Mitomycin C is not metabolized by but is an inhibitor of human kidney NAD(P)H: (quinone-acceptor)oxidoreductase

In vivo role of NAD(P)H:quinone oxidoreductase 1 in metabolic activation of mitomycin C and bone marrow cytotoxicity

NAD(P)H:quinone oxidoreductase 1(-/-) (NQO1(-/-)), NQO1(+/-) along with NRH:quinone oxidoreductase 2(-/-) (NQO2(-/-)), and wild-type (WT) mice were exposed to five once weekly doses of mitomycin C. The mice were euthanized 15 weeks after the first dose. Blood cell counts and histologic analyses were done. WT and NQO2(-/-) mice showed hypocellularity and a significant increase in adipocytes in bone marrow. They also showed anemia because of the loss of RBC and hemoglobin. The neutrophils and platelets were reduced, whereas other blood cell types and tissues were normal. Interestingly, NQO1(-/-) mice showed a complete resistance to mitomycin C-induced bone marrow cytotoxicity and reduction in RBC, hemoglobin, and neutrophils. NQO1(+/-) mice also showed limited resistance to mitomycin C-induced bone marrow cytotoxicity. These data show a major in vivo role of NQO1 in metabolic activation of mitomycin C with implications in mitomycin C chemotherapy.

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.

Immunohistochemical analysis of NAD(P)H:quinone oxidoreductase and NADPH cytochrome P450 reductase in human superficial bladder tumours: Relationship between tumour enzymology and clinical outcome following intravesical mitomycin C therapy

A central theme within the concept of enzyme-directed bioreductive drug development is the potential to predict tumour response based on the profiling of enzymes involved in the bioreductive activation process. Mitomycin C (MMC) is the prototypical bioreductive drug that is reduced to active intermediates by several reductases including NAD(P)H:quinone oxidoreductase (NQO1) and NADPH cytochrome P450 reductase (P450R). The purpose of our study was to determine whether NQO1 and P450R protein expression in a panel of low-grade, human superficial bladder tumours correlates with clinical response to MMC. A retrospective clinical study was conducted in which the response to MMC of 92 bladder cancer patients was compared to the immunohistochemical expression of NQO1 and P450R protein in archived paraffin-embedded bladder tumour specimens. A broad spectrum of NQO1 protein levels exists in bladder tumours between individual patients, ranging from intense to no immunohistochemical staining. In contrast, levels of P450R were similar with most tumours having moderate to high levels. All patients were chemotherapy naïve prior to receiving MMC and clinical response was defined as the time to first recurrence. A poor correlation exists between clinical response and NQO1, P450R or the expression patterns of various combinations of the 2 proteins. The results of our study demonstrate that the clinical response of superficial bladder cancers to MMC cannot be predicted on the basis of NQO1 and/or P450R protein expression and suggest that other factors (other reductases or post DNA damage events) have a significant bearing on tumour response.

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.

Evidence that ferric nitrilotriacetate mediates oxidative stress by down-regulating DT-diaphorase activity: implications for carcinogenesis

Ferric nitrilotriacetate (Fe-NTA) is a known complete renal carcinogen as well as renal and hepatic tumor promoter, which acts by generating oxidative stress in the tissue. However, the mechanism by which it generates this stress is not fully understood. In this study, we show that Fe-NTA down-regulates hepatic and renal quinone reductase (QR) activity dose dependently. The maximum decrease in the activity of QR was observed at 12 h in the liver and 6 h in the kidney following Fe-NTA treatment. However, at all other time points studied, QR activity was reduced. In addition, a parallel increase in protein carbonyl content, a sensitive indicator of tissue oxidative stress was observed both in the liver and kidney. The pretreatment of animals with antioxidants, butylated hydroxyanisole and butylated hydroxytoluene, prevented the observed inhibition in the activity of QR and enhanced the formation of protein carbonyl in both organs. These studies suggest that Fe-NTA-mediated generation of oxidant free radicals down-regulates QR activity which may be responsible, at least in part, for the observed renal and hepatic injury and carcinogenic properties of Fe-NTA.

Enzymology of mitomycin C metabolic activation in tumour tissue. Characterization of a novel mitochondrial reductase

In this study, the enzymology of mitomycin C (MMC) bioactivation in two murine colon adenocarcinomas, MAC 16 and MAC 26, was examined. Subcellular quinone reductase assessment via cytochrome c reduction confirmed a number of active enzymes. MAC 16 exhibited 22-fold greater levels of cytosolic DT-diaphorase than MAC 26, while microsomal NADPH:cytochrome P-450 reductase levels were similar in both tumour types. Metabolism of MMC by subcellular fractions isolated from both MAC 16 and MAC 26 was quantitated by monitoring the formation of the principle metabolite 2,7-diaminomitosene (2,7-DM) via high-performance liquid chromatography (HPLC). In MAC 16 only, activity displaying the properties of cytosolic DT-diaphorase and microsomal NADPH:cytochrome P-450 reductase was detected and confirmed, using the enzyme inhibitors dicoumarol and cytochrome P-450 reductase antiserum, respectively. The highest level of MMC metabolism was associated with the mitochondrial fraction from both tumours and was the sole enzyme activity detected in MAC 26. The greatest mitochondrial drug metabolism was achieved in the presence of NADPH as cofactor and hypoxia (MAC 16-specific activity, 3.67 +/- 0.58 nmol/30 min/mg; MAC 26 specific-activity, 3.87 +/- 0.71 nmol/30 min/mg) and was unaffected by the addition of the inhibitors dicoumarol and cytochrome P-450 reductase antiserum. NADH-dependent mitochondrial activity was only observed in MAC 16 at approximately 4-fold less than that seen with NADPH. MAC 26 homogenate incubations displayed enhanced metabolism under hypoxia, presumably due to the presence of the identified mitochondrial enzyme. MAC 16 homogenates showed no increase in metabolism under hypoxia, suggesting that other enzyme(s) may be predominant. These data indicate the presence of a novel mitochondrial one-electron reductase capable of metabolising MMC in MAC 16 and MAC 26.

Non-enzymatic and enzymatic activation of mitomycin C: identification of a unique cytosolic activity

Mitomycin C (MMC), an alkylating anti-tumor agent, was activated by non-enzymatic and enzymatic mechanisms leading to DNA binding and adduct formation. However, it was enzymatically, not non-enzymatically, activated MMC which induced inter-strand DNA cross-linking, a major determinant of cell death. The enzymatic activation of MMC was catalyzed by microsomal NADPH:cytochrome P450 reductase (P450 reductase) and cytosolic enzyme activities. Human P450 reductase, transiently expressed from its cDNA in the COSI cells, metabolically activated MMC to generate 9 specific MMC-DNA adducts and induced inter-strand DNA cross-linking. Co-chromatography of the MMC-DNA adducts generated by P450 reductase and sodium borohydride in separate experiments indicated that MMC was metabolized by P450 reductase to produce 2,7-diaminomitosenes that exhibited binding to deoxyguanosine. Several experiments indicated that cytosolic enzymes which catalyzed reductive activation of MMC and DNA cross-linking included NAD(P)H:quinone oxidoreductaseI (NQOI or DT diaphorase) when present in extremely high concentrations and a unique cytosolic activity. The unique cytosolic activity was present in several mammalian cells and mouse colon and liver but absent in mouse kidney. The unique activity had properties of a diaphorase but was distinct from NQOI because of a lack of correlation between NQOI (2,6-dichlorophenolindophenol reduction) activity and the amount of MMC-reductive activation leading to DNA cross-linking. This activity was also distinct from xanthine oxidoreductase and NADH-cytochrome b5 reductase, 2 other enzymes that catalyze metabolic activation of MMC, because the unique activity was not inhibited by allopurinol (an inhibitor of xanthine oxidoreductase) and its activity was the same with NADH and NADPH (cytochrome b5 reductase is specific to NADH).

Establishment and characterization of non-small cell lung cancer cell lines resistant to mitomycin C under aerobic conditions

To elucidate the mechanisms of acquired resistance to mitomycin C (MMC) in non-small cell lung cancer (NSCLC), we established two MMC-resistant NSCLC sublines by continuous exposure to MMC, using PC-9 as a parent cell line. The sublines, PC-9/MC2 and PC-9/MC4, were 6.4- and 10-fold more resistant to MMC than their parent cell line, respectively, at the IC50 value as determined by MTT assay. They exhibited cross-resistance to EO9, but were not resistant to cisplatin, vindesine, etoposide, carboquone, or KW-2149, a novel MMC derivative. They were collaterally sensitive to adriamycin and menadione. Accumulation of the drug was decreased in the resistant sublines to about 60% of that in the parent cells. Cytosolic DT-diaphorase (DTD) activities were decreased to 13.5 +/- 3.2 in PC9/MC2 and 1.3 +/- 0.6 in PC-9/MC4 from 261.5 +/- 92.7 nmol/min/mg protein in the parent PC-9. NADH:cytochrome b5 reductase activities in both of the resistant cell lines were significantly decreased as compared to that in the parent cell line. Addition of dicumarol resulted in a two-fold increase in IC50 value in PC-9, whereas the IC50 value showed no change in PC-9/MC4. Moreover, dicumarol did not affect the sensitivities to KW-2149 but decreased the sensitivities to EO9 in both the parent and the resistant cell lines. Formation of an alkylating metabolite was significantly decreased in the resistant cells, in parallel to the degree of resistance. We concluded that deficient drug activation due to decreased DTD activity was important as a mechanism of resistance to MMC in PC-9, a relatively DTD-rich NSCLC cell line.

Encapsulation of mitomycin C in albumin microspheres markedly alters pharmacokinetics, drug quinone reduction in tumour tissue and antitumour activity. Implications for the drugs' in vivo mechanism of action

Pharmacokinetics and metabolism of mitomycin C (MMC) have been studied in NMRI mice bearing MAC 16 colon adenocarcinoma after direct intratumoural injection of either 500 micrograms free MMC or the same dose incorporated in albumin microspheres. Microspheres produced a tumour pharmacokinetic profile of steady state drug levels, avoiding the much higher early peak (20.5 micrograms/tumour vs 98.9 micrograms/tumour) and lower trough of free MMC, and reducing significantly the levels of drug reaching the systemic circulation (AUC 1.8 micrograms/mL x hr for microspheres vs 6.8 micrograms/mL x hr for free drug). 2,7-Diaminomitosene (2,7-DM), a key intermediate in MMC quinone bioreduction, was used as an indicator of drug metabolic activation in tumour tissue. Peak levels were 10-fold higher (11.2 micrograms/tumour vs 1.1 micrograms/tumour) and area under the curve 5-fold higher after free drug. Even taking into account differences in tumour pharmacokinetic profiles of the parent drug, microspheres actively inhibited 2,7-DM formation 3-fold. However, the microspheres generated a completely different pattern of drug metabolism where four previously uncharacterized mitosane metabolites and elevated levels of cis and trans 1-hydroxy 2,7-diaminomitosene were detected. Despite similar parent drug exposure in tumours, free drug was significantly more active (P < 0.05, Student's t-test) against MAC 16. These results suggest that formation of 2,7-DM correlates more closely with antitumour activity than sustained parent drug levels or appearance of other key metabolites. Potentially, they provide the first direct evidence for an in vivo mechanism of action dependent on bioreductive activation and formation of 2,7-DM.

A comparison of free radical formation by quinone antitumour agents in MCF-7 cells and the role of NAD(P)H (quinone-acceptor) oxidoreductase (DT-diaphorase)

Electron paramagnetic resonance (EPR/ESR) spin trapping studies with DMPO revealed that purified rat liver NAD(P)H (quinone-acceptor) oxidoreductase (QAO) mediated hydroxyl radical formation by a diverse range of quinone-based antitumour agents. However, when MCF-7 S9 cell fraction was the source of QAO, EPR studies distinguished four different interactions by these agents and QAO with respect to hydroxyl radical formation: (i) hydroxyl radical formation by diaziquone (AZQ), menadione, 1AQ; 1,5AQ and 1,8AQ was mediated entirely or partially by QAO in MCF-7 S9 fraction; (ii) hydroxyl radical formation by daunorubicin and Adriamycin was not mediated by QAO in MCF-7 S9 fraction; (iii) hydroxyl radical formation by mitomycin C was stimulated in MCF-7 S9 fraction when QAO was inhibited by dicumarol; (iv) no hydroxyl radical formation was detected for 1,4AQ or mitoxantrone in MCF-7 S9 fraction. This study shows that purified rat liver QAO can mediate hydroxyl radical formation by a variety of diverse quinone antitumour agents. However, QAO did not necessarily contribute to hydroxyl radical formation by these agents in MCF-7 S9 fraction and in the case of mitomycin C, QAO played a protective role against hydroxyl radical formation.

DT-diaphorase in activation and detoxification of quinones. Bioreductive activation of mitomycin C

A role of DTD in the bioreductive activation of mitomycin C was supported by indirect evidence utilizing enzyme inhibitors in cellular systems. Using a cell-free system, we have confirmed that DTD can bioactivate mitomycin C using both purified rat and human DTD. Metabolism and bioactivation of mitomycin C by DTD is pH-dependent. At pH 7.8 alkylation of DTD leading to enzyme inhibition and DTD crosslinking occurs whereas at pH values of 7.4 and below metabolite formation, preservation of catalytic activity of DTD and sequence-selective DNA crosslinking occurs. Bioactivation of mitomycin C by DTD and the cytotoxicity of this drug in DTD-rich cell lines is oxygen-independent. Mitomycin C induces greater DNA crosslinking, even after chemical reduction, at lower pH values. This suggests that if mitomycin C is used in tumors with elevated DTD activity, greater therapeutic activity may be obtained by lowering intratumoral pH. Human NSCLC has elevated DTD activity relative to SCLC and normal lung and may be a target for the development of drugs which can be efficiently bioactivated by DTD. Because of the pH-dependent inactivation of DTD by mitomycin C, however, other drugs which are efficiently metabolized and bioactivated by DTD may be better candidates for the therapy of tumors high in DTD such as NSCLC.

Biological and toxicological consequences of quinone methide formation

Quinone methides are a class of reactive, electrophilic compounds which are capable of alkylating cellular macromolecules. They are formed during xenobiotic biotransformation reactions and are hypothesized to mediate the toxicity of a large number of quinone antitumor drugs as well as several alkylphenols. In addition, oxidation of specific endogenous alkylphenols (e.g. coniferyl alcohol) and alkylcatechols (e.g. N-acetyldopamine, dopa) to quinone methides plays an important role in the synthesis of several complex plant and animal polymers, including lignin, cuticle and melanin. The role of quinone methides in these various processes is reviewed.

Determination of mitomycin C, 2,7-diaminomitosene, 1,2-cis- and 1,2-trans-1-hydroxy-2,7-diaminomitosene in tumour tissue by high-performance liquid chromatography

A high-performance liquid chromatographic method is described for the determination of mitomycin C (MMC) and its metabolites 2,7-diaminomitosene (2,7-DM), 1,2-cis-1-hydroxy-2,7-diaminomitosene (cis-hydro) and 1,2-trans-1-hydroxy-2,7-diaminomitosene (trans-hydro) in tumour tissue. N-la-Methylmitomycin C (porfiromycin, PM) was used as an internal standard. Two factors were critical in resolving the metabolites: pH and buffer ionic strength, where the retention times of the four components were affected in the order 2,7-DM >> cis-hydro >> trans-hydro >> MMC. The optimal isocratic conditions (flow-rate 1 ml/min) were 18 mM sodium phosphate pH 5.8-methanol (74:26) and a column temperature of 40 degrees C on a Spherisorb ODS-2 column (25 cm x 4.6 mm I.D.). Liquid-liquid extraction [twice with chloroform-propan-2-ol-ethyl acetate (2:2:1)] is described for tumour tissue. Recoveries varied depending on the component: MMC, 71.9 +/- 12.4%; PM, 85.5 +/- 27%; 2,7-DM, 51.7 +/- 5.4%; cis-hydro, 52.0 +/- 16.8%; trans-hydro, 62 +/- 8%. When applied to the analysis of a rat mammary carcinoma treated intra-tumourally with 450 micrograms of MMC five drug-related "metabolite" peaks were detected. Three of these co-chromatographed with standards of 2,7-DM, cis- and trans-hydro, and had identical absorption maxima to their respective standards, with the possible exception of trans-hydro.

The one-electron reduction potential of several substrates can be related to their reduction rates by cytochrome P-450 reductase

The rates of reduction of 27 compounds by purified cytochrome P-450 reductase have been studied and correlated with the one-electron reduction potentials E7(X/X.-). For compounds with reduction potentials between approx. -400 and -165 mV, there was a good correlation between the rates and E7(X/X.-); log rate (mumol/min per mg) = (2.38 +/- 0.08) + (0.0061 +/- 0.0003)E7 (mV). Compounds with potentials more positive than -165 mV were shown to deviate from the correlation. It is shown that under the standard conditions of the assay, these compounds undergo two-electron reduction by NADPH rather than one-electron reduction with the enzyme. As a consequence of the study, the rate constants for the reaction of superoxide radicals with native and acetylated cytochrome c were also determined to be (4.5 +/- 0.5).10(5) and (1.5 +/- 0.15).10(5) M-1 s-1, respectively.

EO9: a novel bioreductive alkylating indoloquinone with preferential solid tumour activity and lack of bone marrow toxicity in preclinical models

EO9 is a novel and fully synthetic bioreductive alkylating indoloquinone. Although structurally-related to mitomycin C, EO9 exhibits a distinct preclinical antitumour profile and there are also differences in its biochemical activation. In this study, EO9 was found to demonstrate preferential cytotoxicity against solid tumours in vitro as compared to leukaemia cell lines both in the Corbett two-tumour assay and in the disease-oriented human tumour cell line panel of the U.S. National Cancer Institute. In the latter system activity was particularly apparent in colon, melanoma and central nervous system lines, together with some renal and non-small cell lung lines. Preferential cytotoxicity towards hypoxic versus aerobic EMT6 mouse mammary tumour cells was observed. In vivo, EO9 was inactive against the P388 murine leukaemia, while exerting significant antiproliferative effects against several murine and human solid tumours, including the generally resistant MAC mouse colon tumours and gastric, ovarian and breast xenografts. These results confirmed in vitro observations of preferential solid tumour activity. In animal toxicology studies, EO9 induced vascular congestion in the gastrointestinal tract, but no significant bone marrow toxicity. The LD10 value of EO9 after a single intravenous injection into mice was 9 mg/kg (27 mg/m2). A dose of one-tenth of the mouse equivalent LD10 (2.7 mg/m2), the recommended starting dose for clinical phase I studies, was found to be safe in rats. Considering its distinct mechanism of bioactivation as compared to mitomycin C, its preferential solid tumour activity, its excellent activity against hypoxic cells, and lack of significant bone marrow toxicity in animals studies, EO9 has been selected for clinical evaluation within the framework of the EORTC.

Isolation and characterization of a mitomycin C-resistant variant of human colon carcinoma HT-29 cells

To investigate the resistant mechanisms against MMC in human tumor cells, we isolated an MMC-resistant variant (HT-29/MMC) of HT-29 human colon carcinoma cells. HT-29/MMC cells showed 5-fold resistance to MMC as compared with the parental cell line but did not show cross-resistance to Adriamycin, vincristine, ACNU, bleomycin, or cisplatin. Treatment of the cells with dicoumarol, an inhibitor of DT-diaphorase, reduced the cytotoxicity of MMC in DT-diaphorase proficient HT-29 cells but not in HT-29/MMC cells. HT-29/MMC cells were 5 times more sensitive than HT-29 cells to menadione, which is detoxified by DT-diaphorase, DT-diaphorase was deficient in HT-29/MMC cells as determined by the enzyme activity and immunoblot analysis of the cytoplasmic proteins. Levels of cytochrome P-450 reductase and glutathione S-transferase, however, were comparable in both cell lines. The amount of [3H]-MMC found covalently bound to chromosomal DNA in HT-29/MMC cells was one-fourth that detected in HT-29 cells. Treatment with dicoumarol reduced the DNA-bound MMC in HT-29 cells but not in HT-29/MMC cells. These results indicate that the deficiency in DT-diaphorase, an activating enzyme of MMC, is one of the mechanisms of resistance in HT-29/MMC cells.

Modulation of mitomycin C resistance by glutathione transferase inhibitor ethacrynic acid

This study was undertaken to elucidate the mechanism(s) of cross-resistance (4.9-fold) to mitomycin C (MMC) in a multi-drug-resistant cell line, P388/R-84. Intracellular accumulation of MMC by sensitive (P388/S) and P388/R-84 cells was comparable. Despite a 32% reduction in NADPH cytochrome P-450 reductase activity (responsible for MMC activation) in P388/R-84 cells, the rate of MMC bio-reduction by sensitive and resistant cells was similar. These results suggested that MMC resistance in P388/R-84 cell line must depend on factors other than impaired drug accumulation or bio-activation. Recent studies suggest that glutathione transferase (GST) dependent drug detoxification also contributes to cellular resistance of a variety of alkylating agents. Even though overexpression of GST has been noted in some MMC resistant tumor cells, it is not known if its level affects sensitivity to MMC. We have, therefore, determined the effect of ethacrynic acid (an inhibitor of GST activity) treatment on MMC cytotoxicity in P388/R-84 cells, which have about 2-fold higher GST activity than P388/S cells. The IC50 value for the inhibition of GST activity in vitro by ethacrynic acid (EA) was 16.5 microM (5 micrograms/ml). A depletion in intracellular GSH was also observed by treating P388/R-84 cells with EA alone or in combination with MMC. A non-toxic concentration of EA (1 microgram/ml; 3.3 microM) increased MMC cytotoxicity by 36% in P388/R-84 cells. MMC cytotoxicity was increased 2-fold by EA treatment in glutathione (GSH)-depleted P388/R-84 cells. These results suggest that GST mediated drug inactivation may represent another important mechanism of MMC resistance.

Bioreductive activation of mitomycin C by DT-diaphorase

The role of DT-diaphorase (DTD, EC 1.6.99.2) in the bioreductive activation of mitomycin C was examined using purified rat hepatic DTD. The formation of adducts with reduced glutathione (GSH), binding of [3H]mitomycin C to DNA, and mitomycin C-induced DNA interstrand cross-linking were used as indicators of bioactivation. Mitomycin C was metabolized by DTD in a pH-dependent manner with increasing amounts of metabolism observed as the pH was decreased from 7.8 to 5.8. The major metabolite observed during DTD-mediated reduction of mitomycin C was 2,7-diaminomitosene. GSH adduct formation, binding of [3H]mitomycin C and mitomycin C-induced DNA interstrand cross-linking were observed during DTD-mediated metabolism. In agreement with the pH dependence of metabolism, increased bioactivation was observed at lower pH values. Temporal studies and experiments using authentic material showed that 2,7-diaminomitosene could be further metabolized by DTD resulting in the formation of mitosene adducts with GSH. DNA cross-linking during either chemical (sodium borohydride) or enzymatic (DTD) mediated reduction of mitomycin C could be observed at pH 7.4, but it increased as the pH was decreased to 5.8, showing the critical role of pH in the cross-linking process. These data provide unequivocal evidence that the obligate two-electron reductase DTD can bioactivate mitomycin C to reactive species which can form adducts with GSH and DNA and induce DNA cross-linking. The use of mitomycin C may be a viable approach to the therapy of tumors high in DTD activity, particularly when combined with strategies to lower tumor pH.

Mechanism-based inhibition of thioredoxin reductase by antitumor quinoid compounds

Quinoids undergo metabolism by a number of flavoenzymes. Reactive species formed during the metabolism of some quinoids might be anticipated to inhibit flavoenzyme activity. Several quinoids have been tested for their ability to inhibit rat liver thioredoxin reductase (TR). The antitumor quinones diaziquone and doxorubicin, and the quinoneimine 2,6-dichloroindophenol, were found to be inhibitors of the reduction of 5,5'-dithiobis-2-nitrobenzoic acid (DTNB) by TR. The inhibition was most marked after incubation of the quinoid with NADPH and the enzyme for 60 min before adding DTNB, with Ki values of 0.5 microM for diaziquone, 0.5 microM for doxorubicin, and 0.07 microM for 2,6-dichloroindophenol. The three quinoids all produced a time-dependent and first order loss of TR activity. There was formation of electron spin resonance-detectable semiquinoid free radicals upon incubation of diaziquone, doxorubicin and 2,6-dichloroindophenol with TR and NADPH under anaerobic conditions. Oxygen radicals formed by redox cycling of the quinoids did not make a major contribution to the inhibition of TR by the quinoids, as shown by the absence of significant reversal of the inhibition by anaerobic incubation conditions and the lack of effect of the oxygen radical scavengers dimethyl sulfoxide, superoxide dismutase and catalase. It was not possible to demonstrate NADPH-dependent covalent binding of radiolabeled diaziquone or doxorubicin to the TR apoprotein. It is possible that the quinoids bind noncovalently to the enzyme apoprotein, or bind to the FAD prosthetic group. The results of the study suggest that some antitumor quinoids are mechanism-based inhibitors of TR showing metabolism- and time-dependent irreversible inhibition of enzyme activity.

Enzymology of the reduction of the potent benzotriazine-di-N-oxide hypoxic cell cytotoxin SR 4233 (WIN 59075) by NAD(P)H: (quinone acceptor) oxidoreductase (EC 1.6.99.2) purified from Walker 256 rat tumour cells

3-Amino-1,2,4-benzotriazine-1,4-dioxide (SR 4233; WIN 59075) is a highly selective hypoxic cell cytotoxin soon to enter phase I clinical trial. The compound is thought to exert its action through a toxic one-electron reduced free radical intermediate. Preliminary data have suggested that SR 4233 may be metabolized by DT-diaphorase [NAD(P)H: (quinone acceptor) oxidoreductase (EC 1.6.99.2)] to both two- and four-electron reduced products and that this route of biotransformation may represent a bioprotection pathway. In this study, a highly purified enzyme preparation was employed in order to investigate further the metabolism of SR 4233 by DT-diaphorase and to examine the mechanism of reduction in more detail. Spectrophotometric analysis showed that SR 4233 underwent reduction by DT-diaphorase with an apparent Km of 1.23 +/- 0.27 mM and Vmax of 8.55 +/- 1.67 nmol/min/microgram protein. This reaction was inhibited completely by dicoumarol (100 microM) and partially by an antiserum raised against the purified enzyme. Characterization of the products of SR 4233 reduction by reverse-phase HPLC confirmed that both two- (SR 4317) and four- (SR 4330) electron reduction products were generated, the latter being the predominant metabolite, particularly in prolonged incubations. Further experiments showed that the four-electron reduction product, but not the two-electron reduction product, was also a substrate for DT-diaphorase with an apparent Km of 1.14 mM and a Vmax of 57.12 nmol/min/micrograms protein. The results presented confirm that SR 4233 is indeed a substrate for DT-diaphorase and that a mixture of two-, four- and six-electron reduced products may be formed. The possible toxicological and pharmacodynamic significance of this metabolism is discussed.

Bioreductive mechanisms

The design, development, and application of bioreductive antitumor agents in a rational way requires a detailed understanding of the mechanisms involved in their action. In addition to measuring and manipulating tumor hypoxia, we need to elucidate the particulars of the activation versus bioprotection pathways and the nature and properties of the participating enzymes. These areas are reviewed with particular reference to the development of novel quinone, nitro and N-oxide bioreductives.

The role of NAD(P)H:quinone oxidoreductase in mitomycin C- and porfiromycin-resistant HCT 116 human colon-cancer cells

A mitomycin C (MMC)- and porfiromycin (PFM)-resistant subline of the HCT 116 human colon-cancer cell line was isolated after repeated exposure of HCT 116 cells to increasing concentrations of MMC under aerobic conditions. The MMC-resistant subline (designated HCT 116-R30A) was 5 times more resistant than the parent cells to MMC and PFM under aerobic conditions. Both the MMC-resistant cells and the parent HCT 116 cells accumulated similar amounts of PFM by passive diffusion, but levels of macromolecule-bound PFM were about 50% lower in the resistant cell line, implying a decrease in PFM reductive activation in the resistant cells. The finding that microsomes from either sensitive or resistant cells showed an equal ability to reduce MMC and PFM indicated that the activity of NADPH cytochrome P-450 reductase (EC 1.6.2.4) was not changed in the resistant subline. Soluble extracts of HCT 116 cells reduced MMC and PFM more effectively at pH 6.1, and NADH and NADPH were utilized equally well as electron donors under both aerobic and anaerobic conditions. These data suggest that quinone reductase (EC 1.6.99.2; DT-diaphorase) in soluble extracts is responsible for the reduction of MMC. Quinone reductase activities in soluble extracts of HCT 116-R30A cells for the reduction of dichlorophenol indophenol (DCPIP) and menadione-cytochrome c at optimal pHs were decreased by 95% as compared with those obtained in parent cells. However, the MMC-reducing activity of HCT 116-R30A soluble extracts was only 50% lower than that of the parent cell extracts. The kinetic constants (Km, Vmax) found for quinone reductase in the two cell lines with respect to the substrates DCPIP and menadione differed. Two species of mRNA for quinone reductase (2.7 and 1.2 kb) were detected in both cell lines, and there was no detectable difference between parent and resistant cells in the steady-state level of either of these mRNA species. Furthermore, incubation with the quinone reductase inhibitor dicoumarol rendered HCT 116 cells more resistant to MMC. Alteration of the quinone reductase activity in HCT 116-R30A cells appears to be the mechanism responsible for their resistance to MMC and PFM.

The role of human and rodent DT-diaphorase in the reductive metabolism of hypoxic cell cytotoxins

DT-diaphorase is a unique two electron (2e) donating reductase catalyzing either bioactivation or bioprotection reactions. Using human and rodent DT-diaphorase preparations (cell extracts and purified enzyme) we have characterized the reductive metabolism of the hypoxic cell cytotoxins EO9, mitomycin C (MMC), CB 1954, and SR 4233 in vitro. Drug metabolism was assayed spectrophotometrically or by HPLC, with dicoumarol as a selective inhibitor. DNA damage was measured using an agarose gel mobility technique with plasmid pBR322 DNA. The developmental indoloquinone, EO9, was metabolized by both rat Walker and human HT29 tumor DT-diaphorases. Reduction proceeded 5-fold more efficiently with the rat than the human tumor enzyme and resulted in single-strand breaks in plasmid DNA. The structurally related MMC was metabolized much more slowly than EO9 by the rat Walker tumor enzyme and there was no detectable reaction with the human HT29 tumor DT-diaphorase. No DNA damage was seen with MMC for either enzyme. The dinitrophenylaziridine CB 1954 was reduced by both human and rat enzymes forming, preferentially, the highly toxic 4-hydroxylamine as a 4e reduction product. Rates were 3-fold lower than for the human tumor enzyme. SR 4233 was also reduced by the rat tumor enzyme predominantly via 4e reduction to the benzotriazine SR 4330, in a novel reaction mechanism. This appears to be a bioprotection pathway that bypasses the toxic 1e radical formed by other reductases. Such information may be valuable in the selection of hypoxic cell cytoxins to treat human tumors high or low in DT-diaphorase and should facilitate 'enzyme-directed' analogue development.

Kidney cortical necrosis induced by mitomycin-C: a morphologic experimental study

Experimental studies on mitomycin-C nephrotoxicity are scanty and mention the occurrence of cortical hemorrhage, tubular necrosis, or hydronephrosis secondary to papillomatous hyperplasia of the uroepithelium. To our knowledge, only one experimental study has mentioned morphological lesions similar to the hemolytic uremic syndrome in the human. In the present study 40 female Wistar rats were studied following unilateral renal perfusion of the left kidney with 2 mg/kg of mitomycin-C. Renal lesions corresponded to cortical necrosis with the presence of large bizarre nuclei. The presence of these nuclear atypias supports a direct toxic effect (alkylation) of the mitomycin or its metabolites on cells.

Studies on the mechanism of resistance to mitomycin C and porfiromycin in a human cell strain derived from a cancer-prone individual

The mechanism of aerobic resistance to the quinone-containing anti-tumour agents mitomycin C (MMC) and porfiromycin (PM) has been investigated using non-transformed human cells. One of the cell strains used (3437T) was derived from an afflicted member of a cancer-prone family. This cell strain had been shown previously to be six times more resistant to the cytotoxic effects of these agents under aerobic but not hypoxic conditions when compared to a cell strain derived from an unrelated, normal donor (GM38). Differences could not be detected in the ability of cell sonicates prepared from either cell strain to produce alkylating species under aerobic conditions using a 4-(p-nitrobenzyl)pyridine assay. However, using 3H-labelled PM to monitor rapid drug uptake and subsequent accumulation due to drug metabolism, results were obtained indicating that the resistant cell strain (3437T) was deficient in an enzymatic pathway capable of metabolizing these compounds under aerobic but not hypoxic conditions. Dicumarol, an inhibitor of the quinone reductase DT-diaphorase (EC 1.6.99.2), decreased aerobic drug accumulation and cytotoxicity in the control cell strain, but did not alter the lack of accumulation noted in the resistant cell strain. Under hypoxic conditions, dicumarol increased cytotoxicity and drug accumulation in both cell strains. The mechanism of this enhanced cytotoxicity remains unclear. These results suggested that the resistant cells were deficient in the enzyme DT-diaphorase, a potential activator of PM. Enzymatic assays confirmed this and revealed no alterations in cytochrome P450 reductase (EC 1.6.2.4) activity or glutathione content. No protein characteristic of DT-diaphorase was detected in the resistant cell strain using a polyclonal rabbit-anti-rat antibody raised against this enzyme. Southern blot analysis using a rat DT-diaphorase cDNA probe demonstrated differences between the normal and resistant cell strains in the restriction fragment patterns. The present results are consistent with the hypothesis that decreased DT-diaphorase levels are causally associated with PM and MMC resistance in these cells under aerobic exposure conditions.

The differences in kinetics of rat and human DT diaphorase result in a differential sensitivity of derived cell lines to CB 1954 (5-(aziridin-1-yl)-2,4-dinitrobenzamide)

DT diaphorase (NAD(P)H dehydrogenase (quinone), EC 1.6.99.2) isolated from Walker 256 rat carcinoma cells can convert CB 1954 (5-(aziridin-1-yl)-2,4-dinitrobenzamide) to a cytotoxic DNA interstrand cross-linking agent. This is achieved by reduction of the 4-nitro group of CB 1954 to produce the hydroxylamino species, a bioactivation which accounts for the much greater sensitivity of Walker cells to CB 1954 when compared with other cells which are unable to carry out this reduction (Knox et al., Biochem Pharmacol 37: 4661-4669 and 4671-4677, 1988). As predicted from their measured DT diaphorase activities a number of rat hepatoma and hepatocyte cell lines were also shown to be sensitive to CB 1954. However, no CB 1954-sensitive cell lines of human origin were found, although levels of DT diaphorase similar to those in the sensitive rat cells were present in these cells. The human cells were as sensitive as rat cells to the active form of CB 1954 (5-(aziridin-1-yl)-4-hydroxyla mino-2-nitrobenzamide). DT diaphorase, purified to homogeneity from human Hep G2 cells, did metabolize CB 1954 to this 4-hydroxylamino product, but the rate of CB 1954 reduction and thus production of the cytotoxic product, was much lower than that of purified Walker enzyme (ratio of Kcat = 6.4). In addition, CB 1954 could be considered an inhibitor of, rather than a substrate for, the human form of DT diaphorase. The purified rat and human DT diaphorases possessed otherwise similar biochemical and molecular properties. These findings explain the decreased sensitivity towards CB 1954 of human cell lines when compared to rat cell lines.

The effects of three bioreductive drugs (mitomycin C, RSU-1069 and SR4233) on cell lines selected for their sensitivity to mitomycin C or ionising radiation

We have investigated the cross-sensitivity of a number of cell lines to three different classes of bioreductive drugs under both aerobic and hypoxic conditions. The cell lines used were selected for their sensitivity to DNA-damaging agents and fall into two groups. One group, MMC cells derived from CHO-K1 cells (Robson et al., 1985), show a range of sensitivities to mitomycin C in air. The second group, irs cells were cloned from V79 Chinese hamster fibroblasts (Jones et al., 1987) and exhibit sensitivity to ionising radiation. The sensitivity of both groups of cells to mitomycin C (MMC), RSU-1069 and SR4233 was assessed under aerobic and hypoxic conditions. No difference in aerobic or hypoxic toxicity of MMC was observed for CHO-K1 or MMC sensitive cell lines (MMC-2 and MMC-3). However, the MMC-resistant cell line (MMCr) was 10 times more sensitive under hypoxic than aerobic conditions. This suggests that MMCr cells lack or are deficient in the enzymes responsible for activating MMC under aerobic conditions compared to other MMC cells. In contrast, differential toxicities of between 3 and 30 have been observed for all CHO cells treated with RSU-1069 and SR4233. Treatment of V79 and irs cells with RSU-1069 and SR4233 also resulted in selective toxicity towards hypoxic cells. Differential toxicities between 50 and 100 were observed for V79 cells. For both RSU-1069 and SR4233, the hypoxic toxicities were similar in V79 and irs cells but in air, the radiation sensitive cells were up to 10 times more sensitive than wild type cells.

Cytosolic NAD(P)H:(quinone-acceptor)oxidoreductase in human normal and tumor tissue: effects of cigarette smoking and alcohol

NAD(P)H:(quinone-acceptor)oxidoreductase (QAO), previously known as DT-diaphorase, catalyzes the reduction of quinones to hydroquinones. Enhanced activity of the enzyme has been suggested to protect cells against the cellular toxicity and carcinogenicity of quinones, but may activate some cytotoxic anti-tumor quinones. Cytosolic levels of QAO, carbonyl reductase (CR) and total quinone reductase activity have been measured in normal and tumorous human tissues. QAO was the major component of the total cytosolic quinone reductase activity in all the tissues investigated. CR represented 10 to 28% of the total cytosolic quinone reductase activity in normal tissue. Normal tissue QAO was high in the stomach and kidney, and lower in the lung, liver, colon and breast. Primary tumor from lung, liver, colon and breast had elevated levels of QAO compared to normal tissue, while tumor from kidney and stomach had lower levels. CR was not significantly altered in tumor tissue, except in the case of lung and colon tumor which showed an increase compared to normal tissue. A major determinant of the variability of human lung tumor QAO was the cigarette-smoking history of the donor. Non-smokers and past smokers had high levels of tumor QAO compared to normal tissue. Smokers had levels of tumor QAO that were not significantly different from those of normal tissue QAO. Smokers had a small increase in normal lung QAO compared to non-smokers. Alcohol use was associated with an increase in lung tumor QAO but had no effect on QAO in normal lung. The function of QAO in tumors is not known but the elevated activity of QAO in some tumors and the apparent depressant effect of smoking could influence the response of these tumors to quinone drugs or toxic agents that are metabolized by QAO.

Free radical formation by antitumor quinones

Quinones are among the most frequently used drugs to treat human cancer. All of the antitumor quinones can undergo reversible enzymatic reduction and oxidation, and form semiquinone and oxygen radicals. For several antitumor quinones enzymatic reduction also leads to formation of alkylating species but whether this involves reduction to the semiquinone or the hydroquinone is not always clear. The antitumor activity of quinones is frequently linked to DNA damage caused by alkylating species or oxygen radicals. Some other effects of the antitumor quinones, such as cardiotoxicity and skin toxicity, may also be related to oxygen radical formation. The evidence for a relationship between radical formation and the biological activity of the antitumor quinones is evaluated.