Tirapazamine: a bioreductive anticancer drug that exploits tumour hypoxia

Clinical relevance and therapeutic predictive ability of hypoxia biomarkers in head and neck cancer tumour models

Tumour hypoxia promotes poor patient outcomes, with particularly strong evidence for head and neck squamous cell carcinoma (HNSCC). To effectively target hypoxia, therapies require selection biomarkers and preclinical models that can accurately model tumour hypoxia. We established 20 patient-derived xenograft (PDX) and cell line-derived xenograft (CDX) models of HNSCC that we characterised for their fidelity to represent clinical HNSCC in gene expression, hypoxia status and proliferation and that were evaluated for their sensitivity to hypoxia-activated prodrugs (HAPs). PDX models showed greater fidelity in gene expression to clinical HNSCC than cell lines, as did CDX models relative to their paired cell lines. PDX models were significantly more hypoxic than CDX models, as assessed by hypoxia gene signatures and pimonidazole immunohistochemistry, and showed similar hypoxia gene expression to clinical HNSCC tumours. Hypoxia or proliferation status alone could not determine HAP sensitivity across our 20 HNSCC and two non-HNSCC tumour models by either tumour growth inhibition or killing of hypoxia cells in an ex vivo clonogenic assay. In summary, our tumour models provide clinically relevant HNSCC models that are suitable for evaluating hypoxia-targeting therapies; however, additional biomarkers to hypoxia are required to accurately predict drug sensitivity.

Studies related to the enhanced the effect of 5-aminolevulinic acid-based photodynamic therapy combined with tirapazamine

OBJECTIVE

5-aminolevulinic acid (5-ALA) is a precursor of the photosensitizer and photodynamic therapy (PDT) has been used in clinical practice. However, tumor cellular hypoxia severely affects the efficiency of photodynamic therapy. In this study, photodynamic therapy was combined with tirapazamine to investigate the effects of the combined intervention and the related mechanisms it may involve.

METHODS

Colony formation assays were used to demonstrate cell proliferation. Transwell assays were performed to observe the effect on cell invasion and metastasis after the corresponding intervention. DCFH-DA probe was used to detect the reactive oxygen species content. Flow cytometry was used to detect the effects of the interventions on apoptosis and cell cycle. The relevant pathways that may be involved are explored by examining the expression levels of the relevant proteins and genes.

RESULTS

Colony formation assays indicated that the combined intervention inhibited cell proliferation. Transwell assays demonstrated that PDT combined with TPZ effectively inhibited tumor cell invasion and metastasis. In addition, fluorescence intensity generated by DCFH-DA oxidation was detected indicating that the combined intervention increased the formation of reactive oxygen species. Flow cytometry clearly showed that the combination of PDT and TPZ further increased apoptosis and cell cycle arrest. The results of western blotting and qRT-PCR experiments confirmed that the combination therapy inhibited HIF-1α/VEGF axis and the PI3K/Akt/mTOR pathway activation.

CONCLUSION

5-ALA-PDT combined with TPZ can inhibit cell proliferation, increase apoptosis, and inhibit the PI3K/Akt/mTOR pathway, thus inhibiting tumor growth and metastasis and improving anti-cancer effects.

Effectiveness of radiotherapy and targeted radionuclide therapy for melanoma in preclinical mouse models: A combination treatments overview

Cutaneous melanoma is an aggressive and highly metastatic skin cancer. In recent years, immunotherapy and targeted small-molecule inhibitors have improved the overall survival of patients. Unfortunately, most patients in advanced stages of disease exhibit either intrinsically resistant or rapidly acquire resistance to these approved treatments. However, combination treatments have emerged to overcome resistance, and novel treatments based on radiotherapy (RT) and targeted radionuclide therapy (TRT) have been developed to treat melanoma in the preclinical mouse model, raising the question of whether synergy in combination therapies may motivate and increase their use as primary treatments for melanoma. To help clarify this question, we reviewed the studies in preclinical mouse models where they evaluated RT and TRT in combination with other approved and unapproved therapies from 2016 onwards, focusing on the type of melanoma model used (primary tumor and or metastatic model). PubMed® was the database in which the search was performed using mesh search algorithms resulting in 41 studies that comply with the inclusion rules of screening. Studies reviewed showed that synergy with RT or TRT had strong antitumor effects, such as tumor growth inhibition and fewer metastases, also exhibiting systemic protection. In addition, most studies were carried out on antitumor response for the implanted primary tumor, demonstrating that more studies are needed to evaluate these combined treatments in metastatic models on long-term protocols.

Tirapazamine-loaded CalliSpheres microspheres: Preparation and characterization as a chemoembolization agent for liver cancer

Tirapazamine (TPZ), a hypoxia-selective cytotoxic agent, has proved to exert synergistic tumor-killing activity with transcatheter arterial embolization (TAE) against liver cancer. This advances TPZ to transcatheter therapies for liver cancer, particularly in combination with drug-eluting microspheres. We describe methods for preparing and characterizing TPZ-loaded CalliSpheres microspheres (CSMTPZs) with regard to their properties as a chemoembolization agent, which includes 1) preparation of CSMTPZs and determination of drug loading level, 2) in vitro determination of TPZ release, 3) assessment of CSMTPZ size and appearance, and 4) determination of TPZ pharmacokinetics and intratumoral drug concentration in vivo. These methods can be adapted for further clinical I trial.•This is to our knowledge the first methods for preparing and characterizing tirapazamine-loaded microspheres with regard to their properties as a chemoembolization agent•Detailed protocols for preparation of CSMTPZs, determination of drug loading level, in vitro determination of TPZ release, assessment of CSMTPZ size and appearance, and in vivo determination of TPZ pharmacokinetics and intratumoral drug concentration•Adaptable to experiments on other animal models and clinical trials.

Biomimetic Metal-Organic Framework Nanoparticles for Synergistic Combining of SDT-Chemotherapy Induce Pyroptosis in Gastric Cancer

In recent years, sonodynamic therapy (SDT) has been widely developed for cancer research as a promising non-invasive therapeutic strategy. Here, we synthesized zeolitic imidazole frameworks-8 (ZIF-8) and utilized its properties to encapsulate hydrophobic Chlorin e6 (Ce6) and hydrophilic tirapazamine (TPZ) for a synergistic sonodynamic chemotherapy, which was also accompanied by the modification of cytomembrane of gastric cancer (GC) cells. Thus, we enabled the biomimetic property to achieve targeted delivery. Ce6-mediated SDT, in combination with ultrasound irradiation, could target the release of reactive oxygen species (ROS) to aggravate further hypoxia and activate TPZ. Combining these effects could induce the pyroptosis of GC cells and play the anti-tumor function, which could provide a potential therapeutic method for cancer therapy.

Hypoxic Jumbo Spheroids On-A-Chip (HOnAChip): Insights into Treatment Efficacy

Hypoxia is a key characteristic of the tumor microenvironment, too rarely considered during drug development due to the lack of a user-friendly method to culture naturally hypoxic 3D tumor models. In this study, we used soft lithography to engineer a microfluidic platform allowing the culture of up to 240 naturally hypoxic tumor spheroids within an 80 mm by 82.5 mm chip. These jumbo spheroids on a chip are the largest to date (>750 µm), and express gold-standard hypoxic protein CAIX at their core only, a feature absent from smaller spheroids of the same cell lines. Using histopathology, we investigated response to combined radiotherapy (RT) and hypoxic prodrug Tirapazamine (TPZ) on our jumbo spheroids produced using two sarcoma cell lines (STS117 and SK-LMS-1). Our results demonstrate that TPZ preferentially targets the hypoxic core (STS117: p = 0.0009; SK-LMS-1: p = 0.0038), but the spheroids' hypoxic core harbored as much DNA damage 24 h after irradiation as normoxic spheroid cells. These results validate our microfluidic device and jumbo spheroids as potent fundamental and pre-clinical tools for the study of hypoxia and its effects on treatment response.

The Role of Biomimetic Hypoxia on Cancer Cell Behaviour in 3D Models: A Systematic Review

Simple Summary

Cancer remains one of the leading causes of death worldwide. The advancements in 3D tumour models provide in vitro test-beds to study cancer growth, metastasis and response to therapy. We conducted this systematic review on existing experimental studies in order to identify and summarize key biomimetic tumour microenvironmental features which affect aspects of cancer biology. The review noted the significance of in vitro hypoxia and 3D tumour models on epithelial to mesenchymal transition, drug resistance, invasion and migration of cancer cells. We highlight the importance of various experimental parameters used in these studies and their subsequent effects on cancer cell behaviour.

Abstract

The development of biomimetic, human tissue models is recognized as being an important step for transitioning in vitro research findings to the native in vivo response. Oftentimes, 2D models lack the necessary complexity to truly recapitulate cellular responses. The introduction of physiological features into 3D models informs us of how each component feature alters specific cellular response. We conducted a systematic review of research papers where the focus was the introduction of key biomimetic features into in vitro models of cancer, including 3D culture and hypoxia. We analysed outcomes from these and compiled our findings into distinct groupings to ascertain which biomimetic parameters correlated with specific responses. We found a number of biomimetic features which primed cancer cells to respond in a manner which matched in vivo response.

Cascade Drug-Release Strategy for Enhanced Anticancer Therapy

Chemotherapy serves as one of the most effective approaches in numerous tumor treatments but also suffers from the limitations of low bioavailability and adverse side effects due to premature drug leakage. Therefore, it is crucial to realize accurate on-demand drug release for promoting the application of chemotherapeutic agents. To achieve this, stimuli-responsive nanomedicines that can be activated by delicately designed cascade reactions have been developed in recent years. In general, the nanomedicines are triggered by an internal or external stimulus, generating an intermediate stimulus at tumor site, which can intensify the differences between tumor and normal tissues; the drug release process is then further activated by the intermediate stimulus. In this review, the latest progress made in cascade reactions-driven drug-release modes, based on the intermediate stimuli of heat, hypoxia, and reactive oxygen species, is systematically summarized. The perspectives and challenges of cascade strategy for drug delivery are also discussed.

Patient-Derived Xenograft and Organoid Models for Precision Medicine Targeting of the Tumour Microenvironment in Head and Neck Cancer

Patient survival from head and neck squamous cell carcinoma (HNSCC), the seventh most common cause of cancer, has not markedly improved in recent years despite the approval of targeted therapies and immunotherapy agents. Precision medicine approaches that seek to individualise therapy through the use of predictive biomarkers and stratification strategies offer opportunities to improve therapeutic success in HNSCC. To enable precision medicine of HNSCC, an understanding of the microenvironment that influences tumour growth and response to therapy is required alongside research tools that recapitulate the features of human tumours. In this review, we highlight the importance of the tumour microenvironment in HNSCC, with a focus on tumour hypoxia, and discuss the fidelity of patient-derived xenograft and organoids for modelling human HNSCC and response to therapy. We describe the benefits of patient-derived models over alternative preclinical models and their limitations in clinical relevance and how these impact their utility in precision medicine in HNSCC for the discovery of new therapeutic agents, as well as predictive biomarkers to identify patients' most likely to respond to therapy.

Evaluation of cobalt(III) complexes as potential hypoxia-responsive carriers of esculetin

Differentiation between hypoxic and normoxic tissues have been exploited for the development of selective chemotherapeutic agents. In this context, cobalt(III)-based coordination compounds have been designed and investigated as prospective hypoxia-responsive drug delivery systems. Three cobalt(III) complexes, namely [Co^III(esc)(py₂en)]ClO₄·(CH₃OH)₂ (1) [Co^III(esc)(TPA)]ClO₄·3H₂O (2) and [Co^III(bipy)₂(esc)]ClO₄·2.5H₂O (3) (py₂en = N,N'-bis(pyridin-2-ylmethyl)ethylenediamine, TPA = tris(2-pyridylmethyl)amine, bipy = 2,2'-bipyridine and esc = 6,7-dihydroxycoumarin or esculetin), were prepared and investigated as potential carriers of esculetin. The spectroscopic and electrochemical properties of 1-3 were investigated and compared. Reactions of the complexes with biologically relevant reducing agents, viz. ascorbic acid, cysteine and glutathione, were monitored spectroscopically for 24 h, in pH 6.2 and 7.4 PBS phosphate buffer saline (PBS) solutions at 37 °C, under air, argon and dioxygen atmospheres. Dissociation of esculetin was observed upon Co³⁺/Co²⁺ reduction preferably under hypoxic conditions, with more effective conversion rates for 3 > 2 > 1. These results illustrate the importance to modulate the Co³⁺/Co²⁺ redox potential through the donor-acceptor properties of the ancillary ligands. Complex 3 is cytotoxic against HCT-116 but not against HT-29 and HEK-293 cells. In addition, DNA-binding studies indicate that interactions of 1 and 3 with the biomolecule are electrostatic.

Triethylphosphinegold(I) Complexes with Secnidazole-Derived Thiosemicarbazones: Cytotoxic Activity against HCT-116 Colorectal Cancer Cells under Hypoxia Conditions

Triethylphosphinegold(I) complexes [Au(HL1)P(CH₂CH₃)₃]PF₆ (1), [Au(HL2)P(CH₂CH₃)₃]PF₆ (2), and [Au(HL3)P(CH₂CH₃)₃]PF₆ (3) were obtained with (E)-2-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)hydrazinecarbothioamide (HL1), (E)-N-methyl-2-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)hydrazinecarbothioamide (HL2), and (E)-2-(1-(2-methyl-5-nitro-1H-imidazol-1-yl)propan-2-ylidene)-N-phenylhydrazinecarbothioamide (HL3). All compounds were assayed for their cytotoxic activities against HCT-116 colorectal carcinoma cells under normoxia and hypoxia conditions and against nonmalignant HEK-293 human embryonic kidney cells under normoxia conditions. The thiosemicarbazone ligands HL1-HL3 were inactive against HCT-116 cells under hypoxia but while HL3 was inactive, HL1 and HL2 proved to be cytotoxic to both cell lineages under normoxia conditions. Complexes (1-3) and the triethylphosphinegod(I) precursor proved to be active against both cell lineages in normoxia as well as in hypoxia. While 1 and 3 revealed to be active against HEK-293 and HCT-116 cells, being approximately as active against HCT-116 cells in normoxia as under hypoxia, complex (2) proved to be more active against HCT-116 cells under hypoxia than under normoxia conditions, and more active against HCT-116 cells than against the nonmalignant HEK-293 cells, with the selectivity index, calculated as SI = IC_50HEK-293/IC_{50HCT-116hypoxia}, equal to 3.7, similar to the value obtained for the control drug tirapazamine (tirapazamine (TPZ), SI = 4). Although the compounds showed distinct cytotoxic activities, the electrochemical behaviors of HL1-HL3 were very similar, as were the behaviors of complexes (1-3). Complex (2) deserves special interest since it was significantly more active under hypoxia than under normoxia conditions. Hence, in this case, selective reduction of the nitro group in a low oxygen pressure environment, resulting in toxic reactive oxygen species (ROS) and damage to DNA or other biomolecules, might operate, while for the remaining compounds, other modes of action probably occur.

Three-dimensional cell culture models for anticancer drug screening: Worth the effort?

High attrition of new oncology drug candidates in clinical trials is partially caused by the poor predictive capacity of artificial monolayer cell culture assays early in drug discovery. Monolayer assays do not take the natural three-dimensional (3D) microenvironment of cells into account. As a result, false positive compounds often enter clinical trials, leading to high dropout rates and a waste of time and money. Over the past 2 decades, tissue engineers and cell biologists have developed a broad range of 3D in vitro culturing tools that better represent in vivo cell biology. These tools preserve the 3D architecture of cells and can be used to predict toxicity of and resistance against antitumor agents. Recent progress in tissue engineering further improves 3D models by taking into account the tumor microenvironment, which is important for metastatic progression and vascularization. However, the widespread implementation of 3D cell cultures into cell-based research programs has been limited by various factors, including their cost and reproducibility. In addition, different 3D cell culture techniques often produce spheroids of different size and shape, which can strongly influence drug efficacy and toxicity. Hence, it is imperative to morphometrically characterize multicellular spheroids to avoid generalizations among different spheroid types. Standardized 3D culturing procedures could further reduce data variability and enhance biological relevance. Here, we critically evaluate the benefits and challenges inherent to growing cells in 3D, along with an overview of the techniques used to form spheroids. This is done with a specific focus on antitumor drug screening.

Metabolic targeting of HIF-dependent glycolysis reduces lactate, increases oxygen consumption and enhances response to high-dose single-fraction radiotherapy in hypoxic solid tumors

Background

A high rate of glycolysis leading to elevated lactate content has been linked to poor clinical outcomes in patients with head and neck and cervical cancer treated with radiotherapy. Although the biological explanation for this relationship between lactate and treatment response remains unclear, there is a continued interest in evaluating strategies of targeting metabolism to enhance the effectiveness of radiotherapy. The goal of this study was to investigate the effect of metabolic-targeting through HIF-1α inhibition and the associated changes in glycolysis, oxygen consumption and response on the efficacy of high-dose single-fraction radiotherapy (HD-SFRT).

Methods

HIF-1α wild-type and HIF-1α knockdown FaDu and ME180 xenograft tumors were grown in the hind leg of mice that were placed in an environmental chamber and exposed to different oxygen conditions (air-breathing and hypoxia). Ex vivo bioluminescence microscopy was used to measure lactate and ATP levels and the hypoxic fraction was measured using EF5 immunohistochemical staining. The oxygen consumption rate (OCR) in each cell line in response to in vitro hypoxia was measured using an extracellular flux analyzer. Tumor growth delay in vivo was measured following HD-SFRT irradiation of 20 Gy.

Results

Targeting HIF-1α reduced lactate content, and increased both oxygen consumption and hypoxic fraction in these tumors after exposure to short-term continuous hypoxia. Tumors with intact HIF-1α subjected to HD-SFRT immediately following hypoxia exposure were less responsive to treatment than tumors without functional HIF-1α, and tumors irradiated under air breathing conditions regardless of HIF-1α status.

Conclusions

Blocking the HIF1 response during transient hypoxic stress increased hypoxia, reduced lactate levels and enhanced response to HD-SFRT. This strategy of combining hypofractionated radiotherapy with metabolic reprogramming to inhibit anaerobic metabolism may increase the efficacy of HD-SFRT through increased oxygen consumption and complementary killing of radiosensitive and hypoxic, radioresistant cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3402-6) contains supplementary material, which is available to authorized users.

Tumor-Penetrating Nanoparticles for Enhanced Anticancer Activity of Combined Photodynamic and Hypoxia-Activated Therapy

Poor tumor penetration is a major challenge for the use of nanoparticles in anticancer therapy. Moreover, the inability to reach hypoxic tumor cells that are distant from blood vessels results in inadequate exposure to antitumor therapeutics and contributes to development of chemoresistance and increased metastasis. In the present study, we developed iRGD-modified nanoparticles for simultaneous tumor delivery of a photosensitizer indocyanine green (ICG) and hypoxia-activated prodrug tirapazamine (TPZ). The iRGD-modified nanoparticles loaded with ICG and TPZ showed significantly improved penetration in both 3D tumor spheroids in vitro and orthotopic breast tumors in vivo. ICG-mediated photodynamic therapy upon irradiation with a near-IR laser induced hypoxia, which activated antitumor activity of the codelivered TPZ for synergistic cell-killing effect. In vivo studies demonstrated that the nanoparticles could efficiently deliver the drug combination in 4T1 orthotopic tumors. Primary tumor growth and metastasis were effectively inhibited by the iRGD-modified combination nanoparticles with minimal side effects. The results also showed the anticancer benefits of codelivering ICG and TPZ in a single nanoparticle formulation in contrast to a mixture of nanoparticles containing individual drugs. The study demonstrates the benefits of combining tumor-penetrating nanoparticles with hypoxia-activated drug treatment and establishes a delivery platform for PDT and hypoxia-activated chemotherapy.

Pathophysiological implications of neurovascular P450 in brain disorders

Over the past decades, the significance of cytochrome P450 (CYP) enzymes has expanded beyond their role as peripheral drug metabolizers in the liver and gut. CYP enzymes are also functionally active at the neurovascular interface. CYP expression is modulated by disease states, impacting cellular functions, detoxification, and reactivity to toxic stimuli and brain drug biotransformation. Unveiling the physiological and molecular complexity of brain P450 enzymes will improve our understanding of the mechanisms underlying brain drug availability, pharmacological efficacy, and neurotoxic adverse effects from pharmacotherapy targeting brain disorders.

DNA-crosslinker cisplatin eradicates bacterial persister cells

For all bacteria, nearly every antimicrobial fails since a subpopulation of the bacteria enter a dormant state known as persistence, in which the antimicrobials are rendered ineffective due to the lack of metabolism. This tolerance to antibiotics makes microbial infections the leading cause of death worldwide and makes treating chronic infections, including those of wounds problematic. Here, we show that the FDA-approved anti-cancer drug cisplatin [cis-diamminodichloroplatinum(II)], which mainly forms intra-strand DNA crosslinks, eradicates Escherichia coli K-12 persister cells through a growth-independent mechanism. Additionally, cisplatin is more effective at killing Pseudomonas aeruginosa persister cells than mitomycin C, which forms inter-strand DNA crosslinks, and cisplatin eradicates the persister cells of several pathogens including enterohemorrhagic E. coli, Staphylococcus aureus, and P. aeruginosa. Cisplatin was also highly effective against clinical isolates of S. aureus and P. aeruginosa. Therefore, cisplatin has broad spectrum activity against persister cells. Biotechnol. Bioeng. 2016;113: 1984-1992. © 2016 Wiley Periodicals, Inc.

Bioreductive prodrugs as cancer therapeutics: targeting tumor hypoxia

Hypoxia, a state of low oxygen, is a common feature of solid tumors and is associated with disease progression as well as resistance to radiotherapy and certain chemotherapeutic drugs. Hypoxic regions in tumors, therefore, represent attractive targets for cancer therapy. To date, five distinct classes of bioreactive prodrugs have been developed to target hypoxic cells in solid tumors. These hypoxia-activated prodrugs, including nitro compounds, N-oxides, quinones, and metal complexes, generally share a common mechanism of activation whereby they are reduced by intracellular oxidoreductases in an oxygen-sensitive manner to form cytotoxins. Several examples including PR-104, TH-302, and EO9 are currently undergoing phase II and phase III clinical evaluation. In this review, we discuss the nature of tumor hypoxia as a therapeutic target, focusing on the development of bioreductive prodrugs. We also describe the current knowledge of how each prodrug class is activated and detail the clinical progress of leading examples.

c-Kit-positive cardiac stem cells nested in hypoxic niches are activated by stem cell factor reversing the aging myopathy

RATIONALE

Hypoxia favors stem cell quiescence, whereas normoxia is required for stem cell activation, but whether cardiac stem cell (CSC) function is regulated by the hypoxic/normoxic state of the cell is currently unknown.

OBJECTIVE

A balance between hypoxic and normoxic CSCs may be present in the young heart, although this homeostatic control may be disrupted with aging. Defects in tissue oxygenation occur in the old myocardium, and this phenomenon may expand the pool of hypoxic CSCs, which are no longer involved in myocyte renewal.

METHODS AND RESULTS

Here, we show that the senescent heart is characterized by an increased number of quiescent CSCs with intact telomeres that cannot re-enter the cell cycle and form a differentiated progeny. Conversely, myocyte replacement is controlled only by frequently dividing CSCs with shortened telomeres; these CSCs generate a myocyte population that is chronologically young but phenotypically old. Telomere dysfunction dictates their actual age and mechanical behavior. However, the residual subset of quiescent young CSCs can be stimulated in situ by stem cell factor reversing the aging myopathy.

CONCLUSIONS

Our findings support the notion that strategies targeting CSC activation and growth interfere with the manifestations of myocardial aging in an animal model. Although caution has to be exercised in the translation of animal studies to human beings, our data strongly suggest that a pool of functionally competent CSCs persists in the senescent heart and that this stem cell compartment can promote myocyte regeneration effectively, partly correcting the aging myopathy.

The anticancer drug tirapazamine has antimicrobial activity against Escherichia coli, Staphylococcus aureus and Clostridium difficile

Rapidly increasing bacterial resistance to existing therapies creates an urgent need for the development of new antibacterials. Tirapazamine (TPZ, 3-amino-1,2,4-benzotriazine 1,4 dioxide) is a prodrug undergoing clinical trials for various types of cancers. In this study, we showed that TPZ has antibacterial activity, particularly at low oxygen levels. With Escherichia coli, TPZ was bactericidal under both aerobic and anaerobic conditions. Escherichia coli mutants deficient in homologous recombination were hypersusceptible to TPZ, suggesting that drug toxicity may be due to DNA damage. Moreover, E. coli strains deleted for genes encoding putative reductases were resistant to TPZ, implying that these enzymes are responsible for conversion of the prodrug to a toxic compound. Fluoroquinolone-resistant E. coli strains were as susceptible to TPZ as a wild-type strain. Methicillin-resistant Staphylococcus aureus strains were also susceptible to TPZ (MIC = 0.5 μg mL(-1) ), as were pathogenic strains of Clostridium difficile (MIC = 7.5 ng mL(-1) ). TPZ may merit additional study as a broad-spectrum antibacterial, particularly for anaerobes.

Preliminary studies with a new hypoxia-selective cytotoxin, KS119W, in vitro and in vivo

Agents with selective toxicity to hypoxic cells have shown promise as adjuncts to radiotherapy. Our previous studies showed that the bioreductive alkylating agent KS119 had an extremely large differential toxicity to severely hypoxic and aerobic cells in cell culture, and was effective in killing the hypoxic cells of EMT6 mouse mammary tumors in vivo. However, the limited solubility of that compound precluded its development as an anticancer drug. Here we report our initial studies with KS119W, a water-soluble analog of KS119. The cytotoxicity of KS119W to EMT6 cells in vitro was similar to that of KS119, with both agents producing only minimal cytotoxicity to aerobic cells even after intensive treatments, while producing pronounced cytotoxicity to oxygen-deficient cells. This resulted in large differentials in the toxicities to hypoxic and aerobic cells (>1,000-fold at 10 μM). Low pH had only minimal effects on the cytotoxicity of KS119W. Under hypoxic conditions, EMT6 cells transfected to express high levels of either human or mouse versions of the repair protein O(6)-alkylguanine-DNA alkyltransferase, which is also known as O(6)-methylguanine DNA-methyltransferase, were much more resistant to KS119W than parental EMT6 cells lacking O(6)-alkylguanine-DNA alkyltransferase, confirming the importance of DNA O-6-alkylation to the cytotoxicity of this agent. Studies with EMT6 tumors in BALB/c Rw mice using both tumor cell survival and tumor growth delay assays showed that KS119W was effective as an adjunct to irradiation for the treatment of solid tumors in vivo, producing additive or supra-additive effects in most combination regimens for which the interactions could be evaluated. Our findings encourage additional preclinical studies to examine further the antineoplastic effects of KS119W alone and in combination with radiation, and to examine the pharmacology and toxicology of this new bioreductive alkylating agent so that its potential for clinical use as an adjuvant to radiotherapy can be evaluated.

On the reaction mechanism of tirapazamine reduction chemistry: unimolecular N-OH homolysis, stepwise dehydration, or triazene ring-opening

The initial steps of the activation of tirapazamine (TPZ, 1, 3-amino-1,2,4-benzotriazine 1,4-N,N-dioxide) under hypoxic conditions consist of the one-electron reduction of 1 to radical anion 2 and the protonation of 2 at O(N4) or O(N1) to form neutral radicals 3 and 4, respectively. There are some questions, however, as to whether radicals 3 and/or 4 will then undergo N-OH homolyses 3 → 5 + ·OH and 4 → 6 + ·OH or, alternatively, whether 3 and/or 4 may react by dehydration and form aminyl radicals via 3 → 11 + H(2)O and 4 → 12 + H(2)O or phenyl radicals via 3 → 17 + H(2)O. These outcomes might depend on the chemistry after the homolysis of 3 and/or 4, that is, dehydration may be the result of a two-step sequence that involves N-OH homolysis and formation of ·OH aggregates of 5 and 6 followed by H-abstraction within the ·OH aggregates to form hydrates of aminyls 11 and 12 or of phenyl 17. We studied these processes with configuration interaction theory, perturbation theory, and density functional theory. All stationary structures of OH aggregates of 5 and 6, of H(2)O aggregates of 11, 12, and 17, and of the transition state structures for H-abstraction were located and characterized by vibrational analysis and with methods of electron and spin-density analysis. The doublet radical 17 is a normal spin-polarized radical, whereas the doublet radicals 11 and 12 feature quartet instabilities. The computed reaction energies and activation barriers allow for dehydration in principle, but the productivity of all of these channels should be low for kinetic and dynamic reasons. With a view to plausible scenarios for the generation of latent aryl radical species without dehydration, we scanned the potential energy surfaces of 2-4 as a function of the (O)N1-Y (Y = C5a, N2) and (O)N4-Z (Z = C4a, C3) bond lengths. The elongation of any one of these bonds by 0.5 Å requires less than 25 kcal/mol, and this finding strongly suggests the possibility of bimolecular reactions of the spin-trap molecules with 2-4 concomitant with triazene ring-opening.

Electron and spin-density analysis of tirapazamine reduction chemistry

Tirapazamine (TPZ, 1, 3-amino-1,2,4-benzotriazine 1,4-N,N-dioxide), the radical anion 2 formed by one-electron reduction of 1, and neutral radicals 3 and 4 formed by protonation of 2 at O(N4) or O(N1), respectively, and their N-OH homolyses 3 → 5 + ·OH and 4 → 6 + ·OH have been studied with configuration interaction theory, perturbation theory, and density functional theory. A comprehensive comparative analysis is presented of structures and electronic structures and with focus on the development of an understanding of the spin-density distributions of the radical species. The skeletons of radicals 3 and 4 are distinctly nonplanar, several stereoisomeric structures are discussed, and there exists an intrinsic preference for 3 over 4. The N-oxides 1, 5, and 6 have closed-shell singlet ground states and low-lying, singlet biradical (SP-1, SP-6) or biradicaloid (SP-5) excited states. The doublet radicals 2, 3, and 4 are heavily spin-polarized. Most of the spin density of the doublet radicals 2, 3, and 4 is located in one (N,O)-region, and in particular, 3 and 4 are not C3-centered radicals. Significant amounts of spin density occur in both rings in the singlet biradical(oid) excited states of 1, 5, and 6. The dipole moment of the N2-C3(X) bond is large, and the nature of X provides a powerful handle to modulate the N2-C3 bond polarity with opposite effects on the two NO regions. Our studies show very low proton affinities of radical anion 2 and suggest that the pK(a) of radical [2+H] might be lower than 6. Implications are discussed regarding the formation of hydroxyl from 3 and/or 4, regarding the ability of 5 and 6 to react with carbon-centered radicals in a manner that ultimately leads to oxygen transfer, and regarding the interpretation of the EPR spectra of reduced TPZ species and of their spin-trap adducts.

THEORETICAL STUDY OF ADIABATIC AND VERTICAL ELECTRON AFFINITY OF RADIOSENSITIZERS IN SOLUTION PART 2: ANALOGUES OF TIRAPAZAMINE

Tirapazamine is a radiosensitizer, whose biological activity is associated to its electron affinity (EA). The electron affinity can be divided in two main processes: Vertical and Adiabatic. In this work, we calculated the EAs of nitroimidazoles (Fig. 2) using HF and DFT methods and evaluated solvent effects (water and carbon tetrachloride) on EAs. For water, we combined the Polarized Continuum Model (PCM) and free energy perturbation (Finite Difference Thermodynamic Integration, FDTI) methods. For carbon tetrachloride, we used the FDTI method. The values of adiabatic EA obtained are in agreement with experimental data (deviations of 13.25 meV). The vertical EA were calculated according to Cederbaum's Outer Valence Green Function (OVGF) method. This study, which relays on theoretical aspects of free energy calculations on charged molecules in solution, could be helpful in the rational design of new and more selective bioreductive anticancer drugs.

Detection of hypoxia in microscopic tumors using 131I-labeled iodo-azomycin galactopyranoside (131I-IAZGP) digital autoradiography

PURPOSE

Previous studies have shown that tumors less than 1 mm diameter derived from HT29 colorectal cancer cells are extremely hypoxic when grown intraperitoneally or intradermally in nude mice, whereas those of greater size (approximately 1-4 mm diameter) are not significantly hypoxic. The object of this study was to determine if digital autoradiography using the radiolabeled hypoxia imaging tracer iodo-azomycin galactopyranoside ((131)I-IAZGP) could detect hypoxia in this model.

METHODS

Microscopic HT29 tumors were grown as disseminated peritoneal disease and intradermally in nude mice. Tumors ranged in size from a few hundred microns to several millimeters in diameter. Animals were intravenously administered (131)I-IAZGP and pimonidazole 2 h before sacrifice. Following sacrifice, the intratumoral distribution of (131)I-IAZGP was assessed by digital autoradiography and compared with immunofluorescence microscopic images of pimonidazole binding and carbonic anhydrase IX (CAIX) expression.

RESULTS

The distributions of (131)I-IAZGP, pimonidazole, and CAIX expression were similar. Tumors less than 1 mm diameter displayed high (131)I-IAZGP uptake; these tumors also stained strongly for pimonidazole and CAIX. Larger tumors (approximately 1-4 mm diameter) were not significantly hypoxic and had low (131)I-IAZGP accumulation.

CONCLUSION

(131)I-IAZGP can detect hypoxia in microscopic tumors. Microscopic tumors are useful models for the validation of hypoxia radiotracers, and digital autoradiography is an appropriate technique for studying the distribution of hypoxia radiotracers in microscopic tumors.

Accelerated perinecrotic outgrowth of colorectal liver metastases following radiofrequency ablation is a hypoxia-driven phenomenon

OBJECTIVE

The aim of this study was to assess how thermal ablation of colorectal liver metastases affects the outgrowth of micrometastases in the transition zone (TZ) between ablated tissue and the unaffected reference zone (RZ) in 2 different murine models.

BACKGROUND

Thermal destruction therapies of nonresectable colorectal liver metastases, including radiofrequency ablation (RFA), can provide tumor clearance, but local recurrences are common.

METHODS

Three days after intrasplenic injection of C26 colon carcinoma cells, RFA was applied to the left liver lobe. Perinecrotic microcirculation, tissue hypoxia, hypoxia inducible factor (HIF)-1alpha and HIF-2alpha, and the outgrowth of micrometastases both in the TZ and in the RZ were evaluated over time.

RESULTS

In 2 different animal models, the outgrowth of micrometastases in the TZ following RFA was stimulated approximately 4-fold compared to tumor growth in the RZ. Accelerated tumor growth in the TZ was associated with microcirculatory disturbances, prolonged hypoxia, and stabilization of HIF-1alpha and HIF-2alpha in the tumor cells. In addition, RFA induced the formation of new hepatic vessels that sprouted from existing sinusoids and grew into the generated necrotic lesion. Surprisingly, the accelerated tumor growth was not associated with these vessels. Treatment with 17DMAG prevented HIF-1alpha and HIF-2alpha stabilization and selectively reduced tumor growth in the TZ by approximately 40% without affecting tumor growth in sham-operated mice or in the RZ of RFA-treated mice. PTK787/ZK-222584, a nonselective Vascular Endothelial Growth Factor (VEGF)-receptor inhibitor, reduced RFA-stimulated tumor growth and tumor growth in the RZ to a similar extent.

CONCLUSIONS

We conclude that RFA stimulates the outgrowth of tumor cells at the lesion periphery. Angiogenesis is not the driving force behind RFA-stimulated tumor growth, but other hypoxia/HIF-activated pathways are likely to be important.

Tricyclic [1,2,4]triazine 1,4-dioxides as hypoxia selective cytotoxins

A series of novel tricyclic triazine-di- N-oxides (TTOs) related to tirapazamine have been designed and prepared. A wide range of structural arrangements with cycloalkyl, oxygen-, and nitrogen-containing saturated rings fused to the triazine core, coupled with various side chains linked to either hemisphere, resulted in TTO analogues that displayed hypoxia-selective cytotoxicity in vitro. Optimal rates of hypoxic metabolism and tissue diffusion coefficients were achieved with fused cycloalkyl rings in combination with both the 3-aminoalkyl or 3-alkyl substituents linked to weakly basic soluble amines. The selection was further refined using pharmacokinetic/pharmacodynamic model predictions of the in vivo hypoxic potency (AUC req) and selectivity (HCD) with 12 TTO analogues predicted to be active in vivo, subject to the achievement of adequate plasma pharmacokinetics.

8,5'-Cyclopurine-2'-deoxynucleosides in DNA: mechanisms of formation, measurement, repair and biological effects

8,5'-Cyclo-2'-deoxyadenosine (cdA) and 8,5'-cyclo-2'-deoxyguanosine (cdG) are among the major lesions formed in DNA by hydroxyl radical attack on 2'-deoxyadenosine and 2'-deoxyguanosine, respectively, followed by intramolecular cyclization between C5' and C8. Mechanisms of formation of these unique tandem lesions were elucidated. The 8,5'-cyclization causes an unusual puckering of the sugar moiety giving rise to significant distortion in the DNA double helix. Methodologies were developed for the measurement of these lesions in DNA by mass spectrometry coupled either with gas chromatography or high performance liquid chromatography. Both techniques allowed identification and quantification of both R- and S-diastereomers of cdA and cdG in DNA in vitro and in vivo. Because of the 8,5'-covalent bond between the sugar and base moieties in the same nucleoside, cdA and cdG are repaired by nucleotide excision repair rather than by base excision repair. Thus, these lesions may play a role in diseases with defective nucleotide excision repair. Their biological effects include blocking DNA polymerases, inhibition of gene expression, transcriptional mutagenesis among others. Accumulation of cdA and cdG was observed in tissues in vivo in connection to disease and environmental conditions, suggesting an important role for these lesions in disease processes including carcinogenesis and neuronal death.

P450 oxidoreductase: genetic polymorphisms and implications for drug metabolism and toxicity

BACKGROUND

Cytochrome P450 oxidoreductase (POR) is the only electron donor for all microsomal cytochrome P450 monooxygenases (CYP), some of which are phase I drug-metabolizing enzymes, responsible for oxidation of more than 80% of drugs.

OBJECTIVES

To provide a more thorough understanding of the genetic factors influencing drug metabolism, we address the role of genetic polymorphisms in the POR gene, and their implications for drug metabolism and cytotoxicity.

METHODS

The scope of this review is intended to cover polymorphisms currently identified in the POR gene, assess their functional significance on POR activity, and address their impact on CYP-mediated drug metabolism. POR is also responsible for directly metabolizing several anticancer prodrugs via a 1-electron reduction reaction, so the effect of POR polymorphisms on the direct bioactivation of drugs is also considered.

RESULTS/CONCLUSION

POR is a polymorphic enzyme that can affect CYP-mediated drug metabolism as well as direct bioactivation of prodrugs. Genetic polymorphisms in the POR gene may help to explain altered drug-metabolizing phenotypes.

Hypoxic tumor cell radiosensitization through nitric oxide

Hypoxia is a principal signature of the tumor microenvironment and is considered to be the most important cause of clinical radioresistance and local failure. Oxygen is so far the best radiosensitizer, but tumor oxygenation protocols are compromised by its metabolic consumption and therefore limited diffusion inside tumors. Many chemical radiosensitizers can selectively target hypoxic tumor cells, but their systemic toxicity compromises their adequate clinical use. NO is an efficient hypoxic radiosensitizer, as it may mimic the effects of oxygen on fixation of radiation-induced DNA damage, but the required levels cannot be obtained in vivo because of vasoactive complications. Our laboratory explored whether this problem may be overcome by endogenous production of NO inside tumors. We demonstrated that iNOS, activated by pro-inflammatory cytokines, is capable of radiosensitizing tumor cells through endogenous production of NO, at non-toxic extracellular concentrations. We observed that this radiosensitizing effect is transcriptionally controlled by hypoxia and by NF-kappaB. Tumor-associated immune cells may contribute to the iNOS-mediated radiosensitization by the generation of pro-inflammatory cytokines and NO, which may diffuse towards bystander tumor cells. Our findings indicate a rationale for combining immunostimulatory and radiosensitizing strategies in the future.

Potent and highly selective hypoxia-activated achiral phosphoramidate mustards as anticancer drugs

A series of achiral hypoxia-activated prodrugs were synthesized on the basis of the DNA cross-linking toxin of the prodrug, ifosfamide. The hypoxia-selective cytotoxicity of several of the compounds was improved over previously reported racemic mixtures of chiral bioreductive phosphoramidate prodrugs. Prodrugs activated by 2-nitroimidazole reduction demonstrated up to 400-fold enhanced cytotoxicity toward H460 cells in culture under hypoxia versus their potency under aerobic conditions. Compounds were further assessed for their stability to cytochrome P450 metabolism using a liver microsome assay. The 2-nitroimidazole containing lead compound 3b (TH-302) was selectively potent under hypoxia and stable to liver microsomes. It was active in an in vivo MIA PaCa-2 pancreatic cancer orthotopic xenograft model as a monotherapy and demonstrated dramatic efficacy when used in combination with gemcitabine, extending survival with one of eight animals tumor free at day-44. Compound 3b has emerged as a promising antitumor agent that shows excellent in vivo efficacy and is currently being evaluated in the clinic.

Prediction of Tumour Tissue Diffusion Coefficients of Hypoxia-Activated Prodrugs from Physicochemical Parameters

The therapeutic activity of anticancer agents depends critically on their ability to penetrate through tumour tissue to reach their target cells, a requirement that is especially important for hypoxia-activated prodrugs. Here we use multicellular layers (MCL) grown in vitro from HT29 colon carcinoma cells to measure tissue diffusion coefficients (Dmcl) of 67 structurally diverse benzotriazine di-N-oxides (analogues of the hypoxia-activated prodrug tirapazamine) plus four miscellaneous compounds. An algorithm was developed to predict Dmcl from physicochemical parameters (molecular weight, octanol/water partition coefficient at pH 7.4, number of hydrogen bond donors and acceptors); the fitted multivariate relationship had an explained variance (R2) of 0.907 and predictive power (Q2) of 0.879. Using a subset of nine compounds tested as a single cassette, the algorithm was shown to apply, with some adjustment of coefficients, to MCLs from three other tumour cell lines with differing cell packing densities (SiHa, HCT8-Ea, and HCT8-Ra). The demonstrated relationships provide tools for optimizing extravascular transport of anticancer agents during lead optimization.

Hypoxia-selective 3-alkyl 1,2,4-benzotriazine 1,4-dioxides: the influence of hydrogen bond donors on extravascular transport and antitumor activity

Tirapazamine (TPZ) and related 1,2,4-benzotriazine 1,4 dioxides (BTOs) are selectively toxic under hypoxia, but their ability to kill hypoxic cells in tumors is generally limited by their poor extravascular transport. Here we show that removing hydrogen bond donors by replacing the 3-NH2 group of TPZ with simple alkyl groups increased their tissue diffusion coefficients as measured in multicellular layer cultures. This advantage was largely retained using solubilizing 3-alkylaminoalkyl substituents provided these were sufficiently lipophilic at pH 7.4. The high reduction potentials of such compounds resulted in rates of metabolism too high for optimal penetration into hypoxic tissue, but electron-donating 6- and 7-substituents moderated metabolism. Pharmacokinetic/pharmacodynamic model-guided screening was used to select BTOs with optimal extravascular transport and hypoxic cytotoxicity properties for evaluation against HT29 human tumor xenografts in combination with radiation. This identified four novel 3-alkyl BTOs providing greater clonogenic killing of hypoxic cells than TPZ at equivalent host toxicity, with the 6-morpholinopropyloxy-BTO 22 being 3-fold more active.

Pharmacokinetic/pharmacodynamic model-guided identification of hypoxia-selective 1,2,4-benzotriazine 1,4-dioxides with antitumor activity: the role of extravascular transport

Pharmacokinetic/pharmacodynamic (PK/PD) modeling has shown the antitumor activity of tirapazamine (TPZ), a bioreductive hypoxia-selective cytotoxin, to be limited by poor penetration through hypoxic tumor tissue. We have prepared a series of 1,2,4-benzotriazine 1,4-dioxide (BTO) analogues of TPZ to improve activity against hypoxic cells by increasing extravascular transport. The 6 substituents modified lipophilicity and rates of hypoxic metabolism. 3-Alkylamino substituents increased aqueous solubility and also influenced lipophilicity and hypoxic metabolism. PK/PD model-guided screening was used to select six BTOs for evaluation against hypoxic cells in HT29 human tumor xenografts. All six BTOs were active in vivo, and two provided greater hypoxic cell killing than TPZ because of improved transport and/or plasma PK. This PK/PD model considers two causes of therapeutic failure (limited tumor penetration and poor plasma pharmacokinetics) often not addressed early in drug development and provides a general strategy for selecting candidates for in vivo evaluation during lead optimization.

Hyperthermic enhancement of tumor radiosensitization strategies

Local hyperthermia of living tissue can cause significant increases in blood flow and oxygenation depending on time-temperature history. Increases in perfusion of the abnormal and insufficient vasculature found in solid tumors may increase tumor oxygenation, thereby increasing the radiation sensitivity of the tumor. We hypothesized that local heating of tumor would increase the oxygenation of the tumor tissue and allow other oxygenating agents to further modify tumor oxygenation and radiation response. In the present study the effect of moderate temperature hyperthermia (MTH) at 41.5-42.5 degrees C for 30-60 min, 250 mg/kg nicotinamide, or carbogen breathing (95% O2/5% CO2) on the radiation sensitivity of FSaII murine fibrosarcomas or R3230 AC rat adenocarcinomas was studied. Individually, these treatments increased the tumor cell sensitivity to single dose 10-15 Gy X-irradiation by 1-5 fold on average, as measured by the in vivo/in vitro tumor excision assay. The combination of tumor MTH with nicotinamide or carbogen breathing increased the radiation sensitivity by 3-5 fold in FSaII tumors and 10-30 fold in R3230 tumors with varying levels of statistical significance. Finally, the triple combination of adjuvant MTH, nicotinamide and carbogen breathing increased the radiation-induced cell death in FSaII tumors to a similar extent as the dual combinations of MTH, nicotinamide or heat, carbogen breathing. However, in R3230 AC tumors the triple adjuvant combination significantly increased radiation-induced cell killing compared to all other dual adjuvant treatments (p < 0.04). To interrogate the mechanism by which heating alters tumor physiology, nitric oxide production in tumor and endothelial cells in culture and tumor tissue after heating was studied. Heating caused an increase in nitric oxide production over a 24 h period after treatment. Subsequently, inhibiting the enzymatic production of NO with L-NAME was found to increase heat-induced growth delay of FSaII tumors. The cause and effect of increased nitric oxide production and the response of the tumor vasculature to heat are discussed in the context of the tumor radiosensitization achieved by heating, carbogen breathing and nicotinamide.

Preferential DNA cleavage under anaerobic conditions by a DNA-binding ruthenium dimer

In the absence of dioxygen, the cationic complex [(phen)2Ru(tatpp)Ru(phen)2]4+ (P4+) undergoes in situ reduction by glutathione (GSH) to form a species that induces DNA cleavage. Exposure to air strongly attenuates the cleavage activity, even in the presence of a large excess of reducing agent (e.g., 40 equiv of GSH per P4+), suggesting that the complex may be useful in targeting cells with a low-oxygen microenvironment (hypoxia) for destruction via DNA cleavage. The active species is identified as the doubly reduced, doubly protonated complex H2P4+, and a carbon-based radical species is implicated in the cleavage action. We postulate that the dioxygen concentration regulates the degree to which the carbon radical forms and thus regulates the DNA cleavage activity.

Complete 1H, 13C and 15N NMR assignment of tirapazamine and related 1,2,4-benzotriazine N-oxides

1H, 13C and 15N NMR measurements (1D and 2D including 1H--15N gs-HMBC) have been carried out on 3-amino-1, 2,4-benzotriazine and a series of N-oxides and complete assignments established. N-Oxidation at any position resulted in large upfield shifts of the corresponding N-1 and N-2 resonances and downfield shifts for N-4 with the exception of the 3-amino-1,2,4-benzotriazine 1-oxide in which a small upfield shift of N-4 was observed. Density functional GIAO calculations of the 15N and 13C chemical shifts [B3LYP/6-31G(d)//B3LYP/6-311+G(2d,p)] gave good agreement with experimental values confirming the assignments. The combination of 13C and 15N NMR provides an unambiguous method for assigning the 1H and 13C resonances of N-oxides of 1,2,4-benzotriazines.

Concurrent chemotherapy and radiation therapy as the standard of care for cervical cancer

From 1999-2000, each of five randomized trials demonstrated improved rates of survival and local control when concurrent cisplatin-based chemotherapy was added to radiation therapy in patients with loco-regionally advanced cervical cancer. These studies demonstrated that addition of chemotherapy to radiation therapy improved the outcome of patients treated with radiation therapy and hysterectomy or radiation therapy alone. Although concurrent chemotherapy increases the severity of acute side effects, it does not appear to increase the risk of late side effects of radiation therapy. A sixth randomized trial, published in 2002, failed to demonstrate improved outcome with concurrent weekly cisplatin over radiation therapy alone; however, the earlier trials demonstrated benefit with this chemoradiation regimen. In addition, three of the earlier randomized trials demonstrated improved outcome with combinations of cisplatin and 5-fluorouracil compared with radiation therapy alone. Although cisplatin-based chemoradiation is the most accepted standard, individual trials have suggested that other drugs, including mitomycin and epirubicin, might be beneficial. Randomized trials that investigated the administration of neoadjuvant chemotherapy before radiation therapy have failed to demonstrate a benefit of this approach. Although the evidence for benefit of concurrent chemotherapy is strong for otherwise healthy patients with newly diagnosed, loco-regionally advanced cervical cancers confined to the pelvis, the relative benefits and risks are not well understood for patients who are infirm or who require larger fields of radiation therapy. In such patients, the theoretical benefits and potential risks should be considered carefully before a treatment plan is prescribed.

Radiolytic and cellular reduction of a novel hypoxia-activated cobalt(III) prodrug of a chloromethylbenzindoline DNA minor groove alkylator

Metabolic reduction can be used to activate prodrugs in hypoxic regions of tumours, but reduction by ionising radiation is also theoretically attractive. Previously, we showed that a cobalt(III) complex containing 8-hydroxyquinoline (8-HQ) and cyclen ligands releases 8-HQ efficiently on irradiation in hypoxic solutions [Ahn G-O, Ware DC, Denny WA, Wilson WR. Optimization of the auxiliary ligand shell of cobalt(III)(8-hydroxyquinoline) complexes as model hypoxia-selective radiation-activated prodrugs. Radiat Res 2004;162:315-25]. Here we investigate an analogous Co(III) complex containing the potent DNA minor groove alkylator azachloromethylbenzindoline (azaCBI, 1) to determine whether it releases 1 on radiolytic and/or enzymatic reduction under hypoxia. Monitoring by HPLC, the azaCBI ligand in the Co(III)(cyclen)(azaCBI) complex (2) slowly hydrolysed in aqueous solution, in contrast to the free ligand 1 which readily converted to its reactive cyclopropyl form. Irradiation of 2 (30-50 microM) in hypoxic solutions released 1 with yields of 0.57 micromol/J in formate buffer and 0.13 micromol/J in human plasma. Using bioassay methods, cytotoxic activation by irradiation of 2 at 1 microM in hypoxic plasma was readily detectable at clinically relevant doses (> or = 1 Gy), with a estimated yield of 1 of 0.075 micromol/J. Release of 1 from 2 was also observed in hypoxic HT29 cultures without radiation, with subsequent conversion of 1 to its O-glucuronide. Surprisingly, overexpression of human cytochrome P450 reductase in A549 cells did not increase the rate of metabolic reduction of 2, suggesting that other reductases and/or non-enzymatic reductants are responsible. Thus the cobalt(III) complex 2 is a promising prodrug capable of being activated to release a very potent cytotoxin when reduced by either ionising radiation or cells under hypoxic conditions.

1,2-Bis(methylsulfonyl)-1-(2-chloroethyl)-2-[[1-(4-nitrophenyl)ethoxy]carbonyl]hydrazine: an anticancer agent targeting hypoxic cells

To target malignant cells residing in hypoxic regions of solid tumors, we have designed and synthesized prodrugs generating the cytotoxic alkylating species 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE) after bioreductive activation. We postulate that one of these agents, 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[[1-(4-nitrophenyl)ethoxy]carbonyl]hydrazine (KS119), requires enzymatic nitro reduction to produce 90CE, whereas another agent, 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(4-nitrobenzyloxy)carbonyl]hydrazine (PNBC), can also be activated by nucleophilic attack by thiols such as glutathione (GSH)/GST. We demonstrated that these agents selectively kill hypoxic EMT6 mouse mammary carcinoma and CHO cells. In hypoxia, 50 microM KS119 produced 5 logs of kill of EMT6 cells without discernable cytotoxicity in air; similar effects were observed with CHO cells. PNBC was less efficacious against hypoxic tumor cells and also had some toxicity to aerobic cells, presumably because of GST/thiol activation, making PNBC less interesting as a selective hypoxic-cell cytotoxin. BALB/c mice with established EMT6 solid tumors were used to demonstrate that KS119 could reach and kill hypoxic cells in solid tumors. To gain information on bioreductive enzymes involved in the activation of KS119, cytotoxicity was measured in CHO cell lines overexpressing NADH:cytochrome b5 reductase (NBR), NADPH:cytochrome P450 reductase (NPR), or NADPH: quinone oxidoreductase 1 (NQO1). Increased cytotoxicity occurred in cells overexpressing NBR and NPR, whereas overexpressed NQO1 had no effect. These findings were supported by enzymatic studies using purified NPR and xanthine oxidase to activate KS119. KS119 has significant potential as a hypoxia-selective tumor-cell cytotoxin and is unlikely to cause major toxicity to well oxygenated normal tissues.

Checkpoint kinase inhibitors: SAR and radioprotective properties of a series of 2-arylbenzimidazoles

The discovery of a series of novel, potent, and highly selective inhibitors of the DNA damage control kinase chk2 is disclosed. Here we report the first SAR study around inhibitors of this kinase. High-throughput screening of purified human chk2 led to the identification of a novel series of 2-arylbenzimidazole inhibitors of the kinase. Optimization was facilitated using homology models of chk2 and docking of inhibitors, leading to the highly potent 2-arylbenzimidazole 2h (IC(50) 15 nM). Compound 2h is an ATP-competitive inhibitor of chk2 that dose dependently protects human CD4(+) and CD8(+) T-cells from apoptosis due to ionizing radiation. This work suggests that a selective small molecule inhibitor of chk2 could be a useful adjuvant to radiotherapy, increasing the therapeutic window of such treatment.

Extravascular Transport of Drugs in Tumor Tissue: Effect of Lipophilicity on Diffusion of Tirapazamine Analogues in Multicellular Layer Cultures

The extravascular diffusion of antitumor agents is a key determinant of their therapeutic activity, but the relationships between physicochemical properties of drugs and their extravascular transport are poorly understood. It is well-known that drug lipophilicity plays an important role in transport across biological membranes, but the net effect of lipophilicity on transport through multiple layers of tumor cells is less clear. This study examines the influence of lipophilicity (measured as the octanol-water partition coefficient P) on the extravascular transport properties of the hypoxic cytotoxin tirapazamine (TPZ, 1) and a series of 13 neutral analogues, using multicellular layers (MCLs) of HT29 human colon carcinoma cells as an in vitro model for the extravascular compartment of tumors. Flux of drugs across MCLs was determined using diffusion chambers, with the concentration-time profile on both sides of the MCL measured by HPLC. Diffusion coefficients in the MCLs (D(MCL)) were inversely proportional to M(r)(0.5) (M(r), relative molecular weight), although this was a minor contributor to differences between compounds over the narrow M(r) range investigated. Differences in lipophilicity had a larger effect, with a sigmoidal dependence of D(MCL) on log P. Correcting for M(r) differences, lipophilic compounds (log P > 1.5) had ca. 15-fold higher D(MCL) than hydrophilic compounds (log P < -1). Using a pharmacokinetic/pharmacodynamic (PK/PD) model in which diffusion in the extravascular compartment of tumors is considered explicitly, we demonstrated that hypoxic cell kill is very sensitive to changes in extravascular diffusion coefficient of TPZ analogues within this range. This study shows that simple monosubstitution of TPZ can alter log P enough to markedly improve extravascular transport and activity against target cells, especially if rates of metabolic activation are also optimized.

Enzyme-catalyzed activation of anticancer prodrugs

The rationale fo the development of prodrugs relies upon delivery of higher concentrations of a drug to target cells compared to administration of the drug itself. In the last decades, numerous prodrugs that are enzymatically activated into anti-cancer agents have been developed. This review describes the most important enzymes involved in prodrug activation notably with respect to tissue distribution, up-regulation in tumor cells and turnover rates. The following endogenous enzymes are discussed: aldehyde oxidase, amino acid oxidase, cytochrome P450 reductase, DT-diaphorase, cytochrome P450, tyrosinase, thymidylate synthase, thymidine phosphorylase, glutathione S-transferase, deoxycytidine kinase, carboxylesterase, alkaline phosphatase, beta-glucuronidase and cysteine conjugate beta-lyase. In relation to each of these enzymes, several prodrugs are discussed regarding organ- or tumor-selective activation of clinically relevant prodrugs of 5-fluorouracil, axazaphosphorines (cyclophosphamide, ifosfamide, and trofosfamide), paclitaxel, etoposide, anthracyclines (doxorubicin, daunorubicin, epirubicin), mercaptopurine, thioguanine, cisplatin, melphalan, and other important prodrugs such as menadione, mitomycin C, tirapazamine, 5-(aziridin-1-yl)-2,4-dinitrobenzamide, ganciclovir, irinotecan, dacarbazine, and amifostine. In addition to endogenous enzymes, a number of nonendogenous enzymes, used in antibody-, gene-, and virus-directed enzyme prodrug therapies, are described. It is concluded that the development of prodrugs has been relatively successful; however, all prodrugs lack a complete selectivity. Therefore, more work is needed to explore the differences between tumor and nontumor cells and to develop optimal substrates in terms of substrate affinity and enzyme turnover rates fo prodrug-activating enzymes resulting in more rapid and selective cleavage of the prodrug inside the tumor cells.

Oxidation of 2-deoxyribose by benzotriazinyl radicals of antitumor 3-amino-1,2,4-benzotriazine 1,4-dioxides

Tirapazamine (3-amino-1,2,4-benzotriazine 1,4-dioxide) is the lead bioreductive drug in clinical trials as an anticancer agent to kill refractory hypoxic cells of solid tumors. It has long been known that, upon metabolic one-electron reduction, tirapazamine induces lethal DNA double strand breaks in hypoxic cells. These strand breaks arise from radical damage to the ribose moiety of DNA, and in this pulse radiolysis and product analysis study we examine mechanistic aspects of the dual function of tirapazamine and analogues in producing radicals of sufficient power to oxidize 2-deoxyribose to form radicals, as well as the ability of the compounds to oxidize the resulting deoxyribose radicals to generate the strand breaks. Both the rate of oxidation of 2-deoxyribose and the radical yield increase with the one-electron reduction potentials of the putative benzotriazinyl radicals formed from the benzotriazine 1,4-dioxides. Subsequent oxidation of the 2-deoxyribose radicals by the benzotriazine 1,4-dioxides and 1-oxides proceeds through adduct formation followed by breakdown to form the radical anions of both species. The yield of the radical anions increases with increasing one-electron reduction potentials of the compounds. We have previously presented evidence that oxidizing benzotriazinyl radicals are formed following one-electron reduction of the benzotriazine 1,4-dioxides. The reactions reported in this work represent the kinetic basis of a short chain reaction leading to increased oxidation of 2-deoxyribose, a process which is dependent on the one-electron reduction potential of the benzotriazinyl radicals that are above a threshold value of ca. 1.24 V.