Nitroreductase-mediated cell ablation is very rapid and mediated by a p53-independent apoptotic pathway

Microbial nitroreductases: A versatile tool for biomedical and environmental applications

Nitroreductases, enzymes found mostly in bacteria and also in few eukaryotes, use nicotinamide adenine dinucleotide (NADH) or nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor for their activity and metabolize an enormous list of a diverse nitro group-containing compounds. Nitroreductases that are capable of metabolizing nitroaromatic and nitro heterocyclic compounds have drawn great attention in recent years owing to their biotechnological, biomedical, environmental, and human impact. These enzymes attracted medicinal chemists and pharmacologists because of their prodrug selectivity for activation/reduction of nitro compounds that wipe out pathogens/cancer cells, leaving the host/normal cells unharmed. It is applied in diverse fields of study like prodrug activation in treating cancer and leishmaniasis, designing fluorescent probes for hypoxia detection, cell imaging, ablation of specific cell types, biodegradation of nitro-pollutants, and interpretation of mutagenicity of nitro compounds. Keeping in view the immense prospects of these enzymes and a large number of research contributions in this area, the present review encompasses the enzymatic reaction mechanism, their role in antibiotic resistance, hypoxia sensing, cell imaging, cancer therapy, reduction of recalcitrant nitro chemicals, enzyme variants, and their specificity to substrates, reaction products, and their applications.

Precise spatio-temporal control of rapid optogenetic cell ablation with mem-KillerRed in Zebrafish

The ability to kill individual or groups of cells in vivo is important for studying cellular processes and their physiological function. Cell-specific genetically encoded photosensitizing proteins, such as KillerRed, permit spatiotemporal optogenetic ablation with low-power laser light. We report dramatically improved resolution and speed of cell targeting in the zebrafish kidney through the use of a selective plane illumination microscope (SPIM). Furthermore, through the novel incorporation of a Bessel beam into the SPIM imaging arm, we were able to improve on targeting speed and precision. The low diffraction of the Bessel beam coupled with the ability to tightly focus it through a high NA lens allowed precise, rapid targeting of subsets of cells at anatomical depth in live, developing zebrafish kidneys. We demonstrate that these specific targeting strategies significantly increase the speed of optoablation as well as fish survival.

Immunomodulation-accelerated neuronal regeneration following selective rod photoreceptor cell ablation in the zebrafish retina

Müller glia (MG) function as inducible retinal stem cells in zebrafish, completely repairing the eye after damage. The innate immune system has recently been shown to promote tissue regeneration in which classic wound-healing responses predominate. However, regulatory roles for leukocytes during cellular regeneration-i.e., selective cell-loss paradigms akin to degenerative disease-are less well defined. To investigate possible roles innate immune cells play during retinal cell regeneration, we used intravital microscopy to visualize neutrophil, macrophage, and retinal microglia responses to induced rod photoreceptor apoptosis. Neutrophils displayed no reactivity to rod cell loss. Peripheral macrophage cells responded to rod cell loss, as evidenced by morphological transitions and increased migration, but did not enter the retina. Retinal microglia displayed multiple hallmarks of immune cell activation: increased migration, translocation to the photoreceptor cell layer, proliferation, and phagocytosis of dying cells. To test function during rod cell regeneration, we coablated microglia and rod cells or applied immune suppression and quantified the kinetics of (i) rod cell clearance, (ii) MG/progenitor cell proliferation, and (iii) rod cell replacement. Coablation and immune suppressants applied before cell loss caused delays in MG/progenitor proliferation rates and slowed the rate of rod cell replacement. Conversely, immune suppressants applied after cell loss had been initiated led to accelerated photoreceptor regeneration kinetics, possibly by promoting rapid resolution of an acute immune response. Our findings suggest that microglia control MG responsiveness to photoreceptor loss and support the development of immune-targeted therapeutic strategies for reversing cell loss associated with degenerative retinal conditions.

Caspase-mediated apoptosis induction in zebrafish cerebellar Purkinje neurons

The zebrafish is a well-established model organism in which to study in vivo mechanisms of cell communication, differentiation and function. Existing cell ablation methods are either invasive or they rely on the cellular expression of prokaryotic enzymes and the use of antibiotic drugs as cell death-inducing compounds. We have recently established a novel inducible genetic cell ablation system based on tamoxifen-inducible Caspase 8 activity, thereby exploiting mechanisms of cell death intrinsic to most cell types. Here, we prove its suitability in vivo by monitoring the ablation of cerebellar Purkinje cells (PCs) in transgenic zebrafish that co-express the inducible caspase and a fluorescent reporter. Incubation of larvae in tamoxifen for 8 h activated endogenous Caspase 3 and cell death, whereas incubation for 16 h led to the near-complete loss of PCs by apoptosis. We observed synchronous cell death autonomous to the PC population and phagocytosing microglia in the cerebellum, reminiscent of developmental apoptosis in the forebrain. Thus, induction of apoptosis through targeted activation of caspase by tamoxifen (ATTAC^TM) further expands the repertoire of genetic tools for conditional interrogation of cellular functions.

Conditional Toxin Splicing Using a Split Intein System

Protein toxin splicing mediated by split inteins can be used as a strategy for conditional cell ablation. The approach requires artificial fragmentation of a potent protein toxin and tethering each toxin fragment to a split intein fragment. The toxin-intein fragments are, in turn, fused to dimerization domains, such that addition of a dimerizing agent reconstitutes the split intein. These chimeric toxin-intein fusions remain nontoxic until the dimerizer is added, resulting in activation of intein splicing and ligation of toxin fragments to form an active toxin. Considerations for the engineering and implementation of conditional toxin splicing (CTS) systems include: choice of toxin split site, split site (extein) chemistry, and temperature sensitivity. The following method outlines design criteria and implementation notes for CTS using a previously engineered system for splicing a toxin called sarcin, as well as for developing alternative CTS systems.

Spores of Clostridium engineered for clinical efficacy and safety cause regression and cure of tumors in vivo

Spores of some species of the strictly anaerobic bacteria Clostridium naturally target and partially lyse the hypoxic cores of tumors, which tend to be refractory to conventional therapies. The anti-tumor effect can be augmented by engineering strains to convert a non-toxic prodrug into a cytotoxic drug specifically at the tumor site by expressing a prodrug-converting enzyme (PCE). Safe doses of the favored prodrug CB1954 lead to peak concentrations of 6.3 μM in patient sera, but at these concentration(s) known nitroreductase (NTR) PCEs for this prodrug show low activity. Furthermore, efficacious and safe Clostridium strains that stably express a PCE have not been reported. Here we identify a novel nitroreductase from Neisseria meningitidis, NmeNTR, which is able to activate CB1954 at clinically-achievable serum concentrations. An NmeNTR expression cassette, which does not contain an antibiotic resistance marker, was stably localized to the chromosome of Clostridium sporogenes using a new integration method, and the strain was disabled for safety and containment by making it a uracil auxotroph. The efficacy of Clostridium-Directed Enzyme Prodrug Therapy (CDEPT) using this system was demonstrated in a mouse xenograft model of human colon carcinoma. Substantial tumor suppression was achieved, and several animals were cured. These encouraging data suggest that the novel enzyme and strain engineering approach represent a promising platform for the clinical development of CDEPT.

Baculoviruses as gene therapy vectors for human prostate cancer

Prostate cancer is the most commonly diagnosed cancer in ageing men in the western world. While the primary cancers can be treated with androgen ablation, radiotherapy and surgery, recurrent castration resistant cancers have an extremely poor prognosis, hence promoting research that could lead to a better treatment. Targeted therapeutic gene therapy may provide an attractive option for these patients. By exploiting the natural ability of viruses to target and transfer their genes into cancer cells, either naturally or after genetic manipulation, new generations of biological control can be developed. In this review we present the advantages and practicalities of using baculovirus as a vector for prostate cancer gene therapy and provide evidence for the potential of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) as a safer alternative vehicle for targeting cancer cells. Strategies to target baculovirus binding specifically to prostate cell surfaces are also presented. The large insertion capacity of baculoviruses also permits restricted, prostate-specific gene expression of therapeutic genes by cloning extended human transcriptional control sequences into the baculovirus genome.

Establishment of a transgenic zebrafish line for superficial skin ablation and functional validation of apoptosis modulators in vivo

Background

Zebrafish skin is composed of enveloping and basal layers which form a first-line defense system against pathogens. Zebrafish epidermis contains ionocytes and mucous cells that aid secretion of acid/ions or mucous through skin. Previous studies demonstrated that fish skin is extremely sensitive to external stimuli. However, little is known about the molecular mechanisms that modulate skin cell apoptosis in zebrafish.

Methodology/Principal Findings

This study aimed to create a platform to conduct conditional skin ablation and determine if it is possible to attenuate apoptotic stimuli by overexpressing potential apoptosis modulating genes in the skin of live animals. A transgenic zebrafish line of Tg(krt4:NTR-hKikGR)^cy17 (killer line), which can conditionally trigger apoptosis in superficial skin cells, was first established. When the killer line was incubated with the prodrug metrodinazole, the superficial skin displayed extensive apoptosis as judged by detection of massive TUNEL- and active caspase 3-positive signals. Great reductions in NTR-hKikGR⁺ fluorescent signals accompanied epidermal cell apoptosis. This indicated that NTR-hKikGR⁺ signal fluorescence can be utilized to evaluate apoptotic events in vivo. After removal of metrodinazole, the skin integrity progressively recovered and NTR-hKikGR⁺ fluorescent signals gradually restored. In contrast, either crossing the killer line with testing lines or transiently injecting the killer line with testing vectors that expressed human constitutive active Akt1, mouse constitutive active Stat3, or HPV16 E6 element displayed apoptosis-resistant phenotypes to cytotoxic metrodinazole as judged by the loss of reduction in NTR-hKikGR⁺ fluorescent signaling.

Conclusion/Significance

The killer/testing line binary system established in the current study demonstrates a nitroreductase/metrodinazole system that can be utilized to conditionally perform skin ablation in a real-time manner, and provides a valuable tool to visualize and quantify the anti-apoptotic potential of interesting target genes in vivo. The current work identifies a potential use for transgenic zebrafish as a high-throughput platform to validate potential apoptosis modulators in vivo.

Bystander or no bystander for gene directed enzyme prodrug therapy

Gene directed enzyme prodrug therapy (GDEPT) of cancer aims to improve the selectivity of chemotherapy by gene transfer, thus enabling target cells to convert nontoxic prodrugs to cytotoxic drugs. A zone of cell kill around gene-modified cells due to transfer of toxic metabolites, known as the bystander effect, leads to tumour regression. Here we discuss the implications of either striving for a strong bystander effect to overcome poor gene transfer, or avoiding the bystander effect to reduce potential systemic effects, with the aid of three successful GDEPT systems. This review concentrates on bystander effects and drug development with regard to these enzyme prodrug combinations, namely herpes simplex virus thymidine kinase (HSV-TK) with ganciclovir (GCV), cytosine deaminase (CD) from bacteria or yeast with 5-fluorocytodine (5-FC), and bacterial nitroreductase (NfsB) with 5-(azaridin-1-yl)-2,4-dinitrobenzamide (CB1954), and their respective derivatives.

A mammalianized synthetic nitroreductase gene for high-level expression

Background

The nitroreductase/5-(azaridin-1-yl)-2,4-dinitrobenzamide (NTR/CB1954) enzyme/prodrug system is considered as a promising candidate for anti-cancer strategies by gene-directed enzyme prodrug therapy (GDEPT) and has recently entered clinical trials. It requires the genetic modification of tumor cells to express the E. coli enzyme nitroreductase that bioactivates the prodrug CB1954 to a powerful cytotoxin. This metabolite causes apoptotic cell death by DNA interstrand crosslinking. Enhancing the enzymatic NTR activity for CB1954 should improve the therapeutical potential of this enzyme-prodrug combination in cancer gene therapy.

Methods

We performed de novo synthesis of the bacterial nitroreductase gene adapting codon usage to mammalian preferences. The synthetic gene was investigated for its expression efficacy and ability to sensitize mammalian cells to CB1954 using western blotting analysis and cytotoxicity assays.

Results

In our study, we detected cytoplasmic protein aggregates by expressing GFP-tagged NTR in COS-7 cells, suggesting an impaired translation by divergent codon usage between prokaryotes and eukaryotes. Therefore, we generated a synthetic variant of the nitroreductase gene, called ntro, adapted for high-level expression in mammalian cells. A total of 144 silent base substitutions were made within the bacterial ntr gene to change its codon usage to mammalian preferences. The codon-optimized ntro either tagged to gfp or c-myc showed higher expression levels in mammalian cell lines. Furthermore, the ntro rendered several cell lines ten times more sensitive to the prodrug CB1954 and also resulted in an improved bystander effect.

Conclusion

Our results show that codon optimization overcomes expression limitations of the bacterial ntr gene in mammalian cells, thereby improving the NTR/CB1954 system at translational level for cancer gene therapy in humans.

Nitroreductase-mediated cell ablation in transgenic zebrafish embryos

Prodrug dependent cell ablation is a method that allows inducible and spatially restricted cell destruction. We describe transgenic methods to express the Escherichia coli nfsB in a tissue restricted manner in the zebrafish. This bacterial gene encodes a nitroreductase (NTR) enzyme that can render prodrugs such as metronidazole (Met) cytotoxic. Using the expression of NTR fused to a fluorescent protein, one can simultaneously make cells susceptible to prodrug treatment and visualize cell ablation as it occurs.

E. coli nitroreductase/CB1954 gene-directed enzyme prodrug therapy: role of arylamine N-acetlytransferase 2

Gene-directed enzyme prodrug therapy is a promising approach to the local management of cancer and a number of gene prodrug combinations have entered clinical trials. The antitumor activity of Escherichia coli nitroreductase (NTR) in combination with the prodrug CB1954 relies on the reduction of the nitro groups to reactive N-hydroxylamine intermediates that are toxic in proliferating and nonproliferating cells. We examined whether secondary metabolic activation of the N-hydroxylamines by sulfotransferases or acetyltransferases altered cell responsiveness to the drug. We evaluated the coexpression of NTR with the human cytosolic sulfotransferases SULT1A1, 1A2, 1A3, 1E1 and 2A1, or the human arylamine N-acetyltransferases NAT1 and NAT2 on SKOV3 cell survival. Only NAT2 significantly altered the toxicity of CB1954, decreasing the IC(50) 16-fold from 0.61 to 0.04 microM. These results suggest that one or more of the N-hydroxyl metabolites are a substrate for O-acetylation by NAT2. We also examined the bystander effect of SKOV3 cells expressing NTR or NTR plus NAT2. Addition of the acetyltransferase resulted in a significant decreased bystander effect (P>0.01), possibly due to a lower concentration of reactive metabolites in the culture medium. These results suggest that a combination of bacterial NTR and NAT2 may provide a greater clinical response at therapeutic concentrations of CB1954 provided the reduction in bystander effect is not clinically significant. Moreover, endogenous NAT2, which is localized predominantly in the liver and gut, may be involved in the dose-limiting hepatic toxicity and gastrointestinal side effects seen in patients treated with the higher doses of CB1954.

Ablation of central nervous system progenitor cells in transgenic rats using bacterial nitroreductase system

Specific ablation of central nervous system (CNS) progenitor cells in the brain of live animals is a powerful method to determine the functions of these cells and to reveal novel avenues for the treatment of several CNS-related disorders. To achieve this goal, we generated a line of transgenic rats expressing a bacterial enzyme, Escherichia coli nitroreductase gene (NTR), under control of the nestin promoter. In this system, NTR(+) cells are selectively eliminated upon application of prodrug CB1954, through activation of programmed cell death machineries. At 5 days of age, which is a time when cerebellar development is occurring, transgenic rats bearing the nestin-NTR/green fluorescent protein (GFP) gene are overtly normal and express NTR/GFP in neuronal stem cells, without any toxicity in these cells. The functional consequence of progenitor cell ablation was demonstrated by administering prodrug CB1954 into the cerebellum at this 5-day time point. Stem cell ablation in these neonates resulted in sensorimotor abnormalities, cerebellar degeneration, overall reduction in cerebellar seize, and manifestation of ataxia. In adult rats, GFP expression was not seen in the hippocampal progenitor cells and seen only at very low levels in the lateral ventricles, indicating a different NTR/GFP expression pattern between neonates and adults. In addition, application of CB1954 by intraventricular delivery reduced the number of 5-bromo-2'-deoxyuridine-labeled proliferating cells in the lateral ventricle but not hippocampus of NTR/GFP rats. These findings shows that targeted expression of NTR under a specific promoter might be of significant value in addressing the function of distinct cell population in vivo.

Suicide genes for cancer therapy

The principle of using suicide genes for gene directed enzyme prodrug therapy (GDEPT) of cancer has gained increasing significance during the 20 years since its inception. The astute application of suitable GDEPT systems should permit tumour ablation in the absence of off-target toxicity commonly associated with classical chemotherapy, a hypothesis which is supported by encouraging results in a multitude of pre-clinical animal models. This review provides a clear explanation of the rationale behind the GDEPT principle, outlining the advantages and limitations of different GDEPT strategies with respect to the roles of the bystander effect, the immune system and the selectivity of the activated prodrug in contributing to their therapeutic efficacy. An in-depth analysis of the most widely used suicide gene/prodrug combinations is presented, including details of the latest advances in enzyme and prodrug optimisation and results from the most recent clinical trials.

Targeted ablation of beta cells in the embryonic zebrafish pancreas using E. coli nitroreductase

In order to generate a zebrafish model of beta cell regeneration, we have expressed an Escherichia coli gene called nfsB in the beta cells of embryonic zebrafish. This bacterial gene encodes a nitroreductase (NTR) enzyme, which can convert prodrugs such as metronidazole (Met) to cytotoxins. By fusing nfsB to mCherry, we can simultaneously render beta cells susceptible to prodrug and visualize Met dependent cell ablation. We show that the neighboring alpha and delta cells are unaffected by prodrug treatment and that ablation is beta cell specific. Following drug removal and 36h of recovery, beta cells regenerate. Using ptf1a morphants, it is clear that this beta cell recovery occurs independently of the presence of the exocrine pancreas. Also, by using photoconvertible Kaede to cell lineage trace and BrdU incorporation to label proliferation, we investigate mechanisms for beta regeneration. Therefore, we have developed a unique resource for the study of beta cell regeneration in a living vertebrate organism, which will provide the opportunity to conduct large-scale screens for pharmacological and genetic modifiers of beta cell regeneration.

Repeated cycles of Clostridium-directed enzyme prodrug therapy result in sustained antitumour effects in vivo

The unique properties of the tumour microenvironment can be exploited by using recombinant anaerobic clostridial spores as highly selective gene delivery vectors. Although several recombinant Clostridium species have been generated during the past decade, their efficacy has been limited. Our goal was to substantially improve the prospects of clostridia as a gene delivery vector. Therefore, we have assessed a series of nitroreductase (NTR) enzymes for their capacity to convert the innocuous CB1954 prodrug to its toxic derivative. Among the enzymes tested, one showed superior prodrug turnover characteristics. In addition, we established an efficient gene transfer procedure, based on conjugation, which allows for the first time genetic engineering of Clostridium strains with superior tumour colonisation properties with high success rates. This conjugation procedure was subsequently used to create a recombinant C. sporogenes overexpressing the isolated NTR enzyme. Finally, analogous to a clinical setting situation, we have tested the effect of multiple consecutive treatment cycles, with antibiotic bacterial clearance between cycles. Importantly, this regimen demonstrated that intravenously administered spores of NTR-recombinant C. sporogenes produced significant antitumour efficacy when combined with prodrug administration.

Properties of a telomerase-specific Cre/Lox switch for transcriptionally targeted cancer gene therapy

Telomerase expression represents a good target for cancer gene therapy. The promoters of the core telomerase catalytic [human telomerase reverse transcriptase (hTERT)] and RNA [human telomerase RNA (hTR)] subunits show selective activity in cancer cells but not in normal cells. This property can be harnessed to express therapeutic transgenes in a wide range of cancer cells. Unfortunately, weak hTR and hTERT promoter activities in some cancer cells could limit the target cell range. Therefore, strategies to enhance telomerase-specific gene therapy are of interest. We constructed a Cre/Lox reporter switch coupling telomerase promoter specificity with Cytomegalovirus (CMV) promoter activity, which is generally considered to be constitutively high. In this approach, a telomerase-specific vector expressing Cre recombinase directs excisive recombination on a second vector, removing a transcriptional blockade to CMV-dependent luciferase expression. We tested switch activation in cell lines over a wide range of telomerase promoter activities. However, Cre/Lox-dependent luciferase expression was not enhanced relative to expression using hTR or hTERT promoters directly. Cell-specific differences between telomerase and CMV promoter activities and incomplete sigmoid switch activation were limiting factors. Notably, CMV activity was not always significantly stronger than telomerase promoter activity. Our conclusions provide a general basis for a more rational design of novel recombinase switches in gene therapy.

Antitumor immune responses mediated by adenoviral GDEPT using nitroreductase/CB1954 is enhanced by high-level coexpression of heat shock protein 70

Gene-directed enzyme prodrug therapy (GDEPT) is a promising approach to local management of cancer through targeted chemotherapy. Killing localized tumors by GDEPT in a manner that induces strong antitumor cellular immune responses might improve local management and allow benefit in disseminated cancer. Here we evaluated the combination of nitroreductase (NTR)/CB1954 GDEPT with high-level expression of heat shock protein 70 (HSP70, a stress protein that can shuttle cytosolic peptides into antigen-presenting cells) for induction of antitumor immunity using adenovirus gene delivery in an aggressive and nonimmunogenic BALB/c syngeneic 4T1 breast cancer model. The mechanism of cell death and spectrum of stress proteins induced are likely to be important determinants of the resulting immune responses. We showed that NTR/CB1954 treatment of 4T1 cells gave both apoptotic and nonapoptotic killing. In vivo killing of 4T1 cells expressing NTR gave weak antitumor immunity and very limited induction of stress proteins including HSP70. High-level coexpression of HSP70 during NTR/CB1954-mediated killing of 4T1 cells in vivo gave much greater protection from tumor challenge (67% long-term survivors compared to 17%) and induced 4T1-specific cytotoxic T-cell responses. The enhancement of antitumor responses resulting from HSP70 coexpression was similar to that conferred by coexpression of GM-CSF.

Notch1-expressing cells are indispensable for prostatic branching morphogenesis during development and re-growth following castration and androgen replacement

Notch expression is frequently associated with progenitor cells, and its function is crucial for development. Our recent work showing that Notch1 is selectively expressed in basal epithelial cells of the prostate and higher Notch1 expression during development suggests that Notch1-expressing cells may define progenitor cells in the prostate. To test this hypothesis, we have generated a transgenic mouse line in which the Notch1-expressing cells can be ablated in a controlled manner. Specific targeting was achieved by expressing the bacterial nitroreductase, an enzyme that catalyzes its substrate into a cytotoxin capable of inducing apoptosis, under the Notch1 promoter. Cell death in transgenic prostate was confirmed by histological analyses including terminal dUTP nick-end labeling and caspase 3 immunocytochemical staining. We evaluated the consequences of ablation of Notch1-expressing cells in two systems, organ culture of early postnatal prostates and re-growth of prostate in castrated mice triggered by hormone replacement. Our data show that elimination of Notch1-expressing cells inhibited the branching morphogenesis, growth, and differentiation of early postnatal prostate in culture and impaired prostate re-growth triggered by hormone replacement in castrated mice. Furthermore, we found that Notch1 expression following castration and hormone replacement was concomitant with known basal cell markers p63 and cytokeratin 14 and was high in the proliferative human prostate epithelial cells. Taken together, these data suggest that Notch1-expressing cells define the progenitor cells in the prostatic epithelial cell lineage, which are indispensable for prostatic development and re-growth.

Development of novel selective cell ablation in the mammary gland and brain to study cell-cell interactions and chemoprevention

We have generated transgenic mice which express the gene encoding Escherichia coli nitroreductase (NTR) specifically in the luminal epithelial cells of the mammary gland and the glial cells of the brain. The enzyme activates an antitumour drug CB 1954, to produce a cross-linking agent that kills all cells expressing the enzyme. We have shown that administration of the antitumour drug CB 1954 rapidly and selectively kills these cells. Original experiments demonstrated the ability to ablate the luminal cells in the mammary gland with no apparent bystander effect. Subsequently, astrocytes expressing nitroreductase under the targeting of the GFAP promoter were selectively ablated following administration of the prodrug CB 1954 produces a degeneration of granular neurones due to changes in glutamate levels. Recent experiments demonstrated inhibition of myc-dependent mammary tumours using the same enzyme (nitroreductase)-prodrug (CB 1954), combination. Owing to the ease of control of NTR-mediated cell ablation, we anticipate that this system will supersede herpes simplex virus type 1 thymidine kinase. There are widespread potential applications for this approach in the dissection of complex cellular interactions during development and in the adult organism. The present transgenic models also have important applications for the study in vivo of novel prodrugs that can be selected for variable degrees of bystander effects. Such studies will have particular significance for those groups advocating the use of NTR as an appropriate enzyme for gene-directed enzyme prodrug therapy by providing models of a wide range of human disease for mechanistic and therapeutic experimentation. The results clearly demonstrate that the model has potential to study chemoprevention and fundamental questions on cell-cell interactions in cell biology.

Selective ablation of human cancer cells by telomerase-specific adenoviral suicide gene therapy vectors expressing bacterial nitroreductase

Reactivation of telomerase maintains telomere function and is considered critical to immortalization in most human cancer cells. Elevation of telomerase expression in cancer cells is highly specific: transcription of both RNA (hTR) and protein (hTERT) components is strongly upregulated in cancer cells relative to normal cells. Therefore, telomerase promoters may be useful in cancer gene therapy by selectively expressing suicide genes in cancer cells and not normal cells. One example of suicide gene therapy is the bacterial nitroreductase (NTR) gene, which bioactivates the prodrug CB1954 into an active cytotoxic alkylating agent. We describe construction of adenovirus vectors harbouring the bacterial NTR gene under control of the hTR or hTERT promoters. Western blot analysis of NTR expression in normal and cancer cells infected with adenoviral vectors showed cancer cell-specific nitroreductase expression. Infection with adenoviral telomerase-NTR constructs in a panel of seven cancer cell lines resulted in up to 18-fold sensitization to the prodrug CB1954, an effect that was retained in two drug-resistant ovarian lines. Importantly, no sensitization was observed with either promoter in any of the four normal cell strains. Finally, an efficacious effect was observed in cervical and ovarian xenograft models following single intratumoural injection with low doses of vector, followed by injection with CB1954.

Selective ablation of olfactory receptor neurons without functional impairment of vomeronasal receptor neurons in OMP-ntr transgenic mice

This study used transgenic mice, in which expression of a bacterial nitroreductase (ntr) gene was linked to the expression of olfactory marker protein (OMP). The nitroreductase enzyme is thus expressed in mature chemosensory neurons of these OMP-ntr transgenic mice, and converts the pro-drug CB1954 to a cytotoxic form, specifically killing these neurons. Systemic injections of the pro-drug led to the ablation of receptor neurons in both the main olfactory and vomeronasal epithelia. Due to the anatomical separation of the epithelia, however, when the pro-drug was administered by intranasal infusion only the receptors of the main olfactory epithelium were destroyed. This procedure resulted in a profound deficit in olfactory investigation and discrimination in a habituation-dishabituation test, whereas the pregnancy blocking effect of male pheromones, which is mediated via the vomeronasal system was unaffected. OMP-ntr mice receiving intranasal infusion of pro-drug had not recovered any significant main olfactory function at 24 days following treatment. This novel technique could potentially be applied to selectively ablate olfactory receptor neurons expressing a particular olfactory receptor by linking its expression to that of the nitroreductase enzyme.

Effect of selective ablation of proliferating mammary epithelial cells on MNU induced rat mammary tumorigenesis

Proliferating cells within the terminal end buds of the virgin female rat mammary gland are the most susceptible to chemical carcinogen induced tumorigenesis. We hypothesized that selective ablation of proliferating cells in the mammary gland would reduce mammary tumor incidence upon carcinogen challenge. Selective ablation of proliferating cells was achieved by intraductal injections of Adv-RSV-tk and gancyclovir administration. Despite efficient viral transduction of the thymidine kinase protein and the apparent elimination of >90% of the proliferating cells, the rats exhibited a higher incidence of MNU induced mammary tumors arising with shorter latency as compared to control animals. Several possible explanations of the puzzling relationship between elimination of cycling cells and increased tumor incidence are discussed and alternative strategies for the prevention of breast cancer are proposed.

Inhibition of myc-dependent breast tumor formation in transgenic mice

One of the most promising approaches for cancer gene therapy is the use of the so-called suicide genes, which encode prodrug-activating enzymes and render transduced cells more sensitive to prodrugs. The enzyme nitroreductase (NTR) converts prodrug CB1954 into a cytotoxic DNA interstrand cross-linking agent. We have established transgenic mice in which the pro-oncogene c-myc and NTR were fused to the internal ribosome entry site and coexpressed in luminal cells of the mammary gland under the control of mouse whey acidic protein (WAP) promoter to evaluate NTR mediated ablation of mammary tumors. More than 78% of transgenic females developed in situ or infiltrating carcinomas after three to four pregnancies. By contrast, if the transgenic female mice were given the prodrug CB1954 during their third lactation, the incidence of tumors decreased to less than 40% (P < 0.05). The total number of carcinomas was even more striking with 117 carcinomas identified in 14 non-ablated transgenics compared with only five in 15 treated animals (p < 0.05, student t test). C-myc induced pleomorphic nuclei and mitotic figures were seen as a field change in over 70% of the untreated transgenics compared to 20% in the treated group. Our results suggest that the enzyme pro-drug system NTR-CB1954 efficiently inhibit myc-dependent tumor formation and malignant progression in the mammary gland.

E. coli nitroreductase/CB1954: in vitro studies into a potential system for feline cancer gene therapy

Investigations were carried out to identify a suitable prodrug activating system for feline gene therapy with the eventual aim of treating feline thyroid disease and feline neoplasia. The E. coli nitroreductase (NTR)/CB1954 prodrug activating system was evaluated in vitro in feline cells by transient transfection with a nitroreductase expressing construct and subsequent treatment with the prodrug CB1954. The feline cells successfully expressed E. coli nitroreductase, which was able to activate the prodrug CB1954 resulting in cytotoxicity to both transformed and adjacent cells (a bystander effect) in vitro. In the absence of nitroreductase, CB1954 was non-toxic to feline cells. In addition, the nitroreductase gene was expressed in rat thyroid cells under the control of the cell type specific feline thyroglobulin promoter. This paper demonstrates that the E. coli nitroreductase/CB1954 system may be suitable for in vivo feline gene therapy, and further investigations are warranted.

Conditional ablation of neurones in transgenic mice

Conditional targeted ablation of specific cell populations in living transgenic animals is a very powerful strategy to determine cell functions in vivo. This approach would be of particular value to study the functions of distinct neuronal populations; however, the transgene of choice for conditional cell ablation studies in mice, the herpes simplex virus thymidine kinase gene, cannot be used to ablate neurones as its principal mode of action relies on cell proliferation. Here we report that expression of the E.coli nitroreductase gene (Ntr) and metabolism of the prodrug CB1954 (5-aziridin-1-yl-2-4-dinitrobenzamide) to its cytotoxic derivative can be used to conditionally and acutely ablate specific neuronal populations in vivo. As proof of principal, we have ablated olfactory and vomeronasal receptor neurones by expressing Ntr under the control of the olfactory marker protein (OMP) gene promoter. We demonstrate that following CB1954 administration, olfactory and vomeronasal receptor neurones expressing the transgene were selectively eliminated from the olfactory epithelium (OE), and projections to the olfactory bulb (OB) were lost. The functional efficacy of cell ablation was demonstrated using a highly sensitive behavioural test to show that ablated mice had lost the olfactory ability to discriminate distinct odors and were consequently rendered anosmic. Targeted expression of Ntr to specific neuronal populations using conventional transgenes, as described here, or by "knock-in" gene targeting using embryonic stem cells may be of significant value to address the functions of distinct neuronal populations in vivo.

Gene directed enzyme/prodrug therapy of cancer: historical appraisal and future prospectives

Gene therapy of cancer is a novel approach with the potential to selectively eradicate tumour cells, whilst sparing normal tissue from damage. In particular, gene-directed enzyme prodrug therapy (GDEPT) is based on the delivery of a gene that encodes an enzyme which is non-toxic per se, but is able to convert a prodrug into a potent cytotoxin. Several GDEPT systems have been investigated so far, demonstrating effectiveness in both tissue culture and animal models. Based on these encouraging results, phase I/II clinical trials have been performed and are still ongoing. The aim of this review is to summarise the progress made in the design and application of GDEPT strategies. The most widely used enzyme/prodrug combinations already in clinical trials (e.g., herpes simplex 1 virus thymidine kinase/ganciclovir and cytosine deaminase/5-fluorocytosine), as well as novel approaches (carboxypeptidase G2/CMDA, horseradish peroxidase/indole-3-acetic acid) are described, with a particular attention to translational research and early clinical results.

Pharmacological aspects of targeting cancer gene therapy to endothelial cells

Targeting cancer gene therapy to endothelial cells seems to be a rational approach, because (a) a clear correlation exists between proliferation of tumor vessels and tumor growth and malignancy, (b) differences of cell membrane structures between tumor endothelial cells and normal endothelial cells exist which could be used for targeting of vectors and (c) tumor endothelial cells are accessible to vector vehicles in spite of the peculiarities of the transvascular and interstitial blood flow in tumors. Based on the knowledge on the pharmacokinetics of macromolecules it can be concluded that vectors targeting tumor endothelial cells should own a long blood residence time after intravascular application. This precondition seems to be fulfilled best by vectors exhibiting a slight anionic charge. A long blood residence time would allow the formation of a high amount of complexes between tumor endothelial cells and vector particles. Such high amount of complexes should enable a high transfection rate of tumor endothelial cells. In view of their pharmacokinetic behavior nonviral vectors seem to be more suitable for in vivo targeting tumor endothelial cells than viral vectors. Specific binding of nonviral vectors to tumor endothelial cells should be enhanced by multifunctional ligands and the transduction efficiency should be improved by cationic carriers. Effector genes should encode proteins potent enough to induce reactions which eliminate the tumor tissue. To be effective to that degree such proteins should induce self-amplifying antitumor reactions. Examples for proteins which have the potential to induce such self-amplifying tumor reactions are proteins endowed with antiangiogenic and antiproliferative activity, enzymes which convert prodrugs into drugs and possibly also proteins which induce embolization of tumor vessels. The pharmacological data for such examples are discussed in detail.

Combined adenovirus-mediated nitroreductase gene delivery and CB1954 treatment: a well-tolerated therapy for established solid tumors

Gene-directed enzyme prodrug therapy (GDEPT) is a refinement of cancer chemotherapy that generates a potent cell-killing drug specifically in tumor cells by enzymatic activation of an inert prodrug. We describe in vivo studies that evaluate the efficacy and safety of intratumoral (i.t.) injection of an adenovirus vector (CTL102) expressing Escherichia coli nitroreductase (NTR) combined with systemic prodrug (CB1954) treatment. A single i.t. injection of CTL102 (7.5 x 10(9) to -2 x 10(10) particles) followed by CB1954 treatment produced clear anti-tumor effects in subcutaneous (s.c.) xenograft models of four cancers that are likely candidates for GDEPT (i.e., primary liver, head and neck, colorectal and prostate). Virus dose-response studies (s.c. liver model) revealed a steep increase and subsequent rapid plateauing of both NTR gene delivery and anti-tumor efficacy. Evidence of minor virus spread (toxicity) was observed in a s.c. head and neck xenograft model. This was eliminated by passive immunization with neutralizing anti-Ad5 antibodies prior to virus injection without reducing the magnitude of the anti-tumor effect. Preexisting anti-Ad5 neutralizing antibodies may therefore be an advantage rather than an issue in the clinical use of this new therapy.

Inducible ablation of astrocytes shows that these cells are required for neuronal survival in the adult brain

To study the function of astrocytes in the adult brain, we have targeted the expression of E. coli nitroreductase (NTR) to the astrocytes of transgenic mice under the control of the GFAP promoter. The astrocytes expressing NTR were selectively ablated after administration of the prodrug CB1954, resulting in motor discoordination. Histological examination showed that the region most affected in the brain was the cerebellum, in which the Bergmann glia were eliminated and the granular neurons had degenerated. Specific effects were also noted on the dendrites of the Purkinje cells, and the junction between these neurons and granular layer was disrupted. Astrocyte ablation was associated with a dramatic decrease in the expression of glutamate transporters, which may account for the degeneration of granular neurons since the excitotoxic effects of glutamate result in a similar phenotype. These results provide the first evidence that astrocytes are important for the survival of neurons in the adult brain in vivo.

Prodrug activation enzymes in cancer gene therapy

Among the broad array of genes that have been evaluated for tumor therapy, those encoding prodrug activation enzymes are especially appealing as they directly complement ongoing clinical chemotherapeutic regimes. These enzymes can activate prodrugs that have low inherent toxicity using both bacterial and yeast enzymes, or enhance prodrug activation by mammalian enzymes. The general advantage of the former is the large therapeutic index that can be achieved, and of the latter, the non-immunogenicity (supporting longer periods of prodrug activation) and the fact that the prodrugs will continue to have some efficacy after transgene expression is extinguished. This review article describes 13 different prodrug activation schemes developed over the last 15 years, two of which - activation of ganciclovir by viral thymidine kinase and activation of 5-fluorocytosine to 5-fluorouracil - are currently being evaluated in clinical trials. Essentially all of these prodrug activation enzymes mediate toxicity through disruption of DNA replication, which occurs at differentially high rates in tumor cells compared with most normal cells. In cancer gene therapy, vectors target delivery of therapeutic genes to tumor cells, in contrast to the use of antibodies in antibody-directed prodrug therapy. Vector targeting is usually effected by direct injection into the tumor mass or surrounding tissues, but the efficiency of gene delivery is usually low. Thus it is important that the activated drug is able to act on non-transduced tumor cells. This bystander effect may require cell-to-cell contact or be mediated by facilitated diffusion or extracellular activation to target neighboring tumor cells. Effects at distant sites are believed to be mediated by the immune system, which can be mobilized to recognize tumor antigens by prodrug-activated gene therapy. Prodrug activation schemes can be combined with each other and with other treatments, such as radiation, in a synergistic manner. Use of prodrug wafers for intratumoral drug activation and selective permeabilization of the tumor vasculature to prodrugs and vectors should further increase the value of this new therapeutic modality.

Prodrug activation enzymes in cancer gene therapy

Among the broad array of genes that have been evaluated for tumor therapy, those encoding prodrug activation enzymes are especially appealing as they directly complement ongoing clinical chemotherapeutic regimes. These enzymes can activate prodrugs that have low inherent toxicity using both bacterial and yeast enzymes, or enhance prodrug activation by mammalian enzymes. The general advantage of the former is the large therapeutic index that can be achieved, and of the latter, the non-immunogenicity (supporting longer periods of prodrug activation) and the fact that the prodrugs will continue to have some efficacy after transgene expression is extinguished. This review article describes 13 different prodrug activation schemes developed over the last 15 years, two of which - activation of ganciclovir by viral thymidine kinase and activation of 5-fluorocytosine to 5-fluorouracil - are currently being evaluated in clinical trials. Essentially all of these prodrug activation enzymes mediate toxicity through disruption of DNA replication, which occurs at differentially high rates in tumor cells compared with most normal cells. In cancer gene therapy, vectors target delivery of therapeutic genes to tumor cells, in contrast to the use of antibodies in antibody-directed prodrug therapy. Vector targeting is usually effected by direct injection into the tumor mass or surrounding tissues, but the efficiency of gene delivery is usually low. Thus it is important that the activated drug is able to act on non-transduced tumor cells. This bystander effect may require cell-to-cell contact or be mediated by facilitated diffusion or extracellular activation to target neighboring tumor cells. Effects at distant sites are believed to be mediated by the immune system, which can be mobilized to recognize tumor antigens by prodrug-activated gene therapy. Prodrug activation schemes can be combined with each other and with other treatments, such as radiation, in a synergistic manner. Use of prodrug wafers for intratumoral drug activation and selective permeabilization of the tumor vasculature to prodrugs and vectors should further increase the value of this new therapeutic modality.