Immune enhancement of nitroreductase-induced cytotoxicity: studies using a bicistronic adenovirus vector

Smart nanocarriers for enzyme-activated prodrug therapy

Exogenous enzyme-activated prodrug therapy (EPT) is a potential cancer treatment strategy that delivers non-human enzymes into or on the surface of the cell and subsequently converts a non-toxic prodrug into an active cytotoxic substance at a specific location and time. The development of several pharmacological pairs based on EPT has been the focus of anticancer research for more than three decades. Numerous of these pharmacological pairs have progressed to clinical trials, and a few have achieved application in specific cancer therapies. The current review highlights the potential of enzyme-activated prodrug therapy as a promising anticancer treatment. Different microbial, plant, or viral enzymes and their corresponding prodrugs that advanced to clinical trials have been listed. Additionally, we discuss new trends in the field of enzyme-activated prodrug nanocarriers, including nanobubbles combined with ultrasound (NB/US), mesoscopic-sized polyion complex vesicles (PICsomes), nanoparticles, and extracellular vesicles (EVs), with special emphasis on smart stimuli-triggered drug release, hybrid nanocarriers, and the main application of nanotechnology in improving prodrugs.

Nitroreductase gene-directed enzyme prodrug therapy: insights and advances toward clinical utility

This review examines the vast catalytic and therapeutic potential offered by type I (i.e. oxygen-insensitive) nitroreductase enzymes in partnership with nitroaromatic prodrugs, with particular focus on gene-directed enzyme prodrug therapy (GDEPT; a form of cancer gene therapy). Important first indications of this potential were demonstrated over 20 years ago, for the enzyme-prodrug pairing of Escherichia coli NfsB and CB1954 [5-(aziridin-1-yl)-2,4-dinitrobenzamide]. However, it has become apparent that both the enzyme and the prodrug in this prototypical pairing have limitations that have impeded their clinical progression. Recently, substantial advances have been made in the biodiscovery and engineering of superior nitroreductase variants, in particular development of elegant high-throughput screening capabilities to enable optimization of desirable activities via directed evolution. These advances in enzymology have been paralleled by advances in medicinal chemistry, leading to the development of second- and third-generation nitroaromatic prodrugs that offer substantial advantages over CB1954 for nitroreductase GDEPT, including greater dose-potency and enhanced ability of the activated metabolite(s) to exhibit a local bystander effect. In addition to forging substantial progress towards future clinical trials, this research is supporting other fields, most notably the development and improvement of targeted cellular ablation capabilities in small animal models, such as zebrafish, to enable cell-specific physiology or regeneration studies.

Coexpression of Smad7 and UPA attenuates carbon tetrachloride-induced rat liver fibrosis

Background

There is a great need for developing novel therapies to treat liver fibrosis. Previous studies showed that both Smad7 and uPA were inhibitors of liver fibrosis. Therefore, we explored the therapeutic effects of combinational gene therapy with Smad7 and uPA on CCl4-induced liver fibrosis.

Material/Methods

Smad7 and uPA genes were cloned into an adenovirus vector. To observe the therapeutic effects of coexpression of Smad7 and uPA genes, the recombinant adenovirus were delivered into CCL4-induced fibrosis models. Fibrillar collagen, hydroxyproline, α-SMA, TGF-β₁, MMP-13, TIMP-1, HGF and PCNA were detected to evaluate the fibrosis and to explore the mechanisms underlying the treatment with Smad7 and uPA.

Results

The results showed that single Smad7 or uPA adenovirus reduced CCL4 induced liver fibrosis significantly; while combination of Smad7 and uPA had more significant therapeutic effect on CCl4 induced liver fibrosis. Then the markers underlying the therapeutic effect of combination of Smad7 and uPA were also explored. Over-expression of Smad7 and uPA inhibited the expression of α-SMA and TGF-β₁ significantly. Combinational gene therapy also enhanced extracellular matrix degradation by increasing the expression of MMP-13, inhibiting TIMP-1 expression, and promoted hepatocyte proliferation, while single Smad7 or uPA only induced part of these changes.

Conclusions

These results suggest that combinational gene therapy with Smad7 and uPA inhibited CCl4-induced rat liver fibrosis by simultaneously targeting multiple pathogenic pathways.

Testing double mutants of the enzyme nitroreductase for enhanced cell sensitisation to prodrugs: effects of combining beneficial single mutations

Prodrug activation gene therapy for cancer involves expressing prodrug-activating enzymes in tumour cells, so they can be selectively killed by systemically administered prodrug. For example, Escherichia colinfsB nitroreductase (E.C. 1.6.99.7)(NTR), sensitises cells to the prodrug CB1954 (5-[aziridin-1-yl]-2,4-dinitrobenzamide), which it converts to a potent DNA-crosslinking agent. However, low catalytic efficiency with this non-natural substrate appears to limit the efficacy of this enzyme prodrug combination for eliminating the target cancer cells. To improve this, we aim to engineer NTR for improved prodrug activation. Previously, a number of single amino acid substitutions at six positions around the active site of the enzyme were found to increase activity, resulting in up to approximately 5-fold enhanced cell sensitisation to CB1954. In this study we have made pairwise combinations among some of the best mutants at each of these 6 sites. A total of 53 double mutants were initially screened in E. coli, then the 7 most promising were inserted into an adenovirus vector and compared in SKOV3 human ovarian carcinoma cells for sensitisation to CB1954 and two alternative prodrugs. The most effective mutants, T41L/N71S and T41L/F70A, were 14-17-fold more potent than WT NTR at sensitising the cancer cells to CB1954. The best mutant for activation of the dinitrobenzamide mustard prodrug SN23862 was T41L/F70A (4.8-fold improvement); and S40A/F124M showed 1.7-fold improvement over WT with the nitrobenzylphosphoramide mustard prodrug LH7. In two tumour xenograft models using SKOV3 or human prostate carcinoma PC3, T41L/N71S NTR demonstrated greater CB1954-dependent anti-tumour activity than WT NTR.

A phase I/II clinical trial in localized prostate cancer of an adenovirus expressing nitroreductase with CB1954 [correction of CB1984]

We report a phase I/II clinical trial in prostate cancer (PCa) using direct intraprostatic injection of a replication defective adenovirus vector (CTL102) encoding bacterial nitroreductase (NTR) in conjunction with systemic prodrug CB1954. One group of patients with localized PCa scheduled for radical prostatectomy received virus alone, prior to surgery, in a dose escalation to establish safety, tolerability, and NTR expression. A second group with local failure following primary treatment received virus plus prodrug to establish safety and tolerability. Based on acceptable safety data and indications of prostate-specific antigen (PSA) responses, an extended cohort received virus at a single dose level plus prodrug. The vector was well tolerated with minimal side effects, had a short half-life in the circulation, and stimulated a robust antibody response. Immunohistochemistry of resected prostate demonstrated NTR staining in tumor and glandular epithelium at all dose levels [5 x 10(10)-1 x 10(12) virus particles (vp)]. A total of 19 patients received virus plus prodrug and 14 of these had a repeat treatment; minimal toxicity was observed and there was preliminary evidence of change in PSA kinetics, with an increase in the time to 10% PSA progression in 6 out of 18 patients at 6 months.

Combining gene and immunotherapy for prostate cancer

The nitroreductase (NR)/CB1954 enzyme prodrug system has given promising results in pre-clinical studies and is currently being assessed in phase I and II clinical trials in prostate cancer. Enhanced cell killing by apparent immune-mediated mechanisms has been shown in pancreatic and colorectal cancer models, by co-expressing murine granulocyte macrophage colony-stimulating factor (GM-CSF) with NR in a single replication deficient adenoviral vector. This consists of the CMV immediate early promotor driving expression of NR, with an internal ribosome entry site (IRES) and the gene for murine GM-CSF (mGM-CSF). To examine if similar enhancement of tumour cell killing could be produced in prostate cancer, the TRAMP model was chosen. Results illustrate that the combination of suicide gene therapy using NR and CB1954, with cytokine stimulation with mGM-CSF gives an improved response compared with either modality alone. The mechanism of this improved response is however likely to be non-immune based as it lacks a memory effect.

Targeted-simultaneous expression of Gas1 and p53 using a bicistronic adenoviral vector in gliomas

The targeted expression of transgenes is one of the principal goals of gene therapy, and it is particularly relevant for the treatment of brain tumors. In this study, we examined the effect of the overexpression of human gas1 (growth arrest specific 1) and human p53 cDNAs, both under the transcriptional control of a promoter of the human glial fibrillary acidic protein (gfa2), employing adenoviral expression vectors, in glioma cells. We showed that the targeted overexpression of gas1 and p53 (AdSGas1 and AdSp53, respectively) in rat glioma cells (C6) reduced the number of viable cells and induced apoptosis. Moreover, the adenovirally targeted expression of these genes also reduced tumor growth in vivo. Unexpectedly, there was no additive effect when both gas1 and p53 were simultaneously expressed in the same cells using a bicistronic adenoviral vector. We suggest that Gas1 does not act in combination with p53 in the C6 and U373 glioma cell lines, inducing apoptosis and cell cycle arrest. Our results indicate that the targeted expression of tumor suppressor genes (gas1 and p53) regulated by the gfa2 promoter, together with adenoviral vectors may provide an interesting approach for adjuvant selective glioma gene therapy.

Kinetic and structural characterisation of Escherichia coli nitroreductase mutants showing improved efficacy for the prodrug substrate CB1954

Escherichia coli nitroreductase (NTR) is a flavoprotein that reduces a variety of quinone and nitroaromatic substrates. Among these substrates is the prodrug 5-[aziridin-1-yl]-2,4-dinitrobenzamide (CB1954) that is activated by NTR to form two products, one of which is highly cytotoxic. NTR in combination with CB1954 has entered clinical trials for virus-directed enzyme-prodrug therapy of cancer. Enhancing the catalytic efficiency of NTR for CB1954 is likely to improve the therapeutic potential of this system. We previously identified a number of mutants at six positions around the active site of NTR that showed enhanced sensitisation to CB1954 in an E. coli cell-killing assay. In this study we have purified improved mutants at each of these positions and determined their steady-state kinetic parameters for CB1954 and for the antibiotic nitrofurazone. We have also made a double mutant, combining two of the most beneficial single mutations. All the mutants show enhanced specificity constants for CB1954, and, apart from N71S, the enhancement is selective for CB1954 over nitrofurazone. One mutant, T41L, also shows an increase in selectivity for reducing the 4-nitro group of CB1954 rather than the 2-nitro group. We have determined the three-dimensional structures of selected mutants bound to the substrate analogue nicotinic acid, using X-ray crystallography. The N71S mutation affects interactions of the FMN cofactor, while mutations at T41 and F124 affect the interactions with nicotinic acid. The structure of double mutant N71S/F124K combines the effects of the two individual single mutations, but it gives a greater selective enhancement of activity with CB1954 over nitrofurazone than either of these, and the highest specificity constant for CB1954 of all the mutations studied.

Nitroreductase-based therapy of prostate cancer, enhanced by raising expression of heat shock protein 70, acts through increased anti-tumour immunity

Gene-directed enzyme-prodrug therapy (GDEPT) using nitroreductase (NTR), with efficient adenoviral delivery, and CB1954 (CB), is an effective means of directly killing tumours. However, an immune-mediated bystander effect remains an important product of GDEPT since it is often critical to the elimination of untransduced tumour cells both locally and at distal metastatic sites through generation of tumour-specific immunity without the need for tumour antigen identification or the generation of a personalised vaccine. The mode of induced tumour cell death is thought to contribute to the immunisation process, together with the induction and release of stress proteins. Here, RM-9 murine prostate tumour cells were efficiently killed by adenovirally delivered NTR/CB treatment both in vitro and in vivo, and bystander effects were observed. Cells appeared to die by pathways that suggest necrosis more than that of classical apoptosis. NTR/CB-induced expression of a range of stress proteins was determined by proteomic analysis, revealing chiefly heat shock protein (HSP)25 and HSP70 upregulation, whilst immune responses in vivo were weak. In an attempt to enhance the anti-tumour effect, an adenoviral vector was constructed that co-expressed NTR and HSP70, the latter being a known immune stimulator and chaperone of antigen. This combination elicited significantly enhanced protection over NTR alone for both the treated tumour and a subsequent re-challenge. Protection was CD4+ and CD8+ T cell-dependent and was associated with tumour-specific CTL, IFNgamma and IL-5 responses. The use of such a cytotoxic and immunomodulatory gene combination in cancer therapy warrants further pursuit.

Nitroreductase: a prodrug-activating enzyme for cancer gene therapy

1. The prodrug CB1954 (5-(aziridin-1-yl)-2,4-dinitrobenzamide) is activated by Escherichia coli nitroreductase (NTR) to a potent DNA-crosslinking agent. 2. Virus-mediated expression of NTR in tumour cells sensitizes them to CB1954 in vitro and in vivo, providing the basis for a strategy of cancer gene therapy. 3. A phase I trial of CB1954 in cancer patients has been completed, documenting the pharmacokinetics and establishing an acceptable dose. Subsequent trials of the replication-defective adenovirus CTL102 in patients with resectable tumours have documented expression of NTR in injected colorectal liver metastases, hepatocellular carcinoma, head and neck cancer and prostate cancer. Trials combining CTL102 and CB1954 are underway. 4. An oncolytic (replication-competent) adenovirus vector allowed increased expression of NTR in vitro and in a mouse tumour model, resulting in a greater reduction in tumour growth when combined with CB1954 treatment. 5. Alternative prodrugs may eventually prove superior to CB1954; a nitroaryl phosphoramide mustard prodrug activated by NTR shows a greater therapeutic index than CB1954 in a human ovarian carcinoma. 6. The crystal structure of NTR provided the basis for site-directed mutagenesis, which has identified a number of mutants with improved kinetics of CB1954 activation. These can provide improved cell sensitization to CB1954. Combinations of these are being tested. 7. The basis for a positive selection for improved NTR variants has been demonstrated.

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.

Towards gene therapy in prosthesis loosening: efficient killing of interface cells by gene-directed enzyme prodrug therapy with nitroreductase and the prodrug CB1954

BACKGROUND

Loosening is a major complication in prosthesis surgery. To stabilize loosened orthopedic implants, the interface tissue surrounding the implant must be removed. As an alternative to manual removal, we explored the possibility of removing the tissue by gene-directed enzyme prodrug therapy. In the current study we investigated whether interface cells can be transduced by an HAdV-5 vector carrying the E.coli-derived nitroreductase gene and sensitized to the prodrug CB1954.

METHODS

The gene transfer efficiency into cultures of diploid human interface cells was tested by exposing these cells to various concentrations of Ad.CMV.LacZ. Subsequently, we studied the susceptibility of cells to the NTR/CB1954 combination.

RESULTS

X-gal staining of the Ad.CMV.LacZ-transduced cell cultures revealed that, at 200 plaque-forming units (pfu)/cell, 74% of the cells expressed the LacZ gene. Infection with an NTR construct in interface cell lines resulted in a 60-fold sensitization to the prodrug CB1954. In addition we observed that iotrolan (Isovist) contrast medium had no effect on viability of the cells. However, the presence of the contrast medium completely inhibited adenovirus-mediated gene transfer.

CONCLUSIONS

From these data we conclude that HAdV-5-based vectors carrying nitroreductase can be used to sensitize interface tissue. Instead of contrast medium the clinical protocol will use an alternative visualization procedure.

Enhanced efficacy of Escherichia coli nitroreductase/CB1954 prodrug activation gene therapy using an E1B-55K-deleted oncolytic adenovirus vector

Viruses that replicate selectively in cancer cells constitute an exciting new class of anticancer agent. The conditionally replicating adenovirus (CRAd) dl1520, which lacks the E1B-55K gene, has elicited significant clinical responses in humans when used in combination with chemotherapy. A convergent development has been to use replication-defective viruses to express prodrug-activating enzymes in cancer cells. This can sensitize the cancer to prodrug, but depends upon achieving sufficient level, distribution and specificity of enzyme expression within the tumour. In this study, we have expressed the prodrug-activating enzyme nitroreductase (NTR) in the context of an E1B-55K-deleted adenovirus, CRAd-NTR(PS1217H6). We show that CRAd-NTR(PS1217H6) retains oncolytic growth properties, and expresses substantially more NTR than a comparable, replication-defective adenovirus. The combination of viral oncolysis and NTR expression results in significantly greater sensitization of SW480 and WiDr colorectal cancer cells to the prodrug CB1954 in vitro. In vivo, CRAd-NTR(PS1217H6) was shown to replicate in subcutaneous SW480 tumour xenografts in immunodeficient mice, resulting in more NTR expression and greater sensitization to CB1954 than with replication-defective virus. Combination therapy of CRAd-NTR(PS1217H6) with CB1954 reduced tumour growth from 13.5- to 2.8-fold over 5 weeks, and extended median survival from 42 to 81 days, compared with no treatment.

Clostridia in cancer therapy

During the past decade, the search for an effective system for the selective delivery of high therapeutic doses of anti-cancer agents to tumours has explored a variety of ingenious and increasingly complex biological systems. These systems are most often based on gene therapy and use viral vectors as the delivery vehicle. Invariably, such systems have been found wanting with respect to a lack of tumour specificity, poor levels of transgene expression and inefficient distribution of the vector throughout the tumour mass. By contrast, the ability of intravenously injected clostridial spores to infiltrate, then selectively germinate in, the hypoxic regions of solid tumours seems to be a totally natural phenomenon, which requires no fundamental alterations and is exquisitely specific.

Alternative ways to think about mRNA sequences and proteins that appear to promote internal initiation of translation

Translation of some mRNAs is postulated to occur via an internal initiation mechanism which is said to be augmented by a variety of RNA-binding proteins. A pervasive problem is that the RNA sequences to which the proteins bind were not rigorously proven to function as internal ribosome entry sites (IRESs). Critical examination of the evidence reveals flaws that leave room for alternative interpretations, such as the possibility that IRES elements might function as cryptic promoters, splice sites, or sequences that modulate cleavage by RNases. The growing emphasis on IRES-binding proteins diverts attention from these fundamental unresolved issues. Many of the putative IRES-binding proteins are heterogeneous nuclear ribonucleoproteins that have recognized roles in RNA processing or stability and no recognized role in translation. Thus the mechanism whereby they promote internal initiation, if indeed they do, is not obvious. Some recent experiments were said to support the idea that IRES-binding proteins cause functionally important changes in folding of the RNA, but the evidence is not convincing when examined closely. The proteins that bind to some (not all) viral IRES elements include a subset of authentic initiation factors. This has not been demonstrated with any candidate IRES of cellular origin, however; and even with viral RNAs, the required chase experiment has not been done to prove that a pre-bound initiation factor actually mediates subsequent entry of ribosomes. In short, the focus on IRES-binding proteins has gotten us no closer to understanding the mechanism of internal initiation. Given the aforementioned uncertainty about whether other mechanisms (splicing, cryptic promoters) might underlie what-appears-to-be internal initiation, a temporary solution might be to redefine IRES to mean "internal regulatory expression sequence." This compromise would allow the sequences to be used for gene expression studies, for which they sometimes work, without asserting more than has been proven about the mechanism.

Mechanism of cell death induced by the novel enzyme-prodrug combination, nitroreductase/CB1954, and identification of synergism with 5-fluorouracil

Virus-directed enzyme prodrug therapy (VDEPT) utilising the bacterial enzyme nitroreductase delivered by a replication-defective adenovirus vector to activate the prodrug CB1954 is a promising strategy currently undergoing clinical trials in patients with a range of cancers. An understanding of the mechanism of tumour cell death induced by activated CB1954 will facilitate this clinical development. Here, we report that activated CB1954 kills cells predominantly by caspase-dependent apoptosis. This may have important implications for the generation of immune-mediated bystander effects. Further, the use of a replication-defective adenovirus vector to deliver nitroreductase may negatively affect cellular apoptotic pathways stimulated by activated CB1954. Finally, examination of nitroreductase/CB1954 in combination with conventional chemotherapy reveals a synergistic interaction with 5-fluorouracil. These data will facilitate the further development and future clinical trial design of this novel therapy.