Prodrugs for targeting hypoxic tissues: regiospecific elimination of aspirin from reduced indolequinones

CYP450 Enzymes Effect Oxygen-Dependent Reduction of Azide-Based Fluorogenic Dyes

Azide-containing compounds have broad utility in organic synthesis and chemical biology. Their use as powerful tools for the labeling of biological systems in vitro has enabled insights into complex cellular functions. To date, fluorogenic azide-containing compounds have primarily been employed in the context of click chemistry and as sensitive functionalities for hydrogen sulfide detection. Here, we report an alternative use of this functionality: as fluorogenic probes for the detection of depleted oxygen levels (hypoxia). Oxygen is imperative to all life forms, and probes that enable quantification of oxygen tension are of high utility in many areas of biology. Here we demonstrate the ability of an azide-based dye to image hypoxia in a range of human cancer cell lines. We have found that cytochrome P450 enzymes are able to reduce these probes in an oxygen-dependent manner, while hydrogen sulfide does not play an important role in their reduction. These data indicate that the azide group is a new bioreductive functionality that can be employed in prodrugs and dyes. We have uncovered a novel mechanism for the cellular reduction of azides, which has implications for the use of click chemistry in hypoxia.

Site-directed delivery of nitric oxide to cancers

Nitric oxide (NO) is a reactive gaseous free radical which mediates numerous biological processes. At elevated levels, NO is found to be toxic to cancers and hence, a number of strategies for site-directed delivery of NO to cancers are in development during the past two decades. More recently, the focus of research has been to, in conjunction with other cancer drugs deliver NO to cancers for its secondary effects including inhibition of cellular drug efflux pumps. Among the various approaches toward site-selective delivery of exogenous NO sources, enzyme activated nitric oxide donors belonging to the diazeniumdiolate category afford unique advantages including exquisite control of rates of NO generation and selectivity of NO production. For this prodrug approach, enzymes including esterase, glutathione/glutathione S-transferase, DT-diaphorase, and nitroreductase are utilized. Here, we review the design and development of various approaches to enzymatic site-directed delivery of NO to cancers and their potential.

Bexarotene prodrugs: targeting through cleavage by NQO1 (DT-diaphorase)

Bexarotene, a retinoid X receptor (RXR) agonist, is being tested as a potential disease modifying treatment for neurodegenerative conditions. To limit the peripheral exposure of bexarotene and release it only in the affected areas of the brain, we designed a prodrug strategy based on the enzyme NAD(P)H/quinone oxidoreductase (NQO1) that is elevated in neurodegenerative diseases. A series of indolequinones (known substrates of NQO1) was synthesized and coupled to bexarotene. Bexarotene-3-(hydroxymethyl)-5-methoxy-1,2-dimethyl-1H-indole-4,7-dione ester 7a was cleaved best by NQO1. The prodrugs are not cleaved by esterase.

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.

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.

Indolequinone antitumour agents: correlation between quinone structure and rate of metabolism by recombinant human NAD(P)H:quinone oxidoreductase

A series of indolequinones bearing a range of substituents at the (indol-2-yl)methyl position has been synthesized. The ability of these indolequinones to act as substrates for recombinant human NAD(P)H:quinone oxidoreductase (NQO1), a two-electron reductase upregulated in tumour cells, was determined, along with their toxicity to an isogenic tumour cell line pair that is differentiated as either NQO1-expressing cells (BE-NQ) or NQO1-null cells (BE-WT). Overall, the 2-substituted indolequinones were relatively poor substrates for NQO1. Hydroxymethyl groups at C-2 led to higher rates of reduction, a finding that was observed previously with 3-hydroxymethylated indolequinones. Predictably, the best substrate had an electron-withdrawing ester group at the indole-2-position. The indolequinones were generally non-toxic to both cell lines with the exception of those quinones that had methylaziridine groups at the indole-5-position. These compounds could form DNA cross-links when activated by reduction and were up to 3-fold more toxic to the BE-NQ cells than the BE-WT cells.

Bioreduction activated prodrugs of camptothecin: molecular design, synthesis, activation mechanism and hypoxia selective cytotoxicity

Several water-soluble derivatives (CPT3, CPT3a-d) of camptothecin (CPT) were synthesized, among which CPT3 bearing an N,N'-dimethyl-1-aminoethylcarbamate side-chain was further conjugated with reductively eliminating structural units of indolequinone, 4-nitrobenzyl alcohol and 4-nitrofuryl alcohol to produce novel prodrugs of camptothecin (CPT4-6). All CPT derivatives were of lower cytotoxicity than their parent compound of CPT. In contrast, CPT4 and CPT6 showed higher hypoxia selectivity of cytotoxicity towards tumor cells than CPT. A mechanism by which a representative prodrug CPT4 is activated in the presence of DT-diaphorase to release CPT was also discussed. The bioreduction activated CPT prodrugs including CPT4 and CPT6 are identified to be promising for application to the hypoxia targeting tumor chemotherapy.

3-Substituted-5-aziridinyl-1-methylindole-4,7-diones as NQO1-directed antitumour agents: mechanism of activation and cytotoxicity in vitro

Indolequinone agents are a unique class of bioreductive cytotoxins that can function as dual substrates for both one- and two-electron reductases. This endows them with the potential to be either hypoxia-selective cytotoxins or NAD(P)H:quinone oxidoreductase 1 (NQO1)-directed prodrugs, respectively. We have studied the structure-activity relationships of four novel indolequinone analogues with regard to one- and/or two-electron activation. Single-electron metabolism was achieved by exposing the human carcinoma cell line T47D to each agent under hypoxic conditions, whilst concerted two-electron metabolism was assessed by stably expressing the cDNA for human NQO1 in a cloned cell line of T47D. The C-3 and C-5 positions of the indolequinone nucleus were modified to manipulate reactivity of the reduction products and the four prodrugs were identified as NQO1 substrates of varying specificity. Two of the four prodrugs, in which both C-3 and C-5 groups remained functional, proved to be NQO1-directed cytotoxins with selectivity ratios of 60- to 80-fold in the T47D (WT) versus the NQO1 overexpressing T47D cells. They also retained selectivity as hypoxic cytotoxins with oxic/hypoxic ratios of 20- to 22-fold. Replacement of the C-3 hydroxymethyl leaving group with an aldehyde group ablated all selectivity in air and hypoxia in both cell lines. Addition of a 2-methyl group on the C-5 aziridinyl group to introduce steric hinderance reduced but did not abolish NQO1-dependent metabolism. However, it enhanced single-electron metabolism-dependent DNA cross-linking in a manner that was independent of cytotoxicity. These data demonstrate that subtle structure-activity relationship exists for different cellular reductases and under certain circumstances distinct forms of DNA damage can arise, the cytotoxic consequences of which can vary. This study identifies a candidate indolequinone analogue for further development as a dual hypoxia and NQO1-directed prodrug.

Studies on the mechanisms of activation of indolequinone phosphoramidate prodrugs

Previously a series of 2- and 3-substituted indolequinone phosphoramidate prodrugs was synthesized, and the compounds were shown to be nanomolar inhibitors of cell proliferation. The activation of these compounds following both one- and two-electron reduction has been investigated. (31)P NMR experiments demonstrated that both series of compounds undergo rapid activation following two-electron reduction. Additionally, the 3-series of compounds undergo rapid activation following one-electron reduction, while activation of the 2-series of compounds via this mechanism is very slow. The activation of these prodrugs by direct displacement using sulfur nucleophiles such as glutathione has been examined. Activation via this route is rapid for the 3-regioisomers, but is considerably slower for the 2-substituted analogues under similar conditions. Together these findings suggest that drug delivery via two-electron reduction from the 2-position is the more selective prodrug strategy.

Isosorbide-based aspirin prodrugs. II. Hydrolysis kinetics of isosorbide diaspirinate

Aspirin prodrugs have been intensively investigated in an effort to produce compounds with lower gastric toxicity, greater stability or enhanced percutaneous absorption, relative to aspirin. This report describes the hydrolysis kinetics and aspirin release characteristics of isosorbide diaspirinate (ISDA), the aspirin diester of isosorbide. ISDA underwent rapid hydrolysis when incubated in phosphate buffered human plasma solutions (pH 7.4) at 37 degrees C, producing appreciable quantities of aspirin. In 30% human plasma solution the half-life was 1.1 min and 61% aspirin was liberated relative to the initial ester concentration. The hydrolysis kinetics of ISDA were monitored in aqueous solution at 37 degrees C over the pH range 1.03-9.4. The aqueous hydrolysis followed pseudo-first-order kinetics over several half-lives at all pH values, resulting in a U-shaped pH rate profile. Salicylate esters and salicylic acid were formed during these processes. The hydrolysis characteristics of ISDA were also investigated in pH 7.4 phosphate buffered solutions containing alpha-chymotrypsin [EC 3.1.1.1] (t(1/2)=200.9 min), carboxyl esterase [EC 3.1.1.1] (t(1/2)=31.5 min), human serum albumin (t(1/2)=603 min), purified human serum butyrylcholinesterase [EC 3.1.1.8] (80 micro g/ml; t(1/2)=9.4 min; 55% aspirin), purified horse serum butyrylcholinesterase (100 micro g/ml; t(1/2)=1.85 min;11% aspirin) and in 10% human plasma solution in the presence of physostigmine (3 micro M). The results indicate that a specific enzyme present in human plasma, probably human butyrylcholinesterase, catalyses aspirin release from isosorbide diaspirinate.

Design, synthesis, and biological evaluation of indolequinone phosphoramidate prodrugs targeted to DT-diaphorase

A series of 2- and 3-substituted indolequinone phosphoramidate prodrugs targeted to DT-diaphorase (DTD) have been synthesized and evaluated. These compounds are designed to undergo activation via quinone reduction by DTD followed by expulsion of the phosphoramide mustard substituent from the hydroquinone. Chemical reduction of the phosphoramidate prodrugs led to rapid expulsion of the corresponding phosphoramidate anions in both series of compounds. Compounds substituted at the 2-position are excellent substrates for human DTD (k(cat)/K(M) = (2-5) x 10(6) M(-1) s(-1)); however, compounds substituted at the 3-position are potent inhibitors of the target enzyme. Both series of compounds are toxic in HT-29 and BE human colon cancer cell lines in a clonogenic assay. There was a correlation found between cytotoxicity and DTD activity for the 2-series of phosphoramidates; however, there was no correlation between cytotoxicity and DTD activity in the 3-series of compounds. This finding suggests the presence of an alternative mechanism for the activation of these compounds.

Modifying rates of reductive elimination of leaving groups from indolequinone prodrugs: a key factor in controlling hypoxia-selective drug release

3-(4-Methylcoumarin-7-yloxy)methylindole-4,7-diones were synthesised as model prodrugs in order to investigate the correlation between rates of reductive elimination from the (indolyl-3-yl)methyl position with reductive metabolism by hypoxic tumor cells and NADPH: cytochrome P450. Rates of elimination of the chromophore/fluorophore (7-hydroxy-4-methylcoumarin) following one-electron reduction of indolequinones to their semiquinone radicals (Q*-) was measured by pulse radiolysis utilising spectrophotometric and fluorometric detection. Incorporation of a thienyl or methyl substituent at the (indol-3-yl)CHR-position (where R=thienyl or methyl adjacent to the phenolic ether linking bond) significantly shortened the half-life of reductive elimination from 87 to 6 and 2 ms, respectively. Elimination from the methyl substituted analogue can thus compete effectively with the reaction of the semiquinone radical with oxygen at levels typically present in tumours (half-life approximately 1.8 ms at 0.5% O2). Chemical kinetic predictions were confirmed by metabolism in breast tumour MCF-7 cells between 0-2.1% O2. Rates of reductive release of the fluorophore from the non-fluorescent parent indolequinones (R=H, Me, thienyl) were similar under anoxia ( approximately 1.7 nmol coumarinmin(-1)mg protein(-1)) reflecting the similarity in one-electron reduction potential. Whereas coumarin release from the indolequinone (R=H) was completely inhibited above 0.5% O2, the enhanced rate of reductive elimination when R=thienyl or Me increased the metabolic rate of release to approximately 0.35 and 0.7 nmol coumarinmin(-1)mg protein(-1), respectively at 0.5% O2; complete inhibition occurring by 2.1% O2. Similar 'oxygen profiles' of release were observed with NADPH: cytochrome P450 reductase. In conclusion, it is possible to modify rates of reductive elimination from indolequinones to control the release of drugs over a range of tumour hypoxia.

Bioreductive and gene therapy approaches to hypoxic diseases

Hypoxia is a feature that exists in most, if not all, solid tumours and hypoxia has been shown to exist in a variety of other diseases. Bioreductive prodrugs have been developed to preferentially target the hypoxic cells in tumours. They are prodrugs, that are reductively activated (catalysed by reductive enzymes) to afford their active (toxic) species. More recently, bioreductive delivery agents that "release" a therapeutic entity preferentially under hypoxic conditions have also been developed to target hypoxia, not only in tumours, but also in a host of other diseases. This new technology platform is described in this review. In addition, we discuss the potential of utilising hypoxia to deliver selective gene therapy based upon the transcription factor HIF-1 and the use of unique genetic sequences termed HRE's (hypoxia responsive elements) that specifically control gene expression under hypoxic conditions. Finally, we describe how these drugs and gene-based therapeutic approaches can be combined to potentially deliver a highly selective form of therapy for cancer and other diseases where hypoxia plays a major pathophysiological role.

2-nitroimidazol-5-ylmethyl as a potential bioreductively activated prodrug system: reductively triggered release of the PARP inhibitor 5-bromoisoquinolinone

5-Chloromethyl-1-methyl-2-nitroimidazole reacted efficiently with the anion derived from 5-bromoisoquinolin-1-one to give 5-bromo-2-((1-methyl-2-nitroimidazol-5-yl)methyl)isoquinolin -1-one. Biomimetic reduction effected release of the 5-bromoisoquinolin-1-one. The 2-nitroimidazol-5-ylmethyl unit thus has potential for development as a general prodrug system for selective drug delivery to hypoxic tissues.

Bioreductively-activated prodrugs for targeting hypoxic tissues: elimination of aspirin from 2-nitroimidazole derivatives

2-Nitroimidazoles were synthesised substituted with aspirin or salicylic acid, as leaving groups linked through the (imidazol-5-yl)methyl position. Activation of aqueous solutions by CO2*- (a model one-electron reductant) resulted in release of aspirin or salicylate, probably via the 2-hydroxyaminoimidazole. The analogous 2-nitroimidazole with bromide as leaving group eliminated bromide in < 1 ms via the radical-anion.