Two structurally related diaziridinylbenzoquinones preferentially cross-link DNA at different sites upon reduction with DT-diaphorase

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.

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.

Identification and Validation of Small Molecules That Enhance Recombinant Adeno-associated Virus Transduction following High-Throughput Screens

While the recent success of adeno-associated virus (AAV)-mediated gene therapy in clinical trials is promising, challenges still face the widespread applicability of recombinant AAV(rAAV). A major goal is to enhance the transduction efficiency of vectors in order to achieve therapeutic levels of gene expression at a vector dose that is below the immunological response threshold. In an attempt to identify novel compounds that enhance rAAV transduction, we performed two high-throughput screens comprising 2,396 compounds. We identified 13 compounds that were capable of enhancing transduction, of which 12 demonstrated vector-specific effects and 1 could also enhance vector-independent transgene expression. Many of these compounds had similar properties and could be categorized into five groups: epipodophyllotoxins (group 1), inducers of DNA damage (group 2), effectors of epigenetic modification (group 3), anthracyclines (group 4), and proteasome inhibitors (group 5). We optimized dosing for the identified compounds in several immortalized human cell lines as well as normal diploid cells. We found that the group 1 epipodophyllotoxins (teniposide and etoposide) consistently produced the greatest transduction enhancement. We also explored transduction enhancement among single-stranded, self-complementary, and fragment vectors and found that the compounds could impact fragmented rAAV2 transduction to an even greater extent than single-stranded vectors. In vivo analysis of rAAV2 and all of the clinically relevant compounds revealed that, consistent with our in vitro results, teniposide exhibited the greatest level of transduction enhancement. Finally, we explored the capability of teniposide to enhance transduction of fragment vectors in vivo using an AAV8 capsid that is known to exhibit robust liver tropism. Consistent with our in vitro results, teniposide coadministration greatly enhanced fragmented rAAV8 transduction at 48 h and 8 days. This study provides a foundation based on the rAAV small-molecule screen methodology, which is ideally used for more-diverse libraries of compounds that can be tested for potentiating rAAV transduction.

IMPORTANCE

This study seeks to enhance the capability of adeno-associated viral vectors for therapeutic gene delivery applicable to the treatment of diverse diseases. To do this, a comprehensive panel of FDA-approved drugs were tested in human cells and in animal models to determine if they increased adeno-associated virus gene delivery. The results demonstrate that particular groups of drugs enhance adeno-associated virus gene delivery by unknown mechanisms. In particular, the enhancement of gene delivery was approximately 50 to 100 times better with than without teniposide, a compound that is also used as chemotherapy for cancer. Collectively, these results highlight the potential for FDA-approved drug enhancement of adeno-associated virus gene therapy, which could result in safe and effective treatments for diverse acquired or genetic diseases.

Phase I pharmacokinetic and pharmacodynamic study of the bioreductive drug RH1

BACKGROUND

This trial describes a first-in-man evaluation of RH1, a novel bioreductive drug activated by DT-diaphorase (DTD), an enzyme overexpressed in many tumours.

PATIENTS AND METHODS

A dose-escalation phase I trial of RH1 was carried out. The primary objective was to establish the maximum tolerated dose (MTD) of RH1. Secondary objectives were assessment of toxicity, pharmacokinetic determination of RH1 and pharmacodynamic assessment of drug effect through measurement of DNA cross linking in peripheral blood mononuclear cells (PBMCs) and tumour, DTD activity in tumour and NAD(P)H:quinone oxidoreductase 1 (NQO1) polymorphism status.

RESULTS

Eighteen patients of World Health Organization performance status of zero to one with advanced refractory solid malignancies were enrolled. MTD was 1430 μg/m(2)/day with reversible bone marrow suppression being dose limiting. Plasma pharmacokinetic analysis showed RH1 is rapidly cleared from blood (t(1/2) = 12.3 min), with AUC increasing proportionately with dose. The comet-X assay demonstrated dose-related increases in DNA cross linking in PBMCs. DNA cross linking was demonstrated in tumours, even with low levels of DTD. Only one patient was homozygous for NQO1 polymorphism precluding any conclusion of its effect.

CONCLUSIONS

RH1 was well tolerated with predictable and manageable toxicity. The MTD of 1430 μg/m(2)/day is the dose recommended for phase II trials. The biomarkers of DNA cross linking, DTD activity and NQO1 status have been validated and clinically developed.

Measurement of DNA interstrand crosslinking in naked DNA using gel-based methods

Bifunctional DNA damaging agents continue to be the mainstay in various chemotherapeutic regimens used in the clinic. DNA interstrand crosslinks are considered to be the critical cytotoxic lesions for the biological activity of such agents. Gel-based electrophoretic assays can efficiently separate denatured single-stranded DNA from double-stranded, covalently-linked DNA resulting from the presence of an interstrand crosslink. The methods described here offer a simple way for the assessment of crosslinking efficiencies of bifunctional agents in both long fragments of DNA (e.g. 1-5 kb) and short oligonucleotide DNA duplexes. As the repair of interstrand crosslinks is a key determinant of cellular and clinical chemosensitivity, these methods can be useful for the characterization and isolation of site-directed adducted substrates for use in subsequent biochemical analysis of cellular recognition and DNA repair processes.

Preclinical efficacy of the bioreductive alkylating agent RH1 against paediatric tumours

Background:

Despite substantial improvements in childhood cancer survival, drug resistance remains problematic for several paediatric tumour types. The urgent need to access novel agents to treat drug-resistant disease should be expedited by pre-clinical evaluation of paediatric tumour models during the early stages of drug development in adult cancer patients.

Methods/results:

The novel cytotoxic RH1 (2,5-diaziridinyl-3-[hydroxymethyl]-6-methyl-1,4-benzoquinone) is activated by the obligate two-electron reductase DT-diaphorase (DTD, widely expressed in adult tumour cells) to a potent DNA interstrand cross-linker. In acute viability assays against neuroblastoma, osteosarcoma, and Ewing′s sarcoma cell lines RH1 IC₅₀ values ranged from 1-200 nM and drug potency correlated both with DTD levels and drug-induced apoptosis. However, synergy between RH1 and cisplatin or doxorubicin was only seen in low DTD expressing cell lines. In clonogenic assays RH1 IC₅₀ values ranged from 1.5–7.5 nM and drug potency did not correlate with DTD level. In A673 Ewing's sarcoma and 791T osteosarcoma tumour xenografts in mice RH1 induced apoptosis 24 h after a single bolus injection (0.4 mg/kg) and daily dosing for 5 days delayed tumour growth relative to control.

Conclusion:

The demonstration of RH1 efficacy against paediatric tumour cell lines, which was performed concurrently with the adult Phase 1 Trial, suggests that this agent may have clinical usefulness in childhood cancer.

Role of NADPH cytochrome P450 reductase in activation of RH1

PURPOSE

RH1 is a new bioreductive agent that is an excellent substrate for the two-electron reducing enzyme, NAD(P)H quinone oxidoreductase 1 (NQO1). RH1 may be an effective NQO1-directed antitumor agent for treatment of cancer cells having elevated NQO1 activity. As some studies have indicated that RH1 may also be a substrate for the one-electron reducing enzyme, NADPH cytochrome P450 reductase (P450 Red), P450 Red may contribute to the activation of RH1 where NQO1 activities are low and P450 Red activities are high. The mean P450 Red activity in the human tumor cell line panel used by NCI for evaluation of new anticancer agents is 14.8 nmol min(-1) mg prot(-1), while the mean NQO1 activity in these cell lines is 199.5 nmol min(-1) mg prot(-1). Thus, we investigated whether P450 Red could play a role in activating RH1.

METHODS

Reduction of RH1 by purified human P450 Red was investigated using electron paramagnetic resonance and spectroscopic assays. The ability of RH1 to produce DNA damage following reduction by P450 Red was studied using gel assays. To determine the role of P450 Red in activation of RH1 in cells, cell growth inhibition studies with inhibitors of P450 Red and NQO1 were carried out in T47D human breast cancer cells and T47D cells transfected with the human P450 Red gene (T47D-P450) that have P450 Red activities of 11.5 and 311.8 nmol min(-1) mg prot(-1), respectively.

RESULTS

Reduction studies using purified P450 Red and NQO1 confirmed that RH1 can be reduced by both enzymes, but redox cycling was slower following reduction by NQO1. RH1 produced DNA strand breaks and crosslinks in isolated DNA after reduction by either P450 Red or NQO1. DPIC, an inhibitor of P450 Red, had no effect on cell growth inhibition by RH1 in T47D cells, and had only a small effect on cell growth inhibition by RH1 in the presence of the NQO1 inhibitor, dicoumarol, in T47D-P450 cells.

CONCLUSIONS

These results demonstrated that P450 Red does not contribute to the activation of RH1 in cells with normal P450 Red activity and plays only a minor role in activating this agent in cells with high levels of this enzyme. These studies confirmed that P450 Red could activate RH1 and provided the first direct evidence that RH1 could produce both DNA strand breaks and DNA crosslinks after reduction by P450 Red. However, the results strongly suggest that P450 Red does not play a significant role in activating RH1 in cells with normal P450 Red activity.

Preclinical evaluation of the pharmacodynamic properties of 2,5-diaziridinyl-3-hydroxymethyl-6-methyl-1,4-benzoquinone

PURPOSE

The purpose of our study was to investigate the cellular accumulation, DNA cross-linking ability, and cellular toxicity of RH1 (2,5-diaziridinyl-3-[hydroxymethyl[-6-methyl-1,4-benzoquinone), a novel DNA alkylating agent currently in clinical trials. In addition, the in vivo efficacy of RH1 formulated in different vehicles was also compared.

EXPERIMENTAL DESIGN

RH1 is activated by the two-electron reducing enzyme NQO1 [NADPH:quinone oxidoreductase] forming a potent cytotoxic agent that cross-links DNA. We have used whole blood, cell lines, and primary explanted tumor cultures to measure both the cellular accumulation, DNA cross-linking, and cytotoxicity of RH1. Furthermore, the pharmacokinetic and pharmacodynamic characteristics of RH1 formulated in different vehicles were measured in vivo using the validated comet-X assay in mice bearing human tumor xenografts.

RESULTS

Accumulation of RH1 was shown to be both time and concentration dependent, reaching a maximum after 2 hours and correlated well with DNA cross-linking measurements. DNA cross-linking in vitro could be detected at low (1-10 nmol/L) concentrations after as little as 2 hours exposure. In primary tumor cultures, RH1 induces much higher levels of DNA cross-links at lower doses than either mitomycin C or cisplatin. In vivo efficacy testing using polyvinyl pyrrolidone, saline, or cyclodextrin as vehicles showed DNA cross-links readily detectable in all tissues examined and was enhanced when given in cyclodextrin compared with polyvinyl pyrrolidone or saline.

CONCLUSIONS

RH1 represents a potent bioreductive anticancer drug, which may prove effective in the treatment of cancers, particularly those that overexpress NQO1. DNA cross-linking can be reliably measured in tissue using the validated comet-X assay.

Effect of NQO1 induction on the antitumor activity of RH1 in human tumors in vitro and in vivo

NQO1 is a reductive enzyme that is important for the activation of many bioreductive agents and is a target for an enzyme-directed approach to cancer therapy. It can be selectively induced in many tumor types by a number of compounds including dimethyl fumarate and sulforaphane. Mitomycin C is a bioreductive agent that is used clinically for treatment of solid tumors. RH1 (2,5-diaziridinyl-3-(hydroxymethyl)- 6-methyl-1,4-benzoquinone) is a new bioreductive agent currently in clinical trials. We have shown previously that induction of NQO1 can enhance the antitumor activity of mitomycin C in tumor cells in vitro and in vivo. As RH1 is activated selectively by NQO1 while mitomycin C is activated by many reductive enzymes, we investigated whether induction of NQO1 would produce a greater enhancement of the antitumor activity of RH1 compared with mitomycin C. HCT116 human colon cancer cells and T47D human breast cancer cells were incubated with or without dimethyl fumarate or sulforaphane followed by mitomycin C or RH1 treatment, and cytotoxic activity was measured by a clonogenic (HCT116) or MTT assay (T47D). Dimethyl fumarate and sulforaphane treatment increased NQO1 activity by 1.4- to 2.8-fold and resulted in a significant enhancement of the antitumor activity of mitomycin C, but not of RH1. This appeared to be due to the presence of a sufficient constitutive level of NQO1 activity in the tumor cells to fully activate the RH1. Mice were implanted with HL60 human promyelocytic leukemia cells, which have low levels of NQO1 activity. The mice were fed control or dimethyl fumarate-containing diet and were treated with RH1. NQO1 activity in the tumors increased but RH1 produced no antitumor activity in mice fed control or dimethyl fumarate diet. This is consistent with a narrow window of NQO1 activity between no RH1 activation and maximum RH1 activation. This study suggests that selective induction of NQO1 in tumor cells is not likely to be an effective strategy for enhancing the antitumor activity of RH1. In addition, we found that RH1 treatment produced significant leukopenia in mice that may be of concern in the clinic. These results suggest that the ease of reduction of RH1 by NQO1 makes it a poor candidate for an enzyme-directed approach to cancer therapy.

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.

High throughput studies of gene expression using green fluorescent protein-oxidative stress promoter probe constructs: the potential for living chips

Green fluorescent protein fusions were constructed with several oxidative stress promoters from Escherichia coli. These promoters were chosen for their induction by reactive oxygen species (ROS) such as superoxide, hydrogen peroxide, and hydroxyl radicals. When exposed to various free radical insults, the cells fluoresced with great specificity based on the corresponding ROS. In this work, we propose a way in which these constructs could be used to study the mode of action of a variety of antitumor drugs. This approach offers the possibility of complementing gene chip technology by the creation of living chips for high throughput screening as well as studying differential gene expression.

Cross-linking and sequence-specific alkylation of DNA by aziridinylquinones. 3. Effects of alkyl substituents

The cytotoxicities and DNA cross-linking abilities of several alkyl-substituted diaziridinylquinones have been investigated. The cytotoxicities were determined in DT-diaphorase-rich (H460 and HT29) and -deficient (H596 and BE) cell lines. It was shown that the cytotoxicities in these cell lines correlated with the relative rates of reduction by the purified human enzyme and with the cross-linking efficiencies. The rates of reduction by DT-diaphorase were more dependent on the structures of the compounds than the reduction potentials, as determined by cyclic voltammetry. A computer model was also used to explain high efficiency of cross-linking and the GNC sequence selectivity of the reduced methyl-substituted diaziridinylquinones.

Development and validation of a sensitive solid-phase extraction and high-performance liquid chromatographic assay for the novel bio-reductive anti-tumor agent RH1 in human and mouse plasma

A HPLC assay and solid-phase extraction technique from human plasma has been developed and validated for the experimental anticancer agent, RH1 (2,5-diaziridinyl-3-hydroxymethyl-6-methyl-1,4-benzoquinone) which is currently being evaluated by the CRC phase I/II committee. A 500 mg amino propyl solid-phase extraction cartridge was used to isolate RH1 from human plasma. Analysis was performed on a reversed-phase chromatography system using a 15 cm cyanopropyl column and isocratic elution with a 10% methanol-90% water (double distilled) solution. The lower limit of quantitation for RH1 was found to be 0.00375 microg/ml (3.75 ng/ml+/-8.3%) in water and following extraction from plasma. Recovery of >80%(+/-11.9%) was achieved over a five-day validation study. This method was used to carry out pre-clinical studies in BDF mice (standard strain of hybrid mice) at three dose levels (2, 5 and 10 mg/kg of RH1 in 0.9% (w/v) saline via an intraperotoneal injection). Standard Version of PC Winnonlin pharmacokinetic modelling software was used to model the data. A none-compartmental model was used to describe the disposition of RH1 in mice plasma. RH1 was rapidly eliminated from plasma with a mean plasma clearance of 23.4 ml/min, mean volume of distribution of 321.6 ml and mean t(1/2) alpha and beta decays of 4.8 and 9.6 min, respectively. RH1 in human and mouse whole blood and plasma was found to be stable up to 2 h.

Reactions of halogen-substituted aziridinylbenzoquinones with glutathione. Formation of diglutathionyl conjugates and semiquinones

The reaction between glutathione and 2,5-diaziridinyl-1,4-benzoquinones bearing halogen substituents at C3 and C6 was examined in terms of the formation of glutathionyl-quinone conjugates and semiquinones by HPLC with UV detection, mass spectroscopy and EPR. The reactivity of the halogen atoms toward sulfur substitution is the primary reaction leading to the formation of mono- and di-glutathionyl-substituted quinones. The relative formation of these conjugates depended on the GSH/quinone molar ratios. At GSH/quinone molar ratios below unity, the products observed were the reduced form of the parent quinone, a dithioether derivative and GSSG. Disulfide formation accounted for 60-68% of total GSH consumed. EPR analysis of these reaction mixtures showed a 5-line spectrum (1:2:3:2:1 relative intensities) with 2 equivalent N (aN = 1.98 G) and assigned to the semiquinone form of dichloro- diaziridinylbenzoquinone. Semiquinone quantification by double integration of the EPR signals and interpolation with an adequate standard revealed that the amount of semiquinone formed per GSH consumed was 0.98. At GSH/quinone molar ratios above unity (4, 10 and 100 molar excess of GSH) a pattern of products emerged consisting of 3,6-diglutathionyl quinones with two, one and no aziridinyl moieties, identified by mass spectral analysis. EPR studies revealed that these compounds were minor components of a composite EPR spectrum (a 3-line signal with 1:1:1 relative intensities, 1 equivalent N (aN = 1.73 G) and 1 H (aH = 1.45 G) or a 3-line signal with 1:2:1 relative intensities and 2 equivalent H (aH = 1.4 G). These minor components were assigned to the diglutathionyl conjugates bearing one- or no aziridinyl moiety, respectively. The major component in the EPR signal showed a 3-line spectrum (1:1:1 relative intensity) with 1 equivalent N (aN = 1.7 G) and a g shift of -0.96 G. This spectrum was assigned to a triglutathionyl conjugate of a monoaziridinylbenzoquinone. This major component was also observed when GSH/quinone mixtures were incubated with the two-electron transfer flavoprotein NAD(P)H:quinone oxidoreductase. The semiquinone signals were abolished by superoxide dismutase. In the presence of catalase, the contribution of these components to the overall EPR spectrum was equal. These data are discussed in terms of the one-electron transfer steps encompassed by thiol oxidation and semiquinone formation and the two-electron transfers inherent in sulfur substitution and aziridinyl group loss.

Cross-linking and sequence specific alkylation of DNA by aziridinyl quinones. 2. Structure requirements for sequence selectivity

The cytotoxicities and DNA sequence selectivity for guanine-N7 alkylation of 22 mono- and disubstituted 2,5-diaziridinyl-1,4-benzoquinones have been investigated. Several quinones produced patterns of alkylation following reduction with a selectivity for 5'-TGC-3' sequences. This sequence selectivity appeared to be dependent only on the presence of a hydrogen in position-6 of the quinone. A computer model, based on published crystallographic data, was used to explain this selectivity. The sequence selective quinones were generally more cytotoxic that the quinones which reacted randomly.

Induction of p21 mediated by reactive oxygen species formed during the metabolism of aziridinylbenzoquinones by HCT116 cells

Aziridinylbenzoquinones are a group of antitumor agents that elicit cytotoxicity by generating either alkylating intermediates or reactive oxygen species. The mechanism of toxicity may not always, however, involve profound damage of cellular constituents, but may involve a cytostatic effect through interference with the cell cycle. In this context, we have examined the induction of the cell cycle inhibitor p21 (WAF1, CIP1, or sdi1), whose overexpression suppresses the growth of various tumor cells, in human tumor cells metabolizing 3,6-diaziridinyl-1,4-benzoquinone (DZQ) and its C2,C5-substituted derivatives: 2,5-bis-(carboethoxyamino) (AZQ) and 2, 5-bis-2(-hydroxyethylamino) (BZQ). Both DZQ and AZQ were effectively activated by HCT116 human colonic carcinoma cells; the activation of the former involved largely a dicoumarol-sensitive activity, whereas that of the latter appeared to be accomplished primarily by one-electron transfer reductases. BZQ was not a substrate for the dicoumarol-sensitive enzyme in HCT116 cells. Cellular activation of the first two quinones was associated with formation of oxygen-centered radicals as detected by EPR in conjunction with the spin trap 5,5'-dimethyl-1-pyrroline-N-oxide. The redox transitions of DZQ involved hydroxyl radical formation and were strongly inhibited by catalase, whereas those of AZQ showed a strong superoxide anion component sensitive to superoxide dismutase. These signals were suppressed by N-acetylcysteine with concomitant production of a thiyl radical adduct. This suggests an effective electron transfer between the thiol and free radicals formed during the activation of these quinones. DZQ and AZQ induced significantly the expression of p21 in HCT116 cells, but a 10-fold higher concentration of AZQ was required to achieve the level of induction elicited by DZQ. BZQ had little effect on p21 expression. p21 induction at both mRNA and protein levels correlated with the inhibition of either cyclin-dependent kinase activity or cell proliferation. p21 induction elicited by the above quinones was inhibited by N-acetylcysteine, whereas the non-sulfur analog, N-acetylalanine, was without effect. Catalase and superoxide dismutase did not effect p21 induction by aziridinylbenzoquinones in HCT116 cells, thus suggesting that extracellular sources of oxygen radicals generated by plasma membrane reductases have no influence in the expression of this gene. Hydrogen peroxide, a product of quinone redox cycling, elicited an increase of p21 mRNA levels in HCT116 and K562 human chronic myelogenous leukemia cells. The latter lacks p53, one of the activators of p21 transcription, thus suggesting that p21 expression can be accomplished in a p53-independent manner in these cells. This study suggests that p21 induction is mediated by an increase in the cellular steady-state concentration of oxygen radicals and that the greater effectiveness in p21 induction by DZQ may be related to its efficient metabolism by NAD(P)H:quinone oxidoreductase activity in HCT116 cells.

In vivo activity and hydrophobicity of cytostatic aziridinyl quinones

For a series of 3,6-disubstituted bisaziridinylbenzoquinones the in vivo and in vitro activities against murine tumors, as well as the in vivo toxicity, are analyzed. Properties describing biochemical and physicochemical reactions are also incorporated in the analyses. The important 1-octanol/water partition coefficients were determined, using a fast variation of the shake flask method. New pi'-values were calculated for the substituents in this series. These quinone pi'-values deviate strongly from the standard pi-values, especially for hydrogen-bonding substituents. To discriminate between the toxic and therapeutic activity of the compounds, principal components and partial least squares analyses were applied. Evidence is presented for selective antitumor action of the investigated compounds. The L1210 clonogenic assay only seems to relate to the general cytotoxicity and has no predictive value for in vivo activity for these compounds. The activity is correlated to the hydrophobicity of the quinones. The toxicity correlates with the ease of reduction, contrary to the hypothesis of bioreductive activation as a mechanism for selectivity.

Cross-linking and sequence specific alkylation of DNA BY aziridinylquinones. 1. Quinone methides

The cytotoxicities and DNA cross-linking abilities of 16 1,4-benzoquinones have been investigated. All of the alkylmonoaziridinyl-1,4-benzoquinones were able to interstrand crosslink DNA after reduction and were cytotoxic in vitro. Compounds lacking an aziridine group were unable to cross-link DNA and were less cytotoxic. The methyl analogues were shown to preferentially react at TGC sequences. From comparing the structural requirements for crosslinking and the cytotoxicities, a mechanism has been proposed wherein some hydroquinones can associate and react at TGC sequences in DNA. These hydroquinones can subsequently autoxidize to form a reactive quinone methide which reacts at the opposite strand to form a cross-link.

Molecular dynamics simulations of chlorambucil/DNA adducts. A structural basis for the 5'-GNC interstrand DNA crosslink formed by nitrogen mustards

The alkylation of DNA by chlorambucil has been studied using a computational approach. Molecular dynamics simulations were performed on the fully solvated non-covalent complex, two monoadducts and a crosslinked diadduct of chlorambucil with the d(CGG3G2CGC).-d(GCG1CCCG) duplex, in which the N7 atoms of G1, G2 and G3 are potential alkylation sites. The results provide a structural basis for the preference of nitrogen mustards to crosslink DNA duplexes at a 5'-GNC site (a 1,3 crosslink, G1-G3) rather than at a 5'-GC sites (a 1,2 crosslink, G1-G2). In the non-covalent complex simulation the drug reoriented from a non-interstrand crosslinking location to a position favorable for G1-G3 diadduct formation. It proved possible to construct a G1-G3 diadduct from a structure from the non-covalent simulation, and continue the molecular dynamics calculation without further disruption of the DNA structure. A crosslinked diadduct developed with four BII conformations on the 3' side of each alkylated guanine and of their respective complementary cytosine. In the first monoadduct simulation the starting point was the same DNA conformation used in the crosslinked diadduct simulation with alkylation at G1. In this simulation the DNA deformation was reduced, with the helix returning to a more canonical form. A second monoadduct simulation was started from a canonical DNA conformation alkylated at G3. Here, no significant motion towards a potential crosslinking conformation occurred. Collectively, the results suggest that crosslink formation is dependent upon the drug orientation prior to alkylation and the required deformation of the DNA to permit 1,3 crosslinking can largely be achieved in the non-covalent complex.

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.