Initial characterization of the major mouse cytochrome P450 enzymes involved in the reductive metabolism of the hypoxic cytotoxin 3-amino-1,2,4-benzotriazine-1,4-di-N-oxide (tirapazamine, SR 4233, WIN 59075)

Redox Proteomic Profile of Tirapazamine-Resistant Murine Hepatoma Cells

3-Amino-1,2,4-benzotriazine-1,4-dioxide (tirapazamine, TPZ) and other heteroaromatic N-oxides (ArN→O) exhibit tumoricidal, antibacterial, and antiprotozoal activities. Their action is attributed to the enzymatic single-electron reduction to free radicals that initiate the prooxidant processes. In order to clarify the mechanisms of aerobic mammalian cytotoxicity of ArN→O, we derived a TPZ-resistant subline of murine hepatoma MH22a cells (resistance index, 5.64). The quantitative proteomic of wild-type and TPZ-resistant cells revealed 5818 proteins, of which 237 were up- and 184 down-regulated. The expression of the antioxidant enzymes aldehyde- and alcohol dehydrogenases, carbonyl reductases, catalase, and glutathione reductase was increased 1.6-5.2 times, whereas the changes in the expression of glutathione peroxidase, superoxide dismutase, thioredoxin reductase, and peroxiredoxins were less pronounced. The expression of xenobiotics conjugating glutathione-S-transferases was increased by 1.6-2.6 times. On the other hand, the expression of NADPH:cytochrome P450 reductase was responsible for the single-electron reduction in TPZ and for the 2.1-fold decrease. These data support the fact that the main mechanism of action of TPZ under aerobic conditions is oxidative stress. The unchanged expression of intranuclear antioxidant proteins peroxiredoxin, glutaredoxin, and glutathione peroxidase, and a modest increase in the expression of DNA damage repair proteins, tend to support non-site-specific but not intranuclear oxidative stress as a main factor of TPZ aerobic cytotoxicity.

Kinetics of Flavoenzyme-Catalyzed Reduction of Tirapazamine Derivatives: Implications for Their Prooxidant Cytotoxicity

Derivatives of tirapazamine and other heteroaromatic N-oxides (ArN→O) exhibit promising antibacterial, antiprotozoal, and tumoricidal activities. Their action is typically attributed to bioreductive activation and free radical generation. In this work, we aimed to clarify the mechanism(s) of aerobic mammalian cell cytotoxicity of ArN→O performing the parallel studies of their reactions with NADPH:cytochrome P-450 reductase (P-450R), adrenodoxin reductase/adrenodoxin (ADR/ADX), and NAD(P)H:quinone oxidoreductase (NQO1); we found that in P-450R and ADR/ADX-catalyzed single-electron reduction, the reactivity of ArN→O (n = 9) increased with their single-electron reduction midpoint potential (E¹₇), and correlated with the reactivity of quinones. NQO1 reduced ArN→O at low rates with concomitant superoxide production. The cytotoxicity of ArN→O in murine hepatoma MH22a and human colon adenocarcinoma HCT-116 cells increased with their E¹₇, being systematically higher than that of quinones. The cytotoxicity of both groups of compounds was prooxidant. Inhibitor of NQO1, dicoumarol, and inhibitors of cytochromes P-450 α-naphthoflavone, isoniazid and miconazole statistically significantly (p < 0.02) decreased the toxicity of ArN→O, and potentiated the cytotoxicity of quinones. One may conclude that in spite of similar enzymatic redox cycling rates, the cytotoxicity of ArN→O is higher than that of quinones. This is partly attributed to ArN→O activation by NQO1 and cytochromes P-450. A possible additional factor in the aerobic cytotoxicity of ArN→O is their reductive activation in oxygen-poor cell compartments, leading to the formation of DNA-damaging species similar to those forming under hypoxia.

Discovery and optimization of benzotriazine di-N-oxides targeting replicating and nonreplicating Mycobacterium tuberculosis

Compounds bactericidal against both replicating and nonreplicating Mtb may shorten the length of TB treatment regimens by eliminating infections more rapidly. Screening of a panel of antimicrobial and anticancer drug classes that are bioreduced into cytotoxic species revealed that 1,2,4-benzotriazine di-N-oxides (BTOs) are potently bactericidal against replicating and nonreplicating Mtb. Medicinal chemistry optimization, guided by semiempirical molecular orbital calculations, identified a new lead compound (20q) from this series with an MIC of 0.31 μg/mL against H37Rv and a cytotoxicity (CC(50)) against Vero cells of 25 μg/mL. 20q also had equivalent potency against a panel of single-drug resistant strains of Mtb and remarkably selective activity for Mtb over a panel of other pathogenic bacterial strains. 20q was also negative in a L5178Y MOLY assay, indicating low potential for genetic toxicity. These data along with measurements of the physiochemical properties and pharmacokinetic profile demonstrate that BTOs have the potential to be developed into a new class of antitubercular drugs.

Maximizing bactericidal activity with combinations of bioreduced drugs

Certain antimicrobial and anticancer drugs are only active following bioactivation within the target cell. Nitroimidazoles, nitrofurans and quinoxaline-di-N-oxides represent three chemical classes that are active as anti-tubercular drugs following intracellular bioreduction to reactive intermediates. Two nitroimidazoles are in clinical trials as new anti-tubercular drugs with significant bactericidal activity as well as activity on nonreplicating bacteria. Nitrofurans and quinoxaline-di-N-oxides, which are in preclinical development, also exhibit bactericidal activity and activity on nonreplicating bacteria. Current data indicate these drugs are bioreduced via distinct pathways that yield reactive free radical species. Since flux though each system would become saturated due to enzyme kinetics, cellular uptake or maximum drug concentration attainable in the host, one may propose that using three distinct systems simultaneously could produce a larger burst of free radicals to rapidly and efficiently kill bacteria and shorten the time to cure for tuberculosis. Arguments for the possible development of a novel combination therapy with maximized bacterial cell killing and the possibility of shortening the time to cure will be presented.

In silico pharmacogenetics of warfarin metabolism

Pharmacogenetic approaches can be instrumental for predicting individual differences in response to a therapeutic intervention. Here we used a recently developed murine haplotype-based computational method to identify a genetic factor regulating the metabolism of warfarin, a commonly prescribed anticoagulant with a narrow therapeutic index and a large variation in individual dosing. After quantification of warfarin and nine of its metabolites in plasma from 13 inbred mouse strains, we correlated strain-specific differences in 7-hydroxywarfarin accumulation with genetic variation within a chromosomal region encoding cytochrome P450 2C (Cyp2c) enzymes. This computational prediction was experimentally confirmed by showing that the rate-limiting step in biotransformation of warfarin to its 7-hydroxylated metabolite was inhibited by tolbutamide, a Cyp2c isoform-specific substrate, and that this transformation was mediated by expressed recombinant Cyp2c29. We show that genetic variants responsible for interindividual pharmacokinetic differences in drug metabolism can be identified by computational genetic analysis in mice.

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.

Dimemorfan N-demethylation by mouse liver microsomal cytochrome P450 enzymes

Dimemorfan (d-3-methyl-N-methylmorphinan), an analogue of dextromethorphan, is commonly used as a non-opioid antitussive. To clarify the contribution of cytochrome P450 (P450) in dimemorfan N-demethylation, effects of selective inducers and inhibitors were studied in ICR mice. Phenobarbital (PB)- and dexamethasone (Dex)-treatments caused 5-fold increases of liver microsomal dimemorfan N-demethylation activity. In untreated mouse liver microsomes, demethylation activity was strongly inhibited by a CYP3A inhibitor, ketoconazole. In PB-and Dex-treated mouse liver microsomes, ketoconazole caused strong inhibition, whereas orphenadrine caused a decrease of less than 20%. Pretreatment of control mouse liver microsomes with anti-CYP3A inhibited demethylation activity, whereas pre-treatment with anti-CYP2B had no effect. In PB-and Dex-treated mouse liver microsomes, the demethylation activity was inhibited by both anti-CYP3A and anti-CYP2B. In control mice, the intrinsic clearance of dimemorfan from N-demethylation was 5.8 microl min(-1)mg protein(-1). In PB- and Dex-treated mice, the correlation coefficient of fitting using one-enzyme and two-enzyme models were similar. The intrinsic clearances of induced mouse liver microsomes were similar. These results revealed that CYP3A played a major role in hepatic demethylation in untreated mice. Both CYP3A and CYP2B were involved in this demethylation in PB- and Dex-treated mice.

Paraquat detoxicative system in the mouse liver postmitochondrial fraction

We examined the paraquat detoxicative system in mouse livers. The survival rate of mice receiving 50 mg/kg paraquat was 41% at 7 days and significantly rose to 88, 64, 69% with pretreatment with phenytoin, phenobarbital, and rifampicin, respectively. Phenytoin induced activity in NADPH-cytochrome P450 reductase, CYP3A, CYP2B, and CYP2C that was 3 to 4 times higher than that of the controls. Phenobarbital induced CYP2B and rifampicin induced CYP3A, respectively, in addition to NADPH-cytochrome P450 reductase. 3-Methylcholanthrene did not induce these enzymes and did not alter the survival rate. All the mice pretreated with CoCl(2) (a CYP synthesis inhibitor) or SKF 525-A (a CYP inhibitor) were dead after 5 days, and troleandomycin (a CYP3A-specific inhibitor) also reduced the survival rate. When cell homogenates were incubated with paraquat and NADPH, paraquat decreased and its metabolic intermediate paraquat-monopyridone was formed. Troleandomycin inhibited the decrease in paraquat and increased the monopyridone. After making a subfraction of the homogenate, monopyridone was produced in the postmicrosomal 105,000g supernatant, but not in the microsomes. The pretreatment of mice with phenytoin decreased the monopyridone in the postmitochondrial fraction, but did not affect the supernatant. These results indicated that paraquat was first metabolized in the postmicrosomal supernatant into monopyridone, and that may have been subsequently hydroxylated by the microsomes. Repeated intravenous injections of alpha-tocopherol to paraquat-loaded mice significantly reduced the paraquat mortality and when these mice were pretreated with rifampicin, 100% of them survived. These studies demonstrate that postmitochondrial fractions play an important role in paraquat detoxication metabolism, and that the combination of CYP induction and alpha-tocopherol administration is highly useful for the survival of paraquat-exposed mice.

Establishment of an isogenic human colon tumor model for NQO1 gene expression: application to investigate the role of DT-diaphorase in bioreductive drug activation in vitro and in vivo

Many tumors overexpress the NQO1 gene, which encodes DT-diaphorase (NADPH:quinone oxidoreductase; EC 1.6.99.2). This obligate two-electron reductase deactivates toxins and activates bioreductive anticancer drugs. We describe the establishment of an isogenic human tumor cell model for DT-diaphorase expression. An expression vector was used in which the human elongation factor 1alpha promoter produces a bicistronic message containing the genes for human NQO1 and puromycin resistance. This was transfected into the human colon BE tumor line, which has a disabling point mutation in NQO1. Two clones, BE2 and BE5, were selected that were shown by immunoblotting and enzyme activity to stably express high levels of DT-diaphorase. Drug response was determined using 96-h exposures compared with the BE vector control. Functional validation of the isogenic model was provided by the much greater sensitivity of the NQO1-transfected cells to the known DT-diaphorase substrates and bioreductive agents streptonigrin (113- to 132-fold) and indoloquinone EO9 (17- to 25-fold) and the inhibition of this potentiation by the DT-diaphorase inhibitor dicoumarol. A lower degree of potentiation was seen with the clinically used agent mitomycin C (6- to 7-fold) and the EO9 analogs, EO7 and EO2, that are poorer substrates for DT-diaphorase (5- to 8-fold and 2- to 3-fold potentiation, respectively), and there was no potentiation or protection with menadione and tirapazamine. Exposure time-dependent potentiation was seen with the diaziquone analogs methyl-diaziquone and RH1 [2, 5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone], the latter being an agent in preclinical development. In contrast to the in vitro potentiation, there was no difference in the response to mitomycin C when BE2 and BE vector control were treated as tumor xenografts in vivo. This isogenic model should be valuable for mechanistic studies and bioreductive drug development.

Effect of substituents on microsomal reduction of benzo(c)fluorene N-oxides

The potential benzo(c)fluorene antineoplastic agent benfluron (B) displays high activity against a broad spectrum of experimental tumours in vitro and in vivo. In order to suppress some of its undesirable properties, its structure has been modified. Benfluron N-oxide (B N-oxide) is one of benfluron derivatives tested. The main metabolic pathway of B N-oxide is its reduction to tertiary amine B. A key role of cytochrome P4502B and P4502E1 in B N-oxide reduction has been proposed in the rat. Surprisingly, B N-oxide is reduced also in the presence of oxygen although all other N-oxides undergo reduction only under anaerobic conditions. With the aim to determine the influence of the N-oxide chemical structure and its redox potential on reductase affinity, activity and oxygen sensitivity five relative benzo(c)fluorene N-oxides were prepared. A correlation between the redox potential measured and the non-enzymatic reduction ability of the substrate was found, but no effect of the redox potential on reductase activity was observed. Microsomal reductases display a high affinity to B N-oxide (apparent K(m) congruent with0. 2 mM). A modification of the side-chain or nitrogen substituents has led to only a little change in apparent K(m) values, but a methoxy group substitution on the benzo(c)fluorene moiety induced a significant K(m) increase (ten-fold). Based on kinetic study results, the scheme of mechanism of cytochrome P450 mediated benzo(c)fluorene N-oxides reduction have been proposed. All benzo(c)fluorene N-oxides under study were able to be reduced in the presence of oxygen. Changes in the B N-oxide structure caused an extent of anaerobic conditions preference. The relationship between the benzo(c)fluorene N-oxide structure and the profile of metabolites in microsomal incubation was studied and important differences in the formation of individual N-oxide metabolites were found.

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.

Rat cytochromes P450 (CYP) specifically contribute to the reductive bioactivation of AQ4N, an alkylaminoanthraquinone-di-N-oxide anticancer prodrug

1. The bioreductive activation of the alkylaminoanthraquinone di-N-oxide prodrug AQ4N has been characterized in rat hepatic tissue using HPLC. 2. AQ4N was shown to be metabolized to two products, namely AQM, the two electron reduced mono-N-oxide, and AQ4, the four electron reduced active cytotoxic agent. 3. Metabolism was shown to occur in microsomes with an apparent Km = 30.29 microM and Vmax = 1.05 nmol/mg/min. 4. Bioreduction was dependent on anaerobic conditions and the presence of the reduced cofactor NADPH. Ketoconazole (100 microM) and carbon monoxide both inhibited AQ4N metabolism inferring a role for cytochrome P450 (CYP). 5. Microsomes from phenobarbitone and isoniazid-pretreated animals significantly (p < 0.05) enhanced the formation of AQ4 from AQ4N indicating a role for CYP2B and 2E respectively. The involvement of both CYP2B and 2E was confirmed by the use of CYP-specific inhibitors. 6. In conclusion, the involvement of rat hepatic CYP in the reductive bioactivation of the novel antitumour prodrug AQ4N has been established in detail for the first time. These findings highlight an important interspecies difference between the metabolism of AQ4N in rat and man which was shown earlier to be mediated by CYP3A enzymes. The pharmacological significance of this is discussed.

Importance of P450 reductase activity in determining sensitivity of breast tumour cells to the bioreductive drug, tirapazamine (SR 4233)

P450 reductase (NADPH:cytochrome P450 reductase, EC 1.6.2.4) is known to be important in the reductive activation of the benzotriazene-di-N-oxide tirapazamine (SR 4233). Using a panel of six human breast adenocarcinoma cell lines we have examined the relationship between P450 reductase activity and sensitivity to tirapazamine. The toxicity of tirapazamine was found to correlate strongly with P450 reductase activity following an acute (3 h) exposure under hypoxic conditions, the drug being most toxic in the cell lines with the highest P450 reductase activity. A similar correlation was also observed following a chronic (96 h) exposure to the drug in air but not following acute (3 h) exposure in air. We have also determined the ability of lysates prepared from the cell lines to metabolise tirapazamine to its two-electron reduced product, SR 4317, under hypoxic conditions using NADPH as an electron donor. The rate of SR 4317 formation was found to correlate both with P450 reductase activity and with sensitivity to tirapazamine, the highest rates of SR 4317 formation being associated with the highest levels of P450 reductase activity and the greatest sensitivity to the drug. These findings indicate a major role for P450 reductase in determining the hypoxic toxicity of tirapazamine in breast tumour cell lines.

Development and validation of a sensitive solid-phase-extraction and high-performance liquid chromatography assay for the bioreductive agent tirapazamine and its major metabolites in mouse and human plasma for pharmacokinetically guided dose escalation

A sensitive solid-phase-extraction and high-performance liquid chromatography (HPLC) method has been developed to investigate the pharmacokinetics and metabolism of the hypoxic-cell cytotoxic agent tirapazamine (1,2,4-benzotriazine-3-amine 1,4-di-N-oxide; WIN 59075, SR 4233), currently in phase I/II studies in the United Kingdom and United States. A sample extraction and concentration process was devised using strong cation-exchange Bond Elut cartridges. Tirapazamine, the mono and zero-N-oxide metabolites (WIN 64012, WIN 60109) were isocratically resolved using a microBondapak phenyl HPLC column and measured using photodiode-array detection. The minimal quantifiable level (MQL) of tirapazamine was 40 ng/ml in mouse plasma and 20 ng/ml in human plasma. Recovery was consistently greater than 80% for all compounds over the concentration range of 20 ng/ml to 20 micrograms/ml. No significant decomposition was observed following up to three freeze/thaw cycles and storage at -70 degrees C for 52 days. The assay was accurate and reproducible, with measured values lying within the limits of defined acceptance criteria. Additional studies to investigate the degree of plasma protein binding showed that tirapazamine did not bind extensively to plasma proteins (binding, 9.7% +/- 0.1% and 18.7% +/- 1.3% in mouse and human plasma, respectively). These small species differences in protein binding are unlikely to have any major impact on the extrapolation of pharmacokinetic data from mice to humans. The assay has now been successfully applied to investigate the pharmacokinetics and metabolism of tirapazamine in mice and patients as part of a pharmacokinetically guided dose-escalation strategy for phase I clinical trials.

Molecular mechanisms of tirapazamine (SR 4233, Win 59075)-induced hepatocyte toxicity under low oxygen concentrations

Previously we showed that tirapazamine (SR 4233, Win 59075) is cytotoxic towards hepatocytes under conditions of hypoxia but not in 10% or 95% oxygen and that bioreduction by DT-diaphorase or cytochrome P450 is not a major pathway. In the present study, we report that tirapazamine is highly cytotoxic to isolated rat hepatocytes maintained under 1% oxygen and the molecular cytotoxic mechanism has been elucidated. Cytotoxicity was prevented by the cytochrome P450 2E1 inhibitors phenyl imidazole, isoniazid, isopropanol or ethanol, suggesting that cytochrome P450 2E1 catalysed tirapazamine reductive bioactivation. By contrast, dicoumarol, a DT-diaphorase inhibitor, markedly increased tirapazamine-induced cytotoxicity. Cytotoxicity was also inhibited in normal but not DT-diaphorase-inactivated hepatocytes by increasing cellular NADH levels with lactate or ethanol or the mitochondrial respiratory inhibitors. Evidence that oxygen activation contributed to cytotoxicity was that glutathione oxidation occurred well before cytotoxicity ensued and that tirapazamine was more cytotoxic towards catalase- or glutathione reductase-inactivated hepatocytes. Furthermore, polyphenolic antioxidants such as quercetin, caffeic acid or purpurogallin, the radical trap Tempol or the iron chelator desferrioxamine prevented tirapazamine-mediated cytotoxicity. However, the antioxidants diphenylphenylenediamine, butylated hydroxyanisole or butylated hydroxytoluene did not prevent cytotoxicity and malonaldehyde formation was not increased, suggesting that lipid peroxidation was not important. The above results suggest that DT-diaphorase detoxifies tirapazamine whereas reduced cytochrome P450 reduces tirapazamine to a nitrogen oxide anion radical which forms cytotoxic reactive oxygen species as a result of redox cycling.

The metabolism of zidovudine by human liver microsomes in vitro: formation of 3'-amino-3'-deoxythymidine

The characterization of the enzymatic step(s) involved in the reduction of 3'-azido-3'-deoxythymidine (zidovudine)(ZDV) to 3'-amino-3'-deoxythymidine (AMT) was pursued. AMT formation by human liver microsomes was NADPH dependent, enhanced under anaerobic conditions, and increased by flavin adenine dinucleotide (FAD) and FMN. Carbon monoxide inhibited AMT formation by up to 80%. The effect of theophylline (CYP1A substrate), tolbutamide (CYP2C substrate), chlorzoxazone, thiobenzamide, p-nitrophenol, mercaptoethanol, isoniazid (CYP2E substrates), cortisol (CYP3A substrate), ketoconazole, itraconazole, fluconazole, cimetidine, micronazole (CYP inhibitors), methimazole (flavin-containing mono-oxygenase inhibitor), chloramphenicol (undergoes nitroreduction), allopurinol (xanthine oxidase inhibitor) and dicoumarol (DT-diaphorase inhibitor) on AMT formation were studied to see if the reduction reaction was mediated by a particular isozyme. The greatest inhibition was observed with ketoconazole (concentration producing 50% inhibition = 78.0 microM). At this concentration ketoconazole acted as a non-selective inhibitor of several CYP isozymes. Overall, these data suggested that ZDV reduction was probably mediated by both cytochrome P450 isozymes and NADPH-cytochrome P450 reductase. Formation of AMT, as measured by intrinsic clearance (Clint), was significantly increased in microsomes from rats pre-treated with phenobarbitone, dexamethasone and clofibrate (inducers of CYP2B, CYP3A and CYP4A, respectively). Pre-treatment of rats with beta-naphthoflavone and ethanol (CYP1A and CYP2E1 inducers, respectively) had no effect on AMT formation.

Direct measurement of pO2 distribution and bioreductive enzymes in human malignant brain tumors

PURPOSE

To measure the oxygen status of human malignant brain tumors in vivo and to determine the activities and expression of bioreductive enzymes in these same human brain tumor samples, as a means of assessing their suitability as targets for bioreductive drug therapy.

METHODS AND MATERIALS

A polarographic oxygen electrode was used to measure the intratumoral oxygen tension in twenty patients with malignant brain tumors during open brain surgery, performed under standard anaesthetic conditions. Six different tracks, each with a path length of 22 mm, were recorded per patient representing 192 readings. Following pO2 measurements the tumors were resected and stored in liquid N2 for subsequent bioreductive enzyme analysis. Eight human malignant brain tumors were assessed, by enzyme activity and western blot expression, for the presence of various bioreductive enzymes. These enzymes included DT-diaphorase, NADH cytochrome b5 reductase, and NADPH cytochrome P-450 reductase. Of these eight gliomas analyzed six samples were incubated with the bioreductive drug tirapazamine, in the presence of cofactor(s), to establish whether human brain tumors could metabolize this compound.

RESULTS

Both the high grade intrinsic and metastatic brain tumors showed significant regions of hypoxia. All the tumors subjected to enzyme profiling contained the bioreductive enzymes, DT-diaphorase, NADH cytochrome b5 reductase and NADPH cytochrome P-450 reductase. Also all six of the brain tumors investigated could metabolize tirapazamine to the two-electron reduction product.

CONCLUSION

These findings would favor primary brain tumors as suitable targets for bioreductive therapy.

The role of DT-diaphorase in determining the sensitivity of human tumor cells to tirapazamine (SR 4233)

PURPOSE

To determine the dependency of the aerobic and hypoxic toxicity of tirapazamine on the intracellular activity of DT-diaphorase.

METHODS AND MATERIALS

A panel of 18 human cell lines comprising predominantly small cell and nonsmall cell lung cancer and breast cancer lines were used. The activity of DT-diaphorase was determined in cytosolic preparations from cell lysates. The toxicity of tirapazamine was determined using the MTT assay after either 96 or 3 h aerobic exposure or 3 h treatment in hypoxia.

RESULTS

The cell lines exhibited a 5000-fold range in DT-diaphorase activity. In toxicity experiments, values of IC50 range from 10.2-120 microM and from 155-1230 for 96 and 3 h aerobic exposures, respectively. In N2, IC50s ranged from 8-55 microM. None of the toxicity values correlate with activity of DT-diaphorase, neither did the ratio of aerobic:hypoxic toxicity (differential toxicity).

CONCLUSION

The expression of DT-diaphorase in human tumor cells does not affect the toxicity of tirapazamine.

The experimental development of bioreductive drugs and their role in cancer therapy

Bioreductive drugs undergo metabolic reduction to generate cytotoxic metabolites. This process is facilitated by bioreductive enzymes and the lower oxygen conditions present in solid tumours compared to normal tissues. Because of this specificity, bioreductive drugs have enormous potential to contribute to modern cancer therapy. Examples undergoing clinical trials include N-oxides such as tirapazamine, aziridinylnitroimidazoles RSU 1069/RBU 6145 and quinones such as indoloquinone EO9. Other novel structures are also under study. Here we review the experimental development of bioreductive drugs and their role in cancer therapy.