The importance of radiation chemistry to radiation and free radical biology (The 2008 Silvanus Thompson Memorial Lecture)

Synthesis and biological evaluation of 3-aryl-quinoxaline-2-carbonitrile 1,4-di-N-oxide derivatives as hypoxic selective anti-tumor agents

A series of 3-aryl-2-quinoxaline-carbonitrile 1,4-di-N-oxide derivatives were designed, synthesized and evaluated for hypoxic and normoxic cytotoxic activity against human SMMC-7721, K562, KB, A549 and PC-3 cell lines. Many of these new compounds displayed more potent hypoxic cytotoxic activity compared with TX-402 and TPZ in the tumor cells based evaluation, which confirmed our hypothesis that the replacement of the 3-amine with the substituted aryl ring of TX-402 increases the hypoxic anti-tumor activity. The preliminary SAR revealed that 3-chloro was a favorable substituent in the phenyl ring for hypoxic cytotoxicity and 7-methyl or 7-methoxy substituted derivatives exhibited better hypoxic selectivity against most of the tested cell lines. The most potent compound, 7-methyl-3-(3-chlorophenyl)-quinoxaline-2-carbonitrile 1,4-dioxide (9h) was selected for further anti-tumor evaluation and mechanistic study. It also exhibited significant cytotoxic activity against BEL-7402, HepG2, HL-60, NCI-H460, HCT-116 and CHP126 cell lines in hypoxia with IC₅₀ values ranging from 0.31 to 3.16 μM, and preliminary mechanism study revealed that 9h induced apoptosis in a caspase-dependent pathway.

Quantitative modeling of chronic myeloid leukemia: insights from radiobiology

Mathematical models of chronic myeloid leukemia (CML) cell population dynamics are being developed to improve CML understanding and treatment. We review such models in light of relevant findings from radiobiology, emphasizing 3 points. First, the CML models almost all assert that the latency time, from CML initiation to diagnosis, is at most ∼10 years. Meanwhile, current radiobiologic estimates, based on Japanese atomic bomb survivor data, indicate a substantially higher maximum, suggesting longer-term relapses and extra resistance mutations. Second, different CML models assume different numbers, between 400 and 10(6), of normal HSCs. Radiobiologic estimates favor values>10(6) for the number of normal cells (often assumed to be the HSCs) that are at risk for a CML-initiating BCR-ABL translocation. Moreover, there is some evidence for an HSC dead-band hypothesis, consistent with HSC numbers being very different across different healthy adults. Third, radiobiologists have found that sporadic (background, age-driven) chromosome translocation incidence increases with age during adulthood. BCR-ABL translocation incidence increasing with age would provide a hitherto underanalyzed contribution to observed background adult-onset CML incidence acceleration with age, and would cast some doubt on stage-number inferences from multistage carcinogenesis models in general.

Reactivity toward oxygen radicals and antioxidant action of thiol compounds

Thiol compounds exert diverse functions in the defense network against oxidative stress in vivo. Above all, the role of glutathione in the enzymatic removal of hydrogen peroxide and lipid hydroperoxides has been well established. The scavenging of reactive free radicals is one of the many functions. In this study, the reactivities of several thiol compounds toward oxygen- and nitrogen-centered radicals were measured from their reaction with galvinoxyl and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals and also from their sparing effects on the decay of fluorescein, pyrogallol red, and BODIPY induced by peroxyl radicals. Furthermore, the antioxidant capacity against lipid peroxidation was assessed in the oxidation of methyl linoleate induced by free radicals in micelle systems. Cysteine, homocysteine, and glutathione exhibited considerable reactivity toward galvinoxyl, DPPH, and peroxyl radicals in this order but methionine did not. Bovine serum albumin (BSA) was less reactive toward these radicals than cysteine on molar base. Cysteine, homocysteine, and glutathione suppressed the oxidation of methyl linoleate in micelle systems, but methionine did not. The reactivity toward free radicals and antioxidant capacity of these thiol compounds were less than that of ascorbic acid, but higher than that of uric acid.

Utilization of the ferrous sulfate (Fricke) dosimeter for evaluating the radioprotective potential of cystamine: experiment and Monte Carlo simulation

Cystamine, an organic disulfide (RSSR), is among the best of the known radiation-protective compounds and has been used to protect normal tissues in clinical radiation therapy. Recently, it has also proved to be beneficial in the treatment of disorders of the central nervous system in animal models. However, the underlying mechanism of its action at the chemical level is not yet well understood. The present study aims at using the ferrous sulfate (Fricke) dosimeter to quantitatively evaluate, both experimentally and theoretically, the radioprotective potential of this compound. The well-known radiolysis of the Fricke dosimeter by (60)Co γ rays or fast electrons, based on the oxidation of ferrous ions to ferric ions by the oxidizing species (•)OH, HO(2)(•), and H(2)O(2) produced in the radiolytic decomposition of water, forms the basis for our method. The presence of cystamine in Fricke dosimeter solutions during irradiation prevents the radiolytic oxidation of Fe(2+) and leads to decreased ferric yields (or G values). The observed decrease in G(Fe(3+)) increases upon increasing the concentration of the disulfide compound over the range 0-0.1 M under both aerated and deaerated conditions. To help assess the basic radiation-protective mechanism of this compound, a full Monte Carlo computer code is developed to simulate in complete detail the radiation-induced chemistry of the studied Fricke/cystamine solutions. Benefiting from the fact that cystamine is reasonably well characterized in terms of radiation chemistry, this computer model proposes reaction mechanisms and incorporates specific reactions describing the radiolysis of cystamine in aerated and deaerated Fricke solutions that lead to the observable quantitative chemical yields. Results clearly indicate that the protective effect of cystamine originates from its radical-capturing ability, which allows this compound to act by competing with the ferrous ions for the various free radicals--especially (•)OH radicals and H(•) atoms--formed during irradiation of the surrounding water. Most interestingly, our simulation modeling also shows that the predominant pathway in the oxidation of cystamine by (•)OH radicals involves an electron-transfer mechanism, yielding RSSR(•+) and OH(-). A very good agreement is found between calculated G(Fe(3+)) values and experiment. This study concludes that Monte Carlo simulations represent a very efficient method for understanding indirect radiation damage at the molecular level.

Altered metabolism in head and neck squamous cell carcinoma: an opportunity for identification of novel biomarkers and drug targets

Tumor cells were first shown to exhibit a distinct metabolic phenotype over 80 years ago. Since then, it has become clear that multiple oncogenic events contribute to the development of a metabolic phenotype that supports rapid proliferation. Because this phenotype represents an essential component of tumorigenesis and disease progression, it also represents a potential source of biomarkers associated with aggressive disease. In addition, the addiction of tumor cells to specific nutrients and the up-regulation of key metabolic enzymes provide unique opportunities for pharmacologic manipulation. Despite the use of multimodality treatment, survival rates for patients with advanced head and neck squamous cell carcinoma (HNSCC) remain low, partially attributed to the development of drug resistance. In this review, we evaluate the role of altered HNSCC metabolism as both a source of novel biomarkers and a means to bypass resistance mechanisms to conventional forms of therapy.

Metalloporphyrin synergizes with ascorbic acid to inhibit cancer cell growth through fenton chemistry

Ascorbic acid (AA) has been reported to inhibit tumor cell growth through the generation of extracellular hydrogen peroxide (H(2)O(2)). However, the clinical utility of AA has been limited by relatively low potency and in vivo efficacy. This study reports that the metalloporphyrin, Mn(III) tetrakis(N-methylpyridinium-2-yl)porphyrin(5+) (MnTMPyP), has a potent synergistic cytotoxic effect when combined with AA in a variety of cancer cell lines. In the presence of MnTMPyP, the concentration of AA required to inhibit cancer cell growth was markedly reduced. In vitro (cell-free) experiments demonstrated that AA alone enhanced the Fenton reaction that produces cytotoxic hydroxyl radical (HO(*)); however, this reaction was limited by the low rate by which AA generates H(2)O(2) (Fenton reaction substrate) from O(2). MnTMPyP catalyzed H(2)O(2) generation through the AA-facilitated Mn(II <--> III)TMPyP redox cycle and thereby markedly potentiated the Fenton reaction. Accordingly, MnTMPyP and AA resulted in increased cellular levels of H(2)O(2) and HO(*) in cancer cells, which mediate the synergistic cytotoxicity of this combined treatment. This effect was inhibited by cellular enzymes that metabolize H(2)O(2), such as catalase and glutathione peroxidase, suggesting that selective killing of cancer cells deficient in such enzymes can be achieved in vivo.

Pharmacokinetic/pharmacodynamic modeling identifies SN30000 and SN29751 as tirapazamine analogues with improved tissue penetration and hypoxic cell killing in tumors

PURPOSE

Tirapazamine (TPZ) has attractive features for targeting hypoxic cells in tumors but has limited clinical activity, in part because of poor extravascular penetration. Here, we identify improved TPZ analogues by using a spatially resolved pharmacokinetic/pharmacodynamic (SR-PKPD) model that considers tissue penetration explicitly during lead optimization.

EXPERIMENTAL DESIGN

The SR-PKPD model was used to guide the progression of 281 TPZ analogues through a hierarchical screen. For compounds exceeding hypoxic selectivity thresholds in single-cell cultures, SR-PKPD model parameters (kinetics of bioreductive metabolism, clonogenic cell killing potency, diffusion coefficients in multicellular layers, and plasma pharmacokinetics at well tolerated doses in mice) were measured to prioritize testing in xenograft models in combination with radiation.

RESULTS

SR-PKPD-guided lead optimization identified SN29751 and SN30000 as the most promising hypoxic cytotoxins from two different structural subseries. Both were reduced to the corresponding 1-oxide selectively under hypoxia by HT29 cells, with an oxygen dependence quantitatively similar to that of TPZ. SN30000, in particular, showed higher hypoxic potency and selectivity than TPZ in tumor cell cultures and faster diffusion through HT29 and SiHa multicellular layers. Both compounds also provided superior plasma PK in mice and rats at equivalent toxicity. In agreement with SR-PKPD predictions, both were more active than TPZ with single dose or fractionated radiation against multiple human tumor xenografts.

CONCLUSIONS

SN30000 and SN29751 are improved TPZ analogues with potential for targeting tumor hypoxia in humans. Novel SR-PKPD modeling approaches can be used for lead optimization during anticancer drug development.

Anti-tumor and radiosensitization activities of the iron chelator HDp44mT are mediated by effects on intracellular redox status

A novel iron chelator, HDp44mT, has been reported to have potent anti-proliferative effects on cancer cells; however, the underlying mechanism of action is not well understood. In this study, we characterized the cytotoxic effect of HDp44mT in a chemo- and radio-resistant cell line (PC-3) of prostatic cancer origin. The activity of HDp44mT at nM concentrations was dependent on the intracellular GSH and atmospheric O(2) concentration, rather than iron deprivation. HDp44mT also radiosensitized PC-3 cells in a GSH-dependent manner. Interestingly, this radiosensitizing effect was observed under aerobic and, to a larger extent, hypoxic conditions, suggesting its potential utility as a radiosensitizer for some radioresistant tumors.

Radiation chemistry comes before radiation biology

PURPOSE

This article seeks to illustrate some contributions of radiation chemistry to radiobiology and related science, and to draw attention to examples where radiation chemistry is central to our knowledge of specific aspects. Radiation chemistry is a mature branch of radiation science which is continually evolving and finding wider applications. This is particularly apparent in the study of the roles of free radicals in biology generally, and radiation biology specifically. The chemical viewpoint helps unite the spatial and temporal insight coming from radiation physics with the diversity of biological responses. While historically, the main application of radiation chemistry of relevance to radiation biology has been investigations of the free-radical processes leading to radiation-induced DNA damage and its chemical characterization, two features of radiation chemistry point to its wider importance. First, its emphasis on quantification and characterization at the molecular level helps provide links between DNA damage, biochemical repair processes, and mutagenicity and radiosensitivity. Second, its central pillar of chemical kinetics aids understanding of the roles of 'reactive oxygen species' in cell signalling and diverse biological effects more generally, and application of radiation chemistry in the development of drugs to enhance radiotherapy and as hypoxia-specific cytotoxins or diagnostic agents. The illustrations of the broader applications of radiation chemistry in this article focus on their relevance to radiation biology and demonstrate the importance of synergy in the radiation sciences.

CONCLUSIONS

The past contributions of radiation chemistry to radiation biology are evident, but there remains considerable potential to help advance future biological understanding using the knowledge and techniques of radiation chemistry.