A two-stage exacerbated hypoxia nanoengineering strategy induced amplifying activation of tirapazamine for microwave hyperthermia-chemotherapy of breast cancer

Microwave hyperthermia (MH) is an emerging treatment for solid tumors, such as breast cancer, due to its advantages of minimally invasive and deep tissue penetration. However, MH induced tumor hypoxia is still an obstacle to breast tumor treatment failure. Therefore, an original nanoengineering strategy was proposed to exacerbate hypoxia in two stages, thereby amplifying the efficiency of activating tirapazamine (TPZ). And a novel microwave-sensitized nanomaterial (GdEuMOF@TPZ, GEMT) is designed. GdEuMOF (GEM) nanoparticles are certified excellent microwave (MW) sensitization performance, thus improving tumor selectivity to achieve MH. Meanwhile MW can aggravate the generation of thrombus and caused local circulatory disturbance of tumor, resulting in the Stage I exacerbated hypoxia environment passively. Due to tumor heterogeneity and uneven hypoxia, GEMT nanoparticles under microwave could actively deplete residual oxygen through the chemical reaction, exacerbating hypoxia level more evenly, thus forming the Stage II of exacerbated hypoxia environment. Consequently, a two-stage exacerbated hypoxia GEMT nanoparticles realize amplifying activation of TPZ, significantly enhance the efficacy of microwave hyperthermia and chemotherapy, and effectively inhibit breast cancer. This research provides insights into the development of progressive nanoengineering strategies for effective breast tumor therapy.

Photoresponsive metal-organic framework with combined photodynamic therapy and hypoxia-activated chemotherapy for the targeted treatment of rheumatoid arthritis

Activated M1-type macrophages, which produce inflammatory factors that exacerbate rheumatoid arthritis (RA), represent crucial target cells for inhibiting the disease process. In this study, we developed a novel photoresponsive targeted drug delivery system (TPNPs-HA) that can effectively deliver the hypoxia-activated prodrug tirapazamine (TPZ) specifically to activated macrophages. After administration, this metal-organic framework, PCN-224, constructed uing the photosensitizer porphyrin, exhibits the ability to generate excessive toxic reactive oxygen species (ROS) when exposed to near-infrared light. Additionally, the oxygen-consumed hypoxic environment further activates the chemotherapeutic effect of TPZ, thus creating a synergistic combination of photodynamic therapy (PDT) and hypoxia-activated chemotherapy (HaCT) to promote the elimination of activated M1-type macrophages. The results highlight the significantly potential of this photoresponsive nano-delivery system in providing substantial relief for RA. Furthermore, these findings support its effectiveness in inhibiting the disease process of RA, thereby offering new possibilities for the development of precise and accurate strategies for RA.

Physical Dissolution Combined with Photodynamic Depletion: A Two-Pronged Nanoapproach for Deoxygenation-Driven and Hypoxia-Activated Prodrug Therapy

Hypoxia may enhance the chemoresistance of cancer cells and can significantly compromise the effectiveness of chemotherapy. Many efforts have been made to relieve or reverse hypoxia by introducing more oxygen into the tumor microenvironment (TME). Acting in a diametrically opposite way, in the current study, a novel nanocarrier was designed to further exhaust the oxygen level of the hypoxic TME. By creating such an oxygen depleted TME, the hypoxia-selective cytotoxin can work effectively, and oxygen exhaustion triggered chemotherapy can be achieved. Herein, deoxygenation agent, FDA-approved perfluorocarbon (PFC) and photosensitizer indocyanine green (ICG) for oxygen depletion, along with the hypoxia-activating drug tirapazamine (TPZ), were coincorporated within the poly(lactic-co-glycolic acid) (PLGA) nanoemulsion (ICG/TPZ@PPs) for the treatment of hypoxic tumors. Following hypoxia amplifying through physical oxygen dissolution and photodynamic depletion in tumors, hypoxic chemotherapy could be effectively activated to improve multitreatment synergy. After achieving local tumor enrichment, PFC-mediated oxygen dissolution combined with further ICG-mediated photodynamic therapy (PDT) under near-infrared (NIR) laser irradiation could induce enhanced hypoxia, which would activate the antitumor activity of codelivered TPZ to synergize cytotoxicity. Remarkably, in vivo experimental results exhibited that deoxygenated ICG/TPZ@PPs-based photothermal therapy (PTT), PDT, and hypoxia activated chemotherapy have an excellent synergistic ablation of tumors without obvious side effects, and therefore, a broad prospect of application of this nanocarrier could be expected.

New insights into the antimicrobial action and protective therapeutic effect of tirapazamine towards Escherichia coli-infected mice

OBJECTIVES

Escherichia coli is an important pathogen responsible for numerous cases of diarrhoea worldwide. The bioreductive agent tirapazamine (TPZ), which was clinically used to treat various types of cancers, has obvious antibacterial activity against E. coli strains. In the present study, we aimed to evaluate the protective therapeutic effects of TPZ in E. coli-infected mice and provide insights into its antimicrobial action mechanism.

METHODS

The MIC and MBC tests, drug sensitivity test, crystal violet assay and proteomic analysis were used to detect the in vitro antibacterial activity of TPZ. The clinical symptoms of infected mice, tissue bacteria load, histopathological features and gut microbiota changes were regarded as indicators to evaluation the efficacy of TPZ in vivo.

RESULTS

Interestingly, TPZ-induced the reversal of drug resistance in E. coli by regulating the expression of resistance-related genes, which may have an auxiliary role in the clinical treatment of drug-resistant bacterial infections. More importantly, the proteomics analysis showed that TPZ upregulated 53 proteins and downregulated 47 proteins in E. coli. Among these, the bacterial defence response-related proteins colicin M and colicin B, SOS response-related proteins RecA, UvrABC system protein A, and Holliday junction ATP-dependent DNA helicase RuvB were all significantly upregulated. The quorum sensing-related protein glutamate decarboxylase, ABC transporter-related protein glycerol-3-phosphate transporter polar-binding protein, and ABC transporter polar-binding protein YtfQ were significantly downregulated. The oxidoreductase activity-related proteins pyridine nucleotide-disulfide oxidoreductase, glutaredoxin 2 (Grx2), NAD(+)-dependent aldehyde reductase, and acetaldehyde dehydrogenase, which participate in the elimination of harmful oxygen free radicals in the oxidation-reduction process pathway, were also significantly downregulated. Moreover, TPZ improved the survival rate of infected mice; significantly reduced the bacteria load in the liver, spleen, and colon; and alleviated E. coli-associated pathological damages. The gut microbiota also changed in TPZ-treated mice, and these genera were considerably differentiated: Candidatus Arthromitus, Eubacterium coprostanoligenes group, Prevotellaceae UCG-001, Actinospica, and Bifidobacterium.

CONCLUSIONS

TPZ may represent an effective and promising lead molecule for the development of antimicrobial agents for the treatment of E. coli infections.

Tirapazamine-loaded UiO-66/Cu for ultrasound-mediated promotion of chemodynamic therapy cascade hypoxia-activated anticancer therapy

Chemodynamic therapy (CDT), an emerging oncology treatment, has received considerable attention owing to its high selectivity, less aggressiveness, and endogenous stimulation. However, the complex intra-tumor environment limits the therapeutic effect. In this study, Cu⁺ was directly doped into the structure of the UiO-66 matrix using an in situ one-pot oil bath method. The as-formed UiO-66/Cu possessed a large surface area, making it feasible to modify folic acid (FA) and carry more chemotherapeutic agents like tirapazamine (TPZ), thus forming UiO-66/Cu-FA-TPZ nanoplatforms. For CDT, the nanoplatform catalyzed the cyclic generation of the highly oxidizing hydroxyl radical (·OH) from H₂O₂. Particularly, low-frequency ultrasound enhanced the curative effect. Notably, in a tumor, a severe hypoxic environment and ultrasound can activate more TPZ for safe and efficient chemotherapy, achieving synergistic and hypoxia-activated tumor treatment with a low risk of side effects. Moreover, the nanoplatform exhibits computed tomography imaging functions for combined diagnosis and treatment. Our designed nanoplatform overcomes the dilemma of insufficient efficacy from conventional therapy attributed to a hypoxic environment, expecting to guide the design of future treatment regimens for hypoxic tumors.

Tirapazamine-loaded CalliSpheres microspheres: Preparation and characterization as a chemoembolization agent for liver cancer

Tirapazamine (TPZ), a hypoxia-selective cytotoxic agent, has proved to exert synergistic tumor-killing activity with transcatheter arterial embolization (TAE) against liver cancer. This advances TPZ to transcatheter therapies for liver cancer, particularly in combination with drug-eluting microspheres. We describe methods for preparing and characterizing TPZ-loaded CalliSpheres microspheres (CSMTPZs) with regard to their properties as a chemoembolization agent, which includes 1) preparation of CSMTPZs and determination of drug loading level, 2) in vitro determination of TPZ release, 3) assessment of CSMTPZ size and appearance, and 4) determination of TPZ pharmacokinetics and intratumoral drug concentration in vivo. These methods can be adapted for further clinical I trial.•This is to our knowledge the first methods for preparing and characterizing tirapazamine-loaded microspheres with regard to their properties as a chemoembolization agent•Detailed protocols for preparation of CSMTPZs, determination of drug loading level, in vitro determination of TPZ release, assessment of CSMTPZ size and appearance, and in vivo determination of TPZ pharmacokinetics and intratumoral drug concentration•Adaptable to experiments on other animal models and clinical trials.

Tirapazamine-loaded CalliSpheres microspheres enhance synergy between tirapazamine and embolization against liver cancer in an animal model

Tirapazamine (TPZ) is a promising hypoxia-selective cytotoxic agent that may exert synergistic tumor-killing activity with transcatheter arterial embolization (TAE) for liver cancer. To investigated whether TPZ-loaded microspheres enhance the synergy between TPZ and TAE in liver cancer, we prepared TPZ-loaded CalliSpheres microspheres (CSMTPZs) and characterized their properties as a chemoembolization agent in vitro. Tumor hypoxia after TAE was detected in the rabbit VX2 model of liver cancer using a modified Clark-type microelectrode research system. CSMTPZ therapy was performed in the animal model. The plasma and tumor concentrations of TPZ and its metabolites were measured, and the efficacy and safety of CSMTPZ therapy were evaluated and compared with those of the conventional combination of intraarterial TPZ injection and embolization. The results showed that CSMTPZs displayed favorable in vitro properties including drug loading and release and microsphere size, shape, and surface profiles. TAE induced acute tumor hypoxia, but residual tumor cells responded to hypoxia through hypoxia-inducible factor 1α. CSMTPZ therapy improved TPZ delivery into tumor tissue with minimal systemic exposure. Accordingly, CSMTPZ therapy exhibited advantages in terms of hypoxia-selected cytotoxicity, tumor apoptosis and necrosis, animal survival, and safety over the conventional combination of TPZ and TAE. We revealed the improved synergistic anti-tumor effects of CSMTPZ therapy in the rabbit VX2 liver cancer model. Our data support the clinical evaluation of CSMTPZs in the treatment of hepatocellular carcinoma, and CSMTPZ administration might serve as a successful therapeutic strategy for this malignancy.

Investigation of oxygen influence to the optical properties of tirapazamine

New data on 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine) fluorescence has been obtained using the Perkin-Elmer Lambda 950 UV-Vis-NIR spectrophotometer experimental technique in combination with the extensive DFT-theory approach. Based on the results obtained, we revealed that the optical properties of the molecule under study remain significantly unchanged when the number of oxygen substitutions decreases from 2 to 0. Here we also present the results of the study of the influence of acetonitrile and ethyl acetate on the fluorescence of tirapazamine with the different number of oxygen atoms. Results of our investigation indicate the formation of anion in the case of 3-amino-1,2,4-benzotriazine 1,4-dioxide with two oxygen atoms and their transformation to tirapazamine with one oxygen atom.

Silk fibroin-capped metal-organic framework for tumor-specific redox dyshomeostasis treatment synergized by deoxygenation-driven chemotherapy

Disturbance in redox homeostasis always leads to oxidative damages to cellular components, which inhibits cancer cell proliferation and causes tumor regression. Therefore, synergistic effects arising from cellular redox imbalance together with other treatment modalities are worth further investigation. Herein, a metal-organic framework nanosystem (NMOF) based on coordination between Fe (III) and 4,4,4,4-(porphine-5,10,15,20-tetrayl) tetrakis (benzoic acid) (TCPP) was synthesized through a one-pot method. After surface capping of silk fibroin (SF) to form NMOF@SF nanoparticles (NPs), this nanoplatform can serve as an eligible nanocarrier to deliver tirapazamine (TPZ), a hypoxia-activated precursor. As-developed NS@TPZ (NST) NPs remained inactive in the normal tissue, whereas became highly active upon endocytosis by tumor cells via glutathione (GSH)-mediated reduction of Fe (III) into Fe (II), further enabling Fe (II)-mediated chemodynamic therapy (CDT). Upon optical laser irradiation, TCPP-mediated photodynamic therapy (PDT) coordinated with CDT to aggravate intracellular oxidative stress. Thus, such reactive oxygen species accumulation and GSH deprivation contributed to a deleterious redox dyshomeostasis. On the other hand, local deoxygenation caused by PDT can increase the cytotoxicity of released TPZ, which significantly improved the integral therapeutic effectiveness relying on the combined redox balance disruption and bioreductive chemotherapy. More importantly, severe immunogenic cell death can be triggered by the combinatorial treatment modalities and the presence of SF, which facilitated an almost complete tumor eradication in vivo. Taken together, this paradigm provides an insightful strategy for tumor-specific redox dyshomeostasis treatment synergized by deoxygenation-driven chemotherapy, which can remarkably enhance antitumor efficacy with negligible adverse effects. STATEMENT OF SIGNIFICANCE: Recently, silk fibroin (SF) has been demonstrated to be effective in activating antitumor immune system through polarization tumor-associated macrophages into M1 subtype. However, engineering SF into multifunctional nanocomposites is seldom reported for combination tumor therapy. In another aspect, disruption of redox homeostasis becomes increasingly attractive for tumor suppression with high clinical-relevance. Herein, we established a newfashioned NMOF nanosystem, named as NST, for tumor-specific redox dyshomeostasis treatment synergized by deoxygenation-driven chemotherapy. This platform takes advantages of Fe²⁺/Fe³⁺ coupled Fenton-like reaction and GSH depletion, as well as TCPP-mediated photosensitization for admirable redox unbalancing, which further initiates hypoxia-relevant toxin of TPZ for chemotherapy. Finally, combinatorial treatments and the presence of SF could trigger ICD for rendering a complete tumor eradication in vivo.

Hypoxia-targeted cupric-tirapazamine liposomes potentiate radiotherapy in prostate cancer spheroids

In this study, novel cupric-tirapazamine [Cu(TPZ)₂]-liposomes were developed as an effective hypoxia-targeted therapeutic, which potentiated radiotherapy in a three dimensional (3D) prostate cancer (PCa) model. To overcome the low water solubility of the Cu(TPZ)₂, a remote loading method was developed to efficiently load the lipophilic complex into different liposomal formulations. The effect of pH, temperature, PEGylation, lipid composition, liposome size, lipid: complex ratio on the liposome properties, and drug loading was evaluated. The highest loading efficiency was obtained at neutral pH, which was independent of lipid composition and incubation time. In addition, enhanced drug loading was achieved upon decreasing the lipid:complex molar ratio with minimal effects on liposomes' morphology. Interestingly, the in vitro potency of the developed liposomes was easily manipulated by changing the lipid composition. The hydrophilic nature of our liposomal formulations improved the complex's solubility, leading to enhanced cellular uptake and toxicity, both in PCa monolayers and tumour spheroids. Moreover, Cu(TPZ)₂-loaded liposomes combined with radiation, showed a significant reduction in PCa spheroids growth rate, compared to the free complex or radiation alone, which could potentiate radiotherapy in patients with localised advanced PCa.

Hypoxic Jumbo Spheroids On-A-Chip (HOnAChip): Insights into Treatment Efficacy

Hypoxia is a key characteristic of the tumor microenvironment, too rarely considered during drug development due to the lack of a user-friendly method to culture naturally hypoxic 3D tumor models. In this study, we used soft lithography to engineer a microfluidic platform allowing the culture of up to 240 naturally hypoxic tumor spheroids within an 80 mm by 82.5 mm chip. These jumbo spheroids on a chip are the largest to date (>750 µm), and express gold-standard hypoxic protein CAIX at their core only, a feature absent from smaller spheroids of the same cell lines. Using histopathology, we investigated response to combined radiotherapy (RT) and hypoxic prodrug Tirapazamine (TPZ) on our jumbo spheroids produced using two sarcoma cell lines (STS117 and SK-LMS-1). Our results demonstrate that TPZ preferentially targets the hypoxic core (STS117: p = 0.0009; SK-LMS-1: p = 0.0038), but the spheroids' hypoxic core harbored as much DNA damage 24 h after irradiation as normoxic spheroid cells. These results validate our microfluidic device and jumbo spheroids as potent fundamental and pre-clinical tools for the study of hypoxia and its effects on treatment response.

Electronic structure and reactivity of tirapazamine as a radiosensitizer

Tirapazamine (TP) has been shown to enhance the cytotoxic effects of ionizing radiation in hypoxic cells, thus making it a candidate for a radiosensitizer. This selective behavior is often directly linked to the abundance of O2. In this paper, we study the electronic properties of TP in vacuum, micro-hydrated from one up to three molecules of water and embedded in a continuum of water. We discuss electron affinities, charge distribution, and bond dissociation energies of TP, and find that these properties do not change significantly upon hydration. In agreement with its large electron affinity, and bond breaking triggered by electron attachment requires energies higher than 2.5 eV, ruling out the direct formation of bioactive TP radicals. Our results suggest, therefore, that the selective behavior of TP cannot be explained by a one-electron reduction from a neighboring O2 molecule. Alternatively, we propose that TP's hypoxic selectivity could be a consequence of O2 scavenging hydrogen radicals.

A rat toxicological study of intra-arterial injection of Tirapazamine, a hypoxia-activating Cancer therapeutic agent, followed by hepatic artery ligation

Background Tirapazamine's (TPZ) tolerability after an intra-arterial (IA) injection remains unclear. We investigated TPZ's safety and tolerability in rats by first injecting into the left hepatic artery and then performing a hepatic artery ligation, which recapitulates the transarterial embolization used clinically. Research design and methods: Forty-six rats in five groups were respectively injected with 0, 0.25, 0.50, 1.0, or more than 1.5 mL IA of TPZ (0.7 mg/mL) into the left hepatic artery and then subjected to hepatic artery ligation under laparotomy. Blood samples were collected four times daily up to day 15 after which the rats were euthanized and necropsied. The toxicity profile of IA injection of TPZ followed by hepatic artery ligation was then assessed. Results No significant changes to the rats' body weight and serum total bilirubin were observed. Serum alanine aminotransferase (ALT) levels increased slightly but remained below 100 U/L one day after treatment for most rats. Three rats in Groups 3 and 4 exhibited an over two-fold transient elevation of ALT. All ALT recovered to the baseline at day 14. Liver tissues were collected on day 15 using H&E staining. One rat in Group 3 showed ischemic coagulative necrosis in its liver tissue. Other sporadic pathological changes not related to TPZ doses were observed in Groups 2, 3, 4, and 5. Conclusion TPZ by IA injection followed by embolization is tolerated up to 7 mg/kg. This finding supports the strategy of administering an IA injection of TPZ followed by trans-arterial embolization to the liver.

Putative electron-affinic radiosensitizers and markers of hypoxic tissue: Synthesis and preliminary in vitro biological characterization of C3-amino-substituted benzotriazine dioxides (BTDOs)

INTRODUCTION

The redox characteristics of 1,2,4-benzotriazine-1,4-dioxides (BTDOs) make them potential radiosensitizing agents for hypoxic cells in solid human cancers. Tirapazamine (TPZ) is the most clinically tested BTDO radiosensitizer, despite its toxicity at effective doses. To date, no BTDOs have been developed as diagnostic markers of tissue hypoxia.

HYPOTHESIS

TPZ analogues with appropriate reporting groups can act as potential radiosensitizers and hypoxia selective diagnostics.

EXPERIMENTAL AND RESULTS

3-Chloro-1,2,4-benzotriazine 1-oxide was substituted at the C3 position to afford 3-(2-hydroxyethoxyethyl)-amino-1,2,4-benzotriazine-1-oxide, which was oxidized to 3-(2-hydroxyethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (HO-EOE-TPZ) or converted to 3-(2-tosyloxyethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (Tos-EOE-TPZ). Tos-EOE-TPZ was intended for use as a synthon for preparing 3-(2-azidoethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (N₃-EOE-TPZ) and 3-(2-iodoethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxide (I-EOE-TPZ). The logP values (-0.69 to 0.61) for these molecules bracketed that of TPZ (-0.34). Cell line dependent cytotoxicities (IC₅₀) in air were in the 10-100 μM range, with Hypoxia Cytotoxicity Ratios (HCR; IC₅₀-air/IC₅₀-hypoxia) of 5-10. LUMO calculations indicated that these molecules are in the optimal redox range for radiosensitization, offering cell-line-specific Relative Radiosensitization Ratios (RRSR; SER/OER) of 0.58-0.88, compared to TPZ (0.67-0.76).

CONCLUSION

The LUMO, IC₅₀, HCR and RRSR values of 3-(2-substituted ethoxyethyl)-amino-1,2,4-benzotriazine-1,4-dioxides are similar to the corresponding values for TPZ, supporting the conclusion that these TPZ analogues are potentially useful as hypoxia-activated radiosensitizers. Further studies into their biodistributions in animal models are being pursued to determine the in vivo potential in hypoxia management.

Effect of Tirapazamine, Metformin or Mild Hyperthermia on Recovery From Radiation-Induced Damage in Pimonidazole-Unlabeled Quiescent Tumor Cells

Background

The aim of the study was to examine the effect of tirapazamine (TPZ) on recovery from radiation-induced damage in pimonidazole-unlabeled quiescent (Q) tumor cells compared with that of metformin (Met) or mild temperature hyperthermia (MTH).

Methods

Proliferating (P) cells in EL4 tumors were labeled by continuous 5-bromo-2'-deoxyuridine (BrdU) administration. Tumors received γ-rays at 1 h after pimonidazole administration followed by Met or TPZ treatment or MTH. Twenty-four hours later, the responses of Q and total (P + Q) cells and those of the pimonidazole-unlabeled cells were assessed with micronucleation and apoptosis frequencies using immunofluorescence staining for BrdU and apoptosis frequency using immunofluorescence staining for pimonidazole, respectively.

Results

With γ-rays only, the pimonidazole-unlabeled cell fraction showed significantly enhanced radio-sensitivity compared with the whole cell fraction more remarkably in Q than total cells. However, a significantly greater decrease in radio-sensitivity in the pimonidazole-unlabeled than the whole cell fraction, evaluated using a delayed assay, was more clearly observed in Q than total cells. Post-irradiation MTH or Met treatment more clearly repressed the decrease in radio-sensitivity in the Q than total cells. Post-irradiation TPZ administration produced a large radio-sensitizing effect on both total and Q cells, especially on Q cells. In pimonidazole-unlabeled cell fractions in both total and Q cells, TPZ suppressed the reduction in sensitivity much more efficiently than MTH or Met without any radio-sensitizing effect.

Conclusion

Post-irradiation TPZ administration has the potential to both suppress recovery from radiation-induced damage and enhance the radio-sensitivity both in total and Q tumor cells. Post-irradiation TPZ administration may be useful for controlling tumors.

Downregulation of DNA repair proteins and increased DNA damage in hypoxic colon cancer cells is a therapeutically exploitable vulnerability

Surgical removal of colorectal cancer (CRC) liver metastases generates areas of tissue hypoxia. Hypoxia imposes a stem-like phenotype on residual tumor cells and promotes tumor recurrence. Moreover, in primary CRC, gene expression signatures reflecting hypoxia and a stem-like phenotype are highly expressed in the aggressive Consensus Molecular Subtype 4 (CMS4). Therapeutic strategies eliminating hypoxic stem-like cells may limit recurrence following resection of primary tumors or metastases.

Here we show that expression of DNA repair genes is strongly suppressed in CMS4 and inversely correlated with hypoxia-inducible factor-1 alpha (HIF1α) and HIF-2α co-expression signatures. Tumors with high expression of HIF signatures and low expression of repair proteins showed the worst survival. In human tumors, expression of the repair proteins RAD51, KU70 and RIF1 was strongly suppressed in hypoxic peri-necrotic tumor areas. Experimentally induced hypoxia in patient derived colonospheres in vitro or in vivo (through vascular clamping) was sufficient to downregulate repair protein expression and caused DNA damage. Hypoxia-induced DNA damage was prevented by expressing the hydroperoxide-scavenging enzyme glutathione peroxidase-2 (GPx2), indicating that reactive oxygen species mediate hypoxia-induced DNA damage. Finally, the hypoxia-activated prodrug Tirapazamine greatly augmented DNA damage and reduced the fraction of stem-like (Aldefluor^bright) tumor cells in vitro, and in vivo following vascular clamping.

We conclude that decreased expression of DNA repair proteins and increased DNA damage in hypoxic tumor areas may be therapeutically exploited with hypoxia-activated prodrugs, and that such drugs reduce the fraction of Aldefluor^bright (stem-like) tumor cells.

Targeting the hypoxic fraction of tumours using hypoxia-activated prodrugs

The presence of a microenvironment within most tumours containing regions of low oxygen tension or hypoxia has profound biological and therapeutic implications. Tumour hypoxia is known to promote the development of an aggressive phenotype, resistance to both chemotherapy and radiotherapy and is strongly associated with poor clinical outcome. Paradoxically, it is recognised as a high-priority target and one of the therapeutic strategies designed to eradicate hypoxic cells in tumours is a group of compounds known collectively as hypoxia-activated prodrugs (HAPs) or bioreductive drugs. These drugs are inactive prodrugs that require enzymatic activation (typically by 1 or 2 electron oxidoreductases) to generate cytotoxic species with selectivity for hypoxic cells being determined by (1) the ability of oxygen to either reverse or inhibit the activation process and (2) the presence of elevated expression of oxidoreductases in tumours. The concepts underpinning HAP development were established over 40 years ago and have been refined over the years to produce a new generation of HAPs that are under preclinical and clinical development. The purpose of this article is to describe current progress in the development of HAPs focusing on the mechanisms of action, preclinical properties and clinical progress of leading examples.

Targeting the hypoxic fraction of tumours using hypoxia-activated prodrugs

The presence of a microenvironment within most tumours containing regions of low oxygen tension or hypoxia has profound biological and therapeutic implications. Tumour hypoxia is known to promote the development of an aggressive phenotype, resistance to both chemotherapy and radiotherapy and is strongly associated with poor clinical outcome. Paradoxically, it is recognised as a high-priority target and one of the therapeutic strategies designed to eradicate hypoxic cells in tumours is a group of compounds known collectively as hypoxia-activated prodrugs (HAPs) or bioreductive drugs. These drugs are inactive prodrugs that require enzymatic activation (typically by 1 or 2 electron oxidoreductases) to generate cytotoxic species with selectivity for hypoxic cells being determined by (1) the ability of oxygen to either reverse or inhibit the activation process and (2) the presence of elevated expression of oxidoreductases in tumours. The concepts underpinning HAP development were established over 40 years ago and have been refined over the years to produce a new generation of HAPs that are under preclinical and clinical development. The purpose of this article is to describe current progress in the development of HAPs focusing on the mechanisms of action, preclinical properties and clinical progress of leading examples.

The flavoprotein FOXRED2 reductively activates nitro-chloromethylbenzindolines and other hypoxia-targeting prodrugs

The nitro-chloromethylbenzindoline prodrug SN29428 has been rationally designed to target tumour hypoxia. SN29428 is metabolised to a DNA minor groove alkylator via oxygen-sensitive reductive activation initiated by unknown one-electron reductases. The present study sought to identify reductases capable of activating SN29428 in tumours. Expression of candidate reductases in cell lines was modulated using forced expression and, for P450 (cytochrome) oxidoreductase (POR), by zinc finger nuclease-mediated gene knockout. Affymetrix microarray mRNA expression of flavoreductases was correlated with SN29428 activation in a panel of 23 cancer cell lines. Reductive activation and cytotoxicity of prodrugs were measured using mass spectrometry and antiproliferative assays, respectively. SN29428 activation under hypoxia was strongly attenuated by the pan-flavoprotein inhibitor diphenyliodonium, but less so by knockout of POR suggesting other flavoreductases contribute. Forced expression of 5-methyltetrahydrofolate-homocysteine methyltransferase reductase (MTRR), as well as POR, increased activation of SN29428 in hypoxic HCT 116 cells. SN29428 activation strongly correlated with expression of POR and also FAD-dependent oxidoreductase domain containing 2 (FOXRED2), in cancer cell lines. This association persisted after removing the effect of POR enzyme activity using first-order partial correlation. Forced expression of FOXRED2 increased SN29428 activation and cytotoxicity in hypoxic HEK293 cells and also increased activation of hypoxia-targeted prodrugs PR-104A, tirapazamine and SN30000, and increased cytotoxicity of the clinical-stage prodrug TH-302. Thus this study has identified three flavoreductases capable of enzymatically activating SN29428, one of which (FOXRED2) has not previously been implicated in xenobiotic metabolism. These results will inform future development of biomarkers predictive of SN29428 sensitivity.