Hypoxia-selective activation of 5-fluorodeoxyuridine prodrug possessing indolequinone structure: radiolytic reduction and cytotoxicity characteristics

Bioorthogonal chemistry for prodrug activation in vivo

Prodrugs have emerged as a major strategy for addressing clinical challenges by improving drug pharmacokinetics, reducing toxicity, and enhancing treatment efficacy. The emergence of new bioorthogonal chemistry has greatly facilitated the development of prodrug strategies, enabling their activation through chemical and physical stimuli. This "on-demand" activation using bioorthogonal chemistry has revolutionized the research and development of prodrugs. Consequently, prodrug activation has garnered significant attention and emerged as an exciting field of translational research. This review summarizes the latest advancements in prodrug activation by utilizing bioorthogonal chemistry and mainly focuses on the activation of small-molecule prodrugs and antibody-drug conjugates. In addition, this review also discusses the opportunities and challenges of translating these advancements into clinical practice.

X-Ray-Induced Drug Release for Cancer Therapy

Drug delivery systems (DDSs) are designed to deliver therapeutic agents to specific target sites while minimizing systemic toxicity. Recent developments in drug-loaded DDSs have demonstrated promising characteristics and paved new pathways for cancer treatment. Light, a prevalent external stimulus, is widely utilized to trigger drug release. However, conventional light sources primarily concentrate on the ultraviolet (UV) and visible light regions, which suffer from limited biological tissue penetration. This limitation hinders applications for deep-tissue tumor drug release. Given their deep tissue penetration and well-established application technology, X-rays have recently received attention for the pursuit of controlled drug release. With precise spatiotemporal and dosage controllability, X-rays stand as an ideal stimulus for achieving controlled drug release in deep-tissue cancer therapy. This article explores the recent advancements in using X-rays for stimulus-triggered drug release in DDSs and delves into their action mechanisms.

Two Color Imaging of Different Hypoxia Levels in Cancer Cells

Hypoxia (low oxygen levels) occurs in a range of biological contexts, including plants, bacterial biofilms, and solid tumors; it elicits responses from these biological systems that impact their survival. For example, conditions of low oxygen make treating tumors more difficult and have a negative impact on patient prognosis. Therefore, chemical probes that enable the study of biological hypoxia are valuable tools to increase the understanding of disease-related conditions that involve low oxygen levels, ultimately leading to improved diagnosis and treatment. While small-molecule hypoxia-sensing probes exist, the majority of these image only very severe hypoxia (<1% O₂) and therefore do not give a full picture of heterogeneous biological hypoxia. Commonly used antibody-based imaging tools for hypoxia are less convenient than small molecules, as secondary detection steps involving immunostaining are required. Here, we report the synthesis, electrochemical properties, photophysical analysis, and biological validation of a range of indolequinone-based bioreductive fluorescent probes. We show that these compounds image different levels of hypoxia in 2D and 3D cell cultures. The resorufin-based probe 2 was activated in conditions of 4% O₂ and lower, while the Me-Tokyo Green-based probe 4 was only activated in severe hypoxia─0.5% O₂ and less. Simultaneous application of these compounds in spheroids revealed that compound 2 images similar levels of hypoxia to pimonidazole, while compound 4 images more extreme hypoxia in a manner analogous to EF5. Compounds 2 and 4 are therefore useful tools to study hypoxia in a cellular setting and represent convenient alternatives to antibody-based imaging approaches.

Research advances in NQO1-responsive prodrugs and nanocarriers for cancer treatment

NAD(P)H: quinine oxidoreductase (NQO1) is a class of flavoprotein enzymes commonly expressed in eukaryotic cells. It actively participates in the metabolism of various quinones and their in vivo bioactivation through electron reduction reactions. The expression level of NQO1 is highly upregulated in many solid tumor cells compared with that in normal cells. NQO1 has been considered a candidate molecular target because of its overexpression and bioactivity in different tumors. NQO1-responsive prodrugs and nanocarriers have recently been identified as effective objectives for achieving controlled drug release, reducing adverse reactions and improving clinical efficacy. This review systematically introduces the research advances in applying NQO1-responsive prodrugs and nanocarriers to cancer treatment. It also discusses the existing problems and the developmental prospects of these two antitumor drug delivery systems.

Enzymatic activation of indolequinone-substituted 5-fluorodeoxyuridine prodrugs in hypoxic cells

Among the various enzymes, reductases that catalyze one-electron reduction are involved in the selective activation of functional compounds or materials in hypoxia, which is one of the well-known pathophysiological characteristics of solid tumors. Enzymatic one-electron reduction has been recognized as a useful reaction that can be applied in the design of tumor hypoxia-targeting drugs. In this report, we characterized the enzymatic reaction of 5-fluorodeoxyuridine (FdUrd) prodrug bearing an indolequinone unit (IQ-FdUrd), which is a substrate of reductases. IQ-FdUrd was activated to release FdUrd under hypoxic conditions after treatment with cytochrome NADPH P450 reductase. We also confirmed that IQ-FdUrd showed selective cytotoxicity in hypoxic tumor cells.

Breaking the Depth Dependence by Nanotechnology-Enhanced X-Ray-Excited Deep Cancer Theranostics

The advancements in nanotechnology have created multifunctional nanomaterials aimed at enhancing diagnostic accuracy and treatment efficacy for cancer. However, the ability to target deep-seated tumors remains one of the most critical challenges for certain nanomedicine applications. To this end, X-ray-excited theranostic techniques provide a means of overcoming the limits of light penetration and tissue attenuation. Herein, a comprehensive overview of the recent advances in nanotechnology-enhanced X-ray-excited imaging and therapeutic methodologies is presented, with an emphasis on the design of multifunctional nanomaterials for contrast-enhanced computed tomography (CT) imaging, X-ray-excited optical luminescence (XEOL) imaging, and X-ray-excited multimodal synchronous/synergistic therapy. The latter is based on the concurrent use of radiotherapy with chemotherapy, gas therapy, photodynamic therapy, or immunotherapy. Moreover, the featured biomedical applications of X-ray-excited deep theranostics are discussed to highlight the advantages of X-ray in high-sensitivity detection and efficient elimination of malignant tumors. Finally, key issues and technical challenges associated with this deep theranostic technology are identified, with the intention of advancing its translation into the clinic.

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.

Antitumour indolequinones: synthesis and activity against human pancreatic cancer cells

An important determinant of the growth inhibitory activity of indolequinones against pancreatic cancer cells is substitution on the 2-position with 2-unsubstituted derivatives being markedly more potent. A series of indolequinones bearing a range of substituents on nitrogen and at the indolylcarbinyl position was prepared by copper(II)-mediated reaction of bromoquinones and enamines, followed by functional group interconversions. The compounds were then assayed for their ability to inhibit the growth of pancreatic cancer cells. The pKa of the leaving group at the 3-position was shown to influence growth inhibitory activity that is consistent with the proposed mechanism of action of reduction, loss of leaving group and formation of a reactive iminium species. Substitutions on the indole nitrogen were well tolerated with little influence on growth inhibitory activity while substitutions at the 5- and 6-positions larger than methoxy led to decreased activity. The studies presented define the range of substitutions of 2-unsubstituted indolequinones required for optimal growth inhibitory activity.

Palladium-mediated dealkylation of N-propargyl-floxuridine as a bioorthogonal oxygen-independent prodrug strategy

Herein we report the development and biological screening of a bioorthogonal palladium-labile prodrug of the nucleoside analogue floxuridine, a potent antineoplastic drug used in the clinic to treat advanced cancers. N-propargylation of the N3 position of its uracil ring resulted in a vast reduction of its biological activity (~6,250-fold). Cytotoxic properties were bioorthogonally rescued in cancer cell culture by heterogeneous palladium chemistry both in normoxia and hypoxia. Within the same environment, the reported chemo-reversible prodrug exhibited up to 1,450-fold difference of cytotoxicity whether it was in the absence or presence of the extracellular palladium source, underlining the precise modulation of bioactivity enabled by this bioorthogonally-activated prodrug strategy.

Hypoxic X-irradiation as an external stimulus for conformational change of oligodeoxynucleotides that possess disulfide bond and regulation of DNAzyme function

We achieved a conformational change of oligodeoxynucleotides and the regulation of DNAzyme function by means of a radiolytic strand exchange reaction of disulfide bond. We designed a system in which the DNAzyme function of RNA cleavage was suppressed by the hybridization of an inhibitor strand that possessed disulfide bond with an active DNAzyme. Hypoxic X-irradiation led to the recovery of RNA cleavage because the strand exchange reaction at the disulfide bond in inhibitor strand resulted in a release of inhibitor strand. This strategy may be applicable to gene regulation by hypoxic X-irradiation.

Short oligonucleotide prodrug having 5-fluoro and 5-iodouracil inhibits the proliferation of cancer cells in a photo-responsive manner

Photo-induced C1' hydrogen abstraction of 5-fluoro-2'-deoxyuridine was adopted as the key reaction for releasing 5-fluorouracil (5-FU) anticancer drug from oligonucleotide strands. After photoirradiation following 5-FU release, anticancer activity was expected. We demonstrated that oligonucleotide tetramer, d(A(F)U(I)UA), can release 5-FU under physiological conditions in a photo-responsive manner thorough photo-induced C1' hydrogen abstraction, and that the 5-FU released from d(A(F)U(I)UA) having a phosphorothioate backbone clearly suppresses the proliferation of HeLa cells in a photo-responsive manner.

Copper(II)-Mediated Synthesis of Indolequinones from Bromoquinones and Enamines

The reaction of enamines and bromoquinones in the presence of copper(II) acetate and potassium carbonate results in a regiospecific synthesis of indolequinones. The reaction is broad in scope and scalable and provides a route to the core structure that is present in several biologically interesting natural and synthetic compounds.

Mechanism-based inhibition of quinone reductase 2 (NQO2): selectivity for NQO2 over NQO1 and structural basis for flavoprotein inhibition

A role for the flavoprotein NRH:quinone oxidoreductase 2 (NQO2, QR2) in human diseases such as malaria, leukemia and neurodegeneration has been proposed. In order to explore the potential of NQO2 as a therapeutic target, we have developed potent and selective mechanism-based inhibitors centered on the indolequinone pharmacophore. The compounds show remarkable selectivity for NQO2 over the closely related flavoprotein NQO1, with small structural changes defining selectivity. Biochemical studies confirmed the mechanism-based inhibition, whereas X-ray crystallography and mass spectrometry revealed the nature of the inhibitor interaction with the protein. These indolequinones represent the first mechanism-based inhibitors of NQO2, and their novel mode of action involving alkylation of the flavin cofactor, provides significant advantages over existing competitive inhibitors in terms of potency and irreversibility, and will open new opportunities to define the role of NQO2 in disease.

Radiolytic Reduction Characteristics of Artificial Oligodeoxynucleotides Possessing 2-Oxoalkyl Group or Disulfide Bonds

A number of advances have been made in the development of modified oligodeoxynucleotides (ODNs), and chemical or physical properties of which are controlled by external stimuli. These intelligent ODNs are promising for the next generation of gene diagnostics and therapy. This paper focuses on the molecular design of artificial ODNs that are activated by X-irradiation and their applications to regulation of hybridization properties, conformation change, radiation-activated DNAzyme, and decoy molecules.

Design of anticancer prodrugs for reductive activation

Anticancer prodrugs designed to target specifically tumor cells should increase therapeutic effectiveness and decrease systemic side effects in the treatment of cancer. Over the last 20 years, significant advances have been made in the development of anticancer prodrugs through the incorporation of triggers for reductive activation. Reductively activated prodrugs have been designed to target hypoxic tumor tissues, which are known to overexpress several endogenous reductive enzymes. In addition, exogenous reductive enzymes can be delivered to tumor cells through fusion with tumor-specific antibodies or overexpressed in tumor cells through gene delivery approaches. Many anticancer prodrugs have been designed to use both the endogenous and exogenous reductive enzymes for target-specific activation and these prodrugs often contain functional groups such as quinones, nitroaromatics, N-oxides, and metal complexes. Although no new agents have been approved for clinical use, several reductively activated prodrugs are in various stages of clinical trial. This review mainly focuses on the medicinal chemistry aspects of various classes of reductively activated prodrugs including design principles, structure-activity relationships, and mechanisms of activation and release of active drug molecules.

Radiolytic one-electron reduction characteristics of tyrosine derivative caged by 2-oxopropyl group

We employed X-irradiation to activate a caged amino acid with a 2-oxoalkyl group. We designed and synthesized tyrosine derivative caged by a 2-oxoalkyl group (Tyr(Oxo)) to evaluate its radiolytic one-electron reduction characteristics in aqueous solution. Upon hypoxic X-irradiation, Tyr(Oxo) released a 2-oxopropyl group to form the corresponding uncaged tyrosine. In addition, radiolysis of dipeptides containing Tyr(Oxo) revealed that the efficiency of radiolytic removal of 2-oxopropyl group increased significantly by the presence of neighboring aromatic amino acids.

Radiation- and photo-induced activation of 5-fluorouracil prodrugs as a strategy for the selective treatment of solid tumors

5-Fluorouracil (5-FU) is used widely as an anticancer drug to treat solid cancers, such as colon, breast, rectal, and pancreatic cancers, although its clinical application is limited because 5-FU has gastrointestinal and hematological toxicity. Many groups are searching for prodrugs with functions that are tumor selective in their delivery and can be activated to improve the clinical utility of 5-FU as an important cancer chemotherapeutic agent. UV and ionizing radiation can cause chemical reactions in a localized area of the body, and these have been applied in the development of site-specific drug activation and sensitization. In this review, we describe recent progress in the development of novel 5-FU prodrugs that are activated site specifically by UV light and ionizing radiation in the tumor microenvironment. We also discuss the chemical mechanisms underlying this activation.

Emission under hypoxia: one-electron reduction and fluorescence characteristics of an indolequinone-coumarin conjugate

A characteristic feature of the reactivity of indolequinone derivatives, substituents of which can be removed by one-electron reduction under hypoxic conditions, was applied to the development of a new class of fluorescent probes for disease-relevant hypoxia. A reducing indolequinone parent molecule conjugated with fluorescent coumarin chromophores could suppress efficiently the fluorescence emission of the coumarin moieties by an intramolecular electron-transfer quenching mechanism and a conventional internal-filter effect. Under hypoxic conditions, however, the conjugate, denoted IQ-Cou, underwent a one-electron reduction triggered by X irradiation or the action of a reduction enzyme to release a fluorescent coumarin chromophore, whereupon an intense fluorescence emission with a maximum intensity at 420 nm was observed. The one-electron reduction of IQ-Cou was suppressed by molecular oxygen under aerobic conditions. IQ-Cou also showed intense fluorescence in a hypoxia-selective manner upon incubation with a cell lysate of the human fibrosarcoma cell line HT-1080. The IQ-Cou conjugate has several unique properties that are favorable for a fluorescent probe of hypoxia-specific imaging.

One-electron reductive template-directed ligation of oligodeoxynucleotides possessing a disulfide bond

Hypoxic X-radiolysis of diluted aqueous solutions was performed to generate hydrated electrons that induced one-electron reduction of oligodeoxynucleotides (ODNs) possessing a disulfide bond. Upon hypoxic irradiation of dinucleotides, two forms of dinucleotides were produced via intermolecular exchange of the disulfides and ligation that proceeded with a multiple turnover. In contrast to the efficient reaction induced by hypoxic irradiation, the reaction efficiency was dramatically decreased when irradiation was performed under aerobic conditions, presumably due to capturing reactive hydrated electrons by molecular oxygen. We subsequently applied these unique reaction characteristics to template-directed ligation. In the presence of a complementary template ODN, two ODNs possessing a disulfide bond produced a prescribed ODN with high regioselectivity via interstrand crossing upon hypoxic irradiation.

Natural and synthetic quinones and their reduction by the quinone reductase enzyme NQO1: from synthetic organic chemistry to compounds with anticancer potential

The quinone reductase enzyme NAD(P)H: quinone oxidoreductase 1 (NQO1) is a ubiquitous flavoenzyme that catalyzes the two-electron reduction of quinones. This Perspective briefly reviews the structure and mechanism, physiological role, and upregulation and induction of the enzyme, but focuses on the synthesis of new heterocyclic quinones and their metabolism by recombinant human NQO1. Thus a range of indolequinones, some of which are novel analogues of mitomycin C, benzimidazolequinones, benzothiazolequinones and quinolinequinones have been prepared and evaluated, leading to detailed knowledge of the structural requirements for efficient metabolism by the enzyme. Potent mechanism-based inhibitors (suicide substrates) of NQO1 have also been developed. These indolequinones irreversibly alkylate the protein, preventing its function both in standard enzyme assays and also in cells. Some of these quinones are also potent inhibitors of growth of human pancreatic cancer cells, suggesting a potential role for such compounds as therapeutic agents.

Review: implications of in vitro research on the effect of radiotherapy and chemotherapy under hypoxic conditions

As it is now well established that human solid tumors frequently contain a substantial fraction of cells that are hypoxic, more and more in vitro research is focusing on the impact of hypoxia on the outcome of radiotherapy and chemotherapy. Indeed, the efficacy of irradiation and many cytotoxic drugs relies on an adequate oxygen supply. Consequently, hypoxic regions in solid tumors often contain viable cells that are intrinsically more resistant to treatment with radiotherapy or chemotherapy. Moreover, efforts have been made to exploit hypoxia as a potential difference between malignant and normal tissues.Nowadays, a body of evidence indicates that oxygen deficiency clearly influences some major intracellular pathways such as those involved in cell proliferation, cell cycle progression, apoptosis, cell adhesion, and others. Obviously, when investigating the effects of radiotherapy or chemotherapy or both combined under hypoxic conditions, it is essential to consider the influences of hypoxia itself on the cell. In this review, we first focus on the effects of hypoxia per se on some critical biological pathways. Next, we sketch an overview of preclinical and clinical research on radiotherapy, chemotherapy, and chemoradiation under hypoxic conditions.

Chemical radiosensitizers for use in radiotherapy

Radiosensitizers are intended to enhance tumour cell killing while having much less effect on normal tissues. Some drugs target different physiological characteristics of the tumour, particularly hypoxia associated with radioresistance. Oxygen is the definitive hypoxic cell radiosensitizer, the large differential radiosensitivity of oxic vs hypoxic cells being an attractive factor. The combination of nicotinamide to reduce acute hypoxia with normobaric carbogen breathing is showing clinical promise. 'Electron-affinic' chemicals that react with DNA free radicals have the potential for universal activity to combat hypoxia-associated radioresistance; a nitroimidazole, nimorazole, is clinically effective at tolerable doses. Hypoxia-specific cytotoxins, such as tirapazamine, are valuable adjuncts to radiotherapy. Nitric oxide is a potent hypoxic cell radiosensitizer; variations in endogenous levels might have prognostic significance, and routes to deliver nitric oxide specifically to tumours are being developed. In principle, many drugs can be delivered selectively to hypoxic tumours using either reductase enzymes or radiation-produced free radicals to activate drug release from electron-affinic prodrugs. A redox-active agent based on a gadolinium chelate is being evaluated clinically. Pyrimidines substituted with bromine or iodine are incorporated into DNA and enhance free radical damage; fluoropyrimidines act by different mechanisms. A wide variety of drugs that influence the nature or repair of DNA damage are being evaluated in conjunction with radiation; it is often difficult to define the mechanisms underlying chemoradiation regimens. Drugs being evaluated include topoisomerase inhibitors (e.g. camptothecin, topotecan), and the hypoxia-activated anthraquinone AQ4N; alkylating agents include temozolomide. Drugs involved in DNA repair pathways being investigated include the potent poly(ADP ribose)polymerase inhibitor, AG14,361. Proteins involved in cell signalling, such as the Ras family, are attractive targets linked to radioresistance, as are epidermal growth factor receptors and linked kinases (drugs including vandetanib [ZD6,474], cetuximab and gefitinib), and cyclooxygenase-2 (celecoxib). The suppression of radioprotective thiols seems to offer more potential with alkylating agents than with radiotherapy, although it remains a strategy worthy of exploration.

2-Oxoalkyl caged oligonucleotides: one-electron reductive activation into emergence of ordinary hybridization property by hypoxic X-irradiation

Ionizing radiation triggers the activation of caged oligodeoxynucleotides (ODNs) with a 2-oxoalkyl leaving group to give the corresponding normal uncaged strands. We designed and synthesized ODNs caged by a 2-oxopropyl group at a given thymine N(3) position (d(oxo)T) to evaluate their one-electron reduction characteristics. Upon hypoxic X-radiolysis in aqueous solution, the caged ODNs released the 2-oxopropyl group to produce the corresponding uncaged ODNs. Digestion by a restriction enzyme Swa I revealed that caged ODN pre-irradiated in hypoxia could form an ordinary duplex with its complementary strand.

Radiolytic and cellular reduction of a novel hypoxia-activated cobalt(III) prodrug of a chloromethylbenzindoline DNA minor groove alkylator

Metabolic reduction can be used to activate prodrugs in hypoxic regions of tumours, but reduction by ionising radiation is also theoretically attractive. Previously, we showed that a cobalt(III) complex containing 8-hydroxyquinoline (8-HQ) and cyclen ligands releases 8-HQ efficiently on irradiation in hypoxic solutions [Ahn G-O, Ware DC, Denny WA, Wilson WR. Optimization of the auxiliary ligand shell of cobalt(III)(8-hydroxyquinoline) complexes as model hypoxia-selective radiation-activated prodrugs. Radiat Res 2004;162:315-25]. Here we investigate an analogous Co(III) complex containing the potent DNA minor groove alkylator azachloromethylbenzindoline (azaCBI, 1) to determine whether it releases 1 on radiolytic and/or enzymatic reduction under hypoxia. Monitoring by HPLC, the azaCBI ligand in the Co(III)(cyclen)(azaCBI) complex (2) slowly hydrolysed in aqueous solution, in contrast to the free ligand 1 which readily converted to its reactive cyclopropyl form. Irradiation of 2 (30-50 microM) in hypoxic solutions released 1 with yields of 0.57 micromol/J in formate buffer and 0.13 micromol/J in human plasma. Using bioassay methods, cytotoxic activation by irradiation of 2 at 1 microM in hypoxic plasma was readily detectable at clinically relevant doses (> or = 1 Gy), with a estimated yield of 1 of 0.075 micromol/J. Release of 1 from 2 was also observed in hypoxic HT29 cultures without radiation, with subsequent conversion of 1 to its O-glucuronide. Surprisingly, overexpression of human cytochrome P450 reductase in A549 cells did not increase the rate of metabolic reduction of 2, suggesting that other reductases and/or non-enzymatic reductants are responsible. Thus the cobalt(III) complex 2 is a promising prodrug capable of being activated to release a very potent cytotoxin when reduced by either ionising radiation or cells under hypoxic conditions.

Modulated drug release from the stem-and-loop structured oligodeoxynucleotide upon UV-A irradiation in the presence of target DNA

o-Nitrobenzyl photochemistry as induced by UV-A irradiation was applied to a photoactivated drug releasing system based on a molecular beacon strategy. A stem-and-loop structured oligodeoxynucleotide (ODN) possessing a photoreactive o-nitrobenzyl chromophore at the 3'-end and 1-aminonaphthalene quencher at the 5'-end underwent conformational change into a conventional double strand structure by hybridization with a specified target DNA. The intrinsic stem-and-loop structure suppressed photoactivated release of benzoic acid as a phantom drug from the o-nitrobenzyl chromophore because of intramolecular quenching by the 1-aminonaphthalene unit in close proximity to the chromophore. Formation of the double strand structure in the presence of perfectly matched target DNA minimized occurrence of intramolecular quenching and thereby enhanced the photoactivated drug release.