the tumor response to radiation in vivo, but the cellular mechanisms underlying the radiosensitizing effect are not understood. In the present study, we investigated several possible mechanisms using a murine sarcoma cell culture system.

METHODS AND MATERIALS

Cells derived from a murine sarcoma, designated NFSA, were cultured in vitro and exposed to different (either single or split) doses of radiation with and without a pretreatment of SC-236 (4-[5-(4-chlorophenyl)-3-(trifluoromethyl)-1H-pyrazol-l-yl] benzene sulfonamide), a selective cyclooxygenase-2 (COX-2) inhibitor. The cells were assayed for clonogenic survival to determine the radiosensitizing effect of SC-236. In addition, MTT assay and TUNEL assay were performed to determine the effects of SC-236 and radiation on the cell survival and cell cycle distribution. RNase protection assay was performed on the total RNA extract using probes that encoded for selected cell cycle regulatory proteins, such as cyclins and cyclin-dependent kinases. To monitor the extent of COX-2 activity and its role in radiosensitization, the cellular content of prostaglandin E2, a major metabolite of COX-2 activity on arachidonic acid, was also determined.

RESULTS

The cell clonogenic survival assay showed that SC-236 significantly enhanced tumor cell radiosensitivity: 50 microM SC-236 increased it by a factor of 1.51 at the 0.1 cell survival level. Treatment with SC-236 (50 microM, 3 days) removed the "shoulder" region on the radiation survival curve, suggesting that the drug inhibited repair of sublethal radiation damage. The inhibition was confirmed by split-dose experiments where two doses (3 Gy each) of radiation were given 4 h apart. The cells exposed to radiation only repaired the damage by a factor of 1.44, whereas those treated with SC-236 plus radiation repaired it by a factor of 1.1 only. Whereas SC-236 induced apoptosis in these NFSA cells, radiation did not. No further increase in apoptosis was observed when the cells were exposed to both SC-236 and radiation, suggesting that SC-236 did not render tumor cells more susceptible to radiation-induced apoptosis. The RNase protection assay showed that SC-236 (50 microM, 3 days) inhibited the expression of cyclins A and B, as well as cyclin-dependent kinase-1. Inhibition of these cell cycle regulatory elements by SC-236 was associated with the arrest of cells in the radiosensitive G2-M phase (67%), determined by flow cytometry.

CONCLUSIONS

SC-236 significantly enhanced radiosensitivity of tumor cells; the magnitude of sensitivity was dependent on the drug's concentration. The likely mechanisms involve accumulation of cells in the radiosensitive G2-M phase of the cell cycle and inhibition of repair from sublethal radiation damage.

Chemoradiation with novel agents for rectal cancer

Surgery remains the mainstay of treatment for colorectal cancer. Although the role of radiation therapy in colon cancer is unclear, its role in the management of locally advanced rectal cancer has been extensively studied in clinical trials. The use of postoperative chemoradiotherapy has been shown to improve local control and disease-free survival in patients with locally advanced disease over surgery alone; however, an overall survival advantage remains unproven. Clinical trials evaluating preoperative radiotherapy have demonstrated an improved local control as well as a survival advantage. Randomized studies comparing preoperative versus postoperative combined-modality approaches have failed in the United States, mainly due to the perceived advantages of preoperative treatment: improved patient tolerance, tumor downstaging, and fewer treatment-related complications. While 5-fluorouracil-based chemotherapy remains the standard systemic agent used along with radiation, other novel agents and strategies have recently been developed and are under investigation. In this review, we discuss the use of novel anticancer agents in combination with radiation therapy for the treatment of locally advanced rectal cancer.

Docetaxel/Radiation Combinations: Rationale and Preclinical Findings

Concurrent chemoradiotherapy, administration of chemotherapeutic agents during the course of radiation therapy, has increasingly been used for treatment of advanced locoregional cancer. Improvements in radiation therapy are achieved through independent cytotoxic action of drugs and their ability to sensitize tumor cells to radiation. Laboratory investigations showed that docetaxel is potent in both of these actions. The drug increased the radiosensitivity of in vitro cultured cells and the in vivo tumor radioresponse. In contrast to exerting a strong enhancement of tumor radioresponse, the ability of docetaxel to modify normal tissue radiation damage was much lower. Thus, docetaxel can significantly increase therapeutic gain when combined with radiation therapy. The initial rationale for using docetaxel and other taxanes as radiation enhancers was the ability of these agents to arrest cells in the radiosensitive G2/M phases of the cell cycle. Additional mechanisms were subsequently detected, including the ability of docetaxel to eliminate radioresistant S-phase cells, cause tumor reoxygenation, stimulate antitumor immune resistance mechanisms, and possibly inhibit tumor angiogenesis. Because combined chemoradiotherapy treatments are limited by normal tissue toxicity, additional treatment strategies are needed to improve the antitumor efficacy and to minimize normal tissue toxicity. In this regard, many research avenues are being explored, particularly the possibility of combining chemoradiotherapy with molecular targeting. This overview addresses the rationale for major findings on the interaction of docetaxel and radiation in preclinical models and discusses how these findings may impact practical use of chemoradiotherapy.