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

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.

The anti-tumour compound, RH1, causes mitochondria-mediated apoptosis by activating c-Jun N-terminal kinase

BACKGROUND AND PURPOSE

2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone (RH1) is a bioreductive agent that is activated by the two-electron reductase NAD(P)H quinone oxidoreductase 1 (NQO1). Although the cytotoxic efficacy of RH1 against tumours has been studied extensively, the molecular mechanisms underlying this anti-cancer activity have not yet been fully elucidated.

EXPERIMENTAL APPROACH

2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone-induced apoptosis and related signalling pathways in NQO1-negative and NQO1-overexpressing cells were evaluated. The role of p53 in RH1-induced cell death was investigated using parental and p53-deficient RKO human colorectal cancer cells by assaying clonogenic cell survival. Specific inhibitors and siRNAs targeting factors involved in RH1-induced apoptosis were used to clarify the roles played by such factors in RH1-activated apoptotic signalling pathways.

KEY RESULTS

2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone induced apoptosis and clonogenic death, dependent on NQO1 and p53. Treatment of NQO1-overexpressing cells with RH1 caused rapid disruption of mitochondrial membrane potential, nuclear translocation of apoptosis-inducing factor (AIF) and endonuclease G (Endo G) from mitochondria, and subsequent caspase-independent apoptotic cell death. siRNA targeting AIF and Endo G effectively attenuated RH1-induced apoptotic cell death. Moreover, RH1 induced cleavage of Bax, which targets mitochondria. RH1 significantly activated the c-Jun N-terminal kinase (JNK) pathway, and inhibition of this pathway suppressed RH1-induced mitochondria-mediated apoptosis. RH1-induced generation and mitochondrial translocation of cleaved Bax were blocked by the JNK inhibitor, SP600125. Inhibition of JNK with SP600125 attenuated the mitochondrial translocation of JNK.

CONCLUSIONS AND IMPLICATIONS

2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone activated JNK, resulting in mitochondria-mediated apoptotic cell death that was NQO1-dependent.

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.

Dissecting the role of multiple reductases in bioactivation and cytotoxicity of the antitumor agent 2,5-diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone (RH1)

2,5-Diaziridinyl-3-(hydroxymethyl)-6-methyl-1,4-benzoquinone (RH1) is a novel antitumor diaziridinyl benzoquinone derivative designed to be bioactivated by the two-electron reductase NAD(P)H:quinone oxidoreductase (NQO1) and is currently in clinical trials. NQO1 is expressed at high levels in many solid tumors. RH1 cytotoxicity has been shown previously to be NQO1-dependent. The purpose of this study was to investigate whether other reducing enzymes such as cytochrome b(5) reductase (b5R), cytochrome P450 reductase (P450R), dihydronicotinamide riboside:quinone oxidoreductase 2 (NQO2), and xanthine oxidase/xanthine dehydrogenase (XO/XDH) also contribute to the bioactivation and cytotoxicity of RH1 in human tumor cells. For these studies, we established a series of stable MDA468 breast cancer cell lines overexpressing various levels of NQO1, b5R, P450R, and NQO2 and compared RH1-induced growth inhibition [3-(4,5-dimethylthiazol-2,5-diphenyl)tetrazolium and sulforhodamine B analysis] and interstrand DNA cross-linking (comet analysis) in both parental MDA468 cells and transfected clones. RH1 toxicity correlated with NQO1 and NQO2 but not with either b5R or P450R activity levels in the respective series of transfected MDA468 cell clones. Enzymatic assays showed that RH1 was an in vitro substrate for xanthine oxidase. However, XO/XDH protein and activity could not be detected in a variety of human tumor cell lines. These studies suggest that NQO1 and NQO2 are the principal enzymatic determinants of RH1 bioactivation in MDA468 tumor cells and that b5R, P450R, and XDH/XO are unlikely to play major roles. Our studies also suggest that NQO2 may be particularly relevant as a bioactivation system for RH1 in NQO1-deficient tumors such as leukemias and lymphomas.

Bioreductive drugs: from concept to clinic

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

Validation of the comet-X assay as a pharmacodynamic assay for measuring DNA cross-linking produced by the novel anticancer agent RH1 during a phase I clinical trial

PURPOSE

RH1 is a novel anticancer agent with potent DNA-cross linking activity. RH1 has the potential to be activated within tumors over expressing NQO1, giving maximal antitumour activity with reduced toxicity in normal tissues. RH1 has recently completed a Cancer Research UK sponsored phase I clinical trial at two different centers in the United Kingdom. The comet-X assay was a secondary endpoint in this trial and assay validation was necessary. We describe here this validation process. Whilst it is impossible to cover all variations/conditions of a pharmacodynamic assay, we have strived to evaluate and demonstrate that this assay conforms to the three R's of validation, that is robustness, reliability and reproducibility.

METHODS

K562 and peripheral blood mononuclear cells were treated with either radiation alone, or with a combination of radiation and drug. These samples were then embedded in low melting point agarose and subjected to a modified version of the alkaline single cell gel electrophoresis (Comet) assay, described here as the comet-X assay. Variations in the preparation, electrophoresis, storage and scoring of these samples was investigated. In addition radiation and drug dose response curves were constructed. Finally stability of QC standards was investigated over a 30-month period.

RESULTS

We have demonstrated a linear radiation-dose response in cells up to 20 Gy and drug induced DNA cross-linking up to 50 nM. From the radiation dose response curves we were able to show that the relative inaccuracy measured against a global mean value was less than 25% and the relative (within day) imprecision was less than 30% over all doses. Between day runs produced an intra assay imprecision of 21.2%. Variables involved in the electrophoresis process showed the voltage across all slides in the tank ranged from 3.1 to -2.0 (mV) whilst the current ranged from 0.8-5.5 mA. QC standards were prepared from PBMCs of healthy donors and frozen at -80 degrees C. The stability of these frozen QC standards was measured over a 30-month period. No significant deterioration in any of the control, irradiated or drug treated samples was observed.

CONCLUSIONS

The comet-X assay has been shown to be a robust, reliable and reproducible assay. It is ideally suited for the evaluation of the pharmacodynamic effects of DNA cross-linking agents undergoing early clinical trials. Furthermore, this assay may provide valuable data, in conjunction with pharmacokinetics, when measuring toxicity and efficacy as part of the RH1 phase I clinical trial.