Oxygen transfer from the nitro group of a nitroaromatic radiosensitizer to a DNA sugar damage product

Targeting Hypoxia: Revival of Old Remedies

Tumour hypoxia is significantly correlated with patient survival and treatment outcomes. At the molecular level, hypoxia is a major driving factor for tumour progression and aggressiveness. Despite the accumulative scientific and clinical efforts to target hypoxia, there is still a need to find specific treatments for tumour hypoxia. In this review, we discuss a variety of approaches to alter the low oxygen tumour microenvironment or hypoxia pathways including carbogen breathing, hyperthermia, hypoxia-activated prodrugs, tumour metabolism and hypoxia-inducible factor (HIF) inhibitors. The recent advances in technology and biological understanding reveal the importance of revisiting old therapeutic regimens and repurposing their uses clinically.

Hypoxia, metabolism, and the circadian clock: new links to overcome radiation resistance in high-grade gliomas

Radiotherapy is the cornerstone of treatment of high-grade gliomas (HGGs). It eradicates tumor cells by inducing oxidative stress and subsequent DNA damage. Unfortunately, almost all HGGs recur locally within several months secondary to radioresistance with intricate molecular mechanisms. Therefore, unravelling specific underlying mechanisms of radioresistance is critical to elucidating novel strategies to improve the radiosensitivity of tumor cells, and enhance the efficacy of radiotherapy. This review addresses our current understanding of how hypoxia and the hypoxia-inducible factor 1 (HIF-1) signaling pathway have a profound impact on the response of HGGs to radiotherapy. In addition, intriguing links between hypoxic signaling, circadian rhythms and cell metabolism have been recently discovered, which may provide insights into our fundamental understanding of radioresistance. Cellular pathways involved in the hypoxic response, DNA repair and metabolism can fluctuate over 24-h periods due to circadian regulation. These oscillatory patterns may have consequences for tumor radioresistance. Timing radiotherapy for specific times of the day (chronoradiotherapy) could be beneficial in patients with HGGs and will be discussed.

Clinical Advances of Hypoxia-Activated Prodrugs in Combination With Radiation Therapy

With the increasing incidence of cancer worldwide, the need for specific, effective therapies is ever more urgent. One example of targeted cancer therapeutics is hypoxia-activated prodrugs (HAPs), also known as bioreductive prodrugs. These prodrugs are inactive in cells with normal oxygen levels but in hypoxic cells (with low oxygen levels) undergo chemical reduction to the active compound. Hypoxia is a common feature of solid tumors and is associated with a more aggressive phenotype and resistance to all modes of therapy. Therefore, the combination of radiation therapy and bioreductive drugs presents an attractive opportunity for synergistic effects, because the HAP targets the radiation-resistant hypoxic cells. Hypoxia-activated prodrugs have typically been precursors of DNA-damaging agents, but a new generation of molecularly targeted HAPs is emerging. By targeting proteins associated with tumorigenesis and survival, these compounds may result in greater selectivity over healthy tissue. We review the clinical progress of HAPs as adjuncts to radiation therapy and conclude that the use of HAPs alongside radiation is vastly underexplored at the clinical level.

Characterization of radicals formed following enzymatic reduction of 3-substituted analogues of the hypoxia-selective cytotoxin 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine)

The mechanism by which the 1,2,4-benzotriazine 1,4-dioxide (BTO) class of bioreductive hypoxia-selective prodrugs (HSPs) form reactive radicals that kill cancer cells has been investigated by steady-state radiolysis, pulse radiolysis (PR), electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. Tirapazamine (TPZ, 3-amino BTO, 1) and a series of 3-substituted analogues, -H (2), -methyl (3), -ethyl (4), -methoxy (5), -ethoxymethoxy (6), and -phenyl (7), were reduced in aqueous solution under anaerobic steady-state radiolysis conditions, and their radicals were found to remove the substrates by short chain reactions of different lengths in the presence of formate ions. Multiple carbon-centered radical intermediates, produced upon anaerobic incubation of the compounds with cytochrome P(450) reductase enriched microsomes, were trapped by N-tert-butyl-alpha-phenylnitrone and observed using EPR. The highly oxidizing oxymethyl radical, from compound 5, was identified, and experimental spectra obtained for compounds 1, 2, 3, and 7 were well simulated after the inclusion of aryl radicals. The identification of a range of oxidizing radicals in the metabolism of the BTO compounds gives a new insight into the mechanism by which these HSPs can cause a wide variety of damage to biological targets such as DNA.

DNA strand damage product analysis provides evidence that the tumor cell-specific cytotoxin tirapazamine produces hydroxyl radical and acts as a surrogate for O(2)

The compound 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine, TPZ) is a clinically promising anticancer agent that selectively kills the oxygen-poor (hypoxic) cells found in solid tumors. It has long been known that, under hypoxic conditions, TPZ causes DNA strand damage that is initiated by the abstraction of hydrogen atoms from the deoxyribose phosphate backbone of duplex DNA, but exact chemical mechanisms underlying this process remain unclear. Here we describe detailed characterization of sugar-derived products arising from TPZ-mediated strand damage. We find that the action of TPZ on duplex DNA under hypoxic conditions generates 5-methylene-2-furanone (6), oligonucleotide 3'-phosphoglycolates (7), malondialdehyde equivalents (8 or 9), and furfural (10). These results provide evidence that TPZ-mediated strand damage arises via hydrogen atom abstraction from both the most hindered (C1') and least hindered (C4' and C5') positions of the deoxyribose sugars in the double helix. The products observed are identical to those produced by hydroxyl radical. Additional experiments were conducted to better understand the chemical pathways by which TPZ generates the observed DNA-damage products. Consistent with previous work showing that TPZ can substitute for molecular oxygen in DNA damage reactions, it is found that, under anaerobic conditions, reaction of TPZ with a discrete, photogenerated C1'-radical in a DNA 2'-oligodeoxynucleotide cleanly generates the 2-deoxyribonolactone lesion (5) that serves as the precursor to 5-methylene-2-furanone (6). Overall, the results provide insight regarding the chemical structure of the DNA lesions that confront cellular repair, transcription, and replication machinery following exposure to TPZ and offer new information relevant to the chemical mechanisms underlying TPZ-mediated strand cleavage.

Reaction of the hypoxia-selective antitumor agent tirapazamine with a C1'-radical in single-stranded and double-stranded DNA: the drug and its metabolites can serve as surrogates for molecular oxygen in radical-mediated DNA damage reactions

The compound 3-amino-1,2,4-benzotriazine 1,4-dioxide (1, tirapazamine; also known as SR4233, WIN 59075, and tirazone) is a clinically promising anticancer agent that selectively kills the oxygen-poor (hypoxic) cells found in tumors. When activated by one-electron enzymatic reduction, tirapazamine induces radical-mediated oxidative DNA strand cleavage. Using the ability to generate a single deoxyribose radical at a defined site in an oligonucleotide, we recently provided direct evidence that, in addition to initiating the formation of DNA radicals, tirapazamine can react with these radicals and convert them into base-labile lesions [Daniels et al. (1998) Chem. Res. Toxicol. 11, 1254-1257]. The rate constant for trapping of a C1'-radical in single-stranded DNA by tirapazamine was shown to be approximately 2 x 10(8) M(-1) s(-1), demonstrating that tirapazamine can substitute for molecular oxygen in radical-mediated DNA strand damage reactions. Because reactions of tirapazamine with DNA radicals may play an important role in its ability to damage DNA, we have further characterized the ability of the drug and its metabolites to convert a C1'-DNA radical into a base-labile lesion. We find that tirapazamine reacts with a C1'-radical in double-stranded DNA with a rate constant of 4.6 x 10(6) M(-1) s(-1). The mono-N-oxide (3) stemming from bioreductive metabolism of tirapazamine converts the C1'-radical to an alkaline-labile lesion more effectively than the parent drug. Compound 3 traps a C1'-radical in single-stranded DNA with a rate constant of 4.6 x 10(8) M(-1) s(-1) and in double-stranded DNA with a rate constant of 1.4 x 10(7) M(-)(1) s(-)(1). We have also examined the rate and mechanism of reactions between the C1'-radical and representatives from two known classes of "oxygen mimetic" agents: the nitroxyl radical 2,2,6, 6-tetramethylpiperidin-N-oxyl (4, TEMPO) and the nitroimidazole misonidazole (5). TEMPO traps the C1'-radical in single-stranded DNA (7.2 x 10(7) M(-1) s(-1)) approximately 3 times less effectively than tirapazamine, but 2 times as fast in double-stranded DNA (9.1 x 10(6) M(-1) s(-1)). Misonidazole traps the radical in single- (6. 9 x 10(8) M(-1) s(-1)) and double-stranded DNA (2.9 x 10(7) M(-1) s(-1)) with rate constants that are roughly comparable to those measured for the mono-N-oxide metabolite of tirapazamine. Finally, information regarding the chemical mechanism by which these compounds oxidize a monomeric C1'-nucleoside radical has been provided by product analysis and isotopic labeling studies.

Reaction of hydroxyl radicals with alkyl phosphates and the oxidation of phosphatoalkyl radicals by nitro compounds

The rate constants for reactions of hydroxyl radicals with a number of alkyl phosphates have been determined by competition with KSCN. Hydroxyl radicals react with alkyl phosphates preferentially by H-abstraction at the alpha-position of the phosphate functions. The resulting alpha-phosphatoalkyl radicals are not very efficient one-electron reducing agents towards nitro compounds. They react with tetranitromethane (TNM) by addition to form adduct intermediates with absorption maxima at about 300 nm. The rate constants for decay of these TNM adducts to produce the nitroform anion (NF-) and the corresponding alpha-phosphato-alcohols have been determined by optical and/or conductance detection. The stability of these TNM adducts varies considerably with the chain length (methyl > ethyl > isopropyl) and number (trialkyl > dialkyl > monoalkyl) of the alkyl substituents. Additional formation of proton during or after the decay of the TNM adducts has been tentatively attributed to the hydrolysis of the alpha-phosphato-alcohols. Alpha-Phosphatoalkyl radicals derived from trimethyl, triethyl, triisopropyl, and diethyl phosphates react with p-nitroacetophenone (PNAP) very slowly (k < 5 x 10(7) dm3mol-1S-1) possibly forming adducts. One-electron reduction of PNAP by these radicals to PNAP.- was not observed under pulse radiolysis conditions. The rate constants for the reactions of .OH with glycerol 1-phosphate and glycerol 2-phosphate have been redetermined by competition with KSCN. Using the radical scavengers N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and TNM, the percentage of .OH attack at each carbon atom was obtained. Contrary to the simple alkyl phosphates described above, the alpha-position to the phosphate function is the least favoured (10-15% in glycerol 1-phosphate and 6% in glycerol 2-phosphate). These so-formed alpha-phosphatoalkyl radicals react with TNM also by forming adducts. The beta-phosphatoalkyl radicals in both cases eliminate inorganic phosphate on formation (k > 10(6)S-1). The gamma-phosphatoalkyl radical from glycerol 1-phosphate undergoes base-catalysed water elimination (kobs = 1.8 x 10(5)S-1 at pH 10.6) to give an oxidizing radical. Products in the gamma-radiolysis of N2O-saturated solutions of glycerol 1-phosphate and glycerol 2-phosphate have been identified and their yields determined. The mechanisms for their formation are discussed.

Neocarzinostatin-mediated DNA damage in a model AGT.ACT site: mechanistic studies of thiol-sensitive partitioning of C4' DNA damage products

Double-strand (DS) DNA damage caused by neocarzinostatin (NCS) has been studied in the trinucleotide AGT-ACT sequence in an AP-1 transcription factor binding site. There are strong similarities between bistranded lesions produced at AGT.ACT and AGC-GCT, including the fact that DS lesions outnumber SS lesions on the AGT and AGC strands, while SS exceed DS on the ACT and GCT strands. Structure-function studies revealed that a variety of different thiols produced bistranded lesions in this model by predominantly C4'-hydrogen atom abstraction (84-93%) at the T of AGT and C5'-hydrogen atom abstraction (87-91%) at the T of ACT. Single-strand (SS) lesions were found to represent a variable mixture of C4' and C5' chemistry. The C4'-hydroxylated abasic site occurred in both SS and DS lesions at both sites and accounted for most of the DS damage at AGT (60-83%); the remaining damage consisted of 3'-phosphoglycolate- and 3'-phosphate-ended fragments. The nature of the thiol was found to affect the partitioning of the breakdown products arising from C4' and, to a lesser extent, C5' hydrogen atom abstraction. Production of 3'-phosphoglycolate residues, restricted mainly to the T of AGT in bistranded lesions, correlated with the incidence of direct DS breaks in the AGT.ACT model and in plasmid DNA and appeared to be influenced by the reducing power of the thiol activator. Furthermore, hydrazine and sodium borohydride both inhibited the formation of glycolate, an effect that was exploited to determine the rate constant for 3'-phosphoglycolate formation: 0.06 min-1 at 0 degree C, pH 7.4. Under anaerobic conditions, the nitroaromatic radiation sensitizer misonidazole caused a large increase in glycolate production in both SS and DS lesions formed by NCS, which suggests that the formation of 3'-phosphoglycolate, like 3'-formylphosphate generated by C5' chemistry, involves an oxyradical intermediate. The pathways for DNA damage involving C4' and C5' hydrogen atom abstraction thus share many common features, several of which are consistent with a mechanism for the production of NCS-mediated bistranded lesions at AGT.ACT that involves a tetraoxide bridge joining the lesions on opposite strands of DNA.