The role of hypoxia-activated prodrugs in cancer therapy

Hypoxia-specific drug tirapazamine does not abrogate hypoxic tumor cells in combination therapy with irinotecan and methylselenocysteine in well-differentiated human head and neck squamous cell carcinoma a253 xenografts

Well-differentiated hypoxic regions in head and neck squamous cell carcinoma like in A253 xenografts are avascular and, therefore, hinder drug delivery leading to drug resistance and tumor regrowth. Methylselenocysteine (MSC, 0.2 mg/mouse per day per oral for 35 days starting 7 days before the first irinotecan (CPT-11)) has been found to increase efficacy of a wide variety of chemotherapeutic agents including CPT-11 (100 mg/kg per week x 4 intravenously). Whereas CPT-11 leads to a 10% complete response (CR) in A253 xenografts, the combination of MSC and CPT-11 increased the CR to 70%. Surviving tumors were found to consist largely of avascular hypoxic regions. Here, we investigated the combination of tirapazamine (TPZ, 70 mg/kg per week intraperitoneal x 4 administered 3 or 72 hours before CPT-11), a bioreductive drug in clinical trial with selective toxicity for hypoxic cells, with MSC and CPT-11 in further enhancing the cure rates. Tumor response, change in tumor hypoxic regions, and DNA damage were monitored in vivo. Tirapazamine administered 3 hours before CPT-11 in combination with MSC + CPT-11 led to a lower tumor burden. Tirapazamine did not increase cure rate beyond that of MSC + CPT-11 combination and was instead found to decrease cures with no evidence of an increased DNA damage or a significant reduction in avascular hypoxic tumor regions. CD31 immunostaining in A253 demonstrated disruption of tumor vessels by TPZ that could lower cytotoxic drug delivery to carbonic anhydrase IX-positive hypoxic tumor cells and may explain at least partially these unexpected results.

Role of the nitro functionality in the DNA binding of 3-nitro-10-methylbenzothiazolo[3,2-a]quinolinium chloride

Interest in DNA binding drugs has increased in recent years due to their importance in the treatment of genome-related diseases, like cancer. A new family of water-soluble DNA binding compounds, the benzothiazolo[3,2- a]quinolinium chlorides (BQCls), is studied here as potential candidates for chemical treatment of solid tumor cells that may encounter low-oxygen environments, a condition known as hypoxia. These compounds are good DNA intercalators; however, no studies have been made of these compounds under hypoxic conditions. This work demonstrates the importance of the nitro-functionality in the DNA binding of 3-nitro-10-methylbenzothiazolo[3,2- a]quinolinium chloride (NBQ-91), which possesses nitro-functionality, and 10-methylbenzothiazolo[3,2- a]quinolinium chloride (BQ-106), which does not. Both NBQ-91 and BQ-106 have similar noncovalent binding affinity toward DNA. Dialysis experiments show that NBQ-91 binds DNA under N2-saturated conditions with increasing concentrations of reducing agent, presumably due to reduction of the nitro-functionality. Conversely, because of the lack of nitro-functionality, the presence of a reducing agent had no effect on BQ-106 binding to DNA under both aerobic and N2-saturated conditions. Clonogenic assays were performed to determine the quinolinium chloride cytotoxicities under both aerobic (95% air and 5% CO2) and hypoxic (80% N2 and 20% CO2) conditions. The calculated ratios of cellular toxicity under aerobic to hypoxic conditions caused by the same concentration of test agent (CTR values) show greater levels of cell death under hypoxia than under aerobic conditions for mitomycin C (MC) (CTR = 0.7 at 1 microM) and NBQ-91 (CTR = 0.4 at 200 microM) than for BQ-106 (CTR = 1.0 at 200 microM), which agreed with the previously reported data for MC and confirmed the importance of nitro-functionality for reactivity under hypoxic conditions. There was no correlation between noncovalent binding affinity constants and their cytotoxicity under hypoxic conditions. Adduct formation between the NBQ-91 and 2'-dG was also assessed by reacting 2'-dG or DNA with NBQ-91 and BQ-106 under N2-saturated conditions in the presence of hypoxanthine and xanthine oxidase (HX/XO). DNA covalent adduct formation was analyzed by two techniques: LC-ESI-MS and Sephadex size exclusion chromatography. LC-ESI-MS results clearly indicate the formation of a prominent molecular ion at masses of 266.0 and 530.58 Da, corresponding to the [M + H](+2) and [M](+) molecular ions of the monitored 2'-dG-NBQ-91 adduct. Results from the Sephadex size exclusion chromatography support these findings because the NBQ-91 elution percentage increases in the presence of HX/XO due to the reduction of the nitro-functionality, which results in covalent binding to DNA. This study reports evidence of the DNA binding capacity of this bioreductive drug. The preferential N2-saturated over aerobic conditions for DNA binding makes NBQ-91 a potential bioreductive compound for hypoxic cell killing.

Electronic structures and spin topologies of gamma-picoliniumyl radicals. A study of the homolysis of N-methyl-gamma-picolinium and of benzo-, dibenzo-, and naphthoannulated analogs

Radicals resulting from one-electron reduction of (N-methylpyridinium-4-yl) methyl esters have been reported to yield (N-methylpyridinium-4-yl) methyl radical, or N-methyl-gamma-picoliniumyl for short, by heterolytic cleavage of carboxylate. This new reaction could provide the foundation for a new structural class of bioreductively activated, hypoxia-selective antitumor agents. N-methyl-gamma-picoliniumyl radicals are likely to damage DNA by way of H-abstraction and it is of paramount significance to assess their H-abstraction capabilities. In this context, the benzylic C-H homolyses were studied of toluene (T), gamma-picoline (P, 4-methylpyridine), and N-methyl-gamma-picolinium (1c, 1,4-dimethylpyridinium). With a view to providing capacity for DNA intercalation the properties also were examined of the annulated derivatives 2c (1,4-dimethylquinolinium), 3c (9,10-dimethylacridinium), and 4c (1,4-dimethylbenzo[g]quinolinium). The benzylic C-H homolyses were studied with density functional theory (DFT), perturbation theory (up to MP4SDTQ), and configuration interaction methods (QCISD(T), CCSD(T)). Although there are many similarities between the results obtained here with DFT and CI theory, a number of significant differences occur and these are shown to be caused by methodological differences in the spin density distributions of the radicals. The quality of the wave functions is established by demonstration of internal consistencies and with reference to a number of observable quantities. The analysis of spin polarization emphasizes the need for a clear distinction between "electron delocalization" and "spin delocalization" in annulated radicals. Aside from their relevance for the rational design of new antitumor drugs, the conceptional insights presented here also will inform the understanding of ferromagnetic materials, of spin-based signaling processes, and of spin topologies in metalloenzymes.

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.

Comparison of the nucleic acid covalent binding capacity of two nitro-substituted benzazolo[3,2-a]quinolinium salts upon enzymatic reduction

The DNA binding capacity of two nitro-substituted benzazolo[3,2-a]quinolinium chlorides (NBQs), NBQ-38 and NBQ-95, was evaluated upon their enzymatic reduction with hypoxanthine (HX)/xanthine oxidase (XO) under anaerobic conditions. In the presence of 2'-deoxyguanosine (2'-dG) or calf thymus DNA, covalent-addition products were monitored. Reactions of each NBQ with 2'-dG or DNA differed in the NBQ to HX molar ratio. Control reactions, one without HX/OX and another under aerobic conditions, were also analyzed. Adducts were isolated and characterized by high performance liquid chromatography (HPLC) and electrospray ionization-mass spectrometry (ESI-MS). Authentic samples of the reduced forms of these NBQs, identified as ABQ-38 and ABQ-95, were synthesized as standards to monitor bioreduction processes. HPLC analysis showed that the yield of formation of an unknown product (possibly, 2'-dG-NHBQ-38 adduct) from the reaction of NBQ-38 with 2'-dG and DNA was proportional to the HX to NBQ-38 molar ratio. ESI-MS analysis of the DNA hydrolysates showed evidence of an adduct formed upon bioreduction of NBQ-38 by the ions detection at m/z 528.3 and 454.8, consistent with chemical structures of a 2'-dG-NHBQ-38 adduct and a fragment ion. DNA adducts were not observed with NBQ-95, although the corresponding bioreduction product ABQ-95 was detected by ESI-MS. This study provides mechanistic information of these bioreductively-activated pro-drugs with potential therapeutic applications.

Preclinical safety and antitumor efficacy of insulin combined with irradiation

BACKGROUND AND PURPOSE

We have previously reported that insulin significantly enhances tumor oxygenation (pO(2)) and increases radiation-induced tumor regrowth delay in experimental models. Considering the large radiosensitizing effect, clinical trials might be envisioned. The aim of the present pre-clinical study was to obtain a more complete set of safety and efficacy data which would further justify the commencement of such clinical trials.

MATERIAL AND METHODS

Toxicity on normal (early and late-responding) tissues was measured by the intestinal crypt regeneration assay and the late leg contracture assay. Efficacy in terms of enhancement of pO(2) (measured by in vivo EPR oximetry) and increase in radiation-induced tumor regrowth delay was evaluated with a dose-response study on mice bearing FSaII fibrosarcoma.

RESULTS

The effect on regrowth delay was directly correlated with the effect on the tumor pO(2), with a maximal effect using 400 mU kg(-1) insulin. Importantly, there was no increase in the radiation toxicity for normal tissues. Finally, we found that the hypoglycaemia induced by insulin can be corrected by simultaneous glucose infusion without modification of efficacy.

CONCLUSION

Insulin here demonstrated a therapeutic gain and a lack of toxicity to normal tissues. The results of this study fully justify further larger preclinical assays such as the use of fractionated irradiation and a tumor control dose assay, before determining the utility of insulin as a radiosensitizer for human patients in the clinic.

AnIn vivoTumor Model Exploiting Metabolic Response as a Biomarker for Targeted Drug Development

In vivo models that recapitulate oncogene-dependent tumorigenesis will greatly facilitate development of molecularly targeted anticancer therapies. We have developed a model based on activating mutations in c-KIT in gastrointestinal stromal tumors (GISTs). This model comprises murine tumors of FDC-P1 cell lines expressing c-KIT mutations that render the tumors either responsive (V560G) or resistant (D816V) to the small-molecule c-KIT inhibitor, imatinib. Clinically, GIST response to imatinib is associated with rapid reduction in fluorodeoxyglucose (FDG) uptake on positron emission tomography (PET), preceding changes in conventional response criteria by several weeks. Using the FDC-P1 model in small animal PET, FDG uptake into tumors expressing the c-KIT V560G mutation was significantly reduced as early as 4 hours after imatinib treatment. In contrast, no change in FDG uptake was observed in resistant c-KIT D816V-expressing tumors after 48 hours of imatinib treatment. Consistent with the PET results, expression of the glucose transporter, GLUT1, was significantly reduced in V560G tumors at 4 hours, preceding changes in markers of proliferation by several hours. In vitro, imatinib treatment of V560G cells resulted in a reduction of glucose transporter numbers at the cell surface and decreased glucose uptake well before changes in cell viability. Notably, decreased ambient glucose concentrations enhanced the cytotoxic effect of imatinib. Taken together, these data account for the rapidity and significance of the PET response to imatinib and suggest that metabolic effects may contribute to imatinib cytotoxicity. Further, the FDC-P1 model represents a very useful paradigm for molecularly targeted drug development.

DNA damage induced by a quinoxaline 1,4-di-N-oxide derivative (hypoxic selective agent) in Caco-2 cells evaluated by the comet assay

The DNA damage induced by 7-chloro-3-[[(N,N-dimethylamino)propyl]amino]-2-quinoxalinecarbonitrile 1,4-di-N-oxide hydrochloride (Q-85 HCl) in Caco-2 cells under hypoxic and well-oxygenated conditions has been studied by using the comet assay. This compound has shown a good in vitro profile of high selective toxicity in hypoxia, but its mechanism of action is unknown. The DNA damage has been evaluated by performing the comet assay after a 2-h treatment with Q-85 HCl (0.1, 0.2, 0.4 microM in hypoxia; 20, 40 microM in well-oxygenated conditions). The number of cells in apoptosis has also been assessed by flow cytometry analysis of Annexin V-FITC staining. The capability of the cells to repair the DNA damage and the proliferation rate was evaluated at different times after the treatment (24-168 h). Under hypoxic conditions, a clear dose-dependent increase in the number of nuclei with a comet was observed (comet score: 132 +/- 13, 343 +/- 30 and 399 +/- 1; control comet score: 42 +/- 14). Under well-oxygenated conditions, the number of nuclei with comet increased significantly with respect to the control (comet score: 273 +/- 14 and 312 +/- 9; control comet score: 27 +/- 4). Cells in apoptosis were not detected by the comet assay nor by flow cytometry. The recovery from DNA damage was time- and concentration-dependent in hypoxia (cells treated with the highest concentration still showed DNA damage after 72 h) and rather time-dependent in well-oxygenated conditions (DNA was completely repaired after 24 h). In conclusion, Q-85 HCl acts by DNA damage and not only the reduced intermediate is genotoxic but also some other derivatives and Q-85 HCl itself may be acting.

Mechanism of free radical nitric oxide-mediated Ras guanine nucleotide dissociation

Ras proteins cycle between GDP-bound and GTP-bound states to modulate a diverse array of cellular growth processes. In this study, we have elucidated a mechanism by which nitric oxide, in the presence of oxygen (NO/O2), regulates Ras activity. We show that treatment of Ras with NO/O2 causes conversion of Ras-bound GDP into a free 463.3 Da nucleotide-nitration product. Mass and UV/visible spectroscopic analyses suggest that this nitration product is 5-guanidino-4-nitroimidazole diphosphate (NIm-DP), a degradation product of 5-nitro-GDP. These results indicate that NO/O2 mediates Ras guanine nucleotide exchange (GNE) by conversion of Ras-bound GDP into an unstable 5-nitro-GDP. 5-Nitro-GDP can be produced by radical-based reaction of the GDP guanine base with nitrogen dioxide (*NO2). We also provide evidence that the Ras Phe28 side-chain plays a key role in the formation of a NO/O2-induced Ras 5-nitro-GDP product. We previously proposed a mechanism of NO/O2-mediated Ras GNE, in which *NO2, formed by the reaction of NO with O2, generates a Ras Cys118 thiyl radical (Ras-S118) intermediate. In the present study, we provide evidence for a radical-based mechanism of NO/O2-mediated Ras GNE. According to this mechanism, reaction of NO with O2 produces *NO2. *NO2 then reacts with Ras to produce Ras-S118, which withdraws an electron from the Ras-bound guanine nucleotide base to produce a guanine nucleotide diphosphate cation radical (G(+)-DP) via the Phe28 side-chain. G(+)-DP is subsequently converted to a neutral radical, and can react with another *NO2 to produce 5-nitro-GDP. This radical-based reaction process disrupts key binding interactions between Ras and the guanine base, resulting in release of GDP from Ras and its conversion to free 5-nitro-GDP. This mechanism is likely to be common to other NKCD motif-containing Ras superfamily GTPases, as NO/O2 also facilitates GNE on the redox-active Rap1A and Rab3A GTPases.

The role of functional and molecular imaging in cancer drug discovery and development

Studies of pharmacokinetics (which is what the body does to the drug) and pharmacodynamics (which is what the drug does to the body) are essential components of the modern process of cancer drug discovery and development. Defining the precise relationship between pharmacokinetics and pharmacodynamics is critical. It is especially important to establish a well understood pharmacological "audit trail" that links together all of the essential parameters of drug action, from the molecular target to the clinical effects. The pharmacological audit trail allows us to answer two absolutely crucial questions: (1) how much gets there; and (2) what does it do? During the pre-clinical drug discovery phase, it is essential that pharmacokinetic/pharmacodynamic (PK/PD) properties are optimized, so that the best candidate can be selected for clinical development. As part of contemporary mechanistic, hypothesis-testing clinical trials, construction of the pharmacological PK/PD audit trail facilitates rational decision-making. However, PK/PD endpoints frequently require invasive sampling of body fluids and tissues. Non-invasive molecular measurements, e.g. using MRI or spectroscopy, or positron emission tomography, are therefore very attractive. This review highlights the need for PK/PD endpoints in modern drug design and development, illustrates the value of PK/PD endpoints, and emphasises the importance of non-invasive molecular imaging in drug development. Examples cited include the use of PK/PD endpoints in the development of molecular therapeutic drugs such as the Hsp90 molecular chaperone inhibitor 17AAG, as well as the development of SR-4554 as a non-invasive probe for the detection of tumour hypoxia.

Treatment of HER-2/neu overexpressing breast cancer xenograft models with trastuzumab (Herceptin) and gefitinib (ZD1839): drug combination effects on tumor growth, HER-2/neu and epidermal growth factor receptor expression, and viable hypoxic cell fraction

PURPOSE

The purpose of this research was to assess the effects of single agent and combination treatment with trastuzumab and gefitinib on tumor growth and tumor microenvironment in two HER-2/neu overexpressing breast xenograft models, MDA-MB-435/LCC6(HER-2) (LCC6(HER-2); estrogen receptor negative) and MCF-7(HER-2) (estrogen receptor positive).

EXPERIMENTAL DESIGN

LCC6(HER-2) and MCF-7(HER-2) cells, both in tissue culture and xenografts grown in SCID-Rag 2M mice, were treated with trastuzumab and gefitinib, alone or in combination. The rate of tumor growth was determined. In addition, tumor HER-2/neu and epidermal growth factor receptor expression, cell viability, cell cycle distribution, and proportion of viable hypoxic cells were determined by flow cytometric analyses of single tumor cell suspensions.

RESULTS

Both tumor models were very sensitive to trastuzumab and moderately sensitive to gefitinib in vivo. The combination resulted in therapeutic effects, as judged by inhibition of tumor growth, which was greater (albeit not statistically significant) than that observed with trastuzumab administered as a single agent. Trastuzumab was effective in down-regulating HER-2/neu, and gefitinib mediated a reduction in epidermal growth factor receptor expression on tumor cells. In LCC6(HER-2) tumors, trastuzumab significantly reduced tumor cell viability, which was not improved by the addition of gefitinib. Gefitinib dramatically reduced the proportion of viable hypoxic cells in LCC6(HER-2) and MCF-7(HER-2) tumors. This effect was abrogated by the addition of trastuzumab.

CONCLUSIONS

Although in vivo efficacy studies in two HER-2/neu overexpressing breast xenograft models showed that the combination of trastuzumab and gefitinib was effective, analyses of various cellular parameters failed to reveal beneficial effects and argue that this drug combination may not be favorable.

Reductive activation and thiol reactivity of benzazolo[3,2-a]quinolinium salts

Derivatives of benzazolo[3,2-a]quinolium salts (QSDs) are reductively activated by the enzymatic reducing agents hypoxanthine (or xanthine)/xanthine oxidase and NADH dehydrogenase as evidenced by the increase in rates of ferricytochrome c (Cyt(III)c) reduction and oxygen consumption, respectively. No correlation between Michaelis-Menten parameters and QSDs redox potentials was found regarding anaerobic or aerobic Cyt(III)c reduction, although maximum rates were observed for nitro-containing QSDs. However, oxygen consumption rates correlate with QSDs redox potentials when NADH dehydrogenase is used as reducing agent. QSDs bind covalently to bovine serum albumin (BSA) under anaerobic conditions, in the presence, and less in the absence, of HX/XO and only if the nitro group is present at the QSD. QSDs react with glutathione (GSH) in the presence of HX/XO but not in its absence, under anaerobic conditions. The amount of reacted GSH increases, and the relative amount of GSSG formed decreases, with an increase in the QSD reduction potential, thus indicating that GSH reacts with reduced nitro-containing QSDs mainly in a manner which does not involve the production of GSSG, presumably, through the formation of the nitroso-QSD-GSH conjugate. QSDs are, thus, novel nitro-containing heterocyclic compounds which could be bioreductively activated to react with oxygen and thiols.

Cytochrome P450 enzymes: Novel options for cancer therapeutics

The concept of overexpression of individual forms of cytochrome P450 enzymes in tumor cells is now becoming well recognized. Indeed, a growing body of research highlights the overexpression of P450s, particularly CYP1B1, in tumor cells as representing novel targets for anticancer therapy. The purpose of this review is to outline the novel therapeutic options and opportunities arising from both enhanced endogenous expression of cytochrome P450 in tumors and cytochrome P450-mediated gene therapy.

The Design of Drugs that Target Tumour Hypoxia

The occurrence of hypoxia in solid tumours is increasingly recognized as a limiting factor in the success of both radiotherapy and chemotherapy treatment, but at the same time offers a tumour-specific phenomenon for the activation of prodrugs. However, the design of clinically useful prodrugs that can be selectively activated in hypoxic cells has proved elusive. Specific reasons (activation by oxygen-insensitive two-electron reductases) have been proposed for the failure of quinone-based prodrugs, but a more general contributing factor may be inappropriate clinical trial design, and the failure to understand the critical importance of drug properties, such as efficient extra-vascular diffusion of the prodrug and back-diffusion of the activated drug in the tumour. Activation of prodrugs by therapeutic radiation and the use of hypoxia-selective gene therapy vectors, such as Clostridia, are exciting new mechanisms for prodrug research to explore, but are in much earlier stages of evaluation.

Synthesis and antimycobacterial activity of pyrazine and quinoxaline derivatives

A series of pyrazine and quinoxaline derivatives have been synthesized, and their activity against M. tuberculosis (Mtb) and Mycobacterium avium (MAC) are reported. The 4-acetoxybenzyl ester of pyrazinoic acid and 4'-acetoxybenzyl 2-quinoxalinecarboxylate showed excellent activity against Mtb (MIC ranges of less than 1-6.25 microg/mL) but only modest activity against MAC (MICs of 4-32 microg/mL).

Intratumoral hypoxia resulting in the presence of a fibrotic focus is an independent predictor of early distant relapse in lymph node-negative breast cancer patients

AIMS

To examine the importance of a fibrotic focus-a scar-like area in a carcinoma-as a marker of intratumoral hypoxia that correlates with angiogenesis and with clinical outcome in node-negative breast cancer.

METHODS AND RESULTS

One hundred and four T1-2N0M0 breast carcinoma patients were divided into two groups: group 1 (n=46) showing early distant relapse (median disease-free survival 25 months) and group 2 (n=58) showing no evidence of disease (median follow-up 91.5 months). All tumours were evaluated for medial/lateral location, size, histological grade, mitotic activity, necrosis, fibrotic focus, angiogenesis, vascular permeation and growth pattern. Multiple regression analysis showed that only histological grade and the presence of a fibrotic focus were independent predictors of early distant relapse. A fibrotic focus was present in 53% of the tumours. The relative size (fibrotic focus/tumour ratio) was significantly correlated with an unfavourable outcome. The presence of necrosis inside the fibrotic focus and the absolute and relative size of the fibrotic focus were significantly correlated with angiogenesis. A fibrotic focus was significantly associated with large, expansively growing tumours with high histological grade and numerous mitoses.

CONCLUSION

A fibrotic focus can be used as a surrogate for quantifying angiogenesis and is an independent predictor of early metastasis in lymph node-negative breast cancer.