Tumor radiosensitizers--current status of development of various approaches: report of an International Atomic Energy Agency meeting

Repurposing radiosensitising medicines for radiotherapy: an overview

Repurposing established non-cancer drugs for the treatment of cancer offers potential benefits such as speed of clinical translation and financial efficiencies. In this study, we assess the landscape of repurposing drugs for combined use with radiotherapy (RT) based on their capacity to increase tumour radiosensitivity. Using a literature-based approach, we identified 42 radiosensitising drugs with varied non-cancer indications and mechanisms of action, that have entered or completed clinical trials in combination with RT or with chemoradiotherapy. Two compounds, nicotinamide and nimorazole, have entered routine but limited clinical use in combination with radiotherapy. We provide an overview on these successfully repurposed drugs, and highlight some examples of unsuccessful repurposing efforts and drug candidates with an uncertain prospect of success. Upon reviewing the trials, we identified some common themes behind the unsuccessful efforts, including poor trial reporting, absence of biomarkers and patient selection, sub-optimal pharmacological properties, inappropriate trial design, lack or inadequate consideration of pre-clinical and clinical data, and limited funding support. We point out future directions to mitigate these issues and increase the likelihood of success in repurposing drug treatments for radiotherapy.

Anticancer agent α-sulfoquinovosyl-acylpropanediol enhances the radiosensitivity of human malignant mesothelioma in nude mouse models

Malignant pleural mesothelioma (MPM) is a highly malignant disease that develops after asbestos exposure. Although the number of MPM cases is predicted to increase, no effective standard therapies have been established. The novel radiosensitizer α-sulfoquinovosyl-acylpropanediol (SQAP) enhances the effects of γ-radiation in human lung and prostate cancer cell lines and in animal models. In this study, we explored the radiosensitizing effect of SQAP and its mechanisms in MPM. The human MPM cell lines MSTO-211H and MESO-4 were implanted subcutaneously into the backs and thoracic cavities of immunodeficient KSN/Slc mice, then 2 mg/kg SQAP was intravenously administered with or without irradiation with a total body dose of 8 Gy. In both the orthotopic and ectopic xenograft murine models, the combination of irradiation plus SQAP delayed the implanted human MSTO-211H tumor growth. The analysis of the changes in the relative tumor volume of the MSTO-211H indicated a statistically significant difference after 8 Gy total body combined with 2 mg/kg SQAP, compared to both the untreated control (P = 0.0127) and the radiation treatment alone (P = 0.0171). After the treatment in each case, immunostaining of the harvested tumors revealed decreased cell proliferation, increased apoptosis and normalization of tumor blood vessels in the SQAP- and irradiation-treated group. Furthermore, hypoxia-inducible factor (HIF) 1 mRNA and protein expression were decreased, indicating reoxygenation in this group. In conclusion, SQAP improved hypoxic conditions in tumor tissue and may elicit a radiosensitizing effect in malignant mesothelioma models.

Mefenamic acid as a promising therapeutic medicine against colon cancer in tumor-bearing mice

Although radiotherapy is an effective strategy for cancer treatment, tumor resistance to ionizing radiation (IR) and its toxic effects on normal tissues are limiting its use. The aim of this study is to evaluate the anti-cancer effects of mefenamic acid (MEF), as an approved medicine, and its combination with IR against colon tumor cells in mice. Tumor-bearing mice were received MEF at a dose of 25 mg/kg for 6 successive days. The tumor size was measured. In the second experiment, after MEF treatment, tumor-bearing mice locally received an X-ray at dose 6 Gy. Tumor growth and biochemical, histological, and immunohistological assay (caspase-3) were performed. MEF significantly decreased tumor size in mice in comparison to the control group. IR and/or MEF treatment significantly reduced the tumor volume and inhibited tumor growth by 49%, 55%, and 67% by MEF, IR, and MEF + IR groups as compared with the control group. Administration of MEF in combination with radiation had a synergistic effect on enhanced histopathological changes in tumor tissues. MEF treatment in IR exposure mice showed a significant increase in the immunoreactivity of caspase-3 in the colon tumor tissue. MEF has an anti-tumor effect in colon tumor-bearing mice. MEF in combination with IR increased pathological changes and apoptosis in tumor tissues, suggesting that MEF might be clinically useful in the treatment of colon cancer.

Long non-coding RNA FAM133B-2 represses the radio-resistance of nasopharyngeal cancer cells by targeting miR-34a-5p/CDK6 axis

Long non-coding RNAs (lncRNAs) were found to play roles in various cancers, including nasopharyngeal carcinoma. In this study, we focused on the biological function of the lncRNA FAM133B-2 in the radio-resistance of nasopharyngeal carcinoma. The RNA-seq and qRT-PCR analysis showed that FAM133B-2 is highly expressed in the radio-resistant nasopharyngeal carcinoma cells. The following biochemical assays showed that FAM133B-2 represses the nasopharyngeal carcinoma radio-resistance and also affects the apoptosis and proliferation of nasopharyngeal carcinoma cells. Further investigations suggested that miR-34a-5p targets FAM133B-2 and also regulates the cyclin-dependent kinase 6 (CDK6). All these results suggested that the lncRNA FAM133B-2 might function as a competitive endogenous RNA (ceRNA) for miR-34a-5p in nasopharyngeal carcinoma radio-resistance, thus it may be regarded as a novel prognostic biomarker and therapeutic target in nasopharyngeal carcinoma diagnosis and treatment.

Overcoming Radioresistance: Small Molecule Radiosensitisers and Hypoxia-activated Prodrugs

The role of hypoxia in radiation resistance is well established and many approaches to overcome hypoxia in tumours have been explored, with variable success. Two small molecule strategies for targeting hypoxia have dominated preclinical and clinical efforts. One approach has been the use of electron-affinic nitroheterocycles as oxygen-mimetic sensitisers. These agents are best exemplified by the 5-nitroimidazole nimorazole, which has limited use in conjunction with radiotherapy in head and neck squamous cell carcinoma. The second approach seeks to leverage tumour hypoxia as a tumour-specific address for hypoxia-activated prodrugs. These prodrugs are selectively activated by reductases under hypoxia to release cytotoxins, which in some instances may diffuse to kill surrounding oxic tumour tissue. A number of these hypoxia-activated prodrugs have been examined in clinical trial and the merits and shortcomings of recent examples are discussed. There has been an evolution from delivering DNA-interactive cytotoxins to molecularly targeted agents. Efforts to implement these strategies clinically continue today, but success has been elusive. Several issues have been identified that compromised these clinical campaigns. A failure to consider the extravascular transport and the micropharmacokinetic properties of the prodrugs has reduced efficacy. One key element for these 'targeted' approaches is the need to co-develop biomarkers to identify appropriate patients. Hypoxia-activated prodrugs require biomarkers for hypoxia, but also for appropriate activating reductases in tumours, as well as markers of intrinsic sensitivity to the released drug. The field is still evolving and changes in radiation delivery and the impact of immune-oncology will provide fertile ground for future innovation.

MiR-20a-5p promotes radio-resistance by targeting NPAS2 in nasopharyngeal cancer cells

MicroRNAs (miRNAs) are key players of gene expression involved in diverse biological processes including the cancer radio-resistance, which hinders the effective cancer therapy. Here we found that the miR-20a-5p level is significantly up-regulated in radio-resistant nasopharyngeal cancer (NPC) cells via an RNA-seq and miR-omic analysis. Moreover, we identified that the neuronal PAS domain protein 2 (NPAS2) gene is one of the targets of miR-20a-5p. The involvement of miR-20a-5p and NPAS2 with NPC radio-resistance was further validated by either down- or up-regulation of their levels in NPC cell lines. Taken together, these results not only reveal novel insights into the NPC radio-resistance, but also provide hints for an effective therapeutic strategy to fight against NPC radio-resistance.

MiR-20a-5p promotes radio-resistance by targeting Rab27B in nasopharyngeal cancer cells

BACKGROUND

MicroRNAs (miRNAs) was reported to be involved in cancer radio-resistance, which remains a major obstacle for effective cancer therapy.

METHODS

The differently expressed miRNAs were detected by RNA-seq experiment in nasopharyngeal cancer (NPC) cells. MiR-20a-5p was selected as our target, which was subject to finding its target gene Rab27B via bioinformatics analysis. The qRT-PCR, western blot and the luciferase reporter assays were performed to confirm Rab27B as the target of miR-20a-5p. In addition, the roles of miR-20a-5p in NPC radio-resistance were detected by transfection of either miR-20a-5p-mimic or miR-20a-5p-antagomiR. The involvement of Rab27B with NPC radio-resistance was also detected by the experiments with siRNA-mediated repression of Rab27B or over-expression of GFP-Rab27B. Wound healing and invasion assays were performed to detect the roles of both miR-20a-5p and Rab27B.

RESULTS

MiR-20a-5p promotes NPC radio-resistance. We identified that its target gene Rab27B negatively correlates with miR-20a-5p-mediated NPC radio-resistance by systematic studies of a radio-sensitive (CNE-2) and resistant (CNE-1) NPC cell lines. Repression of Rab27B by siRNA suppresses cell apoptosis and passivates CNE-2 cells, whereas over-expression of Rab27B triggered cell apoptosis and sensitizes CNE-1 cells.

CONCLUSIONS

MiR-20a-5p and its target gene Rab27B might be involved in the NPC radio-resistance. Thus the key players and regulators involved in this pathway might be the potential targets for developing effective therapeutic strategies against NPC.

miR-205 acts as a tumour radiosensitizer by targeting ZEB1 and Ubc13

Tumor cells associated with therapy resistance (radioresistance and drug resistance) are likely to give rise to local recurrence and distant metastatic relapse. Recent studies revealed microRNA (miRNA)-mediated regulation of metastasis and epithelial-mesenchymal transition; however, whether specific miRNAs regulate tumor radioresistance and can be exploited as radiosensitizing agents remains unclear. Here we find that miR-205 promotes radiosensitivity and is downregulated in radioresistant subpopulations of breast cancer cells, and that loss of miR-205 is highly associated with poor distant relapse-free survival in breast cancer patients. Notably, therapeutic delivery of miR-205 mimics via nanoliposomes can sensitize the tumor to radiation in a xenograft model. Mechanistically, radiation suppresses miR-205 expression through ataxia telangiectasia mutated (ATM) and zinc finger E-box binding homeobox 1 (ZEB1). Moreover, miR-205 inhibits DNA damage repair by targeting ZEB1 and the ubiquitin-conjugating enzyme Ubc13. These findings identify miR-205 as a radiosensitizing miRNA and reveal a new therapeutic strategy for radioresistant tumors.

PDCD4 as a predictor of sensitivity to neoadjuvant chemoradiotherapy in locally advanced rectal cancer patients

OBJECTIVE

The purpose of this study was to examine the role of programmed cell death 4 (PDCD4) expression in predicting tumor response to neoadjuvant chemoradiotherapy and outcomes for patients with locally advanced rectal cancer.

METHODS

Clinicopathological factors and expression of PDCD4 were evaluated in 92 patients with LARC treated with nCRT. After the completion of therapy, 4 cases achieved clinical complete response (cCR), and thus the remaining 88 patients underwent a standardized total mesorectal excision procedure. There were 38 patients (41.3%) with a good response (TRG 3-4) and 54 (58.7%) with a poor one (TRG 0-2).

RESULTS

Immunohistochemical staining analyses showed that patients with high expression of PDCD4 were more sensitive to nCRT than those with low PDCD4 expression (P=0.02). High PDCD4 expression before nCRT and good response (TRG3-4) were significantly associated with improved 5-year disease-free survival and 5-year overall survival (P<0.05). Multivariate analysis demonstrated that the pretreatment PDCD4 expression was an independent prognostic factor.

CONCLUSION

Our study demonstrated that high expression of PDCD4 protein is a useful predictive factor for good tumor response to nCRT and good outcomes in patients with LARC.

The prognostic role of EZH2 expression in rectal cancer patients treated with neoadjuvant chemoradiotherapy

BACKGROUND

Neoadjuvant chemoradiotherapy (nCRT) combined with surgery has been implemented as a standard treatment strategy in locally advanced rectal cancer (LARC). However, there is a wide spectrum of response to nCRT. The aim of this study was to determine whether enhancer of zeste homologue 2 (EZH2 ) expression could predict response to nCRT and outcomes for patients in LARC.

METHOD

The study examined the EZH2 expression in 112 biopsies by immohistochemistry. The associations between EZH2 and clinical characters were analyzed.

RESULTS

EZH2 expression in biopsy tissue was significantly related to increased tumor cell proliferation, as assessed by Ki-67 expression with a cutoff value of 37% (p <0.001). High EZH2 expression was correlated closely with low differentiation (p = 0.029), high CEA level (p = 0.041), T4 status (p = 0.011) and node metastasis (p =0.045). By univariate and multivariate analysis, we observed low EZH2 expression could reliably and independently predict the good response to nCRT ( p = 0.026 and p = 0.023) and down-staging ( p = 0.021 and p = 0.027). In univariate analysis, high EZH2 expression was significantly associated with poor 5-year disease-free survival (p = 0.025) and 5-year overall survival (p = 0.032). In multivariate analysis, EZH2 was a prognostic factor for 5-year DFS (HR = 2.287; 95% CI 1.137-4.602, p = 0.020) but not for 5-year OS (HR = 2.182; 95% CI 0.940-5.364, p = 0.069).

CONCLUSION

Our study revealed that low EZH2 expression in biopsy tissue might be a useful predictive factor of good tumor response to nCRT and longer 5-year DFS in patients with LARC. However this is a relatively small retrospective study, to further validate the role of EZH2 in rectal cancer, large consistent cohort studies are needed.

Nonhomologous end-joining repair plays a more important role than homologous recombination repair in defining radiosensitivity after exposure to high-LET radiation

DNA double-strand breaks (DSBs) induced by ionizing radiation pose a major threat to cell survival. The cell can respond to the presence of DSBs through two major repair pathways: homologous recombination (HR) and nonhomologous end joining (NHEJ). Higher levels of cell death are induced by high-linear energy transfer (LET) radiation when compared to low-LET radiation, even at the same physical doses, due to less effective and efficient DNA repair. To clarify whether high-LET radiation inhibits all repair pathways or specifically one repair pathway, studies were designed to examine the effects of radiation with different LET values on DNA DSB repair and radiosensitivity. Embryonic fibroblasts bearing repair gene (NHEJ-related Lig4 and/or HR-related Rad54) knockouts (KO) were used and their responses were compared to wild-type cells. The cells were exposed to X rays, spread-out Bragg peak (SOBP) carbon ion beams as well as with carbon, iron, neon and argon ions. Cell survival was measured with colony-forming assays. The sensitization enhancement ratio (SER) values were calculated using the 10% survival dose of wild-type cells and repair-deficient cells. Cellular radiosensitivity was listed in descending order: double-KO cells > Lig4-KO cells > Rad54-KO cells > wild-type cells. Although Rad54-KO cells had an almost constant SER value, Lig4-KO cells showed a high-SER value when compared to Rad54-KO cells, even with increasing LET values. These results suggest that with carbon-ion therapy, targeting NHEJ repair yields higher radiosensitivity than targeting homologous recombination repair.

ATM-mediated stabilization of ZEB1 promotes DNA damage response and radioresistance through CHK1

Epithelial-mesenchymal transition (EMT) is associated with characteristics of breast cancer stem cells, including chemoresistance and radioresistance. However, it is unclear whether EMT itself or specific EMT regulators play causal roles in these properties. Here we identify an EMT-inducing transcription factor, zinc finger E-box binding homeobox 1 (ZEB1), as a regulator of radiosensitivity and DNA damage response (DDR). Radioresistant subpopulations of breast cancer cells derived from ionizing radiation exhibit hyperactivation of ATM and upregulation of ZEB1, and ZEB1 promotes tumor cell radioresistance in vitro and in vivo. Mechanistically, ATM kinase phosphorylates and stabilizes ZEB1 in response to DNA damage, and ZEB1 in turn directly interacts with USP7 and enhances its ability to deubiquitinate and stabilize CHK1, thereby promoting homologous recombination-dependent DNA repair and resistance to radiation. These findings identify ZEB1 as an ATM substrate linking ATM to CHK1 and as the mechanism underlying the association between EMT and radioresistance.

Compliance and toxicity of the hypoxic radiosensitizer nimorazole in the treatment of patients with head and neck squamous cell carcinoma (HNSCC)

PURPOSE

To evaluate the compliance and toxicity of the hypoxic radiosensitizer nimorazole in head and neck cancer patients.

METHODS

A retrospective study of patients with head and neck squamous cell carcinoma (HNSCC), treated in Denmark between 1990 and 2013. All patients treated with radical radiotherapy (± chemotherapy) [66-70 Gy; 33-35 fractions; 2 Gy/fraction; 5-6 fractions/week] concomitant with the hypoxic radiosensitizer nimorazole. Nimorazole was administered as oral tablets in doses of approximately 1.2 g/m(2) body surface area in connection with the first of each daily radiation treatment. A second daily dose of 1 g was given in connection with the second radiotherapy fraction in the accelerated fractionation regimen. The compliance was estimated as the percentage of the initially prescribed dose, which was received by each patient. The main side effects were recorded.

RESULTS

A total of 1049 patients were investigated. The tolerance to nimorazole was fair: 58% of patients received the full prescribed total dose. Nausea and vomiting were the major complaints: among the 260 patients with dose reductions due to known side effects, (87%) were due to nausea/vomiting. All side effects ceased when treatment was interrupted, and neither severe nor long lasting side effects were observed. Female patients were significantly more likely to have dose reduction (OR 2.02; 95% CI 1.50-2.70), and nausea/vomiting. Patients aged more than 70 years were significantly more likely to have dose reduction. Patients who received less than 1100 mg/m(2) were significantly less likely to have dose reduction (OR 0.58; CI 0.44-0.78), and nausea/vomiting, compared to those who received 1100-1300 mg/m(2). The tolerance was also less in the group of patients received accelerated chemoradiotherapy (OR 1.70; CI 1.20-2.50) with more association with nausea/vomiting (OR 2.09; CI 1.40-3.10).

CONCLUSION

The compliance to nimorazole is fair, with tolerable acute, but neither persistent nor late, toxicity. It can be administered with chemotherapy and different radiotherapy fractionation schedules.

Malignant gliomas: strategies to increase the effectiveness of targeted molecular treatment

Recently, there has been increasing interest in the use of targeted molecular agents for the treatment of malignant gliomas. These agents are generally well tolerated but have demonstrated only modest activity. In this article, the current status of targeted molecular agents for malignant gliomas will be reviewed and strategies to improve their effectiveness will be discussed.

MiR-21 mediates the radiation resistance of glioblastoma cells by regulating PDCD4 and hMSH2

The purpose of this study was to investigate the molecular mechanism by which miR-21 and its target genes mediate radiation resistance of glioblastoma cells. Real-time PCR was employed to detect miR-21 expression in normal brain tissues, glioblastoma tissues and glioblastoma cell lines (A172, T98G and U87MG). T98G cells were transfected with anti-miR-21 oligonucleotides, or plasmids containing PDCD4 or hMSH2 (PDCD4-pcDNA3 and hMSH2-pcDNA3). The survival curve was obtained to investigate the sensitivity of T98G cells to radiation. Cell apoptosis was measured by using the Caspase-3/7 kit and cell cycle by flow cytometry. Western blotting was performed to detect the expression of hMSH2 and PDCD4 in miR-21-inhibiting T98G cells. The results showed that miR-21 expression in glioblastoma cells and tissues was conversely associated with the radiation sensitivity. Over-expression of miR-21 resulted in radiation resistance, while knockdown of miR-21 led to higher sensitivity of glioblastma cells to radiation. After miR-21 knockdown, the apoptosis of T98G cells was significantly increased and the G(2) phase arrest was more significant. In addition, miR-21 knockdown increased the expression of endogenous PDCD4 and hMSH2, which contributed to the apoptosis and G(2) arrest of T98G cells. The findings suggested that miR-21 may mediate the resistance of glioblastoma cells against radiation via its target genes PDCD4 and hMSH2. MiR-21 and its target genes may be used as potential molecular targets for clinical radiotherapy sensitization in the future.

Tumor Growth Inhibitory Effect of Juglone and Its Radiation Sensitizing Potential

The present study aimed at evaluating the anticancer and radiosensitizing potential of juglone against a chemoresistant and radioresistant tumor (B16F1 melanoma) growing on C57BL/6J mice. Volume doubling time, growth delay, and median survival were used to assess the in vivo anticancer and radiosensitizing potential of juglone. In vitro radiosensitizing potential of juglone was studied using clonogenic, comet, and reactive oxygen species induction assays. Treatment of tumor-bearing mice with sublethal doses of juglone caused a dose-dependent inhibition of tumor growth as evident from the growth delay and median survival values. Comet assay using tumor tissue and blood showed differential toxicity of juglone, where higher levels of DNA damage was seen in tumor tissue compared with blood cells. Pretreatment of tumor-bearing mice with optimum dose of juglone before radiation resulted in significant tumor growth inhibition compared with radiation alone. From the clonogenic assay, the authors observed a sensitization enhancement ratio of 1.37 for the combination treatment compared with radiation alone. Furthermore, comet assay studies revealed the potential of juglone to enhance the radiation-induced DNA damage and cause a delay in its repair. Juglone pretreatment before radiation also resulted in a significant elevation in the intracellular reactive oxygen species levels compared with radiation alone. In conclusion, the results of this study show the potential of juglone to inhibit the growth of melanoma in vivo. The study also revealed the potential of juglone to augment the radiation-induced cell death of melanoma cells, which may be attributed to oxidative stress–mediated DNA damage and its delayed repair.

Chemical radiosensitizers for use in radiotherapy

Radiosensitizers are intended to enhance tumour cell killing while having much less effect on normal tissues. Some drugs target different physiological characteristics of the tumour, particularly hypoxia associated with radioresistance. Oxygen is the definitive hypoxic cell radiosensitizer, the large differential radiosensitivity of oxic vs hypoxic cells being an attractive factor. The combination of nicotinamide to reduce acute hypoxia with normobaric carbogen breathing is showing clinical promise. 'Electron-affinic' chemicals that react with DNA free radicals have the potential for universal activity to combat hypoxia-associated radioresistance; a nitroimidazole, nimorazole, is clinically effective at tolerable doses. Hypoxia-specific cytotoxins, such as tirapazamine, are valuable adjuncts to radiotherapy. Nitric oxide is a potent hypoxic cell radiosensitizer; variations in endogenous levels might have prognostic significance, and routes to deliver nitric oxide specifically to tumours are being developed. In principle, many drugs can be delivered selectively to hypoxic tumours using either reductase enzymes or radiation-produced free radicals to activate drug release from electron-affinic prodrugs. A redox-active agent based on a gadolinium chelate is being evaluated clinically. Pyrimidines substituted with bromine or iodine are incorporated into DNA and enhance free radical damage; fluoropyrimidines act by different mechanisms. A wide variety of drugs that influence the nature or repair of DNA damage are being evaluated in conjunction with radiation; it is often difficult to define the mechanisms underlying chemoradiation regimens. Drugs being evaluated include topoisomerase inhibitors (e.g. camptothecin, topotecan), and the hypoxia-activated anthraquinone AQ4N; alkylating agents include temozolomide. Drugs involved in DNA repair pathways being investigated include the potent poly(ADP ribose)polymerase inhibitor, AG14,361. Proteins involved in cell signalling, such as the Ras family, are attractive targets linked to radioresistance, as are epidermal growth factor receptors and linked kinases (drugs including vandetanib [ZD6,474], cetuximab and gefitinib), and cyclooxygenase-2 (celecoxib). The suppression of radioprotective thiols seems to offer more potential with alkylating agents than with radiotherapy, although it remains a strategy worthy of exploration.

Cancer gene therapy: Combination with radiation therapy and the role of bystander cell killing in the anti-tumor effect

Current anti-cancer modalities such as surgery, chemo- and radiation therapies have only limited success in cancer treatment. Gene therapy is a promising new tool to improve outcomes. In this review, first we summarize the various strategies to kill tumor cells, and then focus on the bystander effect of gene therapy. A variety of strategies, such as gene-directed enzyme pro-drug therapy, activation of an anti-tumor immune attack, application of replication-competent and oncolytic viral vectors, tumor-specific as well as radiation- and hypoxiainduced gene expression, might be applied to target tumor cells. We put special emphasis on the combination of these approaches with local tumor irradiation. Using the available vector systems, only a small portion of cancer cells contains the therapeutic genes under clinical situations. However, cells directly targeted by gene therapy will transfer death signals to neighboring cancer cells. This bystander cell killing improves the efficiency of cancer gene therapy. Death signals are delivered by cell-to-cell communication through gap junction intercellular contacts, release of toxic metabolites into the neighborhood or to larger distances, phagocytosis of apoptotic bodies, and the activation of the immune system. Bystander cell killing can be enhanced by the introduction of gap junction proteins into cells, by further activating the immune system with immune-stimulatory molecules, or by introducing genes that help the transfer of cytotoxic genes and/or metabolites into bystander cells. In conclusion, although bystander cell killing can improve therapeutic effects, there should be additional developments in cancer gene therapy for a more efficient clinical application.

Inhibition of homologous recombination repair with Pentoxifylline targets G2 cells generated by radiotherapy and induces major enhancements of the toxicity of cisplatin and melphalan given after irradiation

The presentation reviews the modus operandi of the dose modifying drug Pentoxifylline and the dose enhancement factors which can be achieved in different cell types. Preclinical and clinical data show that Pentoxifylline improves the oxygenation of hypoxic tumours and enhances tumour control by irradiation. In vitro experiments demonstrate that Pentoxifylline also operates when oxygen is not limiting and produces dose modifying factors in the region of 1.2 – 2.0. This oxygen independent effect is poorly understood. In p53 mutant cells irradiation induces a G2 block which is abrogated by Pentoxifylline. The enhancement of cell kill observed when Pentoxifylline and irradiation are given together could arise from rapid entry of damaged tumour cells into mitosis and propagation of DNA lesions as the result of curtailment of repair time. Recovery ratios and repair experiments using CFGE after high dose irradiation demonstrate that Pentoxifylline inhibits repair directly and that curtailment of repair time is not the explanation. Use of the repair defective xrs1 and the parental repair competent CHO-K1 cell line shows that Pentoxifylline inhibits homologous recombination repair which operates predominantly in the G2 phase of the cell cycle. When irradiated cells residing in G2 phase are exposed to very low doses of cisplatin at a toxic dose of 5 %. (TC: 0.05) massive toxicity enhancements up to a factor of 80 are observed in melanoma, squamous carcinoma and prostate tumour cell lines. Enhancements of radiotoxicity seen when Pentoxifylline and radiation are applied together are small and do not exceed a factor of 2.0. The capacity of Pentoxifyline to inhibit homologous recombination repair has not as yet been clinically utilized. A suitable application could be in the treatment of cervical carcinoma where irradiation and cisplatin are standard modality. In vitro data also strongly suggest that regimes where irradiation is used in combination with alkylating drugs may also benefit.