Sensitizers and protectors of radiation and chemotherapy

Effect of Linear Energy Transfer on Cystamine's Radioprotective Activity: A Study Using the Fricke Dosimeter with 6-500 MeV per Nucleon Carbon Ions-Implication for Carbon Ion Hadrontherapy

(1) Background: Radioprotective agents have garnered considerable interest due to their prospective applications in radiotherapy, public health medicine, and situations of large-scale accidental radiation exposure or impending radiological emergencies. Cystamine, an organic diamino-disulfide compound, is recognized for its radiation-protective and antioxidant properties. This study aims to utilize the aqueous ferrous sulfate (Fricke) dosimeter to measure the free-radical scavenging capabilities of cystamine during irradiation by fast carbon ions. This analysis spans an energy range from 6 to 500 MeV per nucleon, which correlates with "linear energy transfer" (LET) values ranging from approximately 248 keV/μm down to 9.3 keV/μm. (2) Methods: Monte Carlo track chemistry calculations were used to simulate the radiation-induced chemistry of aerated Fricke-cystamine solutions across a broad spectrum of cystamine concentrations, ranging from 10-6 to 1 M. (3) Results: In irradiated Fricke solutions containing cystamine, cystamine is observed to hinder the oxidation of Fe2+ ions, an effect triggered by oxidizing agents from the radiolysis of acidic water, resulting in reduced Fe3+ ion production. Our simulations, conducted both with and without accounting for the multiple ionization of water, confirm cystamine's ability to capture free radicals, highlighting its strong antioxidant properties. Aligning with prior research, our simulations also indicate that the protective and antioxidant efficiency of cystamine diminishes with increasing LET of the radiation. This result can be attributed to the changes in the geometry of the track structures when transitioning from lower to higher LETs. (4) Conclusions: If we can apply these fundamental research findings to biological systems at a physiological pH, the use of cystamine alongside carbon-ion hadrontherapy could present a promising approach to further improve the therapeutic ratio in cancer treatments.

Piperazin incorporated Schiff Base derivatives: Assessment of in vitro biological activities, metabolic enzyme inhibition properties, and molecular docking calculations

The cytotoxic activities of the compounds were determined by the 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) method in human breast cancer (MCF-7), human cervical cancer (HeLa), and mouse fibroblast (L929) cell lines. The compounds MAAS-5 and four modified the supercoiled tertiary structure of pBR322 plasmid DNA. MAAS-5 showed the highest cytotoxic activity in HeLa, MCF-7, and L929 cells with IC50 values of 16.76 ± 3.22, 28.83 ± 5.61, and 2.18 ± 1.22 µM, respectively. MAAS-3 was found to have almost the lowest cytotoxic activities with the IC50 values of 93.17 ± 9.28, 181.07 ± 11.54, and 16.86 ± 6.42 µM in HeLa, MCF-7, and L929 cells respectively at 24 h. Moreover, the antiepileptic potentials of these compounds were investigated in this study. To this end, the effect of newly synthesized Schiff base derivatives on the enzyme activities of carbonic anhydrase I and II isozymes (human carbonic anhydrase [hCA] I and hCA II) was evaluated spectrophotometrically. The target compounds demonstrated high inhibitory activities compared with standard inhibitors with Ki values in the range of 4.54 ± 0.86-15.46 ± 8.65 nM for hCA I (Ki value for standard inhibitor =  12.08 ± 2.00 nM), 1.09 ± 0.32-29.94 ± 0.82 nM for hCA II (Ki value for standard inhibitor = 18.22 ± 4.90  nM). Finally, the activities of the compounds were compared with the Gaussian programme in the B3lyp, HF, M062X base sets with 6-31++G (d,p) levels. In addition, the activities of five compounds against various breast cancer proteins and hCA I and II were compared with molecular docking calculations. Also, absorption, distribution, metabolism, excretion, and toxicity analysis was performed to investigate the possibility of using five compounds as drug candidates.

Norcantharidin promotes cancer radiosensitization through Cullin1 neddylation-mediated CDC6 protein degradation

Radiotherapy (RT) is a conventional cancer therapeutic modality. However, cancer cells tend to develop radioresistance after a period of treatment. Diagnostic markers and therapeutic targets for radiosensitivity are severely lacking. Our recently published studies demonstrated that the cell division cycle (CDC6) is a critical molecule contributing to radioresistance, and maybe a potential therapeutic target to overcome radioresistance. In the present study, we for the first time reported that Norcantharidin (NCTD), a demethylated form of cantharidin, re-sensitized radioresistant cancer cells to overcome radioresistance, and synergistically promoted irradiation (IR)-induced cell killing and apoptosis by inducing CDC6 protein degradation. Mechanistically, NCTD induced CDC6 protein degradation through the ubiquitin-proteasome pathways. By using small interfering RNA (siRNA) interference or small compound inhibitors, we further determined that NCTD induced CDC6 protein degradation through a neddylation-dependent pathway, but not through Huwe1, Cyclin F, and APC/C-mediated ubiquitin-proteasome pathways. We screened the six most relevant Cullin subunits (CUL1, 2, 3, 4A, 4B, and 5) using siRNAs. The knockdown of Cullin1 but not the other five cullins remarkably elevated CDC6 protein levels. NCTD promoted the binding of Cullin1 to CDC6, thereby promoting CDC6 protein degradation through a Cullin1 neddylation-mediated ubiquitin-proteasome pathway. NCTD can be used in combination with radiotherapy to achieve better anticancer efficacy, or work as a radiosensitizer to overcome cancer radioresistance.

Electron Attachment Studies with the Potential Radiosensitizer 2-Nitrofuran

Nitrofurans belong to the class of drugs typically used as antibiotics or antimicrobials. The defining structural component is a furan ring with a nitro group attached. In the present investigation, electron attachment to 2-nitrofuran (C₄H₃NO₃), which is considered as a potential radiosensitizer candidate for application in radiotherapy, has been studied in a crossed electron-molecular beams experiment. The present results indicate that low-energy electrons with kinetic energies of about 0-12 eV effectively decompose the molecule. In total, twelve fragment anions were detected within the detection limit of the apparatus, as well as the parent anion of 2-nitrofuran. One major resonance region of ≈0-5 eV is observed in which the most abundant anions NO₂^-, C₄H₃O^-, and C₄H₃NO₃^- are detected. The experimental results are supported by ab initio calculations of electronic states in the resulting anion, thermochemical thresholds, connectivity between electronic states of the anion, and reactivity analysis in the hot ground state.

Electron Ionization of Imidazole and Its Derivative 2-Nitroimidazole

Imidazole (IMI) is a basic building block of many biologically important compounds. Thus, its electron ionization properties are of major interest and essential for the comparison with other molecular targets containing its elemental structure. 2-Nitroimidazole (2NI) contains the imidazole ring together with nitrogen dioxide bound to the C2 position, making it a radiosensitizing compound in hypoxic tumors. In the present study, we investigated electron ionization of IMI and 2NI and determined the mass spectra, the ionization energies, and appearance energies of the most abundant fragment cations. The experiments were complemented by quantum chemical calculations on the thermodynamic thresholds and potential energy surfaces, with particular attention to the calculated transition states for the most important dissociation reactions. In the case of IMI, substantially lower threshold values (up to ~ 1.5 eV) were obtained in the present work compared to the only available previous electron ionization study. Closer agreement was found with recent photon ionization values, albeit the general trend of slightly higher values for the case of electron ionization. The only exception for imidazole was found in the molecular cation at m/z 40 which is tentatively assigned to the quasi-linear HCCNH+/ HCNCH+. Electron ionization of 2NI leads to analogous fragment cations as in imidazole, yet different dissociation pathways must be operative due to the presence of the NO2 group. Regarding the potential radiosensitization properties of 2NI, electron ionization is characterized by dominant parent cation formation and release of the neutral NO radical.

Evaluation of the radioprotective ability of cystamine for 150 keV – 500 MeV proton irradiation: a Monte Carlo track chemistry simulation study

Cystamine, an organic diamino-disulfide, is among the best of the known radiation-protective compounds, although the underlying molecular mechanisms by which it operates remain poorly understood. This study aims to use the aqueous ferrous sulfate (Fricke) dosimeter to evaluate the protective properties of this compound when present during irradiation by fast incident protons in the energy range of 150 keV – 500 MeV, that is, for “linear energy transfer” (LET) values ranging from ∼72.3 to 0.23 keV/μm. The presence of cystamine in irradiated Fricke solutions prevents the oxidation of Fe2+ ions by the oxidizing species produced in the radiolysis of acidic water, resulting in reduced Fe3+ ion yields. A Monte Carlo computer code is used to simulate the radiation-induced chemistry of the studied Fricke–cystamine solutions under aerated conditions while covering a wide range of cystamine concentrations from 5 × 10−7 to 1 mol/L. Results indicate that the protective activity of cystamine is due to its radical-capturing ability, a clear signature of the strong antioxidant profile of this compound. In addition, our simulations show that at low and intermediate concentrations of cystamine, its protective efficiency decreases with increasing LET, which is consistent with previous work. This finding stems from differences in the geometry of the track structures that change from low-LET isolated spherical “spurs” to high-LET dense continuous cylindrical tracks as LET increases. This study concludes that Monte Carlo simulations represent a powerful method for understanding, at the molecular level, indirect radiation damage to complex molecules such as cystamine.

Effect of Radiation on the Expression of Taurine Transporter in the Intestine of Mouse

There has been a growing interest on the effects of radiation since the Fukushima nuclear power plant accident of 2011. Taurine has been reported to have a radioprotective effect in irradiated mice. However, the detailed mechanism of this radioprotective effect is still awaiting clarification. The aim of this study was to investigation how radiation affects the expression of taurine and to shed light on the mechanism accounting for radioprotective and radiation mitigating effect. Six-week-old male mice were randomly divided into two groups: IR group (7 Gy irradiation) and IR + Tau group (7 Gy irradiation + taurine 3000 mg/kg/day). We examined the survival rate, the expression of taurine and taurine transporter in the small intestine and the urinary taurine concentration. In this study, no statistically significant difference was found in the survival rate between IR Group and IR + Tau Group. Three days and 7 days after irradiation, the urinary taurine concentration of IR + Tau group increased more than that of IR group. Three days and 10 days after irradiation, the expression of taurine and taurine transporter in the small intestine of IR group and IR + Tau group decreased more than that of normal small intestine. It is reported that radiation exposure increases the urinary taurine concentration. We found that the radiation exposure decreases the expression of the taurine transporter in the small intestine of mouse. This finding suggests that a decrease in the expression of the taurine transporter promotes the release of taurine from the tissue into the urine.

Gold nanoparticles as a potent radiosensitizer in neutron therapy

The purpose of this study was to investigate the potential of gold nanoparticles as radiosensitizer for use in neutron therapy against hepatocellular carcinoma.

The hepatocellular carcinoma cells lines Huh7 and HepG2 were irradiated with γ and neutron radiation in the presence or absence of gold nanoparticles. Effects were evaluated by transmission electron microscopy, cell survival, cell cycle, DNA damage, migration, and invasiveness.

Gold nanoparticles significantly enhanced the radiosensitivity of Huh7 and HepG2 cells to γ-rays by 1.41- and 1.16-fold, respectively, and by 1.80- and 1.35-fold to neutron radiation, which has high linear energy transfer. Accordingly, exposure to neutron radiation in the presence of gold nanoparticles induced cell cycle arrest, DNA damage, and cell death to a significantly higher extent, and suppressed cell migration and invasiveness more robustly. These effects are presumably due to the ability of gold nanoparticles to amplify the effective dose from neutron radiation more efficiently.

The data suggest that gold nanoparticles may be clinically useful in combination therapy against hepatocellular carcinoma by enhancing the toxicity of radiation with high linear energy transfer.

Methoxyamine Enhances 5-Fluorouracil-Induced Radiosensitization in Colon Cancer Cell Line HT29

Objective

This study intended to observe the effects of methoxyamine (Mx) on cytotoxic effects and DNA damage caused by 5-Fluorouracil (5-FU) in combination with gamma radiation in a human colon cancer cell line, HT29.

Materials and Methods

In this experimental study, HT29 cells were cultured as a monolayer and treated with different concentrations of 5-FU along with 1 mM Mx for 24 hours. Next, the cells were irradiated with 2 Gy gamma radiation. After the treatments, we assessed for DNA damage, cytotoxicity, and viability by alkaline comet, clonogenic survival, and trypan blue dye exclusion assays.

Results

Cytotoxicity and DNA damage increased with increasing 5-FU concentration. The 1 mM Mx concentration had no significant effect on cytotoxicity and DNA damage from 5-FU; however, it increased the cytotoxic and genotoxic effects of different concentrations of 5-FU when used in combination with 2 Gy gamma radiation.

Conclusion

Mx combined with 5-FU enhanced the radiosensitivity of colon cancer cells.

Synergistic locoregional chemoradiotherapy using a composite liposome-in-gel system as an injectable drug depot

The use of radiosensitizers in clinical radiotherapy is limited by systemic toxicity. The biopolymeric, biodegradable, injectable liposome-in-gel-paclitaxel (LG-PTX) system was developed for regional delivery of the radiosensitizer paclitaxel (PTX), and its efficacy was evaluated with concurrent fractionated radiation. LG-PTX is composed of nano-sized drug-loaded fluidizing liposomes, which are incorporated into a porous biodegradable gellan hydrogel. This allows enhanced drug permeation while maintaining a localization of the drug depot. LG-PTX had an IC₅₀ of 325±117 nM in B16F10 melanoma cells, and cytotoxicity with concurrent doses of fractionated radiation showed significant increase in apoptotic cells (75%) compared to radiation (39%) or LG-PTX (43%) alone. Peri-tumoral injection in tumor-bearing mice showed PTX localization in the tumor 2 hours after administration, with no drug detected in plasma or other organs. LG-PTX administration with doses of focal radiation (5×3 Gy) significantly reduced tumor volumes compared to control (6.4 times) and radiation alone (1.6 times) and improved animal survival. LG-PTX thus efficiently localizes the drug at the tumor site and synergistically enhances the effect of concurrent radiotherapy. This novel liposome-in-gel system can potentially be used as a platform technology for the delivery of radiosensitizing drugs to enhance the efficacy of chemoradiotherapy.

Site- and state-selected photofragmentation of 2Br-pyrimidine

The fragmentation of the 2Br-pyrimidine molecule following direct valence photoionization or inner shell excitation has been studied by electron-ion coincidence experiments. 2Br-pyrimidine has been chosen as a model for the class of pyrimidinic building blocks of three nucleic acids and several radiosensitizers. It is known that the site- and state-localization of energy deposition, typical of inner shell excitation, results in the enhancement of the total ion yield as well as in changes in the relative intensity of the different fragmentation channels. Here we address the question of the origin of this selective fragmentation by using electron-ion coincidence techniques. The results show that the fragmentation is strongly selective in the final singly charged ion state, independently of the process that leads to the population of that state, and the dominant fragmentation patterns correlate with the nearest appearance potential.

Mechanisms for SU5416 as a radiosensitizer of endothelial cells

Endothelial cells (ECs), that comprise the tumor vasculature, are critical targets for anticancer radiotherapy. The aim of this work was to study the mechanism by which SU5416, a known anti-angiogenesis inhibitor, modifies the radiation responses of human vascular ECs. Two human endothelial cell lines (HUVEC and 2H11) were treated with SU5416 alone, radiation alone, or a combination of both. In vitro tests were performed using colony forming assays, FACS analysis, western blotting, immunohistochemistry, migration assay, invasion assays and endothelial tube formation assays. The combination of radiation and SU5416 significantly inhibited cell survival, the repair of radiation-induced DNA damage, and induced apoptosis. It also caused cell cycle arrest, inhibited cell migration and invasion, and suppressed angiogenesis. In this study, our results first provide a scientific rationale to combine SU5416 with radiotherapy to target ECs and suggest its clinical application in combination cancer treatment with radiotherapy.

Biomarkers for enhancing the radiosensitivity of nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is a common head and neck malignancy. The incidence of NPC is higher in Southern China and Southeast Asia compared with Western countries. Given its high radiosensitivity, the standard treatment for NPC is radiotherapy. However, radioresistance remains a serious obstacle to successful treatment. Radioresistance can cause local recurrence and distant metastases in some patients after treatment by radiation. Thus, special emphasis has been given to the discovery of effective radiosensitizers. This review aims to discuss the biomarkers, classified according to the main mechanisms of radiosensitization, which can enhance the sensitivity of NPC cells to ionizing radiation.

Gamma-tocopherol-N,N-dimethylglycine ester as a potent post-irradiation mitigator against whole body X-irradiation-induced bone marrow death in mice

We examined the radioprotective and mitigative effects of gamma-tocopherol-N,N-dimethylglycine ester (GTDMG), a novel water-soluble gamma-tocopherol derivative, against X-irradiation-induced bone marrow death in mice. Mice (C3H, 10 weeks, male) were injected intraperitoneally with GTDMG suspended in a 0.5% methyl cellulose solution before or after receiving of 7.5-Gy whole body X-irradiation. GTDMG significantly enhanced the 30-day survival rate when given 30 min before or immediately after the irradiation. Its mitigative activity (administered after exposure) was examined further in detail. The optimal concentration of GTDMG given immediately after irradiation was around 100 mg/kg body weight (bw) and the 30-day survival rate was 97.6 ± 2.4%. When GTDMG was administered 1, 10 and 24 h post-irradiation, the survival rate was 85.7 ± 7.6, 75.0 ± 9.7 and 36.7 ± 8.8%, respectively, showing significant mitigation even at 24 h after irradiation (P < 0.05). The value of the dose reduction factor (100 mg/kg bw, given intraperitoneally (i.p.) immediately after irradiation) was 1.25. GTDMG enhanced the recovery of red blood cell-, white blood cell-, and platelet-counts after irradiation and significantly increased the number of endogenous spleen colonies (P < 0.05). Subcutaneous (s.c.) administration also had mitigative effects. In conclusion, GTDMG is a potent radiation mitigator.

Evaluation of (S)- and (R)-misonidazole as GPX inhibitors: synthesis, characterization including circular dichroism and in vitro testing on bovine GPx-1

Racemic misonidazole, a radiosensitizer formally used in radiation therapy of cancer and to date still applied, was once reported to exhibit strong inhibitory effects on mouse glutathione peroxidases (GPX). This appeared to qualify misonidazole as a lead structure for the development of novel GPX inhibitors to cause oxidative stress in chemotherapy-resistant tumors. A unique feature of misonidazole as an inhibitor of GPX is the absence of a thiol functionality. Therefore, it was expected to selectively target inhibition devoid of promiscuous interactions with cations and sulfhydryl groups. We synthesized the isomers of misonidazole and analyzed the ability of chiroptical high-performance liquid chromatography (HPLC) to identify the particular enantiomers. Due to the chiral pool synthesis, the assignment of the correct configuration could be verified. Finally, we evaluated both isomers for their inhibitory activities on bovine erythrocyte GPx-1, which is 87% homologous to the human enzyme. Despite the previously reported inhibition of racemic misonidazole on the less homologous mouse GPx-1, we did not find any significant inhibitory activity on the bovine enzyme for either isomer. Though misonidazole appears unlikely to be an inhibitor of human GPx-1 activity, we still spotlight misonidazole as a promising fragment-like lead structure in general.

Raman spectroscopy demonstrates Amifostine induced preservation of bone mineralization patterns in the irradiated murine mandible

PURPOSE

Adjuvant radiotherapy in the management of head and neck cancer remains severely debilitating. Fortunately, newly developed agents aimed at decreasing radiation-induced damage have shown great promise. Amifostine (AMF) is a compound, which confers radio-protection to the exposed normal tissues, such as bone. Our intent is to utilize Raman spectroscopy to demonstrate how AMF preserves the mineral composition of the murine mandible following human equivalent radiation.

METHODS

Sprague Dawley rats were randomized into 3 experimental groups: control (n=5), XRT (n=5), and AMF-XRT (n=5). Both XRT and AMF groups underwent bioequivalent radiation of 70Gy in 5 fractions to the left hemimandible. AMF-XRT received Amifostine prior to radiation. Fifty-six days post-radiation, the hemimandibles were harvested, and Raman spectra were taken in the region of interest spanning 2mm behind the last molar. Bone mineral and matrix-specific Raman bands were analyzed using one-way ANOVA, with statistical significance at p<0.05.

RESULTS

The full-width at half-maximum of the primary phosphate band (FWHM) and the ratio of carbonate/phosphate intensities demonstrated significant differences between AMF-XRT versus XRT (p<0.01) and XRT versus control (p<0.01). There was no difference between AMF-XRT and control (p>0.05) in both Raman metrics. Computer-aided spectral subtraction further confirmed these results where AMF-XRT was spectrally similar to the control. Interestingly, the collagen cross-link ratio did not differ between XRT and AMF-XRT (p<0.01) but was significantly different from the control (p<0.01).

CONCLUSION

Our novel findings demonstrate that AMF prophylaxis maintains and protects bone mineral quality in the setting of radiation. Raman spectroscopy is an emerging and exceptionally attractive clinical translational technology to investigate and monitor both the destructive effects of radiation and the therapeutic remediation of AMF on the structural, physical and chemical qualities of bone.

Thrombospondin-1 and CD47 limit cell and tissue survival of radiation injury

Radiation, a primary mode of cancer therapy, acutely damages cellular macromolecules and DNA and elicits stress responses that lead to cell death. The known cytoprotective activity of nitric oxide (NO) is blocked by thrombospondin-1, a potent antagonist of NO/cGMP signaling in ischemic soft tissues, suggesting that thrombospondin-1 signaling via its receptor CD47 could correspondingly increase radiosensitivity. We show here that soft tissues in thrombospondin-1-null mice are remarkably resistant to radiation injury. Twelve hours after 25-Gy hindlimb irradiation, thrombospondin-1-null mice showed significantly less cell death in both muscle and bone marrow. Two months after irradiation, skin and muscle units in null mice showed minimal histological evidence of radiation injury and near full retention of mitochondrial function. Additionally, both tissue perfusion and acute vascular responses to NO were preserved in irradiated thrombospondin-1-null hindlimbs. The role of thrombospondin-1 in radiosensitization is specific because thrombospondin-2-null mice were not protected. However, mice lacking CD47 showed radioresistance similar to thrombospondin-1-null mice. Both thrombospondin-1- and CD47-dependent radiosensitization is cell autonomous because vascular cells isolated from the respective null mice showed dramatically increased survival and improved proliferative capacity after irradiation in vitro. Therefore, thrombospondin-1/CD47 antagonists may have selective radioprotective activity for normal tissues.

Heat-treated mineral-yeast as a potent post-irradiation radioprotector

In vivo radioprotection of C3H mice by i.p. administration of Zn-, Mn-, Cu-, or Se-containing heat-treated Saccharomyces serevisiae yeast sample was examined. The 30-day survival of the group treated 30 min before 7.5 Gy whole-body X-irradiation with mineral-containing yeast powders suspended in 0.5% methylcellulose was significantly higher than that of control group. When mineral-yeast was administered immediately after irradiation, the survival rate was even higher and Zn- or Cu-yeast showed the highest rate (more than 90%). Although treatment with simple yeast showed a high survival rate (73%), it was significantly lower than that obtained by the Zn-yeast treatment. The effects of Zn-yeast were studied further. When the interval between irradiation and administration was varied, the protective activity of Zn-yeast decreased gradually by increasing the interval but was still significantly high for the administration at 10 h post-irradiation. The dose reduction factor of Zn-yeast (100 mg/kg, i.p. administration immediately after irradiation) was about 1.2. When the suspension of Zn-yeast was fractionated by centrifugation, the insoluble fraction showed a potent effect, while the soluble fraction had only a moderate effect. In conclusion, mineral-yeast, especially Zn-yeast, provides remarkable post-irradiation protection against lethal whole body X-irradiation. The activity is mainly attributable to the insoluble fraction, whereas some soluble components might contribute to the additional protective activity.

Radioprotection of craniofacial bone growth

In this review, the potential of pharmacologic therapy for prevention of radiation-induced bone growth inhibition is discussed. Significant radioprotection using the radioprotector Amifostine has been achieved in animal models of radiation-induced retardation of long and craniofacial bone growth. Moreover, radioprotection in vitro has been achieved in a number of cell lines, including osteoblast-like, endothelial, and fibroblastic. This evidence may support future clinical investigations of radioprotector Amifostine or similar substances for radioprotection of the growing craniofacial skeleton.

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.

Prodrug Modification Increases Potassium Tricyclo[5.2.1.02,6]-decan-8-yl Dithiocarbonate (D609) Chemical Stability and Cytotoxicity against U937 Leukemia Cells

Potassium tricyclo[5.2.1.0(2,6)]-decan-8-yl dithiocarbonate (D609) is a selective antitumor agent, potent antioxidant, and cytoprotectant. It has the potential to be developed as a unique chemotherapeutic agent that may provide dual therapeutic benefits against cancer, e.g., enhancing tumor cell death while protecting normal tissues from damage. However, D609 contains a dithiocarbonate (xanthate) group [O-C(=S)S(-)/O-C(=S)SH], which is chemically unstable, being readily oxidized to form a disulfide bond with subsequent loss of all biological activities. Therefore, we developed the synthesis of a series of S-(alkoxyacyl) D609 prodrugs by connecting the xanthate group of D609 to an ester via a self-immolative methyleneoxyl group. These S-(alkoxylacyl)-D609 prodrugs are designed to release D609 in two steps: esterase-catalyzed hydrolysis of the acyl ester bond followed by conversion of the resulting hydroxymethyl D609 to formaldehyde and D609. Three S-(alkoxyacyl) D609 prodrugs were synthesized by varying the steric bulkiness of the acyl group. These prodrugs are stable to ambient conditions, but readily hydrolyzed by esterases to liberate D609 in a controlled manner. More importantly, the lead prodrug methyleneoxybutyryl D609 is biologically more effective than D609 in inhibiting sphingomyelin synthase, thereby increasing the level of ceramide and inducing apoptosis in U937 leukemia cells. The prodrug has a significantly lower LD(50) value than that of D609 (56.6 versus 117 microM) against U937 cells. These findings demonstrate that prodrug modification of the xanthate moiety with an alkoxyacyl group can improve D609 oxidative stability and enhance its antitumor activity.

Novel radiosensitizers for locally advanced epithelial tumors: inhibition of the PI3K/Akt survival pathway in tumor cells and in tumor-associated endothelial cells as a novel treatment strategy?

In locally advanced epithelial malignancies, local control can be achieved with high doses of radiotherapy (RT). Concurrent chemoradiotherapy can improve tumor control in selected solid epithelial adult tumors; however, treatment-related toxicity is of major concern and the therapeutic window often small. Therefore, novel pharmacologic radiosensitizers with a tumor-specific molecular target and a broad therapeutic window are attractive. Because of clonal heterogeneity and the high mutation rate of these tumors, combined treatment with single molecular target radiosensitizers and RT are unlikely to improve sustained local tumor control substantially. Therefore, radiosensitizers modulating entire tumor cell survival pathways in epithelial tumors are of potential clinical use. We discuss the preclinical efficacy and the mechanism of three different, potential radiosensitizers targeting the PTEN/PI3K/Akt survival pathway. These compounds were initially thought to act as single-target agents against growth factor receptors (PKI 166 and PTK 787) or protein kinase C isoforms (PKC 412). We describe an additional target for these compounds. PKI 166 (an epidermal growth factor [EGF] receptor inhibitor) and PKC 412, target the PTEN/PI3K/Akt pathway mainly in tumor cells, and PTK 787 (a vascular endothelial growth factor [VEGF] receptor inhibitor) in endothelial cells. Even for these broader range molecular radiosensitizers, the benefit could be restricted to human epithelial tumor cell clones with a distinct molecular profile. Therefore, these potential radiosensitizers have to be carefully tested in specific model systems before introduction in early clinical trials.

RAF antisense oligonucleotide as a tumor radiosensitizer

The RAF-1 serine-threonine kinase plays a central role in signal transduction pathways involved in cell survival and proliferation. The concept of RAF-1-targeted disruption of cell signaling for therapeutic purposes was first advanced in 1989 with the demonstration of tumor growth inhibition in athymic mice and radiosensitization of human squamous carcinoma cells transfected with a vector expressing antisense cDNA. However, the clinical application of antisense strategies has awaited the development of improved antisense oligonucleotide technologies and drug delivery methods. Nuclease-resistant phosphorothioated antisense oligonucleotides have been the focus of pharmaceutical industry attention. In vivo delivery of nuclease-sensitive, natural backbone/phosphodiester oligonucleotides has remained a formidable challenge. Liposomal encapsulation of antisense oligonucleotides protects them from degradation and enhances drug delivery. Here, we review the importance of targeting RAF-1 signaling in cancer therapy and the preclinical and clinical experiences with a liposomal formulation of a nuclease-sensitive, ends-modified antisense RAF oligonucleotide.