Factors affecting sensitivity to EO9 in rodent and human tumour cells in vitro: DT-diaphorase activity and hypoxia

Targeting Hypoxia: Hypoxia-Activated Prodrugs in Cancer Therapy

Hypoxia is an important characteristic of most solid malignancies, and is closely related to tumor prognosis and therapeutic resistance. Hypoxia is one of the most important factors associated with resistance to conventional radiotherapy and chemotherapy. Therapies targeting tumor hypoxia have attracted considerable attention. Hypoxia-activated prodrugs (HAPs) are bioreductive drugs that are selectively activated under hypoxic conditions and that can accurately target the hypoxic regions of solid tumors. Both single-agent and combined use with other drugs have shown promising antitumor effects. In this review, we discuss the mechanism of action and the current preclinical and clinical progress of several of the most widely used HAPs, summarize their existing problems and shortcomings, and discuss future research prospects.

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

Efficacy, pharmacokinetic and pharmacodynamic evaluation of apaziquone in the treatment of non-muscle invasive bladder cancer

INTRODUCTION

Apaziquone (also known as EO9 and Qapzola^TM) is a prodrug that is activated to DNA damaging species by oxidoreductases (particularly NQO1) and has the ability to kill aerobic and/or hypoxic cancer cells. Areas covered: Whilst its poor pharmacokinetic properties contributed to its failure in phase II clinical trials when administered intravenously, these properties were ideal for loco-regional therapies. Apaziquone demonstrated good anti-cancer activity against non-muscle invasive bladder cancer (NMIBC) when administered intravesically to marker lesions and was well tolerated with no systemic side effects. However, phase III clinical trials did not reach statistical significance for the primary endpoint of 2-year recurrence in apaziquone over placebo although improvements were observed. Post-hoc analysis of the combined study data did indicate a significant benefit for patients treated with apaziquone, especially when the instillation of apaziquone was given 30 min or more after surgery. A further phase III study is ongoing to test the hypotheses generated in the unsuccessful phase III studies conducted to date. Expert opinion: Because of its specific pharmacological properties, Apaziquone is excellently suited for local therapy such as NMIBC. Future studies should include proper biomarkers.

Targeting the hypoxic fraction of tumours using hypoxia-activated prodrugs

The presence of a microenvironment within most tumours containing regions of low oxygen tension or hypoxia has profound biological and therapeutic implications. Tumour hypoxia is known to promote the development of an aggressive phenotype, resistance to both chemotherapy and radiotherapy and is strongly associated with poor clinical outcome. Paradoxically, it is recognised as a high-priority target and one of the therapeutic strategies designed to eradicate hypoxic cells in tumours is a group of compounds known collectively as hypoxia-activated prodrugs (HAPs) or bioreductive drugs. These drugs are inactive prodrugs that require enzymatic activation (typically by 1 or 2 electron oxidoreductases) to generate cytotoxic species with selectivity for hypoxic cells being determined by (1) the ability of oxygen to either reverse or inhibit the activation process and (2) the presence of elevated expression of oxidoreductases in tumours. The concepts underpinning HAP development were established over 40 years ago and have been refined over the years to produce a new generation of HAPs that are under preclinical and clinical development. The purpose of this article is to describe current progress in the development of HAPs focusing on the mechanisms of action, preclinical properties and clinical progress of leading examples.

Targeting the hypoxic fraction of tumours using hypoxia-activated prodrugs

The presence of a microenvironment within most tumours containing regions of low oxygen tension or hypoxia has profound biological and therapeutic implications. Tumour hypoxia is known to promote the development of an aggressive phenotype, resistance to both chemotherapy and radiotherapy and is strongly associated with poor clinical outcome. Paradoxically, it is recognised as a high-priority target and one of the therapeutic strategies designed to eradicate hypoxic cells in tumours is a group of compounds known collectively as hypoxia-activated prodrugs (HAPs) or bioreductive drugs. These drugs are inactive prodrugs that require enzymatic activation (typically by 1 or 2 electron oxidoreductases) to generate cytotoxic species with selectivity for hypoxic cells being determined by (1) the ability of oxygen to either reverse or inhibit the activation process and (2) the presence of elevated expression of oxidoreductases in tumours. The concepts underpinning HAP development were established over 40 years ago and have been refined over the years to produce a new generation of HAPs that are under preclinical and clinical development. The purpose of this article is to describe current progress in the development of HAPs focusing on the mechanisms of action, preclinical properties and clinical progress of leading examples.

EO9 (Apaziquone): from the clinic to the laboratory and back again

EO9 (Apaziquone) is a bioreductive drug that has a chequered history. It underwent clinical trial but failed to show activity in phase II clinical trials when administered i.v. Poor drug delivery to tumours caused by a combination of rapid pharmacokinetic elimination and poor penetration through avascular tissue were the major factors responsible for EO9's poor efficacy. Based upon an understanding of why EO9 failed, a further clinical trial against patients with superficial transitional cell carcinoma of the bladder was conducted. The rationale for this was that intravesical administration directly into the bladder would circumvent the drug delivery problem, and any drug reaching the blood supply would be rapidly cleared thereby reducing the risk of systemic exposure. EO9 was well tolerated, and clinical activity against marker lesions was recorded in both phase I and II clinical trials. This article charts the pharmacological history of EO9 and discusses the potential implications that 'the EO9 story' has for the development of other loco-regional therapies.

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.

Pharmacological inhibitors of NAD(P)H quinone oxidoreductase, NQO1: structure/activity relationships and functional activity in tumour cells

NAD(P)H quinone oxidoreductase (NQO1) has multiple functions in the cell including an ability to act as a detoxifying enzyme and as a protein chaperone. The latter property is particularly important in oncology as one of the client proteins of NQO1 is p53. The inhibitor, dicoumarol, is classically used to probe the biological properties of NQO1, but interpretation of enzyme function is compromised by the multiple "off-target" effects of this agent. Coumarin-based compounds that are more potent than dicoumarol as inhibitors of recombinant human NQO1 have been identified (Nolan et al., J Med Chem 2009;52:7142-56) The purpose of the work reported here is to demonstrate the functional activity of these agents for inhibiting NQO1 in cells. To do this, advantage was taken of the NQO1-mediated toxicity of the chemotherapeutic drug EO9 (Apaziquone). The toxicity of this drug is substantially reduced when the function of NQO1 is inhibited and many of the coumarin-based compounds are more efficient than dicoumarol for inhibiting EO9 toxicity. The ability to do this appears to be related to their capacity to inhibit NQO1 in cell free systems. In conclusion, agents have been identified that may be more pharmacologically useful than dicoumarol for probing the function of NQO1 in cells and tissues.

Synthesis, metabolism and in vitro cytotoxicity studies on novel lavendamycin antitumor agents

A series of lavendamycin analogues with two, three or four substituents at the C-6, C-7 N, C-2', C-3' and C-11' positions were synthesized via short and efficient methods and evaluated as potential NAD(P)H:quinone oxidoreductase (NQO1)-directed antitumor agents. The compounds were prepared through Pictet-Spengler condensation of the desired 2-formylquinoline-5,8-diones with the required tryptophans followed by further needed transformations. Metabolism and toxicity studies demonstrated that the best substrates for NQO1 were also the most selectively toxic to NQO1-rich tumor cells compared to NQO1-deficient tumor cells.

Design of anticancer prodrugs for reductive activation

Anticancer prodrugs designed to target specifically tumor cells should increase therapeutic effectiveness and decrease systemic side effects in the treatment of cancer. Over the last 20 years, significant advances have been made in the development of anticancer prodrugs through the incorporation of triggers for reductive activation. Reductively activated prodrugs have been designed to target hypoxic tumor tissues, which are known to overexpress several endogenous reductive enzymes. In addition, exogenous reductive enzymes can be delivered to tumor cells through fusion with tumor-specific antibodies or overexpressed in tumor cells through gene delivery approaches. Many anticancer prodrugs have been designed to use both the endogenous and exogenous reductive enzymes for target-specific activation and these prodrugs often contain functional groups such as quinones, nitroaromatics, N-oxides, and metal complexes. Although no new agents have been approved for clinical use, several reductively activated prodrugs are in various stages of clinical trial. This review mainly focuses on the medicinal chemistry aspects of various classes of reductively activated prodrugs including design principles, structure-activity relationships, and mechanisms of activation and release of active drug molecules.

Taking advantage of tumor cell adaptations to hypoxia for developing new tumor markers and treatment strategies

Cancer cells in hypoxic areas of solid tumors are to a large extent protected against the action of radiation as well as many chemotherapeutic drugs. There are, however, two different aspects of the problem caused by tumor hypoxia when cancer therapy is concerned: One is due to the chemical reactions that molecular oxygen enters into therapeutically targeted cells. This results in a direct chemical protection against therapy by the hypoxic microenvironment, which has little to do with cellular biological regulatory processes. This part of the protective effect of hypoxia has been known for more than half a century and has been studied extensively. However, in recent years there has been more focus on the other aspect of hypoxia, namely the effect of this microenvironmental condition on selecting cells with certain genetic prerequisites that are negative with respect to patient prognosis. There are adaptive mechanisms, where hypoxia induces regulatory cascades in cells resulting in a changed metabolism or changes in extracellular signaling. These processes may lead to changes in cellular intrinsic sensitivity to treatment irrespective of oxygenation and, furthermore, may also have consequences for tissue organization. Thus, the adaptive mechanisms induced by hypoxia itself may have a selective effect on cells, with a fine-tuned protection against damage and stress of many kinds. It therefore could be that the adaptive mechanisms may take advantage of for new tumor labeling/imaging and treatment strategies. One of the Achilles' heels of hypoxia research has always been the exact measurements of tissue oxygenation as well as the control of oxygenation in biological tumor models. Thus, development of technology that can ease this control is vital in order to study mechanisms and perform drug development under relevant conditions. An integrated EU Framework project 2004-2009, termed EUROXY, demonstrates several pathways involved in transcription and translation control of the hypoxic cell phenotype and evidence of cross-talk with responses to pH and redox changes. The carbonic anhydrase isoenzyme CA IX was selected for further studies due to its expression on the surface of many types of hypoxic tumors. The effort has led to marketable culture flasks with sensors and incubation equipment, and the synthesis of new drug candidates against new molecular targets. New labeling/imaging methods for cancer diagnosing and imaging of hypoxic cancer tissue are now being tested in xenograft models and are also in early clinical testing, while new potential anti-cancer drugs are undergoing tests using xenografted tumor cancers. The present article describes the above results in individual consortium partner presentations.

Arene cis-dihydrodiols: Useful precursors for the preparation of analogues of the anti-tumour agent, 2-crotonyloxymethyl-(4R,5R,6R)-4,5,6-trihydroxycyclohex-2-enone (COTC)

The synthesis of 6-epi-COTC, a diastereoisomer of Streptomyces metabolite 2-crotonyloxymethyl-(4R,5R,6R)-4,5,6-trihydroxycyclohex-2-enone (COTC), is described. The anti-cancer activities of the novel analogue, in racemic and enantiomerically pure forms, are presented.

Glut-1 as a therapeutic target: increased chemoresistance and HIF-1-independent link with cell turnover is revealed through COMPARE analysis and metabolomic studies

The facilitative glucose transporter Glut-1 is overexpressed and confers poor prognosis in a wide range of solid tumours. The peri-necrotic pattern of expression often seen in human tumour samples is linked with its transcriptional control in hypoxic conditions by hypoxia-inducible factor HIF-1 or through a reduced rate of oxidative phosphorylation. Hypoxia-regulated genes offer promise as novel therapeutic targets as a means of preventing the proliferation and eventual metastatic spread of tissue originating from residual chemically and radio resistant hypoxic cells that have survived treatment. Inhibiting the expression or functionality of Glut-1 may be a way of specifically targeting hypoxic cells within the tumour that depend upon a high rate of glucose uptake for anaerobic glycolysis. We used an array of formalin-fixed, paraffin-embedded samples of the NCI-60 panel of cell lines to carry out immunohistochemical detection of Glut-1 and to select possible candidate lead compounds by COMPARE analysis with agents from the NCI diversity screen, which may work via inhibition of Glut-1 or Glut-1-dependent processes. "Positive" COMPARE hits were mostly conjugated Pseudomonas toxins binding the epidermal growth factor receptor (EGFR). However, correlations with standard anticancer agents were virtually all negative, indicating a link between Glut-1 and chemoresistance. MTT proliferation assays carried out using stable, Glut-1 overexpressing cell lines generated from the bladder EJ138, human fibrosarcoma HT 1080 and the hepatoma wild type Hepa and HIF-1B-deficient c4 tumour cell lines revealed a cell line-dependent increase in chemoresistance to dacarbazine, vincristine and the bioreductive agent EO9 in Glut-1 overexpressing EJ138 relative to WT and empty vector controls. Metabolomic analysis ((31)P-MRS and (1)H MRS) carried out using cell lysates and xenografts generated from Glut-1 overexpressing Hepa and c4 cell lines showed higher glucose levels in Glut-1 overxpressing c4 relative to parental tumour extracts occurred in the absence of an increase in lactate levels, which were in turn significantly higher in the Glut-1 overexpressing Hepa xenografts. This implies that Glut-1 over-expression without a co-ordinate increase in HIF-1-regulated glycolytic enzymes increases glucose uptake but not the rate of glycolysis. Glut-1 overexpressing xenografts also showed higher levels of phosphodiester (PDE), which relates to the metabolite turnover of phospholipids and is involved in membrane lipid degradation, indicating a mechanism by which Glut-1 may increase cell turnover.

Bioreductive drugs: from concept to clinic

One of the key issues for radiobiologists is the importance of hypoxia to the radiotherapy response. This review addresses the reasons for this and primarily focuses on one aspect, the development of bioreductive drugs that are specifically designed to target hypoxic tumour cells. Four classes of compound have been developed since this concept was first proposed: quinones, nitroaromatics, aliphatic and heteroaromatic N-oxides. All share two characteristics: (1) they require hypoxia for activation and (2) this activation is dependent on the presence of specific reductases. The most effective compounds have shown the ability to enhance the anti-tumour efficacy of agents that kill better-oxygenated cells, i.e. radiation and standard cytotoxic chemotherapy agents such as cisplatin and cyclophosphamide. Tirapazamine (TPZ) is the most widely studied of the lead compounds. After successful pre-clinical in vivo combination studies it entered clinical trial; over 20 trials have now been reported. Although TPZ has enhanced some standard regimens, the results are variable and in some combinations toxicity was enhanced. Banoxantrone (AQ4N) is another agent that is showing promise in early phase I/II clinical trials; the drug is well tolerated, is known to locate in the tumour and can be given in high doses without major toxicities. Mitomycin C (MMC), which shows some bioreductive activation in vitro, has been tested in combination trials. However, it is difficult to assign the enhancement of its effects to targeting of the hypoxic cells because of the significant level of its hypoxia-independent toxicity. More specific analogues of MMC, e.g. porfiromycin and apaziquone (EO9), have had variable success in the clinic. Other new drugs that have good pre-clinical profiles are PR 104 and NLCQ-1; data on their clinical safety/efficacy are not yet available. This paper reviews the pre-clinical data and discusses the clinical studies that have been reported.

Analogues of 2-crotonyloxymethyl-(4R,5R,6R)-4,5,6-trihydroxycyclohex-2-enone (COTC) with anti-tumor properties

The syntheses of three novel analogues of the naturally occurring cytotoxic agent COTC are described and the results of bioassays of the target compounds against two lung cancer cell lines are presented.

Phase I/II Pilot Study of Intravesical Apaziquone (EO9) for Superficial Bladder Cancer

PURPOSE

The quinone based bioreductive drug apaziquone (EO9) failed to demonstrate efficacy in previous phase II studies following intravenous administration. We determined the dose of apaziquone that can be safely administered intravesically and explored its activity for superficial bladder transitional cell carcinoma.

MATERIALS AND METHODS

Six patients with multifocal, Ta/T1 and G1/G2 transitional cell carcinoma of the bladder received escalating doses of apaziquone formulated as EOquintrade mark (0.5 mg/40 ml up to 16 mg/40 ml) weekly for 6 weeks. A further 6 patients received weekly apaziquone at the highest nontoxic dose established. Pharmacokinetic parameters were determined in urine and blood, and the pharmacodynamic markers NQO1 (reduced nicotinamide adenine dinucleotide phosphate:quinone oxidoreductase-1) and glucose transporter 1 were also characterized. Efficacy was determined against a marker lesion.

RESULTS

Local toxicity (grades 2 and 3 dysuria, and hematuria) was observed at doses of 8 mg/40 ml and above but 4 mg/40 ml was well tolerated with no systemic or local side effects. Apaziquone in urine increased linearly with the dose but no apaziquone was detected in plasma. In 8 of 12 patients complete macroscopic and histological disappearance of the marker lesion occurred. A correlation between response and NQO1 and/or glucose transporter 1 expression could not be established.

CONCLUSIONS

Intravesical administration of 4 mg/40 ml apaziquone was well tolerated and had ablative activity against superficial bladder cancer marker lesions.

Development of beta-lapachone prodrugs for therapy against human cancer cells with elevated NAD(P)H:quinone oxidoreductase 1 levels

beta-Lapachone, an o-naphthoquinone, induces a novel caspase- and p53-independent apoptotic pathway dependent on NAD(P)H:quinone oxidoreductase 1 (NQO1). NQO1 reduces beta-lapachone to an unstable hydroquinone that rapidly undergoes a two-step oxidation back to the parent compound, perpetuating a futile redox cycle. A deficiency or inhibition of NQO1 rendered cells resistant to beta-lapachone. Thus, beta-lapachone has great potential for the treatment of specific cancers with elevated NQO1 levels (e.g., breast, non-small cell lung, pancreatic, colon, and prostate cancers). We report the development of mono(arylimino) derivatives of beta-lapachone as potential prodrugs. These derivatives are relatively nontoxic and not substrates for NQO1 when initially diluted in water. In solution, however, they undergo hydrolytic conversion to beta-lapachone at rates dependent on the electron-withdrawing strength of their substituent groups and pH of the diluent. NQO1 enzyme assays, UV-visible spectrophotometry, high-performance liquid chromatography-electrospray ionization-mass spectrometry, and nuclear magnetic resonance analyses confirmed and monitored conversion of each derivative to beta-lapachone. Once converted, beta-lapachone derivatives caused NQO1-dependent, mu-calpain-mediated cell death in human cancer cells identical to that caused by beta-lapachone. Interestingly, coadministration of N-acetyl-l-cysteine, prevented derivative-induced cytotoxicity but did not affect beta-lapachone lethality. Nuclear magnetic resonance analyses indicated that prevention of beta-lapachone derivative cytotoxicity was the result of direct modification of these derivatives by N-acetyl-l-cysteine, preventing their conversion to beta-lapachone. The use of beta-lapachone mono(arylimino) prodrug derivatives, or more specifically a derivative converted in a tumor-specific manner (i.e., in the acidic local environment of the tumor tissue), should reduce normal tissue toxicity while eliciting tumor-selective cell killing by NQO1 bioactivation.

Prospects for bioreductive drug development

Bioreductive drugs are inactive prodrugs that are converted into potent cytotoxins under conditions of either low oxygen tension or in the presence of high levels of specific reductases. The biochemical basis for selectivity relies on the ability of oxygen to reverse the activation process and the presence of elevated reductase levels in some tumour types. Key criteria for an ideal bioreductive drug should include poor activity against aerobic cells, activation over a broad range of oxygen tensions and, penetration through the aerobic fraction of cells. In addition, the active drug should be capable of killing non-proliferating cells. Numerous compounds are currently at various stages of drug development but Mitomycin C, which is generally considered to be the prototype bioreductive drug, is the only one in clinical use today. Of the drugs currently being evaluated clinically, tirapazamine has definite clinical activity against a variety of solid tumours when used in combination with cisplatin. Other drugs, such as EO9 and various nitroimidazoles, have not been impressive in the clinic and further development is required to improve properties such as drug delivery in the case of indoloquinones. A novel approach to exploiting tumour hypoxia is the development of a gene-directed enzyme prodrug therapy (GDEPT) strategy, where a gene encoding for a prodrug activating enzyme has been placed under the control of a hypoxia responsive promoter sequence. It is generally recognised that bioreductive drugs must be directed towards patients whose tumours have hypoxic regions or have appropriate enzymological characteristics. In terms of identifying tumour hypoxia, there has been considerable progress in the development of nitroimidazole based hypoxia markers that can be detected either via non-invasive or invasive procedures. Another strategy currently undergoing preclinical evaluation is the use of agents that modulate tumour blood flow and synergistic effects have been reported between bioreductive drugs and photodynamic therapy or inhibitors of nitric oxide synthase for example. The development of clinically useful bioreductive drugs depends therefore on the expertise of scientists and clinicians with varying backgrounds. The purpose of this review is to describe and critically assess recent developments in this field, with particular emphasis being placed on drug development and strategies aimed at optimising bioreductive drug activity.

Pharmacological and biological evaluation of a series of substituted 1,4-naphthoquinone bioreductive drugs

The indolequinone compound EO9 has good pharmacodynamic properties in terms of bioreductive activation and selectivity for either NAD(P)H:quinone oxidoreductase-1 (NQO1)-rich aerobic or NQO1-deficient hypoxic cells. However, its pharmacokinetic properties are poor and this fact is believed to be a major reason for EO9's lack of clinical efficacy. The purpose of this study was to develop quinone-based bioreductive drugs that retained EO9's good properties, in terms of bioreductive activation, but have improved pharmacokinetic properties. Out of 11 naphthoquinone compounds evaluated, 2-aziridinyl-5-hydroxy-1,4-naphthoquinone (compound 2), 2,3-bis(aziridinyl)-5-hydroxy-1,4-naphthoquinone (compound 3), and 2-aziridinyl-6-hydroxymethyl-1,4-naphthoquinone (compound 11) were selected for further evaluation based on good substrate specificity for NQO1 and selectivity towards NQO1-rich cells in vitro. Compound 3 was of particular interest as it also demonstrated selectivity for NQO1-rich cells under hypoxic conditions. Compound 3 was not metabolised by murine whole blood in vitro (in contrast to compounds 2, 11 and EO9) and pharmacokinetic studies in non-tumour-bearing mice in vivo (at the maximum soluble dose of 60 mg kg(-1) administered intraperitoneally) demonstrated significant improvements in plasma half-life (16.2 min) and AUC values (22.5 microM h) compared to EO9 (T(1/2) = 1.8 min, AUC = 0.184 microM h). Compound 3 also demonstrated significant anti-tumour activity against H460 and HCT-116 human tumour xenografts in vivo, whereas EO9 was inactive against these tumours. In conclusion, compound 3 is a promising lead compound that may target both aerobic and hypoxic fractions of NQO1-rich tumours and further studies to elucidate its mechanism of action and improve solubility are warranted.

Potential mechanism of phytochemical-induced apoptosis in human prostate adenocarcinoma cells: Therapeutic synergy in genistein and beta-lapachone combination treatment

BACKGROUND: Prostate cancer is the second leading cause of male death in the United States. The incidence increases most rapidly with age, and multiple genetic and epigenetic factors have been implicated in the initiation, progression, and metastasis of the cancer. Nevertheless, scientific knowledge of the molecular mechanisms underlying the disease is still limited; and hence treatment has only been partially successful. The objective of the current studies was to examine the role of caspase 3 (CPP32) and NAD(P)H:quinone oxidoreductase (NQO1) in the signaling of genistein-and beta-lapachone (bLap)-induced apoptosis in human prostate carcinoma cells PC3. RESULTS: Both genistein and bLap produced dose-dependent growth inhibition and treatment-induced apoptosis in PC3. Treatment with caspase 3 inhibitor, DEVD-fmk before exposure to genistein, significantly inhibited caspase 3 expression and treatment-induced apoptosis; implicating CPP32 as the main target in genistein-induced apoptosis in PC3. Contrary to this observation, inhibition of CPP32 did not significantly influence bLap-induced apoptosis; implying that the major target of bLap-induced apoptosis may not be the caspase. Treatment with NQO1 inhibitor, dicoumarol (50 microM), prior to exposure of PC3 to bLap led to significant decrease in bLap toxicity concurrent with significant decrease in treatment-induced apoptosis; thus implicating NQO1 as the major target in beta-lapachone-induced apoptosis in PC3. In addition, the data demonstrated that NQO1 is the major target in bLap-genistein (combination)-induced apoptosis. On the contrary, blocking NQO1 activity did not significantly affect genistein-induced apoptosis; implying that NQO1 pathway may not be the main target for genistein-induced apoptosis in PC3 cells. Furthermore, blocking NQO1 and CPP32 did not confer 100% protection against genistein-induced or bLap-induced apoptosis. CONCLUSION: The data thus demonstrate that both genistein-and bLap-induced apoptosis are mostly but not completely dependent on CPP32 and NQO1 respectively. Other minor alternate death pathways may be involved. This suggests that some death receptor signals do not utilize the caspase CPP32 and/or the NQO1 death pathways in PC3. The demonstrated synergism between genistein and bLap justifies consideration of these phytochemicals in chemotherapeutic strategic planning.

Novel quinolinequinone antitumor agents: structure-metabolism studies with NAD(P)H:quinone oxidoreductase (NQO1)

A series of quinolinequinones bearing various substituents has been synthesized, and the effects of substituents on the metabolism of the quinones by recombinant human NAD(P)H:quinone oxidoreductase (hNQO1) was studied. A range of quinolinequinones were selected for study, and were specifically designed to probe the effects of aryl substituents at C-2. A range of 28 quinolinequinones 2-29 was prepared using three general strategies: the palladium(0) catalyzed coupling of 2-chloroquinolines, the classical Friedländer synthesis and the double-Vilsmeier reaction of acetanilides. One example of an isoquinolinequinone 30 was also prepared, and the reduction potentials of the quinones were measured by cyclic voltammetry. For simple substituents R(2) at the quinoline 2-position, the rates of quinone metabolism by hNQO1 decrease for R(2)=Cl>H approximately Me>Ph. For aromatic substituents, the rate of reduction decreases dramatically for R(2)=Ph>1-naphthyl>2-naphthyl>4-biphenyl. Compounds containing a pyridine substituent are the best substrates, and the rates decrease as R(2)=4-pyridyl>3-pyridyl>2-pyridyl>4-methyl-2-pyridyl>5-methyl-2-pyridyl. The toxicity toward human colon carcinoma cells with either no detectable activity (H596 or BE-WT) or high NQO1 activity (H460 or BE-NQ) was also studied in representative quinones. Quinones that are good substrates for hNQO1 are more toxic to the NQO1 containing or expressing cell lines (H460 and BE-NQ) than the NQO1 deficient cell lines (H596 and BE-WT).

Cytotoxicity of the bioreductive agent RH1 and its lack of interaction with radiation

BACKGROUND AND PURPOSE

RH1 is a new bioreductive agent that was developed as a cytotoxic agent with selectivity for tumour cells expressing high levels of the enzyme DT-diaphorase (DTD). The aim of the present study was to investigate the cytotoxicity of RH1 in relation to cellular levels of reducing enzymes and any interaction of RH1 with ionizing radiation under oxic and hypoxic conditions.

PATIENTS AND METHODS

The MB-MDA231 human breast cancer cell line (WT) and WT cells transfected with the NQO1 gene encoding DTD (the D7 cell line) were used to examine the dependency of RH1's cytotoxicity on cellular DTD activity. The role of the 1-electron reducing enzyme P450 reductase was also studied using a P450 reductase-transfected isogenic cell line (R4). A clonogenic assay was used to investigate the cytotoxicity of RH1 with and without irradiation in air and in nitrogen. In all cases drug exposure was for 3 h.

RESULTS

DTD levels were around 300-fold higher in D7 compared to WT and R4 cells. RH1 was cytotoxic at nanomolar concentrations to all the cell lines, and was 2-3 times more toxic in the D7 cells with high DTD than in the other two cell lines. Doses of RH1 was around 2-fold more effective in hypoxic than in oxic WT cells, but not by as much in D7 cells. RH1 did not radiosensitise the cells but showed an additive effect when combined with irradiation under oxic and hypoxic conditions.

CONCLUSIONS

RH1 shows high clonogenic cytotoxicity to MDA231 cells with high DTD activity but its selectivity based on the presence of DTD is much less than as shown in previous reports. RH1 showed an additive cell killing effect when combined with irradiation under both oxic and hypoxic conditions.

The importance of DT-diaphorase and hypoxia in the cytotoxicity of RH1 in human breast and non-small cell lung cancer cell lines

The diaziridiny/benzoquinone RH1 is shortly to enter a phase I clinical trial. The drug was originally designed as a substrate for the enzyme DT-diaphorase (DTD) such that metabolic activation of the drug would lead to toxicity. To evaluate this, we have measured the toxicity of RH1 in a pair of non-small cell lung cancer (NSCLC) cell lines of widely differing levels of DTD and in MDA231 breast cancer cells which have been engineered to overexpress DTD. In addition, we have explored the importance of the putative one-electron reductase, P450 reductase, by assessing the toxicity of RH1 in MDA231 cells engineered to overexpress the enzyme. All drug exposures were carried out under hypoxic and aerobic conditions. Those cells with the highest levels of DTD, i.e. D7 versus MDA231 wt and H460 versus H596, are substantially more sensitive to RH1 than the cell lines expressing low DTD activity. Those cells with the lowest levels of DTD activity, i.e. MDA231 wt, R4 and H596, show much greater sensitivity to RH1 under hypoxic conditions compared to aerobic conditions. Finally, overexpression of P450 reductase, i.e. comparing MDA231 wt with R4, has little, if any, impact on the toxicity of RH1 under hypoxic or aerobic conditions. In summary, RH1 can be effective in killing cells containing high levels of DTD and may be useful in treating tumors expressing this enzyme.

3-Substituted-5-aziridinyl-1-methylindole-4,7-diones as NQO1-directed antitumour agents: mechanism of activation and cytotoxicity in vitro

Indolequinone agents are a unique class of bioreductive cytotoxins that can function as dual substrates for both one- and two-electron reductases. This endows them with the potential to be either hypoxia-selective cytotoxins or NAD(P)H:quinone oxidoreductase 1 (NQO1)-directed prodrugs, respectively. We have studied the structure-activity relationships of four novel indolequinone analogues with regard to one- and/or two-electron activation. Single-electron metabolism was achieved by exposing the human carcinoma cell line T47D to each agent under hypoxic conditions, whilst concerted two-electron metabolism was assessed by stably expressing the cDNA for human NQO1 in a cloned cell line of T47D. The C-3 and C-5 positions of the indolequinone nucleus were modified to manipulate reactivity of the reduction products and the four prodrugs were identified as NQO1 substrates of varying specificity. Two of the four prodrugs, in which both C-3 and C-5 groups remained functional, proved to be NQO1-directed cytotoxins with selectivity ratios of 60- to 80-fold in the T47D (WT) versus the NQO1 overexpressing T47D cells. They also retained selectivity as hypoxic cytotoxins with oxic/hypoxic ratios of 20- to 22-fold. Replacement of the C-3 hydroxymethyl leaving group with an aldehyde group ablated all selectivity in air and hypoxia in both cell lines. Addition of a 2-methyl group on the C-5 aziridinyl group to introduce steric hinderance reduced but did not abolish NQO1-dependent metabolism. However, it enhanced single-electron metabolism-dependent DNA cross-linking in a manner that was independent of cytotoxicity. These data demonstrate that subtle structure-activity relationship exists for different cellular reductases and under certain circumstances distinct forms of DNA damage can arise, the cytotoxic consequences of which can vary. This study identifies a candidate indolequinone analogue for further development as a dual hypoxia and NQO1-directed prodrug.

Combining bioreductive drugs and radiation for the treatment of solid tumors

Methods now exist for the identification of human tumors that contain significant numbers of hypoxic cells and are thereby suitable for treatment with bioreductive drugs to eliminate this refractory cell population. However, to fully exploit the potential of bioreductive drugs, they will need to be used in combination with other modalities likely to target the proliferating aerobic cells in the tumor. Radiation is the treatment that is most effective in killing aerobic cells; therefore, the present report reviews the available preclinical data on combined radiation/bioreductive drug treatments.

Pharmacological approach towards the development of indolequinone bioreductive drugs based on the clinically inactive agent EO9

The bioreductive drug EO9 (3-hydroxy-5-aziridinyl-1-methyl-2[indole-4,7-dione]-prop-beta-en-alpha-ol) has good pharmacodynamic properties in vitro, modest anti-tumour activity in experimental tumour models, but failed to show activity in clinical trials. Understanding the reasons for its poor efficacy in vivo is important in terms of progressing second generation analogues into the clinic. In two human tumour xenografts, direct intra-tumoural injection resulted in improved anti-tumour activity compared with intravenous administration suggesting that drug delivery to tumours is suboptimal. Compared with Mitomycin C (MMC) and the experimental agent MeDZQ, EO9 was rapidly cleared from the systemic circulation (t1/2=1.8 min) whereas MMC and MeDZQ had significantly increased plasma t1/2 values (14 and 22 min respectively). These three compounds demonstrated similar pharmacodynamic properties in terms of potency towards the NQO1 (NAD(P)H:Quinone oxidoreductase) rich H460 cell line in vitro but differed significantly in their in vivo activity with growth delays of 17.7, 4.5 and 1.0 days for MMC, MeDZQ and EO9 respectively. EO9 was rapidly metabolized by red blood cells in vitro (t1/2=14.5 min) which must contribute to its rapid pharmacokinetic elimination in vivo whereas MMC and MeDZQ were metabolized at comparatively slower rates (t1/2>120 min and 77.0 min respectively). In conclusion, the development of second generation EO9 analogues should address the issue of drug delivery and analysis of drug metabolism by murine whole blood in vitro could be utilized as a preliminary screen to identify lead compounds that are likely to have improved pharmacokinetic profiles in vivo.

A novel strategy for NQO1 (NAD(P)H:quinone oxidoreductase, EC 1.6.99.2) mediated therapy of bladder cancer based on the pharmacological properties of EO9

The indolequinone EO9 demonstrated good preclinical activity but failed to show clinical efficacy against a range of tumours following intravenous drug administration. A significant factor in EO9's failure in the clinic has been attributed to its rapid pharmacokinetic elimination resulting in poor drug delivery to tumours. Intravesical administration of EO9 would circumvent the problem of drug delivery to tumours and the principal objective of this study is to determine whether or not bladder tumours have elevated levels of the enzyme NQO1 (NAD(P)H:quinone oxidoreductase) which plays a key role in activating EO9 under aerobic conditions. Elevated NQO1 levels in human bladder tumour tissue exist in a subset of patients as measured by both immunohistochemical and enzymatic assays. In a panel of human tumour cell lines, EO9 is selectively toxic towards NQO1 rich cell lines under aerobic conditions and potency can be enhanced by reducing extracellular pH. These studies suggest that a subset of bladder cancer patients exist whose tumours possess the appropriate biochemical machinery required to activate EO9. Administration of EO9 in an acidic vehicle could be employed to reduce possible systemic toxicity as any drug absorbed into the blood stream would become relatively inactive due to an increase in pH.

Indolequinone antitumor agents: correlation between quinone structure and rate of metabolism by recombinant human NAD(P)H:quinone oxidoreductase. Part 2

A series of indolequinones bearing various functional groups has been synthesized, and the effects of substituents on the metabolism of the quinones by recombinant human NAD(P)H:quinone oxidoreductase (NQO1) were studied. Indolequinones were selected for study on the basis of the X-ray crystal structure of the human enzyme, and were designed to probe the effect of substituents particularly at N-1. Metabolism of the quinones by NQO1 revealed that, in general, compounds with electron-withdrawing groups at the indole 3-position were among the best substrates, and that groups larger than methyl at N-1 are clearly tolerated. Compounds with a leaving group at the 3-indolyl methyl position generally inactivated the enzyme. The toxicity toward human colon carcinoma cells with either no detectable activity (BE-WT) or high NQO1 activity (BE-NQ) was also studied in representative quinones. The most toxic compounds were those with a leaving group at the C-3 position; these compounds were 1.1-5.3-fold more toxic to the BE-NQ than the BE-WT cells.

Involvement of NADPH: cytochrome P450 reductase in the activation of indoloquinone EO9 to free radical and DNA damaging species

Evidence suggests that DT-diaphorase is involved in the activation and mechanism of cytotoxicity of the investigational indoloquinone anticancer drug EO9 under aerobic conditions. Data also implicate a role for other enzymes including NADPH: cytochrome P450 reductase, especially in low DT-diaphorase tumour cells and under hypoxic conditions. Here, we used purified rat NADPH: cytochrome P450 reductase to provide additional evidence in support of a role for this enzyme in activation of EO9 to generate free radical and DNA-damaging species. Electron spin resonance spectrometry studies showed that NADPH: cytochrome P450 reductase reduced EO9 to a free radical species, including a drug radical (most likely the semiquinone) and reactive oxygen species. Plasmid DNA experiments showed that reduction of EO9 catalysed by NADPH: cytochrome P450 reductase results in single-strand breaks in DNA. The information obtained may contribute to the understanding of the molecular mechanism of DNA damage and cytotoxicity exerted by EO9 and may be useful in the design of future bioreductive drugs.

The role of NAD(P)H oxidoreductase (DT-Diaphorase) in the bioactivation of quinone-containing antitumor agents: a review

Bioactivation of quinone-containing anticancer agents has been studied extensively within the context of the chemistry and structure of the individual quinones which may result in various mechanisms of bioactivation and activity. In this review we focus on the two electron enzymatic reduction/activation of quinone-containing anticancer agents by DT Diaphorase (DTD). This enzyme has become important in oncopharmacology because its activity varies with tissues and it has been found to be elevated in tumors. Thus, a selective tumor cell kill can exist for agents that are good substrates for this enzyme. In addition, the enzyme can be induced by a variety of agents, a fact that can be used in chemotherapy. That is induction by a nontoxic agent followed by treatment with a good DT-Diaphorase substrate. A wide variety of anticancer drugs are discussed some of which are not good substrates such as Adriamycin, and some of which are excellent substrates. The latter category includes a variety of quinone containing alkylating agents.

The relative importance of NADPH: cytochrome c (P450) reductase for determining the sensitivity of human tumour cells to the indolequinone EO9 and related analogues lacking functionality at the C-2 and C-3 positions

Analogues of EO9 (3-hydroxymethyl-5-aziridinyl-1-methyl-2[1H-indole-4-7-dione]prop-2-e n-1-ol) which lack functionality at either the C-2 or C-3 position were synthesised. The aim was to establish the importance of each group towards toxicity and to give an indication as to whether substitution at either position altered activation and toxicity after metabolism by cellular NADPH: cytochrome c (P450) reductase (P450R). MDA231 breast cancer cells were transfected with the cDNA for human P450R and stable clones were isolated. These high P450R-expressing clones were used to determine the aerobic and hypoxic toxicity of EO9 and the two analogues that lacked functionality at either C-2 or C-3. The results showed that P450R was strongly implicated in the bioactivation of EO9 and its analogues under both of these conditions. This data also showed that the C-3 functionality was primarily implicated in hypoxic toxicity.

Pharmacological properties of a new aziridinylbenzoquinone, RH1 (2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone), in mice

RH1 (2,5-diaziridinyl-3-(hydroxylmethyl)-6-methyl-1,4-benzoquinone) has shown preferential activity against human tumour cell lines which express high levels of DTD (EC 1.6.99.2; NAD(P)H:quinone oxidoreductase, NQO1, DT-diaphorase) and is a candidate for clinical trials. EO9 (3-hydroxy-5-aziridinyl-1-methyl-2-[1H indole-4,7-dione]prop-beta-en-alpha-ol) is a known substrate for DTD but clinical trials were disappointing, as a result of rapid plasma clearance and reversible dose-limiting kidney toxicity. It is an obvious concern that RH1 does not exhibit the same limitations. We therefore describe the antitumour activity and pharmacology of RH1 in mice and compare its pharmacological characteristics to those of EO9. Significant antitumour activity (P = 0.01) was seen for RH1 (0.5 mg/kg, i.p.) against the high DTD-expressing H460 human lung carcinoma. Pharmacokinetic analysis of RH1 in mice showed a t1/2 of 23 min with an area under the curve of 43.0 ng hr mL(-1) resulting in a calculated clearance of 5.1 mL min(-1), 10-fold slower than EO9. RH1 was also more stable than EO9 in murine blood, where the breakdown was thought to be DTD-related. NADH-dependent microsomal metabolism of RH1 and EO9 in both liver and kidney was slow (<100 pmol/min/g tissue), reflecting the low microsomal DTD expression (<35 nmol/mg/min). Liver cytosol metabolism was rapid for both compounds (>4500 pmol/min/g tissue), although DTD levels were low (21.4+/-0.6 nmol/mg/min). DTD activity in the kidney cytosol was high (125+/-8.2 nmol/mg/min) and EO9 was rapidly metabolised (4396+/-1678 pmol/min/g), but the metabolic rate for RH1 was 7-fold slower (608+/-86 pmol/min/g), even though RH1 was shown to be an excellent substrate for DTD (Vmax = 800 micromol/min/mg and a Km of 11.8 microM). The two DTD substrates RH1 and EO9 are clearly metabolised differently, suggesting that RH1 may have different pharmacological properties to those of EO9 in the clinic.

NAD(P)H:Quinone oxidoreductase activity is the principal determinant of beta-lapachone cytotoxicity

beta-Lapachone activates a novel apoptotic response in a number of cell lines. We demonstrate that the enzyme NAD(P)H:quinone oxidoreductase (NQO1) substantially enhances the toxicity of beta-lapachone. NQO1 expression directly correlated with sensitivity to a 4-h pulse of beta-lapachone in a panel of breast cancer cell lines, and the NQO1 inhibitor, dicoumarol, significantly protected NQO1-expressing cells from all aspects of beta-lapachone toxicity. Stable transfection of the NQO1-deficient cell line, MDA-MB-468, with an NQO1 expression plasmid increased apoptotic responses and lethality after beta-lapachone exposure. Dicoumarol blocked both the apoptotic responses and lethality. Biochemical studies suggest that reduction of beta-lapachone by NQO1 leads to a futile cycling between the quinone and hydroquinone forms, with a concomitant loss of reduced NAD(P)H. In addition, the activation of a cysteine protease, which has characteristics consistent with the neutral calcium-dependent protease, calpain, is observed after beta-lapachone treatment. This is the first definitive elucidation of an intracellular target for beta-lapachone in tumor cells. NQO1 could be exploited for gene therapy, radiotherapy, and/or chemopreventive interventions, since the enzyme is elevated in a number of tumor types (i.e. breast and lung) and during neoplastic transformation.

NADPH:cytochrome c (P450) reductase activates tirapazamine (SR4233) to restore hypoxic and oxic cytotoxicity in an aerobic resistant derivative of the A549 lung cancer cell line

Tirapazamine (TPZ, SR4233, WIN 59075) is a bioreductive drug that is activated in regions of low oxygen tension to a cytotoxic radical intermediate. This labile metabolite shows high selective toxicity towards hypoxic cells, such as those found in solid tumours. Under aerobic conditions, redox cycling occurs with subsequent generation of superoxide radicals, which are also cytotoxic. NADPH:cytochrome c (P450) reductase (P450R) is a one-electron reducing enzyme that efficiently activates TPZ. Recently a derivative of the A549 non-small cell lung cancer cell line (A549c50) was generated that showed substantially reduced P450R activity compared to its parental line (Elwell et al (1997) Biochem Pharmacol 54: 249-257). Here, it is demonstrated that the A549c50 cells are markedly more resistant to TPZ under both aerobic and hypoxic conditions. In addition, these cells have a dramatically impaired ability to metabolize TPZ to its two-electron reduction product, SR4317, under hypoxic conditions when compared to wild-type cells. P450R activity in the A549c50 cells was reintroduced to similar levels as that seen in the parental A549 cells by transfection of the full-length cDNA for human P450R. These P450R over-expressing cells exhibit restored sensitivity to TPZ under both aerobic and hypoxic conditions, comparable to that found in the original parental A549 cells. Further, the ability of the transfected cells to metabolize TPZ to SR4317 under hypoxic conditions is also shown to be restored. This provides further evidence that P450R can play an important role in the activation, metabolism and toxicity of this lead bioreductive drug.

Does reductive metabolism predict response to tirapazamine (SR 4233) in human non-small-cell lung cancer cell lines?

The bioreductive drug tirapazamine (TPZ, SR 4233, WIN 59075) is a lead compound in a series of potent cytotoxins that selectively kill hypoxic rodent and human solid tumour cells in vitro and in vivo. Phases II and III trials have demonstrated its efficacy in combination with both fractionated radiotherapy and some chemotherapy. We have evaluated the generality of an enzyme-directed approach to TPZ toxicity by examining the importance of the one-electron reducing enzyme NADPH:cytochrome P450 reductase (P450R) in the metabolism and toxicity of this lead prodrug in a panel of seven human non-small-cell lung cancer cell lines. We relate our findings on TPZ sensitivity in these lung lines with our previously published results on TPZ sensitivity in six human breast cancer cell lines (Patterson et al (1995) Br J Cancer 72: 1144-1150) and with the sensitivity of all these cell types to eight unrelated cancer chemotherapeutic agents with diverse modes of action. Our results demonstrate that P450R plays a significant role in the activation of TPZ in this panel of lung lines, which is consistent with previous observations in a panel of breast cancer cell lines (Patterson et al (1995) Br J Cancer 72: 1144-1150; Patterson et al (1997) Br J Cancer 76: 1338-1347). However, in the lung lines it is likely that it is the inherent ability of these cells to respond to multiple forms of DNA damage, including that arising from P450R-dependent TPZ metabolism, that underlies the ultimate expression of toxicity.

Bioreductive activation of a series of indolequinones by human DT-diaphorase: structure-activity relationships

A series of indolequinones including derivatives of EO9 bearing various functional groups and related indole-2-carboxamides have been studied with a view to identifying molecular features which confer substrate specificity for purified human NAD(P)H:quinone oxidoreductase (DT-diaphorase), bioreductive activation to DNA-damaging species, and selectivity for DT-diaphorase-rich cells in vitro. A broad spectrum of substrate specificity exists, but minor changes to the indolequinone nucleus have a significant effect upon substrate specificity. Modifications at the 2-position are favorable in terms of substrate specificity as these positions are located at the binding site entrance as determined by molecular modeling studies. In contrast, substitutions at the (indol-3-yl)methyl position with bulky leaving groups or a group containing a chlorine atom result in compounds which are poor substrates, some of which inactivate DT-diaphorase. Modeling studies demonstrate that these groups sit close to the mechanistically important amino acids Tyr 156 and His 162 possibly resulting in either alkylation within the active site or disruption of charge-relay mechanisms. An aziridinyl group at the 5-position is essential for potency and selectivity to DT-diaphorase-rich cells under aerobic conditions. The most efficient substrates induced qualitatively greater single-strand DNA breaks in cell-free assays via a redox mechanism involving the production of hydrogen peroxide (catalase inhibitable). This damage is unlikely to form a major part of their mechanism of action in cells since potency does not correlate with extent of DNA damage. In terms of hypoxia selectivity, modifications at the 3-position generate compounds which are poor substrates for DT-diaphorase but have high hypoxic cytotoxicity ratios.

Immune response to haptenized tumor antigen as possible mechanism of anticancer action of hypoxic bioreductive agents at low doses

Possible anticancer effect mechanism of hypoxic bioreductive agents (HBA) at low nongenotoxic doses are reviewed. Experimental and clinical investigations show the process which can develop step-by-step when injecting HBA into the tumor-bearing organism resulting in the stimulation of antitumor immunity: HBA activation in hypoxic tumor tissue, conjugation of activated HBA with proteins of tumor cells, antigen processing, presentation of neoantigen epitops in association with major histocompatibility complex-I and cytolysis of these tumor cells by T-killers. The present process can be a variant of the delayed-type hypersensitivity reaction in the tumor region. The direct regulating influence of HBA on immunocompetent and/or tumor cells as a result of interaction of these drugs with superficial or intercellular receptors is supposed to be realized additively with this process. It is concluded that the ability of HBA to selective activation in tumor tissue and formation of immunogenic conjugates with tumor proteins can be a starting-point for developing drugs with immuno-modulation anticancer properties.

Indolequinone antitumor agents: correlation between quinone structure, rate of metabolism by recombinant human NAD(P)H:quinone oxidoreductase, and in vitro cytotoxicity

A series of indolequinones bearing various functional groups has been synthesized, and the effects of substituents on the metabolism of the quinones by recombinant human NAD(P)H:quinone oxidoreductase (NQO1) were studied. Thus 5-methoxyindolequinones were prepared by the Nenitzescu reaction, followed by functional group interconversions. The methoxy group was subsequently displaced by amine nucleophiles to give a series of amine-substituted quinones. Metabolism of the quinones by NQO1 revealed that, in general, compounds with electron-withdrawing groups at the indole 3-position were among the best substrates, whereas those with amine groups at the 5-position were poor substrates. Compounds with a leaving group at the 3-indolyl methyl position generally inactivated the enzyme. The toxicity toward non-small-cell lung cancer cells with either high NQO1 activity (H460) or no detectable activity (H596) was also studied in representative quinones. Compounds which were good substrates for NQO1 showed the highest selectivity between the two cell lines.

Reduction of the indoloquinone anticancer drug EO9 by purified DT-diaphorase: a detailed kinetic study and analysis of metabolites

DT-diaphorase has been implicated in the activation and mechanism of cytotoxicity of the investigational indoloquinone anticancer drug EO9. Here, we have used a highly purified DT-diaphorase isolated from rat Walker tumour cells to provide unambiguous evidence for the ability of this enzyme to catalyze reduction of EO9 and to provide a more detailed characterization of the reaction. Under the conditions used hypoxia had no effect on the initial rate of this reduction but did effect the nature and stability of metabolites formed. Electron spin resonance (ESR) spectrometry studies showed that DT-diaphorase reduced EO9 to a highly oxygen-sensitive metabolite that is probably the hydroquinone. In the presence of air, this metabolite is auto-oxidized to generate both drug- and oxygen-based radicals. Comproportionation:disproportionation reactions may also be involved in the generation of these radical species. The identification of these metabolites may contribute to the understanding of the molecular mechanism of DNA damage and cytotoxicity exerted by EO9.

Evaluation of a novel in vitro assay for assessing drug penetration into avascular regions of tumours

The poor blood supply to solid tumours introduces many factors that affect the outcome of chemotherapy, one of which is the problem of drug delivery to poorly vascularized regions of tumours. Whereas poor drug penetration has been recognized as a contributing factor to the poor response of many solid tumours, the question of drug penetration through multicell layers has not been thoroughly addressed, largely because of restrictions imposed upon these studies by the requirement for either radiolabelled or naturally fluorescent compounds. The aim of this study is to describe modifications made to a recently published assay that broadens the scope for assessing drug penetration during the early stages of drug development and to characterize the ability of various drugs to penetrate multicell layers. DLD-1 human colon carcinoma cells were cultured on Transwell-COL plastic inserts placed into 24-well culture plates so that a top and bottom chamber were established, the two chambers being separated by a microporous membrane. Drugs were added to the top chamber at doses equivalent to peak plasma concentrations in vivo and the rate of appearance of drugs in the bottom chamber determined by high-performance liquid chromatography (HPLC). Both 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine) and 7-[4'-(2-nitroimidazol-1-yl)-butyl]-theophylline (NITP) rapidly penetrated DLD-1 multicell layers (50.9 +/- 12.1 microm thick) with t(1/2) values of 1.36 and 2.38 h respectively, whereas the rate of penetration of 5-aziridino-3-hydroxymethyl-1-methyl-2-[1H-indole-4,7-dione] prop-beta-en-alpha-ol (EO9) and doxorubicin through multicell layers was significantly slower (t(1/2) = 4.62 and 13.1 h respectively). Inclusion of dicoumarol increases the rate of EO9 penetration, whereas reducing the oxygen tension to 5% causes a reduction in tirapazamine penetration through multicell layers, suggesting that the extent of drug metabolism is one factor that determines the rate at which drugs penetrate multicell layers. The fact that EO9 does not readily penetrate a multicell layer, in conjunction with its rapid elimination in vivo (t(1/2) < 10 min), suggests that EO9 is unlikely to penetrate more than a few microm from a blood vessel within its pharmacokinetic lifespan. These results suggest that the failure of EO9 in the clinic is due to a combination of poor drug penetration and rapid elimination in vivo.

Indolequinone antitumor agents: relationship between quinone structure and rate of metabolism by recombinant human NQO1

A series of indolequinones bearing various functional groups has been synthesized, and the effects of substituents on the metabolism of the quinones by recombinant human NAD(P)H:quinone oxidoreductase (NQO1), and on the toxicity toward nonsmall cell lung cancer cells with either high NQO1 activity (H460) or with no detectable activity (H596) were studied.

Pharmacological and biochemical determinants of the antitumour activity of the indoloquinone EO9

EO9 is a novel bioreductive drug which has recently undergone extensive clinical evaluation. Its mechanism of action remains to be clearly defined. Antitumour activity of EO9 has been determined in 2 human colon cancer xenografts (HT-29 and BE) and 2 murine colon adenocarcinomas (MAC 16 and 26) after intratumoural injection of 250 microg of drug. Levels of the major bioreductive enzymes (DT-diaphorase, cytochrome P-450 reductase and cytochrome b5 reductase) were measured in tumours using cytochrome c reduction and menadione as the intermediate electron acceptor. There was no correlation between chemosensitivity (T/C: HT-29, 15%; BE, 27%; MAC 16, 33% and MAC 26, 60%) and enzyme activity (r2 = 0.47 for DT-diaphorase, r2 = 0.1 for cytochrome P-450 reductase and r2 = 0.52 for cytochrome b5 reductase). Drug metabolism was followed in vitro using tumour homogenates incubated under aerobic and anaerobic conditions. Four metabolites were identified by HPLC and characterised bv UV-visible spectroscopy. With the exception of the hydrolysis product EO5A, all other metabolites appeared to be drug adducts. No correlation was observed between the kinetics of metabolite formation and antitumour activity. A good correlation (r2 = 0.86) was found with the rate of disappearance of parent drug and antitumour activity. These data show that the overall capacity of a tumour to metabolise EO9 is the most important determinant of antitumour activity rather than the expression of the major bioreductive enzymes and that the parent drug rather than a metabolite leads to the active form of the drug.

Overexpression of human NADPH:cytochrome c (P450) reductase confers enhanced sensitivity to both tirapazamine (SR 4233) and RSU 1069

P450 reductase (NADPH: cytochrome c (P450) reductase, EC 1.6.2.4) plays an important role in the reductive activation of the bioreductive drug tirapazamine (SR4233). Thus, in a panel of human breast cancer cell lines, expression of P450 reductase correlated with both the hypoxic toxicity and the metabolism of tirapazamine [Patterson et al (1995) Br J Cancer 72: 1144-1150]. To examine this dependence in more detail, the MDA231 cell line, which has the lowest activity of P450 reductase in our breast cell line panel, was transfected with the human P450 reductase cDNA. Isolated clones expressed a 78-kDa protein, which was detected with anti-P450 reductase antibody, and were shown to have up to a 53-fold increase in activity of the enzyme. Using six stable transfected clones covering the 53-fold range of activity of P450 reductase, it was shown that the enzyme activity correlated directly with both hypoxic and aerobic toxicity of tirapazamine, and metabolism of the drug under hypoxic conditions. No metabolism was detected under aerobic conditions. For RSU1069, toxicity was also correlated with P450 reductase activity, but only under hypoxic conditions. Measurable activity of P450 reductase was found in a selection of 14 primary human breast tumours. Activity covered an 18-fold range, which was generally higher than that seen in cell lines but within the range of activity measured in the transfected clones. These results suggest that if breast tumours have significant areas of low oxygen tension, then they are likely to be highly sensitive to the cytotoxic action of tirapazamine and RSU 1069.

DT-diaphorase and cytochrome B5 reductase in human lung and breast tumours

The level of expression of enzymes that can activate or detoxify bioreductive agents within tumours has emerged as an important feature in the development of these anti-tumour compounds. The levels of two such reductase enzymes have been determined in 19 human non-small-cell lung tumours and 20 human breast tumours, together with the corresponding normal tissue. DT-diaphorase (DTD) enzyme levels (both expression and activity) were determined in these samples. Cytochrome b5 reductase (Cytb5R) activity was also assessed. With the exception of six patients, the levels of DTD activity were below 45 nmol min(-1) mg(-1) in the normal tissues assayed. DTD tumour activity was extremely variable, distinguishing two different groups of patients, one with DTD activity above 79 nmol min(-1) mg(-1) and the other with levels that were in the same range as found for the normal tissues. In 53% of the lung tumour samples, DTD activity was increased with respect to the normal tissue by a factor of 2.4-90.3 (range 79-965 nmol min[-1] mg[-1]). In 70% of the breast tumour samples, DTD activity was over 80 nmol min(-1) mg(-1) (range 83-267 nmol min[-1] mg[-1]). DTD expression measured by Western blot correlated well with the enzyme activity measured in both tumour and normal tissues. The levels of the other reductase enzyme, Cytb5R, were not as variable as those for DTD, being in the same range in both tumour and normal tissue or slightly higher in the normal tissues. The heterogeneous nature of DTD activity and expression reinforces the need to measure enzyme levels in individual patients before therapy with DTD-activated bioreductive drugs.

Involvement of DT-diaphorase (EC 1.6.99.2) in the DNA cross-linking and sequence selectivity of the bioreductive anti-tumour agent EO9

The chemistry of the mitomycin C-related drug indoloquinone EO9 would suggest that its mechanism of action is likely to involve DNA damage after reductive activation. The ability of this agent to induce DNA damage in intact cells has been examined using alkaline filter elution. After treatment with pharmacologically relevant concentrations of EO9, both DNA strand breaks and interstrand cross-links were detected in rat Walker tumour cells and human HT29 colon carcinoma cells. These cell lines express relatively high levels of DT-diaphorase (NAD(P)H: quinone acceptor oxidoreductase), which is believed to be involved in EO9 activation. The extent of DNA damage was increased by approximately 30-fold under hypoxia in BE colon carcinoma cells that express non-functional DT-diaphorase, but this dramatic hypoxia enhancement was not seen in HT-29 cells. These data are consistent with cytotoxicity studies that indicate that DT-diaphorase appears to be important in EO9 activation under aerobic conditions, but other enzymes may be more relevant under hypoxia. The involvement of DT-diaphorase in DNA damage induction was further investigated using cell-free assays. DNA cross-links were detectable in plasmid DNA co-incubated with EO9, cofactor and DT-diaphorase but not in the absence of this enzyme. In contrast, using a Taq polymerase stop assay, monofunctional alkylation was detected in plasmid DNA without metabolic activation, although the sequence selectivity was altered after reduction catalysed by DT-diaphorase.

Bioreductive activation of a series of analogues of 5-aziridinyl-3-hydroxymethyl-1-methyl-2-[1H-indole-4, 7-dione] prop-beta-en-alpha-ol (EO9) by human DT-diaphorase

The enzyme DT-diaphorase (NAD(P)H:quinone acceptor oxidoreductase, EC 1.6.99.2.; DTD) is believed to be a good target for enzyme-directed bioreductive drug development because elevated levels of enzyme activity have been described in several human tumour types and it plays a key role in the bioreductive activation of several quinone-based anticancer drugs. As part of an ongoing program to develop new bioreductive drugs, the ability of a series of indoloquinone compounds to serve as substrates for and to be bioreductively activated by purified recombinant human DTD was investigated. Of the seven compounds evaluated, EO9, EO68 and EO4 were substrates for human DTD, but only EO4 was reduced to a DNA cross-linking species, and this DNA damage was both concentration dependent and inhibited by dicoumarol. A broad spectrum of chemosensitivity was observed in the H460 non-small cell lung cancer cell line, with the most potent compounds being EO4 (IC50 = 23.9 nM), EO9 (IC50 = 34.5 nM) and EO68 (IC50 = 37.8 nM). Relatively minor structural changes resulted in major changes in both substrate specificity and cytotoxic potency. Comparative chemosensitivity studies demonstrated that EO4, EO9 and EO68 are preferentially toxic towards DTD-rich H460 cells compared with DTD-deficient H596 cells (ratio of IC50 values for H596 cells to H460 cells were 113.8, 92.2 and 103.9 respectively). In conclusion, this study has identified two new compounds that are substrates for human DTD, one of which (EO4) is reduced to a DNA cross-linking species. Further studies in a broad panel of cell lines and human tumour xenografts are warranted for EO4 and EO68 based upon the result of this study.