Indolequinone antitumor agents: reductive activation and elimination from (5-methoxy-1-methyl-4,7-dioxoindol-3-yl)methyl derivatives and hypoxia-selective cytotoxicity in vitro

Design and synthesis of NAD(P)H: Quinone oxidoreductase (NQO1)-activated prodrugs of 23-hydroxybetulinic acid with enhanced antitumor properties

A series of NQO1 selectively activated prodrugs were designed and synthesized by introducing indolequinone moiety to the C-3, C-23 or C-28 position of 23-hydroxybetulinic acid (23-HBA) and its analogues. Among them, the representative compound 32j exhibited significant antiproliferative activities against NQO1-overexpressing HT-29 cells and A549 cells, with IC₅₀ values of 1.87 and 2.36 μM, respectively, which were 20-30-fold more potent than those of parent compound 23-HBA. More importantly, it was demonstrated in the in vivo antitumor experiment that 32j effectively suppressed the tumor volume and largely reduced tumor weight by 72.69% with no apparent toxicity, which was more potent than the positive control 5-fluorouracil. This is the first breakthrough in the improvement of in vivo antitumor activities of 23-HBA derivatives. The further molecular mechanism study revealed that 32j blocked cell cycle arrest at G2/M phase, induced cell apoptosis, depolarized mitochondria and elevated the intracellular ROS levels in a dose-dependent manner. Western blot analysis indicated that 32j induced cell apoptosis by interfering with the expression of apoptosis-related proteins. These findings suggest that compound 32j could be considered as a potent antitumor prodrug candidate which deserves to be further investigated for personalized cancer therapy.

Interactions of the antioxidant enzymes NAD(P)H: Quinone oxidoreductase 1 (NQO1) and NRH: Quinone oxidoreductase 2 (NQO2) with pharmacological agents, endogenous biochemicals and environmental contaminants

NAD(P)H

Quinone Oxidoreductase 1 (NQO1) is an antioxidant enzyme that catalyzes the two-electron reduction of several different classes of quinone-like compounds (quinones, quinone imines, nitroaromatics, and azo dyes). One-electron reduction of quinone or quinone-like metabolites is considered to generate semiquinones to initiate redox cycling that is responsible for the generation of reactive oxygen species and oxidative stress and may contribute to the initiation of adverse drug reactions and adverse health effects. On the other hand, the two-electron reduction of quinoid compounds appears important for drug activation (bioreductive activation) via chemical rearrangement or autoxidation. Two-electron reduction decreases quinone levels and opportunities for the generation of reactive species that can deplete intracellular thiol pools. Also, studies have shown that induction or depletion (knockout) of NQO1 were associated with decreased or increased susceptibilities to oxidative stress, respectively. Moreover, another member of the quinone reductase family, NRH: Quinone Oxidoreductase 2 (NQO2), has a significant functional and structural similarity with NQO1. The activity of both antioxidant enzymes, NQO1 and NQO2, becomes critically important when other detoxification pathways are exhausted. Therefore, this article summarizes the interactions of NQO1 and NQO2 with different pharmacological agents, endogenous biochemicals, and environmental contaminants that would be useful in the development of therapeutic approaches to reduce the adverse drug reactions as well as protection against quinone-induced oxidative damage. Also, future directions and areas of further study for NQO1 and NQO2 are discussed.

Designing Hypoxia-Responsive Nanotheranostic Agents for Tumor Imaging and Therapy

Hypoxia, a common feature of most solid tumors, plays an important role in tumor proliferation, metastasis, and invasion, leading to drug, radiation, and photodynamic therapy resistance, and resulting in a sharp reduction in the disease-free survival rate of tumor patients. The lack of sufficient blood supply to the interior regions of tumors hinders the delivery of traditional drugs and contrast agents, interfering with their accumulation in the hypoxic region, and preventing efficient theranostics. Thus, there is a need for the fabrication of novel tumor theranostic agents that overcome these obstacles. Reports, in recent years, of hypoxia-responsive nanomaterials may provide with such means. In this review, a comprehensive description of the physicochemical and biological characteristics of hypoxic tumor tissues is provided, the principles of designing the hypoxia-responsive tumor theranostic agents are discussed, and the recent research into hypoxia-triggered nanomaterials is examined. Additionally, other hypoxia-associated responsive strategies, the current limitations, and future prospects for hypoxia-responsive nanotheranostic agents in tumor treatment are discussed.

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.

Antitumour indolequinones: synthesis and activity against human pancreatic cancer cells

An important determinant of the growth inhibitory activity of indolequinones against pancreatic cancer cells is substitution on the 2-position with 2-unsubstituted derivatives being markedly more potent. A series of indolequinones bearing a range of substituents on nitrogen and at the indolylcarbinyl position was prepared by copper(II)-mediated reaction of bromoquinones and enamines, followed by functional group interconversions. The compounds were then assayed for their ability to inhibit the growth of pancreatic cancer cells. The pKa of the leaving group at the 3-position was shown to influence growth inhibitory activity that is consistent with the proposed mechanism of action of reduction, loss of leaving group and formation of a reactive iminium species. Substitutions on the indole nitrogen were well tolerated with little influence on growth inhibitory activity while substitutions at the 5- and 6-positions larger than methoxy led to decreased activity. The studies presented define the range of substitutions of 2-unsubstituted indolequinones required for optimal growth inhibitory activity.

One-pot synthesis of benzo[f]indole-4,9-diones from 1,4-naphthoquinones and terminal acetylenes

In this paper, a concise one-pot method for the construction of benzo[f]indole-4,9-dione motifs is described. These transformations proceed via a sequential palladium- and copper-catalyzed coupling reaction of 1,4-naphthoquinones with terminal acetylenes, followed by a copper-catalyzed intramolecular cyclization reaction of the resulting coupling product.

NAD(P)H:quinone oxidoreductase 1-compromised human bone marrow endothelial cells exhibit decreased adhesion molecule expression and CD34+ hematopoietic cell adhesion

NAD(P)H:quinone oxidoreductase 1 (NQO1) deficiency resulting from a homozygous NQO1*2 polymorphism has been associated with an increased risk of benzene-induced myeloid toxicity and a variety of de novo and therapy-induced leukemias. Endothelial cells in human bone marrow form one of the two known hematopoietic stem cell microenvironments and are one of the major cell types that express NQO1 in bone marrow. We have used a transformed human bone marrow endothelial cell (TrHBMEC) line to study the potential impact of a lack of NQO1 activity on adhesion molecule [endothelial leukocyte adhesion molecule 1 (E-selectin), vascular cell adhesion molecule (VCAM)-1, and intercellular adhesion molecule (ICAM)-1] expression and functional adhesion to bone marrow progenitor cells. We used both 5-methoxy-1,2-dimethyl-3-[(4-nitrophenoxy)methyl]indole-4,7-dione (ES936), a mechanism-based inhibitor of NQO1, and anti-NQO1 small interfering RNA to abrogate NQO1 activity. Real-time reverse transcription-polymerase chain reaction data demonstrated a significant inhibition of tumor necrosis factor (TNF)alpha-induced E-selectin mRNA levels after ES936 pretreatment. Immunoblot assays demonstrated a significant reduction in TNFalpha-stimulated E-selectin, VCAM-1, and ICAM-1 proteins after inhibition or knockdown of NQO1. The mechanisms underlying this effect remain undefined, but modulation of nuclear factor-kappaB (p65), c-Jun, and activating transcription factor 2, transcriptional regulators of adhesion molecules, were observed after inhibition or knockdown of NQO1. Decreased level of E-selectin, VCAM-1, and ICAM-1 also resulted in a functional deficit in adhesion. A parallel plate flow chamber study demonstrated a marked reduction in CD34(+) cell (KG1a) adhesion to NQO1-deficient TrHBMECs relative to controls. The reduced adhesive ability of TrHBMECs may affect the function of the vascular stem cell niche and also may contribute to the increased susceptibility of polymorphic individuals lacking NQO1 to leukemias and hematotoxicants such as benzene.

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.

Radiation- and photo-induced activation of 5-fluorouracil prodrugs as a strategy for the selective treatment of solid tumors

5-Fluorouracil (5-FU) is used widely as an anticancer drug to treat solid cancers, such as colon, breast, rectal, and pancreatic cancers, although its clinical application is limited because 5-FU has gastrointestinal and hematological toxicity. Many groups are searching for prodrugs with functions that are tumor selective in their delivery and can be activated to improve the clinical utility of 5-FU as an important cancer chemotherapeutic agent. UV and ionizing radiation can cause chemical reactions in a localized area of the body, and these have been applied in the development of site-specific drug activation and sensitization. In this review, we describe recent progress in the development of novel 5-FU prodrugs that are activated site specifically by UV light and ionizing radiation in the tumor microenvironment. We also discuss the chemical mechanisms underlying this activation.

Emission under hypoxia: one-electron reduction and fluorescence characteristics of an indolequinone-coumarin conjugate

A characteristic feature of the reactivity of indolequinone derivatives, substituents of which can be removed by one-electron reduction under hypoxic conditions, was applied to the development of a new class of fluorescent probes for disease-relevant hypoxia. A reducing indolequinone parent molecule conjugated with fluorescent coumarin chromophores could suppress efficiently the fluorescence emission of the coumarin moieties by an intramolecular electron-transfer quenching mechanism and a conventional internal-filter effect. Under hypoxic conditions, however, the conjugate, denoted IQ-Cou, underwent a one-electron reduction triggered by X irradiation or the action of a reduction enzyme to release a fluorescent coumarin chromophore, whereupon an intense fluorescence emission with a maximum intensity at 420 nm was observed. The one-electron reduction of IQ-Cou was suppressed by molecular oxygen under aerobic conditions. IQ-Cou also showed intense fluorescence in a hypoxia-selective manner upon incubation with a cell lysate of the human fibrosarcoma cell line HT-1080. The IQ-Cou conjugate has several unique properties that are favorable for a fluorescent probe of hypoxia-specific imaging.

Natural and synthetic quinones and their reduction by the quinone reductase enzyme NQO1: from synthetic organic chemistry to compounds with anticancer potential

The quinone reductase enzyme NAD(P)H: quinone oxidoreductase 1 (NQO1) is a ubiquitous flavoenzyme that catalyzes the two-electron reduction of quinones. This Perspective briefly reviews the structure and mechanism, physiological role, and upregulation and induction of the enzyme, but focuses on the synthesis of new heterocyclic quinones and their metabolism by recombinant human NQO1. Thus a range of indolequinones, some of which are novel analogues of mitomycin C, benzimidazolequinones, benzothiazolequinones and quinolinequinones have been prepared and evaluated, leading to detailed knowledge of the structural requirements for efficient metabolism by the enzyme. Potent mechanism-based inhibitors (suicide substrates) of NQO1 have also been developed. These indolequinones irreversibly alkylate the protein, preventing its function both in standard enzyme assays and also in cells. Some of these quinones are also potent inhibitors of growth of human pancreatic cancer cells, suggesting a potential role for such compounds as therapeutic agents.

Synthesis and evaluation of 3-aryloxymethyl-1,2-dimethylindole-4,7-diones as mechanism-based inhibitors of NAD(P)H:quinone oxidoreductase 1 (NQO1) activity

NAD(P)H:quinone oxidoreductase 1 is a proposed target in pancreatic cancer. We describe the synthesis of a series of indolequinones, based on the 5- and 6-methoxy-1,2-dimethylindole-4,7-dione chromophores with a range of phenolic leaving groups at the (indol-3-yl)methyl position. The ability of these indolequinones to function as mechanism-based inhibitors of purified recombinant human NQO1 was evaluated, as was their ability to inhibit both NQO1 and cell growth in human pancreatic MIA PaCa-2 tumor cells. The inhibition of rhNQO1 was related to the pKa of the leaving group: compounds with poorer phenolic leaving groups were poor inhibitors whereas those with more acidic leaving groups were more efficient inhibitors. These inhibition data also correlated with the inhibition NQO1 in MIA PaCa-2 cells. However, the data demonstrate that NQO1 inhibition does not correlate with growth inhibitory activity, at least in the MIA PaCa-2 cell line, suggesting that targets in addition to NQO1 need to be considered to explain the potent growth inhibitory activity of this series of indolequinones in human pancreatic cancer cells.

Synthesis by radical cyclization and cytotoxicity of highly potent bioreductive alicyclic ring fused [1,2-a]benzimidazolequinones

The key step in the synthesis of new five, six and seven-membered alicyclic ring [1,2-a]-fused bioreductive benzimidazolequinones was radical cyclisation. Six and seven-membered tributyltin hydride-mediated homolytic aromatic substitutions of nucleophilic N-alkyl radicals onto the benzimidazole-2-position occurred in high yields (63-70 %) when quaternising the pyridine-like 3-N of imidazole with camphorsulfonic acid and using large excesses of the azo-initiator, 1,1'-azobis(cyclohexanecarbonitrile), to supplement the non-chain reaction. Elaboration of benzimidazoles to the benzimidazolequinones occurred in excellent yields. The IC50 values for the cytotoxicity of benzimidazolequinones towards the human skin fibroblast cell line GM00637 were in the nanomolar range, as determined by using the MTT assay. The benzimidazolequinones were much more cytotoxic than indolequinone analogues. 1,2,3,4-Tetrahydropyrido[1,2-a]benzimidazole-6,9-dione was the most potent compound prepared being more than 300 times more cytotoxic than the clinically used bioreductive drug, mitomycin C. The latter benzimidazolequinone was more potent under hypoxic conditions (associated with solid tumors), being 4.4 times more cytotoxic than under aerobic conditions, while mitomycin C was 1.8 times more selective towards hypoxia. The cyclopropane fused pyrido[1,2-a]benzimidazolequinone, 1a,2,3,9b-tetrahydro-1H-cyclopropa[3,4]pyrido[1,2-a]benzimidazole-5,8-dione was less cytotoxic and selective than the five-membered ring analogue, 1,1a,8,8a-tetrahydrocyclopropa[3,4]pyrrolo[1,2-a]benzimidazole-3,6-dione. Modifying the structure of the most potent pyrido[1,2-a]benzimidazolequinone by attaching methyl substituents onto the quinone moiety increased reductive potentials and decreased cytotoxicity and selectivity towards hypoxia.

Novel N-oxide of naphthalimides as prodrug leads against hypoxic solid tumor: Synthesis and biological evaluation

Novel tertiary amine N-oxides of naphthalimides were designed and synthesized as potential anticancer agents against hypoxic solid tumors. Although their ctDNA-binding affinities and cytotoxic activities against usual tumor cell lines were lower than those of corresponding amines, the N-oxides A1 and A4 showed hypoxia preference activities against A375 cells in vitro and might be used as interesting candidates of prodrug leads in hypoxic tumor cells.

Synthesis and evaluation of prodrugs for anti-angiogenic pyrrolylmethylidenyl oxindoles

Potential prodrugs of inhibitors of VEGF-induced angiogenesis have been investigated. The prodrug systems studied were the 4-nitrobenzyl, 2-nitrophenylacetyl and 3-methyl-3-(3,6-dimethylbenzo-1,4-quinon-2-yl)butanoyl groups, readily attached to acidic OH or NH groups in drug molecules, and released upon bioreductive activation. The anti-angiogenic compounds studied were the pyrrolylmethylidenyl oxindole SU5416 (semaxanib) and its novel 6-hydroxy derivative. The potentially pro-anti-angiogenic compounds were assayed for their ability to block VEGF-induced angiogenesis in HUVECS in comparison to the free agents.

Synthesis and biological properties of bioreductively targeted nitrothienyl prodrugs of combretastatin A-4

Nitrothienylprop-2-yl ether formation on the 3'-phenolic position of combretastatin A-4 (1) abolishes the cytotoxicity and tubulin polymerization-inhibitory effects of the drug. 5-Nitrothiophene derivatives of 1 were synthesized following model kinetic studies with analogous coumarin derivatives, and of these, compound 13 represents a promising new lead in bioreductively targeted cytotoxic anticancer therapies. In this compound, optimized gem-dimethyl alpha-carbon substitution enhances both the aerobic metabolic stability and the efficiency of hypoxia-mediated drug release. Only the gem-substituted derivative 13 released 1 under anoxia in either in vitro whole-cell experiments or supersomal suspensions. The rate of release of 1 from the radical anions of these prodrugs is enhanced by greater methyl substitution on the alpha-carbon. Cellular and supersomal studies showed that this alpha-substitution pattern controls the useful range of oxygen concentrations over which 1 can be effectively released by the prodrug.

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

A series of indolequinones bearing a range of substituents at the (indol-2-yl)methyl position has been synthesized. The ability of these indolequinones to act as substrates for recombinant human NAD(P)H:quinone oxidoreductase (NQO1), a two-electron reductase upregulated in tumour cells, was determined, along with their toxicity to an isogenic tumour cell line pair that is differentiated as either NQO1-expressing cells (BE-NQ) or NQO1-null cells (BE-WT). Overall, the 2-substituted indolequinones were relatively poor substrates for NQO1. Hydroxymethyl groups at C-2 led to higher rates of reduction, a finding that was observed previously with 3-hydroxymethylated indolequinones. Predictably, the best substrate had an electron-withdrawing ester group at the indole-2-position. The indolequinones were generally non-toxic to both cell lines with the exception of those quinones that had methylaziridine groups at the indole-5-position. These compounds could form DNA cross-links when activated by reduction and were up to 3-fold more toxic to the BE-NQ cells than the BE-WT cells.

Novel Lavendamycin Analogues as Antitumor Agents: Synthesis, in Vitro Cytotoxicity, Structure−Metabolism, and Computational Molecular Modeling Studies with NAD(P)H:Quinone Oxidoreductase 1

Novel lavendamycin analogues with various substituents were synthesized and evaluated as potential NAD(P)H:quinone oxidoreductase (NQO1)-directed antitumor agents. Pictet-Spengler condensation of quinoline- or quninoline-5,8-dione aldehydes with tryptamine or tryptophans yielded the lavendamycins. Metabolism studies with recombinant human NQO1 revealed that addition of NH2 and CH2OH groups at the quinolinedione-7-position and indolopyridine-2'-position had the greatest positive impact on substrate specificity. The best and poorest substrates were 37 (2'-CH2OH-7-NH2 derivative) and 31 (2'-CONH2-7-NHCOC3H7-n derivative) with reduction rates of 263 +/- 30 and 0.1 +/- 0.1 micromol/min/mg NQO1, respectively. Cytotoxicity toward human colon adenocarcinoma cells was determined for the lavendamycins. The best substrates for NQO1 were also the most selectively toxic to the NQO1-rich BE-NQ cells compared to NQO1-deficient BE-WT cells with 37 as the most selective. Molecular docking supported a model in which the best substrates were capable of efficient hydrogen-bonding interactions with key residues of the active site along with hydride ion reception.

Hypoxia-selective activation of 5-fluorodeoxyuridine prodrug possessing indolequinone structure: radiolytic reduction and cytotoxicity characteristics

We synthesized a 5-fluorodeoxyuridine (5-FdUrd) derivative possessing an indolequinone structure (IQ-FdUrd) to characterize the radiolytic reduction in aqueous solution and the radiation-activated cytotoxicity against EMT6/KU cells under hypoxic conditions. IQ-FdUrd released antitumor agent 5-FdUrd upon hypoxic, but not aerobic, irradiation with the G value of 0.38 x 10(-7) mol J(-1). Laser flash photolysis of IQ-FdUrd in Ar-purged aqueous solution with dimethylaniline as an electron donor gave rise to a transient absorption spectrum characteristic of semiquinone radical anion, which decayed via second order kinetics. It is most likely that bimolecular disproportionation of intermediate semiquinone radicals occurs to release 5-FdUrd. IQ-FdUrd showed enhanced cytotoxicity against EMT6/KU cells in a radiation dose-dependent manner upon hypoxic irradiation. IQ-FdUrd is potentially a prototype compound for new class of radiation-activated antitumor prodrugs that are useful for radiation treatment of hypoxic tumors.

DT-diaphorase: a target for new anticancer drugs

DT-diaphorase (DTD) is an obligate two-electron reductase which bioactivates chemotherapeutic quinones. DTD levels are elevated in a number of tumour types, including non-small cell lung carcinoma, colorectal carcinoma, liver cancers and breast carcinomas, when compared to the surrounding normal tissue. The differential in DTD between tumour and normal tissue should allow targeted activation of chemotherapeutic quinones in the tumour whilst minimising normal tissue toxicity. The prototypical bioreductive drug is Mitomycin C (MMC) which is widely used in clinical practice. However, MMC is actually a relatively poor substrate for DTD and its metabolism is pH-dependent. Other bioreductive drugs have failed because of poor solubility and inability to surpass other agents in use. RH1, a novel diaziridinylbenzoquinone, is a more efficient substrate for DTD. It has been demonstrated to have anti-tumour effects both in vitro and in vivo and demonstrates a relationship between DTD expression levels and drug response. RH1 has recently entered a phase I clinical trial in solid tumours under the auspices of Cancer Research UK. Recent work has demonstrated that DTD is present in the nucleus and is associated with both p53 and the heat shock protein, HSP-70. Furthermore, DTD is inducible by several non-toxic compounds and therefore much interest has focussed on increasing the differential in DTD levels between tumour and normal tissues.

1,2-benzisoxazole phosphorodiamidates as novel anticancer prodrugs requiring bioreductive activation

Several 1,2-benzisoxazole phosphorodiamidates have been designed as prodrugs of phosphoramide mustard requiring bioreductive activation. Enzymatic reduction of 1,2-benziosoxazole moiety is expected to result in the formation of imine intermediate due to the cleavage of the N-O bond. The imine should then be spontaneously hydrolyzed to a ketone metabolite, thereby facilitating base-catalyzed beta-elimination of cytotoxic phosphoramide mustard. As expected, the proposed prodrugs 4, 9, and 12 were at least 3-5-fold more potent cytotoxins than control compounds 5 and 15, which lack in the phosphoramide mustard group. Upon incubation with phenobarb-induced rat liver S-9 fraction, compounds 4, 9, and 12 underwent extensive NADPH-dependent metabolism with concomitant generation of alkylating activity under both hypoxic and oxic conditions. Corresponding ketone metabolites were detected for 9 and 15. NADPH-dependent bioreduction of 15 to its ketone metabolite 16 was located in the microsomal fraction and inhibited by SKF-525A and pCMBA. Compared with phenobarb-induced rat liver microsomal fraction, incubation of 15 with rat or human p450 reductase microsomes showed moderate generation of 16. Microsomal cytochrome p450 and/or p450 reductase appear to be involved in the reductive metabolism of 1,2-benzisoxazole moiety under hypoxic as well as oxic conditions.

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.

Cytotoxicity of RH1 and related aziridinylbenzoquinones: involvement of activation by NAD(P)H:quinone oxidoreductase (NQO1) and oxidative stress

It is supposed that the main cytotoxicity mechanism of antitumour aziridinyl-substituted benzoquinones is their two-electron reduction to alkylating products by NAD(P)H:quinone oxidoreductase (NQO1, DT-diaphorase, EC 1.6.99.2). However, other possible cytotoxicity mechanisms, e.g., oxidative stress, are studied insufficiently. In the single-electron reduction of quinones including a novel compound RH1 (2,5-diaziridinyl- 3-(hydroxymethyl)-6-methyl-1,4-benzoquinone), by NADPH:cytochrome P-450 reductase (EC 1.6.2.4, P-450R), their reactivity increased with an increase in the redox potential of quinone/semiquinone couple (E(1)7), reaching a limiting value at E(1)7> or =-0.1V. The reactivity of quinones towards NQO1 did not depend on their E(1)7. The cytotoxicity of aziridinyl-unsubstituted quinones in bovine leukemia virus-transformed lamb kidney fibroblasts (line FLK) mimics their reactivity in P-450R-catalyzed reactions, exhibiting a parabolic dependence on their E(1)7. The toxicity of aziridinyl-benzoquinones, although being higher, also followed this trend and did not depend on their reactivity towards NQO1. The action of aziridinylbenzoquinones in FLK cells was accompanied by an increase in lipid peroxidation, their toxicity decreased by desferrioxamine and the antioxidant N,N'-diphenyl-p-phenylene diamine, and potentiated by 1,3-bis-(2-chloroethyl)-1-nitrosourea. The inhibitor of NQO1, dicumarol, protected against the toxicity of aziridinyl-benzoquinones except of 2,5-bis-(2'-hydroxyethylamino)-3,6-diaziridinyl-1,4-benzoquinone (BZQ), which was almost inactive as NQO1 substrate. The same events except the absence of pronounced effect of dicumarol were characteristic in the cytotoxicity of aziridinyl-unsubstituted quinones. These findings indicate that in addition to the activation by NQO1, the oxidative stress presumably initiated by single-electron transferring enzymes may be an important factor in the cytotoxicity of aziridinylbenzoquinones. The information obtained may contribute to the understanding of the molecular mechanisms of aziridinylquinone cytotoxicity and may be useful in the design of future bioreductive drugs.

Studies on the mechanisms of activation of indolequinone phosphoramidate prodrugs

Previously a series of 2- and 3-substituted indolequinone phosphoramidate prodrugs was synthesized, and the compounds were shown to be nanomolar inhibitors of cell proliferation. The activation of these compounds following both one- and two-electron reduction has been investigated. (31)P NMR experiments demonstrated that both series of compounds undergo rapid activation following two-electron reduction. Additionally, the 3-series of compounds undergo rapid activation following one-electron reduction, while activation of the 2-series of compounds via this mechanism is very slow. The activation of these prodrugs by direct displacement using sulfur nucleophiles such as glutathione has been examined. Activation via this route is rapid for the 3-regioisomers, but is considerably slower for the 2-substituted analogues under similar conditions. Together these findings suggest that drug delivery via two-electron reduction from the 2-position is the more selective prodrug strategy.

Pathways of reductive fragmentation of heterocyclic nitroarylmethyl quaternary ammonium prodrugs of mechlorethamine

Nitroarylmethyl quaternary (NMQ) ammonium salts have potential as prodrugs for enzymatic or radiolytic reduction to release amine effectors under hypoxia. Earlier studies demonstrated one-electron release of the cytotoxic amine mechlorethamine (HN2) from 4-nitroimidazolyl and 2-nitropyrrolyl NMQ prodrugs (but not from nitrobenzyl analogs) through intramolecular electron transfer. In this study we determined whether this is a general feature of heterocyclic NMQ prodrugs of HN2 and examined the reductive pathways in detail using pulse and steady-state radiolysis. The kinetics of radical fragmentation varied by more than four orders of magnitude, independently of the one-electron reduction potential, within the series of eight nitroheterocycles examined. In addition to the compounds identified previously, new 2-nitropyrrole and 3-nitrothiophene NMQ prodrugs were found to provide efficient HN2 release (G > 0.5 micromol/J in anoxic formate buffer). However, the nitrothiophene was sensitive to nucleophilic displacement of HN2, making it less promising. Product analysis by HPLC/mass spectrometry identified symmetrical dimers arising from benzyl-type radical intermediates but also demonstrated that these dimers are not reliable markers for the intramolecular fragmentation of the initial nitro radical anion. This study elucidated multiple competing pathways for reductive fragmentation of NMQ prodrugs and identified the preferred electron acceptors for use in the development of analogs that release more potent cytotoxins.

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.

Design, synthesis, and biological evaluation of indolequinone phosphoramidate prodrugs targeted to DT-diaphorase

A series of 2- and 3-substituted indolequinone phosphoramidate prodrugs targeted to DT-diaphorase (DTD) have been synthesized and evaluated. These compounds are designed to undergo activation via quinone reduction by DTD followed by expulsion of the phosphoramide mustard substituent from the hydroquinone. Chemical reduction of the phosphoramidate prodrugs led to rapid expulsion of the corresponding phosphoramidate anions in both series of compounds. Compounds substituted at the 2-position are excellent substrates for human DTD (k(cat)/K(M) = (2-5) x 10(6) M(-1) s(-1)); however, compounds substituted at the 3-position are potent inhibitors of the target enzyme. Both series of compounds are toxic in HT-29 and BE human colon cancer cell lines in a clonogenic assay. There was a correlation found between cytotoxicity and DTD activity for the 2-series of phosphoramidates; however, there was no correlation between cytotoxicity and DTD activity in the 3-series of compounds. This finding suggests the presence of an alternative mechanism for the activation of these compounds.

Modifying rates of reductive elimination of leaving groups from indolequinone prodrugs: a key factor in controlling hypoxia-selective drug release

3-(4-Methylcoumarin-7-yloxy)methylindole-4,7-diones were synthesised as model prodrugs in order to investigate the correlation between rates of reductive elimination from the (indolyl-3-yl)methyl position with reductive metabolism by hypoxic tumor cells and NADPH: cytochrome P450. Rates of elimination of the chromophore/fluorophore (7-hydroxy-4-methylcoumarin) following one-electron reduction of indolequinones to their semiquinone radicals (Q*-) was measured by pulse radiolysis utilising spectrophotometric and fluorometric detection. Incorporation of a thienyl or methyl substituent at the (indol-3-yl)CHR-position (where R=thienyl or methyl adjacent to the phenolic ether linking bond) significantly shortened the half-life of reductive elimination from 87 to 6 and 2 ms, respectively. Elimination from the methyl substituted analogue can thus compete effectively with the reaction of the semiquinone radical with oxygen at levels typically present in tumours (half-life approximately 1.8 ms at 0.5% O2). Chemical kinetic predictions were confirmed by metabolism in breast tumour MCF-7 cells between 0-2.1% O2. Rates of reductive release of the fluorophore from the non-fluorescent parent indolequinones (R=H, Me, thienyl) were similar under anoxia ( approximately 1.7 nmol coumarinmin(-1)mg protein(-1)) reflecting the similarity in one-electron reduction potential. Whereas coumarin release from the indolequinone (R=H) was completely inhibited above 0.5% O2, the enhanced rate of reductive elimination when R=thienyl or Me increased the metabolic rate of release to approximately 0.35 and 0.7 nmol coumarinmin(-1)mg protein(-1), respectively at 0.5% O2; complete inhibition occurring by 2.1% O2. Similar 'oxygen profiles' of release were observed with NADPH: cytochrome P450 reductase. In conclusion, it is possible to modify rates of reductive elimination from indolequinones to control the release of drugs over a range of tumour hypoxia.

A comprehensive study of the active site residues of DT-diaphorase: rational design of benzimidazolediones as DT-diaphorase substrates

A series of quinone substrates were modeled into the active site of human DT-diaphorase and minimized. Correlation of these models with the substrate specificity k(cat)/K(m) provided insights into the structural requirements of quinone substrates. The W105, F106, and H194 residues can influence the position of the quinone substrate in the active site resulting in formation of one of the two possible Michael anions resulting from hydride transfer from FADH(2). Electron withdrawing groups on the substrate can stabilize these anions resulting in excellent substrate specificity. Inspection of models indicated that the W-105 and F-106 residues form parallel walls that will accommodate large polycyclic substrates. Thus excellent polycyclic substrates of DT-diaphorase were designed. However, the placement of tetrahedral centers on these polycyclic substrates interfered with the W-105 and the F-106 residues resulting in their exclusion from the active site. The histidine (H194) residue permits recognition of substrate enantiomers as a result of hydrogen bonding interactions. As a result of this study, it will be possible to design poor to excellent substrates of DT-diaphorase and take advantage of varying levels of this enzyme in histologically different cancers.

Synthesis and anticancer and anti-HIV testing of some pyrazino[1,2-a]benzimidazole derivatives

In this study, some 1-methylene-2,3-diaryl-1,2-dihydropyrazino[1,2-a]benzimidazole and some 1-(2-arylvinyl)-3-arylpyrazino[1,2-a]benzimidazole derivatives were synthesised. The structure elucidation of the compounds was performed by IR, 1H-NMR and MASS spectroscopic data and elemental analyses results. Anticancer and anti-HIV activities of the compounds were examined, however no anti-HIV activity was seen; highly notable anticancer activity was obtained. It was also observed that the compounds were more potent against leukaemia cell lines.

Bioreductive and gene therapy approaches to hypoxic diseases

Hypoxia is a feature that exists in most, if not all, solid tumours and hypoxia has been shown to exist in a variety of other diseases. Bioreductive prodrugs have been developed to preferentially target the hypoxic cells in tumours. They are prodrugs, that are reductively activated (catalysed by reductive enzymes) to afford their active (toxic) species. More recently, bioreductive delivery agents that "release" a therapeutic entity preferentially under hypoxic conditions have also been developed to target hypoxia, not only in tumours, but also in a host of other diseases. This new technology platform is described in this review. In addition, we discuss the potential of utilising hypoxia to deliver selective gene therapy based upon the transcription factor HIF-1 and the use of unique genetic sequences termed HRE's (hypoxia responsive elements) that specifically control gene expression under hypoxic conditions. Finally, we describe how these drugs and gene-based therapeutic approaches can be combined to potentially deliver a highly selective form of therapy for cancer and other diseases where hypoxia plays a major pathophysiological role.

Aziridinyl quinone antitumor agents based on indoles and cyclopent[b]indoles: structure-activity relationships for cytotoxicity and antitumor activity

A large number of aziridinyl quinones represented by series 1-9 were studied with respect to their DT-diaphorase substrate activity, DNA reductive alkylation, cytostatic/cytotoxic activity, and in vivo activity. As a result, generalizations have been made with respect with respect to the following: DT-diaphorase substrate design, DT-diaphorase-cytotoxicity quantitative structure-activity relationship (QSAR), and DNA reductive alkylating agent design. A saturating relationship exists between the substrate specificity for human recombinant DT-diaphorase and the cytotoxicity in the human H460 non-small-cell lung cancer cell line. The interpretation of this relationship is that reductive activation is no longer rate-limiting for substrates with high DT-diaphorase substrate specificities. High DT-diaphorase substrate specificity is not desirable in the indole and cylopent[b]indole systems because of the result is the loss of cancer selectivity along with increased toxicity. We conclude that aziridinyl quinones of this type should possess a substrate specificity (V(max)/K(M)) < 10 x 10(-4) s(-1) for DT-diaphorase in order not to be too toxic or nonselective. While some DNA alkylation was required for cytostatic and cytotoxic activity by series 1-9, too much alkylation results in loss of cancer selectivity as well as increased in vivo toxicity. Indeed, the most lethal compounds are the indole systems with a leaving group in the 3alpha-position (like the antitumor agent EO9). We conclude that relatively poor DNA alkylating agents (according to our assay) show the lowest toxicity with the highest antitumor activity.

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.

NAD(P)H:quinone oxidoreductase 1 (NQO1): chemoprotection, bioactivation, gene regulation and genetic polymorphisms

NAD(P)H:quinone oxidoreductase 1 (NQO1) is an obligate two-electron reductase that is involved in chemoprotection and can also bioactivate certain antitumor quinones. This review focuses on detoxification reactions catalyzed by NQO1 and its role in antioxidant defense via the generation of antioxidant forms of ubiquinone and vitamin E. Bioactivation reactions catalyzed by NQO1 are also summarized and the development of new antitumor agents for the therapy of solid tumors with marked NQO1 content is reviewed. NQO1 gene regulation and the role of the antioxidant response element and the xenobiotic response element in transcriptional regulation is summarized. An overview of genetic polymorphisms in NQO1 is presented and biological significance for chemoprotection, cancer susceptibility and antitumor drug action is discussed.

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.