Mechanisms of radiosensitization in iododeoxyuridine-substituted cells

Targeting of radio-enhancing drugs

OBJECTIVE

Toxicity to normal tissue is frequently the dose-limiting factor in the chemotherapy and mixed modality treatments of cancer. If the radio-enhancing drug could be localized at the disease site and released slowly over time, then systemic drug toxicities could be decreased while simultaneously maintaining high drug concentrations in the tumor. These considerations support a role for a sustained release intra-tumoral delivery systems for the delivery of radio-enhancing drugs.

METHODS

Two approaches aimed at achieving the end of localizing the radio-enhancing drug to the tumor are described. First, nanoparticles, which have a prolonged circulation time and facility for enhanced tumor targeting. Structural defects in the walls of the tumor vasculature allow the passage of particles too large to pass through the walls of normal blood vessels. This characteristic of tumor blood vessels, referred to as the enhanced permeability and retention (EPR) effect, allows relatively large entities (typically liposomes, nanoparticles, and macromolecular drugs) to pass from the blood vessels to tumor tissue and as a result nanoparticles accumulate in the tumor while being excluded from normal tissue. Second, biodegradable implanted polymers. In these devices, the radio-enhancing drug is physically trapped within the polymer matrix which is implanted in the tumor. The drug is released as the polymer degrades in response to its local environment. The degradation rate of the polymer device can be adjusted to control the rate of drug release. By this means, the level of radio-enhancing drug can be maintained at the tumor site for the duration of radiation treatment.

RESULTS AND CONCLUSIONS

Results of experiments indicate that for both methods tumor control could be optimized by maintaining the radio-enhancing drug at a useful concentration in the tumor over a period of time compatible with the duration of fractionated radiation treatment. These studies have provided proof of principle support for the further development of this approach. To date, while some of the methods and devices for drug delivery described in this paper have been involved in clinical trials, none have so far been developed for routine clinical application.

Quasi-Free Electron-Mediated Radiation Sensitization by C5-Halopyrimidines

Substitution of the thymidine moiety in DNA by C5-substituted halogenated thymidine analogues causes significant augmentation of radiation damage in living cells. However, the molecular pathway involved in such radiosensitization process has not been clearly elucidated to date in solution at room temperature. So far, low-energy electrons (LEEs; 0-20 eV) under vacuum condition and solvated electrons (esol-) in solution are shown to produce the σ-type C5-centered pyrimidine base radical through dissociative electron attachment involving carbon-halogen bond breakage. Formation of this σ-type radical and its subsequent reactions are proposed to cause cellular radiosensitization. Here, we report time-resolved measurements at room temperature, showing that a radiation-produced quasi-free electron (eqf-) in solution promptly breaks the C5-halogen bond in halopyrimidines forming the σ-type C5 radical via an excited transient anion radical. These results demonstrate the importance of ultrafast reactions of eqf-, which are extremely important in chemistry, physics, and biology, including tumor radiochemotherapy.

A Novel Role of Chromodomain Protein CBX8 in DNA Damage Response

Induction of DNA damage induces a dynamic repair process involving DNA repair factors and epigenetic regulators. Chromatin alterations must occur for DNA repair factors to gain access to DNA lesions and restore original chromatin configuration to preserve the gene expression profile. We characterize the novel role of CBX8, a chromodomain-containing protein with established roles in epigenetic regulation in DNA damage response. CBX8 protein rapidly accumulates at the sites of DNA damage within 30 s and progresses to accumulate until 4 min before gradually dispersing back to its predamage distribution by 15 min. CBX8 recruitment to the sites of DNA damage is dependent upon PARP1 activation and not dependent on ATM activation. CBX8 biochemically interacts with TRIM33, and its recruitment to DNA damage is also dependent on the presence of TRIM33. Knockdown of CBX8 using siRNA significantly reduces the efficiency of both homologous and the other non-homologous recombination, as well as increases sensitivity of cells to ionizing radiation. These findings demonstrate that CBX8 functions in the PARP-dependent DNA damage response partly through interaction with TRIM33 and is required for efficient DNA repair.

Bromopyridone Nucleotide Analogues, Anoxic Selective Radiosensitizing Agents That Are Incorporated in DNA by Polymerases

Ionizing radiation is frequently used to kill tumor cells. However, hypoxic solid tumor cells are more resistant to this treatment, providing the impetus to develop molecules that sensitize cells to ionizing radiation. 5-Bromo-2'-deoxyuridine (BrdU) has been investigated as a radiosensitizing agent in the lab and clinic for almost 5 decades. Recent reports that BrdU yields DNA interstrand cross-links (ICLs) in non-base-paired regions motivated us to develop radiosensitizing agents that generate cross-links in duplex DNA selectively under anoxic conditions. 4-Bromo- and 5-bromopyridone analogues of BrdU were synthesized and incorporated into oligonucleotides via solid-phase synthesis. Upon irradiation, these molecules yield DNA interstrand cross-links under anaerobic conditions. The respective nucleotide triphosphates are substrates for some DNA polymerases. ICLs are produced upon irradiation under anoxic conditions when the 4-bromopyridone is present in a PCR product. Because the nucleoside analogue is a poor phosphorylation substrate for human deoxycytidine kinase, a pro-nucleotide form of the 4-bromopyridone was used to incorporate this analogue into cellular DNA. Despite these efforts, the 4-bromopyridone nucleotide was not detected in cellular DNA. Although these molecules are improvements over previously reported nucleotide analogues designed to be hypoxic radiosensitizing agents, additional advances are needed to create molecules that function in cells.

DNA interstrand cross-linking upon irradiation of aryl halide C-nucleotides

γ-Radiolysis kills cells by damaging DNA via radical processes. Many of the radical pathways are O2 dependent, which results in a reduction in the cytotoxicity of ionizing radiation in hypoxic tumor cells. Consequently, there is a need for chemical agents that increase DNA damage by ionizing radiation under O2-deficient conditions. Modified nucleotides that are incorporated in DNA and produce highly reactive σ-radicals are useful as radiosensitizing agents. Aryl halide C-nucleotides (4-6) were incorporated into oligonucleotides by solid-phase synthesis. Duplex DNA containing 4-6 forms interstrand cross-links upon γ-radiolysis under anaerobic conditions or UV irradiation. Deep Vent (exo(-)) DNA polymerase accepted the nucleotide triphosphate of C-nucleotide 6 as a substrate and preferentially incorporated it opposite pyrimidines, but no further extension was detected. Incorporation of 6 in extended products by Deep Vent (exo(-)) during PCR or by Sequenase during copying of single stranded DNA plasmid was undetectable. Aryl halide nucleotide analogues that produce DNA interstrand cross-links under anaerobic conditions upon irradiation are potentially useful as radiosensitizing agents, but further research is needed to identify molecules that are incorporated by DNA polymerases and do not block further polymerization for this approach to be useful in cells.

Comparison of the bromodeoxyuridine-mediated sensitization effects between low-LET and high-LET ionizing radiation on DNA double-strand breaks

The incorporation of halogenated pyrmidines such as bromo- and iodo-deoxyuridines (BrdU, IdU) into DNA as thymidine analogs enhances cellular radiosensitivity when high-linear energy transfer (LET) radiation is not used. Although it is known that high-LET ionizing radiation confers fewer biological effects resulting from halogenated pyrimidine incorporation, the exact mechanisms of reduced radiosensitivity with high-LET radiation are not clear. We investigated the radiosensitization effects of halogenated pyrimidines with high-LET radiation using accelerated carbon and iron ions. Cells synchronized into the G1 phase after unifilar (1 cell cycle) and bifilar (2 cell cycles) substitution with 10 µM BrdU were exposed to various degrees of LET with heavy ions and X-rays. We then carried out a colony formation assay to measure cell survival. The γ-H2AX focus formation assay provided a measure of DNA double-strand break (DSB) formation and repair kinetics. Chromosomal aberration formations for the first post-irradiation metaphase were also scored. For both low-LET X-rays and carbon ions (13 keV/µm), BrdU incorporation led to impaired DNA repair kinetics, a larger initial number of DNA DSBs more frequent chromosomal aberrations at the first post-irradiated metaphase, and increased radiosensitivity for cell lethality. The enhancement ratio was higher after bifilar substitution. In contrast, no such synergistic enhancements were observed after high-LET irradiation with carbon and iron ions (70 and 200 keV/µm, respectively), even after bifilar substitution. Our results suggest that BrdU substitution did not modify the number and quality of DNA DSBs produced by high-LET radiation. The incorporation of halogenated pyrimidines may produce more complex/clustered DNA damage along with radicals formed by low-LET ionizing radiation. In contrast, the severity of damage produced by high-LET radiation may undermine the effects of BrdU and account for the observed minimal radiosensitization effects.

DNA damage and interstrand cross-link formation upon irradiation of aryl iodide C-nucleotide analogues

The 5-halopyrimidine nucleotides damage DNA upon UV-irradiation or exposure to gamma-radiolysis via the formation of the 2'-deoxyuridin-5-yl sigma-radical. The bromo and iodo derivatives of these molecules are useful tools for probing DNA structure and as therapeutically useful radiosensitizing agents. A series of aryl iodide C-nucleotides were incorporated into synthetic oligonucleotides and exposed to UV-irradiation and gamma-radiolysis. The strand damage produced upon irradiation of DNA containing these molecules is consistent with the generation of highly reactive sigma-radicals. Direct stand breaks and alkali-labile lesions are formed at the nucleotide analogue and flanking nucleotides. The distribution of lesion type and location varies depending upon the position of the aryl ring that is iodinated. Unlike 5-halopyrimidine nucleotides, the aryl iodides produce interstrand cross-links in duplex regions of DNA when exposed to gamma-radiolysis or UV-irradiation. Quenching studies suggest that cross-links are produced by gamma-radiolysis via capture of a solvated electron, and subsequent fragmentation to the sigma-radical. These observations suggest that aryl iodide C-nucleotide analogues may be useful as probes for excess electron transfer and radiosensitizing agents.

Methoxyamine potentiates iododeoxyuridine-induced radiosensitization by altering cell cycle kinetics and enhancing senescence

We previously reported that methoxyamine (an inhibitor of base excision repair) potentiates iododeoxyuridine (IUdR)-induced radiosensitization in human tumor cells. In this study, we investigated the potential mechanisms of this enhanced cell death. Human colorectal carcinoma RKO cells were exposed to IUdR (3 micromol/L) and/or methoxyamine (3 mmol/L) for 48 hours before ionizing radiation (5 Gy). We found that IUdR/methoxyamine altered cell cycle kinetics and led to an increased G1 population but a decreased S population before ionizing radiation. Immediately following ionizing radiation (up to 6 hours), IUdR/methoxyamine-pretreated cells showed a stringent G1-S checkpoint but an insufficient G2-M checkpoint, whereas a prolonged G1 arrest, containing 2CG1 and 4CG1 cells, was found at later times up to 72 hours. Levels of cell cycle-specific markers [p21, p27, cyclin A, cyclin B1, and pcdc2(Y15)] and DNA damage signaling proteins [gammaH2AX, pChk1(S317), and pChk2(T68)] supported these altered cell cycle kinetics. Interestingly, we found that IUdR/methoxyamine pretreatment reduced ionizing radiation-induced apoptosis. Additionally, the extent of cell death through necrosis or autophagy seemed similar in all (IUdR +/- methoxyamine + ionizing radiation) treatment groups. However, a larger population of senescence-activated beta-galactosidase-positive cells was seen in IUdR/methoxyamine/ionizing radiation-treated cells, which was correlated with the increased activation of the senescence factors p53 and pRb. These data indicate that IUdR/methoxyamine pretreatment enhanced the effects of ionizing radiation by causing a prolonged G1 cell cycle arrest and by promoting stress-induced premature senescence. Thus, senescence, a novel ionizing radiation-induced tumor suppression pathway, may be effectively targeted by IUdR/methoxyamine pretreatment, resulting in an improved therapeutic gain for ionizing radiation.

Clinical significance of atomic inner shell ionization (ISI) and Auger cascade for radiosensitization using IUdR, BUdR, platinum salts, or gadolinium porphyrin compounds

PURPOSE

Halogenated pyrimidines (iododeoxyuridine [IUdR] and bromodeoxyuridine [BUdR]), platinum salts, and gadolinium porphyrins are heavy atom compounds used as radiosensitizers. For IUdR, it has been hypothesized that iodine inner shell ionizations (ISI) and Auger cascades could be one of the primary radiosensitization mechanisms. The purpose of this paper is to estimate the number of ISI produced per tumor cell and per 2 Gy irradiation in clinically relevant modelings.

MATERIALS AND METHODS

ISI were evaluated using a two-step method. Photon-induced ISI were calculated using the MCNP-4C Monte Carlo code, heavy atom concentrations from clinical data published in the literature, and at various depths in a water phantom irradiated with 6-MV, (60)Co, (137)Cs, or (192)Ir sources. Electron knock-on induced ISI on K, L, and M atomic shells were evaluated with an hybrid method, using simulated electron spectra and cross-sections derived from the Møller formalism. Using a biological dose equivalence of 0.05 Gy per cell ISI, relative biological effectiveness (RBE) values were calculated for each situation.

RESULTS

For platinum and gadolinium, ISI occurs in far less than 0.1% of the cell, whichever is the configuration. For IUdR and BUdR, ISI occurs in between 45% to 483% of the cell. Due to spectrum degradation, about 3 times more photoelectric ISI are generated at greater than shallower depths, and 10 times more for (192)Ir compared with (60)Co or 6-MV X-rays. Photoelectric ISI are about 3 times more frequent for iodine than bromine, but electron knock-on ISI are more frequent on bromine, and at the end about the same number of ISI are generated for both elements. RBEs were found to be between 1.01 and 1.12 for clinically relevant irradiation settings.

CONCLUSIONS

The mechanisms of radiosensitization for platinum and gadolinium are clearly not related to an Auger cascade. For halogenated pyrimidines, however, clinically relevant numbers of ISI are generated within each cell. For IUdR, ISI appears to be strongly tied to the photon spectra. Halogenated pyrimidines should be evaluated again clinically, but using lower energy photons like a (192)Ir implant.

Hyperthermia and incorporation of halogenated pyrimidines: radiosensitization in cultured rodent and human tumor cells

PURPOSE

To investigate the possible benefit of hyperthermia (HT) in combination with radiosensitization by halogenated pyrimidines (HPs) in rodent as well as in human tumor cells.

METHODS AND MATERIALS

Exponentially growing rodent cells, radiosensitive R-1 and MOS cells and radioresistant RUC-II and V79 cells, and human SW1573 cells, were exposed to 0, 1, 2, and 4 microM of chloro- (CldUrd), bromo- (BrdUrd), or iodo-deoxyuridine (IdUrd) in the culture medium. Survival after irradiation with gamma-rays from a 137Cs source and/or hyperthermic treatment (HT, 60 min at 42 degrees C) was determined by clonogenic assay. Linear-quadratic analyses of the radiation survival curves were performed to assess sensitization in the dose range 1 to 3 Gy relevant to radiotherapy.

RESULTS

The incorporation of HPs sensitized all cell lines to HT and resulted in radiosensitization dependent on the percentage of thymidine replacement. At equal levels of thymidine replacement, IdUrd was the most potent radiosensitizer. HT further increased radiation-induced lethality of cells that had incorporated HPs. Linear-quadratic analyses showed that HT further increased the linear parameter of the LQ formula while the quadratic parameter was not significantly changed.

CONCLUSION

The combination of HT and HPs act additively in increasing the radiosensitivity of rodent tumor cell lines with varying radiosensitivities as well as of a human tumor cell line. In particular, the ratio of the linear parameter to the quadratic parameter, relevant for fractionation effects in radiotherapy, was increased.