Bioreductive Drugs Research Articles

ChemInform Abstract: Novel Bioreductive Anticancer Agents Based on Indolequinones

The background to bioreductive drugs based on indolequinones is surveyed, and the development of novel cyclopropamitosenes as potent anticancer agents is reviewed. Thiazolylindoles were also investigated as potential inhibitors of topoisomerase II.

The Nitroimidazooxazines (PA-824 and Analogs): Structure–Activity Relationship and Mechanistic Studies

PA-824 is an experimental anti-tubercular agent that has a novel mechanism of action. It is effective against both active and persistent forms of the disease and has recently shown early bactericidal activity in a Phase II clinical trial. This review summarizes recent studies on the mode of action of PA-824 and outlines successful efforts to prepare more effective second-generation analogs. PA-824 displays unusual chemistry following both enzymatic and radiolytic reduction, which is clearly related to its activity as an anti-tubercular agent. The nitroreductase enzyme deazaflavin-dependent nitroreductase, reduces PA-824 with loss of the nitro group, generating reactive nitrogen species such as nitric oxide, which appear important in mediating the activity of the drug. Bioreductive drugs such as PA-824 hold the promise of shorter treatment regimens.

HIF-1alpha and HIF-2alpha are differentially activated in distinct cell populations in retinal ischaemia.

BackgroundHypoxia plays a key role in ischaemic and neovascular disorders of the retina. Cellular responses to oxygen are mediated by hypoxia-inducible transcription factors (HIFs) that are stabilised in hypoxia and induce the expression of a diverse range of genes. The purpose of this study was to define the cellular specificities of HIF-1alpha and HIF-2alpha in retinal ischaemia, and to determine their correlation with the pattern of retinal hypoxia and the expression profiles of induced molecular mediators.Methodology/Principal FindingsWe investigated the tissue distribution of retinal hypoxia during oxygen-induced retinopathy (OIR) in mice using the bio-reductive drug pimonidazole. We measured the levels of HIF-1alpha and HIF-2alpha proteins by Western blotting and determined their cellular distribution by immunohistochemistry during the development of OIR. We measured the temporal expression profiles of two downstream mediators, vascular endothelial growth factor (VEGF) and erythropoietin (Epo) by ELISA. Pimonidazole labelling was evident specifically in the inner retina. Labelling peaked at 2 hours after the onset of hypoxia and gradually declined thereafter. Marked binding to Müller glia was evident during the early hypoxic stages of OIR. Both HIF-1alpha and HIF-2alpha protein levels were significantly increased during retinal hypoxia but were evident in distinct cellular distributions; HIF-1alpha stabilisation was evident in neuronal cells throughout the inner retinal layers whereas HIF-2alpha was restricted to Müller glia and astrocytes. Hypoxia and HIF-alpha stabilisation in the retina were closely followed by upregulated expression of the downstream mediators VEGF and EPO.Conclusions/SignificanceBoth HIF-1alpha and HIF-2alpha are activated in close correlation with retinal hypoxia but have contrasting cell specificities, consistent with differential roles in retinal ischaemia. Our findings suggest that HIF-2alpha activation plays a key role in regulating the response of Müller glia to hypoxia.

ChemInform Abstract: A Large-Scale Synthesis of the Bioreductive Drug 1,4-Bis{[2-(dimethylamino)ethyl] amino}-5,8-dihydroxyanthracene-9,10-dione Bis-N-oxide (AQ4N).

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Abstract SY34-03: Hypoxia-metabolic targets for drug development: Identification of targets and their validation

Abstract The low oxygen status of solid tumours relative to normal tissues has been known for over 50 years. However, methods targeting or exploiting the presence of hypoxia in tumours, for therapeutic benefit, have generally met with little success. Initially, these methods included the use of hypoxic cell radiosensitizers and bioreductive drugs. In the light of current knowledge the reason for “failure” was not necessarily a flaw in the hypotheses being tested, but rather an inability to identify those patients most likely to respond to the modifying treatment. It is now known that tumours vary in their level and extent of hypoxia, with those tumours showing the least hypoxia (highest oxygen tension) being the most responsiveness to therapy. The discovery of the hypoxia-dependent transcription factor, HIF-1, and identification of its downstream gene targets revealed new avenues for targeting tumours. Expression of HIF-1, CAIX, and GLUT-1 have been used as molecular markers of hypoxia and there has been some success in their use as prognostic markers. Further, there has been recent development of a hypoxia metagene signature (Cancer Res. 2007, 67, 3441-9) that has shown prognostic significance in multiple cancers. Despite this progress in identifying molecular markers of hypoxia there has only been one well defined example of using this information for developing novel therapeutic targets; this is the identification and validation of lysyl oxidase (Nature 2006, 440, 1222-6). Associated with hypoxia is an abnormal metabolic phenotype often characterized by high levels of lactate in human tumours. Monocarboxylate transporter-4 (mct-4) functions to transport lactate out of the cell preventing intra-cellular accumulation of lactate and therefore enabling sustained high glycolytic rates and maintenance of intra-cellular pH. We have demonstrated that mct-4 is a hypoxia/HIF-dependent gene and we evaluated its potential as a novel therapeutic target. Firstly, we evaluated whether expression of mct-4 protein in a panel of 155 oropharangeal tumours correlated with outcome of treatment with radiotherapy. Using a univariate analysis to assess high mct-4 expression (top 25% of scores) vs low mct-4 expression (lower 75%) we showed that mct-4 is a significant adverse prognostic factor in this series of biopsies. High mct-4 expression correlated with poor loco-regional control (p = 0.017), reduced cancer-specific survival (p = 0.02) and reduced overall survival (p = 0.055). In a multivariate analysis high mct-4 expression retained prognostic significance for poor loco-regional control (p = 0.007). We have therefore identified mct-4 as a potential target in head and neck cancer and we have gone on to validate its therapeutic significance by carrying out various gene knock-down studies in vitro and in vivo. Using siRNA to mct-4 we have shown that cells lacking mct-4 are substantially more radioresponsive under hypoxic conditions. Hence, not only is mct-4 expression an indicator of adverse outcome but it is also a target for the development of potential therapeutics (particularly when combined with radiotherapy). Citation Format: Ian J. Stratford. Hypoxia-metabolic targets for drug development: Identification of targets and their validation [abstract]. In: Proceedings of the AACR 101st Annual Meeting 2010; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr SY34-03

Abstract 1379: Impact of tumor blood flow modulation on sensitivity of 9L gliosarcoma xenografts to the bioreductive drug, AQ4N

Abstract The bioreductive drug AQ4N is reduced enzymatically under hypoxic conditions to yield AQ4, a cytotoxic metabolite and topoisomerase II inhibitor. This study tests the hypothesis that the two electron reductase DT-diaphorase (NQO1) contributes to AQ4N reduction and that modulation of tumor hypoxia increases AQ4N activity in vivo. A panel of human and rodent tumor cells was assayed for AQ4N sensitivity under normoxic and hypoxic conditions (0.1% O2 for 24 hr), followed by 3 days culture under normoxia without drug. 9L rat gliosarcoma and H460 lung carcinoma, but not 10 other tumor cell lines, exhibited substantially higher (≥ 8 fold) AQ4N sensitivity under hypoxic culture conditions. Investigation of the impact of the DT-diaphorase inhibitors dicumarol and flavone-8-acetic acid on AQ4N cytotoxicity and examination of the cell line dependence of DT-diaphorase protein expression indicated that this enzyme does not contribute to AQ4N cytotoxicity. Next, we investigated whether AQ4N chemosensitivity could be increased by modulating tumor hypoxia in s.c. 9L and H460 tumors implanted in immunodeficient mice. In mice treated with the vasodilator hydralazine, tumor hypoxia was significantly increased in 9L, but not H460 tumors, as determined by pimonidazole staining. Hydralazine also decreased tumor perfusion to a greater extent in 9L tumors than H460 tumors, as judged by uptake of Hoechst 33342. However, hydralazine-induced 9L tumor hypoxia did not increase AQ4N mediated tumor growth delay, in either scid or nude mouse models. Furthermore, combination of AQ4N treatment with inhibition of VEGF receptor-dependent angiogenesis, which increases tumor hypoxia in 9L xenografts, did not augment anti-tumor activity beyond that of VEGF receptor inhibition alone. These findings suggest that the enhancement of AQ4N cytotoxicity in these tumors requires a level of tumor hypoxia that is more extensive or prolonged that can be achieved using these vasodilators and anti-angiogenic agents. Supported in part by NIH grant CA49248 (to D.J.W.). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1379.

Abstract SY34-01: New prodrugs for targeting tumor hypoxia

Abstract SY34-01: New prodrugs for targeting tumor hypoxia

PET imaging of hypoxia using [18F]HX4: a phase I trial

Noninvasive PET imaging of tumour hypoxia could help in the selection of those patients who could benefit from chemotherapy or radiation with specific antihypoxic treatments such as bioreductive drugs or hypoxic radiosensitizers. In this phase I trial, we aimed to determine the toxicity of [(18)F]HX4, a member of the 2-nitroimidazole family, at different dose levels. The secondary aim was to analyse image quality related to the HX4 dose and the timing of imaging. Patients with a histologically proven solid cancer without curative treatment options were eligible for this study. A study design with two dose steps was used in which a single dose of a maximum of 222 MBq (step 1) or 444 MBq (step 2) [(18)F]HX4 was injected. Toxicity was scored on day 0 and on days 3 and 7 after injection, according to the CTCAE 3.0 scoring system. PET/CT images of the largest tumour site were acquired 30, 60 and 120 min after injection. Six patients with stage IV carcinoma were included, four with non-small-cell lung carcinoma, one with thymus carcinoma, and one with colon carcinoma. No toxicity was observed in any of the patients at either dose level. The median tumour to muscle ratio 120 min after injection was 1.40 (range 0.63-1.98). The findings of this study showed that [(18)F]HX4 PET imaging for the detection of hypoxia is not associated with any toxicity. Imaging was successful; however, future trials are needed to determine the optimal image parameters.

The Synthesis of a c(RGDyK) Targeted SN38 Prodrug with an Indolequinone Structure for Bioreductive Drug Release

Preparation of a novel c(RGDyK) targeted SN38 prodrug incorporating an indolequinone structure for bioreductively triggered drug release is described. This design yields a prodrug that targets surface molecules on tumor cells (alpha(v)beta(3) integrins) and releases drug under bioreductive conditions. There are three moieties in the prodrug design, namely a therapeutic drug SN38, an indolequinone structure serving as a drug releasing trigger, and an alpha(v)beta(3) integrin targeting peptide c(RGDyK). Preliminary studies showed that SN38 is released in the presence of a bioreductive enzyme (DT-diaphorase).

The ability of selected pyridinium salts to increase the cytotoxic activity of vincristine but not doxorubicin towards sensitive and multidrug resistant promyelocytic leukaemia HL60 cells

The aim of this study was to examine the effect of selected pyridinium salts, 1-methyl-3-nitropyridine chloride (MNP(+)Cl(-)) and 3,3,6,6,10-pentamethyl-3,4,6,7-tetrahydro-[1,8(2H,5H)-dion]acridine chloride (MDION(+)Cl(-)), on the activity of doxorubicin (DOX) and vincristine (VINC) towards human promyelocytic leukaemia HL60 cells as well as its multidrug resistant (MDR) sublines exhibiting two different phenotypes of MDR related to the overexpression of P-glycoprotein (HL60/VINC) or MRP1 (HL60/DOX). MNP and MDION salts were much less cytotoxic themselves (about 100-fold and 2000-fold compared with DOX and VINC, respectively) against HL60 cells but, in contrast to DOX and VINC, they conserved an important cytotoxic activity towards resistant HL60/VINC and HL60/DOX cells (resistance factor, RF = 2-4.5). It was shown that MNP(+)Cl(-) and MDION(+)Cl(-) increased the cytotoxicity of non-bioreductive antitumour agent VINC towards human promyelocytic leukaemia HL60 cells and its resistant sublines HL60/VINC and HL60/DOX. However, in the case of DOX the decrease in its cytotoxic activity towards all studied cell lines was observed in the presence of MNP(+)Cl(-) and MDION(+)Cl(-). Presented data suggest that the bioreductive drug DOX, in contrast to VINC, could compete with pyridinium salts (MNP(+)Cl(-) and MDION(+)Cl(-)) for NADPH-dependent oxidoreductases and for undergoing cellular reductive activation. This could explain the inefficiency of these salts to increase the cytotoxic activity of DOX against examined leukaemic HL60 cell line and its MDR sublines, HL60/VINC and HL60/DOX.

The role of hypoxia and acidosis in promoting metastasis and resistance to chemotherapy

By a multiplicity of mechanisms, hypoxia and acidosis create a nurturing environment for tumor progression and the evolution of metastatic, drug-resistant cells. Acidosis drives mutagenesis and promotes the subversion of checkpoints and apoptotic mechanisms. Hypoxic tissues secrete cytokines that undermine normal anti-tumor surveillance by macrophages, turning them into accomplices and facilitators of invasion and angiogenesis. Invasiveness is also abetted by acidosis, the result of shifting to an anaerobic glycolytic metabolism. These factors explain the generally poor prognosis indicated by tumors expressing hypoxia-inducible factor-1 (HIF-1). However, these insights into the physiology of hypoxic tumors have inspired the development of new chemotherapeutic approaches directed at these tissues, including bioreductive drugs and gene therapies, some of which are in clinical trials. The ability to target the hypoxic compartment should allow longer progression-free survival and overall survival of patients bearing solid tumor malignancies.

Spin Trapping of Radicals Other Than the •OH Radical upon Reduction of the Anticancer Agent Tirapazamine by Cytochrome P450 Reductase

The radical species produced following one-electron reduction of tirapazamine (3-amino-1,2,4-benzotriazine 1,4-dioxide, TPZ) by cytochrome P(450) reductase-enriched microsomes have been investigated using electron paramagnetic resonance (EPR) spectroscopy. Spin trapping with 5,5'-dimethylpyrroline 1-N-oxide (DMPO) gave a composite spectrum of a carbon-centered radical and the well-known DMPO-OH adduct. Using (17)O-labeled water resulted in a change in the EPR spectrum to that of DMPO-(17)OH, indicating that this radical species is formed with solvent involvement and not from release of a (*)OH radical from one-electron-reduced TPZ. Furthermore, using the closely related spin trap 5-diethoxyphosphoryl-5-methylpyrroline N-oxide (DEPMPO), which is less prone to oxidation than DMPO, gave only a carbon-centered radical spectrum without any involvement of a (*)OH radical. Reduction of a more soluble analogue of TPZ, in redox equilibrium with its 1-oxide derivative, led to spin trapping of both a carbon-centered radical and a nitrogen-centered radical by N-tert-butyl-alpha-phenylnitrone (PBN). The multicentered nature of this nitrogen-centered radical spectrum provides support for the formation of a benzotriazinyl radical following one-electron reduction of this class of bioreductive drug.

Increased Tumor Oxygenation and Drug Uptake During Anti-Angiogenic Weekly Low Dose Cyclophosphamide Enhances the Anti-Tumor Effect of Weekly Tirapazamine (Supplementry Material)

Metronomic cyclophosphamide treatment is associated with anti-angiogenic activity and is anticipated to generate exploitable hypoxia using hypoxia-activated prodrugs. Weekly administration of tirapazamine (TPZ; 5 mg/kg body weight i.p.) failed to inhibit the growth of 9L gliosarcoma tumors grown s.c. in scid mice. However, the anti-tumor effect of weekly cyclophosphamide (CPA) treatment (140 mg/kg BW i.p.) was substantially enhanced by weekly TPZ administration. An extended tumor free period and increased frequency of tumor eradication without overt toxicity were observed when TPZ was given 3, 4 or 5 days after each weekly CPA treatment. Following the 2(nd) CPA injection, Electron Paramagnetic Resonance (EPR) Oximetry indicated significant increases in tumor pO(2), starting at 48 hr, which further increased after the 3(rd) CPA injection. pO(2) levels were, however, stable in growing untreated tumors. A strong negative correlation (-0.81) between tumor pO(2) and tumor volume during 21 days of weekly CPA chemotherapy was observed, indicating increasing tumor pO(2) with decreasing tumor volume. Furthermore, CPA treatment resulted in increased tumor uptake of activated CPA. CPA induced increases in VEGF RNA, which reached a maximum on day 1, and in PLGF RNA which was sustained throughout the treatment, while anti-angiogenic host thrombospondin-1 increased dramatically through day 7 post-CPA treatment. Weekly cyclophosphamide treatment was anticipated to generate exploitable hypoxia. However, our findings suggest that weekly CPA treatment induces a functional improvement of tumor vasculature, which is characterized by increased tumor oxygenation and drug uptake in tumors, thus counter-intuitively, benefiting intratumoral activation of TPZ and perhaps other bioreductive drugs.

1249 Combination of platinum standard first front line chemotherapy and anti-idiotype 1E10/aluminum vaccine in patients with advanced non- small- cell lung cancer (NSCLC)

Background and purpose: Drug toxicities are often a limiting factor in long term treatment regimes used in conjunction with radiotherapy. If the drug could be localized to the tumor site and released slowly, then optimal, intra-tumoral drug concentrations could be achieved without the cumulative toxicity associated with repeated systemic drug dosage. In this paper we describe the use of a biodegradable polymer implant for sustained intra-tumoral release of high concentrations of drugs targeting hypoxic cells.Materials and methods: The RIF-1 tumor was implanted subcutaneously or rntramuscularly in C3H mice and irradiated with 60Co gamma rays. The drug delivery device was the co-polymer CPP-SA;20:80 into which the drug was homogeneously incorporated. The hypoxic radiosensitizer Etanidazole or the bioreductive drug Tirapazamine were delivered intra-tumorally by means of implanted polymer rods containing the drugs. Tumor growth delay (TGD) was used as the end point in these experiments.Results: Both Etanidazole and Tirapazamine potentiated the effects of acute and fractionated radiation in the intra-muscular tumors but neither drug was effective in sub-cutaneous tumors. Since both drugs target hypoxic cells we hypothesized that the lack of effect in the subcutaneous tumor was attributable to the smaller size of the hypoxic fraction in this tumor model. This was confirmed using the hypoxia marker EF5.Conclusions: These results indicate that the biodegradable polymer implant is an effective vehicle for the intra-tumoral delivery of Etanidazole and Tirapazamine and that, in conjunction with radiation, this approach could improve treatment outcome in tumors which contain a sub-population of hypoxic, radioresistant cells.

1250 Phase I study of intra-tumoral injection of a newly developed enzyme-targeting radiosensitizer (KORTUC) containing hydrogen peroxide & sodium hyaluronate for unresectable and/or recurrent malignant neoplasms

Background and purpose: Drug toxicities are often a limiting factor in long term treatment regimes used in conjunction with radiotherapy. If the drug could be localized to the tumor site and released slowly, then optimal, intra-tumoral drug concentrations could be achieved without the cumulative toxicity associated with repeated systemic drug dosage. In this paper we describe the use of a biodegradable polymer implant for sustained intra-tumoral release of high concentrations of drugs targeting hypoxic cells.Materials and methods: The RIF-1 tumor was implanted subcutaneously or rntramuscularly in C3H mice and irradiated with 60Co gamma rays. The drug delivery device was the co-polymer CPP-SA;20:80 into which the drug was homogeneously incorporated. The hypoxic radiosensitizer Etanidazole or the bioreductive drug Tirapazamine were delivered intra-tumorally by means of implanted polymer rods containing the drugs. Tumor growth delay (TGD) was used as the end point in these experiments.Results: Both Etanidazole and Tirapazamine potentiated the effects of acute and fractionated radiation in the intra-muscular tumors but neither drug was effective in sub-cutaneous tumors. Since both drugs target hypoxic cells we hypothesized that the lack of effect in the subcutaneous tumor was attributable to the smaller size of the hypoxic fraction in this tumor model. This was confirmed using the hypoxia marker EF5.Conclusions: These results indicate that the biodegradable polymer implant is an effective vehicle for the intra-tumoral delivery of Etanidazole and Tirapazamine and that, in conjunction with radiation, this approach could improve treatment outcome in tumors which contain a sub-population of hypoxic, radioresistant cells.

Radiosensitising agents for the radiotherapy of cancer: advances in traditional and hypoxia targeted radiosensitisers

Background: Radiotherapy is utilised for the treatment of ∼ 50% of patients with solid tumours, but its efficacy is limited by normal tissue toxicity and by the intrinsic or acquired radioresistance of many tumours. The combination of radiotherapy with chemotherapeutic agents that preferentially sensitise tumour cells to its cytotoxic effects has thus long been considered as a strategy to enhance cancer therapy. However, current chemoradiotherapy protocols remain highly unsatisfactory. Therefore, continuing efforts are being conducted to identify improved radiosensitising agents. Objective: To survey the patent literature and associated peer-reviewed publications of the past 4 years pertaining to the development of novel radiosensitising agents, with a focus on anticancer drugs traditionally used as radiosensitisers and on agents targeting radioresistant hypoxic tumour cells. Methods: Patents were searched with a set of relevant keywords using several search engines (ep.espacenet.com/, www.freepatentsonline.com/, patft.uspto.gov/). A Medline search on the same topics was performed in parallel. Results/conclusion: A total of 37 patents/applications were retrieved. Of these, 14 concern the use of conventional anticancer cytotoxic drugs for tumour radiosensitisation. The other patents mostly disclose novel hypoxic radiosensitisers, bioreductive drugs and inhibitors of hypoxia-inducible factor-1. Whether these advances will translate into clinically valuable radiosensitisers is, however, unclear.

Effects of cytokine-induced macrophages on the response of tumor cells to banoxantrone (AQ4N)

Tumor-associated macrophages (TAMs) are found in many solid tumors and have often been shown to accumulate in the hypoxic regions surrounding areas of necrosis. TAMs are the major site of expression of nitric oxide synthase (NOS), a heme-containing homodimeric enzyme consisting of oxygenase and reductase domains. The latter has a high degree of sequence homology to cytochrome P450 reductase and a functional consequence of this is the ability of NOS, under hypoxic conditions, to activate the bioreductive drugs tirapazamine and RSU1069. Banoxantrone (AQ4N) is a bioreductive prodrug activated in hypoxia by an oxygen-dependent two-electron reductive process to yield the topoisomerase II inhibitor AQ4. A feature of this process is that the final product could potentially show bystander cell killing. Thus, in this study, we investigated the ability of inducible NOS (iNOS)-expressing TAMs to activate AQ4N and elicit toxicity in cocultured human tumor cells. Murine macrophages were induced to overexpress iNOS by treatment with a combination of cytokines, mixed with HT1080 and HCT116 human tumor cells, and the toxicity of AQ4N was determined under aerobic or hypoxic conditions. The aerobic toxicity of AQ4N toward tumor cells was not affected through coculturing with macrophages. However, under hypoxic conditions, the induction of iNOS activity in the macrophages was associated with an increase in AQ4N metabolism and a substantial increase in tumor cell toxicity, which was dependent on the proportion of macrophages in the culture. This study is the first demonstration of TAM-mediated prodrug activation to result in bystander killing of human tumor cells.

Novel Drug Delivery Technologies For The Treatment Of Rheumatoid Arthritis

Rheumatoid Arthritis is a chronic autoimmune disease and a major cause of disability. Current treatment of arthritis involves administration of drugs mainly by oral and parenteral route. Frequent dosing often leads to patient non compliance. Accounting this problem, drug delivery technologies should be developed which reduces frequency of dosing along with sustained release of medicament. Various drug delivery systems are documented like albumin-based drug delivery systems, bio-reductive drug delivery systems, microspheres, nanoparticles, liposomes etc. Conventional drugs like indomethacin, magestrol acetate, methotrexate and cannabidiol have been modified into extended release formulations, nano crystal oral suspensions, topical gel and transdermal patch respectively. Targeting αγβ3 integrin has been shown to enhance drug delivery. Drugs delivered by intranasal route significantly reduce disease severity in experimental collagen induced arthritis model. This review reports a comprehensive overview of newly developed drug delivery technologies as therapeutic targets of rheumatoid arthritis which may potentially reduce adverse extra-articular side effects.

Synthesis, Reduction Potentials, and Antitubercular Activity of Ring A/B Analogues of the Bioreductive Drug (6S)-2-Nitro-6-{[4-(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5H-imidazo[2,1-b][1,3]oxazine (PA-824)

The nitroimidazooxazine S-1 (PA-824) is a new class of bioreductive drug for tuberculosis. A series of related bicyclic nitroheterocycles was synthesized, designed to have a wide range of one-electron reduction potentials E(1) (from -570 to -338 mV, compared with -534 mV for S-1). The observed E(1) values closely correlated with the sigma(m) values of the heteroatom at the 4/8-position of the adjacent six-membered ring. Although the compounds spanned a range of E(1) values around that of S-1, only the nitroimidazothiazines showed significant antitubercular activity (at a similar level of potency), suggesting that E(1) is not the main driver of efficacy. Furthermore, there was a correlation between activity and the formation of imidazole ring-reduced products at the two-electron level, pointing to the potential importance of this reduction pathway, which is determined by the nature of the substituent at the 2-position of the 4-nitroimidazole ring.

Gene therapy approaches to enhance bioreductive drug treatment

Hypoxia, or a lack of oxygen, occurs in 50-60% of solid human tumours. Clinical studies have shown that the presence and extent of hypoxia in a tumour cannot be predicted by size or histopathological stage but it is predictive of a poor outcome following radiotherapy, chemotherapy and surgery. However, as a physiological feature of tumours, it can be exploited and researchers have developed many hypoxia-selective chemotherapies or bioreductive drugs that are in varying stages of clinical development. These agents are prodrugs that have two key requirements for their biological activation: they require the reductive environment of a hypoxic tumour cell and the appropriate complement of cellular reductase enzymes. To overcome tumour heterogeneity in reductase enzyme levels and enhance bioreductive drug metabolism a gene therapy strategy can be employed. We have reviewed this field and also present our own pre-clinical research using gene therapy to enhance bioreductive drug treatment for the treatment of cancer. We have specifically focused on studies enhancing lead clinical bioreductive drugs. We consider the metabolic requirements for their activation and we highlight the key in vivo studies supporting the future clinical development of hypoxia-targeted gene-directed enzyme prodrug therapy.